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.

Acenes and perylene dyes have been a focus of fundamental research for decades, motivated partly by applications in organic photovoltaics and light-emitting diodes. The lowest triplet excited state of these molecules is of interest, because these transient species are closely associated with loss processes as well as mechanisms for new applications (e.g. singlet fission, or triplet-triplet annihilation). Vibrational spectroscopy of molecules in the T1 state is motivated because this approach provides structural insights, and is generally more specific than other techniques such as electronic spectroscopy. This work focuses on resonance Raman spectroscopy, because this technique greatly amplifies the signal from specific targets.The first part of this dissertation describes resonance Raman spectroscopy of the ground (S0), excited triplet (T1), and cationic states of pentacene (Pn) as well as a soluble derivative (triisopropylsilylethynyl-pentacene, TIPS-Pn). Unique signatures were found for each state. These spectra serve as a useful reference for Pn and its derivatives, in solution or solid state. Density functional theory computations that account for resonance enhancement were used to assign bands to specific vibrational normal modes. Shifts in the frequencies, some exceeding 30 cm-1, were analyzed in terms of local mode character, and frontier molecular orbitals. The second part of this dissertation describes a resonance Raman study of a representative perylene diimide molecule in its lowest triplet excited-state (3PDI*). The PDI family is one of the most important synthetic dyes, yet the vibrational structures of their excited states are not well characterized. One obstacle is that high fluorescence from PDI, with quantum yields typically near 100%, overwhelm signals from Raman scattering. To overcome this problem, two cofacial PDI dimers were synthesized, both with fluorescence emission reduced by a factor of ~100. Improvements to a two-chamber pressurized flow system permitted rigorous exclusion of oxygen and allowed many hours of data acquisition. The vibrational spectra of the S0 and T1 states are readily distinguished, and the bands in the range 500 -1700 cm-1 are analyzed as described above for pentacene. As a whole, this works extends the use of resonance Raman spectroscopy and accompanying computations with resonance as useful tools for identifying and probing the triplet excited states.

Electrical measurements in the field are one of the most important methods for probing the structure and dynamics of the Earth’s mantle. Certain geographical regions, such as those around cratons, have anomalously high conductivity, and have drawn great interest in the geophysical community. The presence of iron sulfide minerals is one explanation for the unusual conductivities. The validation of this proposal or others depends on measurements in the laboratory, conducted at temperatures and pressures relevant to the mantle.Although electrical properties of pure iron sulfides are a well-studied topic in the laboratory, much less is known about the properties of sulfides as they exist in the mantle. For example, the networks that allow a relatively minor (~5 vol.%) amount of metal sulfide to strongly impact conductivity of a rock are not yet understood. The impact of other elements that are prevalent in the upper mantle, such as oxygen or nickel, have largely been unexplored. Likewise, the effect of annealing sulfide-silicate mixtures on their electrical properties is unknown. The research presented in this dissertation represents progress towards understanding the topics above. Mixtures of iron sulfides in two natural mantle rocks are presented. Impedance spectra from 1 – 106 Hz were acquired, for samples under pressures from 2 – 6 GPa (equivalent to roughly 60 – 360 km depth) over temperatures from 300 – 1400 °C. Large changes in conductivity values, and changes in Arrhenius activation energies, occur when the quantity of iron sulfide increases from 3 to 6.5 vol.%. The duration of an annealing (dwell) was found to have a significant impact on conduction, particularly at temperatures lower than the annealing temperature. Oxygen or nickel alters the conductivity significantly when added to the sulfide phase. The frequency dependence of these data was analyzed and led to a conclusion that instrumental aspects dominated the imaginary response (capacitance). The impedance spectra acquired in situ were complemented with ex situ electron microprobe analyses and images which advances our understanding of grain interior / grain boundary structure and overall conductivity. Applied to the field, the conductivity results were incorporated into an electrical model of sulfide-bearing mantle rocks. This model suggests that a small amount of iron sulfide is consistent with some electromagnetic anomalies around the cratons of Tanzania, Australia, and southern Africa.