Computational study of charge transfer in a porphine: quinone complex and novel alkoxypyridylindolizine derivatives

Abstract

Finding ways to control electron transfer (ET) between molecules or molecular systems and to investigate efficient theoretical methods capable of describing ET is important for being able to design better molecular electronic devices. In this Thesis the capability of computational methods based on density functional theory (DFT) to describe charge transfer is studied in two molecular systems. In addition, the possibility to control ET by strong electric fields is investigated. The molecular structure and excited states of three alkoxypyridylindolizine derivatives are studied by using DFT and time-dependent DFT (TDDFT). In addition, the influence of an external electrostatic field of the order of magnitude of 109 V/m and of an electric field induced by ambient helical peptides on ET in a porphine–2,5-dimethyl-1,4-benzoquinone (PQ) complex is studied with TDDFT and the approximate coupled cluster singles and doubles (CC2) method. The calculations show that the absorption spectra of the studied indolizine derivatives are characterized by a band arising from the intramolecular CT between the indolizine ring and the pyridyl substituent attached to it and the hybrid functionals reproduce the experimental absorption spectra of the derivatives well within TDDFT. However, the same functionals are not as good in describing the fluorescence properties of the indolizine derivatives, namely the CT excited state of a derivative is obtained with too low an energy, which prevents the relaxation of the correct state. This problem can be circumvented by applying an exchange-correlation (XC) functional with a high fraction of Hartree–Fock (HF) exchange but this is done in the expense of the quantitative accuracy. According to the calculations, perturbation generated either by an external electrostatic field or by ambient Aib peptides affects the energies of the locally excited Q and B states of porphine in the PQ complex clearly less than the energy of the lowest CT state. Hence, it is possible to select a locally excited state (Q or B) of the PQ complex whose photoexcitation leads to ET from porphine to quinone. The results presented show that TDDFT applied with the current XC functionals provides an efficient way to study the qualitative picture of the excited states under the influence of an external electric field.