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 transfer from the photo‐excited ZnTPPS to the phthalocyanine core with a rate constant (kENT) in the order of 109 s−1. The population of phthalocyanine radical cations indicates the occurrence of electron transfer from the excited phthalocyanine to the AQ moieties, forming a charge‐separated state. β‐Cyclodextrin‐conjugated zinc(II) phthalocyanines form stable host–guest complexes with a tetrasulfonated porphyrin and sodium 2‐anthraquinonesulfonate (AQ) in water. Upon excitation of the porphyrin unit, they undergo excitation energy transfer followed by electron transfer as shown by various spectroscopic methods, functioning as artificial photosynthetic models.