Cyclooctatetraene (COT) has a tub-shaped conformation in its ground non-aromatic state while dynamically interconverting between its bent conformers. Photoexcitation of cyclooctatetraene to its triplet state leads to a relaxed planar geometry of the COT ring which shows aromatic stabilization. In contrast, a molecular dye constructed by fusing two anthraceneimide chromophores symmetrically across the COT-ring (FLAP1) show a highly Stokes-shifted vibronic emission which indicates a large-amplitude conformational relaxation in the singlet excited state. Similar Stokes-shifted emission was also observed when the two anthraceneimide units were replaced by two phenazine rings (N-FLAP). Here, using broadband transient absorption spectroscopy, we report the absorption signatures of planar FLAP1 and N-FLAP conformers in the excited state and track the subsequent formation of a triplet through classical intersystem crossing. DFT-based computations support the COT planarization in the excited singlet state along a newly-defined flapping reaction coordinate, while indicating that a planar geometry may also be possible for the lowest triplet state. Our studies provide a glimpse of the reaction coordinates that enable a planar singlet excited state in COT-based dyes, and open new questions on the nature of the triplet state potential energy surface.

Excited state planarization in chromophore-fused cyclooctatetraene dyes

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