Fluorescence from the second excited π-singlet state of aromatic hydrocarbons in solution

Abstract

Fluorescence emission from the first, S₁, and second, S₂, excited π-electronic singlet states to the singlet ground state, S₀, has been observed for a number of aromatic molecules in dilute, deoxygenated n-heptane solutions. Detailed fluorescence spectra from S₁ and S₂ for 3,4-benzpyrene and 1,12-benzperylene over the temperature range –90 to + 90°C are reported. The S₂ fluorescence is very strongly dependent on temperature and it results mostly from thermal repopulation of S₂ from S₁. The ratio, R, of the integrated fluorescence quantum intensity from S₂ to that from S₁ is 0.09 at –90°C and 0.8 at +90°C for 1,12-benzperylene. For 3,4-benzpyrene, R= 0.06 at –90°C and R= 0.4 at +90°C. These studies show that fluorescence emission from the second excited π-singlet state of aromatic molecules in solution is not uncommon and that the internal conversion process cannot be regarded as being completely irreversible in these systems.