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.

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.

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.

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.

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).

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.

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.

Luminescent Surface-Tethered Polymer Brush Materials.

Surface-tethered polymers are unique molecular architectures that have been recently used in advanced sensors, electronics and biomedical applications. However, techniques for characterizing these materials in their surface-tethered form remain limited. The incorporation of luminescent functionality into these materials has enabled new characterization methods, while also unlocking new applications in optoelectronics, stenography and sensing. Micron-scale photolithography techniques have recently enabled the preparation of high-resolution patterns, as well as architectures with unique photophysical properties. Herein, we provide an overview of the techniques used to prepare luminescent polymer brush materials and their applications in stimuli-responsive sensors, cell adhesion materials, and optoelectronics. We also provide our perspective on the promising future uses of surface-tethered polymers, as well as the short-term challenges and opportunities in the field.

Estimating Phosphorescent Emission Energies in IrIII Complexes Using Large-Scale Quantum Computing Simulations.

Here we calculate T1 →S0 transition energies in nine phosphorescent iridium complexes using the iterative qubit coupled cluster (iQCC) method to determine if quantum simulations have any advantages over classical methods. These simulations would require a gate-based quantum computer with at least 72 fully-connected logical qubits. Since such devices do not yet exist, we demonstrate the iQCC method using a purpose-built quantum simulator on classical hardware. The results are compared to a selection of common DFT functionals, ab initio methods, and empirical data. iQCC is found to match the accuracy of the best DFT functionals, but with a better correlation coefficient, demonstrating that it is better at predicting the structure-property relationship. Results indicate that the iQCC method has the required accuracy to design organometallic complexes when deployed on emerging quantum hardware and sets an industrially relevant target for demonstrating quantum advantage.

Red-Emissive Cell-Penetrating Polymer Dots Exhibiting Thermally Activated Delayed Fluorescence for Cellular Imaging.

Fluorescence imaging in living cells is key to understanding many biological processes, yet autofluorescence from the sample can lower sensitivity and hinder high-resolution imaging. Time-gated measurements using phosphorescent metal complexes can improve imaging, at the cost of potential toxicity from the use of heavy metals. Here, we describe orange/red-emitting polymer dots (Pdots) exhibiting thermally activated delayed fluorescence (TADF) for time-gated imaging. Inspired by the cell invasion mechanism of the HIV TAT protein, the Pdots were formed from block copolymers composed of a hydrophilic guanidine-rich block as a cell-penetrating peptide mimic, and a rigid organic semiconductor block to provide efficient delayed fluorescence. These all-organic polymer nanoparticles were shown to efficiently enter HeLa, CHO, and HepG2 cells within 30 min, with cell viabilities remaining high for Pdot concentrations up to 25 mg mL-1. Pdot quantum yields were as high as 0.17 in aerated water, with the Pdot structure effectively shielding the TADF emitters from quenching by oxygen. Colocalization experiments revealed that the Pdots primarily accumulate outside of lysosomes, minimizing lysosomal degradation. When used for fixed cellular imaging, Pdot-incubated cells showed high signal-to-background ratios compared to control samples with no Pdot exposure. Using time-resolved spectroscopy, the delayed emission of the TADF materials was effectively separated from that of both a biological serum and a secondary fluorescent dye.

Polymer Dots with Enhanced Photostability, Quantum Yield, and Two-Photon Cross-Section using Structurally Constrained Deep-Blue Fluorophores.

