Since 2006, singlet fission (SF) gained considerable interest due to its potential to increase solar cell efficiencies (J. Appl. Phys. 2006, 100, 074510). SF is a multiple exciton generation (MEG) process by which a singlet exciton (S1) splits into two triplet excitons (2T1), thereby generating two electron-hole pair carriers from each absorbed photon. Ultrafast singlet-triplet conversion takes place through a correlated triplet pair state (1(TT)) that has an overall singlet spin state. Thus, SF is often described as a two-step process involving two-chromophore centers.
Initial research was mainly focused on intermolecular SF (xSF). However, the unpredictable molecular arrangement of the chromophores in the crystal prevents the rational design of new materials with appropriate arrangements for efficient xSF. In contrast, intramolecular SF (iSF) offers a solution to this problem. In 2015 particular attention was placed on donor-acceptor (DA)-type copolymers owing to their promising characteristics in displaying efficient iSF (Nat. Mater. 2015, 14, 426). The clear advantage of the DA copolymer scheme is its modularity, which allows to select different D and A units to build fission-capable polymers. Certainly, progress on iSF polymeric systems can benefit from computational efforts.
The objective of this proposal is to combine state-of-the-art computational tools with fundamental concepts of quantum chemistry to advance the iSF field by means of high-throughput (HT) screening of efficient DA-polymers and quantum dynamics simulations. The first step of this project is to design an automated workflow to screen a large number of SF-capable DA-copolymers based on appropriate descriptors. In a second stage, the SF-performance of the very best potential candidates is evaluated in terms of real time quantum dynamics of the singlet splitting nonadiabatic process to ultimately assess the iSF capabilities of the promising candidates.
After the completion of this project, we have proposed novel routes to the design of iSF capable D-A copolymers, either using a high-throughput screening protocol applied to a diverse database of D-A units (Chem. Mater. 2020, 32, 6515; 2021, 33, 2567), or using a simple molecular approach consisting at heteroatom oxidation of the building blocks in already existing D-A copolymers (Chem. Commun. 2022, 58, 1338). On the other side, our results unravelled the S1-to-1TT excited state decay mechanism in prototypical D-A copolymers (J. Phys. Chem. Lett. 2020, 11, 9788; 2021, 12, 7270) and turned the attention to two fundamental features that need to be considered in the future development of iSF D-A copolymers, which are coplanarity and triplet-pair dissociation (10.1021/acs.chemmater.2c00367).
