On the nature of the lowest electron transitions in the borine dye derivatives benz[cd]indole

[Display omitted] •The new series of cationic cyanine dyes were synthesized.•Cyclization of the both terminal groups by BF2 bridge changes the total charge in the symmetrical dyes.•Cyclization of the both terminal groups by BF2 bridge practically does not change the optical properties of dyes.•High quantum yield of dye molecules is determined by the spatial structure of the flat dye molecular geometry. The fluorescence and absorption spectra of new boron-containing dyes have been analyzed in detail basing on the results of quantum chemical calculations that were performed to obtain the equilibrium molecular geometry and electron structure of the dye molecules. It has been found that the cyclization of both terminal groups by BF2 bridge changes the total charge in the symmetrical dyes and practically does not change the bond lengths in the π-electron system which increases the quantum yield of fluorescence. The possibility to vary the absorption region and total neutral charge makes these dyes promising for using as additives in light emitting layers of OLED deposited by vacuum evaporation.