Internal conversion in 4-substituted 1-naphthylamines. Influence of the electron donor/acceptor substituent character
作者:Kengo Suzuki、Attila Demeter、Wolfgang Kühnle、Erich Tauer、Klaas A. Zachariasse、Seiji Tobita、Haruo Shizuka
DOI:10.1039/a908924a
日期:——
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
in hexane
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.
对4-取代的1-(二甲氨基)萘(14DMX)和1-氨基萘(14ANX,X=CN、Cl、H、CH3和OCH3)中发生的热激活内部转换(IC)进行了研究,实验在三种不同极性的溶剂中进行,分别是己烷、二乙醚和乙腈。在14DMX和14ANX两系列化合物中,当X从CN变化为OCH3时,IC反应的效率显著降低,这一变化的顺序与4-取代基的电子给予特性逐渐增强有关。对于第一组化合物,获得的IC反应速率常数明显更大。这个差异与氨基的基态结构有关,其中14DMX的扭曲程度(约60°)显著高于14ANX(约20°),而在激发态S1中,这两类1-萘基胺的平面性则得以保持。对于每个14DMX和14ANX分子来说,IC过程随着溶剂极性的增加而减慢。取代基X和溶剂极性主要影响IC的激活能EIC。在己烷中的14DMX,EIC从X=CN的10 kJ mol−1增加到X=OCH3的34 kJ mol−1,而以14DMCL为例,EIC在己烷中观察到从16 kJ mol−1到在乙腈中达到28 kJ mol−1的随着溶剂极性增加而变化。EIC的能垒高度由两个最低激发单态之间的能量间隙ΔE(S1,S2)所主导。ΔE(S1,S2)对EIC的影响归因于由于S1和S2状态的近似而导致的振动耦合,这使得S1的势能曲面变平,从而当ΔE(S1,S2)减小时,降低了IC的能垒。假设1-萘基胺的IC反应是通过锥形交叉点进行的,这一交叉点是由于两种状态中不同的氨基扭转角引起的S1和S0表面的相对位移而存在的。