Reversible and Continuous Color-Tunable Persistent Luminescence of Metal-Free Organic Materials by "Self"-Interface Energy Transfer

Synthesis of Thioxanthone 10,10-Dioxides and Sulfone-Fluoresceins via Pd-Catalyzed Sulfonylative Homocoupling

Our report describes the facile and scalable preparation of 9H-thioxanthen-9-one 10,10-dioxides via Pd-catalyzed sulfonylative homocoupling of the appropriately substituted benzophenones. This transformation provides a straightforward route to previously unreported sulfone-fluoresceins and -fluorones. Several examples of these red fluorescent dyes have been prepared, characterized, and evaluated as live-cell permeant labels compatible with super-resolution fluorescence microscopy with 775 nm stimulated emission depletion.

Phosphorescence resonance energy transfer from purely organic supramolecular assembly

Phosphorescence energy transfer systems have been applied in encryption, biomedical imaging and chemical sensing. These systems exhibit ultra-large Stokes shifts, high quantum yields and are colour-tuneable with long-wavelength afterglow fluorescence (particularly in the near-infrared) under ambient conditions. This review discusses triplet-to-singlet PRET or triplet-to-singlet-to-singlet cascaded PRET systems based on macrocyclic or assembly-confined purely organic phosphorescence introducing the critical toles of supramolecular noncovalent interactions in the process. These interactions promote intersystem crossing, restricting the motion of phosphors, minimizing non-radiative decay and organizing donor-acceptor pairs in close proximity. We discuss the applications of these systems and focus on the challenges ahead in facilitating their further development.

Rational molecular and doping strategies to obtain organic polymers with ultralong RTP

Organic-doped polymers and room-temperature phosphorescence (RTP) mechanisms have been widely reported. However, RTP lifetimes >3 s are rare and RTP-enhancing strategies are incompletely understood. Herein, we demonstrate a rational molecular doping strategy to obtain ultralong-lived, yet bright RTP polymers. The n-π* transitions of boron- and nitrogen-containing heterocyclic compounds can promote a triplet-state population, and the grafting of boronic acid onto polyvinyl alcohol can inhibit molecular thermal deactivation. However, excellent RTP properties were achieved by grafting 1-0.1% (N-phenylcarbazol-2-yl)-boronic acid rather than (2-/3-/4-(carbazol-9-yl)phenyl)boronic acids to afford record-breaking ultralong RTP lifetimes up to 3.517-4.444 s. These results showed that regulation of the interacting position between the dopant and matrix molecules to directly confine the triplet chromophore could more effectively stabilize triplet excitons, disclosing a rational molecular-doping strategy for achieving polymers with ultralong RTP. Based on the energy-donor function of blue RTP, an ultralong red fluorescent afterglow was demonstrated by co-doping with an organic dye.

Photoinduced Radical Persistent Luminescence in Semialiphatic Polyimide System with Temperature and Humidity Resistance

Organic persistent luminescence (pL) systems with photoresponsive dynamic features have valuable applications in the fields of data encryption, anticounterfeiting, and bioimaging. Photoinduced radical luminescent materials have a unique luminous mechanism with the potential to achieve dynamic pL. It is extremely challenging to obtain radical pL under ambient conditions; on account of it, it is unstable in air. Herein, a new semialiphatic polyimide-based polymer (A0) is developed, which can achieve dynamic pL through reversible conversion of radical under photoexcitation. A "joint-donor-spacer-acceptor" molecular design strategy is applied to effectively modulate the intramolecular charge-transfer and charge-transfer complex interactions, resulting in effective protection of the radical generated under photoirradiation. Meanwhile, polyimide-based polymers of A1-A4 are obtained by doping different amine-containing fluorescent dyes to modulate the dynamic afterglow color from green to red via the triplet to singlet Förster resonance energy-transfer pathway. Notably, benefiting from the structural characteristics of the polyimide-based polymer, A0-A4 have excellent processability, thermal stability, and mechanical properties and can be applied directly in extreme environments such as high temperatures and humidity.

Efficient and Color-Tunable Dual-Mode Afterglow from Large-Area and Flexible Polymer-Based Transparent Films for Anti-Counterfeiting and Information Encryption

It remains a great challenge to develop polymer-based materials with efficient and color-tunable organic afterglow. Two indolocarbazole derivatives IaCzA and IbCzA have been synthesized and doped into poly(vinyl alcohol) (PVA) matrices. It is found that the resulting films can produce unique dual-mode afterglow, which is composed of persistent thermally activated delayed fluorescence and ultralong organic phosphorescence. Besides, the IbCzA-doped PVA film exhibits intense blue afterglow with Φ_afterglow and τ_afterglow up to 19.8 % and 1.81 s, respectively, representing state-of-the-art dual-mode organic afterglow performance. Moreover, our reported film has high flexibility, excellent transparency, and large-area producibility; and the afterglow color of the film can be linearly tuned by temperature. Inspired by these distinctive properties, the PVA doped with IbCzA was employed as temperature-sensitive security ink for anti-counterfeiting and information encryption.

