Understanding the detailed mechanism of singlet fission is crucial to the development of new chromophores for efficient fission-sensitised solar cells. In typical solid-state materials, the challenge of controlling the molecular packing and relevant energy levels precludes clear determination of the fission pathway. We instead focus on covalent dimers, in which the excited-state progression is purely intramolecular, to reveal new insights into the mechanism of triplet-pair formation. Initial studies on pentacene-based dimers, in which the process is exothermic, revealed rapid and quantitative singlet fission. The rate and yield of triplet formation could be tuned with solvent polarity and polarisability, highlighting the underlying contribution of charge-transfer states. Ultrafast vibrational spectroscopy reveals that this conversion is vibrationally coherent - mediated by an avoided crossing or conical intersection - and together with advanced quantum-mechanical modelling we can reconstruct the fully atomistic molecular movie of the fission process. In the equivalent tetracene dimer fission is strongly endothermic, yet we observe quantitative singlet fission on comparably fast timescales under optimal conditions. This occurs on a background of non-Kasha emission, symmetry-breaking charge transfer and surprisingly long-lived vibrationally excited states. Using a combination of ultrafast absorption and photoluminescence spectroscopies, we demonstrate that fission is mediated by a surprising 'destabilised' charge-transfer state which enables us to tune from incoherent to coherent fission regimes.

Two's company: Rich photophysics of covalent dimers for singlet fission

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.