Charge transfer states impact the triplet pair dynamics of singlet fission polymers

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.

Unraveling Triplet Formation Mechanisms in Acenothiophene Chromophores

The evolution of molecular platforms for singlet fission (SF) chromophores has fueled the quest for new compounds capable of generating triplets quantitatively at fast time scales. As the exploration of molecular motifs for SF has diversified, a key challenge has emerged in identifying when the criteria for SF have been satisfied. Here, we show how covalently bound molecular dimers uniquely provide a set of characteristic optical markers that can be used to distinguish triplet pair formation from processes that generate an individual triplet. These markers are contained within (i) triplet charge-transfer excited state absorption features, (ii) kinetic signatures of triplet-triplet annihilation processes, and (iii) the modulation of triplet formation rates using bridging moieties between chromophores. Our assignments are verified by time-resolved electron paramagnetic resonance (EPR) measurements, which directly identify triplet pairs by their electron spin and polarization patterns. We apply these diagnostic criteria to dimers of acenothiophene derivatives in solution that were recently reported to undergo efficient intermolecular SF in condensed media. While the electronic structure of these heteroatom-containing chromophores can be broadly tuned, the effect of their enhanced spin-orbit coupling and low-energy nonbonding orbitals on their SF dynamics has not been fully determined. We find that SF is fast and efficient in tetracenothiophene but that anthradithiophene exhibits fast intersystem crossing due to modifications of the singlet and triplet excited state energies upon functionalization of the heterocycle. We conclude that it is not sufficient to assign SF based on comparisons of the triplet formation kinetics between monomer and multichromophore systems.

Quantum interference effects elucidate triplet-pair formation dynamics in intramolecular singlet-fission molecules

Quantum interference (QI)-the constructive or destructive interference of conduction pathways through molecular orbitals-plays a fundamental role in enhancing or suppressing charge and spin transport in organic molecular electronics. Graphical models were developed to predict constructive versus destructive interference in polyaromatic hydrocarbons and have successfully estimated the large conductivity differences observed in single-molecule transport measurements. A major challenge lies in extending these models to excitonic (photoexcited) processes, which typically involve distinct orbitals with different symmetries. Here we investigate how QI models can be applied as bridging moieties in intramolecular singlet-fission compounds to predict relative rates of triplet pair formation. In a series of bridged intramolecular singlet-fission dimers, we found that destructive QI always leads to a slower triplet pair formation across different bridge lengths and geometries. A combined experimental and theoretical approach reveals the critical considerations of bridge topology and frontier molecular orbital energies in applying QI conductance principles to predict rates of multiexciton generation.

Direct Exciton Harvesting from a Bound Triplet Pair

Singlet fission is commonly defined as the generation of two triplet excitons from a single absorbed photon. However, ambiguities within this definition arise due to the complexity of the various double triplet states that exist in SF chromophores and the corresponding interconversion processes. To clarify this process, singlet fission is frequently depicted as sequential two-step conversion in which a singlet exciton decays into a bound triplet-pair biexciton state that dissociates into two "free" triplet excitons. However, this model discounts the potential for direct harvesting from the coupled biexciton state. Here, it is demonstrated that individual triplet excitons can be extracted directly from a bound triplet pair. It is demonstrated that due to the requirement for geminate triplet-triplet annihilation in intramolecular singlet fission compounds, unique spectral and kinetic signatures can be used to quantify triplet-pair harvesting yields. An internal quantum efficiency for triplet exciton transfer from the triplet pair of >50%, limited only by the solubility of the compounds is achieved. The harvesting process is not dependent on the net multiplicity of the triplet-pair state, suggesting that an explicit, independent dissociation step is not a requirement for using triplet pairs to do chemical or electrical work.

