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 EIC. With 14DMX in hexane, EIC
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 EIC from 16 kJ mol−1
to 28 kJ mol−1 in acetonitrile is observed. The height of the barrier EIC is governed by the energy gap ΔE(S1,S2) between the two lowest excited singlet states. The influence of ΔE(S1,S2) on EIC 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.
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