Photophysical, electrochemical, and photoelectrochemical properties of new azulene-based dye molecules

Four new azulene-based dye molecules, 3-(azulen-1-yl)-2-cyanoacrylic acid (Azu-1), 3-(7-isopropyl-1,4-dimethylazulen-3-yl)-2-cyanoacrylic acid (Guai-1), 5-(azulen-1-yl)-2-cyanopenta-2,4-dienoic acid (Azu-2), and 5-(7-isopropyl-1,4-dimethylazulen-3-yl)-2-cyanopenta-2,4- dienoic acid (Guai-2), were synthesized and their photoelectrochemical properties were studied in dye-sensitized solar cells (DSSCs). All of them exhibit, in the visible region, a strong absorption band coming from the S-0-S-2 transition and a very weak band coming from the S-0-S-1 transition, and the transition assignments are supported by theoretical calculations using time-dependent density functional theory (TD-DFT) at the B3LYP/6-31G* level. In sensitization of nanocrystalline TiO2 electrodes, reducing their adsorption amount on the TiO2 surface (by co-adsorption with deoxycholic acid) mitigates dye aggregation and improves their photoelectric conversion efficiency greatly. Also, extending the conjugated side chain (Azu-2 vs. Azu-1 or Guai-2 vs. Guai-1) not only shifts their photoelectric response to longer wavelengths and therefore enhances the short-circuit photocurrent, but also increases the open-circuit photovoltage significantly. Moreover, it was found that the electron injection efficiencies varied remarkably with excitation wavelength, suggesting direct electron injection from the S-2 state of these dye molecules.