Substituent-Engineered Spirofluorene-Boron Emitters: Integrating TADF, Visible Afterglow, and Mechanochromism in a Single Material

A series of spirofluorene–triarylborane derivatives bearing tunable triarylamine donors (–H, –OMe, –SMe) were designed and synthesized to systematically investigate substituent effects on their photophysical behaviors. The methoxy-substituted compound ^FXylB-DOBPDA exhibits efficient thermally activated delayed fluorescence (TADF), characterized by a small singlet–triplet energy gap (ΔE_ST) of 0.05 eV, along with notable aggregation-induced emission enhancement (AIEE). Remarkably, through a host–guest doping strategy using triphenylamine (TPA) as the matrix, the system displays clearly visible afterglow under ambient conditions. In particular, the ^FXylB-DOBPDA@TPA codoped system achieves TADF-promoted long afterglow with lifetimes exceeding 100 ms. Additionally, reversible mechanochromic luminescence is observed, indicating multistimuli-responsive behavior. The combination of experimental analyses and theoretical calculations reveals that subtle changes in substituents critically influence intramolecular charge transfer and excited-state dynamics. These results offer valuable insights into molecular design strategies for achieving tunable emission in solid-state materials. With their integrated TADF, afterglow, and stimuli-responsive properties, these multifunctional materials demonstrate significant potential for applications in organic light-emitting diodes (OLEDs), mechanical sensors, anticounterfeiting labels, and optical storage technologies.