Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland

Faculty of Chemistry, Adam Mickiewicz University, Faculty of Chemistry, Adam Mickiewicz University, Poznan, PolandPoznan, Poland

2012/2013 - lecture 12012/2013 - lecture 1

"Molecular Photochemistry - how to "Molecular Photochemistry - how to study mechanisms of photochemical study mechanisms of photochemical

reactionsreactions ? ?""

BronisBronisllaw Marciniakaw Marciniak

ContentsContents

1.1. Introduction and basic principles Introduction and basic principles (physical and chemical properties of molecules in the excited states, (physical and chemical properties of molecules in the excited states, Jablonski diagram, time scale of physical and chemical events, Jablonski diagram, time scale of physical and chemical events, definition of terms used in photochemistry).definition of terms used in photochemistry).

2.2. Qualitative investigation of photoreaction mechanisms - Qualitative investigation of photoreaction mechanisms - steady-state and time resolved methodssteady-state and time resolved methods(analysis of stable products and short-lived reactive intermediates, (analysis of stable products and short-lived reactive intermediates, identification of the excited states responsible for photochemical identification of the excited states responsible for photochemical reactions).reactions).

3.3. Quantitative methodsQuantitative methods(quantum yields, rate constants, lifetimes, kinetic of quenching, (quantum yields, rate constants, lifetimes, kinetic of quenching, experimental problems, e.g. inner filter effects).experimental problems, e.g. inner filter effects).

Contents cont.Contents cont.

4. Laser flash photolysis in the study of photochemical 4. Laser flash photolysis in the study of photochemical reaction mechanisms (10reaction mechanisms (10–3–3 – 10 – 10–12–12s).s).

5. Examples illustrating the investigation of photoreaction 5. Examples illustrating the investigation of photoreaction mechanisms:mechanisms: sensitized photooxidation of sulfur (II)-containing organic sensitized photooxidation of sulfur (II)-containing organic

compounds,compounds,

photoinduced electron transfer and energy transfer processes, photoinduced electron transfer and energy transfer processes,

sensitized photoreduction of 1,3-diketonates of Cu(II),sensitized photoreduction of 1,3-diketonates of Cu(II),

photochemistry of 1,3,5,-trithianes in solution.photochemistry of 1,3,5,-trithianes in solution.

LiteratureLiterature11. . „Metody badania mechanizmów reakcji fotochemicznych”, (How to study „Metody badania mechanizmów reakcji fotochemicznych”, (How to study

mechanisms of photochemical reactions) (in Polish), editormechanisms of photochemical reactions) (in Polish), editor B. Marciniak, B. Marciniak, Wydawnictwo Naukowe UAM, Poznań 1999.Wydawnictwo Naukowe UAM, Poznań 1999.

2. 2. N.J. Turro, N.J. Turro, Modern Molecular PhotochemistryModern Molecular Photochemistry, Benjamin/Cummings, Menlo , Benjamin/Cummings, Menlo ParkPark, , 19781978; ; N.J. Turro,N.J. Turro, V. Ramamurthy, J.C. Scaiano, V. Ramamurthy, J.C. Scaiano, Modern Molecular Modern Molecular PhotochemistryPhotochemistry of Organic Molecules of Organic Molecules, University Science Book, , University Science Book, Sausalito/California, 2010.Sausalito/California, 2010.

3. 3. J.A. Barltrop, J.D. Coyle, J.A. Barltrop, J.D. Coyle, Excited States in Organic ChemistryExcited States in Organic Chemistry, Wiley, New , Wiley, New York, 1978.York, 1978.

4. 4. G.J. Kavarnos, G.J. Kavarnos, „Fundamentals of Photoiduced Elektron Transfer”„Fundamentals of Photoiduced Elektron Transfer” , VCH, New , VCH, New York 1993.York 1993.

5. 5. B. MarciniakB. Marciniak, J. Chem. Education, , J. Chem. Education, 6363, 998 (1986), 998 (1986)"Does Cu(acac)"Does Cu(acac)22 Quench Benzene Fluorescence". Quench Benzene Fluorescence".

6. 6. B. MarciniakB. Marciniak, J. Chem. Education, , J. Chem. Education, 6565, 832 (1988) , 832 (1988) "Photochemistry of Phenylalkyl Ketones. The "Norrish Type II" "Photochemistry of Phenylalkyl Ketones. The "Norrish Type II" Photoreaction".Photoreaction".

