The simultaneous quantum-chemical and spectral investigation of two anionic polymethine dyes derivatives of dioxaborine was carried out. It is shown that there are two paths of relaxation in the excited state: a) cyanine-like path with equalizing the bond lengths and a symmetric charge distribution; b) polyenic path with the considerable bond length alternation and the symmetry breaking in a charge distribution. This assumption is confirmed by the existence of two components in the time-resolved fluorescence spectra measured at low temperatures.