Design, Synthesis and Photo-physical Studies of Donor-Acceptor Systems for Achieving Long-Lived Charge-Separated State

Abstract

Photogenerated radical-ion pairs in linked donor-acceptor (D-A) dyads are extremely short-lived because of the very fast charge recombination (CR) reaction either to the D-A ground state or to a low-lying local triplet (3D* or 3A*). Applications of photoinduced electron transfer (PET) in practically relevant areas such as artificial photosynthesis, solar water splitting and photocatalytic reactions require radical ion pairs (also known as charge separated (CS) state) with near-microsecond lifetimes and hence enhancing the CS state lifetimes is a major goal in photochemistry. Most of the studies in this area envisaged the natural photosynthetic reaction center as a model and designed covalently linked triads, tetrads and higher order systems. However, covalent synthesis of large molecular arrays is highly inefficient and expensive and hence assembling such arrays through non-covalent interactions were also attempted. The covalent and non-covalent approaches have very little success in extending the radical ion pair lifetimes beyond the nanosecond domain. The amount of energy that can be stored in the radical ion pair is also very important. In large molecular arrays, long-lived RP is possible because of sequential electron transfer steps that increases the distance between the charge centers. Energy is lost in each of these steps and as a result the RP state will store only about fifty percent of the excitation energy. In this context, design of compact dyads that can generate long-lived radical ions and also store energy in excess of 2.0 eV, looks extremely attractive and important. All the compact dyads designed till date exhibited sub-nanosecond lifetimes. In this thesis, we have attempted the design and study of donor-acceptor dyads that exhibit charge separated state lifetimes in the near microsecond regime.