High-Lying 31Ag Dark-State-Mediated Singlet Fission

Understanding and Tuning Singlet-Triplet (S1-T1) Energy Gaps in Planar Organic Chromophores

Molecules with large gaps between their first singlet and triplet excited states (ΔEST) are key components of various modern technologies, most prominently singlet fission photovoltaics and triplet-triplet annihilation upconversion (TTA-UC). The design of these molecules is hampered by the fact that only limited rules for maximizing ΔEST exist, other than increasing the overlap between the frontier molecular orbitals (FMO). Here we suggest a new strategy for tuning and maximizing ΔEST based on a detailed analysis of the underlying quantum mechanical energy terms. We present a model based on the transition density and derive three straightforward design rules: ΔEST values can be maximized by (i) minimizing the overall number of π-electrons, (ii) reducing delocalization, and (iii) optimizing specific geometric interactions. The validity of these rules is first exemplified for a set of 18 hydrocarbon backbones before proceeding to a varied set of dye molecules, highlighting their transferability to realistic settings. We believe that the developed rules will provide an enormous boost to the field, enabling rational design instead of trial-and-error screening. More generally, this work demonstrates the power of going beyond the FMO approximation in designing advanced molecular materials.

Multiple effects of aromatic substituents on excited-state properties and singlet fission process in azaquinodimethane systems

Singlet fission (SF) could offset the thermalization loss of high-energy photons via multiexciton generations, thus holding great potential in improving the power conversion efficiency of solar cells. However, the development of SF-based devices has basically remained stagnant so far owing to the limited scope of practical SF materials. Therefore, designing and developing practical SF material systems have been imperative, yet challenging so far. In this work, we comprehensively investigated the effects of aromatic substituents on excited-state properties and SF process of azaquinodimethane systems. Results indicated that the aromatic substituents have a significant influence on molecular diradical characters, thereby determining the excited-state energetics of the SF material system, including optical band gaps and triplet energy. Moreover, the aromatic substituents influenced charge transfer coupling interactions by adjusting molecular packing in the aggregate state to shunt the excited-state population to exert SF process or trap in excimer species. These results not only offer a deep insight into the multiple regulatory effects of the aromatic substituents on excited-state properties and SF process but also provide a practical SF material system, which could lay the foundation for the discovery of new SF-active chromophores and practical applications of new-generation light-harvesting materials.

Conjugation Length Dependence of Intramolecular Singlet Fission in a Series of Regioregular Oligo 3-Alkyl(thienylene-vinylene)s

Activated intramolecular singlet fission is known to occur in the conjugated polymer polythienylene-vinylene (P3TV). Instead, efficient intersystem crossing has been observed in a short 3-alkyl(thienylene-vinylene) dimer. Here, we investigate a series of oligomers covering the conjugation length gap between the dimer and polymer. We confirm that the polymer and longer oligomers undergo activated intramolecular singlet fission, while the shorter oligomers with less than 3 units predominantly undergo efficient intersystem crossing. For the longer oligomers, the intermediate state of singlet fission is assigned to the 3Ag- covalent state of triplet pair character, as predicted by several computational studies. Our results point to potential pitfalls when assigning triplet production pathways solely with transient absorption spectroscopy.

Activated Singlet Fission Dictated by Anti-Kasha Property in a Rylene Imide Dye

A key challenge in the search of new materials capable of singlet fission (SF) arises from the primary energy conservation criterion, i.e., the energy of the triplet exciton has to be half that of the singlet (E(S1) ≥ 2E(T1)), which excludes most photostable organic materials from consideration and confines the design strategy to materials with low energy triplet states. One potential way to overcome this energy requirement and improve the triplet energy is to enable a SF channel from higher energy ("hot") excitonic states (Sn) in a process called activated SF. Herein, we demonstrate that efficient activated SF is achieved in a rylene imide-based derivative acenaphth[l, 2-a]acenaphthylene diimide (AADI). This process is enabled by an increase in the energy gap to greater than 1.0 eV between the S3 and S1 states due to the incorporation of an antiaromatic pentalene unit, which leads to the emergence of anti-Kasha properties in the isolated molecule. Transient spectroscopy studies show that AADI undergoes ultrafast SF from higher singlet excited states in thin film, with excitation wavelength-dependent SF yields. The SF yield of ∼200% is observed upon higher energy excitation, and long-lived free triplets persist on the μs time scale suggesting that AADI can be used in SF-enhanced devices. Our results suggest that enlarging the Sn-S1 energy gap is an effective way to turn on the activated SF channel and shed light on the development of novel, stable SF materials with high triplet energies.

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.

