Deep Blue Asymmetrical Streptocyanine Dyes: Synthesis, Spectroscopic Characterizations, and Ion-Specific Cooperative Adsorption at the Surface of TiO2 Anatase Nanoparticles

Various triflate (TfO–), tetrafluoroborate (BF4–), and perchlorate (ClO4–) salts of (E)-9-(3-(4-(diphenylamino)­phenyl)­allylidene)-9H-carbazol-9-ium chromophores, asymmetrical streptocyanine blue dyes, were synthesized, and their structures and photochemical and electrochemical properties were characterized. Displaying a high absorption band in the wavelength range 620–640 nm in acidified benzonitrile, the dyes provide an intense absorption at longer wavelength thanks to their oxidized π-conjugated systems. The π-electronic structure underwent twisting and bending, counteracting the electron-donating/withdrawing induction of its different substituents with a propensity to cancel its dipole moment and HOMO–LUMO charge-transfer coefficient. Zeta potential measurements of colloidal suspensions of sensitized TiO2 anatase nanoparticles outlined the difference in electric behavior of the organic ionic pairs at the surface of the nanoparticles: The noncoordinating or coordinating characters of the counterions were found governing the molecular density at the surface and the adsorption/desorption kinetics of the ionic pairs. Dye-sensitized solar cell (DSSC) measurements performed with I–/I3– as redox shuttle outlined the availability of the dyes for solar cell sensitization and the conditions for best stability/efficiency of such π-conjugated structures according to the nature of the counterion.