Conjugation-Modulated Excitonic Coupling Brightens Multiple Triplet Excited States

Manipulating room-temperature phosphorescence by electron-phonon coupling

Designing and optimizing efficient organic room-temperature phosphorescent (RTP) materials remains a captivating yet challenging endeavour due to the inherent difficulties in generating and stabilizing triplet excitons. Here, we report a suite of highly efficient phosphors characterized by near-unity intersystem crossing (ISC) yields. Surprisingly, upon doping these dyes into a polyvinyl alcohol matrix, their phosphorescence quantum yields (Φ P) spanned a wide range from 2.7% to 69.6%, governed by the position of the methyl substituent. Theoretical calculations and experimental results indicate that the variation in phosphorescence efficiency is primarily due to the strong electron-phonon coupling caused by the positional variation of the methyl substituents, rather than common factors such as ISC or energy levels. These findings provide a new insight into the design of high-performance organic RTP dyes.

Triggering anti-Kasha organic room temperature phosphorescence of clusteroluminescent materials

Clusterization-triggered emission (CTE) from organic materials without π-conjugated structures for room temperature phosphorescence (RTP) is fascinating with extraordinary photophysical properties and diversified applications, but rather challenging in material design owing to the limited mechanism understanding. Here, we demonstrate a facile strategy to construct CTE polymers with stimuli-responsive emission, anti-Kasha RTP and organic ultralong RTP (OURTP) by introducing ions into the hydrolyzed nonconjugated maleic anhydride and acrylamide copolymers. Thanks to the synergistic effects of hydrogen and ionic bonding with the ion-triggered electrostatic and coordinate interactions to suppress non-radiative decays and promote intersystem crossing, the amorphous copolymers show efficient photoluminescence with quantum efficiencies up to 13.5%, anti-Kasha RTP blue-shift of 29 nm, and OURTP lifetime up to 420 ms. Moreover, the temperature-dependent and water-sensitive anti-Kasha RTP and OURTP are also observed due to the formation of highly emissive CTE structure regulated by ionization. With the excellent processability and flexibility of the copolymer, lifetime-, temperature- and color-encrypted information anti-counterfeiting is designed and explored. The anti-Kasha RTP in CTE materials realized for the first time demonstrates impressive potential for multi-level encryption/anti-counterfeiting applications and more importantly, providing fundamental mechanism understanding for the rational modulation and design of CTE materials with extraordinary photophysical properties.

White light-excited organic room-temperature phosphorescence for improved in vivo bioimaging

Organic phosphorescence materials offer significant advantages for bioimaging applications. However, most of these materials are excited exclusively by ultraviolet (UV) light, which poses risks to living organisms. Herein, six donor-acceptor-type compounds incorporating triazine groups are designed as guests within doped systems. White-light excitable phosphorescent guests enable doped materials to show efficient afterglow under white-light excitation. By leveraging the ability of white-light to penetrate biological tissues, a bioimaging mode in which the materials are first concentrated within the organism and then excited was developed, yielding superior imaging effects compared with the traditional method in which materials are first excited and then concentrated. Furthermore, these materials are applied in imaging diagnosis of atherosclerosis plaques (male Apoe-/- mice) and intestinal diseases (female BALB/c-nude mice), as well as in navigation for in situ liver tumor surgery (female BALB/c-nude mice), achieving excellent imaging outcomes. This work addresses the limitations of phosphorescent materials that rely on UV-light, significantly enhancing their potential for practical applications in clinical imaging.

Water-Resistant Organic Room-Temperature Phosphorescence from Block Copolymers

Room-temperature phosphorescence (RTP) polymers have demonstrated significant potential for various applications due to their unique luminescent properties. However, most conventional RTP polymers are vulnerable to moisture and water, which can disrupt the hydrogen bonding network within the polymer and accelerate the non-radiative decay of triplet excitons of phosphors, leading to the quenching of RTP. Herein, we present a universal strategy to achieve water-resistant RTP polymers by designing amphiphilic block copolymers with microphase-separated structures. Specifically, the rigid hydrophilic phase, which is rich in carboxyl groups, forms hydrogen bonds that suppress non-radiative decay of the chromophores, resulting in RTP. Meanwhile, the hydrophobic phase effectively prevents water molecules from penetrating and disrupting the rigid polymer network. By combining the functions of both the hydrophilic and hydrophobic phases, the resulting RTP copolymers exhibit good water-resistant properties. Even after being immersed in water for one month, the copolymers maintain a green afterglow with a lifetime of 629 ms. Moreover, the water-resistant nature of these RTP polymers has also been demonstrated in potential applications of afterglow displays and anti-counterfeiting. This research offers valuable insights into the design of RTP materials with stability in aqueous environments and broadens the scope of their potential applications in diverse settings.