Unveiling the double triplet nature of the 2Ag state in conjugated stilbenoid compounds to achieve efficient singlet fission

In this investigation, the excited-state evolution in a series of all-trans stilbenoid compounds, displaying a low-lying dark singlet state of 2Ag-like symmetry nearly degenerate with the bright 1Bu state, was unveiled by employing advanced ultrafast spectroscopies while probing the effect of solvent polarizability. Together with the dual emission, femtosecond transient absorption and broadband fluorescence up-conversion disclosed the double nature of the 2Ag-like state showing both singlet features, a lifetime typical of a singlet and the ability to emit, and a triplet character, exhibiting a triplet-like absorption spectrum. The ultrafast formation (in hundreds of femtoseconds) from the non-relaxed upper singlet state led to the identification of 2Ag as the correlated triplet pair of singlet fission. The spectral difference obtained by comparison of transient absorption peaks of the 2Ag (1TT) and the triplet states was found to be in remarkable agreement with the observed triplet yield and the 1(TT) separation rate constant. Indeed, this spectral shift provided an experimental method to gain qualitative insight into the ease of separation of the 1(TT) and the relative SF efficiency. The highly conjugated polyene-like structures enable the ultrafast formation of the double triplet, but then the large binding energy prevents the triplet separation and thus the effective completion of singlet fission. Even though thermodynamically feasible for all the investigated stilbenoids according to TD-DFT calculations, singlet fission resulted to occur efficiently in the case of 1-(pyridyl-4-ylethenyl)-4-(p-nitrostyryl)benzene and nitro-styrylfuran with the triplet yield reaching 120% and 140%, respectively, triggered by their greatly enhanced intramolecular charge transfer character relative to the other compounds in the series.

Robust singlet fission process in strong absorption π-expanded diketopyrrolopyrroles

Singlet fission (SF) has drawn tremendous attention as a multiexciton generation process that could mitigate the thermal loss and boost the efficiency of solar energy conversion. Although a SF-based solar cell with an EQE above 100% has already been fabricated successfully, the practical efficiency of the corresponding devices is plagued by the limited scope of SF materials. Therefore, it is of great importance to design and develop new SF-capable compounds aiming at practical device application. In the current contribution, via a π-expanded strategy, we presented a new series of robust SF chromophores based on polycyclic DPP derivatives, Ex-DPPs. Compared to conventional DPP molecules, Ex-DPPs feature strong absorption with a fivefold extinction coefficient, good molecular rigidity to effectively restrain non-radiative deactivation, and an expanded π-skeleton which endow them with well-suited intermolecular packing geometries for achieving efficient SF process. These results not only provide a new type of high-efficiency SF chromophore but also address some basic guidelines for the design of potential SF materials targeting practical light harvesting applications.

Singlet fission initiating triplet generations of BODIPY derivatives through [Formula: see text]-stacking: a theoretical study

The role of singlet fission (SF) in the triplet-state generation mechanism of 1,3,5,7-tetramethyl-boron-dipyrromethene derivatives is revealed by exploring the cause for the solvent dependence of the generation rate. Comparing the adsorption energy calculations of solvent molecules, i.e., cyclohexane, chloroform and acetonitrile molecules, to the derivatives with the [Formula: see text]-stacking energies of these derivatives surprisingly show that the hierarchy of the solvation energies and [Formula: see text]-stacking energies strongly correlates with the experimentally-suggested solvent dependence of the triplet-state generation of these derivatives for five and more adsorbing solvent molecules. Following this finding, the excitation spectra of these derivatives in acetonitrile solvent are explored using the proprietary spin-flip long-range corrected time-dependent density functional theory. It is, consequently, confirmed that the [Formula: see text]-stacking activates the second lowest singlet excitation to trigger the spin-allowed transition to the singlet doubly-excited tetraradical (TT)[Formula: see text] state, which generates the long-lived quintet (TT)[Formula: see text] state causing the SF. However, it is also found that the [Formula: see text]-stacking also get a slow intersystem crossing active around the intersections of the lowest singlet excitations with the lowest triplet T[Formula: see text] excitations in parallel with the SF due to the charge transfer characters of the lowest singlet excitations. These results suggest that SF initiates the triplet-state generations through near-degenerate low-lying singlet and (TT) excitations with a considerable singlet-triplet energy gap after the [Formula: see text]-stacking of chromophores stronger than but not far from the solvation. Since these derivatives are organic photosensitizers, this study proposes that SF should be taken into consideration in developing novel heavy atom-free organic photosensitizers, which will contribute to a variety of research fields such as medical care, photobiology, energy science, and synthetic chemistry.

Diphenyldihydropentalenediones: Wide Singlet-Triplet Energy Gap Compounds Possessing the Planarly Fixed Diene Subunit

2,2,5,5-Tetramethyl-3,6-diphenyl-2,5-dihydropentalene-1,4-dione (PD-H) and its dimethoxy (PD-OCH₃) and bis(trifluoromethyl) derivatives (PD-CF₃) were developed as a new class of compounds possessing a wide excited singlet-triplet energy gap. The PD derivatives would also have a high energy level of the triplet-excited state (E _T) due to the planarity of the fused-diene subunit. The results of photophysical studies revealed that the energy level of the singlet-excited state (E _S) and E _T of PD-H are 2.88 and 1.43 eV, respectively. These values indicate that PD-H has the energy relationship, E _S > 2E _T, required for it to be a singlet fission (SF) material. Moreover, the introduction of electron-donating or -withdrawing groups on the benzene rings in PD-H enables fine-tuning of E _S and E _T. The results of transient absorption spectroscopic studies show that PD-H, PD-OCH₃, and PD-CF₃ in CH₂Cl₂ have respective T₁ lifetimes of 71, 118, and 107 μs, which are long enough to utilize its triplet exciton in other optoelectronic systems. These findings suggest that the PDs are potential candidates for SF materials with high E _T levels.

