Screening Database to Discover Donor-Acceptor copolymers for intramolecular Singlet Fission

Project Information

D3AiSF

Grant agreement ID: 836849

DOI

10.3030/836849

Project terminated on 31 March 2022

EC signature date 9 April 2019

Start date 1 February 2020

End date 3 May 2022

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 191 149,44

EU contribution

€ 191 149,44

Coordinated by

ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE
Switzerland

Periodic Reporting for period 1 - D3AiSF (Screening Database to Discover Donor-Acceptor copolymers for intramolecular Singlet Fission)

Reporting period: 2020-02-01 to 2022-01-31

Summary of the context and overall objectives of the project

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).

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Work and results:

The project was organized in two work packages (WP). In WP1, we established a cost-effective computational protocol able to determine the capabilities of D-A copolymers for iSF based on the S1 and T1 excited state energies, FMO and excited state character (Chem. Mater. 2020, 32, 6515). Application of this protocol based on TDDFT computations and CT numbers was performed to a large database of substituted D-A pairs (2944) (Chem. Mater. 2021, 33, 2567). The results allowed us to do a comprehensive analysis of the structure-property correlations, which reveal a fundamental trade-off between favourable energetics, S1 CT character and T1 localization (Figure R1). Analysis of the most promising structures allowed us to provide a novel route towards the rational design of iSF D-A candidates (Chem. Commun. 2022, 58, 1338). This consists in the heteroatom oxidation of the sulphur containing units.

In WP2, we first investigated the excited state decay mechanism of the BDT-TDO copolymer prototype. To do so, we built a model Hamiltonian based on the linear vibronic coupling (LVC) model (J. Phys. Chem. Lett. 2020, 11, 9788). With this setup we evaluated the iSF potential of novel promising candidates obtained from the previous high-throughput screening (J. Phys. Chem. Lett. 2021, 12, 7270). In further work we characterized the triplet-triplet dissociation scenario for a series of D-A extended copolymers and disclosed the key molecular features able to optimize both, thermodynamics and kinetics, simultaneously (Chem. Mater. 2022, 10.1021/acs.chemmater.2c00367). Our analysis showed that optimal values can only be achieved when using flexible donors characterized with a coplanar and a non-coplanar conformation such as bithiophene or TVT (Figure R2).

Exploitable results:

Exploitable results are the comprehensive workflow that we propose based on cost-effective DFT and TDDFT computations to find novel iSF compounds, which can be further exploited to reduce its cost by using Machine Learning (ML) approaches. Moreover, we propose a simple molecular design rule such as sulphur oxidation to systematically build promising iSF copolymers from non-oxidase alternatives. This could be easily exploited in synthetic polymer chemistry laboratories to further expand the so far tight list of D-A iSF systems.

Dissemination:

These results were presented as a talk in the 2021 Swiss Association of Computational Chemists meeting and in the International Conference on Photochemistry, ICP 2021. Also in an online webinar in November 2020, for the group of Prof. B. Curchod, and regularly in the LCMD meetings at EPFL. All scientific publications were promoted in social networks (Twitter and ResearchGate). The Computational PhotoChemistry Online Meeting in July 2021 was organized during this project where a total of 16 speakers give a 30 minutes talk about their current research in computational photochemistry.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The results of this project can be further exploited by other researchers working in the development of singlet fission, copolymers and organic optoelectronic materials in general, as well as researchers that want to use the data for Machine Learning and data-mining purposes. All the results of this action have been published in open-access data repositories available for the next years to the scientific community in https://archive.materialscloud.org/.

From the outcomes obtained in the first work package, we propose to further extend the library of D-A combinations, by exploiting the Cambridge Crystallographic Database. This will be the starting point for the search of the next generation of D-A candidates beyond traditional D-A pairs already available in the polymer literature. We anticipate that this will be done in the near future after the completion of this MSCA.

From the outcomes obtained in the second work package, our conclusions push the understanding of the iSF process in conjugated systems, not only in extended D-A copolymers but also in more general covalently-bridged acenes and oligomers. We believe that coplanarity needs to be seen as the main structural feature to consider in through-bond iSF, analogous to the relative disposition of chromophores in through-space SF. On the other side, triplet-pair dissociation is a crucial step for the practical application of SF materials in devices, and thus deserves as much attention as the singlet-triplet splitting.

Correlation between the 5TT-1TT split and the intermediate (or ts) energy of the A-to-A T1 transport

Relationship between the D-to-A CT character of S1 vs the A local of T1, coloured with E(S1-2T1)