Charge-transfer Triplet State Research Articles

Anion-π-Induced Room Temperature Phosphorescence from Emissive Charge-Transfer States.

The burgeoning noncovalent interactions between π-acidic aromatic surfaces and anions have been recently shown to have unique functional relevance in anion transport, ion sensing, and organocatalysis. Despite its potential to instigate charge-transfer (CT) states, modulation of the emission features by toggling between the excited states using anion-π interactions is not yet explored. On the other hand, excited states with CT characteristics play an important role in the ambient triplet harvesting of organic chromophores. In this context, herein we propose an anion-π-based molecular design for the introduction of emissive singlet and triplet CT excited states, thereby expanding the functional scope of these weak supramolecular interactions. In the present study, we investigate the anion-π-induced emission from the singlet (1CT) and triplet (3CT) CT states of a dibromo dicationic pyromellitic diimide derivative. Remarkably, we accomplish dual room temperature phosphorescence emission from the anion-π-mediated 3CT state along with the locally excited triplet state (3LE) in solution phase using an organic-inorganic supramolecular scaffolding strategy. Comprehensive steady-state and time-resolved spectroscopy along with theoretical calculations provide detailed insights into the excited-state manifolds of phosphor. We envisage that the present study will expedite new molecular designs based on weak intermolecular interactions for the excited-state engineering of organic chromophores to facilitate ambient triplet harvesting and CT emission.

Sky Blue and Yellow Cluster Light-Emitting Diodes Based on Asymmetric Cu4I4 Nanocubes.

Controllably optimizing excited-state characteristics is crucial for luminescent nanoclusters but remains a formidable challenge. Herein, we report an effective "ligand-induced asymmetrization" strategy for constructing thermally activated delayed fluorescence-featured cubic Cu4I4 nanoclusters with asymmetric configurations, named [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4. Through changing 3,6-di-tert-butyl-carbazole (tBCz) to phenothiazine (PTZ) with a stronger electron-donating effect, emission color is tuned from greenish blue of [tBCzDBFDP]2Cu4I4 to yellow of [PTZDBFDP]2Cu4I4, as well as the triplet locally excited state of the former to the triplet charge transfer state of the latter. Temperature-correlated spectroscopic investigation indicates that in terms of triplet quenching suppression, [tBCzDBFDP]2Cu4I4 is superior to [PTZDBFDP]2Cu4I4, in accord with the stabilities of their triplet locally excited state and triplet charge transfer state. As a consequence, these asymmetric Cu4I4 nanocubes endowed their cluster light-emitting diodes with the external quantum efficiencies beyond 12% for sky blue and 8% for yellow. These results suggest the significance and effectiveness of ligand engineering for optoelectronic nanoclusters.

String-Attached Oligothiophene Substituents Determine the Fate of Excited States in Ruthenium Complexes for Photodynamic Therapy.

We explore the photophysical properties of a family of Ru(II) complexes, Ru-ip-nT, designed as photosensitizers (PSs) for photodynamic therapy (PDT). The complexes incorporate a 1H-imidazo[4,5-f][1,10]-phenanthroline (ip) ligand appended to one or more thiophene rings. One of the complexes studied herein, Ru-ip-3T (known as TLD1433), is currently in phase II human clinical trials for treating bladder cancer by PDT. The potent photocytotoxicity of Ru-ip-3T is attributed to a long-lived intraligand charge-transfer triplet state. The accessibility of this state changes upon varying the length (n) of the oligothiophene substituent. In this paper, we highlight the impact of n on the ultrafast photoinduced dynamics in Ru-ip-nT, leading to the formation of the function-determining long-lived state. Femtosecond time-resolved transient absorption combined with resonance Raman data was used to map the excited-state relaxation processes from the Franck-Condon point of absorption to the formation of the lowest-energy triplet excited state, which is a triplet metal-to-ligand charge-transfer excited state for Ru-ip-0T-1T and an oligothienyl-localized triplet intraligand charge-transfer excited state for Ru-ip-2T-4T. We establish the structure-activity relationships with regard to changes in the excited-state dynamics as a function of thiophene chain length, which alters the photophysics of the complexes and presumably impacts the photocytotoxicity of these PSs.

