UC San Diego

The work in this dissertation has two parts. The first focuses on carotenoids and an excited-state mechanism known as singlet exciton fission. This mechanism, which forms two triplet excitons from each singlet, has sparked great interest in part because of the prospects of improving the efficiency of solar cells. Two covalent dimers of zeaxanthin were synthesized to investigate the excited-state dynamics, and in particular, the consequences of interchromophore coupling on singlet fission. The structures of the dimers and the mutual orientation of the carotenoids were elucidated with 1D and 2D NMR, resonance Raman spectroscopy, UV-Vis absorption, and circular dichroism spectroscopy. The electronic coupling between the chromophores of the dimers was quantified from absorption spectra acquired from 80 – 295 K; net amplitude was found to be 150 – 500 cm-1 (i.e. weak-coupling limit). The excited-state dynamics of the dimers were probed with femtosecond-to-nanosecond transient absorption spectroscopy, picosecond resonance Raman spectroscopy, and via triplet-sensitization experiments on the microsecond timescale. Following excitation, triplet excited states are generated pairwise on the dimers in yields ranging from 57% to 105%. These triplets decay almost entirely within 100 ps via triplet-triplet annihilation. The triplet yields and lifetimes correlate with the strength of exciton coupling.

The second part of the dissertation focuses on the excited-state dynamics of marine dissolved organic matter (DOM). The amount of this material in the ocean is vast, and roughly equivalent amounts of carbon are found in the DOM as in atmospheric CO2. The complexity of this material has held back an understanding of the photophysics and excited-state dynamics. Samples cultured and extracted from SIO pier water were probed for the first time with femtosecond transient absorption spectroscopy. These studies were accompanied by steady-state measurements including UV-Vis absorption, excitation-emission matrices, NMR, and EPR spectroscopy. Approximately two-thirds of the excited-state decay occurs within 50 ps directly to the ground state. The dominant components are solvent-dependent, consistent with exposed chromophores and some charge-transfer character. The species with microsecond lifetimes were probed with unique triplet energy acceptors: zeaxanthin and crocins. These acceptors establish that the long-lived transients in DOM are triplet excited states, and a kinetic analysis reveals 1 – 16% quantum yields.