Triplet Exciplex Mediated Multi-Color Ultra-Long Afterglow Mate-rials

Organic long-lived phosphorescent materials demonstrating ultra-long photoluminescence have practical advantages owing to their flexible design and easy processability. However, the exact photophysical process underpinning the persistent-luminescent continues to elude full understanding, and the principles governing the compatibility of hosts and guests remain elusive. In this work, a new type of nonradiative energy transfer mechanism is proposed for the bi-component RTP system. Different from Förster resonance energy transfer or Dexter-type energy transfer, this energy transfer mechanism primarily relies on a triplet exciplex to exchange the electron. This facilitates the formation of triplet excitons that are otherwise difficult to excite directly. An evaluation methodology is devised to gauge the potential of a specific dopant-host combination toward generating pronounced afterglow. According to this framework, the enhancement of the afterglow is proportional to the decrease in the activation energy (ΔG≠) associated with the electron transfer reaction between the dopant and the host. Notably, when the ΔG≠ too larger, no observable afterglow occurs, as higher ΔG≠ values significantly impede the electron transfer reaction between the two components. Furthermore, the remarkable dependence of afterglow intensity on the dopant concentration renders the bi-component RTP system highly promising for applications requiring ultra-high sensitivity and broad-spectrum detection capabilities.

Disturbance-Triggered Instant Crystallization Activating Bioinspired Emissive Gels

Many marine organisms feature sensitive sensory-perceptual systems to sense the surrounding environment and respond to disturbance with intense bioluminescence. However, it remains a great challenge to develop artificial materials that can sense external disturbance and simultaneously activate intense luminescence, although such materials are attractive for visual sensing and intelligent displays. Herein, we present a new class of bioinspired smart gels constructed by integrating hydrophilic polymeric networks, metastable supersaturated salt and fluorophores containing heterogenic atoms. Upon external disturbance, the composite gels undergo an instant and reversible soft-rigid state transition, simultaneously turning on intense fluorescence and activating ultralong afterglow emission with a maximum lifetime of 877.15 ms. The experimental results and molecular dynamics simulations reveal that the disturbance-induced luminescence mainly results from the geometrical confinement of aggregated fluorophores and enhanced molecular interactions to immensely suppress the non-radiative dissipation. Given their versatile and sensitive disturbance-responsiveness, dynamic interactive painting and 3D smart optical displays are demonstrated. This study paves a new avenue to achieve disturbance-sensing soft materials and promotes the development of smart visual sensors and interactive optical displays.

Multi-Color Tunable Afterglow Materials Leveraging Energy Transfer Between Host and Guest

Host/guest doping is an effective approach to achieving room-temperature phosphorescence (RTP). However, the influence of the host matrix on doping systems is still unclear, and it is difficult to select the suitable host species for a certain guest emitter. This study prepared a series of host/guest RTP materials with dynamically adjustable time and color by doping a non-RTP guest material in various host materials that were easy to crystallize. The varying afterglow color originated from the difference in Förster energy transfer between the host and guest. Specifically, the change from yellow to green afterglow was realized by varying the host's molecular structure. This study further revealed the importance of proper host energy levels, the ability to generate long-aging triplet excitons, and the Förster energy transfer from host to guest. Additionally, multiple information encryption anti-counterfeiting materials were developed by leveraging the different afterglow colors and durations, reflecting the unique performance advantages of the prepared long-afterglow materials in various RTP applications.

