Fluorescence emission from the first- and the second-excited π-singlet states of aromatic hydrocarbons in solution, and their temperature dependences

Abstract

Earlier studies on the temperature dependence of the fluorescence emission from dilute deoxygenated n-heptane solutions of aromatic hydrocarbons have been extended from 1,12-benzperylene and 3,4-benzpyrene to 1,2-benzanthracene, 2′-methyl-1,2-benzanthracene, 3,4-benzotetraphene, and 3-methylpyrene. For these molecules, the energy separation, Es_2.0–Es_1.0, between the first, S₁, and the second, S₂, excited π-singlet states lies in the range 1200 to 2500 cm^–1. In all cases, fluorescence emission from S₂ to the ground state S_0,n has been clearly observed. For each molecule studied, the ratio of the integrated fluorescence quantum intensity from S₂ to that from S₁ increases with increasing temperature and, for the molecules investigated, it decreases with increasing Es_2,0–Es_1.0. Our present and earlier experimental results indicate that: (i) the S₂ fluorescence is primarily a result of thermal repopulation of S₂ from a lower level, (ii) the S₂ fluorescence is quite intense (especially at elevated temperatures) for aromatic molecules which have close lying first- and second-excited π-singlet states, (iii) the internal conversion process between S₂ and S₁ cannot be regarded as being completely irreversible, and (iv) the fluorescence emission from the second excited π-singlet state of aromatic molecules in solution is not uncommon.