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School of Sciences, Gujarat University, Ahmedabad, INDIA. 2

Chapter 1

Introduction

Abstract The fundamental principles of photochemistry, the process and condition of photo reactions, the process of the photosensitizations, applications of the photochemistry, significance of the study and the brief introduction of the present investigation are described in the introduction section.

1.1 Introduction

Photochemistry is a branch of chemistry that deals with the interactions and chemical reactions

between molecules and electromagnetic radiation (visible, ultraviolet, vacuum ultraviolet radiation).

The interaction of electromagnetic radiation with molecule, photochemistry involves the study of

both chemistry and physics. The study of photochemical processes also involves physical

phenomena that do not involve chemical changes, for example, absorption and emission of the

radiation and energy transfer.

The major source of radiation energy on the earth is the sun's radiation. A number of processes in

living organism involve photochemical reactions such as photosynthesis and vision. Photographic

processes have been in use for well over a century, and these are based on the use of visible

radiation to produce chemical change on the photographic plate. A systematic study of

photochemical reactions started at the end of the 19th century and in the beginning of the 20th

century. The Italian chemist, Giacomo Ciamician, and the German chemist, Paul Stilber, were

among the first to study photochemical reactions at the end of the 19th century using sealed flasks

exposed to sunlight.

The chemical reactions induced in the presence of the light are known as photochemical reactions.

A number of organic compounds from different sources e.g. industry, sewage are discharged into

the water bodies which may be toxic and persistent. These chemicals are not easily removed during

waste water treatment. The natural waters are often contaminated by a variety of organic pollutants

at the trace level. Therefore, it has become very important to establish methods capable of reducing

a significant part of the pollution by destroying the toxic and hazardous organic pollutants. The

classical biological oxidation methods failed in eliminating toxic and persistent organic micro

pollutants to the desired level whereas physiochemical technologies such as flocculation, membrane

filtration or adsorption on activated carbon just transferred them from one phase to another without

School of Sciences, Gujarat University, Ahmedabad, INDIA. Page 13

destroying them. The demand for the efficient oxidation methods has significantly increased over

the last twenty years to improve the quality of waste water for reuse.

Several oxidation technologies suitable for wastewater treatment have been developed such as wet

air oxidation [1], supercritical water oxidation [2] incineration [3] and advanced oxidation processes

(AOPs). The main advantage of AOPs is that it can be used for the treatment of relatively low levels

of pollution in aqueous medium and treatment of the pollutants to more biodegradable compounds

or mineralization into CO2 and other inorganics [4 - 8]. Microbial degradation has become the

primary mechanism for the removal of different pollutant from their flowing aquatic ecosystems [9

– 11]. Radiolabeled dimethyl benzanthracene does not degrade in the dark but is mineralized in

water samples when exposed to the sunlight because of microbial degradation of the photooxidation

products of the hydrocarbon [12]. Photodegradation however does not necessarily render a parent

compound harmless, for example, the photoproducts of aldrin and heptachlor are more toxic to the

test animals than the parent compounds [13].

A refractory compound generally is regarded as being resistant to conventional treatment processes

and as a result is prone to appear in water bodies which receive wastewaters. These compounds

resist natural degradation and are persist in the aquatic environment for long period. Some of these

compounds are toxic to both aquatic life and humans. The organic chloro pesticides are categorized

as refractory. Pesticides are some of the most toxic entities present in the natural environment and

are widespread in surface waters. These compounds are toxic to fish and other aquatic life at low

concentrations, generally having low solubilities in water and many impart taste and odor. Two

structural groups are prominent among the synthetic pesticides, the chlorinated hydrocarbons and

the organic phosphorus compounds. The chlorinated hydrocarbons are the most stable in the natural

environment, residuals being detected in agricultural areas even several years after application. The

presence of pesticides and other refractory compounds in natural waters presents a problem with

public health and the provision of safe public drinking water. The toxicity of these compounds and

their property of accumulating in the body fat of fish to enter food cycle. They are entrapped in our

digestive system and keep accumulation in the human and the animal body and a long term

ingestion of these substances could lead to adverse effects in the humans [14]. The removal of these

compounds from water is of immediate interest. The impact of pesticides and other refractory

compounds on the pollution abatement field is evidenced by the many attempts to provide new

methods of treatment or removal of these compounds from water and waste water streams.

Intensive studies involving the photochemical degradation of organic pollutants like Benzotriazole

[15], Marbofloxacin and Enrofloxacin [16], 1,3-dichloro-2-propanol [17], polycyclic aromatic

hydrocarbons - phenanthrene, pyrene, and fluoranthene[18], 2,4,6-Trichlorophenol [19], Herbicides


[20], different azo dyes [21], removal of Ammonia from indrustrial waste water etc. have been

reported.

Photochemical reactions are of great importance for the life on Earth. Chemical changes taking

place in the atmospheric gases of the earth are initiated by radiations and modified by the suspended

particles. These are very useful for the support of life. The study of upper atmospheric

photochemical reaction have significantly contributed to the knowledge of depletion of ozone layer,

acid rain and global warming.

Photochemical degradation is one of the important techniques to convert toxic organic compounds

into non-toxic material [22, 23]. Generally photochemical techniques are applied for the treatment

of dilute solutions in the concentration range of 10-3 M to 10-6 M [24]. Most of the photosensitized

degradations were carried out with artificial light (visible and near-UV), but good results were also

reported with sunlight. The involvement of sunlight in the removal of synthetic chemicals from the

environment is well documented. The energy of incoming solar spectrum, ultraviolet radiation (λ <

400 nm) accounts to only less that 4%, while the visible light is more than 50%. Hence, effective

utilization of the visible light is an attractive area of photochemical research. The bond dissociation

energy per mole for most of the molecules lies between 150 kJ and 600 kJ. These energies are

available from Avogadro's number of photons of wavelengths lying between 800 nm and 200 nm

respectively, which correspond to the visible and near ultraviolet regions of the electromagnetic

spectrum. The same range of energies is required for electronic transitions in most atoms and

molecules.

The transformation of the parent organic compound takes place in photo degradation in order to

eliminate its toxicity, but the principal objective is to mineralize all pollutants. During the last few

years there has been a growing interest in the use of photo sensitizers for the oxidation of persistent

organic pollutants. In many cases, the photo sensitizers help to mineralize organic pollutants,

forming CO2, H2O, NO3-1, SO4

-2, halide ions, phosphates, oxides, etc., and have been used to

degrade insecticides, herbicides, phenols, PCBs, textile azo dyes and substituted nitrobenzene. The

degradation rates of the parent organic pollutants are determined in the kinetic studies. The decrease

in the concentration of the parent organic pollutant and / or the formation of one or more

degradation products are monitored. A number of methods for photochemical decomposition of

hazardous compounds such as a halogenated benzenes [25], biphenyls [26-30], phenols [31-35],

naphthalenes [36], aniline [37-39], benzoquinones [40], toluenes [41,42], anthraquinones [43-

45],anisoles [46-48] have been reported in the last two decades.


1.2 Fundamental Principles of Photochemistry

Grotthus and Drapper initiated Quantitative approach in the photochemistry in 19th century, by

formulating first law of photochemistry, that states ‘Only the light absorbed can be effective in

producing chemical changes there in’. The radiation absorbed (Iabs) is given by:

(1)

where I0 is the intensity of the incident light and I is the intensity of the transmitted light. These are

related by the well known Beer-Lambert’s law.

(2)

where ‘C’ is the concentration of the absorbing species in moles litre-1, ‘l’ is the path length in cm

and ε’ the molar extinction coefficient and is a function of the frequency of radiation.

The second law of the photo chemistry was first enunciated by stark and later by Einstein (1912).

The law states that “One quantum of light is absorbed per molecule of absorbing species and

reacting substance that disappears”. This law is valid for normal sources of radiation, where the

average number of photon quanta emitted from such sources is between 1013 to 1015 s-1. A number

of exceptions to the above law have been observed using very intense laser radiation, known as

multi-photon excitation.

1.3 Absorption of light

A light wave consists of a continuous range of wavelengths or frequencies. Absorption of

electromagnetic radiation is the way by which the energy of a photon is taken up by molecule. A

particular molecule absorbs radiation of a certain wavelength or frequency. The frequency (ν) of the

radiation absorbed by a molecule is given by Bohr’s frequency rule:

(3)

where E2 and E1 are the energies of the final and the initial states respectively. The energy absorbed

is expressed in terms of ‘Einstein’. This corresponds to the amount of energy of one mole of

photons of a given frequency absorbed by the system. The value of the einstein depends on the

frequency and can be calculated from Planck’s relation. Thus:


(4)

= (6.626 x 10-34

J s x 3 x 108 m s

-1 x 6.02 x 10

23) / λ x 10

-9 m

= (119666 / λ) J mol-1

where λ is expressed in nm. The energy equivalent of einstein corresponding to various

wavelengths is given in the Table 1. It can be seen that in ultraviolet and visible regions the energy

is comparable to the bond energy in a large number of organic and inorganic molecules.

