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Terpyridine (TP) integrated with tetraphenylethylene (TPE-TP), pyrene (Py-TP) and triphenylamine (TPA-TP) exhibited structure controlled molecular packing and distinct fluorescence in the solid state. TPE-TP exhibited infinite aromatic π⋯π stacking between terpyridine units, Py-TP produced an isolated pyrene dimer, and TPA-TP displayed helical assembly via weak intermolecular H-bonding in the crystal lattice. The different molecular assembly of TPE-TP, TPA-TP and Py-TP produced a locally excited (LE) state, charge transfer (CT) state and excimeric emission at 460, 520 and 568 nm, respectively, in the solid state. Interestingly, the different molecular packing of TPE-TP, Py-TP and TPA-TP produced varied mechanofluorochromism. TPA-TP exhibited on–off reversible fluorescence switching upon mechanical crushing and heating, whereas TPE-TP and Py-TP displayed switching between two fluorescence states but in opposite directions. Crushing caused red shift of emission in TPE-TP and blue shift in Py-TP. Both TPA-TP and Py-TP showed strong fluorescence in solution and crystalline states, but TPE-TP showed typical aggregation induced emission (AIE). Further, Py-TP and TPE-TP showed selective fluorescence sensing of picric acid in solution and solid matrix, respectively. TPA-TP exhibited sensing of nitroaromatics both in solution as well as in the solid state. Solid state fluorescence of TPA-TP and TPE-TP was further utilized for fabricating PVA polymer composite thin films and demonstrated reusable fluorescence sensing of nitroaromatics in aqueous medium. The PVA–fluorophore composite film exhibited clear fluorescence quenching upon immersing the thin film into 10−7 M picric acid. Thus, the present study demonstrates the role of molecular structure in developing stimuli-responsive fluorescence materials and reusable sensors. [ABSTRACT FROM AUTHOR]

Interlinked aggregation induced emissive (AIE) fluorophores (triphenylamine (TPA) donor (D) and indanedione acceptor (A)) via non-conjugated spacers (1–3) showed concentration-dependent tunable fluorescence from blue to deep red/NIR in solution and deep red emission in the solid-state. 1–3 exhibited weak broad solid-state fluorescence covering the blue to deep red region (433 to 706 nm, ΦF = 2.3 to 5.3%). It is noted that the non-interlinked TPA-indanedione fluorophore was non-emissive in solution but showed strong solid-state fluorescence at 611 nm (ΦF = 17.7%). In contrast, 1–3 showed strong fluorescence in the solution state (ΦF = 0.001 to 0.09 compared to fluorescein). Importantly, 1–3 exhibited concentration-dependent tunable fluorescence from blue (457 nm) to deep red/NIR (720 nm) in CHCl3 due to varied aggregation of fluorophores. Dynamic light scattering (DLS) studies confirmed the molecular aggregation even at very low concentrations (10−7 M). The increasing concentration lowered the solubility and produced bigger aggregates with wide size distribution that caused tunable fluorescence. The deep red fluorescence of 1 was utilized for live-cell imaging applications using E. coli cells. Fluorescence microscopic images revealed clear red fluorescent E. coli cells with a rod morphology. Furthermore, the non-toxic nature of 1 was confirmed by in vitro as well as in vivo studies. Thus, deep red/NIR emissive fluorophores were achieved by interlinking two AIEgens using a non-conjugated spacer and utilized for live cell imaging. [ABSTRACT FROM AUTHOR]