Singlet-triplet Energy Difference Research Articles

Multifunctional Deep-Blue Thermally Activated Delayed Fluorescence Based on an Oxygen-Bridged Boron Acceptor for Highly Efficient Organic Light-Emitting Diodes.

Until now, thermally activated delayed fluorescence (TADF) materials based on bridged boron-based acceptors have been primarily developed as dopants. However, in this study, we synthesized and characterized multifunctional deep-blue TADF materials─t-OBO-DMAC and t-OBO-DPAC─using bridged boron-based acceptors in combination with dimethylacridine or diphenylacridine as donors. These materials serve as both dopants and hosts. Theoretical calculations and experimentally measured photophysical properties of t-OBO-DMAC reveal a smaller singlet-triplet energy difference, higher photoluminescence quantum yield, and more efficient reverse intersystem crossing compared to t-OBO-DPAC. When evaluated as TADF emitters, t-OBO-DMAC and t-OBO-DPAC exhibited maximum external quantum efficiency (EQE) of 14.4 and 7.3% with deep-blue color coordinates of (0.14, 0.11) and (0.15, 0.07), respectively. Both materials were further assessed as hosts in various configurations, including host-only, TADF, phosphorescent, and phosphor-sensitized fluorescence (PSF)-emitting systems. Notably, t-OBO-DMAC demonstrated a high maximum EQE of 13.9% with deep-blue color coordinates of (0.15, 0.07) in a nondoped host-only device. Remarkably, both materials achieved EQEs exceeding 20% in the PSF devices. Our study marks a critical advancement in the field that breaks the conventional boundaries of the dopant and host and demonstrates unprecedented multifunctionalities for advanced organic light-emitting diodes.

0D and 1D-dimensional Cu(I)-based halides pyridyltriazoles basis: Synthesis, Structures, and photophysical properties

Three binuclear and two polymeric Cu(I) complexes namely, [Cu2(L1)2Cl2(PPh3)2] (1), [Cu2(L1)2Br2(PPh3)2] (2), [µ-L2(CuCl(PPh3)2)2]∙4MeOH (3), [CuBrL2(PPh3)]n (4) and [Cu2Cl2L2(PPh3)2]n (5) (3,5-bis(pyridin-4-yl)-1H-1,2,4-triazole (L1), 3,5-bis(pyridin-4-yl)-1H-1,2,4-triazole (L2) and 5-(pyridin-4-yl)-3-(pyridin-3-yl)-1H-1,2,4-triazole (L3)) were synthesized and structurally characterized. Binuclear complexes 1–3 show strong green luminescence in the crystals, exhibiting about 50 % photoluminescence quantum yields, while polymeric complexes show moderate emission in yellow region. The emission studies reveal that complexes 1–3 emit TADF from the 1(M + X)LCT state at room temperature because of their small singlet–triplet energy difference. Moreover complex 5 exhibited excellent stimuli-responsive photoluminescent performance in the solid state at room temperature.

New Perspectives on Delocalization Pathways in Aromatic Molecular Chameleons.

This study comprehensively analyzes the magnetically induced current density of polycyclic compounds labeled as "aromatic chameleons" since they can arrange their π-electrons to exhibit aromaticity in both the ground and the lowest triplet state. These compounds comprise benzenoid moieties fused to a central skeleton with 4n π-electrons and traditional magnetic descriptors are biased due to the superposition of local magnetic responses. In the S0 state, our analysis reveals that the molecular constituent fragments preserve their (anti)aromatic features in agreement with two types of resonant structures: one associated with aromatic benzenoids and the other with a central antiaromatic ring. Regarding the T1 state, a global and diatropic ring current is revealed. Our aromaticity study is complemented with advanced electronic and geometric descriptors to consider different aspects of aromaticity, particularly important in the evaluation of excited state aromaticity. Remarkably, these descriptors consistently align with the general features on the main delocalization pathways in polycyclic hydrocarbons consisting of fused 4n π-electron rings. Moreover, our study demonstrates an inverse correlation between the singlet-triplet energy difference and the antiaromatic character of the central ring in S0.

