Molecular J Aggregates

In photosynthesis, solar photons are converted into chemical energy with remarkably high efficiency. A thorough understanding of this process and its technological utilization will be a major contribution to a sustainable energy concept. A novel design strategy for artificial light harvesting systems relies on molecular Jelly (J)- or Scheibe aggregates.1 Due to their structural resemblance to natural light harvesting systems, aggregates of cyanine dyes2 are especially interesting candidates for artificial antenna systems. They are chemically versatile and self-assemble in aqueous solution into various extended supramolecular structures. Among them are micro-meter long cylinders, sharing local dye packing features with naturally occurring chlorosomes.3

The chemical structure of a typical cyanine dye monomer is shown in the upper left corner. In aqueous solution, the hydrophobic end groups of the dye will orient towards each other, resulting in bi-layers. This is not the only level of structural organization of the aggregate: as revealed by cryogenic transmission electron microscopy (cryo-TEM),4 the aggregates form tubular strands of ~15 nm outer diameter. In aqueous solution, the aggregation process continues and the single strains coil up to form a superhelical structure. Addition of small amounts of polyvinyl-alcohol (PVA) will stop this last aggregation process by adhesion of PVA to the outer tube’s surface. This shows how minor chemical changes can alter the aggregate’s macromolecular structure drastically.

2D-ES as used in our group is an ideally suited tool for the investigation of molecular J-aggregates and their energy deactivation network. All structural levels of organization will yield closely spaced electronic transitions as shown in the above figure. To study such a system, one can either use narrow bandwidth excitation pulses and lose time resolution or employ temporarily short and broadband pulses; in the latter case, several bands will be excited simultaneously and dynamical information will we “washed out”. 2D-ES overcomes this dilemma: due to its excitation frequency resolved signals, inter-band dynamics in a broad wavelength range can be retrieved with sub-10 fs time resolution.

[1] Scheibe, G. Angewandte Chemie 1937, 50, 212.
[2] Sperling, J.; Nemeth, A.; Hauer, J.; Abramavicius, D.; Mukamel, S.; Kauffmann, H. F.; Milota, F. J Phys Chem A 2010, 114, 8179.
[3] Ganapathy, S.; Oostergetel, G. T.; Wawrzyniak, P. K.; Reus, M.; Chew, A. G. M.; Buda, F.; Boekema, E. J.; Bryant, D. A.; Holzwarth, A. R.; de Groot, H. J. M. P Natl Acad Sci USA 2009, 106, 8525.
[4] von Berlepsch, H.; Kirstein, S.; Hania, R.; Didraga, C.; Pugzlys, A.; Bottcher, C. J Phys Chem B 2003, 107, 14176.