Impurities in Arylboronic Esters Induce Persistent Afterglow.

Several recent reports suggest that arylboronic esters can exhibit room temperature phosphorescence (RTP), an optical property that is desirable for applications in security printing, oxygen sensing, and bioimaging. These findings challenged the fundamental notion that heavy elements or changes in orbital symmetry were required for intersystem crossing to occur in organic compounds. As we had not observed long afterglow in the many arylboronic esters we had synthesized over many years, we suspected that the RTP observed in these systems had a simpler explanation: the materials reported were impure. Herein, we synthesized 12 arylboronic esters that were previously reported to show RTP, and carefully purified them by column chromatography, recrystallization, and sublimation. We re-examined their photophysical properties alongside single-crystal X-ray diffraction analysis and detailed theoretical studies. While 4 of the 12 compounds showed long afterglows as crude products, none of them showed persistent RTP after careful purification. We also successfully isolated the impurity 4-amino-3,5-bis(pinacolatoboryl)benzonitrile (2), identifying it as the impurity responsible for inducing delayed fluorescence in 3,5-bis(pinacolatoboryl)benzonitrile (1). Doping 1 with 1.0 mol % 2 led to a persistent afterglow with a lifetime of 67 ms, which is mediated by a dimer charge transfer state. Our findings call for a re-examination of previous studies reporting RTP from arylboronic esters, highlight the importance of careful purification in photophysical research, and provide a practical strategy for designing organic materials with a long afterglow.

Coassembling Mesoporous Zeolitic Imidazolate Frameworks by Directed Reticular Chemistry.

Conventional microporous zeolitic imidazolate frameworks (ZIFs) face limitations in mass transfer and pore accessibility when dealing with large guest molecules. Here, we describe a technique for the synthesis of mesoporous ZIFs (MesoZIFs) using a strategy we term directed reticular chemistry. MesoZIF-8 was prepared through solvent evaporation-induced coassembly of polystyrene-block-poly(ethylene oxide) (PS-b-PEO), ZIF-8 building blocks, and acetic acid (AcOH), followed by amine-facilitated crystallization of ZIF-8 in the interstices of PS-b-PEO micelles. AcOH prevents the fast coordination of ZIF-8 building blocks, avoiding phase separation during coassembly. The employed amine plays a crucial role in neutralizing the crystallization environment and further deprotonating the 2-methlyimizale linker to coordinate with zinc ions. Ink bottle-shaped mesopores with tunable mesopore sizes were created by adjusting the molecular weight of PS-b-PEO. Compared to microporous ZIF-8, MesoZIF-8 exhibited enhanced performance in Knoevenagel condensation reactions involving large reactants and hydrogen storage capacity. With this study, we establish an efficient approach for synthesizing MesoZIFs with highly accessible mesopores to enhance ZIF performance in targeted applications.

The Future is Bright: The Emergence of Glassy Organic Dots for Biological Applications.

Glassy organic dots (g-Odots) are an emerging class of luminescent nanoparticles that offer enhanced photostability, superior brightness, and modular tunability compared to other commonly employed nanoparticles. In the last several years, they have been used as bioimaging probes for single- and multi-photon cellular imaging, exhibiting low cytotoxicity even after several days. While they are emerging as promising materials for use in biological applications, g-Odots face several key challenges before their use can become widespread. In this concept, we outline the state of the literature on g-Odots and highlight a few ways in which their design and use can be improved upon.

Reversible Nucleophilic Ring-Opening of Tetraoxapentacene Derivatives: Accessing New Materials for Thermally Activated Delayed Fluorescence.

We report the unexpected nucleophilic ring-opening reaction of electron deficient dioxins in the presence of carbazole under basic conditions. This nucleophilic ring-opening reaction is reversible under basic conditions in the absence of nucleophiles. Further, we demonstrate that this unexpected reactivity can be used to prepare novel donor-acceptor compounds that are emissive in solution and as thin films and exhibit thermally activated delayed fluorescence (TADF).

Triplet-Triplet Annihilation Upconversion from Red to Blue Light Using a TADF Sensitizer Based Polymer.

