Dual-State Photophysical Modulation via Bifurcated Hydrogen Bonding in a U-Shaped Dipyridophenazine-Cored Donor–π–Acceptor–π–Donor Fluorophore

Modulating photophysical properties via hydrogen bonding offers a powerful strategy for designing supramolecular functional materials and responsive optical systems. Here, we present a U-shaped dipyridophenazine (DPyPHZ)-cored donor–π–acceptor–π–donor (D–π–A–π–D) scaffold that serves as a hydrogen-bond receptor capable of forming bifurcated hydrogen bonds, enabling simultaneous modulation of both ground and excited states. We developed an efficient synthetic route to this p-conjugated system, and demonstrated that its absorption and emission spectra undergo pronounced red-shifts upon 1:1 complexation with neutral hydrogen-bond donors such as sulfonamides and even water. Spectroscopic analyses reveal that the hydrogen-bonding interaction stabilizes the charge-transfer excited state, leading to enhanced photoluminescence quantum yield (PLQY) and color modulation. Notably, these optical responses persist in the solid state when embedded in a polymer matrix, enabling emission color switching without fluorophore aggregation. Single-crystal X-ray analysis confirmed the formation of a well-defined bifurcated hydrogen-bonding complex in the solid state. Comparative studies with structural analogues highlight the importance of the electron-density hotspot in DPyPHZ for selective and directional hydrogen bonding. This work introduces a supramolecular design strategy for precise dual-state photophysical control, providing a platform for environmentally responsive luminescent materials with potential in sensing, imaging, and optoelectronic applications.