Multistimuli Fluorescence-Responsive TPA/TPE-Imidazo[1,2-a]pyridine π-Conjugated Donor–Acceptor Derivatives and Effect of Donor Structures

Abstract

Triphenylamine (TPA) and tetraphenylethylene (TPE) πconjugated imidazo[1,2-a]pyridine (IP) donor−acceptor compounds, TPA-IP and TPE-IP, were synthesized, and the TPA/TPE-dependent stimuliresponsive fluorescence switching in the solid and solution state was investigated. TPA-IP and TPE-IP revealed a twisted molecular conformation and adopted opposite molecular dipole arrangement in the solid state. TPA-IP exhibited dual-state fluorescence [λ max = 441 nm (solid) and 387 nm (hexane), quantum yield (Φ f ) = 1.86% (solid) and 0.089 (compared to quinine sulfate)], whereas TPE-IP showed typical aggregation-induced emission in the solid state (λ max = 445, Φ f = 6.21%). Mechanoresponsive studies of TPE-IP showed self-reversible fluorescence switching upon mechanical grinding and acid exposure. In contrast, TPA-IP exhibited reversible fluorescence switching upon mechanical grinding/heating and acid/base exposure. In solution, TPA-IP exhibited pH-dependent tuneable fluorescence from blue to yellow due to protonation of imidazo-pyridine nitrogen including white light emission at optimized acid concentrations. Computational studies of TPA-IP indicated clear charge transfer from TPA donor to IP acceptor, whereas electron density mostly occupied in the IP in highest occupied molecular orbital and lowest unoccupied molecular orbital of TPE-IP, indicating locally excited state emission. This was further supported by electrochemical cyclic voltammetry that showed a relatively low band gap for TPA-IP compared to TPE-IP. Thus, integrating twisted donor (TPA/ TPE) with the imidazo-pyridine acceptor produced fluorophores with donor-dependent tuneable fluorescence and multistimuliresponsive fluorescence switching.