Table 1.1: Energy Conversion Table.

Sr No λ (nm) V (1014 Hz) Einstein (KJ / mol)

1 200 15.0 598.3

2 250 12.0 478.6

3 300 10.0 398.9

4 400 7.5 299.2

5 500 6.0 239.3

6 600 5.0 199.5

7 700 4.286 170.9

Ultraviolet (UV) light is electromagnetic radiation with a wavelength 200 nm to 400 nm. The

sources used for the UV irradiations are Hydrogen lamp, Deuterium lamp. The radiation excites π

→ π* transition and is absorbed by colorless compounds as their absorption bands are obtained

below 300 nm [49]. Visible light waves are the only electromagnetic waves we can see. The range

of visible radiations is 400-800 nm. The tungsten lamp and halogen lamp are the sources for the

visible radiation, which excites some π → π* and n → π* transitions and is useful for coloured

compounds as their absorption bands are obtained above 300 nm.

1.4 Difference between Thermal and Photochemical Reactions

Photochemical reactions have made a considerable impact in synthetic chemistry, both in research

laboratories and in commercial process. Photochemical reactions take place on the absorption of

radiations (photons) by molecules, whereas thermal reactions are initiated by the absorption of heat

energy, normally by an increase of the temperature of the reaction medium. The difference between

thermal chemistry and photochemistry lies in the difference in the distribution of the energy in the


ground and excited electronic states of a molecule, which can lead to major alteration of chemical

behaviour. An electronically excited state of a molecule has a higher internal energy than the

ground state, there exists a much greater choice of reaction product for the excited state on

thermodynamic ground. The comparison is between a reaction A → B and a reaction A → A* → B,

and there will be many systems for which A* → B is thermodynamically favorable where as the

corresponding reaction A → B is not.

Fig 1.1 Absorption Spectrum

1.5 Absorption Spectrum

Molecules in their ground electronic states, exist in their lowest vibrational level (i.e. v = 0, where v

is the vibrational quantum number). When radiation is absorbed by a molecule, the molecules are

excited both electronically and vibrationally and a broad band is observed and the energy

distribution of the broad band is governed by Franck-Condon principle. The principle states that

“the electronic excitation and deactivation in a molecule occurs so rapidly (~ 10-15 s) that the nuclei

retain their relative positions and velocity remains unchanged”, also known as vertical transition.

When an electron absorbs a photon of proper energy, it goes from lower quantum state to an upper

quantum state. The time required in this absorption act (electronic excitation) is very small (i.e. 10-

15 s) and thus the relative positions and velocities of the nuclei before and after the electronic

transition are nearly same. The intensity distribution of each band is proportional to the square of

the transition moment integral <M>, defined as:

<M> = ∫ Ψini Σeri Ψfin dτ (5)


Ψini and Ψfin are the wave functions of the initial and the final quantum states respectively and eri

is the electronic dipole moment operator. Transition is allowed if <M> ≠ 0 and not allowed

(forbidden) if <M> = 0. Due to this absorption transitions from any vibrational level of the ground

electronic state to any vibrational level of the upper electronic state will be observed so long <M> is

finite. Hence a number of transitions are observed. Since the value of each integral is different for

different combination of initial and final quantum states, the energy of each transition will be

different. The probability of a transition between different vibrational levels is not equal and the

intensity of the transition is represented by the height of the arrow in the spectra (Figure 1).

Every molecule possesses electronic states or molecular orbitals (m. o.) and energy levels

corresponding to each electronic state. The classification depicts three kinds of m. o.’s, i.e. σ, π and

n (non-bonding), as shown in Figure 2. Which also depicts the two kinds of anti-bonding orbitals

(σ*, π*) of energies larger than their respective bonding orbitals. The type of transitions observed

when the electron is excited from different bonding or non-bonding m. o. are classified as: n → π*,

n → σ*, π → π* σ → σ* etc. The transitions are governed by the selection rules, based on quantum

mechanical expressions. n → π* and n → σ* are forbidden (or weak) transitions possessing

molecular extinction coefficient of ~ 10 - 102 cm-1 mol-1 cm3 while σ → σ* and π → π* are very

strong transitions.

The energy gap between different m. o.’s are in order of ∆E (n → π*) < (π → π*) < (σ → σ*), i.e.

λmax, (n → π*) > λmax (π → π*) > λmax (σ → σ*). σ → σ* transition are observed in vacuum

UV. The wavelengths and intensities of these transitions are affected by the presence of the

different substituents on the parent molecules.

1.6 The Deactivation Process

A molecule excited to a higher energy state must return to the ground state eventually, unless it gets

involved in a photochemical reaction and loses its identity. This process is accompanied either with

change of mass in the reactants or change in the molecular structure (for example, isomerization).

There are more than one pathways available to the excited molecule for dissipation of the excitation

energy. The different pathways are grouped under photophysical processes in electronically excited

molecules. Some are intrinsic properties of the molecule and are unimolecular while some others

depend on external perturbations and may involve bimolecular collisions. All the photophysical

processes must occur in a time period less than the natural radiative lifetime of the molecule and

priorities are established by their relative rate constants. The loss of the energy of the excited

molecule can take place by two processes.


(A) Radiative process

(B) Non Radiative process.

Radiative process: This involves three kinds of processes.

(1) Direct Deactivation: The light energy excites the bonding electrons of the molecule which

absorb energy and get excited to higher energy singlet state (S1) in a time period 10-15 s

obeying Franck – Condon principle, having opposite spin pair of electrons return to the

ground state by emitting radiation of the same wave length. The loss of energy is fast and it

does not result in the photochemical process.

(2) Fluorescence: The excited molecules in the Singlet excited state can quickly dissipate some

of their excess energy to the surrounding media following vibrational relaxation to come

down to the ground vibrational level by a mechanism known as “Internal conversion”(IC).

The rate constant of the internal conversion is 1014 – 1011 s-1 and is radiationless transition

between states of the same spin multiplicity (e.g. Sn ~~> Sm<n or Tn ~~> Tm<n). When the

molecule returns from the zero vibrational level of the first excited singlet state to ground

state by radiative process, in the absence of photochemical reaction called “Fluorescence”.

The Fluorescence emission has a time period of 10-9 to 10-7 s. Fluorescence is a radiative

decay between states of the same spin multiplicity (i.e. ∆s=0).

(3) Phosphorescence: The singlet excited (S) state molecule can switch over to the triplet

excited state of the molecule by “Inter System Crossing” (ISC) process, which has lower

energy than singlet excited state [S]. When the molecule returns from the zero vibrational

level of the first excited triplet state to ground state, the radiation is called

“Phosphorescence”. It has a time period of 10-3 to 10-2 s. Phosphorescence is a radiative

decay between states of different spin multiplicity (i.e. ∆s≠ 0).


Fig 1.2 Jablonski Energy Diagram

Non-Radiative process

The photo chemical reaction occurs when the singlet excited state does not loose energy by the

radiative process but excess energy is transferred to vibration energy level. The molecule undergoes

different type of photo reaction. The excited molecule without emitting the energy in the form of

the radiation undergoes the process of internal conversion of energy to vibrational level. The

excited vibrational level may lead to the decomposition of molecule giving a new product.

The singlet-excited state in some cases is transformed into triplet state through ISC and vibrational

energy levels of the molecule are excited and the triplet state of the molecule may decompose to

give product.


1.7 Process and condition of photo reaction:

The process of the transfer of the energy to the molecule can be of two type.

(1)Direct Absorption of Light

(2) Indirect energy transfer

(1) Direct absorption of light

The direct absorption of light and photo product formation takes place in Ultra - Violet region of the

light. The sample molecule absorbs U.V. light and goes to the excited state which can give the

photochemical reaction. A mercury vapor lamp is used as an UV source which gives radiation

254nm, 270nm and 300nm. Direct irradiation of the sample leads to the promotion of the molecule

to its excited singlet state (IC), which may undergo inter system crossing (ISC) to produce triplet

state.

The triplet state then can undergo several processes such as homolysis, heterolysis, or photo

ionization [50]. The energy of UV light is enough to cause fission of a single and sometimes even a

double bond present in a molecule. The bond-breaking reactions are of the homolytic kind, yielding

a pair of radicals or the fission of the heterolytic kind, yielding a cation and an anion. The

photolysis of alkyl, vinyl and some phenyl halides in solution gives ion-derived products.