The formation of exciplex and triplet-triplet transfer in organic room temperature phosphorescent guest-host materials

Organic materials typically do not phosphoresce at room temperature because both intersystem crossing (ISC) and phosphorescence back to the electronic ground state are slow, compared to the nonradiative decay processes. A group of organic guest-host molecules breaks this rule. Their phosphorescence at room temperature can last seconds with a quantum efficiency of over 10%. This extraordinary phenomenon is investigated with comprehensive static and transient spectroscopic techniques. Time-resolved vibrational and fluorescence spectral results suggest that a singlet guest-host exciplex quickly forms after excitation. The formation of exciplex reduces the singlet-triplet energy gap and helps facilitate charge separation that can further diffuse into the host matrix. The heavy atoms (P or As) of the host molecule can also help enhance the spin orbital coupling of the guest molecule. Both boost the rate of ISC. After the singlet exciplex transforms into the triplet exciplex through the ISC process, UV-visible transient absorption spectroscopic measurements support that the triplet exciplex quickly transforms into the guest molecule triplet state that is at a lower energy level, thereby reducing the reverse ISC-induced triplet population loss. Finally, the long-lasting separated charges that diffused into the host matrix can diffuse back to the guest hole to form new triplets, and the dilution effect of the host molecules can effectively reduce the triplet quenching. All these factors contribute to the dramatic enhancement of phosphorescence at room temperature.

Site Effect of Electron Acceptors on Ultralong Organic Room-Temperature Phosphorescence

Herein, we successfully observe the site effect of electron acceptors on ultralong organic room-temperature phosphorescence (UORTP) in the case of 7H-benzo[c]carbazole (BCz) derivatives: cyanophenyl on the nitrogen site can promote intersystem crossing (ISC) efficiency and enhance phosphorescence intensity by facilitating n-π* transitions but make a slight change to the phosphorescence wavelength; cyanophenyl on the naphthalene site can cause a remarkable red shift of phosphorescence wavelength by reducing the T1 energy level of BCz derivatives and also enhance phosphorescence intensity by promoting ISC but weaken phosphorescence intensity by lowering the molecular symmetry. Three BCz derivatives (1-BCzPhCN, 2-BCzPhCN, and 3-BCzPhCN) with the electron acceptor cyanophenyl at different sites (nitrogen site and naphthalene site) were synthesized through a combination of the nucleophilic substitution reaction and the Suzuki coupling reaction. The phosphorescence properties of 1-BCzPhCN, 2-BCzPhCN, and 3-BCzPhCN in toluene solution, in a copolymerized MMA film, and in a PVA film were measured and analyzed. 1-BCzPhCN emits intrinsic green ultralong phosphorescence at ∼500, ∼536, and ∼580 nm, while 2-BCzPhCN and 3-BCzPhCN give out intrinsic yellow ultralong phosphorescence with a red shift of 27 and 40 nm, showing that cyanophenyl on the naphthalene site leads to a remarkable red shift of the intrinsic phosphorescence wavelength, but cyanophenyl on the nitrogen site makes a slight difference to the intrinsic phosphorescence wavelength. Under the same condition, the phosphorescence intensity is usually ranked as 1-BCzPhCN/3-BCzPhCN > 2-BCzPhCN, demonstrating that cyanophenyl on the nitrogen site promotes ISC and enhances phosphorescence intensity, but cyanophenyl on the naphthalene site reduces molecular symmetry and accelerates nonradiative dissipation. Time-dependent density functional theory calculations verify that cyanophenyl on the naphthalene site shifts the phosphorescence wavelength by reducing the T1 energy level, and cyanophenyl on the nitrogen site facilitates n-π* transitions to strengthen the phosphorescence intensity. Moreover, three BCz derivatives were doped into DMAP and BBP, separately. The BCz derivatives exhibited different phosphorescence colors and shifts due to interactions with the host materials. We believe this work will give an insight into the structure-property relationship of organic phosphorescence molecules and pave a way for design of colorful UORTP materials.

Hierarchical Dual-Mode Efficient Tunable Afterglow via J-Aggregates in Single-Phosphor-Doped Polymer

The development of polymer-based persistent luminescence materials with color-tunable organic afterglow and multiple responses is highly desirable for applications in anti-counterfeiting, flexible displays, and data-storage. However, achieving efficient persistent luminescence from a single-phosphor system with multiple responses remains a challenging task. Herein, by doping 9H-pyrido[3,4-b]indole (PI2) into an amorphous polyacrylamide matrix, a hierarchical dual-mode emission system is developed, which exhibits color-tunable afterglow due to excitation-, temperature-, and humidity-dependence. Notably, the coexistence of the isolated state and J-aggregate state of the guest molecule not only provides an excitation-dependent afterglow color, but also leads to a hierarchical temperature-dependent afterglow color resulting from different thermally activated delayed fluorescence (TADF) and ultralong organic phosphorescence (UOP) behaviors of the isolated and aggregated states. The complex responsiveness based on the hierarchical dual-mode emission can serve for security features through inkjet printing and ink-writing. These findings may provide further insight into the regulated persistent luminescence by isolated and aggregated phosphors in doped polymer systems and expand the scope of stimuli-responsive organic afterglow materials for broader applications.

