Dual-State Fluorescence with Efficiency Close to Unity from D-π-A-Type Triarylborane-Based Naphthalenes

Fluorophores exhibiting bright fluorescence in both dilute solution and the solid state, termed dual-state emission (DSE), are more attractive for practical applications than those that fluoresce in a single state. Herein, we disclose two DSE-active naphthalene derivatives, BNPh-Nap and BNOPh-Nap, featuring a simple D-π-A structure with an electron-accepting dimesitylboryl (BMes2) and electron-donating diphenylamino (NPh2) or N,N'-di(4-methoxyphenyl)amino (NOMePh2) substituents at the 2,6-positions of the naphthalene core unit. Despite the very flat structure of the naphthalene core unit, the steric hindrance of the BMes2 and diarylamino groups can effectively suppress intermolecular π-π interactions in the solid state. As a result, BNPh-Nap exhibits strong fluorescence not only in solution (ΦF = 0.97 in cyclohexane) but also in the solid state, including powder (ΦF = 0.67) and neat film (ΦF = 0.51). From BNPh-Nap to BNOPh-Nap, although the fluorescence efficiency decreases slightly in solution (ΦF = 0.71 in cyclohexane), the enhanced electron-donating ability of the amino group greatly boosts the solid-state fluorescence, yielding near-unity ΦF values in both powder (ΦF = 0.91) and film (ΦF = 0.99). These results demonstrate that simple modification of a conventional planar π-core with BMes2 and diarylamino groups enables high-performance DSE without complex twisted architectures, providing an effective strategy for developing DSE-active fluorophores.