Investigation of electronic and optical properties of thiophene containing oligomers by density functional theory

The most widely used energy source on the earth are fossil fuels, but unfortunately they are about to die out. Because of the extinction and the known detriments of this source, scientists are looking for new energy resources. Nuclear powerplants can be considered as an alternative, but because of problems arising from the management of nuclear wastes and accidents having hazardous consequences, nuclear energy also needs to be replaced with safer solutions. For these reasons, the renewable energy resources are getting more focus and attention in recent years. Wind energy and solar energy can be considered as renewable energy resources. Wind power is still controversial since it has ecological side effects. Solar energy is becoming more considered and researched. The first method invented to use solar power was to use inorganic semiconductors. Even though people have reached limits on those semiconductors, the usage of solar power is still under expectations. Organic polymers with photovoltaic features are known for a long period, however the popularity of inorganic polymers have dominated the market since early 2000s, so the researches about organic polymers have not been taken into consideration. The limitations of inorganic polymers and low costs of organic polymers encouraged researchers to focus on organic polymers, and with their increasing popularity and focus, those photovoltaics polymers have started to be used in more areas. The working principle of photovoltaic devices can be summarized as; the electron of the molecule, which is on the orbital with the highest energy, becomes a donor by the photon and is switched to the next orbital. The electron hole pairs created after this excitation needs to be separated and arrive into two different electrodes. To accomplish this, two kinds of electrodes with different ionization energies are used in photovoltaic devices. We can mention two kinds of design methods of the organic polymer devices. The first one is bilayer method, which works with separating donor and acceptor as two different layers. Second, one is the bulk heterojunction method, which merges the donor and acceptor in one layer, which allows expanding the surface of contact to increase efficiency and performance. The biggest problem encountered after the increased usage of organic polymers is the wavelength of the light absorbed by the polymers. Most of the synthesized polymers absorb light in the visible region and the late researches are mostly focused on synthesizing low band gap organic polymers, which absorbs photons beyond visible region. Conjugated polymers are the most popular polymers in designing organic materials for electronic applications. The main goal is to reduce bandwidth by bringing donor and acceptor molecules to the same polymer structure. The π conjugation length is inversely proportional with bandwidth, which allows creating polymers that are more effective by increasing chain length. However, resolution is decreasing with increased mass of polymer. To avoid this problem, both bullet points need to be taken into consideration, i.e. the designs of main and side chains should allow high resolution of polymers while keeping the band gap low. Computational methods have become achievable and effective parallel to the rapid developments in the computer technology. They are highly convenient to offer preliminary information about polymers a priori syntheses. Among the computational methods, density functional theory (DFT) methods have become very popular because of the less demanding computational cost and applicability on large systems. Although the choice of an appropriate DFT method is still challenging, B3LYP is most widely used almost standard hybrid method for polymeric systems. In this thesis, 13 different molecules in the form of monomer , dimer and trimer which include some of the thiophene (T), bithiazole (NBT), dithionethiophene (DTT) and thienothiophene (TT) units in their backbone investigated using density functional theory method and B3LYP hybrid exchange and correlation functional was used. First, the low energy conformations of the monomer molecules were searched. 3 lowest energy conformers were selected by using molecular mechanical calculations. The geometries of the chosen monomers were optimized with B3LYP/6-31+G (d, p) level of theory and one conformer with the lowest energy among the optimized ones chosen for further studies. The UV-Visible spectra of the molecules in the gas phase also in acetonitrile solvent were obtained by using Time-Dependent DFT (TDDFT) method at B3LYP/6-31+G (d, p) level for the monomers. The dimers and trimers of the selected monomers were modeled and initially subjected to geometry optimization using semi empirical AM1 method then using DFT method at B3LYP/6-31G(d,p) level. The HOMO-LUMO molecular orbital energies, HOMO-LUMO band gaps, as well as maximum wavelengths, onset wavelengths and optical band gaps calculated from onset wavelengths of the UV-VIS spectra were reported for all molecules. The results investigated for a few different ways. Firstly, conjugation length and band gap relation searched and the results were parallel to researches. Band gap values decreased while conjugation length increasing for monomeric structures. The same reduction in band gap also seen when the results of dimer and trimer results inspected both for gas phase and for solvent phase. For the phenyl to methoxyphenyl changes in side chain there was no clear band gap difference between two versions of structures and so that we cannot say that this change cause a great effect on band gap. To investigate properties of these structures acetonitrile chosen as solvent. The conductor like polarizable continuum model (CPCM) used to add acetonitrile as solvent to system. While results investigated also relationship between band gap values and linearity of molecules also checked. While preparing conjugated donor acceptor monomers can be said that linearity of molecules decreased. However, dimer and trimer results showed that dihedral angles getting more linear while chain elongation. The nonyl side chains which removed while computational works to also have an effect to make structures more linear. For band gap values can be said that while molecules getting more linear structure getting reduced but also this reduction can be caused by increase of conjugation length. As a last step results compared with experimental values and seen that B3LYP method shows lower band gap values for polymeric results. For monomeric structures, band gap values are bigger than experimental values but after dimer and trimer results band gap values getting below of the experimental values. As a reason for that for monomers B3LYP/6-31+G(d,p) level of theory used but for dimers and trimers B3LYP/6-31G(d,p) level of theory was used and this change in basis set can be cause this difference. In addition, for future of the work decided that this basis set difference will be removed and the new results will compared with experimental results. Still the computational results showed similar patterns with experimental results like decrease of band gap while conjugation length increasing. While experimental results compared with B3LYP methods' results, also different DFT results found in literature and B3LYP and MN12-SX methodology results compared against each other. Results of both functionals showed that these two methodologies' expect lower band gap results when compared with experimental results. But as for B3LYP also MN12-SX predicts same pattern like experimental values.