Designing Singlet Fission Candidates from Donor–Acceptor Copolymers

Abstract

Singlet Fission (SF) has demonstrated significant promise for boosting the power conversion efficiency (PCE) of solar cells. Traditionally, SF is targeted as an intermolecular process, however its dependence on crystal packing makes molecular design difficult. In contrast, intramolecular SF (iSF) enables the exploration of tunable bi-chromophoric systems following well-defined structure-property relationships. In this work, we propose a set of parameters to screen conjugated donor-acceptor copolymer candidates with potential iSF behaviour. We focus our analysis on the E(S 1)>2E(T 1) thermodynamic condition and on the appropriate charge transfer (CT) character of S 1. We map the CT character with respect to the frontier molecular orbital (FMO) energies of the constituent monomers, providing a cost-effective protocol for an accelerated screening of promising iSF donor-acceptor pairs, while minimizing the number of computations. These parameters are applied to a chemically diverse, curated library of 81 truncated dimers of synthetically feasible donor-acceptor copolymers. From our dataset, four candidates are flagged for iSF, two of which were previously experimentally reported. This protocol is envisioned to be scaled up for the high-throughput screening of large databases of donor-acceptor dimers for the design and identification of conjugated polymers capable of iSF. File list (2) download file view on ChemRxiv Main_Text.pdf (1.29 MiB) download file view on ChemRxiv ESI.pdf (6.63 MiB)

Identifying the Trade-off between Intramolecular Singlet Fission Requirements in Donor–Acceptor Copolymers

“…and ∆ , we resort to the analysis of the FMO expressions and (see Scheme 1), which we designed to enable a fragmentbased analysis of these key descriptors, in a computationally-efficient manner. 25 These two variables suffice to describe the four FMO energies of the individual D and A monomers, and to predict their behavior once they are coupled in a DA dimer (see Scheme 1). Systems with both values above 1 constitute the target regime, since then the D-to-A charge transfer (CT, green arrows) excitation will be favored over the local excitation in either the acceptor (red arrows) or the donor (blue arrows), thus fulfilling the coupling criterion.…”

“…61 Full details for the choice of method and benchmarking are given in previous work. 25 The character of the excited states are evaluated using the charge transfer numbers (Ω → ) gathered from the transition density matrices of a given excited state E, which express the accumulation of hole and electron density on molecular fragments i and j, respectively. 62 These values are obtained by parsing the Gaussian output files with cclib 63 and using TheoDORE (version 1.7.2) 64,65 to compute the quantity of hole and electron density accumulated on the donor and acceptor fragments of the dimer.…”

“…1,23,24 The first condition is purely thermodynamic, the second ensures efficient fission from the 1 to the 1 state, and the third limits the reverse process of triplet-triplet annihilation back to a singlet. These parameters have been outlined in the context of D-A polymer-based iSF [15][16][17][18][19][20]25 and can be seen as follows: the donor chromophore acts as the site of photon absorption and singlet formation, the coupling between the donor and the acceptor through the conjugated chain leads to the triplet pair being formed on acceptors adjacent to the absorbing donor, and the acceptor has a low triplet energy so as to accommodate the triplets. The spatial separation of the acceptors along the polymer chain ensures that the triplets, once independent, do not recombine.…”

Women in the Singlet Fission World: Pearls in a Semi-Open Shell

“…Molecules 2021, 26, x FOR PEER REVIEW 10 of 14 a cost-effective computational protocol "for an accelerated screening of promising iSF donor-acceptor pairs" on her shoulders while minimizing the number of computations [76].…”

“…The good charge transport properties and tunable structures have made donor–acceptor copolymers the materials of choice for Clémence Corminboeuf in her theoretical investigations. She is the woman who took the task to guide the way towards providing a cost-effective computational protocol “for an accelerated screening of promising iSF donor–acceptor pairs” on her shoulders while minimizing the number of computations [ 76 ]. In a very recent study, she and her co-workers built a library of 81 dimers of donor–acceptor copolymers, and this was only their first step towards the creation of a database of such dimers for engineering iSF copolymers.…”