Homolytic cleavage of C–Si bond of p-trimethylsilylmethylacetophenone [51], chlorobenzene [52],

C-O bonds excited triplet states of biphenyl derivatives [53] have been reported. The compounds

having a 9-anthryl or a 1-pyrenyl group in the structure are reported to give the heterolytic C---O

bond cleavage giving products e.g. 1-hydroxy-2-pyridone and arylmethyl methyl ether in

methanolic solution [54].

A number of papers have been reported on direct photo degradation of inorganic and organic

compounds by direct UV irradiation [55 - 60]. Direct degradation of pharmaceutical products like

Glaphenine [61], Carprofen [62], Diclofenac [63], Tiaprofenic acid [64, 65], Suprofen [66, 67],

Promethazine [68], Diazepam [69], Midazolam [70] have been quoted.

Darren L et al [71] have reported the UV - induced degradation rates of 1,3,5-Triamino-2,4,6-

Trinitrobenzene (TATB) with variations in temperature, humidity, and illumination.

The effect of UV irradiation on herbicide 2,4-D [72], atrazine [73] , atmospheric degradation of 2-

Butanol, 2-Methyl-2-butanol, 2,3-Dimethyl-2-butanol by OH-initiated reaction [74], endocrine


disrupting chemicals (EDCs) like bisphenol A, ethinyl estradiol and estradiol [75], acrylonitrile-

butadiene-styrene and polycarbonate [76] have been reported.

UV degradation of plastic materials e.g. PMMA plastic optical fibers, polyisoprenes [77]

polyethylene, polypropylene and an ethylene–propylene copolymer [78], poly(methyl methacrylate)

and its vinyltriethoxysilane containing copolymers [79], endocrine disruptor di-n-butyl phthalate

[80], nisoldipine [81], polypropylene [82] have been quoted.

(2)Indirect Transfer of energy:

All the compounds absorb radiation but they do not give photochemical reaction. An alternative

method has been described to induce compounds which do not give product by direct absorption to

give photo chemical reaction. The following three types of methods are used for the transfer of the

energy indirectly to the sample.

(a) UV exposure in the presence of different reagent

(b) Photosensitized reaction

(c) Photo-catalysis

(a) UV exposure in the presence of different reagent

UV degradation of many compound in the presence of different reagents have been reported. The

systems like hydrogen peroxide / UV, ozone / UV, Fe (II) / H2O2 / UV, AgSbO3 / UV, S2O82-/UV,

H2O2/UV have been used for the degradation of paracetamol, dichloroacetic acid, trichloroethene,

poly (ethylene glycol), diethyl phthalate, rhodamine B, dimethyl sulfoxide, n-butylparaben and 4-

tert-octylphenol isoprene and 1, 2, 9, 10 - tetrachloro decane in the aqueous medium.

Photodegradation of pharmaceuticals and personal care products in the presence of UV and

UV/H2O2 has been reported by Ilho Kim et. al. [83]. Photodegradation of malachite green [84], 4-

aminoantipyrine [85], diethyl phthalate [86] and many others by UV/H2O2 have been reported.

Hideyuki Katsumata et al [87] have reported the degradation of polychlorinated dibenzo-p-dioxins

in aqueous solution by Fe (II) / H2O2 / UV system.

Photocatalytic degradation of organic compounds under UV Light irradiation in the presence of

AgSbO3 was reported by J. Singh and S. Uma [88] and the aqueous degradation of butylated


hydroxyanisole by UV / S2O82- has been reported by Tim K. Lau, Wei Chu, and Nigel J. D. Graham

[89].

Degradation of carbofuran [90], quinoline [91], methyl tert-butyl ether [92], tert-butyl alcohol [93],

tert-butyl formate and its intermediates [94] by an ozone / UV process in dilute aqueous solution

have been reported.

(b) Photosensitized reaction:

Photosensitized phenomenon involves the photo reactions of the molecules which are different from

those absorbing the radiations. The reactions are shown to be of great significance in photobiology

and also provide valuable insights in to photophysical processes.

Photosensitization is the indirect method of the excitation of the sample molecule using another

molecule which absorbs energy. A suitable molecule is excited by exposing it to the radiation which

can transfer its energy to the sample molecule by a number of energy transfer method. The

molecule which is used for absorption of the radiation and transfer of energy to the sample

molecule is called ‘sensitizer’ and the process is called ‘photo sensitization’ [95]. The sensitizer

molecule does not undergo chemical change itself but only transfers energy to the substrate

molecule which undergoes photo chemical reaction. The method allows photo physical and

photochemical changes in the energy acceptor molecule by the electronically excited energy donor

molecule. The photosensitized reaction by the electronic energy transfer mechanism has become

one of the most useful processes in photochemistry. Photosensitized reaction is one of the important

techniques for the decomposition of organic molecules. Photosensitized reaction allows sample

molecules to accept energy which are called the acceptor molecule by the electronically excited

sensitizer molecule which donates energy to the receptor molecule and returns to the ground state.

D + hυ → D* (6)

D* + A → D + A* (7)

The direct light absorption forms electronically excited donor (D*) molecule. It can transfer the

electronic energy to suitable acceptor molecule (A), present in the solution resulting in the de-

excitation of D* to D and electronic excitation of A to A*. The energy transfer occurs before D*

looses energy by radiative process.

The acceptor molecule thus excited indirectly, can undergo various photo physical and

photochemical processes. The characteristic feature of the photosensitized reaction is that light-


absorbing species remains unchanged while acceptor molecule undergoes chemical changes. One of

the very well known and important photosensitized chemical reactions is the photosynthesis by

plants, in which the green chlorophyll molecules of leaves are light absorbing molecules and CO2

and H2O are acceptor molecules.

Electron transfer and hydrogen transfer are the two important processes widely seen in the

photosensitized reaction although other types of material transfer reaction can occur. These

processes play a significant role in many chemical and biological processes.

Photosensitized electron-transfer reactions: The electron transfer process proceeds with the

participation of a donor excited state to a non-excited acceptor state. A number of biological

processes involve ET reactions including oxygen binding, photosynthesis, respiration, and

detoxification routes. Ghosh et. al. [290] have reported the electron transfer from dimethylaniline to

coumarin dye in hydroxypropyl γ-cyclodextrin. Photosensitized electron-transfer-reactions of some

cyclopropene derivatives [96], polymethine-cyanine dye [97], several interfacial systems e.g. lipid

bilayer membranes (vesicles), water-in-oil microemulsions and a solid SiO2 colloidal interface [98],

beta-cyclodextrin [99], 2, 5-diaryl-1,5-hexadiene and related compounds [100] have been reported.

Photosensitized proton - transfer reactions: The transfer of a proton from one molecule to

another may be considered as one of the most general and important reactions in chemistry [1]. The

electron transfer reactions which involve only exchange of charges only between the reactants, but

the transfer of a proton also results in the transport of the mass. The proton transfer reactions could

be fast enough to complete relatively fast non-radiative transitions from higher excited states (S2,

S3 etc.). The proton transfer normally takes place from equilibrated lowest vibrational level from

first excited electronic state, Wavelength dependence in some cases suggests that higher vibrational

levels may also be involved [7-9]. Since proton transfer reaction is associated with a charge

separation along with a mass transfer, the process gets modified remarkably in the excited state

compared with that in the ground state because of the charge redistribution upon photoexcitation.

There are a number of excellent reviews covering different aspects of the excited state proton

transfer (ESPT) process. Fahrni et. at. [291] have reported the excited-state intramolecular proton

transfer in 2-(2'-Tosylaminophenyl) benzimidazole. Excited state proton transfer effect of 7-

hydroxyquinoline in dimethyl sulfoxide solvent has been quoted by Yang-xue et. al. [292]. Shizuka

[294] has reported the excited-state proton-transfer reactions and proton-induced quenching of

aromatic compounds [295]. Excited state proton transfer process in selenourea [296], 3-

hydroxyflavone [297] have been quoted.


The important factors for photosensitization are:

(a) Absorption spectrum of the donor (sensitizer) and acceptor (substrate) molecules

(b) The singlet and the triplet energies of the donor (sensitizer) and acceptor (substrate) molecules

(c) The quantum yield of the triplet formation of the sensitizer

(a) Absorption spectrum of the donor (sensitizer) and acceptor (substrate) molecules

The energy transfer is large if the absorption spectrum of the acceptor and the emission spectrum of

the donor overlap. The molecules have characteristic vibrational energy levels and hence they can

provide a large number of approximately resonant paths for energy transfer. The greater the number

of such resonant paths, the larger will be the energy transfer probability. Therefore, the probability

of the transfer of energy is a function of the extent of overlap between emission spectrum of the

donor and the absorption spectrum of the acceptor. The stokes shift for the vibrationally relaxed

systems (the rate of transfer < the rate of vibrational relaxation), the transfer between like molecules

is less efficient than that between unlike molecules when acceptor is at a lower energy level.

