Understanding the Control of Singlet-Triplet Splitting for Organic Exciton Manipulating: A Combined Theoretical and Experimental Approach.
ABSTRACT: Developing organic optoelectronic materials with desired photophysical properties has always been at the forefront of organic electronics. The variation of singlet-triplet splitting (?EST) can provide useful means in modulating organic excitons for diversified photophysical phenomena, but controlling ?EST in a desired manner within a large tuning scope remains a daunting challenge. Here, we demonstrate a convenient and quantitative approach to relate ?EST to the frontier orbital overlap and separation distance via a set of newly developed parameters using natural transition orbital analysis to consider whole pictures of electron transitions for both the lowest singlet (S1) and triplet (T1) excited states. These critical parameters revealed that both separated S1 and T1 states leads to ultralow ?EST; separated S1 and overlapped T1 states results in small ?EST; and both overlapped S1 and T1 states induces large ?EST. Importantly, we realized a widely-tuned ?EST in a range from ultralow (0.0003 eV) to extra-large (1.47 eV) via a subtle symmetric control of triazine molecules, based on time-dependent density functional theory calculations combined with experimental explorations. These findings provide keen insights into ?EST control for feasible excited state tuning, offering valuable guidelines for the construction of molecules with desired optoelectronic properties.
Project description:The design of organic compounds with nearly no gap between the first excited singlet (S1) and triplet (T1) states has been demonstrated to result in an efficient spin-flip transition from the T1 to S1 state, that is, reverse intersystem crossing (RISC), and facilitate light emission as thermally activated delayed fluorescence (TADF). However, many TADF molecules have shown that a relatively appreciable energy difference between the S1 and T1 states (~0.2 eV) could also result in a high RISC rate. We revealed from a comprehensive study of optical properties of TADF molecules that the formation of delocalized states is the key to efficient RISC and identified a chemical template for these materials. In addition, simple structural confinement further enhances RISC by suppressing structural relaxation in the triplet states. Our findings aid in designing advanced organic molecules with a high rate of RISC and, thus, achieving the maximum theoretical electroluminescence efficiency in organic light-emitting diodes.
Project description:We observe the diffusion anisotropy difference between singlet and triplet excitons in organic crystals; that is, singlet and triplet excitons may have completely different spatial direction preference for diffusion. This phenomenon can be ascribed to the distinct dependence of different excitonic couplings (Coulomb Förster vs. exchange Dexter) existing in singlet and triplet excitons on their intermolecular distance and intermolecular orientation. Such a discovery provides insights for understanding the fundamental photophysical process in a vast range of organic condensed-phase systems and optimizing the efficiency of organic optoelectronic materials.
Project description:Thermally activated delayed florescence (TADF) is a mechanism that increases the electroluminescence efficiency in organic light-emitting diodes by harnessing both singlet and triplet excitons. TADF is facilitated by a small energy difference between the first singlet (S1) and triplet (T1) excited states (? E(ST)), which is minimized by spatial separation of the donor and acceptor moieties. The resultant charge-transfer (CT) excited states are difficult to model using time-dependent density functional theory (TDDFT) because of the delocalization error present in standard density functional approximations to the exchange-correlation energy. In this work we explore the application of the particle-particle random phase approximation (pp-RPA) for the determination of both S1 and T1 excitation energies. We demonstrate that the accuracy of the pp-RPA is functional dependent and that, when combined with the hybrid functional B3LYP, the pp-RPA computed ? E(ST) have a mean absolute deviation (MAD) of 0.12 eV for the set of examined molecules. A key advantage of the pp-RPA approach is that the S1 and T1 states are characterized as CT states for all of experimentally reported TADF molecules examined here, which allows for an estimate of the singlet-triplet CT excited state energy gap (? E(ST) = 1CT - 3CT). For experimentally known TADF molecules with a small (<0.2 eV) ? E(ST) in this data set, a high accuracy is demonstrated for the prediction of both the S1 (MAD = 0.18 eV) and T1 (MAD = 0.20 eV) excitation energies as well as ? E(ST) (MAD = 0.05 eV). This result is attributed to the consideration of correct antisymmetry in the particle-particle interaction leading to the use of full exchange kernel in addition to the Coulomb contribution, as well as a consistent treatment of both singlet and triplet excited states. The computational efficiency of this approach is similar to that of TDDFT, and the cost can be reduced significantly by using the active-space approach.
