J-aggregation in indolenine dyes drives fluorescent crystals with large Stokes shifts and two-photon excited emission

Organic fluorescent solids are highly demanded for applications such as sensing and bioimaging, yet most of these systems require co-crystallization with additional components to avoid aggregation-caused quenching. Here, the intermolecular assembly and resulting photophysical properties of a series of indolenine-based dyes are described. These dipolar donor-π-acceptor (D-π-A) structures are composed to favor head-to-tail interactions, rendering most of them with an exceptional ability for J -aggregate formation. Due to their tailored molecular architecture, J -aggregation features pair with unusually large Stokes shifts in both concentrated solutions and solid state (up to 3,272 cm −1 in aggregates and 6,135 cm −1 in solid state), enabling two-photon excited fluorescence (2PEF). Unlike most 2PEF-active condensed organic systems, these dyes spontaneously form single-component fluorescent crystals upon solvent evaporation, i.e., without requiring co-crystallization with additional components. The resulting crystals exhibit a pronounced and fully reversible vapochromic response, readily allowing the development of simple and robust sensing devices.