Internal Conversion in 1-Aminonaphthalenes. Influence of Amino Twist Angle

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 S 0, 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, 1 H NMR spectra, ground-state dipole moments, and ab initio calculations. For the five 1-aminonaphthalenes in the equilibrated S 1 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 S 1 and S 0 . A correlation is found between the IC efficiency in these molecules and the twist angle θ. The IC process to S 0 starts from the equilibrated S 1 state, which is vibronically coupled with S 2 due to a small energy gap Δ E( S 1,S 2 ). 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.