Polymethine dyes derived from 2,2-difluoro-3,1,2-(2H)-oxaoxoniaboratines with polymethylene bridge groups in the chromophore

Symmetric and unsymmetric carbocyanines derived from 2,2-difluoro-3,1,2-(2H)-oxaoxoniaboratines have been synthesized which contain the polymethylene bridge groups (CH2)n (n = 1, 2, 3) in the chromophore. As shown, these groups acting as electron-donor substituents deepen the dye colour according to the Foerster–Dewar–Knott rule. The enhanced steric hindrances caused by them give rise to the broken planarity of the dye molecules. The 1H NMR chemical shifts for the protons at the meso-position of the chromophore have been correlated with the π-charges on the corresponding carbon atoms calculated by the Pariser–Parr–Pople method.