Synergistic Effect of Sr-O Divacancy and Exposing Facets in SrTiO3 Micro/Nano Particle: Accelerating Exciton Formation and Splitting, Highly Efficient Co2+ Photooxidation

As a typical perovskite-type crystal, polyhedral strontium titanate (SrTiO₃ ) has shown anisotropic charge transport behavior in recent studies, however, the carrier transportation and transition of which has not been explained very clearly. This work present the existence of Sr and O divacancies in the novel rhombicuboctahedron SrTiO₃ micro/nano particles (Sr_{1- x} TiO_{3- x} /TiO_{2- x} ) with exposing (100), (110) and (111) facets and the diameter of 300-700 nm synthesized via hydrothermal synthesis, and also summarizes the dissociation mechanism of self-trapped excitons (STEs) caused by the divacancy and facet effect. In addition, most importantly, the metastable STEs with ultra-low binding energy (E_b  < 3 meV) under illumination are discovered. Combining the model of S-scheme heterojunction, a conversion mechanism of photoinduced carriers is proposed. The photocatalytic reaction of Co²⁺ is used as the probe reaction, and the unique Sr_{1- x} TiO_{3- x} /TiO_{2- x} possesses a high photooxidation efficiency of Co²⁺ , by which 70.3% of Co²⁺ is oxidized to Co³⁺ (CoOOH) in 5 min. This finding may provide a guideline for an optimal design of the photocatalytic materials for the recovery and extraction of metal ions based on SrTiO₃ .

strong synergistic effect of the (110) and (100) facets of the SrTiO3 perovskite micro/nanocrystal: decreasing the binding energy of exciton and superb photooxidation capability for Co2

Crystal facet regulation is an effective method for preparing SrTiO₃ or other perovskite semiconductor materials with high photochemical catalysis performance. In general, the edge-truncated cube of SrTiO₃ micro-nano particles has been widely reported because of the multiple crystal facets exposed at the same time. However, the effect of the (110) facet and the interaction between the (100) and (110) facets on the properties of photo-induced carriers is still not very clear. In this article, we have designed and prepared two edge-truncated cube SrTiO₃-a small and large area proportion of the (110) facet, respectively. In addition to the morphological and structural characterization, high-resolution XPS and femtosecond multiphoton transient absorption (fs-TA) spectroscopy were used to detect the atomic vacancy and were applied to confirm the state of carrier transition. The results showed that the larger (110) facet led to two influences-more Sr vacancies and more self-trapping excitons (STEs) with an ultra-low binding energy (E_b = 2.13 meV), about 1.17 meV lower than that of the sample with the smaller (110) facet. In particular, the larger (110) facet also caused a much higher photooxidation performance for Co²⁺ to Co³⁺. This study not only enriches the arsenal of SrTiO₃ materials but also sheds new insights into the understanding of the synergistic effect essence of the (100) and (110) facets, which could promote the development of new perovskite photocatalytic materials, particularly in the recovery of heavy metals.

Singlet Fission Materials for Photovoltaics: from Small Molecules to Macromolecules

Singlet fission (SF) is a spin-allowed process in which a singlet state splits into two triplet states. Materials that enable SF have attracted great attention in the last decade, mainly stemming from the potential of overcoming the Shockley-Queisser (SQ) limit in photoenergy conversion. In the past decade, a large number of new molecules exhibiting SF have been explored and many devices based on SF materials have been studied, though the mechanistic understanding is still obscure. This review focuses on the recent developments of SF materials, including small molecules, oligomers and polymers. The molecular design strategies and related mechanisms of SF are discussed. Then the dynamics of charge transfer and energy transfer between SF materials and other materials are introduced. Further, we discuss the progresses of implementing SF in photovoltaics. It is hoped that a comprehensive understanding to the SF materials, devices and mechanism may pave a new way for the design of next generation photovoltaics. This article is protected by copyright. All rights reserved.

H-Type-like Aggregation-Accelerated Singlet Fission Process in Dipyrrolonaphthyridinedione Thin Film: The Role of Charge Transfer/Excimer Mixed Intermediate State

Through the combination of transient spectroscopy and theoretical simulations, an accelerated singlet fission (SF) process was evidently observed in the strongly coupled H-type-like aggregation thin films of a dipyrrolonaphthyridinedione skeleton. Results elucidate that in this H-type-like aggregation, the substantially stabilized charge transfer (CT) state is close in energy with singlet and excimer states, resulting in a CT/excimer mixed state, which could drive excited-state population escaping from excimer trap and promote an ultrafast and highly efficient SF process. Our results not only enrich the limited capacity of SF materials but also contribute to an in-depth understanding of SF dynamics in H-type aggregation, which is of fundamental importance for designing new SF sensitizers and implementing practical SF applications.