The vibrational spectra of a model boron dipyrromethene molecule in condensed phase: Experimental and theoretical insights

Boron dipyrromethene (BODIPY) derivatives are an important class of organic molecular dyes. Their chemical structure can be easily modified by synthetic methods, introducing various substituent groups on a tricyclic core that represents the characteristic unit of these compounds. As a consequence, BODIPY optical properties can be designed to tune their absorption and emission in an especially wide wavelength range. A large amount of experimental and computational studies has been performed to investigate the electronic excited states of many structurally different BODIPY dyes. On the other hand, vibrational properties are by far less known for these compounds. We have therefore measured detailed infrared and Raman spectra of a model compound, namely, a BODIPY derivative with four methyl groups and a phenyl ring as substituents. The experimental spectra of the solid sample have been contrasted with ab initio spectra computed at the CAM-B3LYP/6-31G+(d) level. Computations have been performed both for the isolated molecule and including the crystalline environment. In the latter case, better agreement has been observed. An assignment is proposed for the 123 vibrational modes. These results form the basis for further spectroscopic studies on complex BODIPY derivatives.