Exciton Dissociation and Recombination Afford Narrowband Organic Afterglow Through Efficient FRET

Organic afterglow with long-persistent luminescence (LPL) after photoexcitation is highly attractive, but the realization of narrowband afterglow with small full-width at half-maximum (FWHM) is a huge challenge since it is intrinsically contradictory to the triplet- and solid-state emission nature of organic afterglow. Here, narrow-band, long-lived, and full-color organic LPL is realized by isolating multi-resonant thermally activated delayed fluorescent (MR-TADF) fluorophores in a glassy steroid-type host through a facile melt-cooling treatment. Such prepared host becomes capable of exciton dissociation and recombination (EDR) upon photoirradiation for both long-lived fluorescence and phosphorescence; and, the efficient Förster resonance energy transfer (FRET) from the host to various MR-TADF emitters leads to high-performance LPL, exhibiting small FWHM of 33 nm, long persistent time over 10 s, and facile color-tuning in a wide range from deep-blue to orange (414-600 nm). Moreover, with the extraordinary narrowband LPL and easy processability of the material, centimeter-scale flexible optical waveguide fibers and integrated FWHM/color/lifetime-resolved multilevel encryption/decryption devices have been designed and fabricated. This novel EDR and singlet/triplet-to-singlet FRET strategy to achieve excellent LPL performances illustrates a promising way for constructing flexible organic afterglow with easy preparation methods, shedding valuable scientific insights into the design of narrow-band emission in organic afterglow.

Wide-Range Excitation-Dependent Phosphorescence of Coordination Polymers Exhibiting Dynamic Anticounterfeiting, White-Light Emission, and Antibacterial Performance

Multicolor-tunable room-temperature phosphorescence (RTP) is attracting wide attention in optoelectronic applications. Here, we propose a coordination-oriented assembly approach to achieve wide-range RTP with a benzimidazole derivative (2,7-diazabenzimidazole, DZBIM) as a luminogen. These two compounds exhibit unexpected excitation-responsive RTP emission, and the phosphorescence emission nearly covers the entire visible region with the change of the excitation wavelength from 360 to 620 nm. To the best of our knowledge, this is the first report of coordination polymers with such a full-color-tunable RTP. Compound 1 also shows white-light emission upon excitation at 280 nm. Experimental and theoretical results demonstrate that multiple intermolecular interactions and emission centers from different aggregates are responsible for the generation of multicolor emission. The white-light emission and multiple anticounterfeiting are explored. Besides, compound 1 exhibits high antibacterial activity benefiting from efficient 1O2 generation. This work provides an efficient way to prepare a color-tunable RTP.

Unveiling the potential of triphenylphosphine salts in tuning organic room temperature phosphorescence

Triphenylphosphine (TPP) salt derivatives, with their rich chemistry of core-substitution, have emerged as promising candidates for ultralong room temperature phosphorescence (RTP) materials owing to their distinct molecular structures, high quantum efficiency and exceptional phosphorescence properties. This feature article highlights the vast potential of TPP salt derivatives in tunable RTP properties by exploring some factors such as the alkyl chains, halogen anions, through-space charge transfer states, etc., and recent advancements in multi-level information encryption, high-level anticounterfeiting tags and X-ray imaging applications. We anticipate that this article will assist in directing future analyses based on the mechanisms underlying the RTP behavior of TPP derivatives and offer guidance for the rational design of high-performance RTP materials.

Narrowband room temperature phosphorescence of closed-loop molecules through the multiple resonance effect

Luminescent materials with narrowband emission show great potential for diverse applications in optoelectronics. Purely organic phosphors with room-temperature phosphorescence (RTP) have made significant success in rationally manipulating quantum efficiency, lifetimes, and colour gamut in the past years, but there is limited attention on the purity of the RTP colours. Herein we report a series of closed-loop molecules with narrowband phosphorescence by multiple resonance effect, which significantly improves the colour purity of RTP. Phosphors show narrowband phosphorescence with full width at half maxima (FWHM) of 30 nm after doping into a rigid benzophenone matrix under ambient conditions, of which the RTP efficiency reaches 51.8%. At 77 K, the FWHM of phosphorescence is only 11 nm. Meanwhile, the colour of narrowband RTP can be tuned from sky blue to green with the modification of methyl groups. Additionally, the potential applications in X-ray imaging and display are demonstrated. This work not only outlines a design principle for developing narrowband RTP materials but also makes a major step forward extending the potential applications of narrowband luminescent materials in optoelectronics.

