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

Since the first observation of the dual fluorescence of p-(dimethylamino)-b enzonitrile (p-DMABN) by Lippert et al. [E. Lippert, W. Luder, E Moll, W. N agele, H. Boos, H. Prigge, I.Seilbold-Blankenstein, Angew. Chem. 21, (1961) 695], many theories were proposed to explain this interesting behaviour. S o far, the model proposed by Rotkiewicz in 1973 [K. Rotkiewicz, K.L. Grellm an, 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. Ku hnle, Pure. Appl. Chem. 65, (1993) 1745; K.A. Zachariasse, Th. von der Haar , U. Leinhos and W. Kuhnle, J. Inf. Rec. Mats. 21, (1994) 501] base their c ritique, beside other experimental observations, on the lack of dual fluore scence in two groups of p-DMABN derivatives. One comprises the methyl subst ituted isomers p-(amino)-benzonitrile (p-ABN) and p- (methylamino)-benzonit rile (p-MABN), the other the cyano substituted isomers m-(dimethylamino) -b enzonitrile (m-DMABN) and 3,5-dicyanodimethylaniline (DCDMA). Although thes e derivatives are structurally not too different from p-DMABN, it is not ob vious whether the similarity is large enough to provide evidence for the in validity of the TICT model in the parent compound. in this contribution, th erefore, we will present the results of a semi-empirical study using a nume rical self-consistent reaction field model to describe the solvent influenc e on the excited state properties of the above mentioned derivatives. As wi ll be demonstrated, the non-existent anomalous fluorescence can be rational ized by the changed electronic properties of the different compounds. The d ifferent photophysical behaviour of the p-DMABN derivatives alone cannot be used to doubt the validity of the TICT model in case of p-DMABN. (C) 1999 Elsevier Science S.A. All rights reserved.