Synthesis and self-aggregation of zinc chlorophylls possessing an ω-hydroxyalkyl group: effect of distance between interactive hydroxy group and chlorin moiety on aggregationAlternative synthetic approach for 12 and 12D and IR spectra of the precipitates of 3D are available as supplementary data. For direct electronic access see http://www.rsc.org/suppdata/p1/b1/b107902f

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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.

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