Achieving remarkable mechanochromism and white-light emission with thermally activated delayed fluorescence through the molecular heredity principle

D–A type luminophores with a twisted molecular conformation constructed by phenoxazine and diphenylsulfone showing high contrast mechanofluorochromism

Two new stimulus-response luminescence materials PXZ-DPS and BPXZ-DPS possessing the twisted molecular conformation exhibit a unique ICT effect, effective solid-state fluorescence, and high-contrast and reversible mechanofluorochromism.

Blue thermally activated delayed fluorescence emitters incorporating acridan analogues with heavy group 14 elements for high-efficiency doped and non-doped OLEDs

Deep-blue thermally activated delayed fluorescence (TADF) emitters are promising alternatives for conventional fluorescence and phosphorescence materials for practical application in organic light-emitting diodes (OLEDs). However, as appropriate bipolar hosts for deep-blue TADF-OLEDs are scarce, the development of efficient deep-blue TADF emitters that are applicable to both doped and non-doped systems is an urgent task. In this study, we developed a new family of blue TADF emitters that demonstrated high photoluminescence (PL) and electroluminescence (EL) quantum efficiencies in both doped and non-doped (neat) systems. Four new donor-acceptor (D-A)-type TADF molecules incorporating phenazasiline, phenazagermine, and tetramethylcarbazole as weak D units and phenothiaborin as a weak A unit were designed and synthesized. By varying the structural rigidity/flexibility as well as the electron-donating ability of the D units, the resulting photophysical and TADF properties of the D-A molecules could be systematically regulated. A comprehensive photophysical investigation revealed that phenazasiline and phenazagermine-based emitters concurrently exhibit blue TADF emissions (464-483 nm), high PL quantum efficiencies (∼100%), extremely fast spin-converting reverse intersystem crossing rates (>107 s-1), and suppressed concentration quenching. These fascinating features in conjunction produced high-performance doped and non-doped blue TADF-OLEDs. The doped and non-doped TADF-OLEDs using the phenazasiline-based emitter demonstrated extremely high maximum external EL quantum efficiencies (η ext) of 27.6% and 20.9%, with CIE chromaticity coordinates of (0.14, 0.26) and (0.14, 0.20), respectively. Further, ultra-low efficiency roll-off behavior for both the doped and non-doped devices was demonstrated by their η ext as high as 26.1% and 18.2%, respectively, measured at a practically high luminance of 1000 cd m-2.

Carbon Dots with Dual-Emissive, Robust, and Aggregation-Induced Room-Temperature Phosphorescence Characteristics

Carbon dots (CDs) with dual-emissive, robust, and aggregation-induced RTP characteristics are reported for the first time. The TA-CDs are prepared via hydrothermal treatment of trimellitic acid and exhibit unique white prompt and yellow RTP emissions in solid state under UV excitation (365 nm) on and off, respectively. The yellow RTP emission of TA-CDs powder should be resulted from the formation of a new excited triplet state due to their aggregation, and the white prompt emission is due to their blue fluorescence and yellow RTP dual-emissive nature. The RTP emission of TA-CDs powder was highly stable under grinding, which is very rare amongst traditional pure organic RTP materials. To employ the unique characteristics of TA-CDs, advanced anti-counterfeiting and information encryption methodologies (water-stimuli-response producing RTP) were preliminarily investigated.

Fluorescence-phosphorescence dual emissive carbon nitride quantum dots show 25% white emission efficiency enabling single-component WLEDs

Developing efficient single-component white light-emitting diodes (WLEDs) is extremely challenging due to the issue of Kasha's rule. Here we report the first demonstration of blue-yellow fluorescence-phosphorescence dual emission from our newly minted single-component white emissive carbon nitride quantum dots (W-CNQDs). The W-CNQDs deliver an overall photoluminescence quantum efficiency of 25%, which is the highest value among white-emitting materials reported to date, based on utilizing both singlet and triplet states. Experimental and theoretical investigations reveal that the carbonyl groups at the rim of the W-CNQDs play a key role in promoting intersystem crossing and inducing intermolecular electronic coupling, affording intensive yellow phosphorescence. Efficient white emission is achieved with a phosphorescence quantum efficiency of 6% under ambient conditions. A WLED is fabricated by integrating W-CNQD phosphors into a UV-LED chip, which shows favorable white light characteristics with CIE coordinates and a CRI of (0.35, 0.39) and 85, respectively, demonstrating good color chromatic stability. This work opens up new opportunities for exploring dual emission mechanisms and designs to facilitate the development of efficient single-component WLEDs.

