Unsymmetrical squaraine dimer with an extended π-electron framework: An approach in harvesting near infra-red photons for energy conversion

A fully conjugated unsymmetrical dimeric squaraine dye was synthesized. The synthetic key step for this dye, a selective Knoevenagel type condensation of end groups to the core moiety, allows the incorporation of a carboxylic acid function essentially for the strong coupling to the titanium dioxide (TiO2) surface. Moreover, the dimer exhibit outstanding optical properties such as absorption maximum in the NIR region with a huge molar absorption coefficient. The solvatochromic effects were also studied. Furthermore the electrochemical properties were determined with cyclic voltammetry. The reduction potential is just high enough to allow an efficient photo induced charge injection into the conduction band of the TiO2 semiconductor and the oxidation potential lies about 300 mV below the potential of the iodide / triiodide redox couple providing a good regeneration driving force. From electron density distribution calculations of the optimized BSQ01 structure we could see the directional charge displacement from the solvent exposed side in the HOMO to the carboxyl anchor group in the LUMO. The dimer was immobilised on TiO2 nanoparticles and a first dye sensitized solar cell was made as a proof-of-concept.