4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene fluorophores (known as BODIPYs) have excellent photophysical properties, including sharp absorption and emission bands in the visible spectral region, high fluorescence quantum yields, and high molar extinction coefficients. The properties of BODIPY dyes can be finely tuned via functionalization at the various peripheral positions, including the meso-8-position. In particular, restricting the rotational and vibrational freedom of the meso-aryl substituent can play a substantial role in the observed fluorescence quantum yield due to decreased probability for non-radiative decay. In the present study, a series of meso-8-phenyl, meso-8-mesityl, and meso-8-triphenylphenyl BODIPYs and their 2,6-dichloro substituted analogs were synthesized and studied both experimentally and computationally. Computational modeling of the rotational barriers demonstrated that the presence of 1,7-methyl groups and ortho-substituents on the meso-8-aryl ring prevents free rotation of the meso-8-aryl group, correlating with the experimentally observed increase in the fluorescence quantum yields. In accord with the experiment, substitution at the 2,6-positions of these BODIPYs with chlorine atoms causes reduced band gaps that result in pronounced red-shifted absorptions and emissions. While previous studies have suggested that electron-withdrawing substituents should significantly reduce the “deleterious influence of the phenyl free rotation on their photophysics”, our computational studies demonstrate that the 2,6-dichloro substitution for the investigated series does not significantly affect the rotational barrier and hence, the experimentally observed fluorescence quantum yields.

Effect of restricted meso-8-rotation on the fluorescence properties of a series of BODIPYs

ACS Spring 2022

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