Triplet conformation in chromophore-fused cyclooctatetraene dyes

Cyclooctatetraene (COT) has a tub-shaped conformation in its ground non-aromatic state, while it is predicted to have a planar aromatic triplet state. However, COT possesses a non-vertical triplet state and therefore its detection has remained elusive through direct photoexcitation. Recently, fluorescent dyes synthesized by fusing COT with anthracene and phenazine "wings" have shown a large Stokes shift implying a significant conformational change in their excited singlet manifold. Here, using broadband transient absorption spectroscopy supported with electronic structure calculations, we demonstrate that intersystem crossing occurs in the planar COT conformation in these flapping dyes. We confirm that such a planar conformation in the triplet state does not impart aromaticity to the COT backbone, as previously predicted. Our work therefore provides the basis for the requisite structural design principles that would allow for triplet formation from excited singlet states in COT-based functional molecules. Cyclooctatetraene (COT) is predicted to have a planar aromatic triplet state. Using femtosecond-to-microsecond transient absorption spectroscopy, we show that acene chromophore-fused COT dyes also have a planar COT ring in their triplet states, although they are not aromatic.