A semiempirical study of the fluorescence properties of large charge transfer compounds: N,N‐dimethylanilino‐bis‐pyrazolopyridine

Abstract

Abstract4‐(4′‐N,N‐dimethylaminophenyl)‐3,5‐dimethyl‐1,7‐diphenyl‐bis‐pyrazolo‐[3,4‐b;4′,3′e]‐pyridine (DMA‐DMPP), was recently synthesized as an example of a bulky electron donor‐acceptor compound and was examined by spectroscopic methods. The molecule consists of several subunits of different redox properties and the observed behavior was thus interpreted by an intramolecular charge transfer model. This hypothesis is validated by semiempirical excited state calculations. They are focused on the coupling of intramolecular charge transfer to various relaxation modes, e.g. rotation about single bonds leading to decoupling π systems. Rotation about the single bond connecting the dimethylanilino and heteroaromatic subunit results as the most important relaxation coordinate. The two moieties non orthogonal in the ground state become perpendicularly oriented in the optimum excited state. In this conformation the dipole moment also becomes maximal. These results agree satisfactorily with the observation of a marked red shift of its fluorescence in polar solvents and dual fluorescence found in polar, protic environment.