Mechanism of Ultrafast Triplet Exciton Formation in Single Cocrystals of π-Stacked Electron Donors and Acceptors

Ultrafast triplet formation in donor-acceptor (D-A) systems typically occurs by spin-orbit charge-transfer intersystem crossing (SOCT-ISC), which requires a significant orbital angular momentum change and is thus usually observed when the adjacent π systems of D and A are orthogonal; however, the results presented here show that subnanosecond triplet formation occurs in a series of D-A cocrystals that form one-dimensional cofacial π stacks. Using ultrafast transient absorption microscopy, photoexcitation of D-A single cocrystals, where D is coronene (Cor) or pyrene (Pyr) and A is N,N-bis(3'-pentyl)-perylene-3,4:9,10-bis(dicarboximide) (C₅PDI) or naphthalene-1,4:5,8-tetracarboxydianhydride (NDA), results in formation of the charge transfer (CT) excitons Cor^•+-C₅PDI^•-, Pyr^•+-C₅PDI^•-, Cor^•+-NDA^•-, and Pyr^•+-NDA^•- in <300 fs, while triplet exciton formation occurs in τ = 125, 106, 484, and 958 ps, respectively. TDDFT calculations show that the SOCT-ISC rates correlate with charge delocalization in the CT exciton state. In addition, time-resolved EPR spectroscopy shows that Cor^•+-C₅PDI^•- and Pyr^•+-C₅PDI^•- recombine to form localized ^3*C₅PDI excitons with zero-field splittings of |D| = 1170 and 1250 MHz, respectively. In contrast, Cor^•+-NDA^•- and Pyr^•+-NDA^•- give triplet excitons in which |D| is only 1240 and 690 MHz, respectively, compared to that of NDA (2091 MHz), which is the lowest energy localized triplet exciton, indicating that the Cor-NDA and Pyr-NDA triplet excitons have significant CT character. These results show that charge delocalization in CT excitons impacts both ultrafast triplet formation as well as the CT character of the resultant triplet states.

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.

Quantifying Exciton Transport in Singlet Fission Diblock Copolymers

Singlet fission (SF) is a mechanism of exciton multiplication in organic chromophores, which has potential to drive highly efficient optoelectronic devices. Creating effective device architectures that operate by SF critically depends on electronic interactions across multiple length scales─from individual molecules to interchromophore interactions that facilitate multiexciton dephasing and exciton diffusion toward donor-acceptor interfaces. Therefore, it is imperative to understand the underpinnings of multiexciton transport and interfacial energy transfer in multichromophore systems. Interestingly, block copolymers (BCPs) can be designed to control multiscale interactions by tailoring the nature of the building blocks, yet SF dynamics are not well understood in these macromolecules. Here, we designed diblock copolymers comprising an inherent energy cleft at the interface between a block with pendent pentacene chromophores and an additional block with pendent tetracene chromophores. The singlet and triplet energy offset between the two blocks creates a driving force for exciton transport along the BCP chain in dilute solution. Using time-resolved optical spectroscopy, we have quantified the yields of key energy transfer steps, including both singlet and triplet energy transfer processes across the pentacene-tetracene interface. From this modular BCP architecture, we correlate the energy transfer time scales and relative yields with the length of each block. The ability to quantify these energy transfer processes provides valuable insights into exciton transport at critical length scales between bulk crystalline systems and small-molecule dimers─an area that has been underexplored.

Exploring a new class of singlet fission fluorene derivatives with high-energy triplets

We found that a stronger push–pull character favours SF, as long as the ICT does not act as a trap. The unique property of generating high-energy triplets (ca. 2 eV) via SF makes these materials outstanding candidates for photovoltaic applications.

Exploiting a Neutral BODIPY Copolymer as an Effective Agent for Photodynamic Antimicrobial Inactivation

We report the synthesis and photophysical properties of a neutral BODIPY photosensitizing copolymer (poly-8-(4-hydroxymethylphenyl)-4,4-difluoro-2,6-diethynyl-4-bora-3a,4a-diaza-s-indacene) containing ethynylbenzene links between the BODIPY units. The copolymer absorbs further towards the red in the UV-vis spectrum compared to the BODIPY precursor. Photolysis of the polymer produces a singlet excited state which crosses to the triplet surface in less than 300 ps. This triplet state was used to form singlet oxygen with a quantum yield of 0.34. The steps leading to population of the triplet state were studied using time-resolved spectroscopic techniques spanning the pico- to nanosecond timescales. The ability of the BODIPY polymer to generate a biocidal species for bactericidal activity in both solution- and coating-based studies was assessed. When the BODIPY copolymer was dropcast onto a surface, 4 log and 6 log reductions in colony forming units/ml representative of Gram-positive and Gram-negative bacteria, respectively, under illumination at 525 nm were observed. The potent broad-spectrum antimicrobial activity of a neutral metal-free copolymer when exposed to visible light conditions may have potential clinical applications in infection management.