Abstract
Nonlinear optical phenomena, which are primarily caused due to the interactions of matter with strong (or intense) electric fields (mostly intense fields of laser beams), have received a lot of attention, and significant amounts of experimental as well as theoretical research have been carried out in the recent few decades. As such types of materials have the potential to find applications in a wide range of scientific, engineering, and technological fields. In the recent few decades, a large number of research activities have been focused mostly on the studies related to organic NLO materials. Moreover, most of those works are centered around the investigations of the molecular 1st hyperpolarizability (second order NLO response) of organic NLO-phores. Amongst various organic NLO active materials studies till the present day, the majority of research activities are focused on Push-Pull types of organic molecules and are of Donor-π-Acceptor (D-π-A) types. Though these organic molecular materials have many advantages, they are also closely associated with several problems. To address these issues, in the past researchers have used a variety of design approaches. One effective approach is the manipulation of the bridge in those D-π-A chromophores, like for example, replacing or exchanging the π-bridge with different types of linkers, to establish the link between the D and A. Detailed examinations of previous studies reveal that the nature and types of bridges play critical roles in the structure-property relationships. To attain much higher values of 1st hyperpolarizabilities, this is owing to the zwitterionic natures (intramolecular charge separated state) of such molecules, where the charged donor and acceptor impact significantly on their charge transfer characteristics and many related properties. Many such zwitterions have been proposed as the most suitable candidates for device applications as poled polymer-based materials, as they have strong dipole moments which make them easy get
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incorporated into a polymer matrix and aligning them with the help of electrical or optical poling techniques.
Furthermore, in addition to their larger nearly unidirectional dipole moments (mostly one major dipole component), zwitterions are also known to have practically unidirectional charge transfer paths (from D to A, almost in a uniaxial way), during their ground to excited state excitation processes. To understand the structure and property relationships of the molecular zwitterionic chromophores and embedded nonlinear optical (NLO) polymer hybrids, for applications in organic electro-optic (EO) materials, mainly two groups of zwitterionic chromophores, (I) Brooker’s types of zwitterions and (II) Reichardt's types of zwitterions are widely investigated in past literature. However, while in most of those studies either D-A directly connected (without any bridge). some studies also reported the roles of conjugated linear π-bridges. But any theoretical or experimental studies in which the effects of various types of bridges (any chemical entity which can possibly act as a bridge) on linear and nonlinear optical properties of zwitterionic chromophores could not be found in earlier literature. Hence, in this thesis, a systematic computational study is presented to account for the roles of various bridges on (optoelectronic properties) intramolecular charge transfers, dipole moments, molecular first hyperpolarizabilities (nonlinear optical properties), frontier molecular orbitals, vertical excitations, electronic absorption properties (linear optical properties), etc. zwitterionic chromophores, and comparative analysis is presented to account for the relative efficiencies of those bridges with the focus on two different types of bridges (aromatic vs. non-aromatic). Using Hartree-Fock (HF) Theory and Density Functional Theory (DFT). Some cases Finite Field (FF) Theory (a simplified version of the linear response theory) is also used to compute some of the response properties. To explain the origin of optical nonlinearities, most often Two-State Model is used. Many fascinating findings reported in this thesis demonstrated that the
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nature of bridges (especially the aromatic bridges), and the donor-acceptor combinations not only serve as frameworks, but also play critical roles in charge transfer excitation processes, as well as related linear and nonlinear optical characteristics. The works presented in this thesis are also published as international journal articles.