“On-off” switch between red thermally activated delayed fluorescence and conventional fluorescence by isomeric regulation

Marching toward highly efficient long-wavelength organic light-emitting diodes (OLEDs) is paramount but formidable challenge, and rational manipulation of molecular configuration and excited state dynamic processes are of great importance to harvest dark triplet exciton and suppress non-radiative transition for excellent light emission properties. Herein, we introduced a novel electron acceptor 11H-indeno [1,2-b] quinoxalin-11-one (IQ) as a building-block to construct two bright red light-emitting regioisomers of IQ-oTPA and IQ-pTPA, which exhibit entirely different exciton dynamic processes of thermally activated delayed fluorescence (TADF) and conventional fluorescence (CF) respectively. According to theoretical calculation and photophysical characterization, the energy gap, energy level alignment and spin–orbit coupling (SOC) effects of charge-transfer singlet state (1CT), local excited triplet state (3LE) and charge-transfer triplet state (3CT) together with the molecular geometry rigidity play a critical role in the triplet exciton up-conversion processes. Maximum external quantum efficiencies of 20.6% and 3.5% with emission peaks of 604 and 642 nm were achieved for the proof-of-concept electroluminescent devices based on IQ-oTPA and IQ-pTPA respectively, which are among the state-of-the-art performance for red-emission TADF and CF OLEDs and provide new insights for the molecular design tactic of high-performance red OLEDs.

Electron spin resonance resolves intermediate triplet states in delayed fluorescence

Molecular organic fluorophores are currently used in organic light-emitting diodes, though non-emissive triplet excitons generated in devices incorporating conventional fluorophores limit the efficiency. This limit can be overcome in materials that have intramolecular charge-transfer excitonic states and associated small singlet-triplet energy separations; triplets can then be converted to emissive singlet excitons resulting in efficient delayed fluorescence. However, the mechanistic details of the spin interconversion have not yet been fully resolved. We report transient electron spin resonance studies that allow direct probing of the spin conversion in a series of delayed fluorescence fluorophores with varying energy gaps between local excitation and charge-transfer triplet states. The observation of distinct triplet signals, unusual in transient electron spin resonance, suggests that multiple triplet states mediate the photophysics for efficient light emission in delayed fluorescence emitters. We reveal that as the energy separation between local excitation and charge-transfer triplet states decreases, spin interconversion changes from a direct, singlet-triplet mechanism to an indirect mechanism involving intermediate states.

Charge Recycling Mechanism Through a Triplet Charge-Transfer State in Ternary-Blend Organic Solar Cells Containing a Nonfullerene Acceptor

Charge Recycling Mechanism Through a Triplet Charge-Transfer State in Ternary-Blend Organic Solar Cells Containing a Nonfullerene Acceptor

Dihedral Angle Distribution of Thermally Activated Delayed Fluorescence Molecules in Solids Induces Dual Phosphorescence from Charge-Transfer and Local Triplet States

Dihedral Angle Distribution of Thermally Activated Delayed Fluorescence Molecules in Solids Induces Dual Phosphorescence from Charge-Transfer and Local Triplet States

Nonradiative Triplet Loss Suppressed in Organic Photovoltaic Blends with Fluoridated Nonfullerene Acceptors

In organic photovoltaic (OPV) blends, photogenerated excitons dissociate into charge-separated electrons and holes at donor/acceptor interfaces. The bimolecular recombination of spin-uncorrelated electrons and holes may cause nonradiative loss by forming the low-lying triplet excited states (T1) via the intermediate charge-transfer triplet states. Here, we show that such a spin-related loss channel can be suppressed in the OPV blends with fluorinated nonfullerene acceptors (NFAs). By combining ultrafast optical spectroscopy and triplet sensitization measurements, the T1 states at the acceptors have been observed to generate from the charge-separated electrons and holes in the OPV blends with a same polymer donor and two sets of NFAs with and without fluorination. The triplet formation is largely suppressed and the lifetime of charge carrier is markedly prolonged in the blends with fluorinated NFAs. The fluorination effect on the charge dynamics can be ascribed to the modified energy alignment between the triplet excited states of charge-transfer and locally excited characters as supported by quantum chemical computation. Our findings explain the mechanism responsible for the improved photocurrent generation in the OPV blends with fluorinated NFAs, suggesting that manipulating the energy landscape of triplet excited states is a promising strategy for further optimizing OPV devices.

Efficient Direct Reverse Intersystem Crossing between Charge Transfer-Type Singlet and Triplet States in a Purely Organic Molecule.

