Phenoxazinophanes: Synthesis, structure, spectral, redox and theoretical studies

Three fluorescent phenoxazinophanes, each incorporating two phenoxazine units bridged by two ethene linkers, were synthesized in a three-step sequence starting from commercially available phenoxazine. Substituents such as methyl and phenyl groups were introduced at the ethene bridges for the first time, resulting in significant modulation of the electronic properties of phenoxazinophanes. The X-ray structure revealed that phenoxazine moiety in phenoxazinophane exhibits a butterfly-shaped, non-planar structure with a significant reduction in dihedral angle (36.7°) between the planes of the two benzene rings compared to the angle in free phenoxazine (167°). The spectroscopic, and electrochemical studies revealed that the properties of phenoxazinophanes markedly distinct from those of previously reported phenothiazinophanes and depends on the type of substituents present at the bridged ethene carbons. Remarkably, the phenoxazinophanes exhibit green fluorescence in the solid state with a broad emission in the region of 450–650 nm and quantum yields were in the range of 0.32–0.35. Preliminary studies indicated that phenoxazinophanes exhibit aggregation-induced emission. The electrochemical studies revealed that phenoxazinophanes are highly electron rich and DFT/TD-DFT studies were in agreement with the experimental observations.