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

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 (k_q ≈ 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 corroborated by the Rehm−Weller equation using the measured cyclic voltammetry and spectroscopic data. Exergonic values for PET were calculated for bithiophene quenching with 13 (ΔG° = −120 kJ/mol). The PET deactivation mode can be suppressed by protonating the azomethine resulting in increased fluorescence. Although PET deactivates the singlet excited state, intersystem crossing to the triplet manifold also occurs, confirmed by steady-state phosphorescence measurements. The absence of detectable triplet by laser flash photolysis confirms that the triplet formed is rapidly quenched by energy transfer. This was kinetically corroborated by quenching both fluorene and bithiophene triplets with 13 leading to bimolecular quenching rate constants of 3 × 10⁹ and 2 × 10⁷ M⁻¹ s⁻¹, respectively, in acetonitrile. These data confirm that the heteroatomic azomethine bond efficiently quenches both the singlet excited and triplet states by intramolecular photoinduced electron transfer and energy transfer, respectively.