Overcoming Thermal Quenching in X-ray Scintillators through Multi-Excited State Switching

X-ray scintillators have gained significant attention in medical diagnostics and industrial applications. Despite their widespread utility, scintillator development faces a significant hurdle when exposed to elevated temperatures, as it usually results in reduced scintillation efficiency and diminished luminescence output. Here we report a molecular design strategy based on a hybrid perovskite (TpyBiCl5) that overcomes thermal quenching through multi-excited state switching. The structure of perovskite provides a platform to modulate the luminescence centers. The rigid framework constructed by this perovskite structure stabilized its triplet states, resulting in TpyBiCl5 exhibiting an approximately 12 times higher (45 % vs. 3.8 %) photoluminescence quantum yield of room temperature phosphorescence than that of its organic ligand (Tpy). Most importantly, the interactions between the components of this perovskite enable the mixing of different excited states, which has been revealed by experimental and theoretical investigations. The TpyBiCl5 scintillator exhibits a detection limit of 38.92 nGy s-1 at 213 K and a detection limit of 196.31 nGy s-1 at 353 K through scintillation mode switching between thermally activated delayed fluorescence and phosphorescence. This work opens up the possibility of solving the thermal quenching in X-ray scintillators by tuning different excited states.

Dopants Induce Persistent Room Temperature Phosphorescence in Triarylamine Boronate Esters

Purely organic materials exhibiting room temperature phosphorescence (RTP) are promising candidates for oxygen sensors and information encryption owing to their cost-effective and environmentally friendly nature. Herein, we report a bimolecular RTP system where DTBU acts as the guest and TBBU serves as the host. In contrast to previously reported results, we find that both pure DTBU and TBBU do not exhibit RTP in the solid state even under N2 atmosphere. A DTBU/TBBU system with a low doping ratio (0.1 mol %) exhibits persistent yellowish-green afterglow with a lifetime of 340 ms and is highly sensitive to oxygen. A DTBU/TBBU system with a higher doping ratio (10 mol %) maintains a phosphorescence lifetime of 179 ms under air. Applications of DTBU/TBBU at varied doping ratios in both oxygen sensing and information encryption are demonstrated. We propose that the T1 state of TBBU acts as an energy transfer intermediate between Tn and T1 of DTBU, ultimately leading to the generation of persistent RTP. Overall, this work demonstrates the critical importance of material purity in the design of RTP systems, and how an understanding of host-guest doping enables their photophysical properties to be precisely tuned.

Thermochromic Afterglow from Benzene-1,4-Diboronic Acid-Doped Co-crystals

The accurate thermosensing requires a minimum impact of autofluorescence and light scattering from the samples. In this study, we discovered that commercially available benzene-1,4-diboronic acid (BDBA) doped co-crystals with trimethylolpropane (TMP) exhibit excellent thermochromic dual phosphorescence properties over a wide temperature range from -132 to 40 °C, despite its simple structure that does not have any donor-acceptor linkage. The dual phosphorescence was consisted of monomeric benzene-1,4-diboronate (BDBA ester) and aggregation-stabilized species. With an increase in temperature, the emission intensity from the monomeric state significantly decreased, whereas that originating from the aggregated state remained almost constant owing to the difference in their thermal stabilities. Further investigation revealed that molecular distortions in singlet excited states enable efficient intersystem crossing, causing efficient phosphorescence from the monomeric state of BDBA ester.

Twofold rigidity activates ultralong organic high-temperature phosphorescence

A strategy is pioneered for achieving high-temperature phosphorescence using planar rigid molecules as guests and rigid polymers as host matrix. The planar rigid configuration can resist the thermal vibration of the guest at high temperatures, and the rigidity of the matrix further enhances the high-temperature resistance of the guest. The doped materials exhibit an afterglow of 40 s at 293 K, 20 s at 373 K, 6 s at 413 K, and a 1 s afterglow at 433 K. The experimental results indicate that as the rotational ability of the groups connected to the guests gradually increases, the high-temperature phosphorescence performance of the doped materials gradually decreases. In addition, utilizing the property of doped materials that can emit phosphorescence at high temperatures and in high smoke, the attempt is made to use organic phosphorescence materials to identify rescue workers and trapped personnel in fires.