Oxidation State-Dependent Electronic Properties of Sulfur-Containing Thermally Activated Delayed Fluorescence Molecules

Comparative studies of a series of sulfur-containing thermally activated delayed fluorescence (TADF) molecules and their oxidized compounds are carried out by means of electronic structure calculations. Aiming at investigating the effects of oxidation of bridged sulfur on the modulation of electronic structures of sulfur-containing TADF molecules, their geometrical structures, singlet (S₁) and triple (T₁) energies and their gap (ΔE_ST), the transition dipole moment, the spin-orbit coupling (SOC) between S₁ and T₁ states, the ionization potentials, and electron affinities are analyzed in detail to determine the structure-property relationships in these investigated TADF molecules and their corresponding oxidized counterparts. The electronic structure calculations show that the oxidation of bridged sulfur into the corresponding sulfoxide and sulfone significantly changes the electronic properties of TADF molecules. Interestingly, a substantial reduction in the singlet-triplet energy difference is possible with an increase in the oxidation state of the sulfur atom in the core. Moreover, the sulfone-containing molecules exhibit both S₁ and T₁ states having a large charge transfer (CT) excitation characteristic, which helps reduce the singlet-triplet energy gap and facilitates the reverse intersystem crossing (RISC) from the triplet state to the singlet state. SOC values increase with an increase in the oxidation state of the sulfur atom. Particularly, a sulfoxide-containing core moiety exhibits higher SOC values when compared with the sulfone-containing acceptor core.

Side chain effects on the solid-state emission behaviours and mechano-fluorochromic activities of 10H-phenothiazinylbenzo[d]imidazoles

A family of 4-cyanophenyl-substituted 10H-phenothiazinylbenzo[d]imidazoles with different side chains at the 10-position are prepared and their physical properties are studied. The detailed structure-property research demonstrates that the cold crystallization temperature of ground samples and the emission wavelengths of pristine samples are in good accordance with the packing density, conformation distortion and intermolecular interactions, but emission wavelengths of ground samples are slightly chain-dependent. For benzimidazoles with alkyl chains, longer and more branched chains can produce looser packings, which cause pristine samples to display red-shifted emission and reduced MFC activity. For benzimidazoles with a phenyl chain, the emission wavelengths of both the pristine and the ground samples are remarkably red-shifted. Moreover, the degree of conformation distortion is larger, and the cold crystallization temperature is higher. Interestingly, the homologue with the n-hexyl chain displays an intense ML effect that is mainly attributed to the heavy discharge quantity on largely enhanced discharge areas under force stimuli due to the great fragility of this crystal.

The solid-state emission behaviours and MFC performances of 10H-phenothiazinylbenzo[d]imidazoles with different side chains are investigated.

Rotaxane-Based Mechanophores Enable Polymers with Mechanically Switchable White Photoluminescence

Three mechanoresponsive polyurethane elastomers whose blue, green, and orange photoluminescence can be reversibly turned on by mechanical force were prepared and combined to create a blend that exhibits deformation-induced white photoluminescence. The three polyurethanes contain rotaxane-based supramolecular mechanoluminophores based on π-extended pyrene, anthracene, or 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) luminophores, respectively, and 1,4,5,8-naphthalenetetracarboxylic diimide as an electronically matched quencher. Each polymer shows instantly reversible, strain-dependent switching of its photoluminescence intensity when stretched and relaxed, as deformation leads to a spatial separation of the luminophore and quencher. The present study shows that the photoluminescence color can easily be tailored by variation of the luminophore and also by combining several mechanophores in one material and demonstrates that adaptability is a key advantage of supramolecular approaches to create mechanoresponsive polymers.

Recent Advancements in and the Future of Organic Emitters: TADF- and RTP-Active Multifunctional Organic Materials

Organic emitting compounds that are based on π-conjugated skeletons have emerged as promising next-generation materials for application in optoelectronic devices. In this Minireview, recent advances in the development of organic emitters that irradiate room-temperature phosphorescence and/or thermally activated delayed fluorescence with extraordinary luminescence properties, such as aggregation-induced emission, mechanochromic luminescence, and circularly polarized luminescence, are discussed.

