Excited State Properties Research Articles

Panchromatic Absorption of Unsymmetrical Donor-Acceptor Dyes Based on a Meta-conjugated 1,3,5-Heteroaryltriazine Core.

Attachment of three different heterocycles with electron donor or acceptor character to a central 1,3,5-triazine core generates readily soluble side-chain free dyes with two displaying soft crystalline mesomorphism and one displaying a nematic liquid crystal phase as confirmed by polarized optical microscopy, calorimetry, gravimetric analysis, and powder X-ray diffraction. Equally intriguing is the dyes' relatively strong electronic communication between donor and acceptor subchromophores that are meta-conjugated to one another, which is experimentally observed as a broad intramolecular charge-transfer absorption that can extend over 100 nm past the most intense absorption event and is computationally confirmed through density functional theory (DFT) evaluations of the molecular ground- and excited-state properties. This molecular design permits the preparation of dyes with panchromatic absorption not just based on the additive absorption of individual subchromophores.

Low Temperature Emissive Cyclometalated Cobalt(III) Complexes.

A series of CoIII complexes [Co(RImP)2][PF6], with HMeImP = 1,1'-(1,3-phenylene)bis(3-methyl-1-imidazole-2-ylidene)) and R = Me, Et, iPr, nBu, is presented in this work. The influence of the strong donor ligand on the ground and excited-state photophysical properties was investigated in the context of different alkyl substituents at the imidazole nitrogen. X-ray diffraction revealed no significant alterations of the structures and all differences in the series emerge from the electronic structures. These were probed via cyclic voltammetry and UV-vis spectroscopy, detailing the influence of the different alkyl substituents on the ground-state properties. All complexes are emissive at 77 K from a 3MC state, which exhibits lifetimes in the range of 1-5 ns at room temperature, depending on the alkyl substituent. Therefore, it is clearly shown that even small differences in the electronic structure have a large impact on the details of the excited state landscape. The observed behavior was rationalized by a detailed DFT analysis, which shows that the minimum-energy crossing point to the ground-state is located only slightly above the MC energy: Consequently, nonradiative decay to the ground state at room temperature is enabled, while at 77 K this path is prohibited, leading to low-temperature 3MC emission.

Optimizing Electronic and Photovoltaic Properties of D-π-A Organic Dyes for DSSCs Using Density Functional Theory.

This research utilizes density functional theory to investigate the ground and excited-state properties of a new series of organic dyes with D-π-A configurations (D1-D6) for their potential application in dye-sensitized solar cells. The study focuses on modifying these dyes using various functional groups as π-bridges to optimize their electronic properties and improve their efficiency as sensitizers in DSSCs. The frontier molecular orbitals (HOMO and LUMO) were analysed to evaluate electron transfer properties. The energy gaps, ranging from 2.449 to 2.6979eV, indicate favourable electron injection capabilities. Further analysis included molecular electrostatic potential, electron localization function, and localized orbital locator for all dyes. The maximum absorption wavelengths were found to range from 272.98nm to 624.76nm, covering both the UV and visible spectra. A significant redshift was observed with the addition of electron-withdrawing groups to the D-π-A structures, contributing to enhanced light-harvesting capabilities. The results indicate that all dyes exhibit improved open-circuit photovoltage, enhanced light-harvesting efficiency, and higher electron injection when compared to the reference dye (Dye1). Additionally, parameters such as oxidation potential, free energy change, redox potentials, electron transfer, and dye regeneration showed promising values, pointing to excellent photovoltaic efficiency. Electron injection from the dyes into the conduction band of TiO2, followed by efficient dye regeneration, was confirmed. The choice of the π-bridge group, in particular, plays a crucial role in optimizing dye performance. Based on the theoretical findings, all of the studied dyes demonstrate strong potential as effective photosensitizers for DSSCs applications.

How Structure and Hydrostatic Pressure Impact Excited-State Properties of Organic Room-Temperature Phosphorescence Molecules: A Theoretical Perspective.

