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

Zinc chlorophyll derivatives Zn-1-3 possessing a tertiary amino group at the C3¹ position have been synthesized through reductive amination of methyl pyropheophorbide-d obtained from naturally occurring chlorophyll-a. In a dilute CH₂ Cl₂ solution as well as in a dilute 10 %(v/v) CH₂ Cl₂ /hexane solution, Zn-1 possessing a dimethylamino group at the C3¹ 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 3¹ -N atom of the dimethylamino group. However, Zn-2/3 bearing 3¹ -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 CH₂ Cl₂ , while Zn-2/3 gave open and flexible dimers in a concentrated CH₂ Cl₂ 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.