Project description:Thermally activated delayed fluorescence (TADF) mechanism is a significant method that enables the harvesting of both triplet and singlet excitons for emission. However, up to now most efforts have been devoted to dealing with the relation between singlet-triplet splitting (?EST) and fluorescence efficiency, while the significance of spin-orbit coupling (SOC) is usually ignored. In this contribution, a new method is developed to realize high-efficiency TADF-based devices through simple device-structure optimizations. By inserting an ultrathin external heavy-atom (EHA) perturber layer in a desired manner, it provides useful means of accelerating the T1???S1 reverse intersystem crossing (RISC) in TADF molecules without affecting the corresponding S1???T1 process heavily. Furthermore, this strategy also promotes the utilization of host triplets through Förster mechanism during host???guest energy transfer (ET) processes, which helps to get rid of the solely dependence upon Dexter mechanism. Based on this strategy, we have successfully raised the external quantum efficiency (EQE) in 4CzPN-based devices by nearly 38% in comparison to control devices. These findings provide keen insights into the role of EHA played in TADF-based devices, offering valuable guidelines for utilizing certain TADF dyes which possess high radiative transition rate but relatively inefficient RISC.
Project description:Singlet exciton fission photovoltaic technology requires chromophores with their lowest excited states arranged so that 2E(T1) < E(S1) and E(S1) < E(T2). Herein, qualitative theory and quantum chemical calculations are used to develop explicit strategies on how to use Baird's 4n rule on excited-state aromaticity, combined with Hückel's 4n + 2 rule for ground-state aromaticity, to tailor new potential chromophores for singlet fission. We first analyze the E(T1), E(S1), and E(T2) of benzene and cyclobutadiene (CBD) as excited-state antiaromatic and aromatic archetypes, respectively, and reveal that CBD fulfills the criteria on the state ordering for a singlet fission chromophore. We then look at fulvenes, a class of compounds that can be tuned by choice of substituents from Baird-antiaromatic to Baird-aromatic in T1 and S1 and from Hückel-aromatic to Hückel-antiaromatic in S0. The T1 and S1 states of most substituted fulvenes (159 of 225) are described by singly excited HOMO ? LUMO configurations, providing a rational for the simultaneous tuning of E(T1) and E(S1) along an approximate (anti)aromaticity coordinate. Key to the tunability is the exchange integral (KH,L), which ideally is constant throughout the compound class, providing a constant ?E(S1 - T1). This leads us to a geometric model for the identification of singlet fission chromophores, and we explore what factors limit the model. Candidates with calculated E(T1) values of ?1 eV or higher are identified among benzannelated 4n?-electron compound classes and siloles. In brief, it is clarified how the joint utilization of Baird's 4n and Hückel's 4n + 2 rules, together with substituent effects (electronic and steric) and benzannelation, can be used to tailor new chromophores with potential use in singlet fission photovoltaics.
Project description:Developing high-efficient afterglow from metal-free organic molecules remains a formidable challenge due to the intrinsically spin-forbidden phosphorescence emission nature of organic afterglow, and only a few examples exhibit afterglow efficiency over 10%. Here, we demonstrate that the organic afterglow can be enhanced dramatically by thermally activated processes to release the excitons on the stabilized triplet state (T1*) to the lowest triplet state (T1) and to the singlet excited state (S1) for spin-allowed emission. Designed in a twisted donor-acceptor architecture with small singlet-triplet splitting energy and shallow exciton trapping depth, the thermally activated organic afterglow shows an efficiency up to 45%. This afterglow is an extraordinary tri-mode emission at room temperature from the radiative decays of S1, T1, and T1*. With the highest afterglow efficiency reported so far, the tri-mode afterglow represents an important concept advance in designing high-efficient organic afterglow materials through facilitating thermally activated release of stabilized triplet excitons.