Organic room-temperature phosphorescence (RTP) emitters with long lifetimes, high exciton utilizations, and tunable emission properties show promising applications in organic light-emitting diodes (OLEDs) and biomedical fields. Their excited-state properties are highly related to single molecular structure, aggregation morphology, and external stimulus (such as hydrostatic pressure effect). To gain a deeper understanding and effectively regulate the key factors of luminescent efficiency and lifetime for RTP emitters, we employ the thermal vibration correlation function (TVCF) theory coupled with quantum mechanics/molecular mechanics (QM/MM) calculations to investigate the photophysical properties of three reported RTP crystals (Bp-OEt, Xan-OEt, and Xan-OMe) with elastic/plastic deformation. By analyzing the geometric structures and stacking modes of these crystals, we observe that the geometric structure variations influence the electronic structures, subsequently modifying the transition properties and the energy consumption processes. Specifically, the presence of strong π-π interactions and hydrogen bonds in the Xan-OEt crystal inhibits a nonradiative decay process, thereby realizing long-lived emission. Additionally, the hybridized local and charge-transfer (HLCT) excited-state feature with the largest charge transfer excitation contributions (57.74%) for Xan-OEt stabilizes the triplet excitons and facilitates the radiative decay process, ultimately achieving high efficiency and long lifetime emissions. Furthermore, by applying high hydrostatic pressure for the Bp-OEt crystal, the RTP emission efficiencies and lifetimes are enhanced and blue-shifted. All of these results demonstrate the crucial role of molecular structure and stacking modes as well as the hydrostatic pressure effect in regulating RTP properties. Thus, our findings reveal the structure-packing-property relationship and highlight the control of molecular packing and the related tunable approaches, which could provide prospective strategies for constructing stimuli-responsive RTP emitters in practical applications.

Modeling the effect of substituents on the electronically excited states of indole derivatives.

A proper understanding of excited state properties of indole derivatives can lead to rational design of efficient fluorescent probes. The optically active and excited states of a series of substituted indoles, where a substituent was placed on position four, were calculated using equation of motion coupled cluster and time dependent density functional theory. The results indicate that most substituted indoles have a brighter second excited state corresponding to experimental absorption maxima, but a few with electron withdrawing substituents absorb more on the first excited state. Absorption on the first excited state may increase their fluorescence quantum yield, making them better probes. Electronic structure methods were found to predict the energies of the systems with electron withdrawing substituents more accurately than those with electron donating substituents. The excited states of both states correlated well with electrophilicity, similar to the experimental trends for the absorption maxima. Overall, these computational studies indicate that theory can be used to predict excited state properties of substituted indoles, when the substituent is an electron withdrawing group.

Hydrogen Bond "Double-Edged Sword Effect" on Organic Room-Temperature Phosphorescence Properties: A Theoretical Perspective.

The strategy of designing efficient room-temperature phosphorescence (RTP) emitters based on hydrogen bond interactions has attracted great attention in recent years. However, the regulation mechanism of the hydrogen bond on the RTP property remains unclear, and corresponding theoretical investigations are highly desired. Herein, the structure-property relationship and the internal mechanism of the hydrogen bond effect in regulating the RTP property are studied through the combination of quantum mechanics and molecular mechanics methods (QM/MM) coupled with the thermal vibration correlation function method. Intermolecular interactions, excited-state transition properties, reorganization energies, radiative and nonradiative decay rates, and the intersystem crossing rates are analyzed in detail. Results show that intermolecular hydrogen bonds can effectively delocalize molecular orbitals, enhance spin-orbit coupling (SOC) effect, and thus accelerate intersystem crossing (ISC) processes. In addition, an intermolecular hydrogen bond can also suppress nonradiative transition by restricting molecular motion, thereby promoting generation of phosphorescence. However, an excessively enhanced intermolecular hydrogen bond effect promotes molecular vibrations, leading to increased reorganization energies and thus facilitating nonradiative energy consumption process. The hydrogen bond "double-edged sword" effect on RTP properties and nonradiative decay process is theoretically revealed. Therefore, reasonable control of the hydrogen bond strength is beneficial for the development of efficient RTP emitters. Our research provides rational explanations for previous measurements and highlights the hydrogen bond effect in constructing efficient RTP emitters.

Theoretical Understanding of Photoluminescence and Singlet Oxygen Quantum Yields in a Few Halogenated Fluorescein Dyes.

