1- The Jablonski diagram (or the state diagram of diamagnetic molecules)2- Various natures of excited states and basics in molecular orbitals3- Vibronic coupling and the Franck-Condon term4- Excited state distortion vs bond order5- The radiative and non-radiative processes6- The Kasha rule and its exceptions7- Photo-induced emission and excitation spectra (+ electroluminescence)8- Polarized emission and excitation spectra (tool for assignments)9- Delayed fluorescence10- Excimers and exciplexes11- Basics in spin-orbit couplings12- The measurements of some of the photophysical parameters13- Bimolecular reactions, Stern-Volmer plots, and sensors14- Time-resolved spectroscopy15- 2-Photon spectroscopy, flash photolysis and transient spectra16- Exciton coupling and delocalized exciton in extended systems17- Photo-induced energy transfer and the antenna effect18- Photo-induced electron transfer and the Marcus inverted region

Chapter 5: The radiative and non-radiative processes

Explanation:

k = rate constantR = rate constantstraight arrow = radiativewave line = non-radiativea = absoptionf = fluorescencep = phosphorescencei = internal conversionisc = inter-system crossingip = internal conversion

from the triplet state

kisc

A = ε l [c]

I1/I0 = 10-εl[c]

A = -log (I1/Io)

A is absorbanceI0 is the intensity of the incident light I1 is the intensity after passing through the materiall is the distance that the light travels through the material (the path length) c is the concentration of absorbing species in the materialε is the absorption coefficient or the molar absorptivity of the absorber

Absorption is a radiative process

The Beer-Lambert-Bouguer law (Beer-Lambert)

http://en.wikipedia.org/wiki/Beer-Lambert_law

Limitations of the Beer-Lambert law: The linearity of the Beer-Lambert law is limited by chemical and instrumental factors.

Causes of nonlinearity include:

1- deviations in absorptivity coefficients at high concentrations (>0.01M) due to electrostatic interactions between molecules in close proximity

2- scattering of light due to particulates in the sample3- fluoresecence or phosphorescence of the sample4- changes in refractive index at high analyte concentration 5- shifts in chemical equilibria as a function of concentration 6- non-monochromatic radiation,

http://en.wikipedia.org/wiki/Beer-Lambert_law

For air-tight samples

∫ ε dν

The oscillator strength (f)

f = 4.3 x 10-9 ∫ ε dν (unitless)

∫ ε dν

Theoretical radiative rate constant: ke

o = 3 x 10-9 (νo)2 ∫ ε dν

N. J. Turro, Modern Molecular Photochemistry, Benjamen/Cummings, Menlo Park, 1978.

in cm-1

τe0 = 22.5 x 10-9 s

τe = 16.0 x 10-9 sτe

0 = 8.9 x 10-9 sτe = 8.8 x 10-9 s

τe0 = 5.1 x 10-9 s

τe = 5.6 x 10-9 s

Examples

9,10-diphenyl(anthracene) rubrene perylene

Relationship between absorptivity and oscillator strength

∫(Ie/n) dν

Theoretical radiative rate constantaccording Birk and Dyson (+ accurate):

keo = 2.88 x 10-9 ∫ (ε/n) dν ∫ (n3) (Ie) dν

FluorescenceAbsorption

Ie = fluorescence intensity f(ν).n = refractive index f(ν).

Birk & Dyson, Proc. Roy. Soc. 1963, A275, 135.

also τe0 = 1/ke

o

Examples

N

Et

kF0 = 25(2) x 106 s-1

kF(exp) = 30(2) x 106 s-1

N

HC

N

kF0 = 46(2) x 106 s-1

kF(exp) = 48(4) x 106 s-1

N N

kF0 = 88(9) x 106 s-1

kF(exp) = 23(2) x 106 s-1

(structures S0 = S1)

N

HO

OEt

kF0 = 68(3) x 106 s-1

kF(exp) = 3.2(0.4) x 106 s-1

N

H OOEt

N

H OOEt

*

Photochemical reaction as a supplementary process.

Definitions:

Emission = a general term meaning a radiative process regardelessof its origin and nature of the excited state.

Fluorescence = a radiative process occuring between two states of the same multiplicity (it is a spin allowed process).ex.: S1 → S0

Phosphorescence = a radiative process occuring between two states of different multiplicities (it is a spin forbidden process).ex.: T1 → T0

Luminescence = a general term also meaning a radiative processregardeless of its origin and nature of the excited state(idem as emission) but often employed when the statemultiplicity is not pure such as in heavy atom-containingspecies (coordination complexes, clusters….).