Molecules capable of thermally activated delayed fluorescence (TADF) can exhibit triplet lifetimes on the order of µs-ms as well as low energy losses in the intersystem crossing (ISC) process. As a result, they have great potential to be used as sensitizers in triplet-triplet annihilation upconversion (TTA-UC) systems with high anti-Stokes shifts, replacing traditional phosphorescent sensitizers. In this study, we employ a red-absorbing boron difluoride curcuminoid-based TADF molecule as the sensitizer and a 9,10-diphenylanthracene derivative as the annihilator. We polymerize them to synthesize a polymer-based red-to-blue TTA-UC system with an anti-Stokes shift of up to 182 nm (0.9 eV) and an upconversion photoluminescence quantum yield (PLQY) of 0.77%. To our knowledge, this is the first report of a TTA-UC polymer containing a TADF photosensitizer. The upconversion properties were investigated through power-dependent experiments and photoluminescence decay measurements. This work provides a more detailed understanding of TTA-UC polymers, highlights the strength of TADF sensitizers in improving anti-Stokes shifts in TTA-UC systems, and demonstrates the feasibility of building polymer-based upconversion systems. This development will expand the application of purely organic TTA-UC.

Organelle-Targeting Polymer Dots Exhibiting Thermally Activated Delayed Fluorescence for Subcellular Imaging.

Selective imaging of specific subcellular structures provides valuable information about the cellular microenvironment. Materials exhibiting thermally activated delayed fluorescence (TADF) are rapidly emerging as metal-free probes with long-lived emission for intracellular time-gated imaging applications. Polymers incorporating TADF emitters can self-assemble into luminescent nanoparticles, termed polymer dots (Pdots), and this strategy enables them to circumvent the limitations of commercial organelle trackers and small molecule TADF emitters. In this study, diblock copolymers comprised of a hydrophilic block containing organelle-targeting monomers and a hydrophobic TADF-active block were synthesized by ring-opening metathesis polymerization (ROMP). Oxanorbornene-based monomers incorporating morpholine and triphenylphosphonium groups for lysosome and mitochondria targeting, respectively, were also synthesized. ROMP by sequential addition yielded well-defined diblock copolymers with dispersities <1.28. To analyze the effect of tuning the hydrophilic corona on cellular viability and uptake, we prepared Pdots with poly(ethylene glycol) (PEG) and bis-guanidinium (BGN) coronas, resulting in limited and efficient cellular uptake, respectively. Red-emissive Pdots with BGN-based coronas and organelle-targeting functionality were obtained with quantum yields up to 12% in water under air. Colocalization analysis confirmed that lysosome and mitochondria labeling in live HeLa cells was accomplished within 2 h of incubation, affording Pearson's correlation coefficients of 0.37 and 0.70, respectively. The potential application of these Pdots for time-resolved imaging is highlighted by a proof of concept using time-gated spectroscopy, which effectively separates the delayed emission of the TADF Pdots from the background autofluorescence of biological serum.

Triplet dynamics reveal loss pathways in multi-resonance thermally activated delayed fluorescence emitters.

Multi-resonance thermally activated delayed fluorescence (MR-TADF) materials are of interest for light-emitting applications due to their narrow emission bandwidths and high photoluminescence quantum yields. Whilst there have been numerous examples of multi-resonance molecules exhibiting efficient TADF, the photophysics and mechanism of TADF in multi-resonance emitters have not been investigated to the same extent as the more conventional spatially separated donor-acceptor TADF materials, limiting the development of MR-TADF devices. Here we study the photophysics of a multi-resonance TADF material, OQAO(mes)2, using transient absorption spectroscopy to spectrally resolve the triplet population(s). We identify multiple triplet populations with distinct spectral contributions, and resolve the dynamics between them. Unlike conventional donor-acceptor TADF materials that have previously been studied, we find these triplet states are not formed in equilibrium, instead exhibiting a slow evolution from a high-energy triplet to a low-energy triplet. Delayed fluorescence predominantly reflects the lifetime of the high-energy triplet state, indicating that the formation of the low-energy triplet is a loss pathway for TADF. We also find that greater amounts of the low-energy triplet are formed in a higher dielectric environment, which leads to less delayed fluorescence. These triplet dynamics have significant implications for TADF in devices, as depending on the identity of the triplet formed by electrical excitation, there will either be a significant barrier to TADF, or a competing nonradiative decay pathway.

