Intersystem crossing time scale

The time-scale of molecular vibrations is of the order of 10 13 s, just outside the ps range. Internal conversions and in particular vibrational cascades therefore fall into the femtosecond (10-15s) time-scale. However, the spin-forbidden processes of intersystem crossing take place in times of a few ps to several ns. The case of benzophenone is a good example of the compensation between spin and orbital angular momentum. The rise of the triplet state absorption shows that intersystem crossing is completed within some 20 ps. 

Some fluorescence lifetimes are observed in ps times, although these are unusual cases. In organic molecules the Sj—S0 fluorescence has natural lifetimes of the order of ns but the observed lifetimes can be much shorter if there is some competitive non-radiative deactivation (as seen above for the case of cyanine dyes). A few organic molecules show fluorescence from an upper singlet state (e.g. azulene) and here the emission lifetimes come within the ps time-scale because internal conversion to S and intersystem crossing compete with the radiative process. To take one example, the S2-S0 fluorescence lifetime of xanthione is 18 ps in benzene, 43 ps in iso-octane. 

Finally, transient absorption measurements were deemed necessary to confirm the photoproducts in 21a,b and 22a,b. Due to overlapping absorptions of Cso, oFL and ZnP, which would impede a clear analysis, we have focused first on the selective excitation of ZnP. To this end, transient absorption spectra of the reference compounds (19 and 20a,b) reveal the instantaneous formation of the ZnP singlet excited state with maxima at 460 and 800 nm and minima at 565 and 605 nm. Furthermore, an isosbestic point at 500 nm as it develops on a time scale of 3000 ps reflects the intersystem crossing process at which end the triplet excited state of ZnP stands. The latter includes maxima at 530, 580 and 640 nm (Fig. 9.57a). Equally important is the fact that the decay of the singlet excited state matches the formation of the triplet excited state kinetics (Fig. 9.57b). 

The observation in propynal of both internal conversion and intersystem crossing on the same time scale, together with collision-induced transitions on both pathways, make this... 

The transitions between levels with non-equilibrium populations will be in the direction towards restoring the normal Boltzmann population their signal intensities will depend on the extent of non-equilibrium population. The observed effects are optimal for radical pairs with lifetimes in the nanosecond range. On a shorter time-scale, hyperfine induced intersystem crossing is negligible whereas on a longer time-scale the polarization decays owing to spin-lattice relaxation in the radicals. 

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