Remarkable mechanochromism and force-induced thermally activated delayed fluorescence enhancement from white-light-emitting organic luminogens with aggregation-induced emission

Toward High Contrast and Noninvasive Fluorescence Switches via an O-Fused Ring 5,7-Dihydroxy-4-methyl-2,2,3-triphenylbenzofuran-6(2H)-one Strategy

An unprecedented O-fused ring 5,7-dihydroxy-4-methyl-2,2,3-triphenylbenzofuran-6(2H)-one (3) was first time synthesized. Further, a series of novel dialkyl/fluoroalkyl derivatives of compound 3, 5,7-dialkoxy/fluoroalkoxy-4-methyl-2,2,3-triphenylbenzofuran-6(2H)-one, were obtained with noninvasive fluorescence switching characteristics and aggregation-induced emission properties. Compared with fluoroalkyl derivatives, the alkyl analogs exhibited a significant bathochromic shift in solid-state fluorescence emission. Notably, these noninvasive fluorescent molecular switches could be facilely tuned through light and heat stimulation, which successfully achieved high contrast and reversible fluorescent emission between orange and yellow endowing them with potential applications in data encryption materials. In addition, the single crystal data of compounds 3 and 7-CF3 displayed weak intermolecular interactions in different directions, resulting in twisted conformation and antiparallel slip stacking. Interestingly, the polymer dimethyl silicone film doped with 7-C3F7 also showed an evident light-responsive behavior, meeting the criterion for fluorescent materials in the optical field.

Understanding H-aggregates crystallization induced emissive behavior: insights from theory

We conducted a theoretical investigation into how the molecular stacking effect impacts the photophysical properties in solid phases. Our findings indicated that in the aggregated state, the out-of-plane distorted vibration and imidazole ring stretching vibration of triimidazo-[1,3,5] triazinethe are significantly suppressed, which decreased the Huang-Rhys factor and the corresponding reorganization energy of the photophysical process, as a result, this restricted intramolecular motions and dissipation pathways of excess energy in the excited state, therefore, aggregation induced enhancement emission (AIEE) was found for the title compound from dichloromethane solution to solid state. Analysis of the emission spectrum through discrete spectral lines revealed that the main peak was affected by the vibrational modes with lower frequencies, while the middle-frequency modes influenced the shoulder peak. Furthermore, the predicted intersystem crossing rate (k_iosk) and reverse intersystem crossing rate (k_risc) using Marcus theory confirmed that an electron can successfully shift from its S₁ state to the T₁ state, however, the reverse T₁ → S₁ process can not come into being due to very small k_risc (10^-6-10^-9 s^-1), therefore the phosphorescence can be observed. At last, we explored the influence of charge transfer process of the title compound, our theoretical data declared this process can be ignored due to its low transfer rate.

Color-Tunable Dual-Mode Organic Afterglow from Classical Aggregation-Caused Quenching Compounds for White-Light-Manipulated Anti-Counterfeiting

Color-tunable dual-mode organic afterglow excited by ultraviolet (UV) and white light was achieved from classical aggregation-caused quenching compounds for the first time. Specifically, two luminescent systems, which could produce significant organic afterglow composed of persistent thermally activated delayed fluorescence and ultralong organic phosphorescence under ambient conditions, were constructed by doping fluorescein sodium and calcein sodium into aluminum sulfate. Their lifetimes surpassed 600 ms, and the dopant concentrations were as low as 5×10^-6  wt %. Moreover, the persistent luminescence colors of the materials could be tuned from blue to green and then to yellow by simply varying the concentrations of guest compounds or the temperature in the range of 260-340 K. Inspired by these exciting results, the afterglow materials were used for UV- and white-light-manipulated anti-counterfeiting and preparation of elastomers with different colors of persistent luminescence.

Conformational isomeric thermally activated delayed fluorescence (TADF) emitters: mechanism, applications, and perspectives

Thermally activated delayed fluorescence (TADF) materials have received enormous attention and the mechanism behind them has been investigated in depth. It has been found that some donor-acceptor (D-A) type TADF emitters could obviously exhibit dual stable conformations in the ground states and their distributions significantly affect the physical properties and device performances. Therefore, professional analysis and a summary of the relationship between molecular structures and performances are very important. In this review, we first summarize the mechanism and properties of TADF emitters with conformational isomerism. We also classify their recent progress according to their different applications, and provide an outlook on their perspectives.

Bright Organic Mechanoluminescence and Remarkable Mechanofluorochromism from Circularly Polarized TADF Enantiomers with Aggregation-Induced Emission Properties

The development of circularly polarized thermally activated delayed fluorescence (CP-TADF) luminogens with stimuli-response characteristics remains challenging. Herein, a pair of organic enantiomers, S-CzTA and R-CzTA, with aggregation-induced emission properties, have been successfully developed by introducing chiral 1,2,3,4-tetrahydronaphthalene and carbazole to phthalimide. They present CP-TADF properties in toluene solutions, giving dissymmetric factors of 0.84×10^-3 and -1.03×10^-3 , respectively. In the crystalline state, both S-CzTA and R-CzTA can emit intense blue TADF and produce very bright sky-blue mechanoluminescence (ML) and remarkable mechanofluorochromism (MFC) under the stimuli of mechanical force. Single-crystal analysis and theoretical calculation results suggest that their ML activities are probably associated with their chiral and polar molecular structures and unique non-centrosymmetric molecular packing modes. Furthermore, the MFC properties of the enantiomers likely originate from the destruction of crystal structure, leading to the planarization of molecular conformation. This work may provide helpful guidance for developing new CP-TADF materials with force-stimuli-responsive properties.

Unexpected synthesis, delayed emission and solid-state acidochromism of novel 2,7-naphthyridine derivatives obtained from 2-(3,5-diaryl-4H-pyran-4-ylidene)malononitrile

Two novel 2,7-naphthyridine derivatives are unexpectedly synthesized by the reaction of 2-(3,5-diaryl-4H-pyran-4-ylidene)malononitrile and benzylamine, and are achieved through different ring-closing mechanisms. These two derivatives with twisted molecular conformations display phosphorescence, thermally activated delayed fluorescence, and high contrast solid-state acidochromism due to special chemical structures.