7. 7. B. Marciniak, G.E. Buono-CoreB. Marciniak, G.E. Buono-Core, J. Photochem. Photobiol. A.: Chemistry, , J. Photochem. Photobiol. A.: Chemistry, 5252, 1 , 1 (1990)(1990)"Photochemical Properties of 1,3-Diketonate Transition Metal Chelates". "Photochemical Properties of 1,3-Diketonate Transition Metal Chelates".

8. 8. B. Marciniak, G.L. Hug, B. Marciniak, G.L. Hug, Coord. Chem. RevCoord. Chem. Rev., ., 159159, 55 (1997), 55 (1997)

“Quenching of Triplet States of Organic Compounds by 1,3-Diketonate “Quenching of Triplet States of Organic Compounds by 1,3-Diketonate Transition-Metal Chelates in Solution. Energy and/or Electron Transfer”. Transition-Metal Chelates in Solution. Energy and/or Electron Transfer”.

9. 9. K. Bobrowski, B. Marciniak, G.L. HugK. Bobrowski, B. Marciniak, G.L. Hug, J. Am. Chem. Soc., , J. Am. Chem. Soc., 114114, 10279, 10279 (1992) (1992) "4-Carboxybenzophenone Sensitized Photooxidation of Sulfur- Containing "4-Carboxybenzophenone Sensitized Photooxidation of Sulfur- Containing Amino Acids. Nanosecond Laser Flash Photolysis and Pulse Radiolysis Amino Acids. Nanosecond Laser Flash Photolysis and Pulse Radiolysis Studies".Studies".

11.11. B. Marciniak, G.L. Hug, J. Rozwadowski, K. Bobrowski, B. Marciniak, G.L. Hug, J. Rozwadowski, K. Bobrowski, J. Am. Chem. SocJ. Am. Chem. Soc., ., 117117, 127, 127 (1995) (1995) "Excited Triplet State of N-(9-methylpurin-6-yl)pyridinium Cation as an "Excited Triplet State of N-(9-methylpurin-6-yl)pyridinium Cation as an Efficient Photosensitizer in the Oxidation of Sulfur-Containing Amino Efficient Photosensitizer in the Oxidation of Sulfur-Containing Amino Acids. Laser Flash and Steady-State Photolysis Studies". Acids. Laser Flash and Steady-State Photolysis Studies".

12.12. E. Janeba-Bartoszewicz, G.L. Hug, E. Andrzejewska, B. Marciniak,E. Janeba-Bartoszewicz, G.L. Hug, E. Andrzejewska, B. Marciniak, J. Photochem. J. Photochem. Photobiol. A: Chemistry,Photobiol. A: Chemistry, 177177, 17-23 (2006) , 17-23 (2006) "Photochemistry of 1,3,5-trithianes in solution. Steady-state and laser flash "Photochemistry of 1,3,5-trithianes in solution. Steady-state and laser flash photolysis studies".photolysis studies".

Literature cont.Literature cont.

Textbooks on photochemistryTextbooks on photochemistry1. 1. N.J. Turro, N.J. Turro, Modern Molecular PhotochemistryModern Molecular Photochemistry, Benjamin/Cummings, , Benjamin/Cummings,

Menlo Park, 1978. Menlo Park, 1978.

22.. A. Barltrop, J.D. Coyle, A. Barltrop, J.D. Coyle, Excited States in Organic ChemistryExcited States in Organic Chemistry, Wiley, , Wiley, New York, 1978.New York, 1978.

33. . A. Gilbert, J. Baggott, A. Gilbert, J. Baggott, Essentials of Molecular PhotochemistryEssentials of Molecular Photochemistry, , Blackwell Scientific Publications, Oxford, 1991. Blackwell Scientific Publications, Oxford, 1991.

44.. R.P. Wayne, R.P. Wayne, Principles and Applications of PhotochemistryPrinciples and Applications of Photochemistry, Oxford , Oxford University Press, 1988.University Press, 1988.

55.. J.F. Rabek, J.F. Rabek, Experimental Methods in Photochemistry and PhotophysicsExperimental Methods in Photochemistry and Photophysics, , volumsvolums 1 i 2, Wiley, New York, 1982 1 i 2, Wiley, New York, 1982

66.. S.L. Murov, J. Carmichael, G.L. Hug, S.L. Murov, J. Carmichael, G.L. Hug, Handbook of PhotochemistryHandbook of Photochemistry, , Marcel Dekker, New York, 1993. Marcel Dekker, New York, 1993.

7. 7. M. Montalti, A. Credi, L. Prodi, M.T. Gandolfi, M. Montalti, A. Credi, L. Prodi, M.T. Gandolfi, Handbook of Handbook of PhotochemistryPhotochemistry, , CRC Press,CRC Press, Boca RatonBoca Raton, , 20062006..

