Thermally Activated Delayed Fluorescence Dyes Research Articles

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.

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

AbstractBy 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.

Shedding light on thermally-activated delayed fluorescence.

Thermally activated delayed fluorescence (TADF) is a hot research topic in view of its impressive applications in a wide variety of fields from organic LEDs to photodynamic therapy and metal-free photocatalysis. TADF is a rare and fragile phenomenon that requires a delicate equilibrium between tiny singlet-triplet gaps, sizable spin-orbit couplings, conformational flexibility and a balanced contribution of charge transfer and local excited states. To make the picture more complex, this precarious equilibrium is non-trivially affected by the interaction of the TADF dye with its local environment. The concurrent optimization of the dye and of the embedding medium is therefore of paramount importance to boost practical applications of TADF. Towards this aim, refined theoretical and computational approaches must be cleverly exploited, paying attention to the reliability of adopted approximations. In this perspective, we will address some of the most important issues in the field. Specifically, we will critically review theoretical and computational approaches to TADF rates, highlighting the limits of widespread approaches. Environmental effects on the TADF photophysics are discussed in detail, focusing on the major role played by dielectric and conformational disorder in liquid solutions and amorphous matrices.

Modulating optoelectronic properties of red TADF emitters based on acenaphtho[1,2-b]quinoxaline-3,4-dicarbonitrile through isomeric engineering

Developing new electron acceptors with high rigidity and structural adjustability is significant for realizing efficient long-wavelength thermally activated delayed fluorescence (TADF) emitters. Herein, a new acceptor group, acenaphtho[1,2-b]quinoxaline-3,4-dicarbonitrile (AQCN), featuring a π-extended, rigid skeleton with high electron affinity, is developed for the construction of red TADF dyes. Two red light-emitting regioisomers, LB-Y and LB-T, featuring a Y-shape and T-shape D-A-D molecular configuration, respectively, were prepared by adjusting the relatively substituting position of two triphenylamine electron-rich moieties on AQCN. Through the isomeric engineering, it is found that LB-Y shows a high degree of molecular rigidity, matched excited-state energy level alignment, and decent interplay among singlet charge transfer excited state (1CT), triplet locally excited state (3LE) and triplet hybridized local and charge transfer excited state (3HLCT). Therefore, LB-Y has a higher radiative transition rate (8.36 × 107 s−1), reverse intersystem crossing rate (1.02 × 105 s−1), and better electroluminescent performances (λEL = 614 nm, EQEmax = 4.1%) than LB-T (λEL = 612 nm, EQEmax = 2.6%). This study reports a promising electron acceptor and provides an efficient approach to modulating the optoelectronic properties of red TADF dyes.

Design, synthesis, and electroluminescence of red TADF dyes based on cyanophenanthrene

Two rigid electron-deficient cyanophenanthrene frameworks with limited π-conjugation, phenanthrene-9-carbonitrile (CNP) and phenanthrene-9,10-dicarbonitrile (DCNP), are employed as the electron acceptors to construct thermally activated delayed fluorescence (TADF) dyes. The new TADF emitters exhibit long-wavelength emission without strong long-range π-π stacking or interchromophore coupling, as supported by the photophysical and crystallographic results. These unique features are suitable to construct red organic light-emitting diodes (OLEDs). An OLED based on CNP-TPXZ showed an orange-red emission with a peak at 586 nm and a maximum EQE of 8.4%, while that of DCNP-DPXZ exhibited red emission at 614 and a maximum EQE of 6.6%. These results demonstrate that DCNP and CNP are potential electron acceptors for the development of red TADF emitters and OLEDs.

Microdroplet-Facilitated Assembly of Thermally Activated Delayed Fluorescence-Encoded Microparticles with Non-interfering Color Signals.

Encoded microparticles (EMPs) have shown demonstrative value for multiplexed high-throughput bioassays such as drug discovery and diagnostics. Herein, we propose for the first time the incorporation of thermally activated delayed fluorescence (TADF) dyes with low-cost, heavy metal-free, and long-lived luminescence properties into polymer matrices via a microfluidic droplet-facilitated assembly technique. Benefiting from the uniform droplet template sizes and polymer-encapsulated structures, the resulting composite EMPs are highly monodispersed, efficiently shield TADF dyes from singlet oxygen, well preserve TADF emission, and greatly increase the delayed fluorescence lifetime. Furthermore, by combining with phase separation of polymer blends in the drying droplets, TADF dyes with distinct luminescent colors can be spatially separated within each EMP. It eliminates optical signal interference and generates multiple fluorescence colors in a compact system. Additionally, in vitro studies reveal that the resulting EMPs show good biocompatibility and allow cells to adhere and grow on the surface, thereby making them promising optically EMPs for biolabeling.

