Quantum-Chemical Studies on the Effects of Substituents on Photographic Action of N-Phenyl-benzimidacarbocyanine

Abstract

As part of the investigation of the photographic actions of cyanine dyes, quantum-chemical studies have been made of the effect of substituents on the sensitizing power of N-phenyl-benzimidacarbocyanine (I) by using HMO method. HMO calculations have indicated that the lowest vacant electronic energy level siv of N-(o-substituted) phenyl-I and N-(p-substituted) phenyl-I (II and III resp.) are higher by 0-0.3 eV shan that of I, and that siv of N-(m-substituted) phenyl-I (IV) is nearly equal to that of I. This tendency increases with decreasing the electronegativity of substituents and with decreasing the C-R bond length, where R is a substituent. In the lowest vacant orbital, the nature of C-R bond is antibonding. Therefore, the more electron cloud gathers at C-R bond, the higher (the more unstable) the lowest vacant level is. On the contrary to the prediction by Neunhoeffer and keiler, there is no correlation between positive charge at N atom and sensitizing power of dyes. The observation by Neunhoeffer and Keiler that II and III have stronger sensitizing power than I and IV has been explained by the result that sty of II and III are higher than those of I and IV.