Quantum Chemical Investigations Of Structure,electronic Spectroscopy And Nonlinear Optical Properties Of Benzimidazole/Coumarin Derivatives

Heterocyclic compounds containing nitrogen,oxygen,and sulfur atoms are present in a wide range of naturally occurring and synthetic compounds.The incorporation of heteroatoms in typical carbocyclic compounds leads to the redistribution of electron density,which imparts fascinating physical and chemical properties.The nature and position of the heteroatoms are responsible for the modulation of various molecular properties such as chemical reactivity,spectroscopic behavior and nonlinear optical?NLO?properties.The NLO materials have drawn much attention for their potential applications in optical data transmission,integrated photonics,optical imaging,and sensor protection.The main objective of this thesis is to characterize and optimize the multifunctional heterocyclic materials.The geometric structure and charge distribution of a series of molecules are studied in detail,further,the relationships between the second-order nonlinear optical coefficient???and structural parameters have been discussed.The designing ideas of several potential nonlinear optical functional materials are proposed.Firstly,we have designed and synthesized new benzimidazole based acetamides,substituted with various electron donor/acceptor functional groups i.e.?CH₃,?OCH₃ and?NO₂.2-?1H-Benzo[d]imidazol-2-ylthio?-N-o-tolylacetamide?2a?2-?1H-Benzo[d]imidazol-2-ylthio?-N-?2-nitrophenyl?acetamide?2b?2-?1H-Benzo[d]imidazol-2-ylthio?-N-?2-methoxyphenyl?acetamide?2c? Literature shows that high performance functional materials can be designed and modulated by modifying the electron donor/acceptor functional groups.In order to predict and explain the structure and characteristics of the above molecules,we use the combination of theory and experiment.In the experiment,the structure and electronic spectrum were characterized by infrared?IR?,nuclear magnetic resonance?NMR?spectroscopy and mass spectrometry?MS?.In theory,the relationship between the structure and properties of these compounds has been systematically discussed,and the second-order nonlinear optical properties??₀?of systems have been studied.Due to the larger electron-withdrawing-NO₂ on the primary amine,the transition energy of 2b is decreased,there is a large?₀ value?1550 au?.However,2c with the electron donor?-OCH₃?has the opposite result.We find that these types of compounds have the potential to be used as nonlinear optical materials.In our second work,the alkali metal and alkaline earth metal ions(Li⁺,Na⁺,K⁺,and Ca²⁺)are used to doped into heterocyclic molecules to develop a series of coumarin based complexes.The structure and nonlinear optical properties of these complexes have been systematically studied by density functional method?DFT?.Based on the open and closed-forms of coumarin derivatives,the switching ratio effect on the nonlinear optical properties has been investigated.Interestingly,the Mero-Li⁺shows a significant nonlinear optical response and its HRS value of 7607 au,is about 9 times higher than that of the corresponding spiro?closed?structure?846 au?.In this study,the relationship between the open-/closed-form of coumarin derivatives with regard to different metal ions and the effect on second-order nonlinear optical properties was studied,hence the new idea of nonlinear optical molecular switching was proposed.In the third work,the structure and nonlinear optical properties of the complexes of coumarin based N-?omethoxyphenyl?aza-15-crown-5-ether derivative?TZC?with alkali?Li⁺,Na⁺,K⁺?,alkali earth(Mg²⁺,Ca²⁺)and transition metal cations(Ni²⁺,Zn²⁺)have been studied.The results show that the bonding abilities of alkali metal cations have high ionic characteristics while alkaline earth metal cations and transition metal cations have significant covalent characteristics as depicted from the natural bond orbital?NBO?and interaction energy?Eint?analysis.Interestingly,we find that the atomic number,size and charge of metal cations,as well as the O-and N-atoms of crown ether,are important factors affecting?value.As compared to TZC,the doping of metal cations leads to a significant increase in the?values of these systems.In addition,frontier molecular orbital?FMO?analysis and TD-DFT studies revealed that the transition energies can be interpreted to explain the relationship between the charge and size of the metal atoms and the variations of the first hyperpolarizability values.