3 Main non-reactive decay routes following S1 excitation Non-radiativeIC 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)RadiativeFluorescence to S0ISC to T1 then phosphorescence to S0Delayed 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 glassFluorescenceRapid (10-8s) decay - spin allowed
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 stateGood mirror image: anthracene, rhodamine, fluoresceinPoor 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 etcNear-field scanning optical microscopy – optical fibre delivers laser light to spot size nmMaintain 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 dℓItI0Intensity decreases as it passes through cellc = 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-1Nb 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 nmC6H5- (* ) at 255 nm[Cu(H2O)6] at 810 nmmax
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 PhotolysisUse a short pulse of light to produce a large population of S1 state.Follow decay of S1 after excitation switched offFluorescence in real timeDelayed ‘probe’ pulse to detect ‘product’ absorption (e.g., T1 T2).Choose light source according to timescale of process under studyConventional flashlamp ms - sQ switched laser ns - sMode locked laser ps – nsColliding pulse mode locked laser fs - ps
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 emissiontypically kf 108 s-1= frequency of transitioni and f are the initial and final states
24 First order decayDefine 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 lifetimef
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.
28 Fluorescence quantum yields show strong dependence on type of compound excited
29 Fluorescence quenching and the Stern Volmer equation Continuous illuminationIabskf[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 actinometerTo determine a fluorescence quantum yield need an accurate measure of photon intensityA chemical actinometer uses a reaction with known quantum yield, and known absorption coefficient at a given wavelength to determine the light intensity.
33 Fluorescence quantum yield Alternatively; define f0 as the fluorescence quantum yield in the absence of quencherThus
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 rightThus the fraction passing on to T1 state is 1- fFraction of T1 molecules undergoing phosphorescenceThus’ is observed phosphorescence lifetime