Design of Stimuli-Responsive Phenothiazine Derivatives with Triplet-Related Dual Emission and High-Contrast Mechanochromism Guided by Polymorph Prediction

The development of high-contrast stimulus-responsive materials with excited triplet emission is of great significance for anti-counterfeiting, sensor and memory applications, but remains a challenge. Here, we report a strategy for the rational design of stimulus-responsive phenothiazine derivatives with triplet-related dual emissions and high-contrast mechanochromism guided by Polymorph Prediction. The designed phenothiazine derivatives have the characters of simple structures, a facile synthetic procedure, and a good crystalline nature. We found that the crystals of those derivatives with the potential to form both quasi-axial (ax) and quasi-equatorial (eq) conformations could undergo conformation transition and show significant emission difference (Δλ_em >100 nm) under mechanical force. Meanwhile, all these phenothiazine derivatives exhibit aggregation-induced emission and emit room-temperature phosphorescence or thermally activated delayed fluorescence. The significant luminescent change of these materials under different stimuli gives them promise for applications in encryption and anti-counterfeiting.

Modulating Room-Temperature Phosphorescence-To-Phosphorescence Mechanochromism by Halogen Exchange

Modulating the stimulus-responsiveness of a luminescent crystal is challenging owing to the complex interdependent nature of its controlling factors, such as molecular structure, molecular conformation, crystal packing, optical properties, and amorphization behavior. Herein, we demonstrate a halogen-exchange approach that disentangles this problem, thereby realizing the modulation of room-temperature phosphorescence-to-phosphorescence mechanochromism. Replacing the bromine atoms in a brominated thienyl diketone with chlorine atoms afforded isostructural crystals; i.e., molecules with different halogen atoms exhibited the same molecular conformation and crystal packing. Consequently, amorphization behavior toward mechanical stimulation was also the same, and the phosphorescence of amorphous states originated from the same conformer of each diketone. In contrast, the phosphorescence properties of each conformer were modulated differently, which is ascribable to heavy atom effects, resulting in the modulation of the mechanochromism. Thus, halogen exchange is a promising approach for modulating the stimulus-responsive photofunctions of crystals involving spin-forbidden processes.

Multistimulus Response of Two Tautomeric Zn(II) Complexes and Their White-Light Emission Based on Different Mechanisms

A triphenylamine (TPA)-based 2H-quinazoline Zn(II) complex (Q-TPA-Zn) exhibiting dual fluorescence and phosphorescence emission in the solid state was designed and prepared. It possesses mechanochromic luminescence and thermochromic luminescence properties. In the solid state, the white afterglow luminescence could be observed at 77 K (CIE_xy: 0.27, 0.33) while cyan luminescence could be observed at 297 K. After thermolysis at 300 °C, Q-TPA-Zn could be transformed into Schiff base complex S-TPA-Zn with white fluorescence in the powder state (CIE_xy: 0.32, 0.38), in methanol (CIE_xy: 0.32, 0.39), and in dimethylformamide (CIE_xy: 0.26, 0.32) at room temperature. This arises from dual emission of normal* emission and tautomeric* emission induced by excited-state intramolecular proton transfer (ESIPT) from the benzimidazole NH group to the Schiff base N atom. Q-TPA-Zn could also be transformed into its isomeric form, S-TPA-Zn, through photochemical ring-opening reaction upon irradiation under 365 nm in the solution, exhibiting high-contrast photochromic luminescence. Interestingly, S-TPA-Zn could further be transformed into its zwitterionic isomer after continuous irradiation. The same ring-opening reaction could also take place for the orgainc compound Q-TPA via heating or 365 nm irradiation. The ring-opening reaction mechanism and ESIPT emission were interpreted via theoretical calculation.

New [2,2]Fluorenophanes Give Insights into Asymmetric Charge Transfer-Mediated Exciton Delocalization along the π-π Packing Direction

A series of new [2,2]fluorenophanes has been synthesized and characterized; among them, molecules of crystallographically asymmetric anti-[2.2](1,4)(4,1)fluorenophane (K2C-2) aggregate to form one-dimensional supramolecular chain structures through effective intermolecular π-π overlapping. This, in combination with the synergistic intramolecular π-π interaction, leads to prominent dual emission mediated by charge transfer (CT) exciton delocalization. Support of this new insight is given by mapping the transition density along the π-π packing direction where the intramolecular excitation and intermolecular CT coexist in K2C-2.