Project description:As structure defined cutouts of the graphene lattice, nanographene molecules have gained plenty of attention because of their high potential for versatile applications in organic electronics and energy conversion devices and as ideal model systems for the better understanding of intrinsic structure-property correlations of graphenes. In this study, well-defined nanographenes with sp2 carbon networks of different sizes, hexa-peri-hexabenzocoronene (HBC) and its rectangularly π-extended version, a short graphene nanoribbon (GNR), have been covalently functionalized with photoactive porphyrin molecules. On the basis of their spectroscopic studies, the photodynamics of the porphyrin-linked nanographenes was found to be influenced substantially by the size of the nanographenes. Photoexcitation of the porphyrin-HBC linked system led to exclusive energy transfer (EnT) from the first singlet excited state (S1) of the nanographene to the porphyrin, whereas opposite selective EnT occurred from the first and second singlet excited states (S1 and S2) of the porphyrin to the nanographene in the porphyrin-GNR linked system. In particular, ultrafast efficient EnTs from both the S2 and S1 states of the porphyrin to GNR mimic the corresponding ultrafast EnTs from the S2 and S1 states of carotenoids to chlorophylls in light-harvesting systems of natural photosynthesis. Such unique photophysical properties will be useful for the rational design of carbon-based photofunctional nanomaterials for optoelectronics and solar energy conversion devices.
Project description:Singlet fission (SF) is a photophysical process in which one of two adjacent organic molecules absorbs a single photon, resulting in rapid formation of a correlated triplet pair (T1T1) state whose spin dynamics influence the successful generation of uncorrelated triplets (T1). Femtosecond transient visible and near-infrared absorption spectroscopy of a linear terrylene-3,4:11,12-bis(dicarboximide) dimer (TDI2), in which the two TDI molecules are directly linked at one of their imide positions, reveals ultrafast formation of the (T1T1) state. The spin dynamics of the (T1T1) state and the processes leading to uncoupled triplets (T1) were studied at room temperature for TDI2 aligned in 4-cyano-4'-pentylbiphenyl (5CB), a nematic liquid crystal. Time-resolved electron paramagnetic resonance spectroscopy shows that the (T1T1) state has mixed 5(T1T1) and 3(T1T1) character at room temperature. This mixing is magnetic field dependent, resulting in a maximum triplet yield at ∼200 mT. The accessibility of the 3(T1T1) state opens a pathway for triplet-triplet annihilation that produces a single uncorrelated T1 state. The presence of the 5(T1T1) state at room temperature and its relationship with the 1(T1T1) and 3(T1T1) states emphasize that understanding the relationship among different (T1T1) spin states is critical for ensuring high-yield T1 formation from singlet fission.
Project description:Organic singlet diradicaloids promise application in non-linear optics, electronic devices and singlet fission. The stabilization of carbon allotropes/cumulenes (C1, C2, C4) by carbenes has been equally an area of high activity. Combining these fields, we showed recently that carbene scaffolds allow as well for the design of diradicaloids. Herein, we report a comprehensive computational investigation (CASSCF/NEVPT2; fractional occupation DFT) on the electronic properties of carbene-bridge-carbene type diradicaloids. We delineate how to adjust the properties of these ensembles through the choice of carbene and bridge and show that already a short C2 bridge results in remarkable diradicaloid character. The choice of the carbene separately tunes the energies of the S1 and T1 excited states, whereas the bridge adjusts the overall energy level of the excited states. Accordingly, we develop guidelines on how to tailor the electronic properties of these molecules. Of particular note, fractional occupation DFT is an excellent tool to predict singlet-triplet gaps.
Project description:Over the last decade, we have investigated and exploited the photophysical properties of triangulenium dyes. Azadioxatriangulenium (ADOTA) and diazaoxatriangulenium (DAOTA), in particular, have features that make them useful in various fluorescence-based technologies (e.g., bioimaging). Through our work with ADOTA and DAOTA, we became aware that the reported fluorescence quantum yields (?fl) for these dyes are lower than their actual values. We thus set out to further investigate the fundamental structure-property relationships in these unique conjugated cationic systems. The nonradiative processes in the systems were explored using transient absorption spectroscopy and time-resolved emission spectroscopy in combination with computational chemistry. The influence of molecular oxygen on the fluorescence properties was explored, and the singlet oxygen sensitization efficiencies of ADOTA and DAOTA were determined. We conclude that, for these dyes, the amount of nonradiative deactivation of the first excited singlet state (S1) of the azaoxa-triangulenium fluorophores is low, that the rate of such deactivation is slower than what is observed in common cationic dyes, that there are no observable radiative transitions occurring from the first excited triplet state (T1) of these dyes, and that the efficiency of sensitized singlet oxygen production is low (?? ? 10%). These photophysical results provide a solid base upon which technological applications of these fluorescent dyes can be built.