Visible-light absorbing metal-free organic dyes are of increasing demand for various optoelectronic applications because of their great structure-function tunability through chemical means. Several dyes also show huge potential in triplet photosensitization, generating reactive singlet oxygen. Understanding the structure-property relationships of many well-known fluorescein dyes is of paramount importance in designing next-generation energy efficient dyes, which is currently limited. For example, the role of heavy atoms in the excited-state deactivations is not fully understood for these dyes. Herein, 9 halogenated (Cl, Br, I) fluorescein dyes with varied halogen concentrations and positions are studied using time-dependent range-separated hybrid combined with polarizable continuum model with water dielectric for accounting of polarization and screening effects. Excited state energies of these dyes and their deactivations via radiative and non-radiative pathways are well described using 0-0 corrected excitation energies. Calculated results are in reasonable agreement with the available experimental data. However, no systematic correlation is found between the heavy-atom effect and calculated intersystem crossing/fluorescence rates. Not surprisingly, heavy-atom effect is found to be more pronounced in iodinated dyes compared to their brominated analogues. Halogen position also plays a critical role in determining the excited-state deactivation rates. All dyes show similar fluorescence rates of ~108 . Whereas, the intersystem crossing rates are much smaller and vary from ~102 to 104 . Nevertheless, lower fluorescence quantum yields for some dyes are attributed to the large internal conversion. Microscopic understanding on the excited-state properties of several halogenated fluorescein dyes reported here will aid in developing advanced fluorescein dye based energy efficient photosensitizers and also emitters.

Excited state properties from the Bethe-Salpeter equation: State-to-state transitions and spin-orbit coupling.

The formalism to calculate excited state properties from the GW-Bethe-Salpeter equation (BSE) method is introduced, providing convenient access to excited state absorption, excited state circular dichroism, and excited state optical rotation in the framework of the GW-BSE method. This is achieved using the second-order transition density, which can be obtained by solving a set of auxiliary equations similar to time-dependent density functional theory (TD-DFT). The proposed formulation therefore leads to no increase in the formal computational complexity when compared to the corresponding ground state properties. We further outline the calculation of fully relaxed spin-orbit coupling matrix elements within the GW-BSE method, allowing us to include perturbative corrections for spin-orbit coupling in aforementioned properties. These corrections are also extended to TD-DFT. Excited state absorption and perturbative spin-orbit coupling corrections within GW-BSE are evaluated for a selected set of molecular systems, yielding promising results.

Approaching coupled-cluster accuracy for molecular electronic structures with multi-task learning.

Machine learning plays an important role in quantum chemistry, providing fast-to-evaluate predictive models for various properties of molecules; however, most existing machine learning models for molecular electronic properties use density functional theory (DFT) databases as ground truth in training, and their prediction accuracy cannot surpass that of DFT. In this work we developed a unified machine learning method for electronic structures of organic molecules using the gold-standard CCSD(T) calculations as training data. Tested on hydrocarbon molecules, our model outperforms DFT with several widely used hybrid and double-hybrid functionals in terms of both computational cost and prediction accuracy of various quantum chemical properties. We apply the model to aromatic compounds and semiconducting polymers, evaluating both ground- and excited-state properties. The results demonstrate the model's accuracy and generalization capability to complex systems that cannot be calculated using CCSD(T)-level methods due to scaling.

Directly Fused Porphyrin-Tetracyanopentacenequinone Conjugates: Role of the Cross-Conjugated, Powerful Electron Acceptor in Promoting Highly Efficient Charge Separation.

Tetracyanopentacenequinone, a powerful electron acceptor, is fused directly to the porphyrin π-system to create a new class of donor-acceptor conjugates. Owing to the direct fusion and electron-deficient property of tetracyanopentacenequinone, strong intramolecular charge transfer both in the ground and excited states was witnessed. As a control, porphyrin fused with pentacenequinone was also investigated. Upon complete spectral and electrochemical characterization, the excited state properties were initially probed by time-dependent DFT studies, and the occurrence of electron transfer from different excited states was established. Free-energy calculations revealed higher exothermic electron transfer (> 600 mV) than the control pentacenequinone-porphyrin systems. Pump-probe studies covering broad spatial and temporal regions revealed efficient excited state charge separation. This was unlike the control pentacenequinone-porphyrin system, where slow charge separation was witnessed only in the case of the zinc derivatives but not the free-base ones. The lifetimes of the charge-separated states ranged between 30-500 ps depending on the solvent and metal ion in the porphyrin cavity. The significance of cross-conjugated tetracyanopentacenequinone fused directly to the porphyrin π system in promoting highly exothermic and efficient charge separation, irrespective of its cross conjugation, is borne out from this study.