Thermally Activated Delayed Fluorescence: Polarity, Rigidity, and Disorder in Condensed Phases.

We present a detailed and comprehensive picture of the photophysics of thermally activated delayed fluorescence (TADF). The approach relies on a few-state model, parametrized ab initio on a prototypical TADF dye, that explicitly accounts for the nonadiabatic coupling between electrons and vibrational and conformational motion, crucial to properly address (reverse) intersystem crossing rates. The Onsager model is exploited to account for the medium polarity and polarizability, with careful consideration of the different time scales of relevant degrees of freedom. TADF photophysics is then quantitatively addressed in a coherent and exhaustive approach that accurately reproduces the complex temporal evolution of emission spectra in liquid solvents as well as in solid organic matrices. The different rigidity of the two environments is responsible for the appearance in matrices of important inhomogeneous broadening phenomena that are ascribed to the intertwined contribution from (quasi)static conformational and dielectric disorder.

Elucidating the Electronic Structure of a Delayed Fluorescence Emitter via Orbital Interactions, Excitation Energy Components, Charge-Transfer Numbers, and Vibrational Reorganization Energies.

Recently, Wang and co-workers carried out frontier molecule orbital engineering in the design of m-Cz-BNCz, a thermally activated delayed fluorescence (TADF) molecule that emits pure green light at an external quantum efficiency of 27%. To further understand the underlying molecular design principles, we employed four advanced electronic structure analysis tools. First, an absolutely localized molecular orbitals (ALMO-) based analysis indicates an antibonding combination between the highest occupied molecular orbitals (HOMOs) of the donor 3,6-di-tert-butylcarbazole fragment and the acceptor BNCz fragment, which raises the HOMO energy and red-shifts the fluorescence emission wavelength. Second, excitation energy component analysis reveals that the S1-T1 gap is dominated by two-electron components of the excitation energies. Third, charge transfer number analysis, which is extended to use fragment-based Hirshfeld weights, indicates that the S1 and T1 excited states of m-Cz-BNCz (within time-dependent density functional theory) have notable charge transfer characters (27% for S1 and 12% for T1). This provides a balance between a small single-triplet gap and a substantial fluorescence intensity. Last, a vibrational reorganization energy analysis pinpoints the torsional motion between the BNCz and Cz moieties of m-Cz-BNCz as the source for its wider emission peak than that of p-Cz-BNCz. These four types of analyses are expected to be very valuable in the study and design of other TADF and functional dye molecules.

A Plastic Scintillator Based on an Efficient Thermally Activated Delayed Fluorescence Emitter 9‐(4‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)‐2‐methylphenyl)‐3,6‐dioctyl‐9H‐carbazole for Pulse Shape Discrimination Measurement

AbstractPulse shape discrimination (PSD)‐capable plastic scintillator is in demand for the detection of high‐energy neutrons in the presence of gamma radiation background. Conventional PSD plastics are based on delayed fluorescent dye via triplet–triplet annihilation of a fluorescent dye additive, which is inefficient to harvest the energy of triplet excitons. In recent years, thermally activated delayed fluorescence (TADF) emitters have gained substantial success in organic light‐emitting diodes due to their efficient utilization of the energy of triplet excitons and thereby achieving theoretical unity internal quantum efficiencies. Herein, a highly efficient TADF dye 9‐(4‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)‐2‐methylphenyl)‐3,6‐dioctyl‐9H‐carbazole is reported for the PSD application. This TADF dye may be loaded up to 30 wt% in polyvinyltoluene matrix and still retains high optical transparency. High scintillation light yield and PSD figures of merit are obtained from the TADF plastic. With the introduction of a secondary dye to further increase the utilization efficiency of the excitation energy, the light yield is increased to 6948 photons MeV−1. The measured alpha/gamma PSD figure of merit is 1.12 at the energy threshold of 100 keVee and neutron/gamma PSD figure of merit is 1.32 at the threshold of 1000 keVee.

Thermally Activated Delayed Fluorescence Benzyl Cellulose Derivatives for Nondoped Organic Light-Emitting Diodes

Thermally activated delayed fluorescence (TADF) benzyl cellulose derivatives (TBC-X), which contained both carbazole (host) and phthalimide-based TADF dye (guest) moieties, were prepared from 2,3-d...

Silica nanoparticles with thermally activated delayed fluorescence for live cell imaging.

