The Microscopic Mechanism Of Nonlinear Optical Polarizabilities And Charge Transport In Organic Polymers

Conjugated organic polymers are of immense interest, both for electronic, photonic andmagnetic properties they possess and for potential applications in nonlinear optical materials,field effect transistors, light emitting diodes, photovoltaic cell and so on. We study thenonlinear optical polarizabilities and the properties of charge transport in this article.First, on the basis of the calculation of ab initio, we discuss the influence of substituentson the nonlinear polarizabilities. Moreover, we analysis the nonlinear optical responses ofindividual parts using the local atomic charge derivatives, which help us to understand theorigin of nonlinear optical polarizabilities and the relation between the structure and theproperties further. The results show that the nonlinear optical polarizabilities not only dependon the substituents and the charged conformational defects, but also the various combinationof the substituents and the charged conformational defects. For example, substituents andcharged conformational defects both can enhance the nonlinear optical polarizabilities, whilethe combination of acceptor substituents and positive charged conformational defects (thecombination of donor substituents and negative charged conformational defects) can enhancethe nonlinear optical polarizabilities further, and the combination of donor substituents andpositive charged conformational defects (the combination of acceptor substituents andnegative charged conformational defects) restrain the nonlinear optical polarizabilities. Inaddition, the effect of the combination of substituents and charged conformational defects onpolarizabilities for different chain length is investigated.Second, there are two major parameters that determine the charge mobility: theelectronic coupling (transfer integral t) between adjacent molecules and the reorganizationenergy in organic materials. Based on the semi empirical molecular orbital theory and thedensity functional theory, we discuss the effect of chain length and the substituents on thecharge transport. Furthermore, we pay attention to the contribution of each vibrational modeto reorganization energy by using the normal mode method, and understand the microscopicmechanism of charge transport qualitatively.The results show that : 1) The reorganization energy of polyacetylene and oligoacenes decrease with thechain length, and the electronic coupling decrease as well.2) Due to the interactions between substituents and their neighbor atoms, thecontribution of modes which are related to the vibration of substituents and its neighbor atomsto reorganization energy increase, consequently the reorganization energy increases. Besides,substituents enhance the diagonal interactions by which the direct interactions arecompensated for, and reduce the electronic coupling.