Convenient One-Pot Synthesis of 1,2,3,4-Thiatriazoles Towards a Novel Electron Acceptor for Highly-Efficient Thermally-Activated Delayed-Fluorescence Emitters

A novel and unexpected convenient one-pot synthesis of 1,2,3,4-thiatriazoles has been discovered while investigating the classical tetrazine "Pinner synthesis". The synthetic route starts from commercially-available nitrile derivatives and gives good to high yields (51-80 %) with no need to isolate any thioacylating agents. The crucial impact of the solvent on the outcome of the modified "Pinner synthesis" is moreover examined and discussed. Using this new synthetic route, a novel donor-acceptor thiatriazole derivative has been prepared, which exhibits prominent thermally-activated delayed fluorescence (TADF) in both solution and film. The photoluminescence quantum yield (PLQY) in methylcyclohexane (MCH) and Zeonex (a cyclo olefin polymer) in oxygen-free conditions were determined to be 76 and 99 %, respectively. This work provides an efficient and practical synthetic approach to functionalized 1,2,3,4-thiatriazole derivatives, and will noticeably facilitate the application of 1,2,3,4-thiatriazole as an electron acceptor in organic electronics.

Mechanofluorochromic behaviors of triphenylamine functionalized salicylaldimine difluoroboron complexes

The solid-state emission of D–A type difluoroboron complexes is tuned by external forces.

Multitasking behaviour of a small organic compound: solid state bright white-light emission, mechanochromism and ratiometric sensing of Al(iii) and pyrophosphate

Solid state bright white-light emission, mechanochromism and ratiometric fluorescence sensing of Al³⁺ and pyrophosphate by a single organic molecule are demonstrated.

Thiophene-containing tetraphenylethene derivatives with different aggregation-induced emission (AIE) and mechanofluorochromic characteristics

Four thiophene-containing tetraphenylethene derivatives were successfully synthesized and characterized. All these highly fluorescent compounds showed typical aggregation-induced emission (AIE) characteristics and emitted different fluorescence colors including blue-green, green, yellow and orange in the aggregation state. In addition, these luminogens also exhibited various mechanofluorochromic phenomena.

Four thiophene-containing AIE-active TPE derivatives were synthesized. Furthermore, these luminogens exhibited various mechanofluorochromic phenomena.

Mechanochromic fluorescence based on a combination of acceptor and bulky donor moieties: tuning emission color and regulating emission change direction

Mechanochromic fluorescence based on a conventional strategy using a donor–acceptor structure bearing nonplanar three-dimensional donor moieties was studied.

Excited-state dynamics of a molecular dyad with two orthogonally-oriented fluorophores

The excited-state dynamics of a T-shaped bichromophoric molecule, consisting of two strong fluorophores, diphenyloxazole and diphenylpyrazoline, directly linked in an orthogonal geometry, was investigated. Despite the weak coupling ensured by this geometry and confirmed by the electronic absorption spectra, this dyad exhibits only weak fluorescence in both apolar and polar solvents, with fluorescence lifetimes ranging from 200 ps in CHX to 10 ps in ACN. Ultrafast spectroscopic measurements reveal that the fluorescence quenching in polar solvents is due to the population of a charge-separated state. In non-polar solvents, this process is energetically not feasible, and a quenching due to an efficient intersystem crossing (ISC) to the triplet manifold is proposed, based on quantum-chemical calculations. This process occurs via the spin-orbit charge-transfer (SOCT) ISC mechanism, which is enabled by the charge-transfer character acquired by the S1 state of the dyad upon structural relaxation and by the orthogonal arrangement of the molecular orbitals involved in the transition. The same mechanism is proposed to explain why the recombination of the charge-separated state is faster in medium than in highly polar solvents, as well as to account for the fast decay of the lowest triplet state to the ground state.

Photomechanochromic vs. mechanochromic fluorescence of a unichromophoric bimodal molecular solid: multicolour fluorescence patterning

The multicolour fluorescence responses to external mechanical forces vs. internal photomechanical stresses of a molecular solid are demonstrated and compared.

