The Effect of Molecular Conformations and Simulated "Self-Doping" in Phenothiazine Derivatives on Room-Temperature Phosphorescence.

The research of purely organic room-temperature phosphorescence (RTP) materials has drawn great attention for their wide potential applications. Besides single-component and host-guest doping systems, the self-doping with same molecule but different conformations in one state is also a possible way to construct RTP materials, regardless of its rare investigation. In this work, twenty-four phenothiazine derivatives with two distinct molecular conformations were designed and their RTP behaviors in different states were systematically studied, with the aim to deeply understand the self-doping effect on the corresponding RTP property. While the phenothiazine derivatives with quasi-axial (ax) conformation presented better RTP performance in aggregated state, the quasi-equatorial (eq) ones were better in isolated state. Accordingly, the much promoted RTP performance was achieved in the stimulated self-doping state with ax-conformer as host and eq-one as guest, demonstrating the significant influence of self-doping on RTP effect.

Photo-Response with Radical Afterglow by Regulation of Spin Populations and Hole-Electron Distributions.

No matter photoinduced organic radicals have been reported frequently, they are usually non-luminescent at ambient conditions. The internal mechanism on stability and electronic transitions of photoinduced radicals, is thus crucial for the development of relevant materials. Herein, a series of photoinduced radical emission systems were developed conveniently through doping benzoic acids into the hydrogen donor polyvinyl alcohol (PVA) matrix. Visual photoinduced radical emission and photochromism could be observed on Ph-3COOH@PVA film with the formation of cyclohexadienyl-type structure. For the first time, radical afterglow appeared with energy transfer from triplet state. The appropriate introduction of carboxylic groups to three nonadjacent carbon atoms on the benzene ring was the best for decreasing spin population and promoting electronic transitions of the radical. This study largely expands the radical emission property from both internal mechanism and practical application.

Multistage Stimulus-Responsive Room Temperature Phosphorescence Based on Host-Guest Doping Systems.

Compared with inorganic long-lasting luminescent materials, organic room temperature phosphorescent (RTP) ones show several advantages, such as flexibility, transparency, solubility and color adjustability. However, organic RTP materials close to commercialization are still to be developed. In this work, we developed a new host-guest doping system with stimulus-responsive RTP characteristics, in which triphenylphosphine oxide (OPph3 ) acted host and benzo(dibenzo)phenothiazine dioxide derivatives as guests. Turn-on RTP effect was realized by mixing them together through co-crystallization or grinding, in which the efficient energy transfer from host to guest and the strong intersystem crossing (ISC) ability of the guest have played significant role. Further on, multistage stimulus-responsive RTP characteristics from grinding to chemical stimulus were achieved via introducing pyridine group into the guest molecule. In addition, the anti-counterfeiting printings were realized for these materials through various methods, including stylus printing, thermal printing and inkjet printing, which brings RTP materials closer to commercialization.

Force-Induced Turn-On Persistent Room-Temperature Phosphorescence in Purely Organic Luminogen.

Research of purely organic room-temperature phosphorescence (RTP) materials has been a hot topic, especially for those with stimulus response character. Herein, an abnormal stimulus-responsive RTP effect is reported, in which, purely organic luminogen of Czs-ph-3F shows turn-on persistent phosphorescence under grinding. Careful analyses of experimental results, coupled with the theoretical calculations, show that the transition of molecular conformation from quasi-axial to quasi-equatorial of the phenothiazine group should be mainly responsible for this exciting result. Furthermore, the applications of stylus printing and thermal printing are both successfully realized, based on the unique RTP effect of Czs-ph-3F.

Organic Dyes based on Tetraaryl-1,4-dihydropyrrolo-[3,2-b]pyrroles for Photovoltaic and Photocatalysis Applications with the Suppressed Electron Recombination.

A tetraaryl-1,4-dihydropyrrolo-[3,2-b]pyrroles (TAPP) moiety with the combination of two pyrrole rings and four phenyl moieties demonstrated strong electron-donating ability and nonplanar configuration simultaneously. Once incorporated into the organic dyes as a novel electron donor, it was beneficial for the enhancement of light-harvesting ability and suppression of electron recombination in the photovoltaic and photocatalysis systems. With the linkage of tunable conjugated bridges and electron acceptor, the corresponding organic dyes exhibited improved photovoltaic performance in dye-sensitized solar cells and facilitated photocatalytic hydrogen generation with a highest turnover number (TON) of 4337. Through the detailed investigation of relationship between molecular structures and photovoltaic/photocatalysis property, the connection and difference in molecular design for these two systems are well explained, with the aim to promote the application of dye-sensitized technology in various fields.

