The photophysics of tertiary aliphatic amines

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The fluorescence properties of the three simple tertiary amines, trimethylamine (TMA), triethylamine (TEA), and tri- n-propylamine (TPA) have been investigated in the vapour phase with particular reference to the effect of varying the excitation energy. For excitation into the lowest energy absorption band, attributed to an n → 3s Rydberg excitation, single exponential decay characteristics were observed, whereas at higher energies simultaneous excitation of n → 3s and n → 3p excitations resulted in dual exponential decay. This is explained on the basis of different distributions of emitting vibronic levels of the lower Rydberg state being produced by internal conversion from the n → 3p upper state from those produced upon direct excitation of the lower state followed by partial redistribution. As would be expected form this analysis, rate constants for radiative decay of both sets of levels are found to be identical in TMA and TEA, whereas the rate constants for non-radiative decay, assumed to be decomposition, are not identical for the two sets of levels. The analysis permits separation of the overlapping absorptions of the n → 3s and n → 3p Rydberg transitions in TMA and TEA.