Mechanofluorochromism (MFC) of molecular solids generally results from the variation of intermolecular interactions induced by external mechanical forces. However, the use of internal photomechanical forces to perturb intermolecular interactions for multicolour fluorescence responses has yet to be demonstrated. Herein we report a unichromophoric anthracene–pentiptycene derivative (1) that displays both MFC and photomechanofluorochromism (PMFC), which lead to various fluorescence colours including red-green-blue (RGB) and near-pure white-light emission. Compound 1 crystallizes in two polymorphs, the yellow (Y) and green (G) emissive forms, in which the pairwise stacked anthracene groups undergo [4 + 4] photodimerization to form the UV (black) emissive photodimer 2 and meanwhile exert photomechanical stresses on the neighbouring molecules. While the photomechanical stresses cause an excimer-to-monomer switching that results in a blue fluorescent state for the Y form, a red-emissive “super dimer” is photomechanically produced for the G form. The recovery of the Y form demands heating, but the G form could be restored by selective photoexcitation of the super dimer. X-ray crystal structures of the Y and G forms and the photodimer 2 generated through single-crystal-to-single-crystal transformation provide a clue to the origins of polymorph-dependent PMFC. The corresponding MFC and mechano-activated vapofluorochromism (VFC) of 1 also shed light on the structure–property relationship. The ability to spatially and temporally control the fluorochromicity of 1 is demonstrated by a series of multicolour fluorescence patterning of “angelfishes”.

Sensitized triplet-triplet annihilation upconversion in water and its application to photochemical transformations

Sensitized triplet-triplet annihilation (TTA) is a promising mechanism for solar energy conversion, but so far its application has been practically completely limited to organic solvents and self-assembled or solid state systems. Combining water-soluble ruthenium complex-pyrene dyads with particularly long excited-state lifetimes as sensitizers and highly fluorescent commercial anthracenes as acceptors/annihilators, we were able to achieve green-to-violet upconversion with unprecedented quantum yields in pure water. Compared to the only known system exploiting sensitized TTA in homogeneous aqueous solution, we improve the overall photon upconversion efficiency by a full order of magnitude and present the very first example for a chemical transformation on a laboratory scale via upconversion in water. Specifically, we found that a thermodynamically challenging carbon-chlorine bond activation can be driven by green photons from an inexpensive continuous wave light source in the presence of dissolved oxygen. Our study is thus potentially relevant in the context of cleaning water from halogenated (toxic) contaminants and for sustainable photochemistry in the most environmentally friendly solvent.

Optically Triggered Planarization of Boryl-Substituted Phenoxazine: Another Horizon of TADF Molecules and High-Performance OLEDs

We report the unprecedented dual properties of excited-state structural planarization and thermally activated delayed fluorescence (TADF) of 10-dimesitylboryl phenoxazine, i.e., PXZBM. Bearing a nonplanar phenoxazine moiety, PXZBM shows the lowest lying absorption onset at ∼390 nm in nonpolar solvents such as cyclohexane but reveals an anomalously large Stokes-shifted (∼14 500 cm^-1) emission maximized at 595 nm. In sharp contrast, when a phenylene spacer is added between phenoxazine and dimesitylboryl moieties of PXZBM, the 10-(4-dimesitylborylphenyl)phenoxazine PXZPBM in cyclohexane reveals a much blue-shifted emission at 470 nm despite its red-shifted absorption maximized at 420 nm (cf. PXZBM). The emission of PXZBM further reveals solvent polarity dependence, being red-shifted from 595 nm in cyclohexane to 645 nm in CH₂Cl₂. For rationalization, the steric hindrance between phenoxazine and the dimesitylboryl unit in PXZBM caused a puckered arrangement of phenoxazine at the ground state. Upon electronic excitation, as supported by the femtosecond early relaxation dynamics, spectral-temporal evolution and energetics calculated along the reaction potential energy surfaces, the diminution of N → B electron transfer reduces π-conjugation and elongates the N-B bond length, inducing the fast phenoxazine planarization with a time constant of 890 ± 100 fs. The associated charge-transfer reaction from phenoxazine (donor) to dimesitylboryl unit (acceptor) results in a further red-shifted emission in polar solvents. In stark contrast, PXZPBM shows a planar phenoxazine and undergoes excited-state charge transfer only. Despite the distinct difference in excited-state relaxation dynamics, both PXZBM and PXZPBM exhibit efficient TADF capable of producing highly efficient orange and green organic light emitting diodes with peak efficiencies of 10.9% (30.3 cd A^-1 and 18.7 lm W^-1) and 22.6% (67.7 cd A^-1 and 50.0 lm W^-1).

