Internal conversion in 1-aminonaphthalenes. Influence of amino twist angle

With the I-aminonaphthalenes 1N5 and IDMAN a fast radiationless process occ urs in n;hexane, diethyl ether, and acetonitrile, which is shown to be inte rnal conversion (IC). The IC reaction is slower with 1N4 and much less effi cient with 1MAN and 1AN. This IC process is thermally activated and slows d own with increasing solvent polarity, due to a larger IC activation energy. In the ground state So, the amino twist angle theta relative to the naphth alene plane increases in the order 1MAN, 1AN, 1N4, 1N5, IDMAN, as derived f rom absorption and fluorescence spectra, II-I NMR spectra, ground-state dip ole moments, and ab initio calculations. For the five l-aminonaphthalenes i n the equilibrated S-1 state, the twist angle and the radiative rate consta nt have similar values. The different IC efficiences of these molecules are therefore determined by the structural differences (amino twist angle) bet ween SI and So. A correlation is found between the IC efficiency in these m olecules and the twist angle theta. The IC process to So starts from the eq uilibrated S1 state, which is vibronically coupled with Sg due to a small e nergy gap Delta E(S-1,S-2). It is therefore concluded-that the extent of vi bronic coupling and the magnitude of the twist angle theta are the determin ing factors in the IC process.