Substituent Effects on the Photophysical Properties of a Series of 8(meso)-Pyridyl-BODIPYs: A Computational Analysis of the Experimental Data

Recently, a series of 8(meso)-pyridyl-BODIPYs (2-pyridyl, 3-pyridyl, and 4-pyridyl) and their 2,6-substituted derivatives were synthesized and their structure and photophysical properties were studied both experimentally and computationally. One of the main observed trends was that the 2-pyridyl-BODIPYs were consistently less fluorescent than their 3-pyridyl and 4-pyridyl analogs, regardless of the 2,6-substituents. Herein, we extend our previous computational studies and model not only the ground but also the excited states of the entire series of previously synthesized meso-pyridyl-BODIPYs with the aim of explaining the observed differences in the emission quantum yields. To better understand the trends and the effect of 2- and 2,6-substitution on the photophysical and electron-density-related properties, we also model the ground and excited states of BODIPYs that were not synthesized experimentally, however represent a logical part of the series. We calculate a variety of molecular properties and propose that the experimentally observed low quantum yields for all 2-pyridyl-BODIPYs could be due to the very flat potential energy surfaces with respect to the rotation of the 2-pyridyl ring in the excited states, and the stability of a non-planar and significantly less fluorescent meso-2-pyridyl-BODIPY structure.