Transient and Persistent Room-Temperature Mechanoluminescence from a White-Light-Emitting AIEgen with Tricolor Emission Switching Triggered by Light

Persistent luminescence from purely organic materials is basically triggered by light and electricity, which largely confines its practical applications. A purely organic AIEgen exhibits not only persistent photoluminescence, but also transient and persistent room-temperature mechanoluminescence. By simply turning on and off a UV lamp, tricolor emission switching between blue, white, and yellow was achieved. The data from single-crystal structure analysis and theoretical calculation suggest that mechanism of the observed persistent mechanoluminescence (pML) is correlated with the strong spin-orbit coupling of the bromine atom, as well as the formation of H-aggregates and restriction of intramolecular motions in noncentrosymmetric crystal structure. These results outline a fundamental principle for the development of new pML materials, providing an important step forward in expanding the application scope of persistent luminescence.

Molecular stacking dependent phosphorescence-fluorescence dual emission in a single luminophore for self-recoverable mechanoconversion of multicolor luminescence

A self-recoverable mechanoconversion of multicolor luminescence was observed by crystallization-induced self-assembly. Such a self-assembly led to the formation of nanorods with different molecular stacking modes, thus making the phosphorescence/fluorescence proportion alterable. Therefore, multicolor luminescence of the single luminophore can be achieved by a straightforward reversible mechanical stimulus.

Developing the structure-property relationship to design solid state multi-stimuli responsive materials and their potential applications in different fields

Establishing the structure–property relationship for multi-stimuli responsive mechanochromic materials based on charge transfer luminogens.

Prediction of multi-stimuli responsive behavior in newly developed luminogens is an appealing yet challenging puzzle, since no concrete design strategy has been developed so far. In this article, we demonstrate a potent strategy to gain a deep understanding of the structure–property relationship to design multi-stimuli responsive mechanochromic materials. To achieve our goal, a variety of new isoindolinone core based charge transfer luminogens exhibiting aggregation-induced emission (AIE) have been prepared through C–H bond activation using a cost-effective ruthenium (Ru) metal catalyzed one-pot synthetic strategy. We have shown that slight tuning of the donor moiety is found to be highly effective in controlling molecular packing and metastable energy states in solid states, and thus, optical properties and multi-stimuli responsive behaviors. The flexibility and twisting of donor moieties afford a loosely bound ‘herringbone’ packing, enabling reversible transformation under multiple mechanical stimuli. The cyclized derivative of the donor exhibits a completely different packing mode (i.e., cross packing), and subsequently, does not give rise to mechanochromism. The Hirshfeld surface analysis from a single crystal infers that non-covalent interactions (specifically C–H···π and π···π) are extremely important to yield mechanochromism under external force. Correlating all solid-state behavior with the molecular structure, we conclude that the synergistic effect between the twisting and conformational flexibility of donor moieties along with numerous non-covalent interactions gives rise to multi-stimuli responsive behaviors. Finally, the newly designed molecules are found to be highly emissive in solution and potentially applicable in fluorescence thermometer construction, lighting up cells, acid–base sensors and rewritable devices.

Dimethylamine substituted bisbenzocoumarins: solvatochromic, mechanochromic and acidochromic properties

Highly fluorescent emissive dimethylamine substituted bisbenzocoumarins (DB-Cns) show reversible fluorescence color changes under mechanical force stimuli and acidochromic properties.

Facile preparation and investigation of the properties of single molecular POSS-based white-light-emitting hybrid materials using click chemistry

Recently, organic white light emitting devices (OWLEDs) have attracted great interest in flexible displays and solid state lighting devices.

An organic–inorganic hybrid zinc phosphite framework with room temperature phosphorescence

An organic–inorganic hybrid zinc phosphite framework with large channels and fascinating room temperature phosphorescence has been presented.