Thermally activated delayed fluorescence (TADF) has revolutionized the field of organic light emitting diodes owing to the possibility of harvesting non-emissive triplet states and converting them in emissive singlet states. This mechanism generates a long-lived delayed fluorescence component which can also be used in sensing oxygen concentration, measuring local temperature, or on imaging. Despite this strong potential, only recently TADF has emerged as a powerful tool to develop metal-free long-lived luminescent probes for imaging and sensing. The application of TADF molecules in aqueous and/or biological media requires specific structural features that allow complexation with biomolecules or enable emission in the aggregated state, in order to retain the delayed fluorescence that is characteristic of these compounds. Herein we demonstrate a facile method that maintains the optical properties of solvated dyes by dispersing TADF molecules in nanoparticles. TADF dye-doped silica nanoparticles are prepared using a modified fluorescein fluorophore. However, the strategy can be used with many other TADF dyes. The covalent grafting of the TADF emitter into the inorganic matrix effectively preserves and transfers the optical properties of the free dye into the luminescent nanomaterials. Importantly, the silica matrix is efficient in shielding the dye from solvent polarity effects and increases delayed fluorescence lifetime. The prepared nanoparticles are effectively internalized by human cells, even at low incubation concentrations, localizing primarily in the cytosol, enabling fluorescence microscopy imaging at low dye concentrations.

Excited-state engineering of universal ambipolar hosts for highly efficient blue phosphorescence and thermally activated delayed fluorescence organic light-emitting diodes

The development of high-performance universal host materials for both blue electrophosphorescence and thermally activated delayed fluorescence (TADF) is one of the main challenges for organic light-emitting diode (OLED) technology. In this work, based on four quaternary ambipolar hosts with insulated donor-acceptor (D-A) structures, the influence of the excited-state characteristics for these hosts on their device performance was systematically investigated. It was showed that on the premise of the similar electrical properties and excited-state energy levels for positive host-dopant energy transfer (ET), the weak exciton capture ability, embodied as low exciton binding energy (Eb), for the hosts could facilitate the Dexter ET to the dopant through charge migration. Since the charge-transfer -featured excited states and small Eb of blue TADF emitters, in contrast to the blue phosphorescence (PH) counterparts, the influence of the host Eb on the device efficiencies of blue TADF dyes was dramatically enlarged, reflecting the higher sensitivity of the latter to host characteristics. In consequence, 9CzFDBFDPO with the smallest Eb endowed its blue TADF and PH devices with the state-of-the-art performance, e.g. EQE beyond 20% and EQE roll-off within 20% at 500 cd m−2. This work showed that based on the systematically optimized excited-state characteristics, the ambipolar hosts can be competent in both high-efficiency blue PH and TADF OLEDs, which is crucial for their commercial applications.

Plastic scintillators based on thermally activated delayed fluorescence dyes

Abstract Thermally activated delayed fluorescence (TADF) dyes are a class of luminescent organic compounds capable of the conversion of triplet to singlet excited states for increased photon emission efficiency, and these classes of dyes have recently attracted a great deal of attention for their potential use in highly efficient organic light emitting diodes (OLEDs). For organic scintillators utilizing conventional fluorescent dyes, triplet excited states generated from energy deposition of the incident particle’s energy can be converted to singlets via triplet–triplet annihilation, which leads to the delayed pulse tail characteristic of organic scintillators, but is not overall a particularly efficient process for triplet to singlet conversion. In this work, we investigated the applicability of the use of TADF dyes in organic scintillators, in particular for their capability for triplet to singlet conversion, and how these unique properties impact the overall scintillation photophysics. In particular, the prospects of using TADF dyes as a means for increased light yield as well as for efficient pulse shape discrimination (PSD) properties are directly addressed. In doing do, we explored the prominent effect that the dye’s singlet–triplet splitting energy ( Δ E ST ) – a parameter fundamental to the realization of efficient TADF processes – has on determining these properties, and our findings suggest that TADF dyes could eventually enable an entirely new generation of high performance organic scintillators.

Spirobicyclic host material with pseudo-intramolecular charge transfer: Improving color purity of high-performance pure-blue and white thermally activated delayed fluorescence diodes

Developing high-performance pure-blue and white organic light-emitting diodes (OLED) is one of the biggest challenges for thermally activated delayed fluorescence (TADF) technology, although great efforts have been made in designing new blue TADF emitters. Herein, we demonstrate that by virtue of solvatochromic characteristics for TADF molecules, host engineering can provide a feasible alternative to realize the high color purity and the state-of-the-art efficiencies on the basis of conventional blue TADF dyes. An electron-withdrawing diphenylphosphine oxide (DPPO) group is introduced at 4 position of symmetrical spirobi[xanthene] to construct 9,9′-spirobi[xanthen]-4-yldiphenylphosphine oxide (SXSPO) with the asymmetry of configuration and optoelectronic properties. Besides forming the ambipolar property and suppressing quenching, the steric and inductive effects of DPPO are spatially confined, which prevents the formation of high-polarity charge transfer excited states and the interaction between polar PO group and TADF dopants. Based on a conventional TADF emitter bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS), SXSPO dramatically improves the color purity of its pure-blue and white devices and realizes the external quantum efficiencies (ηEQE) beyond 20%. The power efficiency of the white devices reaches to ∼60 lm W−1, demonstrating the potential applications as lighting source.

