Tuning Emission Wavelength of Polymorphous Crystal via Controllable Alkyl Chain Stacking and Its Vapor‐ and Thermo‐Responsive Fluorescence

Abstract

Tuning fluorescence colour of solid‐state materials has become a topic of increasing interest for both fundamental mechanism study and practical applications such as sensors, optical recording and security printing. In this work, a fluorescent colour tuneable molecule BA‐C16 is rationally designed and facilely synthesized by attaching flexible long alkyl chains to 2‐hydroxybenzophenone azine (BA), which shows both aggregation‐induced emission (AIE) and excited‐state intramolecular proton transfer (ESIPT) characteristics. Compared to BA, the simple introduction of long alkyl chains in BA‐C16 leads to an emission wavelength redshift from 542 to 558 nm. This strategy of extending emission wavelength is rarely reported, and is ascribed to the enlarged through‐space π‐conjugation between interplanar molecules in the aggregate of BA‐C16. Three crystals of BA‐C16 are obtained with green, yellowish green and yellow emission. According to characterization by X‐ray crystallography, X‐ray powder diffraction and differential scanning calorimetry, alkyl chains play an important role in inducing different stacking modes of the three crystals, which further leads to polymorph‐dependent fluorescence colour. BA‐C16 exhibits tuneable solid‐state fluorescence upon vapor fumigation, or annealing based on a transition between a “near‐monomer” crystalline state and a “dimer” crystalline state. BA‐C16 is further applied for rewritable fluorescence printing tuned by vapor‐ and thermal‐treatment.

ESIPT‐based AIE luminogens: Design strategies, applications, and mechanisms

“…[193] In addition, different polymorphs of the same ESIPT-based AIEgen can also be converted by griding, melting, vapor, annealing, and so on. [178,179,214] As a result, the ESIPT-based AIEgens will change the color and can be applied as stimuli-responsive materials.…”

Interplay of Dual-Proton Transfer Relay to Achieve Full-Color Panel Luminescence in Excited-State Intramolecular Proton Transfer (ESIPT) Fluorophores

“…The design and study of organic molecules exhibiting excited-state intramolecular proton transfer (ESIPT) properties have recently gained much momentum, notably because such fluorophores have a wide variety of potential applications. − Typically, ESIPT is a four-level photochemical cycle (E → E* → K* → K) involving photoexcitation of a molecule in the ground enol-state (E) to an excited enol*-state (E*), followed by fast tautomerization ( k ESIPT ≈ 10 12 s –1 ), leading to the excited keto*-state (K*), which then decays radiatively to its ground keto-state (K). − This usually results in large Stokes shifts and dual emission when the radiative decay occurs from both E* and K* states. − In the past few years, important efforts have been made to merge ESIPT properties with useful photophysical phenomena. Hence, diverse molecular designs allowing the combination of ESIPT with thermally activated fluorescence, phosphorescence or long persistent luminescence, aggregation-induced emission, amplified spontaneous emission, as well as multiple stimuli-responsive emissions have been reported, leading to the potential in different fields including sensing, displaying, lasing, imaging, and so forth. − …”