Photophysics and peripheral ring size dependent aggregate emission of cross-conjugated enediynes: applications to white light emission and vapor sensing

Photophysical understanding of organic fluorophores with π-conjugated scaffolds is crucial as such dyes are central to optoelectronic applications. This work presents a detailed photophysical investigation of a class of cross-conjugated homo- and hetero-enediynes (Y-shaped) peripherally attached to common aromatic moieties such as benzene, naphthalene, and anthracene. The cross-communicated electronic communication among the three aromatic units located at the tri-poles of the Y-shaped enediynes results in a broad S0 → S1 absorption band and locally excited (LE) emission signals. In addition to the LE emission band, a red-shifted aggregate emission is observed for some of the dyes in non-aqueous solvents where a clear size dependence of the peripheral aromatic rings is noted for the appearance of the aggregate fluorescence. The aggregates are static in nature as is evident from ground-state absorption spectral changes and the absence of rise-time in the time-resolved fluorescence decay studies, which are substantiated further through nuclear magnetic resonance spectroscopy and single-crystal X-ray diffraction experiments. Molecular orbital calculations support the local nature of the dominant electronic transition. The optimized ground state geometries of the dyes from partially to fully propeller shaped structures confirm the ring-size dependence of the aggregates. The LE and aggregate state emissions are judiciously exploited to generate single-component white light emission in binary solvent mixtures. The excited state photophysics are further applied toward polar aprotic vapor sensing in the solid state.