Molecular Conformation-Dependent Mechanoluminescence: Same Mechanical Stimulus but Different Emissive Color over Time.

A phenothiazine derivative of FCO-CzS with changeable mechanoluminescence is reported, which, upon continuous mechanical stimulus, shows mechanoluminescent emission from blue to white and yellow. Careful analysis of the experimental results, coupled with the well-understood photoluminescence theory, show that the molecular conformation transition of the phenothiazine group from quasi-axial to quasi-equatorial is responsible for this dynamic mechanoluminescence effect.

Elucidating the Excited State of Mechanoluminescence in Organic Luminogens with Room-Temperature Phosphorescence.

Two stable, purely organic luminogens exhibit both mechano- (ML) and photoluminescence (PL) with dual fluorescence-phosphorescence emissions at room temperature. Careful analysis of the crystal structures, coupled with theoretical calculations, demonstrate that room-temperature phosphorescence and ML properties are strongly related to molecular packing. In particular, the formation and fracture of molecular dimers with intermolecular charge-transfer properties has a significant effect on intersystem crossing, as well as excited triplet state emissions, in both PL and ML processes.

Pure room temperature phosphorescence emission in nondoped OLEDs: adjustable oxidation states and excited-state modulation.

Purely organic room temperature phosphorescence (RTP) materials are a new kind of triplet emitter, which can harvest both singlet and triplet excitons in theory, thus showing great application potential for organic light-emitting diodes (OLEDs). However, nondoped OLEDs based on RTP emitters have been rarely explored owing to challenges in realizing efficient phosphorescence in single-component systems. Herein, three donor-acceptor-type luminogens were designed and synthesized in which phenothiazine, with different oxidation degrees, acted as the electron donor and acetophenone as the acceptor. The adjustable oxidation states of phenothiazine enabled the modulation of excited states, facilitating the transition from dual RTP and thermally activated delayed fluorescence emissions to pure RTP. A nondoped OLED device was then fabricated based on the pure RTP emitter, achieving a high exciton utilization efficiency of 86%, clearly demonstrating the enhancement of electroluminescence performance through RTP properties.

Phenothiazine Derivatives as Small-Molecule Organic Cathodes with Adjustable Dropout Voltage and Cycle Performance.

Compared with conventional inorganic materials, organic electrodes are competitive candidates for secondary battery cathodes due to their resourcefulness, environmental friendliness, and cost-effectiveness. Much effort is devoted at the level of chemical structure, while ignoring the impact of molecular aggregation on battery behavior. Herein, this work designs a series of organic molecules with two electrochemically active phenothiazine groups linked by different lengths of alkyl chain to regulate molecular symmetry and crystallinity. The results emphasize the equally important role of molecular aggregation and chemical structure for battery performance. Among them, 2PTZ-C7H14|Li cell exhibits the most impressive cycle and rate performance. At the high rate of 50 C, it can still deliver a capacity of 63.4 mA h g-1 and 74.5% capacity retention after 10 000 cycles. Besides, the dropout voltage of 2PTZ-C9H18|Li cell is only 52 mV, which is among the lowest reported for lithium-organic batteries to the best of the author's knowledge.

Ultralong Room-Temperature Phosphorescence with Second-level Lifetime in Water Based on Cyclodextrin Supramolecular Assembly.

Organic solid-state materials with long-lived room-temperature phosphorescence (RTP) emission have been widely developed and applied in many fields, while works in developing solution-phase phosphorescence materials were rarely reported owing to the ultrafast nonradiative relaxation and quenchers from the liquid medium. Herein, we report an ultralong RTP system in water through assembly based on a ß-cyclodextrin host and p-biphenylboronic acid guest with a lifetime of 1.03 s under ambient conditions. It is worth noting that the long-lived phosphorescence depends on the host-guest inclusion as well as intermolecular hydrogen bonding interactions, which suppress nonradiative relaxation and avoid quenchers effectively. Furthermore, the addition of fluorescent dyes to the assembly system achieved the tuning of the afterglow color through radiative energy transfer of reabsorption.

