Quantum chemical calculations of the excited states of porphyrins

Suess, C.J. (2018) Quantum chemical calculations of the excited states of porphyrins. PhD thesis, University of Nottingham.

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Abstract

The development of optical multidimensional spectroscopic techniques has opened up new possibilities for the study of biological processes. Ultrafast two-dimensional ultraviolet spectroscopy experiments have determined the rates of tryptophan → heme electron transfer and excitation energy transfer for the two tryptophan residues in myoglobin. Here we show that accurate prediction of these rates can be achieved using Marcus theory in conjunction with time dependent density functional theory (TDDFT). Key intermediate residues between the donor and acceptor are identified, and in particular the residues Val68 and Ile75 play a critical role in calculations of the electron coupling matrix elements. Our calculations demonstrate how small changes in structure can have a large effect on the rates, and show that the different rates of electron transfer are dictated by the distance between the heme and tryptophan residues, while for excitation energy transfer the orientation of the tryptophan residues relative to the heme is important.

The absorption and fluorescence spectroscopy of a series of porphyrin based systems have been studied. The range of systems has been selected in order to investigate the influence of both the electronic and geometric configuration on the photophysical properties. The origin of the bathochromic shift in the absorption and fluorescence spectra of substituted porphyrins is attributed to both steric distortions of the ring and electronic effects of the substituents. Three DFT based approaches have been used to model and calculate these properties. The approach using the maximum overlap method (MOM) predicted the largest discrepancy from Excited States of Porphyrins experimental results whilst TDDFT calculated shifts within 0.05 eV of experimental values. Finally a third method labelled as a ‘hybrid’ approach has been used, where the MOM is employed to optimise excited state geometries and single point TDDFT calculations are used to evaluate the vertical excitation energies. This approach improves on the excitation energies predicted by the MOM but does not improve on the values that a full TDDFT calculation produced. However, this ‘hybrid’ approach is computationally less demanding. There is distinct trade-off between accuracy and feasibility of calculations, where this ‘hybrid’ method of MOM and TDDFT becomes beneficial and useful.

High resolution spectra of both free base and metallocentred complexes of porphyrin are calculated for the Q band region. Calculations on the vibronic structure of porphyrin are performed using DFT and TDDFT. Both Franck–Condon (FC) and Herzberg–Teller (HT) approaches have been used to predict the frequency and intensity of vibronic bands in the simulated absorption spectra with respect to the S0 → S1 electronic transition as the summation of contributions from both schemes characterise the electronic transitions and provide a high resolution description of the Q-band. Chapter five shows that the first electronic transition into the singlet excited state is vibronically active in the Qx region of the absorption spectrum, in good agreement with experimental data. The HT scheme appears to have more significant contributions and provides more insight into resolving the vibrationally active area of the absorption spectra. HT contributions to the electronic transition dipole moments are essential to assign the weak vibrational transitions and reproduce the experimental spectral profile. In order to provide a detailed account of the vibronic structure it is necessary to assign the vibrational transitions using both HT and FC schemes. The importance of theoretical calculations are highlighted here and can help the general understanding of the photophysical properties of porphyrins.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Besley, N.A.
Hirst, J.D.
Keywords: porphyrins; electron transfer
Subjects: Q Science > QC Physics > QC350 Optics. Light, including spectroscopy
Q Science > QD Chemistry > QD 71 Analytical chemistry
Faculties/Schools: UK Campuses > Faculty of Science > School of Chemistry
Item ID: 48422
Depositing User: Suess, Christian
Date Deposited: 19 Jul 2018 04:40
Last Modified: 19 Jul 2018 04:40
URI: http://eprints.nottingham.ac.uk/id/eprint/48422

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