2.9.1 GaussView
All molecular structures, before geometry optimisations and when not taken from experiment, have been built using GaussView [66].
2.9.2 Turbomole
All Density Functional Theory and Time Dependent Functional Theory calculations have been completed using version 6.6 of the Turbomole program [67] unless otherwise indicated. Tur-bomole calculations have employed the Ahlrichs style basis sets of polarised triple-zeta quality (def2-TZVP) [68].
2.9.3 Molcas
All CASSCF and RASSCF calculations have been completed using the Molcas 8.0 program [69]
and employed the relativistically contracted atomic natural orbital (ANO-RCC) basis sets [70]
of polarised triple-zeta quality. Scalar relativistic effects were included via the use of the 2nd order Douglas-Kroll-Hess Hamiltonian in all calculations [71].
Where second order perturbation theory calculations (PT2) have been completed, these have been performed on the converged CAS/RASSCF wavefunctions and adds in the effect of dy-namical correlation. All perturbation theory calculations included an imaginary level shift of 0.2 a.u. which was included to reduce the effect of intruder states. Additionally all perturbation calculations have also been performed both with the standard IPEA shift of 0.25 a.u.
2.9.4 Aimall
All QTAIM data presented in this thesis has been obtained using the Aimall package on wave-functions obtained from the programs described above [72].
Porphyrin Complexes
3.1 Introduction
3.1.1 Overview
Porphyrin ring complexes are some of the most widely studied and investigated molecules in the chemical world. Their necessity to life, in processes such as photosynthesis in plants and oxygen transport in the human body, and the colourful complexes they form have earned them the nickname The Pigments of Life [73], in fact the word Porphyrin is derived from the Greek word for purple, porphura.
The simplest porphyrin molecule, porphine, is shown in Figure 3.1, the backbone structure consists of four pyrrole rings joined together by four methine bridges. Porphyrins can be eas-ily substituted at the eight β positions on the four pyrrolic rings as well as at the four meso positions of the methine bridge. This wide range of potential substitution positions on the porphyrin ring backbone, in addition to the vast number of groups which can be substituted, gives rise to a huge number of potential molecules which can be synthesised.
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Figure 3.1: Porphine - the simplest free base porphyrin (FBP) consisting of four pyrrole joined together by four methyne bridges.
Porphine is an example of a Free Base Porphyrin (FBP), a porphyrin ring that is not complexed to a metal. Complexation of porphyrin rings to metals can occur when the porphyrin ring is deprotonated and an anion with a -2 charge is formed. If the many different metals which can complex to the ring are included, the total number of potential porphyrin molecules increases even further. By differing the metal and the substitution on the basic porphyrin backbone structure the electronic energy gap between occupied and unoccupied orbitals can be altered.
By varying this energy gap we can fine tune potentially useful porphyrin molecules for various applications, including but not limited to photosensitisers for cancer therapy and dye sensitised solar cells (DSSC), described later in Section 3.1.4.
3.1.2 Gouterman’s Four Orbital Model
The optical absorption spectra of porphyrins has been rationalised by Goutermans four orbital model [74], which considers the main electronic transitions of porphyrin complexes to be between the HOMO (highest occupied molecular orbital) and HOMO-1 to the LUMO (lowest unoccupied molecular orbital) and LUMO+1 of the porphyrin ring, shown in Figure 3.2 (right).
Figure 3.2: Absorption Spectra of a FBP showing the difference in intensity of the Q and B bands (left) & Energy level diagram of Gouterman Orbitals and molecular orbital pictures (right). Adapted from [4, 5, 6].
The main transitions seen in the absorption spectra of porphyrin complexes are called the Q and B bands. The B band is a very intense absorption band which is characterised by its large oscillator strength and is commonly referred to as the Soret Band (or Peak). The Q band is weaker than the B band and exists as two separate components for porphine due to its D2h symmetry. Figure 3.2 above shows an example absorption spectra of a FBP (left) and the Gouterman orbitals and their energies (right) for a D4h porphyrin.
The problem with considering only these four ‘Gouterman’ orbitals is that it is a very simplistic view of what is a highly conjugated and complex molecule. In truth the electronic transitions of porphyrin molecules are composed of transitions from the entire conjugated π-system. This is why when the substitution on the ligand backbone is altered or the metal complexed changed, the excitation energies obtained from an absorption spectra are also altered. For example if a conjugated chain were to be added to one or more of the meso or β positions on the Porphyrin ring then the number of π electrons in the system and the overall conjugation of the molecule would be increased. This in turn would affect the energy levels of the Gouterman orbitals
and hence also the excitation energies. Generally speaking it can be predicted what would happen if an electron donating or withdrawing group is added to the porphyrin backbone, however synthesising the molecule and experimentation is the only way to quantify the change in excitation energies.
3.1.3 The Need for Computational Models
Obviously not every porphyrin molecule will have the potential for useful applications, so how can the screening of every potential molecule without the waste of time, effort and resources be achieved? To efficiently test for potentially useful molecules it is wise to utilise computa-tional chemical simulations and calculations. In order to assess the effect of different metals and substitutions on the electronic energy gap and the absorption spectrum, excited states of the molecules in question also need to be simulated. Excited states can be simulated using Time Dependent Density Functional Theory (TDDFT) or by using multiconfigurational meth-ods (such as CASSCF), an overview of both of these methmeth-ods is given in Section 2.
But what is the most accurate and efficient way of describing the electronic structure of both free base and metal porphyrins? Can an accurate generic description of the frontier orbitals of a metal porphyrin be defined which can be transposed onto any metal porphyrin system that is to be investigated? The aim of this section of the thesis is to develop an understanding of the transitions that comprise the excitations of porphryin system and consequently a method of selecting orbitals that is not particular to one transition metal porphryin system.
3.1.4 Potential Uses of Porphyrin Complexes
As mentioned previously the substitution on the ring and complexation to different metals affects the electronic energy gap. This is evident in the very different colours of haemoglobin (Fe) and chlorophyll (Mg), both of which are porphyrinLOL complexes (both shown in Figure 3.3).
Figure 3.3: Chemical structures of Chlorophyll (Left) and Haemoglobin (Right) showing the porphryin ring backbone structure.
Recently uses of porphyrins have expanded into molecular electronics, specifically as dye-sensitised solar cells (DSSC). A dye dye-sensitised solar cell is a variation of thin-film solar cells, they are a semi-conductor formed between the photo-sensitised anode and an electrolyte. DSSCs are potential sources of renewable energy as although they typically possess low conversion rates they are cheap to make, very tunable and easily fabricated [75]. Typically porphyrin based DSSCs contain zinc porphyrin as the photosensitised dye [75, 76, 77] but there are also a great deal of other transition metals that have been used in the literature including but not limited to Ruthenium [78, 79] and Cobalt [80].
Another use of porphyrins molecules is in medical imaging and photodynamic therapy (PDT).
Photodynamic therapy is the use of light-sensitive molecules to kill abnormal cells. Porphyrin molecules are ideal molecules for photodynamic therapy because they are soluble in water, meaning that they can be administered intravenously, as well as having an energy gap that is tunable, as has been mentioned previously. Porphyrins as photosensitisers in PDT will be revisited in Chapter 5.