(b) Singlet and triplet energies of the donor and acceptor molecules

The energy transfer will be efficient if the excited state of the acceptor to be populated is of the

lower energy than that of the donor. The spin selection rule requires that the total multiplicity of the

donor and the acceptor, prior to and after the act of transfer, must be preserved.

When ETD > ETA, i.e., transfer is exothermic by 3 - 4 kcal/mol, transfer occurs on nearly every

collision, i.e. The rate of transfer is diffusion-controlled.

When ETD ≈ ETA, the quenching rate drops suddenly, probably due to the possibility of back

transfer and the energy transfer may show temperatute dependence.

When ETD < ETA by 3 – 4 kcal or more, the quenching rate becomes at least a million times slower

than the diffusion – controlled rate.

The singlet-singlet and triplet-triplet energy transfers can take place in a suitable donor-acceptor

system so chosen as to provide conveniently located energy levels. Aromatic carbonyl compounds

are good sensitizers of triplet state because of small singlet-triplet splitting and aromatic

hydrocarbons and olefines have convenient singlet-triplet levels to act as good triplet quenchers or

acceptors.


(c) The quantum yield of the triplet formation of the sensitizer

High quantum yield of the triplet formation of the sensitizer is favourable for photosensitization.

Engel and Monroes have reviewed the photosensitization reactions, giving singlet and triplet

energies of various compounds, quantum yields of fluorescence and phosphorescence, internal

conversion and intersystem crossing and various types of reactions the compounds can undergo.

A number of sensitizers are reported for the photo chemical reaction. Different class of compounds

such as (A) hydrocarbons like naphthalene, benzene (B) aliphatic amines [101,102], aromatic

amines [103], (C) dyes like eosin [104], methylene blue [105], phenothiazine [106], and ketones

like hydroquinone [107,108].benzophenone [109,110] have been used as photo sensitizer.

Photo sensitizers have a stable electronic configuration which is in a singlet state in their ground

state energy level. The absorption of a photon of light of specific wavelength by the molecule

promotes it to an excited state, which is also a singlet state and is short lived. The photo sensitizer

returns to the ground state by emitting a photon or by internal conversion with a part of the energy

loss as heat. It is also possible that the molecule may convert to the triplet state via intersystem

crossing over which involves a change in the spin of an electron. The triplet excited state of the

photo sensitizer has lower energy than the singlet excited state, but has a longer lifetime (typically >

500 ns for photo sensitizers) and this increases the probability of energy transfer to the substrate

molecules. The tendency of a photo sensitizer to reach the triplet state is measured by the triplet

state quantum yield, which measures the probability of the formation of the triplet state per photon

absorbed (depending on the interaction of the singlet species with other substrates producing

fluorescent quenching).

The Process of the photosensitization:

The photosensitized reaction can be of several types which depend upon the process of energy

transfer from the excited sensitizer molecule to the substrate molecule. There are four different

types of path way to transfer the energy to the substrate molecules.

(1)Dipole-Dipole energy transfer:

(2)Energy transfer by exciplex formation:

(3)Quenching of fluorescence:


(4)Energy transfer to triplet oxygen:

(1)Dipole-Dipole energy transfer:

Dipole moments depend upon the electro negativity of atom in the molecule. The ground state

dipole moment has a magnitude Mr but in the excited state Me and also direction is different. The

interaction between the excited sensitiser molecule and the ground state substrate molecule can

takes place without contact with each other by dipole-dipole inter action and energy can be

transfered from sensitizer to substate molecule. Dipole – Dipole energy transfer from several

fluorinated and methylated benzenes to cyclopentanone and I-pyrazoline [111] and carotenoids to

chlorophyll [112] have been measured by G.M. Breuer and K. Razi Naqvi respectively.

(2) Exciplex Formation:

A Transient complex formation between the excited state of the sensitizer molecule and the

substrate molecule may be involved which is called exciplex. The exciplex may or may not have

their own characteristic emission or absorption spectrum. The excited Sensitizer molecule forms an

exciplex with substrate molecule which do not form complex in the ground state. The process can

take place:

A + hυ → A*1

(8)

A*1

+ B → (AB) *1

→ A + B* + heat (9)

The exciplex may become an ion pair exciplex if the charge transfer is complete. The ion pair has

longer lifetime. The dissociation may occur via ion pair complex.

(A) *1

+ Q → (A1Q)

* → (A

+/----Q

-/+) → A + Q (10)

The presence of a heavy atom in the substrate molecule facilitate the exciplex dissociation via Inter

system crossing over and the formation of the triplet state.

A*1

+ Q → (A1Q)

* → A

3 + Q (11)

J.P.Soumallion and B.Dewolf have reported the formation of a radical cation via the triplet excimer

and nucleophilic attack in the solvent in chloro methoxy benzene. Chesta et al. [293] have reported

decomposition of chlorobenzenes using naphthalene as sensitizer and the reaction has been

explained via exciplex mechanism [113]. Energy-transfer study of a triplet exciplex of

cyclohexanone and mesitylene has been reported by Wilson et. al. [114]


Solvent role in exciplex formation

Solvent molecules interact with excited molecule, which comes to the ground state by losing excess

energy as heat.

A* + S → A + S + heat (12)

Polar solvents show solute-solvent interaction forming polar exciplex, which loses excess energy as

heat and comes to the ground state.

A* + S → (A

- S

+)* → A + S + heat (13)

Larger is the solute-solvent interaction; more will be the energy loss. The dispersion force

interactions are important in the non-polar solvents. Some solvents, which contain heavy atoms can

produce enhancement of phosphorescence at the cost of the fluorescence. e.g. ethyl iodine,

nitromethane and carbon disulphide.

(3) Quenching of the fluorescence:

The sensitizer molecule absorbs energy at longer wavelength and is excited to the singlet excited

state and undergoes internal conversion process and molecule emits fluorescence due to the

deexcitation process. The energy of the excited state molecule which emits fluorescence is

transferred to another molecule by non-radiative mechanism. The second molecule gets energy and

goes to the excited state and undergoes various photo physical and photochemical reactions. A

Bimolecular reaction process inhibits emission of radiation because frequency of bimolecular

collisions in the solution as well as in the gas phase competes with the fluorescence emission. This

process is called “quenching.”

Quenching by adding impurity in BpBr and cis 1-3 pentadiene system has been reported [115].

A*1

+ Q → A + Q*1

(14)

BpBr + hv → BpBr*

→ Bp+ + Br

- (15)

Bp+*1

+ cis 1-3 pentadiene → Bp+ + cis 1-3 pentadiene

*1 (16)

(Quench the fluorescence) (17)


Quenching by added substances occurs by two mechanisms [116]:

1) Charge transfer mechanism

2) Energy transfer mechanism

1) Charge transfer mechanism: when two dissimilar molecules collide, attractive tendencies

are usually larger, depending on the polar and the polarizability properties. The interaction

between two molecules involves some degree of charge transfer and the complexes between

excited fluorescent molecules and added foreign molecules are formed which are called

“exciplex”. The absorption spectra remain unchanged, in contrast to the ground state

interactions, because of the short life of the complexes, but if high concentractions can be

built up by the use of very intense light then the absorption spectrum can be observed.

A*1

+ Q → (A1 Q)

* → A

3 + Q → A

1 + Q (18)

In the absence of heavy atom effect where the extent of charge transfer is large enough, the exciplex

lifetimes may be long enough for them to degrade by light emission.

Nanosecond flash photolysis study of anthracene, 9-methylanthracene, 9, 10-diphenylanthracene

and pyrene in the presence of diethylaniline as a fluorescence quencher shows the formation of the

triplet state by the charge-transfer mechanism [117]. Charge - transfer quenching of singlet oxygen

O2 (1∆g) by amines and aromatic hydrocarbons has been quoted by Darmanyan and Jenks [118].

Stern-Volmer equation

The lifetime of the excited state molecule and the quenching constant are determinable quantities

using Stern-Volmer equation. The loss of the energy of the excited state is given in term of lifetime.

The lifetime of the excited state and the quenching constant can be determined from the plot of the

ratio of quantum yield in the absence of the quencher molecule and quantum yield in the presence

of the quencher (φf0 / φf) Vs. concentration of the quencher [Q]. The plot is called Stern-Volmer

plot. The plot is straight line and the slope of the plot gives the quenching constant of the excited

state.

