Issue 27, 2020

Photophysical implications of ring fusion, linker length, and twisting angle in a series of perylenediimide–thienoacene dimers

Abstract

Perylenediimide (PDI) derivatives have been widely studied as electron acceptor alternatives to fullerenes in organic photovoltaics (OPVs) because of their tunable absorption in the visible range, inexpensive synthesis, and photochemical stability. A common motif for improving device efficiency involves joining multiple PDIs together through electron-rich linkers to form a twisted acceptor–donor–acceptor molecule. Molecular features such as ring fusion are further employed to modify the structure locally and in films. These synthetic efforts have greatly enhanced OPV device efficiencies, however it remains unclear how the increasingly elaborate structural modifications affect the photophysical processes integral to efficient photon-to-charge conversion. Here we carry out a systematic study of a series of PDI dimers with thienoacene linkers in which the twist angle, linker length, and degree of ring fusion are varied to investigate the effects of these structural features on the molecular excited states and exciton recombination dynamics. Spectroscopic characterization of the dimers suggest that ring fusion causes greater coupling between the donor and acceptor components and greatly enhances the lifetime of a thienoacene to PDI charge transfer state. The lifetime of this CT state also correlates well with the linker–PDI dihedral angle, with smaller dihedral angle resulting in longer lifetime. DFT and two-photon absorption TDDFT calculations were developed in-house to model the ground state and excited transitions, providing theoretical insight into the reasons for the observed photophysical properties and identifying the charge transfer state in the excited state absorption spectra. These results highlight how the longevity of the excited state species, important for the efficient conversion of excitons to free carriers in OPV devices, can be chemically tuned by controlling ring fusion and by using steric effects to control the relative orientations of the molecular fragments. The results provide a successful rationalization of the behavior of solar cells involving these acceptor molecules.

Graphical abstract: Photophysical implications of ring fusion, linker length, and twisting angle in a series of perylenediimide–thienoacene dimers

Supplementary files

Article information

Article type
Edge Article
Submitted
21 5 2020
Accepted
17 6 2020
First published
01 7 2020
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY license

Chem. Sci., 2020,11, 7133-7143

Photophysical implications of ring fusion, linker length, and twisting angle in a series of perylenediimide–thienoacene dimers

A. A. Leonard, M. A. Mosquera, L. O. Jones, Z. Cai, T. J. Fauvell, M. S. Kirschner, D. J. Gosztola, G. C. Schatz, R. D. Schaller, L. Yu and L. X. Chen, Chem. Sci., 2020, 11, 7133 DOI: 10.1039/D0SC02862B

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given.

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