In the field of organic light-emitting diodes, thermally activated delayed fluorescence (TADF) materials have achieved great performance. The key factor for this performance is the small energy gap (ΔEST ) between the lowest triplet (T1 ) and singlet excited (S1 ) states, which can be realized in a well-separated donor-acceptor system. Such systems are likely to possess similar charge transfer (CT)-type T1 and S1 states. Recent investigations have suggested that the intervention of other type-states, such as locally excited triplet state(s), is necessary for efficient reverse intersystem crossing (RISC). Here, we theoretically and experimentally demonstrate that our blue TADF material exhibits efficient RISC even between singlet CT and triplet CT states without any additional states. The key factor is dynamic flexibility of the torsion angle between the donor and acceptor, which enhances spin-orbit coupling even between the charge transfer-type T1 and S1 states, without sacrificing the small ΔEST . This results in excellent photoluminescence and electroluminescence performances in all the host materials we investigate, with sky-blue to deep-blue emissions. Among the hosts investigated, the deepest blue emission with CIE coordinates of (0.15, 0.16) and the highest EQEMAX of 23.9 % are achieved simultaneously.

TD-DFT and Experimental Methods for Unraveling the Energy Distribution of Charge-Transfer Triplet/Singlet States of a TADF Molecule in a Frozen Matrix.

Reverse intersystem crossing (RISC) rate of a thermally activated delayed fluorescence (TADF) molecule is sensitive to the energy alignment of the singlet charge-transfer state (1CT), triplet charge-transfer state (3CT), and locally excited triplet state (3LE). However, the energy distribution of the charge-transfer states originating from the conformational distribution of TADF molecules in a solid matrix inevitably generated during the preparation of a solid sample due to the rotatable donor-acceptor linkage is rarely considered. Moreover, the investigation of the energy distribution of the 3CT state is both theoretically and experimentally difficult due to the triplet instabilities of time-dependent density functional (TD-DFT) calculations and difficulties in phosphorescence measurements, respectively. As a result, the relationships between conformational distribution, configurations of excited state transition orbitals, and excited state energies/dynamics have not been clearly explained. In this work, we determined the energy distribution of CT states of the TADF emitter TPSA in frozen toluene at 77 K by the measurement of time-resolved spectra in the full time range (1 ns to 30 s) of emission including prompt fluorescence, TADF, 3CT phosphorescence, and 3LE phosphorescence. We obtained the energy band of CT states where 1CT and 3CT states are distributed in the range of 2.85-3.00 and 2.64-2.96 eV, respectively. We tested various global hybrid and long-range corrected functionals for the TD-DFT calculation of 3CT energy of TPSA and found that only the M11 functional shows consistent results without triplet instability. We performed TD-DFT with the M11* functional optimized for a robust dihedral angle scan of 3CT states without triplet instability and reproduced the energy band structure obtained from the experiment. Through TD-DFT and experimental investigations, it is estimated that the dihedral angles of donor-acceptor (θD-A) and acceptor-linker (θA) of TPSA in frozen toluene lie within the range 70° ≤ θD-A ≤ 90° and 0° ≤ θA ≤ 30° respectively. Our results show that the dihedral angle distribution must be considered for further investigation of the photophysics of TADF molecules and the development of stable and efficient TADF emitters.

Spiral Donor Design Strategy for Blue Thermally Activated Delayed Fluorescence Emitters.

Thermally activated delayed fluorescence (TADF) emitters with a spiral donor show tremendous potential toward high-level efficient blue organic light-emitting diodes (OLEDs). However, the underlying design strategy of the spiral donor used for blue TADF emitters remains unclear. As a consequence, researchers often do "try and error" work in the development of new functional spiral donor fragments, making it slow and inefficient. Herein, we demonstrate that the energy level relationships between the spiral donor and the luminophore lead to a significant effect on the photoluminescent quantum yields (PLQYs) of the target materials. In addition, a method involving quantum chemistry simulations that can accurately predict the aforementioned energy level relationships by simulating the spin density distributions of the triplet excited states of the spiral donor and corresponding TADF emitters and the triplet excited natural transition orbitals of the TADF emitters is established. Moreover, it also revealed that the steric hindrance in this series of molecules can form a nearly unchanged singlet (S1) state geometry, leading to a reduced nonradiative decay and high PLQY, while a moderated donor-acceptor (D-A) torsion in the triplet (T1) state can induce a strong vibronic coupling between the charge-transfer triplet (3CT) state and the local triplet (3LE) state, achieving an effective reverse intersystem crossing (RISC) process. Furthermore, an electric-magnetic coupling is formed between the high-lying 3LE state and the charge-transfer singlet (1CT) state, which may open another RISC channel. Remarkably, in company with the optimized molecular structure and energy alignment, the pivotal TADF emitter DspiroS-TRZ achieved 99.9% PLQY, an external quantum efficiency (EQE) of 38.4%, which is the highest among all blue TADF emitters reported to date.

