Conjugated Organic Oligomers Second-order Stark Effect And The Ground State Of Quantum Transport

This dissertation discusses two subjects.One subject is the theoretical investigation of the excess polarizabilities of organic conjugated oligomers.Corresponding discussions were presented in chapter 1,2,and 3.The other subject is the theoretical study of the quantum transport in conjugated molecules.The excess polarizability is a parameter characterizing the behavior of the excitation energy under the external electric field.Nowadays,the theoretical investigation of the excess polarizabilities of organic conjugated oligomers is in its infancy.The reason is that the large sizes of organic conjugated oligomers will make conventional high-level excited-state methods become extremely expensive in predicting the excited-state electronic structures.The time-dependent density functional theory(TDDFT) invented in 1980s is a powerful theoretical method to investigate the excited-state properties of large-sized molecules.The TDDFT approach is relatively cheap but has the considerable accuracy.The exchange-correlation(xc) potential is the spirit of the density functional theory(DFT).With the development of the xc potential, the TDDFT approach will become more and more accurate.Our investigation of the excess polarizability involves two major aspects.First, we evaluated the performance of the TDDFT approach in calculating the excess polarizabilities of organic conjugated oligomers by comparing TDDFT predictions with the experimental results.Then,we studied the influence on the excess polarizability induced by the geometry relaxation upon excitation. The model molecules studied in this work have been widely investigated experimentally and theoretically.We confined our interest in the first dipoleallowed excite state.For our model molecules,their first dipole-allowed excited states can be easily obtained from the ground state by the visible light, so that this excited state is one of the main states for optoelectronic devices.The investigation of the quantum transport of the molecule provides the fundamental information for the molecular device designing.We calculated the quantum transport of the molecule quantitatively by combining the nonequilibrium Green's function with the DFT theol.The delocalizedπ orbitals in the conjugated molecule have large contributions to the quantum transport.It is known that theπbond is sensitive to the torsion.Therefore, the delocalization extent of theπorbital will be decreased when the conjugated molecule is twisted.We studied the relation between the current and the change in the delocalization extent of theπorbital.Our results provide information on controlling the current flowing through the molecule.