Thermally activated delayed fluorescence (TADF) dyes as efficient organic photosensitizers for photocatalytic water reduction

An efficient water reduction system was developed using thermally activated delayed fluorescence (TADF) dyes as organic photosensitizers and a palladium complex as water reduction catalyst. 2,4,5,6-tetrakis(carbazol-9-yl)-1,3-dicyanobenzene (4CzIPN) that has the best fluorescence efficiency displayed the highest photoinduced productivity (TON up to 2567) and was active for 5 days. Based on time-resolved photoluminescence experiments on the photocatalytic system, reductive quenching of the TADF dye was found as main pathway occurring before intersystem crossing takes place.

Optimizing the Intralayer and Interlayer Compatibility for High-Efficiency Blue Thermally Activated Delayed Fluorescence Diodes.

A series of phosphine oxide hosts, 4,6-bis(diphenylphosphoryl) dibenzothiophene (DBTDPO) and 4- diphenylphosphoryldibenzothiophene (DBTSPO), and electron transporting materials (ETM), 2-(diphenylphosphoryl)dibenzothiophene sulfone (2DBSOSPO), 3-(diphenylphosphoryl)dibenzothiophene sulfone (3DBSOSPO) and 4-(diphenylphosphoryl)dibenzothiophene sulfone (4DBSOSPO) were developed to support blue thermally activated delayed fluorescence (TADF) devices with high performance through optimizing intralayer and interlayer compatibility of emissive layers. On the basis of the triplet energy of ~3.0 eV for the hosts and ETMs, excitons can be effectively confined on DMAC-DPS. Compared to DBTSPO, DBTDPO can support the excellent distribution uniformity to blue TADF dye bis[4-(9,9-dimethyl–9,10-dihydroacridine) phenyl] sulfone (DMAC-DPS), owing to their configuration similarity; while 3DBSOSPO and 4DBSOSPO are superior in compatibility with the hosts due to the similar molecular polarity or configuration. Through adjusting the molecular configuration, the electrical performance of ETMs can be feasibly tuned, including the excellent electron mobility (μe) by the order of 10−3 cm2 V−1 s−1. As the result, DBTDPO and 4DBSOSPO endowed their four-layer blue TADF devices with the maximum current efficiency of 33.5 cd A−1 and the maximum external quantum efficiency more than 17%, which are impressive among the best blue TADF devices. It is showed that intralayer compatibility determines the maximum efficiencies, while interlayer compatibility influences efficiency stability.

Solid state photon upconversion utilizing thermally activated delayed fluorescence molecules as triplet sensitizer

The ability to upconvert light is useful for a range of applications, from biological imaging to solar cells. But modern technologies have struggled to upconvert incoherent incident light at low intensities. Here, we report solid state photon upconversion employing triplet-triplet exciton annihilation in an organic semiconductor, sensitized by a thermally activated-delayed fluorescence (TADF) dye. Compared to conventional phosphorescent sensitizers, the TADF dye maximizes the wavelength shift in upconversion due to its small singlet-triplet splitting. The efficiency of energy transfer from the TADF dye is 9.1%, and the conversion yield of sensitizer exciton pairs to singlet excitons in the annihilator is 1.1%. Our results demonstrate upconversion in solid state geometries and with non-heavy metal-based sensitizer materials.

Dibenzothiophene-Based Phosphine Oxide Host and Electron-Transporting Materials for Efficient Blue Thermally Activated Delayed Fluorescence Diodes through Compatibility Optimization

Thermally activated delayed fluorescence (TADF) organic light-emitting diodes arise from the development of high-performance host materials and carrier transporting materials fitting for TADF dyes with optimized respective properties and interplays, making simultaneous performance improvement and device structure simplification feasible. In this work, a highly efficient blue TADF diode with simplified four-layer structure was successfully achieved by utilizing bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS) as blue emitter, 4,6-bis(diphenylphosphoryl)dibenzothiophene (DBTDPO) as host, and 4,6-bis(diphenylphosphoryl)dibenzothiophene sulfone (46DBSODPO) as electron-transporting layer. The compatibilities between DBTDPO and DMAC-DPS and DBTDPO and 46DBSODPO were optimized with respect to configuration, polarity, energy level, and interfacial interaction, resulting in the unchanged roughness of ∼0.25 nm before and after doping, high photoluminescence quantum yield over 85%, and reduced inte...