| dc.description.abstract | Light-activated chemistry begins with the excitation of a ground-state molecule to a higher-lying excited state. Once in the excited state, the nuclei in the molecule respond by trying to minimize the energy of the system. The nuclear motions sometimes propagate along a pathway that facilitates a reaction, such as isomerization or dissociation. However, in the condensed phase, the environment impacts these fundamental dynamics, thereby changing the excitation process and altering the nuclear motions of the molecule. In order to investigate the kinetics and dynamics for a variety of light sensitive systems we use time-resolved pump-probe spectroscopy. Transient optical and x-ray absorption spectroscopy provide valuable insight into the different reactive pathways and fundamental chemistry in light-activated systems. First, we show how spatial confinement for a series of stilbene and azobenzene deivatives profoundly impacts the isomerization dynamics involving large amplitude structural rearrangements of a molecule. This work uses ultrafast spectroscopy to probe the effects of confinement for molecules encapsulated in a supramolecular host-guest complex. The supramolecular complex distorts the ground- and excited-state potential energy surfaces, including the conical intersection connecting those states, which results in hindered nuclear motions and different reactive pathways. For stilbene derivatives, the transient absorption measurements reveal broader excited-state absorption spectra, longer excited-state lifetimes, and reduced quantum yields for isomerization in the restricted environment. The organic capsule disrupts the equilibrium structure and restricts torsional rotation around the central C=C double bond in the excited-state, which is an important motion for the relaxation of trans-stilbene from S1 to S0. Unlike stilbene, azobenzene derivatives also have an in-plane inversion pathway for isomerization, in additon to out-of-plane rotation around the N=N bond. The transient absorption spectroscopy of the encapsulated azobenzene derivative reveals formation of the cis isomer in the excited state after exciting the trans geometry, indicating a direct excited-state isomerization channel that is not observed in solution. The confined environment provides new mechanistic insight on the relative roles of inversion and rotation in the ultrafast photoisomerization of azobenzene derivatives. The solvent affects the photodissociation reactions of manganese tricarbonyl complexes. Interest in manganese tricarbonyl complexes stems from the possible use as earth abundant catalysts for CO2 reduction, but the complexes decompose under visible light. To investigate the photodecomposition of manganese tricarbonyl complexes, we use ultrafast transient absorption spectroscopy and time-resolved x-ray absorption spectroscopy. The optical transient absorption measurement probes the femtosecond relaxation of a metal-to-ligand charge transfer state via back-electron-transfer, which facilitates the loss of a CO ligand to form a 5-coordinate Mn species. The 5-coordinate species has two competing reaction pathways, the first is solvent coordination to form a stable 6-coordinate Mn species and the second pathway involves the 5-coordinate species reducing the starting Mn(I) complex to a Mn(0) species. In order to distinguish between the two pathways, we use a non-coordinating solvent which prevents the formation of the 6-coordinate complex and results in the survival of a 5-coordinate species in solution. Time-resolved x-ray absorption measurements provide structural information about the 5-coordinate and solvent-coordinated manganese species on the picosecond to nanosecond timescale. The combination of optical and x-ray techniques provides new insight into the photodecomposition pathway for manganese tricarbonyl complexes. Finally, we examine the excitation event directly for photochromic molecules in the plasmonic field of an array of gold nanorods. Incident laser light on a gold nanostructure creates an enhanced electric field capable of driving higher-order excitation processes. To investigate the spatial dependence and anisotropy of optical activation in the field surrounding gold nanorods, we use pulsed 800 nm laser light incident on a plasmonic array of gold nanorods to induce an oscillating plasmonic field that interacts with an overlaid film of photochromic molecules through nonlinear two-photon excitation. Following nonlinear excitation, the photochromic molecules isomerize. The enhanced electric field results in a larger fraction of molecules excited in the near-field in comparison with the photochromic molecules outside of the enhanced field. The conversion rate is greatest at the tips of the nanorod where the enhanced electric field is the strongest. However, the observed conversion rate depends on the difference in polarization vectors for the 800 nm and probe light near the nanoparticle surface. The excitation and probe fields have different alignment based on the wavelength-dependence of the plasmonic resonance. Using a simple simulation of the enhanced near-field, we model the conversion of molecules around a single gold nanorod. | |
