Photochemical Sciences Ph.D. Dissertations

Impact of Vibrational Mode Coupling on the Isomerization of Natural and Synthetic Molecular Switches

Date of Award

2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Massimo Olivucci (Committee Chair)

Second Advisor

Josh Halfpap (Other)

Third Advisor

Hong Lu (Committee Member)

Fourth Advisor

Alexey Zayak (Committee Member)

Abstract

This dissertation investigates the photoisomerization process of natural and synthetic molecular switches, with a focus on the influence of vibrational mode coupling on photoisomerization quantum efficiency. Utilizing semi-classical trajectories and quantum-mechanic molecular-mechanic (QM/MM) models and examining electronic and geometric parameters in the ground-state and excited-state, helps unravel the mechanistic aspects of light-induced transformation while gaining valuable insight into the complex photoisomerization processes. The dissertation aims to enhance our understanding of molecular switches and their quantum efficiency, enabling the design of more efficient molecular machines for diverse applications. The research result is divided into three chapters, each exploring distinct aspects of photoisomerization. Chapter III investigates the role of torsional deformation and vibrational excitation, particularly stretching motions, in modulating the charge-density distribution of a donor-bridge-acceptor molecule in solution. Transient absorption spectroscopy and semi-classical molecular dynamics simulations reveal the connection between reactive nuclear and electronic motions in the ground state. The findings highlight the influence of pre-twisted molecular geometries on enhancing vibrational energy transfer, with potential applications in infrared-mediated chemistry. Chapter IV presents a comparative study between rhodopsin, a natural dim-light visual pigment, and a synthetic biomimetic molecular rotor. Quantum-classical dynamics simulations uncover the factors affecting light-energy conversion. The results emphasize the significance of auxiliary molecular vibrations and their synchronization with rotary motions in maximizing quantum efficiency. The mechanisms employed by rhodopsin and its synthetic analog, MeO-NAIP, are investigated, shedding light on vibrational mode coupling, quantum efficiency, and the role of solvent. Chapter V examines the complete photocycle of a photon-only rotary molecular motor. Quantum-classical trajectories and state-interaction quantum chemical methods unveil the factors governing the motor's quantum efficiency. The analysis highlights the importance of rotational directionality, Franck-Condon relaxation, intersection space, and conformational helix inversions in determining the efficiency and dynamics of the motor's photocycle. Considering the asynchronous dynamical structure of the motor is crucial when utilizing it as a transducer of light energy into molecular-level mechanical motion. In conclusion, this dissertation provides a comprehensive exploration of the photoisomerization processes in molecular switches. It offers insights into mechanistic aspects and quantum efficiency modulation, laying the foundation for future advancements in designing more effective and efficient molecular machines for various applications.

Link to Full Text

Share

COinS