Theoretical Study On Non-classical Thiophene And Molecular Design Of Organic Conductor

In this dissertation, the electronic structures of non-classical thiophene and borazine are investigated by theoretical method. From monomer to oligomer and to polymer, the conductivity of non-classical thiophene polymers are studied. The aim of those strudies is to understand two relationships, which the electronic structure among of monomers, oligomers and polymers, and which between the electronic structure of monomers, oligomers and the conductivity of polymers. We hope to comprehend the conductive mechanism of polymers. The main researched content and result are listed following.In Chapter 1, we introduce briefly the development and study status of non-classical thiophene and organic conductor. The computation methods (density function theory) including basis principle and some methods are presented. In addition, including current density and NICS, and NBO-NCS, and AIM theory, some theoretical measures and methods are presented.In Chapter 2, we investigated the geometries, electronic structures and aromaticity of nonclassical thiophene, thieno[3,4-f]isothianaphthene and its five N-substitutes. The bond lengths and Wierg bond indexes of S–C bond and S–N bond are in the range between single bond and double bond. The NBO charge of sulfur is always positive, and it indicated that the sulfur atom is a better electron donor in nonclassical thiophenes. Analyzed by NBO, there are several 3-center 4-electron bonds in those resonance structures. Both the result of NBO and topological properties calculation shows they delocalize strongly in those molecules. Furthermore, compared with the classical thiophene, the adiabatic splitting energies of studied molecules are quitely low, and it indicates the structure of those molecules have diradical structure. The facts that all of the NICS values at the RCP are negative suggest that all of those molecules have aromatic character, and there is electron current. According to small vertical ionization energies and low HOMO–LUMO energy gaps, those tricyclic nonclassical thiophenes may be potential candidates for conductive polymers.Because of the special structure, non-classical thiophenes attracted many people's attentions. In Chapter 3, a type of transtactic block polymerization by Thieno[3,4-c][1,2,5]thiadiazole (TT) and its co-polymer with benzene and thiophene are studied. We intend to understand the intrinsic electronic characteristics and also to comprehend the relationship of electronic structures between monomers, oligomers and polymers. In this study, we discussed in detail the properties of central bond in oligomers and polymers. The change of NICS as a method examining conjugational degree is reported. The change of NICS shows that the conjugation in central section are stronger than that in outer section, and the structure in central are closer to quinoid structure than that in outer section. The band structure, partial density of states (PDOS) and the crystal orbital overlap populations (COOP) are calculated by period boundary condition. The groups'contribution for highest bond band and lowest conductive band are disscused. The band structure and density of stated studies show that the benzene and thiophene substitute compound TTB have narrow energy band (0.23 eV and 0.32 eV, respectively), and they can be consider as intrinsic conductor.In Chapter 4, a series molecules that two vertex carbon atoms of pantalene are substituted by X atoms [X=NH(pyrrolo[3,4-c]pyrrole),O(furo[3,4-c]furan),PH ( phospholo[3,4-c]phosphole ), S ( thieno[3,4-c]thiophene ), Se(selenopheno[3,4-c]selenophene)], as well as their dimers and polymers, are researched. The electronic structures of monomers show the conjugation of those bi-ring compounds are stronger than that of single ring compounds. With polymeric number increase, the aromatic of ring near by poly axe are decreased, and the ring currents are reduced and the conjugation degrees are increased. The results of band structure and density of state show that poly-dipyrrole has low energy gap (0.3 eV) and that it may be a candidate for conductor.Thieno[3,4-c]Thiophene and Thieno[3,4-f]isothianaphthene are two normal non-classical thiophenes. The structure of Benzo[c][1,2,5]thiadiazole is between classical thiophene and non-classical thiophene. In Chapter 5, we studied electronic structures of 11 types of non-classical thiophene, which composed by benzene, thiophene, pyrazine and thiadiazole, respectively. The studied results show that the conjugational degree is increased along with the increase of polymeric number. We also studied the band structure and density of state (DOS) of these polymers. The results show that the energy gaps of bicyclical polymers are smaller than that of tri-cyclical polymers; substituted by N atom, the polymer's energy bands are decreased. The energy gaps of ethylene bond with thiophene are smaller than that of ethylene bond with benzene. In all polymers, thieno[3,4-b] pyrazene have the smallest energy gap (0.4 eV), and it can be a candidate for conductor.Ladder-type polymers have one dimension structure, which can reduce and remove the electron localization of single and double bond, and can decrease the alternation between single bond and double bond. This sort of molecules usually have low band energy gap. In Chapter 6, four classical aromatic ring compounds (acene, pyridine, pyrrole and thiophene) are studied by theoretical method. The results show that the aromatic structures of two six-member-ring (acene and pyridine) are changed to quinonoid structure, along with the increase of polymeric chain. Nevertheless, the structures have little changes in five-member-ring. The same result is obtained by NICS analysis. The energy gaps of two six-member-ring (acene and pyridine) ladder polymers are 0.02 and 0.08 eV, respectively. It shows that they are excellent conductor. Whereas, the results show that the five-member-ring ladder polymers (pyrrole and thiophene) have no conductivity.In Chapter 7, the research results show that borazine and its derivatives have rigid planes and high symmetries and the similar B–N bond lengths. For all the examined compounds, the lone pair VSCCs of the N atoms are found. The Laplacian of BCPs at the B–N bonds are positive, which imply that the B–N bonds are of ionic nature. The interaction energies ofπ--π* and LP--π* are single direction in ring compounds. The electron delocalization of borazine and its fused ring derivatives are limited in B–N bond, and there are a few electron currents above molecular planes. For all the examined ring borazines, the aromaticities are very weak due to a small NICS. Furthermore, the aromaticity of borazines is weakened with the fused ring number increasing.