Book Description

In this PhD thesis, the results of molecular dynamics simulation studies of structural properties of nano-aggregates and experimental time-resolved spectroscopy studies of exciton dynamics in nano-structures of chromophores are presented. The OPLS force field parameters of chlorophyll a, astaxanthin (a carotenoid) and phenyltrimethoxysilane molecules are developed to study their structural, physical and thermodynamic properties in solution using classical molecular dynamics simulations. Simulations of chlorophyll a in different solvents show formation of monomeric, dimeric and multimeric structures in methanol, benzene and water, respectively. The structures of the aggregates show that different functional groups present in the ring of the molecule, hydrophobicity of the phytol tail and the water molecules coordinated to the Mg of the chlorin ring play important role in aggregation. Simulations of astaxanthin in water and ethanol mixtures show formation of aggregates in the mixtures in which the water content is more than 50\%. The results show that hydrophobicity of the conjugated chain in astaxanthin plays a major role in aggregation. Apart from the natural systems like light-harvesting complexes, chlorophylls and carotenoids also aggregate on surfaces. In light-harvesting complexes, the aggregation is controlled by proteins in such a way that the aggregates efficiently collect sunlight, which the plants use for photosynthesis. Such a controlled aggregation is also necessary to develop nano-antennas of these chromophores for artificial photosynthesis or other photovoltaic systems. One of the ways to control their aggregation in surfaces is to change the hydrophobicity of the surface. For this reason, a molecular model of the phenyltrimethoxysilane has been parameterized to model hydrophobic phenyl-functionalized inorganic surfaces like silica surface. Functioning of nano-assemblies of chromophores for photovoltaic application relies on formation of excitons, their motion, energy dissipation, charge separation, etc. that follow the absorption of photons. The processes like formation of excitons and charge separation are desirable while energy dissipation by vibrational relaxation are undesirable. In order to control aggregation such that the desirable functions are maximized, the different processes occurring in the nano-aggregates need to investigated. These processes, which occur in femto-second to pico-second timescales, can be studied using different techniques of time-resolved spectroscopy. However, the widely used techniques in time-resolved spectroscopy do not have spatial resolution high enough to study dynamics in individual nano-structures or nano-meter or sub-nanometer thin layers of chromophores. The experimental work presented here present the development and implementation of two techniques: near-field pump-probe technique to study the ultra-fast processes in nano-structures with 100 nm spatial resolution, and transient grating technique to study ultra-fast processes in few to sub-nanometer thin films of chromophores. Results of the investigation of exciton dynamics using the two techniques on 3,4,9,10 Perylenetetracarboxylic dianhydride show ultra-fast exciton annihilation and self-trapping of excitons at high exciton densities. The results also show that the pump-probe spectroscopy using the near field technique allows one to quantify the annihilation rate and diffusion constant of the excitons in nano-crystals. These techniques can also be used to investigate ultra-fast processes in the nano-structures of chlorophylls, carotenoids and their derivatives on functionalized surfaces.