•• Fluorescence was first observed from quinine Fluorescence was first observed from quinine

by Sir J.F.W. Herschel in 1845by Sir J.F.W. Herschel in 1845

Blue glass Blue glass Filter Filter

Church Window!Church Window!

<400nm<400nm

Quinine Quinine SolutionSolution

Yellow glass of wineYellow glass of wineEm filter > 400 nmEm filter > 400 nm

1853 G.G. Stokes 1853 G.G. Stokes coined term coined term ““fluorescencefluorescence””

Emission spectra measurements:steady state (Harvey group)

Dewar assembly (77 K) water cooledPMT (Hamatsu R 950)

Xe lamp (450 W)

computer

Screen

cover

double-monochromatorat the emission

double-monochromatorat the excitation

samplecompartment

Time-resolved emission and excitation spectra measurementsin the microsecond time scale (single monochromator)

(Harvey group)

+ nanosecond photon counting system(FWHM lamp = 2.5 ns)

Sample compartment

Dye laser

N2 laser

Detectors

Computer

Time correlator

Power supply

High purity N2

http://www.jobinyvon.com/usadivisions/fluorescence/images/VLD_cuvette.jpg

Dye laser module from PTI (Harvey group)

Drawing from PTI

Emission and excitation spectra

emission spectrum (—)excitation wavelength is fixed and emission wavelength is scanned

excitation spectrum (---) excitation wavelength is scanned and emission wavelength is fixed

Abs.

Fluorescence

The non-radiative processes

(Drawings are generally missleading!The up-down linesare not really adequate, but rather right-left are better. )

Importance of the non-radiative rate constants.

P

P

P

P P

P

P

POO

P =

P P

P

P

P P

P

POO

OO

P

Pt2(pcp)44- Pt2(pop)4

4-

d(Pt...Pt)/Å 2.980(1) 2.925(1)

HH

low-frequency wagging and twisting modes

λabs (1dσ*pσ)/nm 382 367

λabs (3dσ*pσ)/nm 470 452

εabs (1dσ*pσ)/M-1cm-1 29000 35000

εabs (3dσ*pσ)/M-1cm-1 142 120

ν(Pt-Pt)/cm-1 113 115

ν(Pt-Pt)*/cm-1 146 155

τe/µs 0.055 9.5

Φe 0.0024(3) 0.5

Exp. emission lifetime (298K)

Emission intensity (298K)

Max Roundhill and collaborators J. Am. Chem. Soc., 1986, 108, 5626

= Structurallyand electronicallyquasi-identical!

1- The Jablonski diagram (or the state diagram of diamagnetic molecules)2- Various natures of excited states and basics in molecular orbitals3- Vibronic coupling and the Franck-Condon term4- Excited state distortion vs bond order5- The radiative and non-radiative processes6- The Kasha rule and its exceptions7- Photo-induced emission and excitation spectra (+ electroluminescence)8- Polarized emission and excitation spectra (tool for assignments)9- Delayed fluorescence10- Excimers and exciplexes11- Basics in spin-orbit couplings12- The measurements of some of the photophysical parameters13- Bimolecular reactions, Stern-Volmer plots, and sensors14- Time-resolved spectroscopy15- 2-Photon spectroscopy, flash photolysis and transient spectra16- Exciton coupling and delocalized exciton in extended systems17- Photo-induced energy transfer and the antenna effect18- Photo-induced electron transfer and the Marcus inverted region

6- The Kasha rule and its exceptions

Emission always arises from the lowestenergy excited state (i.e. S1 and T1)

Exception to the Kasha`s rules:

S0

S1

S2

Large

ΦF = 0.02τF = 1 ns

Harvey and collaborators, Inorg. Chem. 2001, 40, 4134-4142

Another example: bis[(porphyrine)gallium(III)]

N NNN

N NNN

Ga

Ga

OMe

OMe

X-ray

1- The Jablonski diagram (or the state diagram of diamagnetic molecules)2- Various natures of excited states and basics in molecular orbitals3- Vibronic coupling and the Franck-Condon term4- Excited state distortion vs bond order5- The radiative and non-radiative processes6- The Kasha rule and its exceptions7- Photo-induced emission, excitation spectra and electroluminescence8- Polarized emission and excitation spectra (tool for assignments)9- Delayed fluorescence10- Excimers and exciplexes11- Basics in spin-orbit couplings12- The measurements of some of the photophysical parameters13- Bimolecular reactions, Stern-Volmer plots, and sensors14- Time-resolved spectroscopy15- 2-Photon spectroscopy, flash photolysis and transient spectra16- Exciton coupling and delocalized exciton in extended systems17- Photo-induced energy transfer and the antenna effect18- Photo-induced electron transfer and the Marcus inverted region