If the reaction takes place from the singlet excited state in competition with the reaction via the

triplet excimer, the Stern-Volmer plot will be linear with a positive slope, at the low concentrations

of the quencher but would be flattened as the concentration of the quencher increases [119].


This indicates that when all the triplet states are quenched at high concentration of the quencher, the

singlet state continues to react. Thus the triplet excimer is the sole product determining intermediate

for the photodecomposition reaction.

Now,

(19)

(20)

Where,

Ia = Rate of absorption or rate of formation of activated molecule

Kq = Rate constant for bimolecular quenching

Kf = Rate constant for fluorescence

KIC = Rate constant for internal conversion

KISC = Rate constant for inter system crossing

ΣKi = Sum of KIC + KISC

[A*]0 = Fluorescer concentration in the absence of the quencher

[A*] = Fluorescer concentration in the presence of the quencher

(21)

(22)

Where,

φf0 = Quantum yield in the absence of quencher

φf = Quantum yield in the presence of quencher


The ratio of two quantum yields is,

(23)

(24)

(25)

Where,

KSV = Kqτ = Stern Volmer constant = the ratio of bimolecular quenching constant to unimolecular

decay constant (liter/mole)

τ = 1/Kf + ΣKi = Actual time of fluorescer molecule in the absence of bimolecular quenching

The value of KSV is obtained from the slope of the plot of φf0/φf Vs [Q]. If τ is measured

independently with the knowledge of KSV, the rate constant Kq for bimolecular quenching step can

be determined.

(c) Energy transfer mechanism by electron transfer:

The energy transfer mechanism by electron transfer has become one of the most useful processes in

photochemistry. It has wide application as a mechanistic tool and in photochemical synthesis. It

allows photo physical and photochemical changes in the acceptor molecule by the electronically

excited donor molecule. There are two mechanisms by which the triplet state photo sensitizer can

react with substrate molecule; these are known as the Type I and Type II reactions.

Type I reaction mechanism

Type I reaction involves the photochemical cleavage or homolysis of aldehydes and ketones into

two free radical intermediates. The carbonyl group accepts a photon and is excited to a singlet state

and forms triplet state by intersystem crossing. The cleavage of the α-carbon carbon bond from

either state, two radical fragments are obtained which react rapidly, usually with the triplet state

oxygen, resulting in the production of highly reactive singlet state oxygen species (e.g. the

superoxide and the peroxide anions).


Type II reaction mechanism

A type II reaction is the photochemical intramolecular abstraction of a γ - hydrogen (which is a

hydrogen atom three carbon positions removed from the carbonyl group) by the excited carbonyl

compound to produce a 1, 4-biradical as a primary photoproduct.

M. Nowakowska et al. [120,121] have reported that carbazole is very useful photo sensitizer for the

dechlorination of pentachloro phenols and polychlorinated benzenes. The process has been

explained involving electron transfer mechanism from excited carbazole molecule to substrate

molecule.

(4)Energy transfer to triplet oxygen:

The excited sensitizer molecule produces the excited and highly reactive state of the oxygen known

as the singlet oxygen. The excited triplet state of the photo sensitizer directly transfers energy to the

triplet ground state molecular oxygen which results in the photo sensitizer returning to its singlet

ground state and the formation of singlet excited oxygen.

Fig. 1.3. Scheme of electronic transitions between the ground and the lowest singlet states of

molecular oxygen.

Numbers in brackets denote the vibrational transitions. The wavelengths indicate the main maxima

of the absorption and luminescence spectra in the gas phase.


Photooxygenation:

Singlet oxygen was first observed in 1924 and found as more reactive form of oxygen. The main

method of production of singlet oxygen is photosensitization. Kautsky first proposed that singlet

oxygen might be a reaction intermediate in the dye sensitized photo oxygenation. It has been

demonstrated that 1O2 can react with different molecules and give oxidation product.

Fig. 1.4 Energy diagram showing mechanisms of generation of singlet oxygen by the singlet

and the triplet states of dye molecules (Terenin’s mechanisms). Horizontal arrows show that

deactivation of the excited states of the dye are accompanied by transition of the oxygen molecule

into the singlet state.

Sensitizer molecule absorbs the energy of the light that raises the molecule to an excited singlet

state and is converted to the triplet state by ISC. The excitation energy of the sensitizer molecule is

transferred to 3O2 molecule, and Converts it to the singlet state, while photo sensitizer molecule

returns to the ground state.


S + hv → 1S* (26)

(27)

3S* +

3O2 → S0 +

1O2* (28)

A + 1O2* → AO2 (29)

S= Sensitizer, A = Substrate

Photosensitized oxidation by singlet oxygen of some pesticides like methylparathion [122],

endosulfan [123] and atrazine [124] has been reported. The photo-oxidation of some organo-

nitrogen compounds have also been reported [125-127]. Kinetic study of the singlet molecular

oxygen mediated photo degradation of some hetero aromatic compounds has been reported [128].

Methylene blue photosensitized hydroxylation and oxidation with singlet oxygen have been

reported in the literature [129- 133].

The effect of the dissolved oxygen on the treatment of 2-chlorophenol [134,135] has been reported.

Palumbo et al. [136] have reported the kinetic and mechanistic study for the dye sensitized photo-

oxidation of mononitrophenols and monochlorophenols via singlet molecular oxygen.

Sensitizers:

A number of different photo sensitizers have been used for the photo reaction of organic

compounds. Reviews have been published on the photosensitized degradation of some important

pesticides with different pathways and the reaction mechanisms. Different class of compounds have

been used as sensitizers for the photo chemical reaction (A) hydrocarbons like naphthalene,

benzene, anthracene, rubrene. (B) aliphatic amines [101], aromatic amines [102], (C) dyes like

eosin [103], methylene blue [104], rose bengal phenothiazine, crystal violet [105,106] and ketones

like hydroquinone [107,108] benzophenone [109,110].

A brief discussion is presented for the different photo sensitizers used.

Hydrocarbons:

The aromatic hydrocarbons are fluorescent. The fluorescence efficiency increases with increase of

the number of the condensed ring as the planarity of the ring increases. The substituents on the ring

which produce steric hindrance reduce mobility of π electrons and decrease the fluorescence. The


aromatic hydrocarbons transfer the energy to the substrate molecule by the quenching of the

fluorescence or the exciplex formation.

Chesta et al. [182] have reported decomposition of chloro-benzenes using naphthalene as sensitizer

and the reaction has been explained via exciplex mechanism. Soumillion et al. [183] have reported

the use of anionic sensitizer such as naphthoxide anion as excited donor for the photo

dechlorination of chloroaromatics and compared it with other sensitized systems. 1- and 2-

naphthols have been widely used as photo sensitizer in photochemical reactions for the study of

energy transfer reactions [184,185]. Soltermann et al. [186] have reported photo-dechlorination of

trichloro- benzenes (TCBs) sensitized by naphthalene-triethylamine. The mechanism suggested is

via exciplex formation.

Aliphatic and aromatic amines:

Aliphatic and aromatic amines contains non bonding electrons on nitrogen atom which absorbs the

energy from the incident light and get excited to the singlet state and form triplet excited state via

ISC. The T1 excited state is formed by n→π* excitation.The triplet excited state transfers the energy

to the substrate molecules. The amino compounds give electron transfer reaction via triplet excited

state.

Davidson et al. [148] have discussed the mechanistic aspects of the triethylamine assisted photo

induced dehalogenation of some halo aromatic compounds. Occhiucci et al. [149] have suggested

that the photolysis of PCBs was enhanced by using triethylamine as a sensitizer. Lin et al. [150]

have reported the diethylamine as an efficient photo sensitizer for the photo degradation of PCB

congeners using sunsimulated light.

M. Ohashi et al. [151] have analyzed the Stern-Volmer Plot of the aliphatic and aromatic amines

assisted photo dechlorination of 4-chlorobiphenyl and showed that the reaction from the singlet

state is more efficient than that from the triplet state. The authors have also reported the

dechlorination of meso-substituted mono- and dichloroanthracene via their radical anions, produced

by diffusion controlled reaction of the singlet excited states of chloro anthracene with the ground

state of aliphatic and aromatic amines in acetonitrile at room temperature [152].

Several reports have been published in last few years on photochemical reactions, in which aliphatic

and aromatic amines are used as photo sensitizer [153]. C. A. Chesta et al. [154] have studied the

photosensitized dechlorination of chlorinated benzenes by using N, N-dimethylaniline and the

electron transfer mechanism from both the singlet and the triplet state has been suggested. Chen. et.


al. [155] have reported the use of triethylamine hydroiodide as a supporting electrolyte in the dye-

sensitized solar cells.