Unraveling the Configuration Modulation in Spiro-Based Through-Space Charge Transfer Materials.

Thermally activated delayed fluorescence (TADF) materials hold promise for optoelectronic applications. Among various design strategies, through-space charge transfer (TSCT) systems offer the potential for enhanced performance. However, the relationship between molecular configuration and TSCT properties remains unclear compared to traditional through-band charge transfer materials. In this study, we investigated the influence of spatial configuration on TSCT features and electronic properties of triplet excited states in these TSCT materials. By manipulating the spatial arrangement between donor and acceptor segments using different spiro skeletons, a series of TSCT materials (DMB2-DMB5) was synthesized. Together with the parent molecule, DM-B, these materials exhibited completely different TADF characteristics, demonstrating the impact of spatial arrangements on their optoelectronic properties. Thus, the external quantum efficiency of these materials ranged from as high as 28.0 % (DMB2) to as low as 3.6 % (DMB5) due to variations in their TADF characteristics. Our findings highlight the significance of spatial configuration, beyond distance alone, in influencing TSCT properties when donor and acceptor segments are sufficiently close. This insight provides valuable guidance for developing advanced TSCT materials and advancing TADF systems with improved performance and functionality.

Unraveling the Configuration Modulation in Spiro‐Based Through‐Space Charge Transfer Materials

AbstractThermally activated delayed fluorescence (TADF) materials hold promise for optoelectronic applications. Among various design strategies, through‐space charge transfer (TSCT) systems offer the potential for enhanced performance. However, the relationship between molecular configuration and TSCT properties remains unclear compared to traditional through‐band charge transfer materials. In this study, we investigated the influence of spatial configuration on TSCT features and electronic properties of triplet excited states in these TSCT materials. By manipulating the spatial arrangement between donor and acceptor segments using different spiro skeletons, a series of TSCT materials (DMB2‐DMB5) was synthesized. Together with the parent molecule, DM‐B, these materials exhibited completely different TADF characteristics, demonstrating the impact of spatial arrangements on their optoelectronic properties. Thus, the external quantum efficiency of these materials ranged from as high as 28.0 % (DMB2) to as low as 3.6 % (DMB5) due to variations in their TADF characteristics. Our findings highlight the significance of spatial configuration, beyond distance alone, in influencing TSCT properties when donor and acceptor segments are sufficiently close. This insight provides valuable guidance for developing advanced TSCT materials and advancing TADF systems with improved performance and functionality.

Deep Blue Emitter with a Combination of Hybridized Local and Charge Transfer Excited State and Aggregation-Induced Emission Features for Efficient Non-Doped OLED.

Herein, a deep blue emitter (PI-TPB-CN) with a synergistic effect of hybridized local and charge transfer excited state (HLCT) and aggregation-induced emission (AIE) properties is successfully designed and synthesized to improve the performance of deep blue organic light-emitting diodes (OLEDs). It is constructed using a 1,2,4,5-tetraphenylbenzene (TPB) as an π-conjugated AIE core being asymmetrically functionalized with a phenanthro[9,10-d]imidazole (PI) as a weak donor (D) and a benzonitrile (CN) as an acceptor (A), thereby formulating D-π-A type fluorophore. Its HLCT and AIE properties verified by theoretical calculations, solvatochromic effects, and transient photoluminescence decay experiments, bring about a strong blue emission (452 nm) with a high photoluminescence quantum yield of 74 % in the thin film. PI-TPB-CN is successfully employed as a blue emitter in OLEDs. Non-doped OLED with the structure of ITO/HAT-CN (6 nm)/NPB (30 nm)/TCTA (10 nm)/PI-TPB-CN (30 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) demonstrates excellent electroluminescence (EL) performance with blue emission (451 nm) and maximum external quantum efficiency (EQEmax) of 7.38 %. The device with a thinner layer of PI-TPB-CN (20 nm) and TPBi (30 nm) exhibits a deeper blue emission (444 nm) with CIE coordinates of (0.156, 0.096), a low turn-on voltage of 3.0 V, and EQEmax of 6.45 %.

Encapsulation-induced hypsochromic shift of emission properties from a cationic Ir(III) complex in a hydrogen-bonded organic cage: A theoretical study.

