1. Lecture 3 Kinetics of electronically excited states
Photochemistry
Lecture 3
Kinetics of electronically excited states

2. Jablonski diagram S0 S1 T1

3. Main non-reactive decay routes following S1 excitation
Non-radiative
IC to S0 followed by vibrational relaxation.
ISC to T1 then ISC’’ to S0 with vibrational relaxation after each step.
Collisional quenching (before or after ISC)
Radiative
Fluorescence to S0
ISC to T1 then phosphorescence to S0
Delayed fluorescence

4. Fluorescence and phosphorescence in solution
Weak and slow – spin forbidden (ms – s)
Competing collisional processes may eliminate – unless frozen out e.g., in glass
Fluorescence
Rapid (10-8s) decay - spin allowed

5. Mirror image of absorption and fluorescence

6. Fluorescence from v=0 following vibrational relaxation
absorption

7. Mirror image depends on
Molecule being fairly rigid (e.g., as in polyaromatic systems)
No dissociation or proton donation in excited state
Good mirror image: anthracene, rhodamine, fluorescein
Poor mirror image: Biphenyl, phenol, heptane

8. Solvent relaxation leads to a shift of the 0-0 band

9. Absorption and fluorescence in organic dyes
Population inversion between excited electronic state and higher vib levels of ground state.

10. Fluorescence labelling and single molecule spectroscopy
Attaching a fluorescent chromophore to biological molecules etc
Near-field scanning optical microscopy – optical fibre delivers laser light to spot size nm
Maintain sufficient dilution of sample so that single molecules are illuminated

12. Looking at single molecules using near field optical microscopy / fluorescence
Single molecules of pentacene in a p-terphenyl crystal

13. Rate of absorption; Beer Lambert Law
ℓ dℓ It I0
Intensity decreases as it passes through cell
c = concentration, ℓ = length

14. Beer Lambert Law (cont)or
(2.3 log x= ln x)
is known as the molar (decadic) absorption coefficient; it is often given units mol-1dm3 cm-1
Nb Intensity has units Js-1m-2 or Wm-2 and is the light energy per second per unit area

15. Limit of very dilute concentrations
Rate of absorption only proportional to concentration when above approximation is valid (cℓ « 1).

16. Absorption spectrum of chlorophyll in solution
This shows the absorption spectrum of a molecuke

17. Some values for max /(L mol-1 cm-1)
C=C (* ) at 163 nm (strong)
C=0 (* n) at nm
C6H5- (* ) at 255 nm
[Cu(H2O)6] at 810 nm
max 

18. Integrated absorption coefficient
 varies with wavenumber ˜ ˜

19. Integrated absorption coefficient proportional to square of electronic transition moment
But from lecture 1, Einstein coefficient of absorption

20. Determining spontaneous emission rates
By measuring the area under the absorption profile, we can determine the transition probability and hence the rate coefficients for stimulated absorption/emission (Bif), and also for spontaneous emission (Aif).

21. Flash Photolysis
Use a short pulse of light to produce a large population of S1 state.
Follow decay of S1 after excitation switched off
Fluorescence in real time
Delayed ‘probe’ pulse to detect ‘product’ absorption (e.g., T1  T2).
Choose light source according to timescale of process under study
Conventional flashlamp ms - s
Q switched laser ns - s
Mode locked laser ps – ns
Colliding pulse mode locked laser fs - ps

22. Modern flash photolysis setup

23. Fluorescence lifetimes
Following pulsed excitation fluorescence would follow first order decay in absence of other processes.
kf is equivalent to the Einstein A coefficient of spontaneous emission
typically kf  108 s-1
= frequency of transition
i and f are the initial and final states

24. First order decay
Define fluorescence lifetime f as time required, after switching off excitation source, for fluorescence to reduce to 1/e (=0.368) times original intensity.
If there are no competing processes, then the fluorescence lifetime is equal to the true radiative lifetime f

25. Observed fluorescence lifetime
But if there are competing processes:
Decay is still first order but as the rate of fluorescence is proportional to [S1] the observed fluorescence lifetime is reduced to

26. Branching ratio and quantum yield
The fraction of molecules undergoing fluorescence (branching ratio into that decay channel), is equal to the rate of fluorescence divided by the rate of all processes.
In the present case the above quantity is equal to the quantum yield f – see below.

27. Quantum Yield
Definition:

28. Fluorescence quantum yields show strong dependence on type of compound excited

29. Fluorescence quenching and the Stern Volmer equation
Continuous illumination
Iabs
kf[S1]
kisc[S1]
kQ[S1][Q]
Apply SSA

30. Fluorescence quantum yield
Can determine ratios of kQ/kf and kisc/kf from suitable plot.

31. Chemical actinometer
To determine a fluorescence quantum yield need an accurate measure of photon intensity
A chemical actinometer uses a reaction with known quantum yield, and known absorption coefficient at a given wavelength to determine the light intensity.

32. Chemical actinometer systems

33. Fluorescence quantum yield
Alternatively; define f0 as the fluorescence quantum yield in the absence of quencher
Thus

34. If assume diffusion limited rate constant for kQ ( 5 x 109 M-1s-1) then can determine kf + kisc.
Alternatively can recognise 1/(kf+kisc) as the observed fluorescence lifetime; if this is known can measure kQ.

35. The quantum yield represents a branching ratio
Fraction of molecules initially excited to S1 that subsequently fluoresce; for the scheme on the right
Thus the fraction passing on to T1 state is 1- f
Fraction of T1 molecules undergoing phosphorescence
Thus
’ is observed phosphorescence lifetime