The Leap from Organic Light-Emitting Diodes to Organic Semiconductor Laser Diodes

In recent years, organic light-emitting device technology has expanded from organic light-emitting diodes (OLEDs) to organic semiconductor laser diodes (OSLDs) with the progress of sophisticated mo...

Computationally Guided Molecular Design to Minimize the LE/CT Gap in D-π-A Fluorinated Triarylboranes for Efficient TADF via D and π-Bridge Tuning.

In this combined experimental and theoretical study, a computational protocol is reported to predict the excited states in D‐π‐A compounds containing the B(FXyl)2 (FXyl = 2,6‐bis(trifluoromethyl)phenyl) acceptor group for the design of new thermally activated delayed fluorescence (TADF) emitters. To this end, the effect of different donor and π‐bridge moieties on the energy gaps between local and charge‐transfer singlet and triplet states is examined. To prove this computationally aided design concept, the D‐π‐B(FXyl)2 compounds 1–5 were synthesized and fully characterized. The photophysical properties of these compounds in various solvents, polymeric film, and in a frozen matrix were investigated in detail and show excellent agreement with the computationally obtained data. Furthermore, a simple structure–property relationship is presented on the basis of the molecular fragment orbitals of the donor and the π‐bridge, which minimize the relevant singlet–triplet gaps to achieve efficient TADF emitters.

Efficient Intersystem Crossing in the Tröger's Base Derived From 4-Amino-1,8-naphthalimide and Application as a Potent Photodynamic Therapy Reagent.

Intersystem crossing (ISC) was observed for naphthalimide (NI)-derived Tröger's base, and the ISC was confirmed to occur by a spin-orbital charge-transfer (SOCT) mechanism. Conventional electron donor/acceptor dyads showing SOCT-ISC have semirigid linkers. In contrast, the linker between the two chromophores in Tröger's base is rigid and torsion is completely inhibited, which is beneficial for efficient SOCT-ISC. Femtosecond transient absorption (TA) spectra demonstrated charge-separation and charge-recombination-induced ISC processes. Nanosecond TA spectroscopy confirmed the ISC, and the triplet state is long-lived (46 μs, room temperature). The ISC quantum yield is dependent on solvent polarity (8-41 %). The triplet state was studied by pulsed-laser-excited time-resolved EPR spectroscopy, and both the NI-localized triplet state and triplet charge-transfer state were observed, which is in good agreement with the spin-density analysis. The Tröger's base was confirmed to be a potent photodynamic therapy reagent with HeLa cells (EC50 =5.0 nm).

Sensitizing TADF Absorption Using Variable Length Oligo(phenylene ethynylene) Antennae.

Beyond their applications in organic light-emitting diodes (OLEDs), thermally activated delayed fluorescence (TADF) materials can also make good photonic markers. Time-gated measurement of their delayed emission enables “background-free” imaging in, for example, biological systems, because no naturally-occurring compounds exhibit such long-lived emission. Attaching a strongly-absorbing antenna, such as a phenylene ethynylene oligomer, to the TADF core would be of interest to increase their brightness as photonic markers. With this motivation, we study a sequence of TADF-oligomer conjugates with oligomers of varying length and show that, even when the absorption of the oligomer is almost resonant with the charge-transfer absorption of the TADF core, the antenna transfers energy to the TADF core. We study this series of compounds with time resolved emission and transient absorption spectroscopy and find that the delayed fluorescence is essentially turned-off for the longer antennae. Interestingly, we find that the turn-off of the delayed fluorescence is not caused by quenching of the TADF charge-transfer triplet state due to triplet energy transfer of the lower-lying triplet state to the antenna, but must be associated with a decrease in the reverse intersystem crossing rate. These results are of relevance for the further development of TADF “dyes” and also, in the broader context, for understanding the dynamics of TADF molecules in the vicinity of energy donors/acceptors (i.e., in fluorescent OLEDs wherein TADF molecules are used as an assistant dopant).

