Hydrogen Migration-Triggered Diradicaloid Singlet-Fission Switch

Reducing the excessive exoergicity through a helically locked tether-driven approach for high-efficiency singlet fission chromophores

Singlet fission (SF) is a process in which the absorption of a single photon results in the generation of a pair of triplet excited states, showing potential for enhancing solar conversion efficiency. The thermodynamic driving force behind SF is determined by the energy difference between the first singlet excited state and the first triplet excited state, denoted as ΔE1 = E(S1) - 2E(T1). In general, an excessively large ΔE1 value (i.e., excessive exoergicity) can facilitate alternative relaxation pathways for excitons, thereby diminishing SF efficiency. Consequently, when designing high-efficiency SF chromophores, optimization of ΔE1 becomes crucial. Herein, we introduce a helically locked tethering strategy to optimize ΔE1 for low-efficiency SF chromophores. Specifically, different dihedral angles are induced by tethering tethers of different lengths (Cn = -(CH2)n-, n = 1-6) to tetraazaacenes, allowing us to systematically monitor the variational characteristic as a function of the dihedral angle. Tethered products show strong chirality with a high energy barrier to twist back and forth. A tunable ΔE1 has been realized by adjusting the tether length, allowing us to identify the optimal ΔE1 of 0.29, 0.26, and 0.11 eV at tether lengths of n = 3 or 2. Our results suggest that this strategy could be applied to existing low-efficiency SF databases that are not typically considered for future application in the SF field, thereby creating novel high-efficiency and stable SF chromophores. This strategy not only makes full use of the existing resources but also greatly expands the SF arsenal.

Heterofission-induced room temperature phosphorescence from range-separated hybrids: in search of the qualified blending components

Heterofission, as the conversion mechanism of a singlet excitation on one chromophore to two triplet excitations on two different chromophores, has been known to play imperative roles to boost the efficiency of photovoltaics. Most recently, the heterofission mechanism has been proposed to explain the room temperature phosphorescence (RTP) of organic materials in the form of host/guest (H/G) systems. Herein, the heterofission-induced RTP in the H/G systems is thoroughly investigated with the help of optimally tuned range-separated hybrid functionals (OT-RSHs). Several experimentally known ultralong RTP H/G systems have been considered as working models. For reliable prediction of the energy level matching criteria for the heterofission-induced RTP in these systems, we have proposed and validated variants of the OT-RSHs, their counterparts based on the linear-response and state-specific formalisms within the polarizable continuum model with both the equilibrium and nonequilibrium solvation regimes, and their screened versions accounting for the screening effects through the scalar dielectric constant. In this line, we scrutinize the role of the related ingredients including the underlying density functional approximations, short-range (α) and long-range Hartree-Fock (HF) exchange, and range-separation parameter. Perusing the results reveals that a particular compromise among the involved parameters is needed for well describing the heterofission-induced RTP. Accordingly, the full time-dependent density functional theory computations in the gas phase using the Perdew-Burke-Ernzerhof (PBE)-based OT-RSH (α = 0.0) with the correct asymptotic behavior in the long-range limit as the best performer are preferred. The proposed method also outperforms the standard RSHs with the default parameters, screened-exchange models, and conventional hybrids with both fixed and interelectronic distance-dependent HF exchange. Lastly, the applicability of our developed approximation is put into broader perspective, where it has been used for computational design of several H/G systems as promising candidates prone to be utilized in heterofission-induced RTP materials. We envisage that the recommended OT-RSH in this study can function as an affordable method for both computational modeling of heterofission-induced RTP and verifying the related experimental observations.

