Internal conversion in 4-substituted 1-naphthylamines. Influence of the electron donor/acceptor substituent character

Abstract

The thermally activated internal conversion (IC) taking place in 4-substituted 1-(dimethylamino)naphthalenes (14DMX) and 1-aminonaphthalenes (14ANX) with X=CN, Cl, H, CH₃ and OCH₃ was investigated in three solvents spanning the polarity scale, hexane, diethyl ether and acetonitrile. In both series 14DMX and 14ANX, the efficiency of the IC reaction decreases substantially when X changes from CN to OCH₃, the order in which the electron donor character of the 4-substituent increases. Considerably larger IC reaction rate constants are obtained for the first group of compounds. This difference is connected with the ground state structure of the amino group, which is more strongly twisted for 14DMX (ca. 60°) than for 14ANX (ca. 20°), whereas both sets of 1-naphthylamines are planarised in the S₁ excited state. The IC process slows down with increasing solvent polarity for each of the 14DMX and 14ANX molecules. The substituent X and the solvent polarity mainly affect the IC activation energy E_IC. With 14DMX in hexane, E_IC increases from 10 kJ mol⁻¹ for X=CN to 34 kJ mol⁻¹ for X=OCH₃, whereas with, e.g., 14DMCL a solvent polarity dependent increase of E_IC from 16 kJ mol⁻¹ in hexane to 28 kJ mol⁻¹ in acetonitrile is observed. The height of the barrier E_IC is governed by the energy gap ΔE(S₁,S₂) between the two lowest excited singlet states. The influence of ΔE(S₁,S₂) on E_IC is attributed to vibronic coupling caused by the proximity of the S₁ and S₂ states, which flattens the S₁ potential energy surface and thereby lowers the IC barrier when ΔE(S₁,S₂) becomes smaller. It is assumed that the IC reaction of the 1-naphthylamines passes through a conical intersection, which exists as a consequence of the relative displacement of the S₁ and S₀ surfaces caused by the different amino twist angles in the two states.