Synthesis and self-aggregation of zinc chlorophylls possessing an ω-hydroxyalkyl group: effect of distance between interactive hydroxy group and chlorin moiety on aggregation

Abstract

Zinc 13¹-oxochlorins 2, 3 and 3D possessing 2-hydroxyethyl, 3-hydroxypropyl and 3-hydroxyprop-1-enyl groups, respectively, at the 3-position are synthesized as models for self-aggregative antenna chlorophylls in green photosynthetic bacteria. Self-aggregation of 2, 3 and 3D in nonpolar organic solvents and in the solid state is compared with that of 1 possessing a 3-hydroxymethyl group to determine the effect of the distance between the hydroxy group and the chlorin moiety on the self-aggregation. Visible spectral analyses in hexane containing a small amount of THF reveal that the aggregation abilities decrease in the order of 1 → 2 → 3, with an increase of conformational flexibility of the ω-hydroxyalkyl group in a molecule. Aggregated 2 and 3 give absorption maxima at 701 and 702 nm, respectively, red-shifted from the corresponding monomeric absorption (644 nm). These red-shifts are smaller than that of 1 (647 → 740 nm), which is attributable to the expanded chlorin π–π plane distance in the self-aggregates of 2 and 3. Furthermore, their CD and IR spectra reveal that the aggregates of 3 are relatively disordered and have weak intermolecular noncovalent interactions among the hydroxy group, central zinc and keto group in the supramolecule. Aggregated 3D shows a relatively small red-shift of absorption from monomer to aggregates (654 → 680 nm) due to the decreased overlap between the chlorin π-planes in the aggregated state. However, 3D easily forms precipitates composed of structurally ordered large aggregates, indicating that 3D is favorable for molecular packing in the aggregates.