The novel thermally activated delayed fluorescence compounds based on trifluoromethylpyridine were synthesized for blue OLEDs

Seeking efficient blue thermally activated delayed fluorescence (TADF) luminous materials with a simple structure and applying them to organic light-emitting diodes (OLEDs) is still an urgent challenge. Herein, two novel D-π-A type blue TADF emitters with simple molecular structure, namely p-AcBPyCF3 and m-AcBPyCF3, featuring a trifluoromethylpyridine (PyCF3) as acceptor moiety, 9,9-dimethyl-tetrahydroacridine (Ac) as donor unit and benzene (B) as π-bridge, respectively, are first developed and compared in parallel. The analysis of single crystal reveals that p-AcBPyCF3 possesses clearly twisted geometry between the donor and acceptor units, as well as expanded molecular rigidity, thus leading to a smaller singlet-triplet energy difference and greatly improved emission efficiency. The p-AcBPyCF3 exhibits a photoluminescence quantum yield (PLQY) of 28.80 %, surpassing the PLQY value of 18.70 % for the m-AcBPyCF3. Compared to m-AcBPyCF3, the TADF-OLEDs based on p-AcBPyCF3 demonstrated exceptional properties, which suppresses the rotation of molecular conformation and can achieve high PLQY by reducing non radiative transformation processes, resulting in a maximum external quantum efficiency (EQE) of 9.26 % with peak at 470 nm and blue color coordinates of (0.18, 0.25). This work highlights that the simple PyCF3 group can be a promising electron acceptor moiety for the design of blue TADF materials.

Projective Measurement-Based Quantum Phase Difference Estimation Algorithm for the Direct Computation of Eigenenergy Differences on a Quantum Computer.

Quantum computers are capable of calculating the energy difference of two electronic states using the quantum phase difference estimation (QPDE) algorithm. The Bayesian inference-based implementations for the QPDE have been reported so far, but in this approach, the quality of the calculated energy difference depends on the input wave functions being used. Here, we report the inverse quantum Fourier transformation-based QPDE with Na of ancillary qubits, which allows us to compute the difference of eigenenergies based on the single-shot projective measurement. As proof-of-concept demonstrations, we report numerical experiments for the singlet-triplet energy difference of the hydrogen molecule and the vertical excitation energies of halogen-substituted methylenes (CHF, CHCl, CF2, CFCl, and CCl2) and formaldehyde (HCHO).

Substituent influence of heteroatom on the novel remote N-heterocyclic silylenes (RNHSis) using density functional theory

In this work, we have compared and contrasted the stability, polarity, polarizability, frequency, band gap, charge distribution, and global reactivity of singlet (s) and triplet (t) states of the fused benzo-4-sila-1,4-dihydropyridine-4,4-diyl as the reference structure (1-s and 1-t) as well as the fused pyrrole congeners (2x-s, and 2x-t, x = NH, PH, AsH, O, S, and Se), using density-functional theory (DFT). According to frequency data, all singlet and triplet remote N-heterocyclic silylene (RNHSi) structures appear as minimum displaying a positive force constant. Every singlet silylene as ground state exhibits more stability than its corresponding triplet congener. Consistent with Hoffmannʼs findings, the fusion of one pyrrole ring with different heteroatoms stabilizes the corresponding RNHSi more than the fusion of one benzene ring. In going from 1-s to 2x-s species and substituting of NH, PH, AsH, O, S, and Se groups, higher singlet–triplet energy difference (ΔE s-t = E t – E s) and the higher band gap is found for 2NH-s and 2 O-s structures. Interestingly, the higher thermodynamic and kinetic stability of the optimized RNHSis than the synthesized Kiraʼs silylene makes them worthy of synthetic exploration. According to the distributed charge and FMO (the frontier molecular orbital) shapes, the more negative region is localized on pyridineʼs nitrogen and the less negative region is confined over pyrroleʼs nitrogen, while the positive charge is distributed on silylenic center. This phenomenon is inconsistent with the expected canonical forms for RNHSis.