Investigating Hydrogen Bonding in Quinoxaline-Based Thermally Activated Delayed Fluorescent Materials.

In recent years, hydrogen bonding (H bonding) as an intramolecular locking strategy has been proposed to enhance photoluminescence, color purity, and photostability in thermally activated delayed fluorescence (TADF) materials. Rigidification as a design strategy is particularly relevant when using electron-deficient N-heterocycles as electron acceptors, because these materials often suffer from poor performance as orange to near-infrared emitters as a result of the energy gap law. To critically evaluate the presence of H bonding in such materials, two TADF-active donor-acceptor dyads, ACR-DQ and ACR-PQ, were synthesized. Despite their potential sites for intramolecular H bonding and emissions spanning yellow to deep red, computational analyses (including frequency, natural bond orbital, non-covalent interaction, and potential energy surface assessments) and crystal structure examinations collectively suggest the absence of H bonding in these materials. Our results indicate that invoking intramolecular H bonding should be done with caution in the design of rigidified TADF materials.

Polymer Dots with Delayed Fluorescence and Tunable Cellular Uptake for Photodynamic Therapy and Time-Gated Imaging.

By combining bioimaging and photodynamic therapy (PDT), it is possible to treat cancer through a theranostic approach with targeted action for minimum invasiveness and side effects. Thermally activated delayed fluorescence (TADF) probes have gained recent interest in theranostics due to their ability to generate singlet oxygen (1O2) while providing delayed emission that can be used in time-gated imaging. However, it is still challenging to design systems that simultaneously show (1) high contrast for imaging, (2) low dark toxicity but high phototoxicity and (3) tunable biological uptake. Here, we circumvent shortcomings of TADF systems by designing block copolymers and their corresponding semiconducting polymer dots (Pdots) that encapsulate a TADF dye in the core and expose an additional boron-dipyrromethene (BODIPY) oxygen sensitizer in the corona. This architecture provides orange-red luminescent particles (ΦPL up to 18 %) that can efficiently promote PDT (1O2 QY=42 %) of HeLa cells with very low photosensitizer loading (IC50 ~0.05-0.13 µg/mL after 30 min). Additionally, we design Pdots with tunable cellular uptake but similar PDT efficiencies using either polyethylene glycol or guanidinium-based coronas. Finally, we demonstrate that these Pdots can be used for time-gated imaging to effectively filter out background fluorescence from biological samples and improve image contrast.

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.

Excited-state dynamics of C3-symmetric heptazine-based thermally activated delayed-fluorescence emitters.

Heptazine-based materials have recently emerged as a promising motif for thermally activated delayed fluorescence, as their near-zero or negative singlet-triplet energy gaps enable extremely fast reverse intersystem crossing (rISC) rates. Another method for achieving a high rate of rISC is through the use of highly symmetric emitters, which benefit from energy-level degeneracies and a high density of states. Here, we investigate the effect of combining these two design strategies on the excited-state dynamics of C3-symmetric emitters containing heptazine cores. We find that in two of the four emitters studied, the S1 state has a high degree of locally excited (LE) character with density on the heptazine moiety, preventing excited-state localization and a loss of symmetry in the energy-minimized S1 geometry. Surprisingly, these symmetric molecules still suffer from a loss of density of triplet states below the S1 state. Overall, we find that maintaining C3 symmetry will not necessarily maintain density of states, but that heptazine-based materials with LE S1 states still benefit from maximized rISC rates via increased spin-orbit coupling with low-lying charge-transfer triplet states and exhibit advantageous photophysical properties, such as near-unity photoluminescence quantum yields and high colour purity.

Organic Photothermal Materials Obtained Using Thermally Activated Delayed Fluorescence Design Principles.

