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With 3,5-dimethyl-4-(methylamino)benzonitrile (MHD) in n-hexane and n-hexadecane at 25°C, relatively small values are obtained for the fluorescence quantum yield Φ f  (0.006) and the lifetime τ (0.103 ns). From Φ f  and the yield of intersystem crossing Φ ISC  (0.024), it follows that MHD in these solvents undergoes efficient internal conversion with a yield Φ IC  of 0.96 and an IC rate constant k IC  of 9×10 9  s -1 . An IC activation energy of 12 kJ/mol is obtained from the measurement of τ as a function of temperature in n-hexane. The amino group of MHD in the electronic ground state S 0  is twisted relative to the benzonitrile moiety, as deduced from quantum chemical calculations supported by experiments. It is assumed that the IC reaction is a consequence of the planarisation of the amino group upon excitation to the relaxed S 1  state and passes through a conical intersection. In support of this mechanism, efficient IC is not observed with the planar derivative 4-amino-3,5-dimethylbenzonitrile (HHD), in contrast to the strongly twisted 3,5-dimethyl-4-(dimethylamino)benzonitrile (MMD).

Internal Conversion with 3,5-Dimethyl-4-(methylamino)benzonitrile in Alkane Solvents