Thiophene-Phenyl Azomethines with Varying Rotational Barriers - Model Compounds for Examining Imine Fluorescence Deactivation

Abstract

Alkylated derivatives of 2, 4, 6-aniline and 3-thiophene carbaldehydes were used for preparing aldol and ketal imines (1−9). The effect of substitution on the photophysical properties of alkylated azomethines was examined in order to understand the origins of complete fluorescence suppression exhibited by all azomethines. Introducing steric elements for increasing the barrier of rotation around the N-aryl and CH-thiophene bonds did not result in increased fluorescence either at room temperature or at 77 K. This confirmed that azomethine fluorescence quenching is not by nonradiative energy dissipation by bond rotation. Stern−Volmer fluorescence measurements of both fluorene and bithiophene lead to diffusion controlled quenching rates (kq ≈ 1010 M−1 s−1) when quenched by both an aliphatic aldolimine (13) and a conjugated azomethine (3). Photoinduced electron transfer (PET) from the fluorophore to the imine is the major mode of deactivation of the singlet excited state. This deactivation mode was corroborate...