Organic small molecules with high photothermal conversion efficiencies that absorb near-infrared light are desirable for photothermal therapy due to their improved biocompatibility compared to inorganic materials and their ability to absorb light in the biological transparency window (650-1350 nm). Here we report three donor-acceptor organic materials DM-ANDI, O-ANDI, and S-ANDI that show high photothermal conversion efficiencies of 46-68 % with near-infrared absorption. The design of these molecules is based on the rational modification of a thermally activated delayed fluorescence material to favour a low photoluminescence quantum yield by reducing HOMO-LUMO overlap. Encapsulating these materials into either neat nanoparticles or aggregated organic dots modulates their photothermal conversion efficiencies, and also facilitates dispersion in water.

Semiconducting Polymer Dots Directly Stabilized with Serum Albumin: Preparation, Characterization, and Cellular Immunolabeling.

Semiconducting polymer dots (Pdots) are brightly fluorescent nanoparticles of growing interest for bioanalysis and imaging. A recurring challenge with these materials is obtaining robust physical and colloidal stability and low nonspecific binding. Here, we prepared and characterized Pdots with bovine serum albumin (BSA) as the stabilizing agent (BSA-Pdots) instead of a more conventionally used amphiphilic polymer, both without and with cross-linking of the protein using glutaraldehyde (BSA(GA)-Pdots) or disuccinimidyl glutarate. Characterization included fluorescence properties; colloidal stability as a function of pH, ionic strength, and solvent perturbation; shape retention and hardness; and nonspecific binding with common assay substrates, fixed cells, and live cells. These properties were contrasted with the same properties for amphiphilic polymer-stabilized Pdots and silica-coated Pdots. On balance, the BSA-stabilized Pdots were similar or more favorable in their properties, with BSA(GA)-Pdots being especially advantageous. Bioconjugation of the BSA-stabilized Pdots was possible using amine-reactive active-ester chemistry, including biotinylation and bioorthogonal functionalization for immunoconjugation via tetrazine-strained-alkene click chemistry. These approaches were used for selective fluorescent labeling of cells based on ligand-receptor and antibody-antigen binding, respectively. Overall, direct BSA stabilization is a very promising strategy for preparing Pdots with improved physical and colloidal stability, reduced nonspecific interactions, and utility for in vitro diagnostics and other bioanalyses and imaging.

Red-Shifted Emission in Multiple Resonance Thermally Activated Delayed Fluorescent Materials through Malononitrile Incorporation.

Multiple resonance thermally activated delayed fluorescent (MR-TADF) materials offer higher color purity than conventional TADF materials but suffer from aggregation-caused quenching (ACQ) and rarely exhibit red emission. Herein, two malononitrile-substituted emitters are synthesized from a quinolino[3,2,1-de]acridine-5,9-dione (QAO) MR-TADF precursor. Both materials maintain MR-TADF, while they display red-shifted fluorescence. They also overcome ACQ, displaying enhanced emission upon aggregation in solution and forming red-emissive J-aggregates in the solid state with photoluminescent quantum yields of 9 and 11%.

Imidazophenothiazine-Based Thermally Activated Delayed Fluorescence Materials with Ultra-Long-Lived Excited States for Energy Transfer Photocatalysis.

Triplet-triplet energy transfer (EnT) is a powerful activation pathway in photocatalysis that unlocks new organic transformations and improves the sustainability of organic synthesis. Many current examples, however, still rely on platinum-group metal complexes as photosensitizers, with associated high costs and environmental impacts. Photosensitizers that exhibit thermally activated delayed fluorescence (TADF) are attractive fully organic alternatives in EnT photocatalysis. However, TADF photocatalysts incorporating heavy atoms remain rare, despite their utility in inducing efficient spin-orbit-coupling, intersystem-crossing, and consequently a high triplet population. Here, we describe the synthesis of imidazo-phenothiazine (IPTZ), a sulfur-containing heterocycle with a locked planar structure and a shallow LUMO level. This acceptor is used to prepare seven TADF-active photocatalysts with triplet energies up to 63.9 kcal mol-1. We show that sulfur incorporation improves spin-orbit coupling and increases triplet lifetimes up to 3.64 ms, while also allowing for tuning of photophysical properties via oxidation at the sulfur atom. These IPTZ materials are applied as photocatalysts in five seminal EnT reactions: [2 + 2] cycloaddition, the disulfide-ene reaction, and Ni-mediated C-O and C-N cross-coupling to afford etherification, esterification, and amination products, outcompeting the industry-standard TADF photocatalyst 2CzPN in four of the five studied scenarios. Detailed photophysical and theoretical studies are used to understand structure-activity relationships and to demonstrate the key role of the heavy atom effect in the design of TADF materials with superior photocatalytic performance.

