1- The Jablonski diagram (or the state diagram of diamagnetic molecules) 2- Various natures of excited states and basics in molecular orbitals 3- Vibronic coupling and the Franck-Condon term 4- Excited state distortion vs bond order 5- The radiative and non-radiative processes 6- The Kasha rule and its exceptions 7- Photo-induced emission and excitation spectra (+ electroluminescence) 8- Polarized emission and excitation spectra (tool for assignments) 9- Delayed fluorescence 10- Excimers and exciplexes 11- Basics in spin-orbit couplings 12- The measurements of some of the photophysical parameters 13- Bimolecular reactions, Stern-Volmer plots, and sensors 14- Time-resolved spectroscopy 15- 2-Photon spectroscopy, flash photolysis and transient spectra 16- Exciton coupling and delocalized exciton in extended systems 17- Photo-induced energy transfer and the antenna effect 18- Photo-induced electron transfer and the Marcus inverted region Chapter 5: The radiative and non-radiative processes Explanation: k = rate constant R = rate constant straight arrow = radiative wave line = non-radiative a = absoption f = fluorescence p = phosphorescence i = internal conversion isc = inter-system crossing ip = internal conversion from the triplet state k isc
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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
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)
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)
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).
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!
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).
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)