Dataset Information

Pathway complexity in the self-assembly of a zinc chlorin model system of natural bacteriochlorophyll J-aggregates.

ABSTRACT: Whilst bacteriochlorophyll c, d, and e dyes self-assemble into the most efficient light harvesting J-aggregate systems found in nature, their supramolecular packing arrangements are still a matter of debate and a significant number of models have been suggested for their local and long-range ordering. Here we reveal for a synthetic model system based on a zinc chlorin (ZnChl) dye an intriguing interplay of two competing aggregation pathways by kinetic and thermodynamic studies in MeOH/water solvent mixtures: the formation of kinetically controlled off-pathway nanoparticles consisting of excitonically coupled J-dimers versus the formation of thermodynamically more stable one-dimensional helical fibers consisting of J-coupled extended aggregates. The higher order of the latter is evidenced by atomic force microscopy and a more narrow absorption spectrum of the J-aggregates. Based on a recently developed thermodynamic model that combines the cooperative K2-K growth model with a competing dimerization model, an energy landscape could be derived that describes the pathway complexity of this biomimetic system. Our studies reveal that the kinetic stability of the off-pathway nanoparticles increases with increasing concentration of ZnChl or water content in a MeOH/water solvent mixture. For a water content >90% deeply trapped off-pathway nanoparticle products are formed that do not transform anymore to the more ordered thermodynamic product within reasonable time scales. Based on these observations, we hypothesize that out-of-equilibrium aggregate structures of natural BChl dyes may also exist in the natural chlorosomes of green bacteria.

SUBMITTER: Ogi S

PROVIDER: S-EPMC5914135 | biostudies-literature | 2018 Mar

REPOSITORIES: biostudies-literature

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Pathway complexity in the self-assembly of a zinc chlorin model system of natural bacteriochlorophyll J-aggregates.

Ogi Soichiro S Grzeszkiewicz Charlotte C Würthner Frank F

Chemical science 20180214 10

Whilst bacteriochlorophyll c, d, and e dyes self-assemble into the most efficient light harvesting J-aggregate systems found in nature, their supramolecular packing arrangements are still a matter of debate and a significant number of models have been suggested for their local and long-range ordering. Here we reveal for a synthetic model system based on a zinc chlorin (ZnChl) dye an intriguing interplay of two competing aggregation pathways by kinetic and thermodynamic studi ...[more]

PMID: 29732062

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Project description:A bis(squaraine) dye equipped with alkyl and oligoethyleneglycol chains was synthesized by connecting two dicyanomethylene substituted squaraine dyes with a phenylene spacer unit. The aggregation behavior of this bis(squaraine) was investigated in non-polar toluene/tetrachloroethane (98:2) solvent mixture, which revealed competing cooperative self-assembly pathways into two supramolecular polymorphs with entirely different packing structures and UV/Vis/NIR absorption properties. The self-assembly pathway can be controlled by the cooling rate from a heated solution of the monomers. For both polymorphs, quasi-equilibrium conditions between monomers and the respective aggregates can be established to derive thermodynamic parameters and insights into the self-assembly mechanisms. AFM measurements revealed a nanosheet structure with a height of 2 nm for the thermodynamically more stable polymorph and a tubular nanorod structure with a helical pitch of 13 nm and a diameter of 5 nm for the kinetically favored polymorph. Together with wide angle X-ray scattering measurements, packing models were derived: the thermodynamic polymorph consists of brick-work type nanosheets that exhibit red-shifted absorption bands as typical for J-aggregates, while the nanorod polymorph consists of eight supramolecular polymer strands of the bis(squaraine) intertwined to form a chimney-type tubular structure. The absorption of this aggregate covers a large spectral range from 550 to 875 nm, which cannot be rationalized by the conventional exciton theory. By applying the Essential States Model and considering intermolecular charge transfer, the aggregate spectrum was adequately reproduced, revealing that the broad absorption spectrum is due to pronounced donor-acceptor overlap within the bis(squaraine) nanorods. The latter is also responsible for the pronounced bathochromic shift observed for the nanosheet structure as a result of the slip-stacked arranged squaraine chromophores.

Dataset Information

Pathway complexity in the self-assembly of a zinc chlorin model system of natural bacteriochlorophyll J-aggregates.

Publications

Pathway complexity in the self-assembly of a zinc chlorin model system of natural bacteriochlorophyll J-aggregates.

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