Encapsulation of coordination complexes within the confined spaces of self-assembled hosts is an effective method for creating supramolecular assemblies with distinct chemical and physical properties. Recent studies with calix-resorcin[4]arene hydrogen-bonded hexameric capsules revealed that encapsulated metal complexes exhibit enhanced and blue-shifted photoluminescence compared to their unencapsulated forms. The photophysical change has been hypothetically attributed to encapsulation-induced confinement, which isolates the metal complex from the solvent, suppressing stabilization of the excited state of the guest by solvent reorganization and structural relaxation, and altering the local environment, such as solvent polarity and viscosity, around the guest. In this study, density-functional theory calculations were conducted to explore how encapsulation affects the photophysical properties of a cationic iridium complex within a hydrogen-bonded hexameric capsule. The encapsulation-induced emission shift was analyzed by separating it into three factors: suppression of solvent reorganization, suppression of structural relaxation of the complex, and electronic interactions between the complex and the capsule. The findings indicate that the photoluminescence modulation is driven by the electronic interaction between the host and guest, which affects the energy levels of the molecular orbitals involved in the T1 excited state and the suppression of excited-state structural relaxation of the Ir complex due to the presence of the host. This study advances our understanding of the photophysical dynamics of coordination complexes within the confined spaces of hexameric capsules, providing a valuable approach for tuning the excited state properties of guest molecules.

Ultrafast Spectroscopic Investigation of the Aggregation Induced TADF from High-Level Reversed Intersystem Crossing.

The thermally activated delayed fluorescence (TADF) originating from high-level intersystem crossing (hRISC) presents great potential in realizing a more full utilization of triplet excitons. In this study, DPA-FBP and TPA-FBP were doped in a PMMA film with different weight fractions to study the effect of aggregation on the luminescence properties. As a result, the TADF feature from hRISC was only found in the 50 wt % doped film, whereas the 1 wt % doped film only shows prompt fluorescence. The fs-TA spectroscopy results reveal that the 50 wt % film will generate charge transfer species to lower the energy gap, so that the high-lying triplet exciton can transition back to the singlet state, whereas that of the 1 wt % film will quickly transition to the lowest triplet state due to the unfavorable energy splitting. This study provides a new insight into aggregation effects on the excited-state properties of hot exciton materials and the solid-state photodynamic.

Linear Response pCCD-Based Methods: LR-pCCD and LR-pCCD+S Approaches for the Efficient and Reliable Modeling of Excited State Properties.

In this work, we derive working equations for the linear response pair coupled cluster doubles (LR-pCCD) ansatz and its extension to singles (S), LR-pCCD+S. These methods allow us to compute electronic excitation energies and transition dipole moments based on a pCCD reference function. We benchmark the LR-pCCD+S model against the linear response coupled-cluster singles and doubles method for modeling electronic spectra (excitation energies and transition dipole moments) of the BH, H2O, H2CO, and furan molecules. We also analyze the effect of orbital optimization within pCCD on the resulting LR-pCCD+S transition dipole moments and oscillator strengths and perform a statistical error analysis. We show that the LR-pCCD+S method can correctly reproduce the transition dipole moments features, thus representing a reliable and cost-effective alternative to standard, more expensive electronic structure methods for modeling electronic spectra of simple molecules. Specifically, the proposed models require only mean-field-like computational cost, while excited-state properties may approach the CCSD level of accuracy. Moreover, we demonstrate the capability of our model to simulate electronic transitions with non-negligible contributions of double excitations and the electronic spectra of polyenes of various chain lengths, for which standard electronic structure methods perform purely.

Highly Efficient Blue Organic Light‐Emitting Devices Based on “Cross”‐Shaped Hot Exciton Emitters

AbstractThe development of blue electroluminescent (EL) materials remains a significant challenge in organic light‐emitting diode (OLED) technology. In this study, a novel design strategy is proposed for blue hot exciton (HE) materials, which involves utilizing a “cross” shaped molecular structure characterized by substantial steric hindrance and a highly twisted conformation. The unique cross‐shaped molecular architecture with distinct “arms” enables flexible control over the excited state properties of the molecule, thereby facilitating precise modulation of high‐lying triplet and low‐lying singlet excited state energy levels. Furthermore, the 3D spatial configuration of the molecule effectively reduces close molecular packing, thereby minimizing the risk of material concentration quenching. The proof‐of‐concept HE emitters CN‐PI and TP‐PI exhibit non‐π‐π stacking configurations in single crystals, achieving high photoluminescence quantum yield (PLQY) values up to 51.3% and 46.5% in non‐doped thin films, respectively, along with rapid radiation decay rates and reasonable distribution of Tm (m ≤ 5) and S1 states. Non‐doped OLEDs incorporating these emitters demonstrate exceptional external quantum efficiencies (EQE), reaching 7.3% and 6.4%, respectively, while exhibiting minimal efficiency roll‐off at high luminance. This research introduces a promising approach for developing high‐performance blue HE emitters.

