Assembling Artificial Photosynthetic Models in Water Using β-Cyclodextrin-Conjugated Phthalocyanines as Building Blocks

Two water-soluble zinc(II) phthalocyanines substituted with two or four permethylated β-cyclodextrin (β-CD) moieties at the α positions have been utilized as building blocks for the construction of artificial photosynthetic models in water. The hydrophilic and bulky β-CD moieties not only can increase the water solubility of the phthalocyanine core and prevent its stacking in water but can also bind with a tetrasulfonated zinc(II) porphyrin (ZnTPPS) and/or sodium 2-anthraquinonesulfonate (AQ) in water through host-guest interactions. The binding interactions of these species have been studied spectroscopically, while the photoinduced processes of the resulting complexes have been investigated using steady-state and time-resolved spectroscopic methods. In the ternary complexes, the ZnTPPS units serve as light-harvesting antennas to capture the light energy and transfer it to the phthalocyanine core via efficient excitation energy transfer. The excited phthalocyanine is subsequently quenched by the electron-deficient AQ units through electron transfer. Femtosecond transient absorption spectroscopy provides clear evidence for the singlet-singlet energy transferfrom the photo-excited ZnTPPS to the phthalocyanine core with a rate constant (kENT) in the order of 109 s-1. Thepopulation of phthalocyanine radical cations indicates the occurrence of electron transfer from the excited phthalocyanine to the AQ moieties, forming a charge-separated state.