Bridging Small Molecules to Conjugated Polymers: Efficient Thermally Activated Delayed Fluorescence with a Methyl-Substituted Phenylene Linker.

Based on a "TADF + Linker" strategy (TADF=thermally activated delayed fluorescence), demonstrated here is the successful construction of conjugated polymers that allow highly efficient delayed fluorescence. Small molecular TADF blocks are linked together using a methyl-substituted phenylene linker to form polymers. With the growing number of methyl groups on the phenylene, the energy level of the local excited triplet state (3 LEb ) from the delocalized polymer backbone gradually increases, and finally surpasses the charge-transfer triplet state (3 CT). As a result, the diminished delayed fluorescence can be recovered for the tetramethyl phenylene containing polymer, revealing a record-high external quantum efficiency (EQE) of 23.5 % (68.8 cd A-1 , 60.0 lm W-1 ) and Commission Internationale de l'Eclairage (CIE) coordinates of (0.25, 0.52). Combined with an orange-red TADF emitter, a bright white electroluminescence is also obtained with a peak EQE of 20.9 % (61.1 cd A-1 , 56.4 lm W-1 ) and CIE coordinates of (0.36, 0.51).

Emission wavelength dependence on the rISC rate in TADF compounds with large conformational disorder.

Large vibronic coupling between the local and charge-transfer triplet states is required for efficient reverse intersystem crossing in TADF compounds. This is ensured by low steric hindrance between donor and acceptor molecular units. However, flexible molecular cores show large conformational disorder and emission wavelength instability in solid films.

Balancing charge-transfer strength and triplet states for deep-blue thermally activated delayed fluorescence with an unconventional electron rich dibenzothiophene acceptor

Manipulation of the emission properties of deep-blue emitters exhibiting thermally activated delayed fluorescence (TADF) through molecular design is reported.

Experimental Evidence for a Triplet Biradical Excited-State Mechanism in the Photoreactivity of N,C-Chelate Organoboron Compounds.

N,C-chelate organoborates represent an emerging class of photoresponsive materials due to their photochromic switching at a boron center. Despite the promising applicability of such systems, little is known about the excited-state processes that lead to their unique photoreactivity, which is detrimental to the design of next-generation smart materials based on boron. As part of our ongoing effort to understand and improve the utility of these organoboron compounds, we report some of the first experimental evidence to support an excited-state mechanism for N,C-chelate organoborates. Femtosecond transient absorption spectroscopy combined with steady-state UV/vis and fluorescence measurements gives direct insight into their underlying photochemical processes, such as the formation of a common triplet charge-transfer state which either relaxes radiatively or undergoes the desired photoisomerization through a biradical intermediate. With this information, a complete mechanistic picture of the excited-state reactivity of N,C-chelate organoborates has been established, which is anticipated to lead to new smart materials with improved performance.

Donor-Acceptor Fluorophores for Energy-Transfer-Mediated Photocatalysis.

Triplet-triplet energy transfer (EnT) is a fundamental activation pathway in photocatalysis. In this work, we report the mechanistic origins of the triplet excited state of carbazole-cyanobenzene donor-acceptor (D-A) fluorophores in EnT-based photocatalytic reactions and demonstrate the key factors that control the accessibility of the 3LE (locally excited triplet state) and 3CT (charge-transfer triplet state) via a combined photochemical and transient absorption spectroscopic study. We found that the energy order between 1CT (charge transfer singlet state) and 3LE dictates the accessibility of 3LE/3CT for EnT, which can be effectively engineered by varying solvent polarity and D-A character to depopulate 3LE and facilitate EnT from the chemically more tunable 3CT state for photosensitization. Following the above design principle, a new D-A fluorophore with strong D-A character and weak redox potential is identified, which exhibits high efficiency for Ni(II)-catalyzed cross-coupling of carboxylic acids and aryl halides with a wide substrate scope and high selectivity. Our results not only provide key fundamental insight on the EnT mechanism of D-A fluorophores but also establish its wide utility in EnT-mediated photocatalytic reactions.