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

• Published in: Chemistry, an Asian journal Vol. 12; no. 7; pp. 759 - 767
• Main Authors: Watanabe, Hiroaki, Kamatani, Yusuke, Tamiaki, Hitoshi
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 04.04.2017
• Subjects: Amines - chemistry; Chemistry; Chlorophyll; Chlorophyll - chemistry; Circular Dichroism; coordination; Dimerization; DOSY; Magnetic Resonance Spectroscopy; Models, Molecular; Molecular Structure; steric hindrance; Zinc; Zinc - chemistry; zinc chlorophyll; dimerization; DOSY; zinc chlorophyll; steric hindrance; coordination
• ISSN: 1861-4728; 1861-471X
• DOI: 10.1002/asia.201700015

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 CH2Cl2 solution as well as in a dilute 10 %(v/v) CH2Cl2/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 1H 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 CH2Cl2, while Zn‐2/3 gave open and flexible dimers in a concentrated CH2Cl2 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. Coordinated movements: Synthetic zinc chlorophyll derivative Zn‐1 possessing a dimethylamino group at the C31 position dimerized in CH2Cl2 by one‐by‐one coordination to give J‐type opened‐to‐closed dimers. These dimers were partially π‐overlapped along the y‐axis with a red‐shifted Qy band, depending on concentration and temperature.