Organic photochemistry:Organic photochemistry:1. J.A. Barltrop, J.D. Coyle, 1. J.A. Barltrop, J.D. Coyle, Excited States in Organic PhotochemistryExcited States in Organic Photochemistry, ,

Wiley, New York, 1978. Wiley, New York, 1978. 2. M. Klessinger, J. Michl, 2. M. Klessinger, J. Michl, Excited States and Organic PhotochemistryExcited States and Organic Photochemistry, ,

VCH, 1995.VCH, 1995.3.3. J. Kagan, J. Kagan, Organic Photochemistry. Principles and ApplicationsOrganic Photochemistry. Principles and Applications, ,

Academic Press, London, 1993.Academic Press, London, 1993.4. 4. J. Kapecky, J. Kapecky, Organic Photochemistry. A Visual ApproachOrganic Photochemistry. A Visual Approach, VCH, New , VCH, New

York, 1992.York, 1992.5. 5. J. Michl, V. Bonaèiæ-Kouteck, J. Michl, V. Bonaèiæ-Kouteck, Electronic Aspects of Organic Electronic Aspects of Organic

PhotochemistryPhotochemistry, Wiley, New York, 1990., Wiley, New York, 1990.6. 6. Handbook of Organic PhotochemistryHandbook of Organic Photochemistry, , EdEd. J.C. Scaiano, CRL Press, . J.C. Scaiano, CRL Press,

Boca Raton, tomy 1 i 2, 1989. Boca Raton, tomy 1 i 2, 1989. 7. 7. CRC Handbook of Organic PhotochemistryCRC Handbook of Organic Photochemistry, , Ed.Ed. W.M. Horspool, CRC W.M. Horspool, CRC

Press, Boca Raton, 1995.Press, Boca Raton, 1995.8. 8. Synthetic Organic PhotochemistrySynthetic Organic Photochemistry, , EdEd. W.M. Horspool, Plenum Press, . W.M. Horspool, Plenum Press,

New York, 1984.New York, 1984.

Textbooks on photochemistryTextbooks on photochemistry

Inorganic photochemistryInorganic photochemistry::1. 1. V. Balzani, V. Carassiti, V. Balzani, V. Carassiti, Photochemistry of Coordination CompoundsPhotochemistry of Coordination Compounds, ,

Academic Press, London, 1970. Academic Press, London, 1970. 2.2. Concepts of Inorganic PhotochemistryConcepts of Inorganic Photochemistry, pod red. A.W. Adamson i P.D. , pod red. A.W. Adamson i P.D.

Fleischauer, Wiley, New York, 1975. Fleischauer, Wiley, New York, 1975. 3.3. G.J. Ferraudi, G.J. Ferraudi, Elements of Inorganic PhotochemistryElements of Inorganic Photochemistry, Wiley, New York, 1988. , Wiley, New York, 1988.

OthersOthers::1.1. V. Balzani, F. Scandola,V. Balzani, F. Scandola, Supramolecular Photochemistry Supramolecular Photochemistry, Ellis Horwood, New , Ellis Horwood, New

York, 1991. York, 1991. 2.2. G.J. Kavarnos, G.J. Kavarnos, Fundamentals of Photoinduced Electron TransferFundamentals of Photoinduced Electron Transfer, VCH, New , VCH, New

York, 1993. York, 1993. 3.3. Photoinduced Electron TransferPhotoinduced Electron Transfer, pod red. M.A. Fox i M. Chanon, tomy 1-4, , pod red. M.A. Fox i M. Chanon, tomy 1-4,

Elsevier, Amsterdam, 1988.Elsevier, Amsterdam, 1988.4.4. J.B. Birks, J.B. Birks, Photophysics of Aromatic MoleculesPhotophysics of Aromatic Molecules, Wiley, New York, 1970. , Wiley, New York, 1970. 5.5. Glossary of Terms Used in PhotochemistryGlossary of Terms Used in Photochemistry, Pure Applied Chemistry , Pure Applied Chemistry 7979, 293–, 293–

465465 (2007)(2007)6.6. J.E. Guillet, J.E. Guillet, Polymer Photophysics and PhotochemistryPolymer Photophysics and Photochemistry, Cambridge University , Cambridge University

Press, Cambridge, 1985 Press, Cambridge, 1985

Textbooks on photochemistryTextbooks on photochemistry

1. 1. Introduction and basic principles Introduction and basic principles

- - physical and chemical properties of molecules physical and chemical properties of molecules in the excited states, in the excited states,

- - Jablonski diagram, Jablonski diagram,

- - time scale of physical and chemical events, time scale of physical and chemical events,

