Research Projects

The first steps in natural light harvesting proceed with remarkably high speed and efficiency. This is exemplified by chlorosomes, nanotubular chlorophyll aggregates which are one of the most efficient natural light-harvesting systems. Photosynthetic growth vitally depends on chlorosomes which fully exploit the incredibly low photon incident rates available to low-light photosynthetic organisms. Efficient photon capture, exciton delocalization and directional energy funneling make chlorosomes a template for artificial light harvesting research, but with several open questions – 1. What governs the design principles that allow exciton delocalization and directional migration faster than known internal conversion and dissipation? 2. Can artificial chlorosome-like assemblies be tailored to replicate these principles? Approaches include synthetic donor-acceptor (D-A) complexes, DNA-templated chromophore architectures, polymer synthesis with repeat D-A units, and supramolecular chemistry approaches with molecules tailored for directed self-assembly. However, several crucial considerations are missing in these approaches, most importantly, the nature of constituent molecule itself. Porphyrin and derivatives are known to have femtosecond internal conversion driven through Herzberg-Teller couplings between the B (Soret) and Q bands, non-orthogonal Qx and Qy transition dipoles and a large number of Raman vibrations delocalized on the ring skeleton with small Huang Rhys factors. The latter feature is vital for non-adiabatic mixing of vibrational and electronic degrees of freedom in photosynthetic excitons. For example, D-A systems with 10x larger HR factor and electronic couplings operate in parameter regimes distinctly different from photosynthetic excitons. Any inferences about quantum dynamics observed in time-resolved experiments have limited value for understanding and mimicking structure-function relationships that govern chlorosomes. The current proposal addresses the above gaps by following a distinctly different approach. Using porphyrin derivatives, with essentially same features as chlorophylls, and creating a structural hierarchy of self-assembled aggregates – monomer, dimers, rings, tubes and bundles, we propose to systematically elucidate the interplay between internal conversion, exciton delocalization, diffusion and exciton-exciton annihilation as a function of aggregate hierarchy. Our choice of experimental tool, a home-built and patented two-dimensional electronic spectrometer (2DES), is ideal for spectrally congested systems with ability to resolve femtosecond quantum dynamics along excitation, emission and coherence axes, and mathematically unique inversion of observed rates to state-to-state population transfer. Our systematic study is expected to probe the hypothesized emergence of enhanced exciton delocalization and suppression of internal conversion as large number of molecules self-assemble under coherent electronic coupling.