A Donor–Acceptor Cage for Thermally Activated Delayed Fluorescence: toward a New Kind of TADF Exciplex Emitters

We present the rational design of an organic cage combining an electron-donating (D) triazatruxene and electron-accepting (A) triphenyl triazine connected by three electronically inert pillars. This controlled architecture leads to a precise spatial separation and cofacial alignment of the D and A motifs. Such an architecture induces an efficient intramolecular through-space charge transfer (TSCT), providing a small singlet–triplet energy difference (ΔEST) of 0.049 eV, and thus gives rise to a thermally activated delayed fluorescence (TADF) macrocyclic exciplex emitter.

Excitation energies of polycylic aromatic hydrocarbons by double-hybrid functionals: Assessing the PBE0-DH and PBE-QIDH models and their range-separated versions.

A family of non-empirical double-hybrid (DH) density functionals, such as Perdew-Burke-Ernzerhof (PBE)0-DH, PBE-QIDH, and their range-separated exchange (RSX) versions RSX-0DH and RSX-QIDH, all using Perdew-Burke-Ernzerhof(PBE) exchange and correlationfunctionals, is applied here to calculate the excitation energies for increasingly longer linear and cyclic acenes as part of their intense benchmarking for excited states of all types. The energies for the two lowest-lying singlet 1La and 1Lb states of linear oligoacenes as well as the triplet 3La and 3Lb states, are calculated and compared with experimental results. These functionals clearly outperform the results obtained from hybrid functionals and favorably compare with other double-hybrid expressions also tested here, such as B2-PLYP, B2GP-PLYP, ωB2-PLYP, and ωB2GP-PLYP. The study is complemented by the computation of adiabatic S0-T1 singlet-triplet energy difference for linear acenes as well as the extension of the study to strained cyclic oligomers, showing how the family of non-empirical expressions robustly leads to competitive results.

Polyradical character assessment using multireference calculations and comparison with density-functional derived fractional occupation number weighted density analysis.

The biradicaloid character of different types of polycyclic aromatic hydrocarbons (PAHs) based on small band gaps is an important descriptor to assess their opto-electronic properties. In this work, the unpaired electron densities and numbers of unpaired electrons (NU values) calculated at the high-level multireference averaged quadratic coupled-cluster (MR-AQCC) method are used to develop a test set to assess the capabilities of different biradical descriptors based on density functional theory. A benchmark collection of 29 different compounds has been selected. The DFT descriptors contain primarily the fractional occupation number weighted electron density (FOD) based on simplified thermally-assisted-occupation density functional theory (TAO-DFT) calculations, but the singlet-triplet energy difference and other descriptors denoted as y0 and nLUNO have been considered as well. After adjustment of the literature-recommended finite temperatures, a very good, detailed agreement between unpaired density and FOD analysis is observed which is also manifested in excellent statistical correlations. The other two descriptors also show good correlations even though the absolute scaling is not satisfactory. A new linear fit of FOD data to the MR-AQCC reference values leads to an improved regression relation for determining the recommended finite temperature value in dependence of the Hartree-Fock exchange. This provides the basis for fast and reliable assessment of the biradical character of many classes of PAHs without the need for performing computationally extended MR calculations.

A TADF Emitter with Dual Para-Positioned Donors Enables OLEDs with Improved Efficiency and CIE Coordinates Close to the Rec. 2020 Red Standard