- - definition of terms used in photochemistrydefinition of terms used in photochemistry

Energy level diagramEnergy level diagram

3210

S0

0123

S1

0123

S2

0123

T2

0123

T1

Physical and chemical properties of molecules in the Physical and chemical properties of molecules in the excited states (comparison with the ground state)excited states (comparison with the ground state)

1. 1. Energy (80 Energy (80 400 kJ/mol) 400 kJ/mol)

2. 2. Lifetimes (10Lifetimes (101212 10 1000 s) s)

3. 3. Geometry of excited molecules Geometry of excited molecules (bond lengths, angles)(bond lengths, angles)

4. 4. Dipole moments (redistributions of electron densities)Dipole moments (redistributions of electron densities)

5. 5. Chemical properties (photochemical reactions)Chemical properties (photochemical reactions)

TabelTabelee 1. 1. Energies and lifetimes for lowest excited states Energies and lifetimes for lowest excited states (S(S11 i T i T11) ) organiorganicc molecules in solutions molecules in solutions

CompoundCompoundEESS

(kJ/mol)(kJ/mol)SS

(ns)(ns)EETT

(kJ/mol)(kJ/mol)TT

((s)s)

BenzenBenzenee a) a) 459459 3434 353353

NaNaphphtalentalenee a) a) 385385 9696 253253 175175

AntracenAntracenee a) a) 318318 55..33 178178 670670

TetracenTetracenee a) a) 254254 66..44 123123 400400

BenzoBenzophphenonenonee b)b) 316316 00..0303 287287 66..99

CC60 60 a)a) 193193 11..22 151151 250250

a)a) in nonpolar solventsin nonpolar solvents, , b)b) inin benzen benzenee

TabelTabelee2. 2. Dipole moments of organic molecules in the ground state Dipole moments of organic molecules in the ground state ((SS00) ) and in the lowest excited singlet statesand in the lowest excited singlet states (S (S11) )

CompoundCompoundDipole moment (in Debyes)Dipole moment (in Debyes)

SS00 SS11

FormaldehydFormaldehydee 2.32.3 1.61.6BenzoBenzophephenonnonee 3.03.0 1.21.2p-Nitroanilinp-Nitroanilinee 66 14144-Amino-4'-nitrobi4-Amino-4'-nitrobiphphenylenyl 66 1616

Tabela 3. pKa Tabela 3. pKa values in the ground and lowest exited values in the ground and lowest exited SS11 andand T T11

statesstates for organic compoundsfor organic compounds

CompoundCompound pKpKaa (S (S00)) pKpKaa**(S(S11)) pKpKaa

**(T(T11))

1-1-NNaaphphtoltol 99..22 2,02,0

2-2-NNaaphphtoltol 99..55 2,82,83,1 3,1 7,77,78,18,1

1-1-NNaaphphtotoic acidic acid 33..77 10101212 3,83,84,64,6

Acridine caAcridine cationtion 55..55 10,610,6 5,65,6

2-2-NNaaphphtylamtylamine cationine cation 44..11 22 3,13,13,33,3

R + H3ORH + H2OKa R + H3ORH + H2OKa

Ka* *(R ) + H3O(RH) + H2O*Ka* *(R ) + H3O(RH) + H2O*

Acid -base properties in the excited statesAcid -base properties in the excited states

Photochemical reactions:Photochemical reactions:

hh

- - Photodissociation Photodissociation (photofragmentation)(photofragmentation)

- - PhotocycloadditionPhotocycloaddition- - PhotoisomerizationPhotoisomerization- - PhotorearrangementsPhotorearrangements- - Photo additionPhoto addition- - PhotosubstitutionPhotosubstitution- - PhotooxidationPhotooxidation- - PhotoreductionPhotoreduction- - other Photo....other Photo....

AA A* A* B + CB + C

Intermolecular Excited-State ReactionsIntermolecular Excited-State Reactions

• Energy TransferEnergy Transfer

DD* + * + QQ DD + + QQ**

• Electron TransferElectron Transfer

D* + A D* + A D D++ + A + A

D + A* D + A* D D + A + A++

• Hydrogen AbstractionsHydrogen Abstractions

Note:Have to have excitedstates that live long enoughto find quenching partnerby diffusion

PPhysical and chemical properties of molecules hysical and chemical properties of molecules in the excited statesin the excited states

Conclusion:Conclusion: Molecules in the excited states are characterized Molecules in the excited states are characterized by different physical and chemical propetries in comparison by different physical and chemical propetries in comparison with those in the ground states. with those in the ground states. They act like distinct chemical species.They act like distinct chemical species.