Heterojunction Organic Solar Cells with Efficient Charge Mobility and Separation Capabilities Studied by DFT.

The properties of the active layer materials play a decisive role in determining the power conversion efficiency of organic solar cells (OSCs). Chlorophyll and its derivatives are abundant and environmentally friendly functional organic molecular materials. Using density functional theory (DFT) and time-dependent density functional theory (TD-DFT), we have calculated the absorption spectra and their excited state properties based on optimized ground state structures. It was found that bacteriochlorin exhibits superior structural properties, a smaller energy gap and hole reorganization energy, redshifted absorption spectra, and higher hole mobility compared to the donor D18. This suggests that bacteriochlorin exhibits superior donor properties. Comparative studies between o-AT-2Cl and m-AT-2Cl showed that o-AT-2Cl had superior acceptor properties, implying that differences in substitution positions can influence the physicochemical properties of non-fullerene acceptors (NFAs). Subsequently, six bulk heterojunctions (BHJs) were constructed by combining three donors with nonfused ring electron acceptors, o-AT-2Cl and m-AT-2Cl. The bacteriochlorin-based BHJs performed well among them, with BChl3/o-AT-2Cl and BChl4/o-AT-2Cl having the largest interfacial charge separation rate. The results suggested that BHJs composed of bacteriochlorin and NFAs can improve OSCs' photovoltaic performance, providing a feasible scheme for designing efficient OSCs.

Inverse Design of Singlet‐Fission Materials with Uncertainty‐Controlled Genetic Optimization

AbstractSinglet fission has shown potential for boosting the efficiency of solar cells, but the scarcity of suitable molecular materials hinders its implementation. We introduce an uncertainty‐controlled genetic algorithm (ucGA) based on ensemble machine learning predictions from different molecular representations that concurrently optimizes excited state energies, synthesizability, and exciton size for the discovery of singlet fission materials. The ucGA allows us to efficiently explore the chemical space spanned by the reFORMED fragment database, which consists of 45,000 cores and 5,000 substituents derived from crystallographic structures assembled in the FORMED repository. Running the ucGA in an exploitative setup performs local optimization on variations of known singlet fission scaffolds, such as acenes. In an explorative mode, hitherto unknown candidates displaying excellent excited state properties for singlet fission are generated. We suggest a class of heteroatom‐rich mesoionic compounds as acceptors for charge‐transfer mediated singlet fission. When included in larger donor‐acceptor systems, these units exhibit localization of the triplet state, favorable diradicaloid character and suitable triplet energies for exciton injection into semiconductor solar cells.

Inverse Design of Singlet-Fission Materials with Uncertainty-Controlled Genetic Optimization.

Singlet fission has shown potential for boosting the efficiency of solar cells, but the scarcity of suitable molecular materials hinders its implementation. We introduce an uncertainty-controlled genetic algorithm (ucGA) based on ensemble machine learning predictions from different molecular representations that concurrently optimizes excited state energies, synthesizability, and exciton size for the discovery of singlet fission materials. The ucGA allows us to efficiently explore the chemical space spanned by the reFORMED fragment database, which consists of 45,000 cores and 5,000 substituents derived from crystallographic structures assembled in the FORMED repository. Running the ucGA in an exploitative setup performs local optimization on variations of known singlet fission scaffolds, such as acenes. In an explorative mode, hitherto unknown candidates displaying excellent excited state properties for singlet fission are generated. We suggest a class of heteroatom-rich mesoionic compounds as acceptors for charge-transfer mediated singlet fission. When included in larger donor-acceptor systems, these units exhibit localization of the triplet state, favorable diradicaloid character and suitable triplet energies for exciton injection into semiconductor solar cells.