Optimizing the Thermodynamics and Kinetics of the Triplet-Pair Dissociation in Donor–Acceptor Copolymers for Intramolecular Singlet Fission

Abstract

Singlet fission (SF) is a two-step process in which a singlet splits into two triplets throughout the so-called correlated triplet-pair ( 1 TT) state. Intramolecular SF (iSF) materials, in particular, have attracted growing interest as they can be easily implemented in single-junction solar cells and boost their power conversion efficiency. Still, the potential of iSF materials such as polymers and oligomers for photovoltaic applications has been partially hindered by their ability to go beyond the 1 TT intermediate and generate free triplets, whose mechanism remains poorly understood. In this work, the main aspects governing the 1 TT dissociation in donor–acceptor copolymers and the key features that optimize this process are exposed. First, we show that both thermodynamics and kinetics play a crucial role in the intramolecular triplet-pair separation and second, we uncover the inherent flexibility of the donor unit as the fundamental ingredient to optimize them simultaneously. Overall, these results provide a better understanding of the intramolecular 1 TT dissociation process and establish a new paradigm for the development of novel iSF active materials.

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

“…Recently, Corminboeuf et al have proposed key aspects governing the triplet dissociation process and highlighted the essential features for optimizing this process in D-A copolymers. [137] They found that both thermodynamics and kinetics played a significant role in the intramolecular triplet-pair separation. Utilizing flexible donors with at least one rotatable sigma bond allows modulation of the coplanarity differently, leading to a reduction in triplet-pair binding energy and an acceleration of dissociation.…”

Molecular design of two-dimensional donor–acceptor covalent organic frameworks for intramolecular singlet fission

“…Previous work showed that donors such as 2,2 0 -bithiophene and thiophene-vinyl-thiophene (TVT) are able to optimize both the thermodynamics and kinetics of the triplet-pair dissociation process in D-A copolymers due to their ability to adopt pseudo coplanar and non-coplanar conformations. 15 In this case, the TVT unit was selected for D 2 because it can be extended into tetrathienylethene (TTE) as a 4-arms linker to built the 2D network. DFT and TDDFT(TDA) calculations were done for TTE at the same level ob97xD/6-31g* using Gaussian 09 22 to collect the HOMO, LUMO, S 1 and T 1 energies of this ligand (Table 1).…”

“…9,10 Finally, intramolecular triplet–triplet dissociation rates rely upon a small acceptor–acceptor interaction through the donor core, and its ability to promote triplet energy transfer along the D–A copolymer chain. 15 In this way, the role of the donor core is two-fold in the iSF process: first, to promote CT for efficient singlet splitting and second, to allow favorable and fast intrachain triplet energy transfer for triplet–triplet dissociation (Fig. 1c).…”

“…To do that, the database containing the HOMO, LUMO, S 1 and T 1 energies of 92 donors and 32 acceptor units reported by Blaskovits et al based on DFT and TDDFT calculations has been used, 10 together with the triplet–triplet dissociation energies and triplet energy transfer barriers analysis previously reported. 15 Both datasets are based on ω b97xD/6-31g* calculations benchmarked in front of refence wave function based methods. 9 First, each unit is selected based on the aforementioned criteria.…”

Data-driven discovery of organic electronic materials enabled by hybrid top-down/bottom-up design

“…We privilege small units to preserve the notion of 'donor' and 'acceptor' within an extended π-conjugated manifold and to promote intrachain triplet transport in an eventual polymer, as it has been shown that shorter distances between acceptors promotes efficient separation of the intramolecular triplet-pair state ( 1 [TT]) 92 into independent triplets (T1 + T1). 93 This pre-screening of acceptors and donors, automated cross-coupling of appropriate D-A combinations and subsequent semiempirical geometry optimization (GFN2-xTB) results in 1,002,268 unique dimers. The models discussed above are employed to predict their excited state energies, and the score function outlined in Figure S10 is then used to rank the resulting compounds based on potential SF exergonicity and T1 energy.…”