Anthracene, radiationless transitions

As an example of the effect of level shifts in the crystalline state, as just described, consider the observed rates of radiationless transitions in anthracene.45 The first excited 1BSu of the isolated anthracene molecule is located about 600 cm-1 above the second triplet state. Hence, 8 < vv and the intersystem crossing process is quite rapid at room temperature. The fluorescence quantum yield is about 0.3 for this molecule in the gas phase and in solution. In the crystal the first excited singlet state is red shifted (from the gas level) by about 1880 cm- while the second triplet state is hardly affected, so that in this case the energy gap between those two states increases in the crystal. Then the coupling term, v, is smaller in the crystalline state than in solution, thereby leading to a decrease in the rate of the intersystem crossing. The result is that the fluorescence yield in the crystal is close to unity.40... 

Until recently the lifetimes of the triplet states of aromatic hydrocarbons in fluid solution at room temperature had been investigated exclusively by the technique of flash absorption spectroscopy. The lifetimes reported for many hydrocarbons, e.g., anthracene or phenan-threne, had been below 1 msec, and it had been assumed that radiationless conversion processes were so rapid under these conditions that the competing radiative triplet-singlet transition would be too slow to per-... 

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