Dye:

Different type of dyes has received considerable attention as laser dyes, textile industries and also

used as a photo initiator for photo polymerization. Dyes have λmax in the visible range and absorb

visible radiation and form excited molecule. The absorption of the visible radiation at their λmax

results in the singlet excited state [156]. The cationic dyes give two type of reaction (a) Electron

transfer and (b) Photo oxygenation via singlet excited state of oxygen. A number of cationic dyes

have been used as photosensitizer.

Electron transfer by Methylene Blue sensitizer: Tanielian et al [157] have explained mechanistic

and kinetic aspects of photosensitization by methylene blue. Methylene blue sensitized degradation

through photo induced normal electron transfer [158] and electron transfer in the upper excited state

[159] has been reported by Kojima et al. [160] and Das et al. [161] have reported methylene blue

sensitized decarboxylation of the substituted carboxylic acids via photochemical electron transfer

mechanism across liq/liq interface. A Photo induced electron transfer process between ketone triplet

states and organic dyes (methylene blue, thiopyrinine, safranine and phenosafranine) has been

reported by Jockusch et al. [162].

Photo-oxidation by Methylene Blue sensitizer: Methylene blue sensitized photo-oxidation of the

hydroxyl and amino derivatives of naphthalenes have been investigated by Chawla et al. [163].

Silva et. al have reported methylene blue sensitized photo oxidation by singlet oxygen of Lysozyme

[164].

Rose Bengal Dye as a Sensitizer:

T. Sehili et al. [165] have used rose bengal as photo sensitizer for the photo-oxidation of

chloropheol (CP) in alkaline aqueous solution and identified photoproducts. A wavelength

dependent mechanism for rose bengal-sensitized photoinhibition of red cell acetylcholinesterase

was reported by Tedd Allen et al [166]. Rose bengal-sensitized photo-oxygenation of the

tryptophan alkyl esters [167], 2-chlorophenol [168], sulfathiazole and succinylsulfathiazole [169]

have been reported. The effects and mechanism of the methylene blue or the chlorophyll sensitized

photo-oxidations of soyabean oil have been studied by Ramshaw and coworkers [170]. Chlorophyll

sensitized photoreduction of methyl red and crystal violet [171], peroxidation of the saturated fatty


acid esters [172], photoreduction of nitro compounds in a heterogeneous system [173] indoleacetic

acid [174], safranine [175] have been reported.

Riboflavin as a Sensitizer:

Riboflavin-sensitized photoreaction of the tryptophan is reported by Silva E et.al. [176]. Riboflavin

sensitized photooxidation of tyrosine [177], horseradish apoperoxidase [178], ascorbic acid [179]

have been reported. The riboflavin-sensitized photo-oxidation of choline metabolism in cultured rat

lenses have been reported by Howard M. Jernigan et.al. [180]

Riboflavin-sensitized photooxidation of isohumulones and derivatives has been reported by Kevin

Huvaere et.al [181].

Ketones

Ketones absorb incident light and are excited to the singlet excited state and give the triplet excited

state via ISC and transfer the energy to the triplet state of the substrate molecules. The MO suggests

that a pair of nonbonding electron of the carbonyl oxygen in aldehydes and ketones which are

directed in the plane of the molecule and perpendicular to the C=O bond are excited. Ketones as a

sensitizer transfer the energy to the substrate molecule via triplet n, π* excited state.

When an electron releasing group is suitably attached to the aromatic ring of the ketones, the charge

migrate to the carbonyl oxygen producing charge transfer state (CT) e.g. p-methyl amino

benzophenone [137].The different type of the excited states have different type of electron

distribution and exhibit difference in their photochemical reactivity.

Aliphatic Ketone:

G. G. Choudhry et al. [138] have studied acetone sensitized and non-sensitized photolysis of tetra-,

penta- and hexachlorobenzenes in acetonitrile-water mixture and identified the photoproducts. J.

Hawari et al. [139] have discussed acetone induced photodechlorination of aroclor-1254 in the

alkaline 2-propanol and proposed a probable mechanism by thermolysis in the presence of di-tert-

butyl peroxide.

Aromatic ketones:

Teijiro Ichimura et al [140] have reported the quenching of the rate of DPA by triplet

benzophenone. Benzophenone-sensitized photo-degradation of polyolefins, phenothiazine,


polyethylene, polystyrene films, polypropylene and rearrangement of furylidentetralones have also

been reported. Patrick J. Kerzic et. al. [141] have reported the inhibition of NF-κB by hydroquinone

sensitized human bone marrow progenitor cells to TNF-α-induced apoptosis. Koji Yamada et. al.

[142] have reported that Nitrile-forming radical elimination reactions of 1-naphthaldehyde O-(4-

substituted benzoyl) oximes activated by triplet benzophenone.

Ketones show two types of reaction

(1) proton abstraction

(2) Photo-oxidation

Electron/proton transfer reaction of sensitizer:

R.G.Brinson et.al [143] have reported the proton abstraction and electron transfer photo reaction by

anthraquinone. R.E.Galian et.al. [144] have reported that the intramolecular electron transfer

between tyrosin and trytrophan photo sensitized by aromatic ketone. Cai et. al. [145] have

suggested the mechanism of the sensitized reactions by benzophenone in the triplet excited state.

Photo oxidation by Singlet oxygen:

The excited triplet state of ketones can excite triplet state oxygen by triplet – triplet energy transfer

and produce the singlet state oxygen. Santi Nonell et al. [297] have reported that aromatic ketones

as standards for singlet molecular oxygen photosensitization. Canonica, S. et. al. [147] have

reported the oxidation of phenylurea herbicides by triplet aromatic ketones via singlet O2 in

aqueous medium. A laser flash photolysis study for the reactivity of aromatic amines with triplet 1,8

dihydroxy anthraquinone has been reported by Y. Pan et. al [146].

(b) Photo-catalysis:

A semiconductor is used to transfer energy to the substrate molecules in the photo catalysed

reactions. The electron of the valence band of semiconductor is promoted to the conduction band

on exposure to light, leaving a (+) ve hole in the valence band and an excited electron in the

conduction band. The substrate molecule can directly react with (+) ve hole or excited electron or

through free radical OH• generated by water molecules. Photo-catalysis is one of the techniques for

the degradation of environmental organic pollutants [187]. The semiconductor does not take part in

the chemical reaction. The photo catalytic reactions can be initiated so as to drive the reaction in a

particular direction depending on the band gap between valence band and the conduction band of

the semiconductor The process is called ‘Photo catalysis’ [188].


4. Photo catalytic Reaction:

The metals are good conductors of electricity while non-metals are insulator and do not conduct

electricity. Some elements and their compounds are metalloids, which have ability to conduct

electricity to some extent. The elements like B, Bi, Ge, As, Sb, Te, Si, Ti, Zr, Mo and Cd are known

as semiconductors. Some of the compounds e.g. SiO2, TiO2, ZnO, ZnS, CdS show properties of

semiconductor. There exits two energy bands for each semiconductor (a) Valence Band (VB) and

(b) Conduction Band (CB). The two energy bands are separated by energy gap of particular energy.

This energy gap is different for different semiconductor. Chemical reactions in the presence of a

semiconductor and light are known as photo catalytic reactions. Semiconductors with suitable

band gaps can act as quantum collectors.

The conductive mechanism of a semiconductor can be illustrated by silicon or germanium. Silicon

and germanium exist in the crystalline diamond structure and four other atoms surround each atom.

Each pair of electrons is covalently shared between adjacent atoms. The electrons at the lower

temperature in the ground state are not free to move. The excitation of the electron is done by

exposing them to the light corresponding to energy gap of conduction band and valence band. The

place within the crystal from where the electron is removed is termed as a ‘Hole’. An electron

jumps from near by atom to fill this hole and this helps in migrating the hole. The semiconductor is

irradiated by light which have the higher energy than the energy gap between the valence band and

the conduction band. The substrate molecule coming in contact with the surface of the

semiconductor undergoes chemical reaction.

The surface electrons of the semiconductor are excited by the absorption of light radiation, which

are promoted to the conduction band from the valence band, which results in positive holes in the

valence band. Surface attracts hydroxyl group from the solution yielding absorbed OH° radicals.

The OH° radicals attack on substrate molecule and products are formed.

Mechanism

SC + hυ → SC* (30)

SC* → e

- (CB) + h

+ (VB) (31)

h+ + H2O → H

+ + OH° (32)

S + OH° → Products (33)


An electron transfer process is involved in a photo catalytic reaction from semiconductor to

substrate or vice-versa. Following reactions can occur on the surface of an irradiated

semiconductor:

The oxidation or reduction of the substrate molecule occurs via electron transfer from the

conduction band electron to the substrate molecule or electron transfer from the substrate molecule

to the valence band of the semiconductor. The substrate molecule thus undergoes chemical reaction.