Developing red thermally activated delayed fluorescence (TADF) emitters concurrently with high efficiency and emission color close to the BT.2020 red standard is an ongoing challenge. Herein, we developed a new red TADF emitter BCN-TPA, in which two identical donors are attached at the para-positions of one fused phenyl ring in the acceptor framework. Such an arrangement mode can lead the donors with an obvious superimposed effect comparing the conventional arrangement with edge-capped donors on the acceptor. Thus, BCN-TPA yields enhanced overall donor strength with numerous superiorities, such as high oscillator strength and narrow singlet-triplet energy difference, thus giving rise to red-shifted emission with improved overall exciton utilization. In an organic light-emitting diode, BCN-TPA presents efficient deep-red electroluminescence with a maximum external quantum efficiency of 27.6% and a peak at 656 nm, corresponding to CIE coordinates of (0.686, 0.304), which are very close to the red primary in the BT.2020 standard. To the best of our knowledge, this is one of the topmost efficiencies in the field of deep-red TADF OLEDs. This work exemplifies an easy design principle for constructing high-performance deep-red TADF emitters, providing unique molecular-level insights toward improving color quality and elevating efficiency based on conventional D-A type molecular frameworks.

Highly Efficient Blue Thermally Activated Delayed Fluorescence Emitters with a Triphenylamine‐Based Macrocyclic Donor

AbstractThis work reports the incorporation of a triphenylamine‐based macrocyclic donor to design new donor‐π‐acceptor‐configured blue thermally activated delayed fluorescence (TADF) emitters. The X‐ray structure analyses manifest the degree of twisted conformations that can be modulated by methyl substituents of the π‐bridge and macrocyclic donor, leading to well‐separated highest occupied natural transition orbital and lowest unoccupied natural transition orbital frontier orbitals, thus sufficiently small singlet–triplet energy difference (ΔEST) for TADF. The theoretical analyses elucidate the structure–property relationship and reveal the beneficial effect of macrocyclic donor on increasing reverse intersystem crossing (RISC) process that can contribute to improved triplet‐upconversion efficiency. The blue device employing c‐NN‐TRZ as emitter gave a maximum external quantum efficiecny (EQEmax) of 26.3% as compared to that (19.1%) of the device using the model compound DPA‐MeTRZ without the macrocyclic donor, suggesting the contribution of macrocyclic donor to enhance device performance. Benefiting from the combined advantages of macrocyclic donor and methyl substituents, the device incorporating c‐NN‐MeTRZ as emitter achieves an outstanding EQEmax of 32.2%, which is attributed to the more horizontally oriented emission dipoles as well as the significantly accelerated RISC rate constant (kRISC) resulting from reduced ΔEST. This work represents a new strategy of designing twisted TADF emitter incorporating macrocyclic donor to achieve highly efficient blue device.

A Dislocated Twin-Locking Acceptor-Donor-Acceptor Configuration for Efficient Delayed Fluorescence with Multiple Through-Space Charge Transfer.

Organic materials featuring intramolecular through-space charge transfer (TSCT) excited states are advantageous for efficient thermally activated delayed fluorescence (TADF), although the realization of multiple TSCT systems remains challenging. Herein, a rigid molecule with a three-dimensional dislocated sandwich acceptor-donor-acceptor configuration has been developed by a linking biphenazine (2PXZ) donor and 2,4,6-triphenyl-1,3,5-triazine (TRZ) acceptor through the twin-locking of two spiro-fluorene bridges. The twin-locking construction with multiple TSCT effects suppresses the intramolecular rotations of various segments in 2PXZ-2TRZ, leading to a small singlet-triplet energy difference, a fast reverse intersystem crossing process, and high photoluminescence quantum yield. This material simultaneously possesses the capabilities of TADF and aggregation-induced emission. The device employing 2PXZ-2TRZ as a dopant displays an optimal external quantum efficiency of 27.1 % and a low efficiency roll-off.