1. Energy (80 1. Energy (80 400 kJ/mol) 400 kJ/mol)2. Lifetimes (102. Lifetimes (1012 12 10 100 s) s)3.Geometry of excited molecules 3.Geometry of excited molecules

( bond lengths, angles)( bond lengths, angles)4. Dipole moments (redistributions of electron densities)4. Dipole moments (redistributions of electron densities)5. Chemical properties (photochemical reactions)5. Chemical properties (photochemical reactions)

Stable Stable productsproducts

Scheme of photochemical reactionScheme of photochemical reaction

AA A* A* I B + CI B + Chh

IntermediatesIntermediates

Reactive IntermediatesReactive Intermediates

• Want to see time development of excited states Want to see time development of excited states and free radicalsand free radicals

• Excited states and free radicals act as Excited states and free radicals act as individual chemical species during their individual chemical species during their existence.existence.

• They are species of particular interest because They are species of particular interest because of their high energy content.of their high energy content.

• If you can capture their energy content, you If you can capture their energy content, you can do chemistry that you cannot do in ground can do chemistry that you cannot do in ground states.states.

How to Utilize the Energy Content?How to Utilize the Energy Content?

• If excited states channel their energy into specific If excited states channel their energy into specific bonds, then photochemistry can occur.bonds, then photochemistry can occur.

• If scavengers or quenchers can find the excited state If scavengers or quenchers can find the excited state or free radical in time, then the electronic or chemical or free radical in time, then the electronic or chemical energy can be captured by the, ordinarily, stable energy can be captured by the, ordinarily, stable scavenger or quencher.scavenger or quencher.

Jablonski diagramJablonski diagram

S0

T2

T1

2S

1SIC

ISC IC

A F IC Ph ISC

+Q

R

+Q

R

Alexander JabłońskiAlexander Jabłoński(1898(18981980)1980)

before 1939 University of Warsaw, Institute of Experimental before 1939 University of Warsaw, Institute of Experimental PhysicsPhysics

194319431945 Edinburgh Medical School1945 Edinburgh Medical School

194619461980 Copernicus University in Toruń1980 Copernicus University in Toruń

about 70 scientific papers on atomic and molecular spectroscopyabout 70 scientific papers on atomic and molecular spectroscopy

A. JabłońskiA. JabłońskiNatureNature 1933, 839 1933, 839

S1

excited singlet state

S1

JabJabłonski - diagramłonski - diagram

IC

T1

ISC

phosphorescence

excited triplet state

ISC

T1

radiationlessdeactivation

heat

S0

singletground state

S0

fluorescence

Radiationless TransitionsRadiationless TransitionsShowing Nuclear ContributionsShowing Nuclear Contributions

““Stokes” shiftStokes” shiftAbsorption vs EmissionAbsorption vs Emission

EE = = hhc / c /

In most of photochemical reactions of organic In most of photochemical reactions of organic compounds only the lowest excited states (Scompounds only the lowest excited states (S11 and T and T11) ) are reactive statesare reactive states(rapid radiationless conversion to S(rapid radiationless conversion to S11 or T or T11))

Exceptions: emission from SExceptions: emission from S22 excited singlets for excited singlets for azulene, thioketones azulene, thioketones

Kasha’s ruleKasha’s rule

Energy Gap LawEnergy Gap Law

• The rate of radiationless transitions goes as the The rate of radiationless transitions goes as the exponential of the energy gap between the 0-0 exponential of the energy gap between the 0-0 vibronic levels of the two electronically excited vibronic levels of the two electronically excited statesstates..

„ „the smaller the energy gap the bigger the rate”the smaller the energy gap the bigger the rate”

A*(SA*(S11) + Q ) + Q A(S A(S00) + Q*) + Q*

A(SA(S11)* + Q )* + Q ( A( A++...Q...Q) ) A(SA(S00) + Q) + Q ( A( A...Q...Q++) ) A(S A(S00) + Q) + Q

Processes from SProcesses from S11 state: state:

- fluorescence (F)- fluorescence (F)- internal conversion (IC)- internal conversion (IC)- intersystem crossing (ISC) S- intersystem crossing (ISC) S11 T T11

- chemical reaction (R- chemical reaction (RSS))- quenching (+Q):- quenching (+Q):

- phosphorescence (P)- phosphorescence (P)

- intersystem crossing (ISC) T- intersystem crossing (ISC) T11 SS00

- chemical reaction (R- chemical reaction (RTT))

- quenching (+Q)- quenching (+Q)

Processes from TProcesses from T11 state: state:

Schematic of the network of processes of interest to a molecular photochemist [Turro] Schematic of the network of processes of interest to a molecular photochemist [Turro]