Effect of Differential Self-Assembly on Mechanochromic Luminescence of Fluorene-Benzothiadiazole-Based Fluorophores

Supramolecular self-assembly is an excellent tool for controlling the optical and electronic properties of chromophore-based molecular systems. Herein, we demonstrate how differential self-assembly affects mechanoresponsive luminescence of fluorene-benzothiadiazole-based fluorophores. We have synthesized two donor-acceptor-donor-type conjugated oligomers consisting of fluorene as the donor and benzothiadiazole as the acceptor. For facile self-assembly, both molecules are end-functionalized with hydrogen-bonding amide groups. Differential self-assembly was induced by attaching alkyl chains of different lengths onto the fluorene moiety: hexyl (FB-C6) and dodecyl (FB-C12). The molecules self-assemble to form well-defined nanostructures in nonpolar solvents and solvent mixtures. Although their optical properties in solution are not affected by the alkyl chain length, significant effects were observed in the self-assembled state, particularly in the excitation energy migration properties. As a result, remarkable differences were observed in the mechanochromic luminescence properties of the molecules. A precise structure-property correlation is made using UV-visible absorption and fluorescence spectroscopy, time-correlated single-photon counting analysis, scanning electron microscopy, and X-ray diffraction spectroscopy.

Single-Molecule-based White-Light Emissive Organic Solids with Molecular-Packing-Dependent Thermally Activated Delayed Fluorescence

White-light-emitting single molecules have attracted broad attention because of their great potential for use in flat-panel displays and future light sources. We report a unique molecule of 3-(diphenylamino)-9H-xanthen-9-one (3-DPH-XO), which was found to exhibit bright white-light emission in the solid state caused by the spontaneous formation of a mixture with different polymorphs. Single-crystal analyses demonstrate that noncovalent interactions (such as π···π stacking, hydrogen bonding, and C-H···π interactions) induce different stacking arrangements (polymorphs A, B, and C) with different photophysical properties in a molecular solid. In addition, crystals B and C with the acceptor···acceptor stacking feature show the thermally activated delayed fluorescence (TADF) characteristics, indicating that appropriate noncovalent interactions could enhance the reverse intersystem crossing process and consequently lead to delayed fluorescence. This discovery provides an effective strategy for the design of new white-light-emitting single molecules as well as TADF materials.

White light emission from a single organic molecule with dual phosphorescence at room temperature

The development of single molecule white light emitters is extremely challenging for pure phosphorescent metal-free system at room temperature. Here we report a single pure organic phosphor, namely 4-chlorobenzoyldibenzothiophene, emitting white room temperature phosphorescence with Commission Internationale de l’Éclair-age coordinates of (0.33, 0.35). Experimental and theoretical investigations reveal that the white light emission is emerged from dual phosphorescence, which emit from the first and second excited triplet states. We also demonstrate the validity of the strategy to achieve metal-free pure phosphorescent single molecule white light emitters by intrasystem mixing dual room temperature phosphorescence arising from the low- and high-lying triplet states.

The development of single molecule white light-emitters is extremely challenging for pure phosphorescent metal-free systems at room temperature. Here the authors show a single pure organic room temperature phosphor, 4-chlorobenzoyldibenzothiophene, utilizing the emission from both T₁ and T₂ states.

Thermally activated delayed fluorescent phenothiazine-dibenzo[a,j]phenazine-phenothiazine triads exhibiting tricolor-changing mechanochromic luminescence

Novel U-shaped donor-acceptor-donor (D-A-D) π-conjugated multi-functional molecules comprising dibenzo[a,j]phenazine (DBPHZ) as an acceptor and phenothiazines (PTZ) as donors have been developed. Most importantly, the D-A-D compounds exhibit not only distinct tricolor-changeable mechanochromic luminescence (MCL) properties but also efficient thermally activated delayed fluorescence (TADF). Quantum chemical calculations, X-ray diffraction analysis, and systematic studies on the photophysical properties indicated that the "two-conformation-switchable" PTZ units play a highly important role in achieving multi-color-changing MCL. Time-resolved photophysical measurements revealed that the developed D-A-D compounds also exhibit efficient orange-TADF. Furthermore, organic light-emitting diode (OLED) devices fabricated with the new TADF emitters have achieved high external quantum efficiencies (EQEs) up to 16.8%, which significantly exceeds the theoretical maximum (∼5%) of conventional fluorescent emitters.