Excited-state dynamic planarization processes play a crucial role in determining exciton size in cyclic systems, as reported for π-conjugated linear oligomers. Herein, we report time-resolved fluorescence spectra and molecular dynamics simulations of π-conjugated cyclic oligothiophenes in which the number of subunits was chosen to show the size-dependent dynamic planarization in the vicinity of a ring-to-linear behavioral turning point. Analyses on the evolution of the total fluorescence intensity and the ratio between 0-1 to 0-0 vibronic bands suggest that excitons formed in a cyclic oligothiophene composed of six subunits fully delocalize over the cyclic carbon backbone, whereas those formed in larger systems fail to achieve complete delocalization. With the aid of molecular dynamics simulations, it is shown that distorted structures unfavorable for efficient exciton delocalization are more easily populated as the size of the cyclic system increases. Going around in circles: Excited-state dynamic planarization processes of π-conjugated cyclic oligothiophenes were investigated using time-resolved fluorescence spectra and molecular dynamics simulations. Excitons formed in a cyclic oligothiophene composed of six subunits fully delocalize ("cyclic exciton"), whereas those formed in larger systems fail to achieve complete delocalization ("acyclic exciton").
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