Absorption of lightAbsorption of light

Electronic excitationElectronic excitation

Dissipation mechanismDissipation mechanism

Radiative mechanismRadiative mechanism Radiationless mechanismRadiationless mechanism

(1) Fluorescence(1) Fluorescence(2) Phosphorescence(2) Phosphorescence

ChemicalChemical(1) Singlet(1) Singlet(2) Triplet(2) Triplet

PhysicalPhysical(1) Internal conversion(1) Internal conversion(2) Intersystem crossing(2) Intersystem crossing

ProducesProduces

Net effectNet effect Net effectNet effect Net effectNet effect

Light Light Light Lighthh h h’’

Light Light Chemistry Chemistryhh GG

Light Light Heat Heathh Q Q

Comparison of Comparison of time scaletime scaless of physical and chemical events of physical and chemical events of of photochemical interest (10photochemical interest (10-15-15 s - 1s) [Turro] s - 1s) [Turro]

time scale (s)

femto 10-15 electronic motion

pico 10-12 vibrational motion bond cleavage (weak)nano

Fluorescence 10-9 rotational and translational motion (small molecules fluid)micro 10-6 rotational and translational motion (large molecules fluid) ultrafast chemical reaction

Phosphorescence milli 10-3 rotational and translational motion (large molecules, very viscous)

100 fast chemical reactions

Lifetimes:Lifetimes:

Lifetime of a molecular entity, which decays by first-order kinetics, Lifetime of a molecular entity, which decays by first-order kinetics, is the time needed for a concentration of the entity to decrease to 1/e is the time needed for a concentration of the entity to decrease to 1/e of its original value, i.e., of its original value, i.e., cc(t =(t =) = ) = cc(t = 0)/e. (t = 0)/e. It is equal to the reciprocal of the sum of the first-order rate constants It is equal to the reciprocal of the sum of the first-order rate constants of all processes causing the decay of the molecular entity.of all processes causing the decay of the molecular entity.

DDefinition of terms used in photochemistryefinition of terms used in photochemistry

2007 IUPAC, S. E. Braslavsky, 2007 IUPAC, S. E. Braslavsky, Pure and Applied Chemistry Pure and Applied Chemistry 7979, 293–465, 293–465

][11

QSSqrISCICf

ii kkkkkk

][11

''' QTTqrISCp

ii kkkkk

Lifetimes:Lifetimes:



0.0

0.5

1.0

tS

1/e

SeS[S 11

t

0][]0][]

1

1

S[S



Quantum yields Quantum yields ::

Number of defined events occurring per Number of defined events occurring per photon photon absorbed absorbed by the system.by the system.

Integral quantum yield:Integral quantum yield:

For a photochemical reaction AFor a photochemical reaction A BB : :hvhv

RRamount of reactant consumed or product formedamount of reactant consumed or product formed

amount of photons absorbedamount of photons absorbed

number of eventsnumber of eventsnumber of photons absorbednumber of photons absorbed

DDifferential quantum yield:ifferential quantum yield:



Integral quantum yield:Integral quantum yield:

At

at

A A

I dt

[ ] [ ]0

0

Bt

a

tB

I dt

[ ]

0

Aa

d AdtI

[ ]

Ba

d BdtI

[ ]

For a photochemical reaction AFor a photochemical reaction A BB : :hvhv

Experimental parameters characterizing Experimental parameters characterizing fluorescence properties of moleculesfluorescence properties of molecules

00f f is radiative lifetime (Einstein coefficient of spontaneous is radiative lifetime (Einstein coefficient of spontaneous

emission)emission)

2. 2. ff

3. 3. SS

1. k1. kff = =1100

ffk df ~ 0

2

ff

ii

fk

kk

S S

si

ik

1S

CompoundCompound FF maxmax kkff kkISCISC ConfigConfiguration uration (s(s-1-1)) (s (s-1-1) ) of Sof S11

BenzeneBenzene ~0.2~0.2 250250 22101066 101077 , ,

NaphthaleneNaphthalene ~0.2~0.2 270270 22101066 55101066 , ,

AnthraceneAnthracene ~0.4~0.4 85008500 55101077 ~5~5101077 , ,

9,10-Diphenylanthracene9,10-Diphenylanthracene ~1.0~1.0 1260012600 ~5~5101088 <10<1077 , ,