Time-resolved emission and excitation spectra measurementsin the microsecond time scale (single monochromator)

(Harvey group)

+ nanosecond photon counting system(FWHM lamp = 2.5 ns)

Image: http://www.chromatography-online.org/rs_13/image022.gif

excitation spectrum = excitationwavelength is scanned and emission wavelength is fixed

emission spectrum = excitation wavelength is fixed and emission wavelength is scanned

Photo-induced emission, excitation spectra

Photoinduced emission, excitation spectra & electroluminescence

Perylene

Image from: http://www.jp.jobinyvon.horiba.com/product_j/spex/principle/image/i_10.gif

What is a Stokes shift?

Low-temperature measurements

(EPR) Dewar N2(l) made of quartz (77 K) for frozen solutions

closed cycle cryostat He(l) 10-RT for solids

http://www.ciam.unibo.it/photochem/equip2.jpghttp://www.aurumresearch.com/images/front_left.jpg

Low-temperature cellN2(l) (77 K) for solids

N2(l)

Harveygroup

Harveygroup

N2(l)vacuumNMR tube

NN

N

NN

N

Cu Cu

P. D. Harvey Inorg. Chem. 1995, 34, 2019-2024.

Anomalies in the excitation spectra

Image: http://www.bgsu.edu/departments/chem/faculty/pavel/OLED.gifand: http://www.hlphys.jku.at/fkphys/epitaxy/insitu_fig1.jpg

Principle of electroluminescence

molecule anion 1 (charge) + molecule cation 2 (hole)

excited molecule 1* + relaxed molecule 2

Light Emitting Diodes (LED)

(Solution and gas phase devices also exist!)

http://static.howstuffworks.com/gif/oled-1.jpg

http://www.okulla.de/images/oled.jpghttp://us1.webpublications.com.au/static/images/articles/i306/30650_2mg.jpg

1- The Jablonski diagram (or the state diagram of diamagnetic molecules)2- Various natures of excited states and basics in molecular orbitals3- Vibronic coupling and the Franck-Condon term4- Excited state distortion vs bond order5- The radiative and non-radiative processes6- The Kasha rule and its exceptions7- Photo-induced emission, excitation spectra and electroluminescence8- Polarized emission and excitation spectra (tool for assignments)9- Delayed fluorescence10- Excimers and exciplexes11- Basics in spin-orbit couplings12- The measurements of some of the photophysical parameters13- Bimolecular reactions, Stern-Volmer plots, and sensors14- Time-resolved spectroscopy15- 2-Photon spectroscopy, flash photolysis and transient spectra16- Exciton coupling and delocalized exciton in extended systems17- Photo-induced energy transfer and the antenna effect18- Photo-induced electron transfer and the Marcus inverted region

Principle of polarized light and transition moment

oriented molcecule

transition moment

There is a match between excitation polarization and transition moment, so the light will be absorbed, and so, luminescence can be seen.

Y. Kim, N. Minami and S. Kazaoui, Appl. Phys. Lett. 86, 073103 (2005).

Active in light absorption and emission

Inactive in light absorption and emission

air-tight valve

NMR tube

frozen solution

EPR Dewar

Excitation polarizer Emission polarizer

electro-magnet

I

I

I

I

x

x

H

V

N =

Polarization ratio

Polarization ratio: a tool for assignement

Wavelength (nm)

Wavenumbers (x 10-3 cm-1) Wavenumbers (x 10-3 cm-1)

Wavelength (nm)

I

I

I

I

x

x

H

V

N =

Anisotropy

r=

Grating Factor

G=

V

H V

Hx

y

IVV

IVH

IHH

IHV

IVV

IVH

-

+2 G

Gx

x

Image: http://www.rub.ruc.dk/dis/chem/psos/2002/anthan5.gif

Another example of « photo-selection »

Why the polarization ratio (or the anisotropy constant) varies along an electronic band?

P = ∫ ψe ψv(µe) ψv*ψe* dτ

N

The electronic band is not fully allowed (ε).

→The probability integrals are not separable.

→The vibronic coupling is important.

→Non totally symmetric modes are active.

→Non symmetric modes depolarize the emission.

→Useful for band assigment.

