Decoupling excitons from high-frequency vibrations in organic molecules

Pratyush Ghosh, Antonios M. Alvertis, Rituparno Chowdhury, Petri Murto, Alexander J. Gillett, Shengzhi Dong, Alexander J. Sneyd, Hwan-Hee Cho, Emrys W. Evans, Bartomeu Monserrat, Feng Li, Christoph Schnedermann, Hugo Bronstein, Richard H. Friend and Akshay Rao ()
Additional contact information
Pratyush Ghosh: University of Cambridge
Antonios M. Alvertis: NASA Ames Research Center
Rituparno Chowdhury: University of Cambridge
Petri Murto: University of Cambridge
Alexander J. Gillett: University of Cambridge
Shengzhi Dong: Jilin University
Alexander J. Sneyd: University of Cambridge
Hwan-Hee Cho: University of Cambridge
Emrys W. Evans: University of Cambridge
Bartomeu Monserrat: University of Cambridge
Feng Li: Jilin University
Christoph Schnedermann: University of Cambridge
Hugo Bronstein: University of Cambridge
Richard H. Friend: University of Cambridge
Akshay Rao: University of Cambridge

Nature, 2024, vol. 629, issue 8011, 355-362

Abstract: Abstract The coupling of excitons in π-conjugated molecules to high-frequency vibrational modes, particularly carbon–carbon stretch modes (1,000–1,600 cm−1) has been thought to be unavoidable1,2. These high-frequency modes accelerate non-radiative losses and limit the performance of light-emitting diodes, fluorescent biomarkers and photovoltaic devices. Here, by combining broadband impulsive vibrational spectroscopy, first-principles modelling and synthetic chemistry, we explore exciton–vibration coupling in a range of π-conjugated molecules. We uncover two design rules that decouple excitons from high-frequency vibrations. First, when the exciton wavefunction has a substantial charge-transfer character with spatially disjoint electron and hole densities, we find that high-frequency modes can be localized to either the donor or acceptor moiety, so that they do not significantly perturb the exciton energy or its spatial distribution. Second, it is possible to select materials such that the participating molecular orbitals have a symmetry-imposed non-bonding character and are, thus, decoupled from the high-frequency vibrational modes that modulate the π-bond order. We exemplify both these design rules by creating a series of spin radical systems that have very efficient near-infrared emission (680–800 nm) from charge-transfer excitons. We show that these systems have substantial coupling to vibrational modes only below 250 cm−1, frequencies that are too low to allow fast non-radiative decay. This enables non-radiative decay rates to be suppressed by nearly two orders of magnitude in comparison to π-conjugated molecules with similar bandgaps. Our results show that losses due to coupling to high-frequency modes need not be a fundamental property of these systems.

Date: 2024
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DOI: 10.1038/s41586-024-07246-x

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