Theoretical Design And Investigation Of The Nonlinear Optical Materials Based On The Aromatic Systems

With the fast development of the information technology, thedesign and investigation of high-performance nonlinear optical (NLO)materials with large NLO response have been attracting more and moreattention, where the focused species mainly concern the inorganic NLOcrystals, the organic π-conjugated systems modified withDonor/Acceptor group (D-π-bridge-A), and the intriguing electride withthe excess electron etc. Above of these investigations can considerablyenrich our knowledge about designing the high-performance NLOmaterials. In this thesis, we mainly focus on the design for the novelNLO materials based on the fascinating organic (e.g. carbon nanotube)and inorganic (e.g. P4molecule) systems with aromatic characteristic, inview of the case that these acromatic systems can usually possessunique electronic structure and the excellent stability. Through systemicstudies, we proposed that employing the new concept of mixedπ-conjugated bridge and introducing the excess electron by the newinteraction mode can significantly improve the NLO properties. Clearly,our work can provide the new strategies and valuable theoreticalinformation for the experimental design of the new type of NLOmaterials based on the intriguing aromatic systems. The relatedcomputed results can be presented in detail as follows:1. Carbon nanotube (CNT)-based systems with the mixedπ-conjugated bridges. Under a D–π–A framework, we firstly propose anew concept of mixed π-bridge, which is constructed by linking a–(CH=CH)x–NH2chain to the end of CNT. It is revealed that elongatingthe–(CH=CH)x–NH2chain in the mixed π-bridge can effectivelyincrease the first hyperpolarizability of the CNT–(CH=CH)x–NH2system monotonously, much larger than those of the correspondingdirectly NH2-modified CNT systems with the same conjugated length.Particularly, applying the–(CH=CH)x–NH2chain with comparableconjugated size to modify the end of CNT can achieve a larger NLOresponse in the CNT-based system with a D–π–A framework. Clearly, this new concept of mixed π-conjugated bridge can overcome thebottleneck encountered with the pure CNTs acting as a π-conjugatedbridge, that is, further elongating tube length will not bring any positiveeffect on improving the first hyperpolarizability of the CNT-NH2series.These fascinating findings can be immensely valuable insights for thedesign of novel high-performance NLO materials based on CNTs.2. The (super)alkali doping the inorganic P4molecule withspherical aromaticity. A novel approach was proposed to achieve theexcess electron via (super)alkali interacting with σ electron cloud.Under high-level ab initio calculations, the geometrical structures andnonlinear optical properties of M@P4(M=Li, Na, K and Li3O) areinvestigated. The computational results revealed that doping the(super)alkali atom M can break the original double spherical aromaticityof the tetrahedral P4molecule, and the σ electron cloud is formed at theinteracting surface of the P4moiety with the M atom in the M@P4series.Upon interaction with the newly formed σ electron cloud, the valenceelectron of the (super)alkali atom is pushed out to produce a diffuseexcess electron, further resulting in a considerably large β0value.Moreover, it was found that the β0values of the M@P4(M=Li, Na andK) series is nonmonotonic dependency on the alkali atomic number,where the distance d between the alkali atom M and the interactingsurface with the σ electron cloud can play a crucial role. Clearly, thenovel mode through the (super)alkali atom interacting with the σelectron cloud to produce diffuse excess electrons, can intrinsicallyextend the effective concept of the excess electron towards the newdesign domain of high-performance NLO materials.