Benefits of the Exciplex-like Framework in Reducing the Singlet-Triplet Energy Difference: A Theoretical Perspective on the Role of the Exciton Binding Energy

By means of post-Hartree-Fock and density functional theory calculations, we compare the exciplex-like U-type and conventional S-type thermally activated delayed fluorescence emitters which are composed of electron-donor (D), linker (L), and electron-acceptor (A) units: 10-phenyl-9,10-dihydroacridine, fluorene, and 2,4,6-triphenyl-1,3,5-triazine analogues, respectively. We found that the singlet-triplet energy difference, ΔEST, consistently decreases in going from the S-type emitters to their U-type counterparts, and this reduction in ΔEST is ascribed to the substantially more stable S1 state of the latter, while their T1 states remain similar in energy. Natural transition orbital pictures and excitation energy decomposition analyses demonstrate that the S1 states of the emitters are dominated by the charge transfer (CT) character and stabilized by the exciton binding energy, EB, which substantially enhances when the hole and electron are in close proximity. Without relying on the vague notion of through-space vs through-bond CT characters, we clearly showed that the exciplex-like molecular framework can effectively reduce ΔEST by taking advantage of the short distance between the D and A units and subsequently reinforcing EB for the D-to-A CT state.

Molecular design of blue thermally activated delayed fluorescent emitters for high efficiency solution processable OLED via an intramolecular locking strategy

Realizing high-efficiency blue emission in solution processable organic light-emitting diode (OLED) with thermally activated delayed fluorescent (TADF) emitters is still a big challenge. To suppress the non-radiative process in TADF emitters, compound DPS-BF-Ac featuring a donor-π-acceptor skeleton is prepared via intramolecular cyclization, in which diphenylsulfone (DPS), acridine (Ac) and furan derivatives are regarded as the donor, acceptor and π linker, respectively. Then, compounds DPS-Ph-Ac, DPS-OMe-Ac and DPS-OH-Ac with different π linkers are investigated to further explore the molecular structure–property relationship. All compounds show promising blue emission with a clear TADF character. Single crystal analysis demonstrate that compound DPS-BF-Ac possesses almost a perpendicular geometry between donor and acceptor groups and expanded molecular rigidity, leading to a smaller singlet–triplet energy difference and greatly improved emission efficiency. A remarkable external quantum efficiency (EQEmax) of ∼ 25 % is achieved for the DPS-BF-Ac based solution OLED, concomitant with the emission peak at ∼ 480 nm. Using DPS-BF-Ac and PO-01 as the blue and red dopant, respectively, the white OLED exhibits an EQEmax of ∼ 29 %. This research shows that intramolecular cyclization is an effective strategy for designing high efficiency solution-processable blue TADF emitter.

Synthesis and structural characterization of the first stable cycloheptatrienyl metal complexes bearing a CF3 moiety. DFT investigations of structures, energetics, NBO charges, and frontier MOs of W-CF3 and Mo-CF3 with η7-C7H7 and η5-C5H5

Thermally stable cycloheptatrienyl metal complexes bearing the trifluoromethyl moiety are reported for the first time. While the syntheses, properties and structural characterizations of transition metal complexes of the type [(η5–C5H5)MLnCF3] are well known in organometallic chemistry, analogous chemistry of the elusive [(η7–C7H7)MLnCF3] complexes was just recently disclosed by the syntheses of Mo complexes such as [(η7–C7H7)Mo(CO)(PMe3)CF3] (I). Nevertheless, complex I and similar complexes were found to be of extreme thermal instability, indicative in the fact that their purification and isolations require low temperature chromatography. Advancing past the stability hindrance, we report herein the syntheses of the W complexes [(η7-C7H7)W(CO)2CF3] (II), and [(η7–C7H7)W(CO)(PMe3)CF3] (III), which are found to be indefinitely thermally stable, a fact that allowed the study of complex II by X-ray crystallography. In addition, the first DFT studies on I-III, as well as their η5–C5H5 analogues, were performed both in the gas phase and with the implicit diethyl ether effects included, the structures, energetics, NBO charges, and frontier MOs were investigated. For the η7-C7H7-complexes, the metal change from Mo to W was shown to significantly increase the singlet–triplet energy difference, whereas for the η5-C5H5-complexes the metal change essentially has no effect on this difference, which is in line with the experimental findings. Comparison of the calculated structure of complex II with its X-ray structure showed quite reasonable agreement with the experimental data. The negative NBO charges on W in the C5H5-complexes were calculated to be significantly lower than in the C7H7-complexes, which again is completely in line with the stability profile of the complex.