However semiconductor remains unchanged. The reactions can be initiated by an irradiated

semiconductor and be driven in a particular direction depending on the energy of the band gap of

the semiconductor. Thus many reactions can be initiated under favorable conditions in the presence

of the semiconductor.

Environmental applications and some mechanisms of semiconductors as photocatalysts have been

reviewed in the literature for the decomposition of pollutants [187]. Ameta et al. have reviewed the

applications of the semiconductors in photo catalytic oxidation, reduction and degradation [188].

Different semiconductors have been used in different conditions for the degradation of different

compounds.

Titanium Dioxide (TiO2)

Titanium dioxide is a widely used semiconductor in photo catalysis. A number of papers have been

published on the photo degradation of the halo aromatic pollutants [189-191]. Bekbolet et al. [192]

have studied the photo catalytic decomposition of the chlorinated benzaldehyde. Photo catalytic

degradation of some chlorinated benzenes have been reported in the literature [193,194].

Rao et al. [195] have reported the photo degradation of some chloro-hydrocarbons in the aqueous

suspension of MO3 / TiO2 (M = Mo or W). The studies of photodechlorination of

polychlorobiphenyls (PCBs) in the presence of TiO2 have been reported [196-198]. The photo

catalytic decomposition of phenols and their chlorinated derivatives using titania alone or the

combination of it with different other catalysts have been reported. [199].

The use of TiO2 and UV light irradiation for the wastewater treatment and for the decomposition of

chlorinated phenols has been reported [200,201]. The influence of H2O2 on the photo catalytic

mineralization rate of 3- and 4- chlorophenol has been reported in the suspension of TiO2 exposed

to UV radiation [202]. Complete mineralization of 4-chlorophenol in the presence of hydroquinone

[203] and 4- chlorocatechol [204] has been achieved by photo catalytic degradation of oxygenated

solutions containing suspended TiO2. The photo catalytic degradation of polychlorophenols have


been carried out using TiO2 suspension and UV irradiation for the purpose of wastewater treatment

[205].

Zinc Oxide (ZnO)

Zinc oxide (ZnO) a semiconductor has been found very useful photo catalyst. T. Sehili et al. [206-

208] have studied photo catalyzed transformation of chlorinated aromatic compounds especially

chlorinated phenols using ZnO and have also reported the photo catalyzed transformation of

dichlorobenzene in aqueous suspension of ZnO [209].

Miscellaneous

H. Kawaguchi et al. [210] have studied the kinetics of Fe3+ promoted photodecomposition of 2-

chlorophenol. Tang et. al. [211] have studied photo catalytic oxidation of 2, 4-dichlorophenol using

CdS in acidic and alkaline medium. Wada et al. [212] have studied photo reductive dechlorination

of chlorinated benzene derivatives using ZnS nanocrystals. Enhancement of the rate of the photo

catalytic oxidation of the chloro phenols in the presence of Mn2+ in TiO2 suspension has been

studied by Jong - Nan et al [213].

1.8 Quantum Efficiency (φ):

The efficiency of a reaction initiated by the absorption of the photons is expressed in terms of

quantum yield or efficiency (φ). It is a measure of the efficiency of the use of light in

photochemical reaction. This is defined as:

(34)

The quantum efficiency is a fundamental quantity, which is useful in the study of the photochemical

mechanisms. The influence of the experimental variables on the φ-value, gives important

information about the nature of the reaction. The quantum yields are calculated as primary quantum

yield, the product quantum yield, quantum yield of the fluorescence, the decomposition and the

rearrangement. Quantum yield of the product has been proved very useful for the proper evaluation

of a photochemical mechanism.

The rate of the formation of some stable product can be measured by a chemical or instrumental

method, irrespective of whether it is formed directly in primary process or in the secondary reaction


involving free radicals or atoms. The quantum yield of any stable product X from the

photodecomposition of reactant R is defined as:

(35)

Normally maximum quantum efficiency of a primary photochemical reaction is less than 1. The

decomposition of the excited molecule giving the product can have maximum value of φ as 1. In

some cases the product formation may be much higher than the excitation. The formation of a free

radical is normal in such reactions which are chain reactions.

Small quantum yields of all the decomposition products (φ < < 1) indicate deactivation,

fluorescence or other processes that lead to a small chemical change. Large quantum yields (φ > >

1) indicate the photochemical change forming the products [214]. The sensitized quantum yield of

the product formation can be defined as the number of molecules formed from energy acceptors

divided by the number of quanta absorbed by energy donors.

The shape of the plot of inverse of the quantum yield (1/φ) against the inverse of the concentration

of the substrate (1/[C]) gives an important clue to the reaction mechanism. The relationship between

φ and concentration of the substrate suggests that whether the reaction will occur via singlet state or

triplet state [115]:

1) If the plot of the inverse of the quantum yield (1/φ) against the inverse of the concentration of the

substrate (1/[C]) is horizontal with zero slope, the product formation will take place via singlet-

excited state and the quantum yield of the photochemical reaction will be independent of the

substrate concentration.

The reaction is independent of the substrate concentration as the substrate molecule directly

decomposes from singlet-excited state to give product.


substrate (1/[C]) is linear with a positive slope, the product formation will take place via singlet-

excited state and exciplex formation and the quantum yield of the photochemical reaction will be

dependent on the substrate concentration.

The substrate molecule decomposes from singlet-excited state via exciplex or excimer mechanism

to give product, which increases with the increase in the concentration of the substrate.



substrate (1/[C]) is curved, the singlet excited state forms product in competition with the triplet

excited state via excimer formation and the quantum yield of the photochemical reaction will be

dependent on the substrate concentration.

The substrate molecule goes to the triplet state via ISC and then it decomposes via exciplex or

excimer mechanism to give product.

N. J. Bunce [215] has developed expressions to calculate the quantum efficiency of the reaction in

solution, where the reaction products compete with starting material for absorption of the incident

light. He determined the quantum yield of the reaction and tested whether the excimer participates

or not in the selected photoreactions. These expressions are less useful (i) if the products are

themselves photo-labile or (ii) if the products interacts photo chemically with the starting materials

by energy transfer or electron transfer processes.

Φ - Value of some chloroaromatics in different solvents has been reported in the literature [216].

1.9 Product identification

The product formed in the photochemical degradation of the organic compounds can be identified

by using techniques like (TLC), (GC) [150], or (HPLC) [217] by comparison of retention time with

authentic sample. The structure of the photodecomposition product can be identified by following

the fragmentation pattern of the mass spectrum with the help of GC-MS [218]. The formation of the

product in the photochemical reaction can be confirmed by UV-Visible spectrophotometry and

fluorimetry by the comparison of UV-Visible [219] spectra with standard sample.

1.10 Applications

(1) Photo degradation of the Pharmaceutical Compounds

The photo chemical behavior of the pharmaceutical compounds in the water bodies is becoming

important area of the photo chemistryand the use of photo chemistry literature which help

understand and predict the aquatic fate and impact about the pharmaceutical compounds. Some

indications that photochemical degradation may be a central factor to determine the environmental

fate of pharmaceutical and personal care products (PPCPs).


The molecules which are of the pharmaceutical interest may contain a particular group in their

structure or may be photo active compounds and may undergo decomposition. The photo

decomposition products and their structure are of significant interest wether they show toxicity.

It has been found that many PPCPs are not completely removed and degraded during the waste

water treatment. Some compounds create hormonal mimicking effects at part per trillion levels

[7].Many of these compounds feature aromatic ring, hetero atoms and functional groups like

phenol, nitro and also napthoxyl that undergo photo degradation. The presence of propranolol,

naproxen and nabumetone in water bodies has been detected. The compound contain photo active

group in the structure. The compound decomposes on exposure to sun light gives photo products.

Some of the product has been reported to be toxic.

The triclosen is rapidly photo degraded at pH 7.6 and converted to 2,8- dichlorodibenzo-p-dioxin,

which is more harmful compound [154]. The fluoroquinolone antibiotics, which undergo photo

reaction like substitution, fragmentation, oxidation and decarboxylation in water in neutral

condition.

(2) Photo degradation of the Pesticides

Some pesticides like carbaryl and napropamide contain the napthoxyl chromophore in the structure

which can undergo degradation by internal quenching of the fluorescence on exposure to the

sunlight [265, 266]

The toxic effects of halogenated pesticides have been established by various biomedical studies.

