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

Abstract

Intramolecular singlet fission (iSF) has shown potential to improve the power conversion efficiency in photovoltaic devices by promoting the splitting of a photon-absorbing singlet exciton into two triplet excitons within a single molecule. Among different possibilities, the donor-acceptor modular strategy of copolymers has shown great promise in its ability to undergo iSF under certain conditions. However, the number of iSF donor-acceptor copolymers reported in the literature remains remarkably narrow and clear trends for the molecular design of better candidates have not yet been established. In this work, we identify the trade-off between the main iSF requirements of the donor-acceptor strategy and formulate design rules that allow them to be tuned simultaneously in a fragment-based approach. Based on a library of 2944 donor-acceptor copolymers, we establish simple guidelines to build promising novel materials for iSF. These consist in (1st) selecting an acceptor core with high intrinsic singlet-triplet splitting, (2nd) locating a donor with a larger monomer frontier molecular orbital (FMO) gap than that of the acceptor, and (3rd) tuning the relative energy of donor and/or acceptor FMOs through functionalization to promote photoinduced charge transfer in the resulting polymer. Remarkably, systems containing benzothiadiazole and thiophehe-1,1-dioxide acceptors, which have been shown to undergo iSF, fulfill all criteria simultaneously when paired with appropriate donors. This is due to their particular electronic features, which make them highly promising candidates in the quest for iSF materials. File list (2) download file view on ChemRxiv Main_text.pdf (1.26 MiB) download file view on ChemRxiv Supporting_Information.pdf (2.34 MiB)

Heteroatom oxidation controls singlet–triplet energy splitting in singlet fission building blocks

“…All dimers exhibit energy splitting above the vertical threshold Δ E vert ST ≥ −1 eV (Fig. S17, ESI†), which is consistent with our observation 14 that the dimer Δ E ST originates from the monomer with the higher ( i.e. more positive) Δ E ST .…”

“…Here, we identify a chemical functionality, heteroatom oxidation, which modulates the Δ E ST in potential SF chromophores in a foreseeable way (Scheme 1 (iv)). This approach is motivated by the experimental observation that the oxidized form of thiophene (thiophene- S , S -dioxide) is an effective acceptor in donor–acceptor copolymers capable of intramolecular SF (iSF), 12–14 and that nitrone/ N -oxide groups were found in large numbers in a recent screening of thousands of crystal structures for compounds with high Δ E ST . 5 The present results show that, indeed, heteroatom oxidation governs the S 1 and T 1 energies and thus can be used to improve the singlet–triplet splitting of potential SF chromophores.…”

“…We have recently proposed a protocol with which to identify potential polymer candidates for iSF based on the Δ E ST of the constituent monomers and their relative frontier molecular orbital (FMO) energies. 13,14 To assess the performance of these new oxidized units to form iSF-capable D–A pairs, we treat all those in the dataset that fulfill Δ E adia ST ≥ 0 (and Δ E vert ST ≥ −1 eV, vide supra ) as acceptor monomers (34 compounds). The FMOs of each acceptor were compared to all other buildings blocks (2244 monomer pairs), and only those whose FMO arrangement is conducive to CT ( i.e.…”

“…The FMOs of each acceptor were compared to all other buildings blocks (2244 monomer pairs), and only those whose FMO arrangement is conducive to CT ( i.e. donor HOMO higher than acceptor HOMO and donor LUMO higher than acceptor LUMO; see earlier work for details 14 ) were retained. For these 631 D–A combinations, the dimers were generated, their ground state geometries optimized, and their vertical excited states were evaluated at the same level of theory as the monomers.…”

“…We have previously outlined two other requirements beyond Δ E ST for iSF to be possible in D–A systems: S 1 must have significant donor-to-acceptor CT character to drive triplet-pair formation (S 1 → 1 TT), and T 1 must be located on the acceptor to promote dissociation of the triplet states ( 1 TT → T 1 + T 1 ). 13,14 Deactivation of S 1 towards higher energy triplet states is neglected but the S 1 → 1 TT transition required for iSF is expected to be the most efficient decay pathway (see Section S6). Quantum chemical descriptors were introduced to quantify these criteria using the character of excited states (see ESI†).…”