Stepwise Energy Transfer: Near-Infrared Persistent Luminescence from Doped Polymeric Systems

Organic near infrared (NIR) persistent-luminescence systems with bright and long-lived emission are highly valuable for applications in communication, imaging, and sensors. However, realizing these materials (especially lifetime over 0.1 s) is a challenge, mainly because of non-radiative quenching of their long-lived excitons. Herein, a universal strategy of stepwise Förster resonance energy transfer (FRET) for a bright NIR system with remarkable persistent luminescence (up to 0.2 s at 810 nm) is presented, based on a new triphenylene-dye-doped polymer (triphenylene-2-ylboronic acid@poly(vinyl alcohol) (TP@PVA)) with a persistent blue phosphorescence of 3.29 s. This persistent NIR luminescence is demonstrated for application not only in NIR anti-counterfeiting but also NIR bioimaging with penetrating a piece of skin as thick as 2.0 mm. By co-doping a red dye (such as Nile red) and an NIR dye Cyanine 7 (Cy7) into this doped PVA film, the shortage of spectral overlap between TP emission and Cy7 absorbance is successfully solved, through a stepwise FRET process involving triplet to singlet (TS)-FRET from TP to the intermediate red dye and then singlet to singlet (SS)-FRET to Cy7. It is noted that the efficiency of the upper TS-FRET is enhanced significantly by the lower SS-FRET, leading to high efficiencies for the continuous FRETs.

Organic Supramolecular Zippers with Ultralong Organic Phosphorescence by a Dexter Energy Transfer Mechanism

Ultralong organic phosphorescence (UOP) materials glow persistently in the dark, which offers new exciting possibilities in the fields of anti-counterfeiting, photoelectric device and biological imaging. However, development of single component UOP materials remains great challenge. Herein, we firstly develop a single component organic supramolecular zipper system with lifetime up to 0.77 s. Owing to introduction of pyrazol ring into diphenylsulfone group, the 'V' shape molecules were artfully self-assembled into supramolecular zippers via π-π and CH•••π interactions, that are not only of significance in highly efficient generation of triplet excitons, but also facilitate Dexter energy transfer process within supramolecular zippers, that are responsible for the alleviating radiative and non-radiative deactivation decay of triplet excitons, to finally boost the UOP. This finding not only gives a new set of guidelines for the design of single component UOP molecules, but also reveals the UOP mechanism from a new perspective.

Activating Room-Temperature Phosphorescence of Organic Luminophores via External Heavy-Atom Effect and Rigidity of Ionic Polymer Matrix*

Pure organic room-temperature phosphorescence (RTP) materials have attracted wide attention for their easy preparation, low toxicity, and applications in various fields like bioimaging and anti-counterfeiting. Developing phosphorescent systems with more universality and less difficulty in synthesis has long been the pursuit of materials scientists. By employing a polymeric quaternary ammonium salt with an ionic bonding matrix and heavy atoms, commercial fluorescent dyes are directly endowed with phosphorescence emission. In a single amorphous polymer, the external heavy-atom effect generates excited triplet states and the rigid polymer matrix stabilizes them. This study put forward a new general strategy to design and develop pure organic RTP materials starting from existing library of organic dyes without complicated chemical synthesis.

Triple-mode tunable long-persistent luminescence in a 3D zinc-organic hybrid

A 3D zinc-organic hybrid [Zn3(D-Cam)3(tib)2]·2H2O (1) exhibits triple-mode dependent (including excitation wavelength, time and temperature) long-persistent luminescence. Experimental and theoretical calculations support that the long lifetime and color-tunable afterglow may be due to the dispersive electronic state distribution. Furthermore, the hybrid is also used for optical anti-counterfeiting and information encryption applications.

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.