PyrenePyrene ~0.7~0.7 510510 ~10~1066 <10<1055 , ,

TriphenyleneTriphenylene ~0.1~0.1 355355 ~2~2101088 ~10~1077 , ,

PerylenePerylene ~1.0~1.0 3950039500 ~10~1088 <10<1077 , ,

StilbeneStilbene ~0.05~0.05 2400024000 ~10~1088 ~10~1099 , ,

1-Chloronaphthalene1-Chloronaphthalene ~0.05~0.05 ~300~300 ~10~1066 55101088 , ,

1-Bromonaphthalene1-Bromonaphthalene ~0.002~0.002 ~300~300 ~10~1066 ~10~1099 , ,

1-Iodonaphthalene1-Iodonaphthalene ~0.000~0.000 ~300~300 ~10~1066 ~10~101010 , ,

BenzophenoneBenzophenone ~0.000~0.000 ~200~200 ~10~1066 ~10~101111 nn, ,

AcetoneAcetone ~0.00~0.0011 ~20~20 ~10~1055 ~10~1099 nn, ,

PerfluoroacetonePerfluoroacetone ~0.~0.11 ~20~20 ~10~1055 ~10~1077 nn, ,

Some examples of fluorescence quantum yields and Some examples of fluorescence quantum yields and other emission parametersother emission parameters [Turro][Turro]

Experimental parameters characterizing Experimental parameters characterizing phosphorescence properties of moleculesphosphorescence properties of molecules

1. k1. kpp = =1100

ppk dp ~ 0

2S T

2. 2. ISCISC(S(S11 T T11)) ISC ISCk = S

3. 3. p p p ISCp

ii

ISC pk

kk

T T

4. 4. TT T T

1ki

i

ConfigConfiguration uration CompoundCompound 77K 77K 25°C25°C ISCISC kkp p (s(s-1-1)) of Tof T11

BenzeneBenzene ~0.2~0.2 (<10(<10–4–4)) ~0.7~0.7 ~10~10–1–1 , ,

NaphthaleneNaphthalene ~0.05~0.05 (<10(<10–4–4)) ~0.7~0.7 ~10~10–1–1 , ,

1-Fluoronaphthalene1-Fluoronaphthalene ~0.05~0.05 (<10(<10–4–4)) —— ~0.3~0.3 , ,

1-Chloronaphthalene1-Chloronaphthalene ~0.3~0.3 (<10(<10–4–4)) ~1.0~1.0 ~2~2 , ,

1-Bromonaphthalene1-Bromonaphthalene ~0.3~0.3 (<10(<10–4–4)) ~1.0~1.0 ~30~30 , ,

1-Iodonaphthalene1-Iodonaphthalene ~0.4~0.4 —— ~1.0~1.0 ~300~300 , ,

TriphenyleneTriphenylene ~0.5~0.5 (<10(<10–4–4)) ~0.9~0.9 ~10~10–1–1 , ,

BenzophenoneBenzophenone ~0.9~0.9 (~0.1)(~0.1) ~1.0~1.0 ~10~1022 nn, ,

BiacetylBiacetyl ~0.3~0.3 (~0.1)(~0.1) ~1.0~1.0 ~10~1022 nn, ,

AcetoneAcetone ~0.03~0.03 (~0.01)(~0.01) ~1.0~1.0 ~10~1022 nn, ,

4-Phenylbenzophenone4-Phenylbenzophenone —— —— ~1.0~1.0 1.01.0 , ,

AcetophenoneAcetophenone ~0.7~0.7 (~0.03)(~0.03) ~1.0~1.0 ~10~1022 nn, ,

CyclobutanoneCyclobutanone 0.00.0 0.00.0 0.00.0 —— nn, ,

QQuantum yields for phosphorescence and other triplet uantum yields for phosphorescence and other triplet emission parametersemission parameters [Turro][Turro]

PP

Lifetimes & Quantum YieldsLifetimes & Quantum Yields

• Triplet states have much longer lifetimes than Triplet states have much longer lifetimes than singlet statessinglet states

• In solutions, singlets live on the order of In solutions, singlets live on the order of nanoseconds or 10’s of nanosecondsnanoseconds or 10’s of nanoseconds

• Triplets in solution live on the order of 10’ or Triplets in solution live on the order of 10’ or 100’s of microseconds100’s of microseconds

• Triplets rarely phosphoresce in solution Triplets rarely phosphoresce in solution (competitive kinetics)(competitive kinetics)

Important Types of Important Types of Organic Excited StatesOrganic Excited States

,,* states, particularly in aromatics and polyenes* states, particularly in aromatics and polyenes• nn,,* states, particular in carbonyls* states, particular in carbonyls

Example:Example:Lowest electronic statesLowest electronic statesof Benzophenoneof BenzophenoneSS00

TT11

TT22SS11

SS22

33nn,,**

33,,**

11,,**11nn,,**

ISCISC

Why Triplet Quantum Yield is high inWhy Triplet Quantum Yield is high inBenzophenone?Benzophenone?