Semiconducting polymer dots (Pdots) have emerged as versatile probes for bioanalysis and imaging at the single-particle level. Despite their utility in multiplexed analysis, deep blue Pdots remain rare due to their need for high-energy excitation and sensitivity to photobleaching. Here, we describe the design of deep blue fluorophores using structural constraints to improve resistance to photobleaching, two-photon absorption cross sections, and fluorescence quantum yields using the hexamethylazatriangulene motif. Scanning tunneling microscopy was used to characterize the electronic structure of these chromophores on the atomic scale as well as their intrinsic stability. The most promising fluorophore was functionalized with a polymerizable acrylate handle and used to give deep-blue fluorescent acrylic polymers with Mn > 18 kDa and D < 1.2. Nanoprecipitation with amphiphilic polystyrene-graft-(carboxylate-terminated poly(ethylene glycol)) gave water-soluble Pdots with blue fluorescence, quantum yields of 0.81, and molar absorption coefficients of (4 ± 2) × 108 M-1 cm-1. This high brightness facilitated single-particle visualization with dramatically improved signal-to-noise ratio and photobleaching resistance versus an unencapsulated dye. The Pdots were then conjugated with antibodies for immunolabeling of SK-BR3 human breast cancer cells, which were imaged using deep blue fluorescence in both one- and two-photon excitation modes.

Preparation of Patterned and Multilayer Thin Films for Organic Electronics via Oxygen-Tolerant SI-PET-RAFT.

An oxygen-tolerant approach is described for preparing surface-tethered polymer films of organic semiconductors directly from electrode substrates using polymer brush photolithography. A photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) approach was used to prepare multiblock polymer architectures with the structures of multi-layer organic light-emitting diodes (OLEDs), including electron-transport, emissive, and hole-transport layers. The preparation of thermally activated delayed fluorescence (TADF) and thermally assisted fluorescence (TAF) trilayer OLED architectures are described. By using direct photomasking as well as a digital micromirror device, we also show that the surface-initiated (SI)-PET-RAFT approach allows for enhanced control over layer thickness, and spatial resolution in polymer brush patterning at low cost.

Near-Infrared-Emitting Boron-Difluoride-Curcuminoid-Based Polymers Exhibiting Thermally Activated Delayed Fluorescence as Biological Imaging Probes.

Near-infrared-emitting polymers were prepared using four boron-difluoride-curcuminoid-based monomers using ring-opening metathesis polymerization (ROMP). Well-defined polymers with molecular weights of ≈20 kDa and dispersities <1.07 were produced and exhibited near-infrared (NIR) emission in solution and in the solid state with photoluminescence quantum yields (ΦPL ) as high as 0.72 and 0.18, respectively. Time-resolved emission spectroscopy revealed thermally activated delayed fluorescence (TADF) in polymers containing highly planar dopants, whereas room-temperature phosphorescence dominated with twisted species. Density functional theory demonstrated that rotation about the donor-acceptor linker can give rise to TADF, even where none would be expected based on calculations using ground-state geometries. Incorporation of TADF-active materials into water-soluble polymer dots (Pdots) gave NIR-emissive nanoparticles, and conjugation of these Pdots with antibodies enabled immunofluorescent labeling of SK-BR3 human breast-cancer cells.

Thermally Activated Delayed Fluorescence in 1,3,4-Oxadiazoles with π-Extended Donors.

Here, we describe the synthesis of five 1,3,4-oxadiazole-based donor-acceptor materials, using dendritic carbazole-based donors 9'H-9,3':6'9″-tercarbazole (terCBz) and N3,N3,N6,N6-tetra-p-tolyl-9H-carbazole-3,6-diamine (TTAC). Due to the strongly donating and highly twisted nature of the TTAC donor as well as the spatially separated hole-particle wavefunctions, three of the five compounds exhibited thermally activated delayed fluorescence (TADF) in spite of a relatively large ΔEST measured through phosphorimetry (0.33-0.37 eV). These materials demonstrated photoluminescence quantum yields as high as 0.89 in toluene, with emission maxima ranging from 474 to 495 nm in the solid state. Additionally, two materials containing only terCBZ donor(s) exhibited deep blue fluorescence, with Commission Internationale de l'éclairage coordinates of (0.16, 0.05); the weaker nature of the terCBz donor results in a prohibitively large ΔEST (0.68-0.77 eV). A gap-tuned range-separated hybrid functional (ωB97XD*) was used to rigorously calculate triplet energies, while a systematic analysis of electronic structures and photophysical properties provided further insight into the properties of these materials. These findings ultimately contribute a synthetically facile approach toward highly emissive TADF emitters using a 1,3,4-oxadiazole motif.

