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

Jablonski diagram S0 S1 T1

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

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

Mirror image of absorption and fluorescence

Fluorescence from v=0 following vibrational relaxation absorption

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

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

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

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 50-100 nm Maintain sufficient dilution of sample so that single molecules are illuminated

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

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

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

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

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

Some values for max /(L mol-1 cm-1) C=C (* ) 15000 at 163 nm (strong) C=0 (* n) 10-20 at 270- 290nm C6H5- (* ) 200 at 255 nm [Cu(H2O)6]2+ 10 at 810 nm max 

Integrated absorption coefficient  varies with wavenumber ˜ ˜

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

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).

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

Modern flash photolysis setup

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

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

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

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.

Quantum Yield Definition:

Fluorescence quantum yields show strong dependence on type of compound excited

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

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

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.

Chemical actinometer systems

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

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.

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