Acquiring Charge-Transfer-Featured Single-Molecule Ultralong Organic Room Temperature Phosphorescence via Through-Space Electronic Coupling

Although long-lived triplet charge-transfer (3 CT) state with high energy level has gained significant attention, the development of organic small molecules capable of achieving such states remains a major challenge. Herein, by using the through-space electronic coupling effect, we have developed a compound, namely NIC-DMAC, which has a long-lived 3 CT state at the single-molecule level with a lifetime of 210 ms and a high energy level of up to 2.50 eV. Through a combination of experimental and computational approaches, we have elucidated the photophysical processes of NIC-DMAC, which involve sequential transitions from the first singlet excited state (S1 ) that shows a 1 CT character to the first triplet excited state (T1 ) that exhibits a local excited state feature (3 LE), and then to the second triplet excited state (T2 ) that shows a 3 CT character (i.e., S1 (1 CT)→T1 (3 LE)→T2 (3 CT)). The long lifetime and high energy level of its 3 CT state have enabled NIC-DMAC as an initiator for photocuring in double patterning applications.

Unveiling autophagy and aging through time-resolved imaging of lysosomal polarity with a delayed fluorescent emitter

Detecting the lysosomal microenvironmental changes like viscosity, pH, and polarity during their dynamic interorganelle interactions remains an intriguing area that facilitates the elucidation of cellular homeostasis. The subtle variation of physiological conditions can be assessed by deciphering the lysosomal microenvironments during lysosome-organelle interactions, closely related to autophagic pathways leading to various cellular disorders. Herein, we shed light on the dynamic lysosomal polarity in live cells and a multicellular model organism, Caenorhabditis elegans (C. elegans), through time-resolved imaging employing a thermally activated delayed fluorescent probe, DC-Lyso. The highly photostable and cytocompatible DC-Lyso rapidly labels the lysosomes (within 1 min of incubation) and exhibits red luminescence and polarity-sensitive long lifetime under the cellular environment. The distinct variation in the fluorescence lifetime of DC-Lyso suggests an increase in local polarity during the lysosomal dynamics and interorganelle interactions, including lipophagy and mitophagy. The lifetime imaging analysis reveals increasing lysosomal polarity as an indicator for probing the successive development of C. elegans during aging. The in vivo microsecond timescale imaging of various cancerous cell lines and C. elegans, as presented here, therefore, expands the scope of delayed fluorescent emitters for unveiling complex biological processes.

Achieving the Reverse Intersystem Crossing in Chalcone Based Donor-Acceptor System through Down-Conversion of Triplet Exciton

In recent years, understanding the mechanism of thermally activated delayed fluorescence (TADF) has become the primary choice for designing high-efficiency, low-cost, metal-free organic light emitting diodes (OLEDs). Herein, we propose a strategically designed chalcone based donor-acceptor system, where intensification of delayed fluorescence with decrease in temperature (300 K to 100 K) is observed; the theoretical investigations of electronic states and orbital characters uncovered a new cold rISC pathway in donor-acceptor system, where rISC occurs through the down-conversation of higher triplet exciton (from T3 ) to lowest singlet state (S1 ), having negative energy splitting, thus no thermal energy is required. The comprehensive research described herein might open-up new avenues in donor-acceptor system over the conventional up-convention of triplet exciton and demonstrates that not necessarily all delayed fluorescence are thermally activated (TADF).

Color-Tunable, Excitation-Dependent, and Water Stimulus-Responsive Room-Temperature Phosphorescence Cellulose for Versatile Applications

Smart-response materials with ultralong room-temperature phosphorescence (RTP) are highly desirable, but they have rarely been described, especially those originating from sustainable polymers. Herein, a variety of cellulose derivatives with 1,4-dihydropyridine (DHP) rings are synthesized through the Hantzsch reaction, giving impressive RTP with a long lifetime of up to 1251 ms. Specifically, the introduction of acetoacetyl groups and DHP rings promotes the spin-orbit coupling and intersystem crossing process; and multiple interactions between cellulose induce clustering and inhibit the nonradiative transitions, boosting long-live RTP. Furthermore, the resulting transparent and flexible cellulose films also exhibit excitation-dependent and color-tunable afterglows by introducing different extended aromatic groups. More interestingly, the RTP performance of these films is sensitive to water and can be repeated in response to wet/dry stimuli. Inspired by these advantages, the RTP cellulose demonstrates advanced applications in information encryption and anti-counterfeiting. This work not only enriches the photophysical properties of cellulose but also provides a versatile platform for the development of sustainable afterglows.