Achieving White-Light Emission Using Organic Persistent Room Temperature Phosphorescence.

Artificial lighting currently consumes approximately one-fifth of global electricity production. Organic emitters with white persistent RTP have potential for applications in energy-efficient lighting technologies, due to their ability to harvest both singlet and triplet excitons. Compared to heavy metal phosphorescent materials, they have significant advantages in cost, processability, and reduced toxicity. Phosphorescence efficiency can be improved by introducing heteroatoms, heavy atoms, or by incorporating luminophores within a rigid matrix. White-light emission can be achieved by tuning the ratio of fluorescence to phosphorescence intensity or by pure phosphorescence with a broad emission spectrum. This review summarizes recent advances in the design of purely organic RTP materials with white-light emission, describing single-component and host-guest systems. White phosphorescent carbon dots and representative applications of white-light RTP materials are also introduced.

Dibenzodipyridophenazines with Dendritic Electron Donors Exhibiting Deep-Red Emission and Thermally Activated Delayed Fluorescence.

The development of deep-red thermally activated delayed fluorescence (TADF) emitters is important for applications such as organic light-emitting diodes (OLEDs) and biological imaging. Design strategies for red-shifting emission include synthesizing rigid acceptor cores to limit nonradiative decay and employing strong electron-donating groups. In this work, three novel luminescent donor-acceptor compounds based on the dibenzo[a,c]dipyrido[3,2-h:20-30-j]-phenazine-12-yl (BPPZ) acceptor were prepared using dendritic carbazole-based donors 3,3″,6,6″-tetramethoxy-9'H-9,3':6',9″-tercarbazole (TMTC), N3,N3,N6,N6-tetra-p-tolyl-9H-carbazole-3,6-diamine (TTAC), and N3,N3,N6,N6-tetrakis(4-methoxyphenyl)-9H-carbazole-3,6-diamine (TMAC). Here, dimethoxycarbazole, ditolylamine, and bis(4-methoxyphenyl)amine were introduced at the 3,6-positions of carbazole to increase the strength of these donors and induce long-wavelength emission. Substituent effects were investigated with experiments and theoretical calculations. The emission maxima of these materials in toluene were found to be 562, 658, and 680 nm for BPPZ-2TMTC, BPPZ-2TTAC, and BPPZ-2TMAC, respectively, highlighting the exceptional strength of the TMAC donor, which pushes the emission into the deep-red region of the visible spectrum as well as into the biological transparency window (650-1350 nm). Long-lived emission lifetimes were observed in each emitter due to TADF in BPPZ-2TMC and BPPZ-2TTAC, as well as room-temperature phosphorescence in BPPZ-2TMAC. Overall, this work showcases deep-red emissive dendritic donor-acceptor materials which have potential as bioimaging agents with emission in the biological transparency window.

Thermally Activated Delayed Fluorescence and Room-Temperature Phosphorescence in Materials with Imidazo-pyrazine-5,6-dicarbonitrile Acceptors.

Three donor-acceptor compounds based on the imidazo-pyrazine-5,6-dicarbonitrile (IPDC) acceptor were synthesized. The IPDC emitters exhibit blue to near-infrared (NIR) emission with up to 54 % photoluminescent quantum yield. 9,9-Dimethyl-9,10-dihydroacridine (ACR), phenoxazine (POX), and phenothiazine (PTZ) served as electron donors. IPDC-POX displayed NIR emission in toluene solution, while showing room-temperature phosphorescence in the solid state. IPDC-ACR exhibited yellow thermally activated delayed fluorescence. Interestingly, dual-emissive behavior as well as excitation-dependent thermally activated delayed fluorescence (TADF) was found for IPDC-PTZ, arising from the two conformers of phenothiazine derivatives. Overall, this work describes a novel strong electron acceptor from the fusion of imidazole, pyrazine, and nitrile functional groups into one conjugated heterocycle for materials exhibiting NIR emission, TADF, and/or room-temperature phosphorescence (RTP).