Ultrafast Excited-State Dynamics within Organic Chromophores and Transition Metal Complexes: Light-Harvesting and Photo-protection

Abstract

“Embracing nature’s brilliance through the lens of ultrafast spectroscopy” Nature stands as a towering testament to mastery, its each and every flawless design motivates us to replicate its brilliance, yet reminds us that true innovation demands unwavering dedication. In nature, sunlight serves as a key source of energy driving life-processes around us. With the ability of converting solar energy into chemical energy, photosynthesis stands out as one of the most important biological processes on earth. Elucidating the key mechanism of photosynthesis and developing artificial photosynthetic systems to mimic this process are promising approaches to address global energy demands and have attracted significant research interest. Time-resolved measurements with high temporal resolution are capable of resolving the crucial events of absorption, energy transfer and charge separation, which are important facet of the function and efficacy of both natural and artificial light-harvesting systems. The present work explores essential insights into the intricate photophysical processes, such as inter- or intramolecular charge and energy transfer in organic chromophores, different multi-chromophoric systems and transition metal complexes with their potential applications in photovoltaics or artificial light harvesting. The fundamental insights into ultrafast relaxation dynamics in wide range of systems are explored in present thesis, which are crucial for advancing the development of artificial light-harvesting devices. The work aspires to contribute to the development of efficient artificial systems for solar energy conversion for photo-voltaic as well as for opto-electronic applications. Following the motivation behind the work, an overview of combination of techniques utilized in the work including femtosecond pump-broadband probe spectroscopy with details of analysis which is extensively utilized to elucidate the excited-state molecular mechanisms in different light-harvesting systems is discussed. The thesis is divided into two parts, where Part A focuses the energy and charge transfer dynamics within organic chromophores, their aggregates and multichromophoric systems including effect of high-energy Söret-band excitation on the excited state dynamics of bioinspired chlorophyll-astaxanthin system, effect of regioisomerism on the ultrafast symmetry-breaking charge separation in perylene monoimide-embedded multichromophores and charge transfer dynamics within molecular dyads. Part B of the thesis explores excited state dynamics in different transition metal complexes. This part includes including discussion on excited state dynamics of ferrocene based bimetallic iridium (III) polypyridyl complex and charge transfer within a two-fold catenated metal organic framework. In later part of the thesis, taking the specific example of solvation dynamics of C153 to investigate the initial solvation process using femtosecond transient absorption spectroscopy is presented and the limitations/ challenges of the same are addressed. Further the capabilities of an alternative technique, i.e., time-resolved impulsive stimulated Raman spectroscopy is explored to offer new insights into the molecular mechanisms governing solvation dynamics. The thesis concludes with summary of the work along with the discussion on the future prospects of this work, with an emphasis on the development of new organic/inorganic hybrid systems to realize versatile energy harvesting for photo-voltaic as well as opto-electronic applicatio