Room-Temperature Multiple Phosphorescence from Functionalized Corannulenes: Temperature Sensing and Afterglow Organic Light-Emitting Diode

Corannulene-derived materials have been extensively explored in energy storage and solar cells, however, are rarely documented as emitters in light-emitting sensors and organic light-emitting diodes (OLEDs), due to low exciton utilization. Here, we report a family of multi-donor and acceptor (multi-D-A) motifs, TCzPhCor, TDMACPhCor, and TPXZPhCor, using corannulene as the acceptor and carbazole (Cz), 9,10-dihydro-9,10-dimethylacridine (DMAC), and phenoxazine (PXZ) as the donor, respectively. By decorating corannulene with different donors, multiple phosphorescence is realized. Theoretical and photophysical investigations reveal that TCzPhCor shows room-temperature phosphorescence (RTP) from the lowest-lying T1 ; however, for TDMACPhCor, dual RTP originating from a higher-lying T1 (T1 H ) and a lower-lying T1 (T1 L ) can be observed, while for TPXZPhCor, T1 H -dominated RTP occurs resulting from a stabilized high-energy T1 geometry. Benefiting from the high-temperature sensitivity of TPXZPhCor, high color-resolution temperature sensing is achieved. Besides, due to degenerate S1 and T1 H states of TPXZPhCor, the first corannulene-based solution-processed afterglow OLEDs is investigated. The afterglow OLED with TPXZPhCor shows a maximum external quantum efficiency (EQEmax ) and a luminance (Lmax ) of 3.3 % and 5167 cd m-2 , respectively, which is one of the most efficient afterglow RTP OLEDs reported to date.

Synthesis and Photophysical Properties of Carbazole-Functionalized Diazaphosphepines via Sequent P-N Chemistry

Chemical structure tunability of organic π-conjugated molecules (OCMs) is highly appealing for fine-tuning the optoelectronic properties. Herein, we report a new series of carbazole-functionalized diazaphosphepines (DPP-CBZs) via one-pot phosphorus-nitrogen (P-N) chemistry. The one-pot synthesis harnessed the mild and selective P-N chemistry that successively installed carbazole moieties and seven-membered heterocycles at one P-center. Single-crystal structure studies revealed the tweezer-like structures for 1PO, 2PO, and 3PO that maintained the intramolecular donor-acceptor interactions between [d]-aryl moieties and carbazole. DPP-CBZs exhibited a more twisted central-diazaphosphepine ring compared with the reference molecules (1-3MO without carbazole group). DPP-CBZs with strong electron-accepting [d]-Ars generally showed lower photoluminescence quantum yields (PLQYs) than those of the reference molecules, which is probably due to the intramolecular charge transfer (ICT) from electron-donating carbazole to electron-withdrawing [d]-Ars. Upon the oxidation of the P-centers, PLQYs of DPP-CBZs increased. Furthermore, photophysical studies and theoretical studies suggested that the carbazole group had a strong impact on the structures of DPP-CBZs. As a proof of concept, we showed that grinding the mixture of 1PO as the electron-donating tweezer and benzene-1,2,4,5-tetracarbonitrile (BzCN) as the electron acceptor induced the formation of the CT complex.

Switching Singlet Exciton to Triplet for Efficient Pure Organic Room-Temperature Phosphorescence by Rational Molecular Design

The design and regulation of phosphors are attractive but challenging because of the spin-forbidden intersystem crossing (ISC) process. Here, a new perspective on the enhancement of the ISC is proposed and demonstrated. Different from current strategies, the ISC yield (Φ_ISC) is enhanced by decreasing the fluorescence radiative transition rate constant (k_F) via rational molecular designing rather than boosting the spin-orbit coupling by decorating the molecular skeleton with a heavy atom, heteroatom, or carbonyl. The k_F of the designed molecule in this case is associated with the substituent position of the methoxy group, which alters the distribution of the front orbitals. The S₀ → S₁ transition of these compounds evolves from a bright state to a dark state gradually with the variation of the substituent position, accompanied by the decrease of k_F and increase of Φ_ISC. The fluorescence emission is switched to phosphorescence emission successfully by regulating the k_F. This work provides an alternative strategy to design efficient room-temperature phosphorescence material.