Stimulus-responsive room temperature phosphorescence materials with full-color tunability from pure organic amorphous polymers.

Achieving stimulus-responsive ultralong room temperature phosphorescence (RTP) in organic materials especially with full-color tunable emissions is attractive and important but rarely reported. Here, a strategy was reported to realize stimulus-responsive RTP effect with color-tunable emissions by using water as solvent in the preparation process without any organic solvent through covalent linkage of arylboronic acids with different π conjugations and polymer matrix of polyvinyl alcohol. The yielded polymer films exhibit outstanding RTP performance (2.43 s). Furthermore, an excitation-dependent RTP film was obtained, and the afterglow color changes from blue to green, then to red as the excitation wavelength increases. The RTP property of all the above materials is sensitive to water and heat stimuli, because the rigidity of the system could be broken by water. Last, they were successfully applied in a multilevel information encryption and multicolor paper and ink.

Completely aqueous processable stimulus responsive organic room temperature phosphorescence materials with tunable afterglow color.

Many luminescent stimuli responsive materials are based on fluorescence emission, while stimuli-responsive room temperature phosphorescent materials are less explored. Here, we show a kind of stimulus-responsive room temperature phosphorescence materials by the covalent linkage of phosphorescent chromophore of arylboronic acid and polymer matrix of poly(vinylalcohol). Attributed to the rigid environment offered from hydrogen bond and B-O covalent bond between arylboronic acid and poly(vinylalcohol), the yielded polymer film exhibits ultralong room temperature phosphorescence with lifetime of 2.43 s and phosphorescence quantum yield of 7.51%. Interestingly, the RTP property of this film is sensitive to the water and heat stimuli, because water could destroy the hydrogen bonds between adjacent poly(vinylalcohol) polymers, then changing the rigidity of this system. Furthermore, by introducing another two fluorescent dyes to this system, the color of afterglow with stimulus response effect could be adjusted from blue to green to orange through triplet-to-singlet Förster-resonance energy-transfer. Finally, due to the water/heat-sensitive, multicolor and completely aqueous processable feature for these three afterglow hybrids, they are successfully applied in multifunctional ink for anti-counterfeit, screen printing and fingerprint record.

Organic microporous crystals driven by pure C-H⋯π interactions with vapor-induced crystal-to-crystal transformations.

Organic porous crystals constructed by only a single kind of weak molecular interaction are invaluable to understanding the nature of the formation of organic porous materials and developing new types of porous materials. Here, we designed and synthesized two pure organic compounds of PBO and PBS through integrating planar dibenzothiophene/dibenzofuran and two phenothiazine groups together with twisted C-N bonds, which form organic microporous crystals with very good stability against strong acids and bases VIA pure C-H⋯π interactions. Accordingly, the effective absorption of toluene has been successfully realized with an adsorbing capacity of 6.20 mmol g-1, regardless of the interference of water vapor. Excitingly, these microporous materials exhibit interesting crystal-to-crystal transformation (CCT) properties accompanied by changed pore size on being exposed to different organic vapors. Therefore, the desorption process of toluene could be completed through a simple exposure to dichloromethane (DCM) vapor and the second transformation of the crystal occurred in this process.

Direct demonstration of triplet excimer in purely organic room temperature phosphorescence through rational molecular design.

Organic luminogens with room temperature phosphorescence (RTP) have been paid great attention and developed rapidly for their wide application values. Until now, the internal mechanism and source of phosphorescence are still obscure, especially for the relationship between molecular dimer and RTP emission. Hence, we designed and synthesized eight phenothiazine 5,5-dioxide derivatives to directly reveal how the monomer and dimer in packing affect the RTP behavior. Dimers with strong π-π stacking (θ < 20.66°; d < 3.86 Å) lead to pure triplet excimer emission, while those with weak π-π stacking (27.02°< θ < 40.64°; 3.84 Å < d < 4.41 Å) contribute to dual RTP emissions of both monomer and triplet excimer. The valuable information of this work would promote the further development of this research field, as well as others in aggregate.

High Performance of Simple Organic Phosphorescence Host-Guest Materials and their Application in Time-Resolved Bioimaging.