A facile strategy for enhancing reverse intersystem crossing of red thermally activated delayed fluorescence emitters

By enhancing electron-withdrawing abilities of the secondary acceptor attachments into the main acceptor core, their charge transfer triplet ( 3 CT) energy levels are gradually reduced, while the locally excited triplet ( 3 LE A ) levels are well maintained. The well-aligned excited states, consequently, results in the enhancement of the rates of reverse intersystem crossing (RISC), and boosting the device efficiencies in red OLEDs. • A facile strategy to modulate CT states while maintaining LE state is proposed. • CT energy levels are gradually reduced, while the 3 LE A ones are well maintained. • Fast k RISC , small ΔE ST and high PLQY can be realized simultaneously. • mDPBPZ-DPXZ-based red OLED exhibits much higher EQE compared to its counterparts. Realizing highly efficient reverse intersystem crossing (RISC) process for red thermally activated delayed fluorescence (TADF) emitters remains a formidable challenge. Herein we demonstrate that charge transfer (CT) energy levels are gradually reduced with enhancing electron-withdrawing abilities of the secondary acceptor (A) attachments. Meanwhile, their local excited triplet ( 3 LE A ) state levels are well maintained due to the conjugations between the main framework and the attachments are all similar for these A segments. The optimized molecular energy level alignments are obtained in the case of mDPBPZ-DPXZ, in which 3 CT and the 3 LE A states are almost degenerate, and gives rise to a small singlet–triplet energy difference ( ΔE ST ) of 0.02 eV, a fast RISC rate of 1.54 × 10 6 s −1 and a high photoluminescence quantum yield of 93%, simultaneously. And the corresponding device achieves an external quantum efficiency of 21.6%, significantly higher than the other counterparts. This work demonstrates that the energy alignment between the CT and 3 LE A state can be easily managed by introducing secondary A attachments on original A framework, which shifts the 1 CT and 3 CT state to lower energies without affecting the 3 LE A energy. It provides a facile design strategy to promote the RISC process, which is particular useful in designing highly efficient red TADF emitters.

Molecular Insight into Efficient Charge Generation in Low-Driving-Force Nonfullerene Organic Solar Cells.

ConspectusFor organic solar cells (OSCs), charge generation at the donor/acceptor interfaces is regarded as a two-step process: driven by the interfacial energy offsets, the excitons produced by light absorption are first dissociated into the charge-transfer (CT) states, and then the CT states are further separated into free charge carriers of holes and electrons by overcoming their Coulomb attraction. Meanwhile, the CT states can recombine through radiative and nonradiative decay. Owing to the emergence of narrow-band-gap A-D-A small-molecule acceptors, nonfullerene (NF) OSCs have developed rapidly in recent years and the power conversion efficiencies (PCEs) surpass 18% now. The great achievement can be attributed to the high-yield charge generation under low exciton dissociation (ED) driving forces, which ensures both high photocurrent and small voltage loss. However, it is traditionally believed that a considerable driving force (e.g., at least 0.3 eV in fullerene-based OSCs) is essential to provide excess energy for the CT states to achieve efficient charge separation (CS). Therefore, a fundamental question open to the community is how the excitons split into free charge carriers so efficiently under low driving forces in the state-of-the-art NF OSCs.In this Account, we summarize our recent theoretical advances on the charge generation mechanisms in the low-driving-force NF OSCs. First, the A-D-A acceptors are found to dock with the D-A copolymer or A-D-A small-molecule donors mainly via local π-π interaction between their electron-withdrawing units, and such interfacial geometries can provide sufficient electronic couplings, thus ensuring fast ED. Second, the polarization energies of holes and electrons are enhanced during CS, which is beneficial to reduce the CS energy barrier and even leads to barrierless CS in the OSCs based on fluorinated A-D-A acceptors. Moreover, the exciton binding energies (Eb) are substantially decreased by the strong polarization of charge carriers for the A-D-A acceptors; especially for the Y6 system with three-dimensional molecular packing structures, the remarkable small Eb can enable direct photogeneration of free charge carriers. Accordingly, the excess energy becomes unnecessary for CS in the state-of-the-art NF OSCs. Third, to simultaneously decrease the driving force and suppress charge recombination via the triplet channel, it is imperative to reduce the singlet-triplet energy difference (ΔEST) of the narrow-band-gap A-D-A acceptors. Importantly, the intermolecular end-group π-π stacking is demonstrated to effectively decrease the ΔEST while keeping strong light absorption. Finally, hybridization of the CT states with local excitation can be induced by small interfacial energy offset. Such hybridization will result in direct population of thermalized CT states upon light absorption and a significant increase of luminescence quantum efficiency, which is beneficial to concurrently promote CS and reduce nonradiative voltage loss. We hope this Account contributes to the molecular understanding of the mechanisms of efficient charge generation with low driving forces and would be helpful for further improving the performance of organic photovoltaics in the future.