Lowest electronic statesLowest electronic statesof Benzophenoneof Benzophenone

SS00

TT11

TT22SS11

SS22

33nn,,**

33,,**

11,,**11nn,,**

ISCISC

(1)(1) 11nn,,* states have small * states have small kkradrad because of small orbital overlap because of small orbital overlap

(2) (2) kkiscisc is large because of low-lying is large because of low-lying 33,,* and El-Sayed’s Rule* and El-Sayed’s Rule

Selection Rules for ISCSelection Rules for ISC

• El-Sayed’s RuleEl-Sayed’s Rule::

Allowed: Allowed: 11(n,(n,*) *) 33((,,*); *); 33(n,(n,*) *) 11((,,*) *)

Forbidden: Forbidden: 11(n,(n,*) *) 33(n,(n,*); *); 33((,,*) *) 11((,,*) *)

• Intersystem crossing between states of like orbital Intersystem crossing between states of like orbital character is slower than ISC between states of different character is slower than ISC between states of different orbital character.orbital character.

Characteristics ofCharacteristics ofRadiationless TransitionsRadiationless Transitions

• Kasha’s RuleKasha’s Rule• El-Sayed’s RuleEl-Sayed’s Rule• Wavelength Independence of LuminescenceWavelength Independence of Luminescence• Energy Gap LawEnergy Gap Law• Competitive First-Order KineticsCompetitive First-Order Kinetics

Lambert-Beer lawLambert-Beer law

= k= k cc dd lldd IIII

= = cc llII00

IIloglog

A = A = cc llII00 IIaa

II

I = II = I00 10 10cc ll IIaa = I = I00 (1 (11010

cc l l ))

II00

IIA = logA = log 11

TTA = logA = logII

II00T = T =

R + H3ORH + H2OKa R + H3ORH + H2OKa

Ka* *(R ) + H3O(RH) + H2O*Ka* *(R ) + H3O(RH) + H2O*

Acid -base properties in the excited statesAcid -base properties in the excited states

B. Marciniak, H. Kozubek, S. PaszycJ. Chem. Education, 69, 247-249 (1992)"Estimation of pK in the First Excited Singlet State"

S1

S0

S1

S00

E

E2

E1

H

H*

(RH)

(RH)*

R * + H

R + H

S1

S0

S1

S00

E

E2

E1

H

H*

(RH)

(RH)*

R * + H

R + H

Estimation of pK in the First Excited Singlet StateEstimation of pK in the First Excited Singlet State

Thermodynamic FThermodynamic Föörster cyclerster cycle

EE11 EE22 = = HH HH* *

RTEE

KK 2.303

pp 21aa

2

flr1

abs1

A1

hcNE2

flr2

abs2

A2

hcNE

GG = = RT RT ln ln KKaa SS = = SS**

GG = = HH TT SS

GG GG* = * = –– RT (ln RT (ln KKaa –– ln ln KKaa**) = ) = EE11 –– EE22

EE11 EE22 = = HH HH* *

EE11 EE22 = ( = (GG ++ TT SS) ) ( (GG** ++ TT SS*)*)

2-naphtol in HCl2-naphtol in HCl

40 35 30 25 20

EmissionAbsorption

300 350 400 500 (nm)

10 3 1(cm )

1

2

3

4

10

3

1(

cm)

mol

dm1

3

Rel

ativ

e in

tens

ity

S S0 2

S S0 1 S S1 0

~

2-naphtol in NaOH2-naphtol in NaOH

40 35 30 25 20

Emission

300 350 400 500 (nm)

1

2

3

4

5

6

7

10

3

1(

cm)

mol

dm1

3

S S0 2

S S0 1 S S1 0Absorption

10 3 1(cm )~

Rel

ativ

e in

tens

ity

Tabela 3. pKa Tabela 3. pKa values in the ground and lowest exited values in the ground and lowest exited SS11 andand T T11

statesstates for organic compoundsfor organic compounds

CompoundCompound pKpKaa (S (S00)) pKpKaa**(S(S11)) pKpKaa

**(T(T11))

1-1-NNaaphphtoltol 99..22 2,02,0

2-2-NNaaphphtoltol 99..55 2,82,83,1 3,1 7,77,78,18,1

1-1-NNaaphphtotoic acidic acid 33..77 10101212 3,83,84,64,6

Acridine caAcridine cationtion 55..55 10,610,6 5,65,6

2-2-NNaaphphtylamtylamine cationine cation 44..11 22 3,13,13,33,3

Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland

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