Numerical self-consistent reaction field study of the excited-state properties of p-(dimethylamino)-benzonitrile derivatives

Abstract

Since the first observation of the dual fluorescence of p-(dimethylamino)-benzonitrile ( p-DMABN) by Lippert et al. [E. Lippert, W. Lüder, F. Moll, W. Nägele, H. Boos, H. Prigge, I.Seilbold-Blankenstein, Angew. Chem. 21, (1961) 695], many theories were proposed to explain this interesting behaviour. So far, the model proposed by Rotkiewicz in 1973 [K. Rotkiewicz, K.L. Grellman, Z.R. Grabowski, Chem. Phys. Lett. 19, (1973) 315] has gained the widest acceptance, despite doubts being expressed about its validity. Zachariasse et al. [K.A. Zachariasse, Th. von der Haar, A. Hebecker, U. Leinhos, W. Kühnle, Pure. Appl. Chem. 65, (1993) 1745 ; K.A. Zachariasse, Th. von der Haar, U. Leinhos and W. Kühnle, J. Inf. Rec. Mats. 21, (1994) 501] base their critique, beside other experimental observations, on the lack of dual fluorescence in two groups of p-DMABN derivatives. One comprises the methyl substituted isomers p-(amino)-benzonitrile ( p-ABN) and p-(methylamino)-benzonitrile ( p-MABN), the other the cyano substituted isomers m-(dimethylamino)-benzonitrile ( m-DMABN) and 3,5-dicyanodimethylaniline (DCDMA). Although these derivatives are structurally not too different from p-DMABN, it is not obvious whether the similarity is large enough to provide evidence for the invalidity of the TICT model in the parent compound. In this contribution, therefore, we will present the results of a semi-empirical study using a numerical self-consistent reaction field model to describe the solvent influence on the excited state properties of the above mentioned derivatives. As will be demonstrated, the non-existent anomalous fluorescence can be rationalized by the changed electronic properties of the different compounds. The different photophysical behaviour of the p-DMABN derivatives alone cannot be used to doubt the validity of the TICT model in case of p-DMABN.