Using fullerene fragments as acceptors to construct thermally activated delayed fluorescence emitters for high-efficiency organic light-emitting diodes

Fragments of fullerene are used as acceptor segments for the first time to construct two thermally activated delayed fluorescence (TADF) compounds DBCP and FAP. Both compounds exhibit similar highly twisted geometries and separated frontier molecular orbital distributions. While on the other hand, their locally exited triplet energy levels from acceptor moieties (3LEA) are fine-tunable owning to the different structural stress in the fragments, resulting in different lowest triplet excited state (T1) features. In DBCP, the 3LEA is the high lying state and thus DBCP exhibits a small singlet–triplet energy difference of 0.10 eV and a high photoluminescence quantum yield (ΦPL) of 89% with excellent TADF characteristics; while T1 of FAP is 3LEA, and thus, FAP displays poor triplet exciton utilization with ΦPL of only 54%. In organic light-emitting diodes (OLEDs) based on DBCP and FAP as dopants, maximum external quantum efficiencies of 20.2% and 12.8% are respectively achieved. This work not only demonstrates for the first time that fullerene fragments can be used as key building blocks for TADF emitters, it also proves in principle that pure hydrocarbon mobilities without any electronegative heteroatom can also be employed as acceptor segment in high performance TADF emitters.

Aggregation Turns BODIPY Fluorophores into Photosensitizers: Reversibly Switching Intersystem Crossing On and Off for Smart Photodynamic Therapy

Aggregation Turns BODIPY Fluorophores into Photosensitizers: Reversibly Switching Intersystem Crossing On and Off for Smart Photodynamic Therapy

Accurate computed singlet-triplet energy differences for cobalt systems: implication for two-state reactivity.

Accurate singlet-triplet energy differences for cobalt and rhodium complexes were calculated by using several wave function methods, such as MRCISD, CASPT2, CCSD(T) and BCCD(T). Relaxed energy differences were obtained by considering the singlet and triplet complexes, each at the minimum of their potential energy surfaces. Active spaces for multireference calculations were carefully checked to provide accurate results. The considered systems are built by increasing progressively the first coordination sphere around the metal. We included in our set two CpCoX complexes (Cp = cyclopentadienyl, X = alkenyl ligand), which have been suggested as intermediates in cycloaddition reactions. Indeed, cobalt systems have been used for more than a decade as active species in this kind of transformations, for which a two-state reactivity has been proposed. Most of the considered systems display a triplet ground state. However, in the case of a reaction intermediate, while a triplet ground state was predicted on the basis of Density Functional Theory results, our calculations suggest a singlet ground state. This stems from the competition between the exchange term (stabilising the triplet) and the accessibility of an intramolecular coordination (stabilising the singlet). This finding has an impact on the general mechanism of the cycloaddition reaction. Analogous rhodium systems were also studied and, as expected, they have a larger tendency to electron pairing than cobalt species.