Coordination‐Driven Dimerization of Zinc Chlorophyll Derivatives Possessing a Dialkylamino Group

Zinc chlorophyll derivatives Zn‐1 – 3 possessing a tertiary amino group at the C3 1 position have been synthesized through reductive amination of methyl pyropheophorbide‐ d obtained from naturally occurring chlorophyll‐ a . In a dilute CH 2 Cl 2 solution as well as in a dilute 10 %(v/v) CH 2 Cl 2 /hexane solution, Zn‐1 possessing a dimethylamino group at the C3 1 position showed red‐shifted UV/Vis absorption and intensified exciton‐coupling circular dichroism (CD) spectra at room temperature owing to its dimer formation via coordination to the central zinc by the 3 1 ‐N atom of the dimethylamino group. However, Zn‐2/3 bearing 3 1 ‐ethylmethylamino/diethylamino groups did not. The difference was dependent on the steric factor of the substituents in the tertiary amino group, where an increase of the carbon numbers on the N atom reduced the intermolecular N⋅⋅⋅Zn coordination. UV/Vis, CD, and 1 H NMR spectroscopic analyses including DOSY measurements revealed that Zn‐1 formed closed‐type dimers via an opened dimer by single‐to‐double axial coordination with an increase in concentration and a temperature decrease in CH 2 Cl 2 , while Zn‐2/3 gave open and flexible dimers in a concentrated CH 2 Cl 2 solution at low temperature. The supramolecular closed dimer structures of Zn‐1 were estimated by molecular modelling calculations, which showed these structures were promising models for the chlorophyll dimer in a photosynthetic reaction center.