Supramolecular structure of self-assembled synthetic zinc-13(1)-oxo-chlorins possessing a primary, secondary or...

Zinc-chlorin 3 (see Fig. 2 in text) possessing a tertiary 3(1)-hydroxyl group and a 13-keto group was synthesized as a model for the antenna chlorophylls of green bacteria. Self-aggregation of 3 in nonpolar organic media was examined and compared to 1 and 2 possessing a primary and secondary 3(1)-hydroxyl group, respectively. Zinc-chlorin 3 self-aggregated in 1 vol% CH2Cl2-hexane to form oligomers and showed a red-shifted Qy maximum at 704 nm compared to the monomer (648 nm in CH2Cl2). This red-shift is larger than that of 2S (648-->697 nm) and comparable to that of 2R (648-->705 nm), but smaller than that of 1 (648-->740 nm), indicating that while a single 3(1)-methyl group (prim-OH-->sec-OH) suppressed close and/or higher aggregation, the additional 3(1)-methyl group (sec-OH-->tert-OH) did not further suppress aggregation. The relative stability of the aggregates was in the order 1 > 2R-3 > 2S as determined by visible spectral analyses. Molecular modeling calculations on dodecamers of zinc-chlorins 1, 2R and 3 gave similar well-ordered energy-minimized structures, while 1 stacked more tightly than 2R and 3. In contrast, 2S gave a relatively disordered (twisted) structure. The calculated dodecameric structures could explain the visible spectral data of 1-3 in nonpolar organic media.