No abstract

With the 1-aminonaphthalenes 1N5 and 1DMAN a fast radiationless process occurs in n-hexane, diethyl ether, and acetonitrile, which is shown to be internal conversion (IC). The IC reaction is slower with 1N4 and much less efficient with 1MAN and 1AN. This IC process is thermally activated and slows down with increasing solvent polarity, due to a larger IC activation energy. In the ground state S0, the amino twist angle θ relative to the naphthalene plane increases in the order 1MAN, 1AN, 1N4, 1N5, 1DMAN, as derived from absorption and fluorescence spectra, 1H NMR spectra, ground-state dipole moments, and ab initio calculations. For the five 1-aminonaphthalenes in the equilibrated S1 state, the twist angle and the radiative rate constant have similar values. The different IC efficiences of these molecules are therefore determined by the structural differences (amino twist angle) between S1 and S0. A correlation is found between the IC efficiency in these molecules and the twist angle θ. The IC process to S0 starts from the equilibrated S1 state, which is vibronically coupled with S2 due to a small energy gap ΔE(S1,S2). It is therefore concluded that the extent of vibronic coupling and the magnitude of the twist angle θ are the determining factors in the IC process.

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 Φf (0.006) and the lifetime τ (0.103 ns). From Φf and the yield of intersystem crossing ΦISC (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 ϫ 10 9 s Ϫ1 . 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 S0 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).