Elucidating the effect of solvent friction on the excited-state reaction dynamics of light-responsive molecular switches
Title: Elucidating the effect of solvent friction on the excited-state reaction dynamics of light-responsive molecular switches
DNr: SNIC 2022/5-566
Project Type: SNIC Medium Compute
Principal Investigator: Bo Durbeej <bo.durbeej@liu.se>
Affiliation: Linköpings universitet
Duration: 2022-12-01 – 2023-12-01
Classification: 10407 21001 10405
Homepage: https://liu.se/en/employee/bodu88


Light-responsive molecular motors and switches are molecules that can perform many useful functions in nanotechnology by absorbing light energy and converting it to other forms of energy. In the last decade, we have initiated a line of research unique to Swedish academia in which more useful and potent light-responsive molecular motors and switches are designed through computational studies in theoretical chemistry. Thereby, we have discovered a number of ways to improve the functions of such systems. In particular, in the most recent years, our research in this field has focused on understanding how mechanical friction from solvent molecules and surfaces affects the dynamics of the photochemical reactions through which molecular motors convert light energy into rotary motion. From a modeling perspective, this is a challenging task requiring accurate simulation of reaction dynamics in electronically excited states, combined with a simultaneous description of the molecular collisions causing the friction. In our work, the strategy to meet this challenge has been to couple semi-classical, non-adiabatic molecular dynamics (NAMD) simulations of the motors based on quantum chemical methods, to MD simulations of the solvent or surface environments (in which the motors are operated) based on classical force fields. In this way, we have been able to model the photochemical reaction dynamics of the motors as they experience friction in a solvent or from a surface. Specifically, by calculating the photochemical quantum yields of the motors in such environments, we have been able to assess how mechanical friction affects their overall performance, which is a central question in this field of research. These achievements have been made possible by the development, in the previous and earlier SNIC Medium Compute projects upon which the present proposal is based (SNIC 2021/5-553 and SNIC 2020/5-618), of so-called "wrapper" codes that integrate in-house codes for NAMD simulations with commercial electronic-structure software packages. In this continuation project, we will now use these codes to identify ways to avoid the negative influence of solvent friction on the ability of light-responsive molecular switches (a.k.a. molecular photoswitches) to store solar energy. In fact, while such molecular photoswitches offer a well-recognized means to store solar energy through the photochemical conversion of a sunlight-absorbing isomer of the switch into an energy-storing isomer, this technique is limited by the fact that solvent friction can have a pronounced negative effect on the quantum yields of the conversion processes. Accordingly, finding ways to circumvent this problem is a very worthwhile goal in this field of research. Focusing on both well-established classes of solar-energy storing photoswitches and photoswitches that we have developed ourselves, and building on results obtained in the previous SNIC Medium Compute project, the goal of this continuation project is to identify both intrinsic properties of the switches (e.g., bulkiness and substitution patterns) and solvents properties (e.g., polarity and bulkiness) that maximally reduce the detrimental effect of solvent friction on the quantum yields.