Molecular excitons are prerequisites for important processes of light energy conversion, such as photosynthesis and operation of organic photovoltaic devices. Therefore, the detailed insight into the exciton dynamics induced upon absorption of photons is a primary task. In this contribution, we report on ultrafast exciton localization during excited-state dynamics of a recently synthesized covalently-linked tetracene trimer. To tackle the problem, we employ surface hopping approach to nonadiabatic molecular dynamics in conjunction with long-range-corrected time-dependent density functional tight-binding (LC-TDDFTB) method. We address the exciton dynamics within a manifold of singlet excited states to elucidate the first steps of photophysics occurring in the trimer. Utilizing a set of descriptors based on transition density matrix, we perform comprehensive analysis of the exciton dynamics. While the exciton is delocalized for a ground-state minimum geometry, conformational disorder resulting from thermal motion in the ground state leads to somewhat more localized picture. Subsequently, excited-state dynamics promotes the ultrafast exciton localization (on a time scale of ~17 fs) to a single tetracene unit of the trimer. This localized picture is then maintained throughout the whole simulation period of 500 fs. The localized excitons are transferred between the fragments of the trimer, mainly via transient delocalization mechanism. Our results may serve as a starting point for investigation of singlet fission.

Ultrafast exciton localization during excited-state dynamics of a tetracene trimer: Insight from surface hopping LC-TDDFTB dynamics

256th National Meeting (2018)

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