Modelling quantum dynamics in molecular photoswitches

Quantum dynamics and effects in complex systems is an increasingly important
area of research with wide impact to several areas of physics, chemistry,
and biology. Progress in this field promises the advancement of our
understanding of the fundamental behaviour of nature, and crucial developments
for future technologies. Molecular photoswitches present a key
example with applications to medicine, molecular motors and machines,
and understanding the first stages of vision. Significant challenges arise in
the modelling of these complex quantum systems due to the need to account
for several degrees of freedom in combination with quantum effects.
Furthermore, such systems constitute a difficult regime of quantum dynamics
far from equilibrium that necessitates an anharmonic description. This
thesis addresses the challenge with the development of efficient quantum
stochastic methods. As a result, a method is produced capable of simulating
quantum dynamics on several anharmonic surfaces in contact with an
environment.

Firstly, model systems that go beyond the standard harmonic approximation
are used to investigate molecular photoswitch potentials, and tools
are identified to predict potential energy surface curvature and assist in the
interpretation of linear absorption spectroscopy.
Secondly, a stochastic Schrodinger equation approach is developed for
the investigation of the absorption spectra of anharmonic systems, in the
presence of an environment. Through this it is demonstrated that photoswitching
may be stabilised via damping caused by interaction with the
environment.

Lastly, a numerically exact method, the hierarchy of stochastic pure
states, is extended for the study of molecular photoswitch dynamics. It
is found that a combination of potential energy surface shape, damped
quantum dynamics, thermally activated dynamics, and environment memory
play significant roles in the observed dynamics of the stiff-stilbene photoswitch.
Subsequently, it is established that dynamics produced with the
method and single coordinate models agree well with transient absorption
spectroscopy experiments.