Temperature dependence of intersystem crossing in substituted anthracenes

Abstract

Fluorescence quantum yields of 9- or 9, or 10-substituted anthracenes in liquid solid matrices increase with decreasing temperature, while the initial absorbance of the triplet-triplet absorption shows a converse temperature dependence. At 77°K, the fluorescence quantum yields of these compounds are unity and concurrently no triplet formation can be detected. An Arrhenius plot analysis of the emission and absorption data definitely establishes that the temperature dependence of fluorescence of these compounds is caused by the temperature dependence of the lowest excited singlet state to triplet state intersystem crossing. It is proposed that the activated intersystem crossing is the result of higher triplet state (assumed to be the second triplet state) or states lying above the lowest excited singlet state, participating in the radiationless transition. The “direct” intersystem crossing involving the lowest excited singlet and the lowest triplet states appears to be incapable of competing with rapid fluorescence because of the large energy gap between these two states. Experimental results suggesting the unimportance of the lowest excited singlet to ground singlet and higher excited singlet to triplet radiationless transitions are also discussed.