Controlling structural and photophysical properties in sequentially methylated phenoxazines

Abstract

The “magic methyl” effect is widely regarded as one of the most extraordinary features in small-molecule drug design and has recently been recognized as a subtle yet powerful tool for fine-tuning the photophysical properties of luminophores. In this context, we investigated how steric pressure influences the structural and photophysical attributes in a series of sequentially methylated phenoxazines. Structural elucidation using X-ray diffraction revealed that the steric strain of ortho-positioned methyl groups induces out-of-plane twisting of the N-arylated rings, which significantly alters the packing interactions by preventing dense π stacking. Complementary quantum chemical calculations indicate changes in the antiaromatic character due to partial rehybridization of the bridging nitrogen atom from sp² to a more trigonal pyramidal geometry. Steady-state and time-resolved spectroscopy further highlighted the correlations between out-of-plane bending and emissive behavior, as characterized by distinctly pronounced changes in molar absorptivity and Stokes shifts. Finally, an effective strategy to overcome steric hindrance was demonstrated by extending the overall molecular conjugation, resulting in intense emission with high absolute photoluminescence quantum yields in both solution and the solid state.