0

0.1

0.2

0.3

0.4

0.5

0 20 40 60 80 100 120

[Protein], µM

An

iso

tro

py,

rMonomers: SmallRapid rotationUnhinderedLow anisotropyDepolarized

Dimers: LargerSlow rotationHindered by ViscosityHigh anisotropyPolarized

IVV

IVHIVV

IVH

-

+2

xr =

G

G

1- The Jablonski diagram (or the state diagram of diamagnetic molecules)2- Various natures of excited states and basics in molecular orbitals3- Vibronic coupling and the Franck-Condon term4- Excited state distortion vs bond order5- The radiative and non-radiative processes6- The Kasha rule and its exceptions7- Photo-induced emission, excitation spectra and electroluminescence8- Polarized emission and excitation spectra (tool for assignments)9- Delayed fluorescence10- Excimers and exciplexes11- Basics in spin-orbit couplings12- The measurements of some of the photophysical parameters13- Bimolecular reactions, Stern-Volmer plots, and sensors14- Time-resolved spectroscopy15- 2-Photon spectroscopy, flash photolysis and transient spectra16- Exciton coupling and delocalized exciton in extended systems17- Photo-induced energy transfer and the antenna effect18- Photo-induced electron transfer and the Marcus inverted region

Chapter 9: Delayed fluorescence

The excited molecule has the singlet state energybut the longer lived triplet state lifetime! But, themolecules have to be able to « communicate » efficiently!

http://micro.magnet.fsu.edu/primer/java/jablonski/lightandcolor/jablonskijavafigure1.jpg

Example of delayed fluorescence

O

Solid state emissionAt 298 K

Molecule(T1)* + Molecule(T1)* → Molecule(S1)* + Molecule(S0)

Harvey and collaborators, J. Photochem. Photobio. A., 1991, 57, 465-477.

http://tesla.desy.de/new_pages/FEL_figures/05_X-ray_Optics/Pictures/fig-5_3_11b.jpg

Delayed chlorophyll fluorescence images. Luminescence from leavesof Arabidopsis (A) and Tradescantia (B). Images are 5-minute exposurestaken as soon as possible after transfer of leaves to the equipment. A conventional photograph of the Tradescantia leaf imaged in (B) is shownto illustrate the pattern of variegation (C).

Delayed fluorescence in nature

http://www.biomedcentral.com/content/figures/1471-2229-4-19-2.jpg

1- The Jablonski diagram (or the state diagram of diamagnetic molecules)2- Various natures of excited states and basics in molecular orbitals3- Vibronic coupling and the Franck-Condon term4- Excited state distortion vs bond order5- The radiative and non-radiative processes6- The Kasha rule and its exceptions7- Photo-induced emission, excitation spectra and electroluminescence8- Polarized emission and excitation spectra (tool for assignments)9- Delayed fluorescence10- Excimers and exciplexes11- Basics in spin-orbit couplings12- The measurements of some of the photophysical parameters13- Bimolecular reactions, Stern-Volmer plots, and sensors14- Time-resolved spectroscopy15- 2-Photon spectroscopy, flash photolysis and transient spectra16- Exciton coupling and delocalized exciton in extended systems17- Photo-induced energy transfer and the antenna effect18- Photo-induced electron transfer and the Marcus inverted region

Excimer formation: a dimerization in the excited states(There is no ground state dimer observable)

Molecule(S1)* + Molecule(S0) → (Molecule)2*

http://probes.invitrogen.com/handbook/images/g000234.gif

HOMO HOMO

M MM...M

LUMO LUMO

HOMO HOMO

M MM...M

http://www-organik.chemie.uni-wuerzburg.de/ak_wuert/research/pictures/figure12_b.jpg

Perylene bisimide luminescence in toluene vs concentration. 10-6, 10-5, 10-4, 10-3, 10-2 M.

NN

O

O

O

O

R

R

R

R

R

R

R = C12H25

HOMO

HOMO

M MM...M `

LUMOLUMO

HOMOHOMO

M MM...M `

Exciplexes are heterodimers in the excited states(There is no ground state complex observed)

N. J. Turro, Modern Molecular Photochemistry, Benjamen/Cummings, Menlo Park, 1978.

http://www.gbvision.com/images/prk.jpg

http://www.nzlaser.co.nz/images/peter-operating.jpg

Medical applications of excimer lasers

N. J. Turro, Modern Molecular Photochemistry, Benjamen/Cummings, Menlo Park, 1978.

Dynamic characteristics of exciplex

∗ ∗