Amphiphilic Diazapyrenes with Multiple Stimuli-Responsive Properties

A series of amphiphilic diazapyrenes exhibiting switchable fluorescence under the stimulus of mechanical force or water vapor are reported for the first time. Comprehensive studies of their photophysical properties in different states based on UV-Vis absorption, FL emission, FT-IR spectroscopy, and XRD analysis have revealed a stimuli-induced excimer-based sensing mechanism. The relationship between molecular structures and optical responsive properties of these diazapyrene derivatives is illustrated. Moreover, the unique fluorescent, stimuli-responsive behaviors of these diazapyrene compounds in the solid state are used to fabricate sensory films for successively and orthogonally sensing mechanical force and water vapor. In contrast to the well-established knowledge on the transformation between the pyrene monomer and excimer, our study offers valuable information about the unknown diazapyrene excimers and demonstrates their potential applications in biocompatible force sensors, data storage, and humidity sensors.

Room-Temperature Phosphorescence Resonance Energy Transfer for Construction of Near-Infrared Afterglow Imaging Agents

Afterglow imaging that detects photons after cessation of optical excitation avoids tissue autofluorescence and thus possesses higher sensitivity than traditional fluorescence imaging. Purely organic molecules with room-temperature phosphorescence (RTP) have emerged as a new library of benign afterglow agents. However, most RTP luminogens only emit visible light with shallow tissue penetration, constraining their in vivo applications. This study presents an organic RTP nanoprobe (mTPA-N) with emission in the NIR range for in vivo afterglow imaging. Such a probe is composed of RTP molecule (mTPA) as the phosphorescent generator and an NIR-fluorescent dye as the energy acceptor to enable room-temperature phosphorescence resonance energy transfer (RT-PRET), ultimately resulting in redshifted phosphorescent emission at 780 nm. Because of the elimination of background noise and redshifted afterglow luminescence in a biologically transparent window, mTPA-N permits imaging of lymph nodes in living mice with a high signal-to-noise ratio. This study thus opens up a universal approach to develop organic RTP luminogens into NIR afterglow imaging agents via construction of RT-PRET.

Stimuli-Responsive Aggregation-Induced Delayed Fluorescence Emitters Featuring the Asymmetric D-A Structure with a Novel Diarylketone Acceptor Toward Efficient OLEDs with Negligible Efficiency Roll-Off

Multifunctional luminescent materials with aggregation-induced delayed fluorescence (AIDF) are capable of suppressing concentration-caused emission quenching and exciton annihilation when used as organic light-emitting diode (OLED) emitters. In this contribution, three stimuli-responsive AIDF luminogens, pipd-BZ-PXZ, pipd-BZ-PTZ, and pipd-BZ-DMAC, featuring a D-A asymmetric framework based on a fused N-heterocycle diarylketone acceptor (imid-azo[1,2-a]pyridin-2-yl(phenyl)methanone pipd) are designed and synthesized. Interestingly, pipd-BZ-PTZ forms two different kinds of crystals (G-crystal and O-crystal) with distinct intermolecular interactions between pipd moieties. The G-crystal with a looser packing mode presents significant morphology-dependent stimuli-responsive behavior with a shifted emission wavelength of 56 nm. Generated by a strong intramolecular charge transfer effect, pipd-BZ-PXZ and pipd-BZ-PTZ exhibit orange to red emission in solution and neat films. Both nondoped and doped devices are fabricated for comparison. Nondoped devices present moderate performance with external quantum efficiencies and current efficiency that reach 7.04% and 19.86 cd A^-1, respectively, and the corresponding efficiency roll off at 1000 cd m^-2 is as small as 2.3%, which is among the best records of AIDF-OLEDs with an emission wavelength over 570 nm. Doped devices show better performance with corresponding efficiencies of up to 55.41 cd A^-1 and 15.77%.

Phosphorescence Energy Transfer: Ambient Afterglow Fluorescence from Water-Processable and Purely Organic Dyes via Delayed Sensitization

Ambient afterglow luminescence from metal-free organic chromophores would provide a promising alternative to the well-explored inorganic phosphors. However, the realization of air-stable and solution-processable organic afterglow systems with long-lived triplet or singlet states remains a formidable challenge. In the present study, a delayed sensitization of the singlet state of organic dyes via phosphorescence energy transfer from organic phosphors is proposed as an alternative strategy to realize "afterglow fluorescence". This concept is demonstrated with a long-lived phosphor as the energy donor and commercially available fluorescent dyes as the energy acceptor. Triplet-to-singlet Förster-resonance energy-transfer (TS-FRET) between donor and acceptor chromophores, which are co-organized in an amorphous polymer matrix, results in tuneable yellow and red afterglow from the fluorescent acceptors. Moreover, these afterglow fluorescent hybrids are highly solution-processable and show excellent air-stability with good quantum yields.