Morisue Mitsuhiko, Haruta Noriko, Kalita Dipak, Kobuke Yoshiaki
Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan.
Chemistry. 2006 Oct 25;12(31):8123-35. doi: 10.1002/chem.200600304.
A novel surface fabrication methodology has been accomplished, aimed at efficient anodic photocurrent generation by a photoexcited porphyrin on an ITO (indium-tin oxide) electrode. The ITO electrode was submitted to a surface sol-gel process with titanium n-butoxide in order to deposit a titanium monolayer. Subsequently, porphyrins were assembled as monolayers on the titanium-treated ITO surface via phosphonate, isophthalate, and thiolate groups. Slipped-cofacial porphyrin dimers, the so-called artificial special pair at the photoreaction center, were organized through imidazolyl-to-zinc complementary coordination of imidazolylporphyrinatozinc(II) units, which were covalently immobilized by ring-closing olefin metathesis of allyl side chains. The modified surfaces were analyzed by means of X-ray photoelectron spectroscopy. Photoirradiation of the porphyrin dimer generated a large anodic photocurrent in aqueous electrolyte solution containing hydroquinone as an electron sacrificer, due to the small reorganization energy of the dimer. The use of different linker groups led to significant differences in the efficiencies of anodic photocurrent generation. The apparent flat-band potentials evaluated from the photocurrent properties at various pH values and under biased conditions imply that the band structure of the ITO electrode is modified by the anchoring species. The quantum yield for the anodic photocurrent generation by photoexcitation at the Soret band is increased to 15 %, a surprisingly high value without a redox cascade structure on the ITO electrode surface, while excitation at the Q band is not so significant. Extensive exploration of the photocurrent properties has revealed that hot injection of the photoexcited electron from the S2 level into the conduction band of the ITO electrode takes place before internal conversion to the S1* state, through the strong electronic communication of the phosphonyl anchor with the sol-gel-modified ITO surface.
已实现一种新型的表面制备方法，旨在通过光激发的卟啉在氧化铟锡（ITO）电极上高效产生阳极光电流。将ITO电极用正丁醇钛进行表面溶胶 - 凝胶处理，以沉积钛单分子层。随后，通过膦酸酯、间苯二甲酸酯和硫醇盐基团将卟啉组装成单分子层覆盖在经钛处理的ITO表面。通过咪唑基卟啉锌（II）单元的咪唑基与锌的互补配位构建了滑移共面卟啉二聚体，即光反应中心的所谓人工特殊对，其通过烯丙基侧链的闭环烯烃复分解反应共价固定。通过X射线光电子能谱对改性表面进行了分析。由于二聚体的重组能较小，卟啉二聚体的光照射在含有对苯二酚作为电子牺牲剂的水性电解质溶液中产生了大的阳极光电流。使用不同的连接基团导致阳极光电流产生效率存在显著差异。从各种pH值和偏置条件下的光电流特性评估得到的表观平带电位表明，ITO电极的能带结构被锚定物种改变。在Soret带光激发产生阳极光电流的量子产率提高到了15%（在ITO电极表面没有氧化还原级联结构的情况下，这是一个惊人的高值），而在Q带激发则不那么显著。对光电流特性的广泛探索表明，通过膦酰基锚与溶胶 - 凝胶改性的ITO表面的强电子通信，光激发电子从S2能级热注入到ITO电极的导带发生在内部转换到S1*态之前。
