Graduate Thesis Or Dissertation
Control of Photophysical Processes Through Nuclear Motions — Design and Characterization Public Deposited
In order to achieve third-generation solar devices, the basic photophysics of light-absorbing materials must be thoroughly understood. Herein, vibrational motions are explored as potential methods by which excited-state dynamics can be controlled. In an effort to prevent charge-recombination, vibrationally activated photoinduced dissociative electron transfer (ET) of the type [RuII(A)n(L–X)]2+ + hν⟶ [RuIII(A)n(L–X)•–]2+*⟶ [RuIII (A)n(L•)]3+ + X- (L = polypyridine ligand; X = halogen; A = ancillary ligand) is explored computationally and experimentally. Density functional theory (DFT) calculations employed a thermochemical cycle to determine structural and electronic factors influencing ΔErxn. Intramolecular strain is shown to decrease ΔErxn and the formation of a contact ion pair (CIP) state is determined to be a favored product. Thus, parent complex [Ru(tpy)2]2+ (1) (tpy = 2,2':6',2''-terpyridine) is compared with two compounds [Ru(6,6''-Br2-tpy)(tpy)]2+ (2) and [Ru(6,6''-Br2-tpy)2]2+ (3), that incorporate interligand strain. The crystal structure of 3 is distorted due to strain as compared to 1. While electronic absorption in 2 and 3 is weakened relative to transitions in 1, a strong interligand charge transfer (CT) transition is observed. Ultrafast transient absorption spectroscopy revealed coherent vibrational dynamics in 3 and 2 that were assigned to Br motion. In spite of additional strain, the excited-state lifetime of 3 is ~6x longer than 2. Constrained-DFT calculations shows the strain-induced geometric distortions in 3 causes a nesting of excited state surfaces, extending excited-state lifetime. Kinetic evidence is presented for C–Br bond scission in 3 and formation of the predicted CIP.
A secondary project explores singlet fission (SF) — a process where one photon produces two excited-states. DFT calculations are used to explore a series of bistetracene (BT) molecular dimers connected by norbonyl bridges that exhibit through-space and through-bond electronic coupling. Dimer orientation and separation is shown to significantly affect SF driving force, Davydov splitting, and the magnitude of matrix elements important to CT-mediated SF. It is determined that BT1-cis, a norbonyl bridged dimer connected at the 1,2 tetracene positions with both tetracenes extending in the same direction, is most favorable for SF due to an exoergic SF driving force (–119 meV) and increased magnitudes of Davydov splitting and ET matrix elements.
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