Structure–Property Relationships for Self‐Assembled Zinc Chlorin Light‐Harvesting Dye Aggregates

Abstract

A series of zinc 3(1)-hydroxymethyl chlorins 10 a-e and zinc 3(1)-hydroxyethyl chlorins 17 with varied structural features were synthesized by modifying naturally occurring chlorophyll a. Solvent-, temperature-, and concentration-dependent UV/Vis and CD spectroscopic methods as well as microscopic investigations were performed to explore the importance of particular functional groups and steric effects on the self-assembly behavior of these zinc chlorins. Semisynthetic zinc chlorins 10 a-e possess the three functional units relevant for self-assembly found in their natural bacteriochlorophyll (BChl) counterparts, namely, the 3(1)-OH group, a central metal ion, and the 13(1) C==O moiety along the Qy axis, and they contain various 17(2)-substituents. Depending on whether the zinc chlorins have 17(2)-hydrophobic or hydrophilic side chains, they self-assemble in nonpolar organic solvents or in aqueous media, respectively. Zinc chlorins possessing at least two long side chains provide soluble self-aggregates that are stable in solution for a prolonged time, thus facilitating elucidation of their properties by optical spectroscopy. The morphology of the zinc chlorin aggregates was elucidated by atomic force microscopy (AFM) studies, revealing well-defined nanoscale rod structures for zinc chlorin 10 b with a height of about 6 nm. It is worth noting that this size is in good accordance with a tubular arrangement of the dyes similar to that observed in their natural BChl counterparts in the light-harvesting chlorosomes of green bacteria. Furthermore, for the epimeric 3(1)-hydroxyethyl zinc chlorins 17 with hydrophobic side chains, the influence of the chirality center at the 3(1)-position on the aggregation behavior was studied in detail by UV/Vis and CD spectroscopy. Unlike zinc chlorins 10, the 3(1)-hydroxyethyl zinc chlorins 17 formed only small oligomers and not higher rod aggregate structures, which can be attributed to the steric effect imposed by the additional methyl group at the 3(1)-position.

Columnar Mesophases Based on Zinc Chlorophyll Derivatives Functionalized with Peripheral Dendron Wedges

“…Chl a was extracted from cyanobacterium Spirulina platensis and was converted to pheophorbide a (pheo a) according to literature procedures. [27] The latter compound was transformed into 3 1 -methoxypheo a [27b] and was further hydrolyzed with conc. hydrochloric acid into the corresponding 17 2 -carboxylic acid derivative.…”

Zinc chlorins for artificial light-harvesting self-assemble into antiparallel stacks forming a microcrystalline solid-state material

“…S4 and Table S3) allowing us to eliminate this model. It is also known that compounds 1 and 2 in non polar solvents (14,23) show an aggregate bathochromic shift of the Q y bands of Ϸ80-100 nm, and such shifts are not possible for dimers. Because similar solvent combinations are used to obtain solid aggregates, closed-dimers are unlikely.…”

“…Zinc chlorins 1 and 2 were synthesized starting from Chl a, extracted from Spirulina platensis cyanobacteria (13,14,23). The intermediate compounds and the final zinc chlorin dyes 1 and 2 were purified by silica-gel column chromatography and subsequently by reverse-phase HPLC, and were characterized by solution-state 1 H NMR and high resolution mass spectrometry (HRMS).…”

Coarse-grained molecular dynamics studies of the structure and stability of peptide-based drug amphiphile filaments

“…Moreover, the strength of the π-π stacking interactions between aromatic groups in competition with additional intermolecular forces such as hydrogen bonding and electrostatics may play a significant role in determining the chirality of supramolecular systems. The methodologies developed here can be applied to study the structure and stability of additional supramolecular assemblies governed by a competition of aromatic and hydrogen-bonding interactions 94–96 , as well as peptide-amphiphiles where electrostatic interactions play a key role. 5 …”