Metal-Free Organic Luminophores that Exhibit Dual Fluorescence and Phosphorescence Emission at Room Temperature

Dual-fluorescent-phosphorescent compounds have attracted increasing attention in various fields, such as bio-imaging, data protection/encryption, ratiometric luminescence sensing, and white-light emission. Conventional dual-emissive compounds contain a phosphorescent organometallic complex of a precious metal, such as iridium or platinum. However, the use of precious metals in organic materials has several drawbacks. This Minireview focuses on precious-metal-free organic light-emitting materials that exhibit dual fluorescence and phosphorescence emission in the solid state at room temperature to produce bimodal steady-state emission spectra. The dual emitters presented herein are categorized into the following six compound classes: (1) difluoroboron diaroylmethanes, (2) diarylketones, (3) diarylsulfones, (4) triazines and pyrimidines, (5) fused phenazines, and (6) N-arylcarbazoles.

Excitation-Dependent Triplet-Singlet Intensity from Organic Host-Guest Materials: Tunable Color, White-Light Emission, and Room-Temperature Phosphorescence

A series of organic host-guest materials with multifunctional luminescence were constructed. Four isoquinoline derivatives were used as the guests, and benzophenone was used as the host. The doped system exhibited excellent dual emission with cyan fluorescence and orange-yellow room-temperature phosphorescence, and the dual emission could be combined into almost pure white-light emission. Importantly, the relative intensity of the fluorescence-phosphorescence could be adjusted by changing the excitation wavelength, with the phosphorescence intensity being significantly higher than the fluorescence intensity under shorter excitation wavelengths and vice versa under longer excitation wavelengths. Therefore, three-color emission switching among cyan, white, and orange could be achieved by simply adjusting the excitation wavelength. The results of experimental and theoretical calculations indicated that the excitation-dependent emission colors were caused by different transfer paths for excitons under different excitation wavelengths. These materials with multifunctional luminescence could be used as writable inks for advanced anticounterfeiting.

Preparation and Photoluminescent Properties of Three 5-Amino Benzothiadiazoles (5-amBTDs)

Three D-A compounds were designed and synthesized based on a benzothiadiazole acceptor. Azepane (AP), iminodubenzyl (IDB) and iminostilbene (ISB) were used, respectively, as donors and installed on the 5-position of BTD to afford 1, 2 and 3, respectively. Their photophysical properties in different states (solution, film, crystal, and powder) are systematically investigated. Among them, AIE-active compounds 2 and 3 were found to have good sensitivity toward viscosity and display quite good linear relationship with an increase in viscosity. Compound 2 displayed dual emission in solutions which largely depended on the polarity of the solvent. Meanwhile, compound 2 exhibits a mechanochromic character with disappearance and reappearance of a dual-emissive peak induced by mechanical grinding and solvent fuming. Furthermore, these three compounds can be used in the fabrication of blue OLED devices.

Single-Molecular White-Light Emitters and Their Potential WOLED Applications

White organic light-emitting diodes (WOLEDs) are superior to traditional incandescent light bulbs and compact fluorescent lamps in terms of their merits in ensuring pure white-light emission, low-energy consumption, large-area thin-film fabrication, etc. Unfortunately, WOLEDs based on multilayered or multicomponent (red, green, and blue (RGB)) emissive layers can suffer from some remarkable disadvantages, such as intricate device fabrication and voltage-dependent emission color, etc. Single molecules, which can emit white light, can be used to replace multiple emitters, leading to a simplified fabrication process, stable and reproducible WOLEDs. Recently, the performance of WOLEDs by using single molecules is catching up with that of the state-of-the-art devices fabricated by multicomponent emitters. Therefore, an increasing attention has been paid on single white-light-emitting materials for efficient WOLEDs. In this review, different mechanisms of white-light emission from a single molecule and the performance of single-molecule-based WOLEDs are collected and expounded, hoping to light up the interesting subject on single-molecule white-light-emitting materials, which have great potential as white-light emitters for illumination and lighting applications in the world.

Selective Expression of Chromophores in a Single Molecule: Soft Organic Crystals Exhibiting Full-Colour Tunability and Dynamic Triplet-Exciton Behaviours

Soft luminescent materials are attractive for optoelectronic applications, however, switching dominant chromophores for property enrichment remains a challenge. Herein, we report the first case of a soft organic molecule (DOS) featuring selective expression of chromophores. In response to various external stimuli, different chromophores of DOS can take turns working through conformation changes, exhibiting full-colour emissions peaking from 469 nm to 583 nm from ten individual single crystals. Dynamic triplet-exciton behaviours including thermally activated delayed fluorescence (TADF), room-temperature phosphorescence (RTP), mechanoluminescence (ML), and distinct mechano-responsive luminescence (MRL) can all be realized. This novel designed DOS molecule provides a multifunctional platform for detection of volatile organic compounds (VOCs), multicolour dynamic displays, sensing, anticounterfeiting, and hopefully many others.