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

• Published in: Crystal growth & design Vol. 24; no. 8; pp. 3388 - 3398
• Main Authors: Issac, Jerome, Ravi, Sasikala, Chidambaranathan, Kamalanathan, Karthikeyan, Subramanian, Pannipara, Mehboobali, Al-Sehemi, Abdullah G., Anthony, Savarimuthu Philip, Madhu, Vedichi
• Format: Journal Article
• Language: English
• Published: American Chemical Society 17.04.2024
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/acs.cgd.4c00109

Triphenylamine (TPA) and tetraphenylethylene (TPE) π-conjugated imidazo­[1,2-a]­pyridine (IP) donor–acceptor compounds, TPA-IP and TPE-IP, were synthesized, and the TPA/TPE-dependent stimuli-responsive 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 [λmax = 441 nm (solid) and 387 nm (hexane), quantum yield (Φf) = 1.86% (solid) and 0.089 (compared to quinine sulfate)], whereas TPE-IP showed typical aggregation-induced emission in the solid state (λmax = 445, Φf = 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 multistimuli-responsive fluorescence switching.