A series of new benzothiazolium hemicyanine dyes (HC-1, HC-2, HC-3, HC-4, and HC-5 in Scheme 1) were designed and synthesized for sensitization of nanocrystalline TiO 2 electrodes by introducing carboxyl, hydroxyl, or sulfonate anchoring groups onto the dyes' skeletons. A naphthothiazolium hemicyanine with both sulfonate and hydroxyl (HC-6) was also prepared for comparison. The photophysical and photoelectrochemical studies revealed that three kinds of efficiencies, i.e. the fluorescence quenching efficiencies of the dyes by colloidal TiO 2 , the monochromatic incident photon-to-current conversion efficiencies (IPCEs) for the dye-sensitized TiO 2 electrodes, and the overall photoelectric conversion efficiencies (g) for the dye-sensitized solar cells (DSSCs) based on these hemicyanines, all depended strongly on the anchoring group types and decreased in the order: carboxyl + hydroxyl . carboxyl . sulfonate + hydroxyl, indicating the importance of the dyes' adsorbing groups for their sensitization effects in DSSCs. The combination of carboxyl and hydroxyl as anchoring groups led to highly efficient IPCEs over a wide spectrum region with the maximum IPCE of 73.6% and a g of 5.2% under AM1.5 Global simulated light (80 mW cm 22 ) for the HC-1 based DSSC, which may result from the complex formation between HC-1 and TiO 2 and the cathodic shift of the excited state oxidation potential.{ Electronic supplementary information (ESI) available: 1 H NMR and MALDI-TOF data for the hemicyanine dyes. See
The electron spin resonance spectrum of superoxide anion radical adsorbed on the surface of colloidal TiO 2 was detected directly at room temperature for the first time. This signal was generated partially by porphyrinsensitized titanium dioxide, i.e., the reduction of adsorbed oxygen on the TiO 2 nanocluster surface by the just injected electron from the excited porphyrin.
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