The study of purely organic room-temperature phosphorescence (RTP) has drawn increasing attention because of its considerable theoretical research and practical application value. Currently, organic RTP materials with both high efficiency (ΦP  > 20%) and a long lifetime (τP  > 10 s) in air are still scarce due to the lack of related design guidance. Here, a new strategy to increase the phosphorescence performance of organic materials by integrating the RTP host and RTP guest in one doping system to form a triplet exciplex, is reported. With these materials, the high-contrast labeling of tumors in living mice and encrypted patterns in thermal printing are both successfully realized by taking advantage of both the long afterglow time (up to 25 min in aqueous media) and high phosphorescence efficiency (43%).

Tunable Photoresponsive Behaviors Based on Triphenylamine Derivatives: The Pivotal Role of π-Conjugated Structure and Corresponding Application.

Photoresponsive materials have drawn much attention and are widely applied in daily life for their reversible changes in luminous color or appearance color under light irradiation. In this work, a new photoresponsive system based on triarylamine derivatives is developed. With the changed aryl substituents, adjustable photoresponsive properties, including photoactivated phosphorescence and photochromism after being dispersed into the poly(methyl methacrylate) (PMMA) matrix, are demonstrated. According to the theoretical calculations and experimental data, the competition between the formations of triplet excitons and cationic radicals under photoirradiation should be the main reason for their different photoresponsive properties. Excitingly, the applications of rewritable photopatterning, anticounterfeiting, information encryption, and decryption are realized conveniently, in addition to the successful model of sunglasses to protect eyes away from ultraviolet radiation and strong light in the sunlight. These studies present a simple and efficient design strategy for the development of photoresponsive materials on modulating the phosphorescence and photochromic property.

Persistent organic room temperature phosphorescence: what is the role of molecular dimers?

Molecular dimers have been frequently found to play an important role in room temperature phosphorescence (RTP), but its inherent working mechanism has remained unclear. Herein a series of unique characteristics, including singlet excimer emission and thermally activated delayed fluorescence, were successfully integrated into a new RTP luminogen of CS-2COOCH3 to clearly reveal the excited-state process of RTP and the special role of molecular dimers in persistent RTP emission.

Different Effect of the Additional Electron-Withdrawing Cyano Group in Different Conjugation Bridge: The Adjusted Molecular Energy Levels and Largely Improved Photovoltaic Performance.

Four organic sensitizers (LI-68-LI-71) bearing various conjugated bridges were designed and synthesized, in which the only difference between LI-68 and LI-69 (or LI-70 and LI-71) was the absence/presence of the CN group as the auxiliary electron acceptor. Interestingly, compared to the reference dye of LI-68, LI-69 bearing the additional CN group exhibited the bad performance with the decreased Jsc and Voc values. However, once one thiophene moiety near the anchor group was replaced by pyrrole with the electron-rich property, the resultant LI-71 exhibited a photoelectric conversion efficiency increase by about 3 folds from 2.75% (LI-69) to 7.95% (LI-71), displaying the synergistic effect of the two moieties (CN and pyrrole). Computational analysis disclosed that pyrrole as the auxiliary electron donor (D') in the conjugated bridge can compensate for the lower negative charge in the electron acceptor, which was caused by the CN group as the electron trap, leading to the more efficient electron injection and better photovoltaic performance.

Conjugated or Broken: The Introduction of Isolation Spacer ahead of the Anchoring Moiety and the Improved Device Performance.

Acceptors in traditional dyes are generally designed closed to TiO2 substrate to form a strong electronic coupling with each other (e.g., cyanoacrylic acid) to enhance the electron injection for the high performance of the corresponding solar cells. However, some newly developed dyes with chromophores or main acceptors isolated from anchoring groups also exhibit comparable or even higher performances. To investigate the relatively untouched electronic coupling effect in dye-sensitized solar cells, a relatively precise method is proposed in which the strength is adjusted gradually by changing isolation spacers between main acceptors and anchoring groups to partially control the electronic interaction. After an analysis of 3 different groups of 11 sensitizers, it is inferred that the electronic coupling should be kept at a suitable level to balance the electron injection and recombination. Based on a reference dye LI-81 possessing a cyanoacrylic acid as acceptor and anchoring group, both photocurrent and photovoltage are synergistically improved after the properties of isolation spacers were changed through the adjustment of the length, steric hindrance, and push-pull electronic characteristic. Accordingly, the rationally designed dye LI-87 with an isolation spacer of thiophene ethylene gives an efficiency of 8.54% and further improved to 9.07% in the presence of CDCA, showing a new way to develop efficient sensitizers.