The direct contact of halogenated solvent has been reported to cause irritation, pulmonary edema,

and inflammation of the respiratory tract. The injury of cornea, liver damage, neurological effects,

irritation of eyes, nose and throat have also been reported in the case of sever exposure [267, 268].

The chlorinated pesticides induce chronic effects in the human beings and animals. The pesticides

are lipophilic in nature and also contain some specific functional groups. The pesticides undergo

biochemical reactions in the body, which can produce toxic effects. It has been reported that higher

concentration of the chlorinated pesticides in human body can cause liver injury,

pathomorphological changes in liver, urological problems, carcinogenesis, neurotoxicity and effect

on the reproductive system [269 - 279].

(3)The compounds in personal care products

Photochemical reactions occur in skin exposed to the sunlight and synthesis of many compounds,


such as vitamin D, is triggered by light. At the same time, exposure to the sunlight could induce

unwanted photochemical reactions in the skin and prolonged exposure to sun causes skin cancer

and sunburn, which may be prevented by applying sunscreens. The role of the sunscreen is to

protect the skin from the "damaging" UVA and UVB radiation by absorbing this radiation and

releasing the energy as heat.

The free radicals cause damage to DNA and cell, which are produced in the body upon interaction

of different chemical agents with light and exposure to sun is related to various types of cancer,

aging, and cataract. The free radicals are produced either by direct photolysis or by

photosensitization, a process during which molecules in excited states transfer their energy to other

molecules in ground state e,g, triplet oxygen is a good energy acceptor and can be excited to its

singlet state forming superoxide species which can induce DNA damage.

The personal care products available in the market especially sunscreen lotion and de pigmentation

product contain P-amino benzoic acid, Benzophenone and hydroquinoe. The compounds are very

stable and are known as photo sensitizer. The small concentrations of these compounds are reported

to be present in water bodies. The exposure of the compounds used in personal care products to

sunlight excites them and excited molecule can give two types of reaction.

(a) Proton abstraction from other molecule.

(b) Energy transfer to triplet oxygen to convert into singlet oxygen.

The other organic molecules present in the water bodies can undergo sensitized photoreaction or

oxidation reaction from singlet oxygen. The photo products thus produced have to be evaluated for

their toxic effects.

(4) Photo degradation of the Dyes

The direct exposure of chlorinated dyes has been reported to cause sarcoma, cathartic effects,

pathological changes in liver and urinary bladder, tubular degeneration of kidneys, high rate of

mortality in human beings as well as in animals [280 - 282].

Nitro aromatic compounds are hazardous for human beings as well as animals in very trace amount

[267]. T. Tabuchi et al. [273] have reported the toxicity to skin and urinary system caused by p-

chloronitrobenzene. Chloronitrobenzenes also show toxicity to eyes, blood and immune system.

These are hazardous substances and are grouped under environmental pollutants [283].


(5) Photo degradation of the Insecticides

An insecticide is a pesticide used against insects. Insecticides are used in agriculture and the

household. Nearly all insecticides have the potential to significantly alter ecosystems; many are

toxic to humans; and others are concentrated in the food chain. Schwack et. al. [284] have reported

the photodegradation of the carbamate insecticide propoxur. Naman et.al. [285] have reported the

photo-oxidative degradation of insecticide dichlorovos by a combined semiconductors and organic

sensitizers in aqueous media. Photodegradation of organophosphorus insecticides azinphos-methyl,

chlorpyrifos, malathion and malaoxon in aqueous solution have been reported by Kralj et. al. [286].

Photodegradation of the carbamate insecticide ethiofencarb [287], nitromethylene neonicotinoids

[288], organophosphorus insecticides (ethyl-parathion, methyl-parathion, fenitrothion, fenthion

[289] have been reported.

1.11 Significance of the Study

The number of possible heterocyclic systems is almost limitless. An enormous number of

heterocyclic compounds are known and their photochemistry has been studied.

The photochemistry of heterocyclic compounds which are widely distributed in nature and are

essential to life are reported. They play a vital role in the metabolism of all living cell, for example

the pyrimidine [221 - 223] and purine [224 - 227] bases of genetic material DNA; the essential

amino acids proline [228, 229], histidine [230 - 232] and tryptophan [233 – 235]; the vitamins and

coenzyme precursors thiamine [236, 237], riboflavine [238-242], pyridoxine [243 - 245], folic acid

[246 – 248] and biotin; the B12 and E families of vitamins; the photosynthesizing pigment

chlorophyll; the oxygen transporting pigment hemoglobin and its breakdown products the bile

pigments; the hormones kinetin [249], hetroauxin, serotonin [ 250, 251] and most of the sugars.

The photochemistry of a vast number of pharmacologically active hetrocyclic compounds, many of

which are in regular clinical use have been studied. Some of these are natural products for example

antibiotics such as penicillin and cephalosorin, alkaloids such as vinblastine, ellipticine and cardiac

glycosides such as those of digitalis. The photochemistry of anti – inflammatory and analgesic

drugs such as suprofen [66, 67], thiorphan [252, 253], tiaprofenic acid [64, 65], antipyretic drugs

such as apazone [254], Phenylbutazone[255], phenazone [256], antihistamic drugs like ranitidine

[257], azathioprine[258], cimetidine [259], drugs acting on the central nervous system such as

barbital [260], barbituric acid [252], probarbital [261], thiobarbital [260, 261], thoiphenobarbital

[260, 261] have been reported. The photochemistry of many sulfa drugs such as sulfamethoxazole


[262], sulfadiazine [263], sulfadimethoxime [264] and many others and thier environmental effect

have been studied.

The photochemical study of many pesticides, insecticides, weedkillers and rodenticides have been

quoted. The photochemical effect of many thio containing dyes, copolymers, photographic

sensitizers and devlopers, antioxidants and vulcanization accelerators in the rubber indrustry have

been studied.

1.12 The Present Investigation

The study reports photosensitized reaction of thio organic heterocyclic compounds. The sensitizer

used are cationic dye methylene blue and benzophenone. The thio compound used for the study are

2 – Thiobarbituric Acid, 2 – Aminothiazole, 2 - (Methylthio) Thiazole, 2 – Mercaptobenzothiazole,

2 – Mercaptothiazoline, 4 – Methylthiazole, 5 – Methylthiazole and 2 – Methylthiazoline.

The chapter 1 introduces the photochemistry, different types of photo processes and methods of

transfer of energy to the molecule. The literature survey of the photoreactions, different type of the

photoreactions, processes involved in the photo reactions, methods used to study the photo

chemical reaction and establishing the mechanism are described. The applications of the

photochemical reactions are also presented.

The preparation of reagents and solutions, the procedure and the instrumental techniques used in the

experiments have been discussed in the chapter 2. The isolation from the reaction mixture of the

photo product and methods for their identification has been discussed. The details of GC analysis,

mass spectrometric techniques and IC analysis have been described. The method of the calculations

of the rate of the reaction and φ value of the photosensitized reaction has been discussed.

The photosensitized reaction of 2 – Thiobarbituric acid, 2 – Amino Thiazole and 2-(Methylthio)

Thiazole have been studied in the presence of Methylene blue as a sensitizer in the aqueous alkaline

medium on irradiation with visible light are presented in the chapter 3,4 and 5. The effect of the pH,

the concentration of the sensitizer, the concentration of the substrate, the intensity of the light and

the effect of the solvent on the rate of the photo sensitized reaction have been studied. The quantum

efficiency of the photo reaction has been evaluated using potassium ferrioxalate actinometer. The

reaction products are isolated and identified. The mechanism of the photoreaction has been

suggested.


The study of the photosensitized reaction of 2 – Mercaptobenzothiazole and 2 – Mercaptothiazoline

using methylene blue as a photo sensitizer in the aqueous alkaline medium on irradiation with

visible light have been presented in Chapter 6. The photo products have been identified with the use

of mass spectrometry and GC. The effects of the pH, the concentration of the sensitizer, the

concentration of the substrate, the intensity of the light and the effect of the solvent on the rate of

the reaction have been studied. The quantum efficiency of the photoreaction has been evaluated

using potassium ferrioxalate actinometer. The reaction products are isolated and identified. The

mechanism of the photo reaction has been suggested.

The photosensitized reaction of 4 – Methylthiazole, 5 – Methylthiazole and 2 – Methyl 2 –

Thiazoline using benzophenone as a photo sensitizer in the aqueous alkaline medium on irradiation

with visible light have been presented in Chapter 7. The effects of the pH, the concentration of the

sensitizer, the concentration of the substrate, the intensity of the light and the effect of the solvent

on the rate of the reaction have been studied. The quantum efficiency of the photoreaction has been

evaluated using potassium ferrioxalate actinometer. The mechanism of the photo reactions have

been suggested.


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