a-Polymethine-Substituted Boron Dipyrromethenes - BODIPY-Based NIR Cyanine-Like Dyes

A series of dyes bearing polymethine chromophore units in boron dipyrromethene (BODIPY) a-positions were prepared by condensation of 3,5-dimethyl boron dipyrromethene with various hemicyanines. One or two methyl groups of the starting material can react, yielding the corresponding mono- and disubstituted derivatives. The deeply coloured monosubstituted dyes belong to the family of merocyanine dyes and exhibit spectral properties and chemical behaviour similar to those of meso-analogues. On the other hand, disubstituted dyes show more complex absorption spectra compared to their monosubstituted counterparts, and their absorption maxima are redshifted (up to 970 nm, the most redshifted absorption known among borondipyrromethene derivatives). Spectral methods and quantum-chemical calculations suggest evolution of the long-wavelength electronic transition from dipyrromethene type into polymethine on changing the character of the peripheral chromophore system. The nature of highest electronic transitions has been analyzed.