Reliable Diradical Characterization via Precise Singlet-Triplet Gap Calculations: Application to Thiele, Chichibabin, and Müller Analogous Diradicals

Accurately calculating the diradical character (y0) of molecular systems remains a significant challenge due to the scarcity of experimental data and the inherent multireference nature of the electronic structure. In this study, various quantum mechanical approaches, including broken symmetry density functional theory (BS-DFT), spin-flip time-dependent density functional theory (SF-TDDFT), mixed-reference spin-flip time-dependent density functional theory (MRSF-TDDFT), complete active space self-consistent field (CASSCF), complete active space second-order perturbation theory (CASPT2), and multiconfigurational pair-density functional theory (MCPDFT), are employed to compute the singlet-triplet energy gaps (EST) and y0 values in Thiele, Chichibabin, and Müller analogous diradicals. By systematically comparing the results from these computational methods, we identify optimally tuned long-range corrected functional CAM-B3LYP in the BS-DFT framework as a most efficient method for accurately and affordably predicting both EST and y0 values. Additionally, our results demonstrate that (i) MRSF-TDDFT performs much better than SF-TDDFT; (ii) the MCPDFT method is robust in determining EST with minimal dependence on the choice of active space. These findings provide insight into the electronic structure and diradical character of the investigated molecules and highlight effective computational strategies for future studies in this domain. Thus, this work not only advances our understanding of diradical systems but also offers practical guidelines for their computational investigation.

Recent Advance in the Development of Singlet-Fission-Capable Polymeric Materials

Singlet fission (SF) is a spin-allowed process in which a higher-energy singlet exciton is converted into two lower-energy triplet excitons via a triplet pair intermediate state. Implementing SF in photovoltaic devices holds the potential to exceed the Shockley-Queisser limit of conventional single-junction solar cells. Although great progress has been made in exploiting the underlying mechanism of SF over the past decades, the scope of materials capable of SF, particularly polymeric materials, remains poor. SF-capable polymer is one of the most potential candidates in the implementation of SF into devices due to their distinct superiorities in flexibility, solution processability and self-assembly behavior. Notably, recent advancements have demonstrated high-performance SF in isolated donor-acceptor (D-A) copolymer chains. This review provides an overview of recent progress in the development of SF-capable polymeric materials, with a significant focus on elucidating the mechanisms of SF in polymers and optimizing the design strategies for SF-capable polymers. Additionally, the paper discusses the challenges encountered in this field and presents future perspectives. It is expected that this comprehensive review will offer valuable insights into the design of novel SF-capable polymeric materials, further advancing the potential for SF implementation in photovoltaic devices.

A Ground-State Dual-Descriptor Strategy for Screening Efficient Singlet Fission Systems

Exploration of singlet fission (SF) materials is vital for enhancing the photoelectric conversion efficiency of photovoltaic devices, and the development of an effective screening means is in great demand. In this work, we for the first time propose a promising dual-descriptor strategy to predict the SF energetics (ΔESF) from ground-state electronic properties, the gap (GapHL) and exchange energy (KHL) between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), where GapHL plays a dominant role and KHL acts as a correction. This strategy is statistically verified through exploring the effect of N-doping on the electronic/energetic properties of the N-doped tetracene derivatives and isomers. Several rules of thumb are suggested, and the reliability of this strategy is validated by comparison with experiments. This work proposes a novel strategy for exploring SF chromophores with insights into the SF energetics from ground-state properties and certainly has fundamental interest and generality in exploring efficient SF-capable materials.

Computational predictions of adaptive aromaticity for the design of singlet fission materials

The concept of adaptive aromaticity has been demonstrated as an alternative strategy for the design of singlet fission materials.

Unconventional singlet fission materials

Singlet fission (SF) is a photophysical downconversion pathway, in which a singlet excitation transforms into two triplet excited states. As such, it constitutes an exciton multiplication generation process, which is currently at the focal point for future integration into solar energy conversion devices. Beyond this, various other exciting applications were proposed, including quantum cryptography or organic light emitting diodes. Also, the mechanistic understanding evolved rapidly during the last year. Unfortunately, the number of suitable SF-chromophores is still limited. This is per se problematic, considering the wide range of envisaged applicability. With that in mind, we emphasize uncommon SF-scaffolds and outline requirements as well as strategies to expand the chromophore pool of SF-materials.