Charge-Separation and Charge-Recombination Rate Constants in a Donor-Acceptor Buckybowl-Based Supramolecular Complex: Multistate and Solvent Effects.

Jesús Cerdá,Juan Aragó,Enrique Ortí,Joaquín Calbo

doi:10.1021/acs.jpca.1c05740

Abstract

The kinetics of the nonradiative photoinduced processes (charge-separation and charge-recombination) experimented in solution by a supramolecular complex formed by an electron-donating bowl-shaped truxene-tetrathiafulvalene (truxTTF) derivative and an electron-accepting fullerene fragment (hemifullerene, C30H12) has been theoretically investigated. The truxTTF·C30H12 heterodimer shows a complex decay mechanism after photoexcitation with the participation of several low-lying excited states of different nature (local and charge-transfer excitations) all close in energy. In this scenario, the absolute rate constants for all of the plausible charge-separation (CS) and charge-recombination (CR) channels have been successfully estimated using the Marcus–Levich–Jortner (MLJ) rate expression, electronic structure calculations, and a multistate diabatization method. The outcomes suggest that for a reasonable estimate of the CS and CR rate constants, it is necessary to include the following: (i) optimally tuned long-range (LC) corrected density functionals, to predict a correct energy ordering of the low-lying excited states; (ii) multistate effects, to account for the electronic couplings; and (iii) environmental solvent effects, to provide a proper stabilization of the charge-transfer excited states and accurate external reorganization energies. The predicted rate constants have been incorporated in a simple but insightful kinetic model that allows estimating global CS and CR rate constants in line with the most generalized three-state model used for the CS and CR processes. The values computed for the global CS and CR rates of the donor–acceptor truxTTF·C30H12 supramolecular complex are found to be in good agreement with the experimental values.

Highlights

Since their discovery,[1,2] organic solar cells (OSCs) have been considered as potential alternatives to silicon photovoltaic cells, mainly due to their low cost, easy processing, and low toxicity.[3]
The values computed for the global CS and CR rates of the donor− acceptor truxTTF·C30H12 supramolecular complex are found to be in good agreement with the experimental values
The suggested theoretical protocol including multistate effects and an accurate state-specific description of the solvent effects is of general application to any other D−A molecular or supramolecular system with potential for organic solar cells

Summary

Introduction

Since their discovery,[1,2] organic solar cells (OSCs) have been considered as potential alternatives to silicon photovoltaic cells, mainly due to their low cost, easy processing, and low toxicity.[3]. The processes occurring in a standard OSC can be summarized as follows: (i) light absorption by the donor compound (exciton formation), (ii) exciton migration to the interface between the donor and acceptor, and (iii) electron transfer from the donor to the acceptor (i.e., charge separation, CS), with the consequent generation of a chargetransfer (CT) state. If we turn our attention to the active materials involved in OSCs, fullerenes and fullerene derivatives are the most used electron-acceptor systems for OSC applications.[6−9] In particular, [6,6]-phenyl-C61-butyric acid methyl ester (known as PCBM) is likely to be the most employed acceptor for bulk heterojunction solar cells.[10−12] The combination of PCBM with poly(3-hexylthiophene) (P3HT), acting as a donor, has been widely studied as a model system to gain insight into the elementary physical processes occurring in OSCs.[3,13−15] In the last years, the quest for novel non-fullerene acceptors for photovoltaic applications has emerged as an active research field to boost the potential and application of OSCs.[16−19] Recently, novel fullerene fragments known as buckybowls

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Conclusion