Methods to simulate photo-excited properties of aromatic molecules

Taylor, Marcus Peter (2019). Methods to simulate photo-excited properties of aromatic molecules. University of Birmingham. Ph.D.

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The study of ultrafast dynamics is crucial for understanding the photophysics of many chemical and fundamental processes. Theoretical models have become vital for providing the rationale for experimental observations. These models require solving the time-dependent Schrodinger equation (TDSE). An efficient way for solving the TDSE is the MCTDH method, an algorithm for efficient propagation of wavepackets. A pre-requisite for performing quantum dynamics is the construction of potential energy surfaces (PES), which often require expensive electronic structure calculations, and are further complicated by non-adiabatic features such as conical intersections.

In this thesis, the photo-excited properties of a range of aromatic molecules are studied, including chromophores of fluorescent proteins, the most renowned being the green fluorescent protein (GFP). Much focus is therefore given to the photochemistry of GFP and its corresponding chromophore, HBDI.

The stability of the HBDI anion and its photoelectron spectrum was understood through considering its molecular components, imidazolide and phenoxide, as well as the totally symmetric analogues bis-imidazoloxy and bis-phenoxy. A vibronic coupling Hamiltonian was developed for each molecule to calculate the photoelectron spectrum. Good agreement with experimental data was achieved, indicating the fitted surfaces provide accurate PES. From the results, the contribution of the phenoxide and imidazolide ring to the photophysics of HBDI was determined.

A simple linear vibronic coupling Hamiltonian has been developed to calculate the photoelectron spectra for the two lowest bands of phenol. The obtained spectra were in good agreement with experiment, however an alternative assignment of the vibrational fine structure is proposed. The existence of a conical intersection between D\(_0\) and D\(_1\) was found and its effect on the photodynamics determined from diabatic state populations. The ion surfaces, along with those for the explicit states, are necessary for modelling time-resolved photoelectron spectra to complement experimental data. To ensure accuracy of the S\(_1\) and S\(_2\) surfaces, the absorption spectrum of phenol was also calculated.

Two alternative methods for calculating PESs are then investigated. The first uses gradients and second derivatives from Hessian calculations to express the potentials for all normal modes as displaced harmonic potentials. This methodology was applied to calculating the absorption and emission spectra for a range of anthracene derivatives and linear polyacenes. The success and limitations of the model are evaluated. The second is the recently developed DD-vMCG method, where Gaussian wavepackets are propagated on PES calculated "on-the-fly". Using this approach, the excited state proton transfer in GFP is studied and the mechanism elucidated. Despite the dynamics not being fully completed, preliminary results provide details on the mechanism, in support of previous studies.

Type of Work: Thesis (Doctorates > Ph.D.)
Award Type: Doctorates > Ph.D.
Licence: All rights reserved
College/Faculty: Colleges (2008 onwards) > College of Engineering & Physical Sciences
School or Department: School of Chemistry
Funders: Engineering and Physical Sciences Research Council
Subjects: Q Science > Q Science (General)
Q Science > QD Chemistry


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