The development of multi-color aggregation-induced emission (AIE)-featured azine molecules with excited-state intramolecular proton transfer (ESIPT) characteristics has drawn significant interest in recent years. In this study, we report the construction of donor-acceptor-structured triphenylamine-functionalized unsymmetrical azine molecules (L₁–L₄) prepared from 4-(hydrazonomethyl)-N,N-diphenylaniline with different salicylaldehyde derivatives. By changing the electron donating ability at the ESIPT moiety, we could tune the optical properties of the newly synthesized molecules. All the compounds exhibited the AIE behavior in a THF/water mixture and the excited-state intramolecular proton transfer phenomenon. Further, the tuning of a peripheral substituent in the salicylaldehyde moiety resulted in different emission colors in the aggregated state. The crystal structure of all the compounds (L₁–L₄) revealed that the multiple weak interactions present in the solid-state structure lead to various supramolecular networks. In addition, these molecules showed a prominent positive solvatochromic effect, which was confirmed by their solvent polarity-dependent emission behavior. The appearance of dual emission bands in the solid as well as in the solution state, lifetime values and HOMO, LUMO energy gap of the keto and enol forms strongly support the occurrence of ESIPT in all the compounds (L₁–L₄). Furthermore, the presence of two different electron donating groups in L₃ (triphenylamine and diethylamino) induced pH-dependent emissive features in solution.

Design and development of multifunctional materials have drawn incredible attraction in recent years. Herein, we report the design and construction of versatile star-shaped intramolecular charge transfer (ICT)-coupled excited-state intramolecular proton transfer (ESIPT)-active mechanoresponsive and aggregation-induced emissive (AIE) luminogen triaminoguanidine-diethylaminophenol (LH₃) conjugate from simple precursors triaminoguanidine hydrochloride and 4-(N,N-diethylamino)salicylaldehyde. Solvent-dependent dual emission in nonpolar to polar protic solvents implies the presence of ICT-coupled ESIPT features in the excited state. Aggregation-enhanced emissive feature of LH₃ was established in the CH₃CN/water mixture. Furthermore, this compound exhibits mechanochromic fluorescence behavior upon external grinding. Fluorescence microscopy images of pristine, crystal, and crushed crystals confirm the naked-eye mechanoresponsive characteristics of LH₃. In addition, LH₃ selectively sensed a Cu²⁺ ion through a colorimetric and fluorescence “turn-off” route, and subsequently, the LH₃-Cu²⁺ ensemble could act as a selective and sensitive sensor for S^2– in a “turn-on” fluorescence manner via a metal displacement approach. Reversible “turn-off–turn-on” features of LH₃ with Cu²⁺/S^2– ions were efficiently demonstrated to construct the IMPLICATION logic gate function. The Cu²⁺/S^2–-responsive sensing behavior of LH₃ was established in the paper strip experiment also, which can easily be characterized by the naked eye under daylight as well as a UV lamp (λ = 365 nm).

Multi-stimuli responsive fluorescent materials have vast applications in the field of chemistry, physics and material science. Among the multifunctional materials, ESIPT-active materials have drawn prodigious attraction in recent years due to their large Stokes shifted emission and dual emissive behavior. Herein, we report a novel “all-in-one” type fluorescent 7-diethylamino-4-hydroxycoumarin-rhodamine B conjugate (DHCRH) which shows aggregation induced emission, acidochromism, and mechanochromic luminescent features along with unique methanol responsiveness. An information protection application was executed via a write-erase-write fluorescent platform using the acidochromic feature of DHCRH. Furthermore, the DHCRH-Zn ensemble was found to exhibit reversible photochromic behavior from a yellow to pink color in the solution state upon irradiation with 365 nm UV light through an intramolecular ring opening and closing mechanism. The complex provided a long lifetime for the ring-opened form and good fatigue resistance for reversible photochromism.