Donor-Acceptor Materials Exhibiting Thermally Activated Delayed Fluorescence Using a Planarized N-Phenylbenzimidazole Acceptor.

An N-phenylbenzimidazole constrained in a coplanar fashion with a methylene tether (IMAC) was designed and used to prepare a series of emitters exhibiting thermally activated delayed fluorescence (TADF). Four novel TADF emitters using 9,9-dimethylacridine, phenoxazine, phenothiazine, and bis(di-p-tolylamino)carbazole as the donor group were designed and synthesized using IMAC as the acceptor. Additionally, two deep-blue fluorescent emitters were prepared with carbazole and tercarbazole as the donor moieties. The twisted conformation between donor and acceptor in these molecules resulted in effective spatial separation of the HOMO and LUMO and small singlet-triplet energy gaps. Crystallographic properties, electronic structures, thermal stabilities, photophysical properties, and energy levels were studied systematically. Ultimately, these findings provide a promising opportunity for the design and synthesis of highly efficient TADF materials based on IMAC derivatives.

Interface-Dependent Aggregation-Induced Delayed Fluorescence in Bottlebrush Polymer Nanofibers.

Bottlebrush copolymers provide a covalent route to multicompartment nanomaterials that remain nanosegregated regardless of environmental conditions. This is particularly advantageous when combining polymers for optoelectronics, where the ability to control the interface between multiple chemically distinct polymers can be key to a device's function. Here we prepare bottlebrush nanofibers from an acridine- and triazine-based donor/acceptor pair, which have been shown to exhibit thermally activated delayed fluorescence (TADF) via through-space charge transfer (TSCT). By controlling the morphology of the donor and acceptor domains within the bottlebrush, random, miktoarm, and block bottlebrush morphologies are obtained. Using these materials, nanofibers may be prepared which (i) strongly exhibit TSCT TADF; (ii) exhibit switchable TSCT TADF based on aggregation of the fibers; or (iii) preserve the properties of the original donor and acceptor components. This work demonstrates that a bottlebrush strategy may be used to either force or prevent interactions between chemically dissimilar optoelectronic polymers in blended thin films. In this way, we establish a convenient method for either maximizing or minimizing donor-acceptor interactions in semiconductor polymer blends, using different arrangements of the same building blocks within a bottlebrush nanofiber.

Aggregation-Induced Energy Transfer in Color-Tunable Multiblock Bottlebrush Nanofibers.

The synthesis of multicomponent nanoscale structures with precisely addressable function is critical to the discovery of both new phenomena and new applications in nanotechnology. Though self-assembly offers low-cost routes to many such materials, these methods often require building blocks with particular structural motifs, thus limiting the scope of nanomaterials that can be prepared in these ways. Herein we use a bottom-up approach based on covalent chemistry to synthesize a series of bottlebrush copolymers from red, green, and blue luminescent macromonomers, which were then used to prepare multiblock organic nanofibers structurally analogous to nanoscale RGB pixels. Efficient energy transfer from a blue fluorophore to red and green phosphors can be modulated, using the solvent polarity as a stimulus, to give aggregation-induced changes in emission color. Aggregation was also accompanied by changes in the emission lifetime of the nanofiber from the nanosecond to microsecond regime. Additionally, changes in energy transfer efficiency and interchromophore distance were quantified using a FRET model. Preliminary demonstration of these materials as polarity-sensitive inks for encryption and encoding were also demonstrated using a red/blue fluorescence switch upon exposure to solvent. Finally, the potential complexity of optoelectronic materials accessible with these methods was demonstrated by combining these building blocks with charge-transporting materials to give organic nanofibers with ordered structures mimicking that of multilayer white OLEDs. Ultimately this work describes the preparation of robust, multicomponent nanofibers from general building blocks, combining their optoelectronic properties in ways that can be both reversibly switched and temporally resolved.