Theoretical Investigations On The Nonlinear Optical Properties Of The Boronheterofullerene-superalkali Systems

With the rapid development of optical telecommunication, optical computing andsignal-processing devices etc, high-perfomance materials with larger nonlinear opticalresponse have gained a great deal of attention. Among all, the π conjugated organicmaterials are of major interest in the nonlinear optical field, due to their largenonlinear optical coefficient, fast nonlinear optical response times, relatively low cost,ease of fabrication and integration into devices, tailorability which allows one to finetune the chemical structure and properties for a given nonlinear optical process, highlaser damage thresholds. Theoretical calculations are capable to obtain the nonlinearoptical cofficients of molecules, which are of great importance in establishing thestructure-property relationship and exploring the inner mechanism, so as to effectivelyimprove the molecular optical nonlinearity, by comparing the computed results withcorresponding experimental results. In this thesis, through constructing(super)alkali—boron-heterofullerene charge-transfer systems, and usingdensity-functional theory combined with finite-field (FF) method, we have performedsystematic theoretical research on their molecular geometries, electronic spectrum,charge transfer and nonlinear optical properties, which would provide new insightsand valuable theoretical clues for the rational design of more applicable andhigh-performance nonlinear optical materials. The main results of our work are asfollows:1. It is found that both alkali K and superalkali M3O (M=Li, Na and K) can beeffectively absorbed on the BC59nanocage. The binding energies (Eb) forK@n-BC59(n=5and6) are in the range of2.50~2.69eV, and those forM3O-BC59(M=Li, Na and K) are in the range of4.24~5.14eV, which showsthat all of these combined compounds can exhibit high stability, especially forthe superalkali-combined systems (owing to the formation of the B-Ochemical bonds). 2. Compared with the parent cluster BC59(619au), both alkali and superalkalidoping can apparently enhance the first hyperpolarizabilities (β0). The β0ofK@n-BC59(n=5and6) and M3O-BC59(M=Li, Na and K) are3352,2621and4921,5440and7800au, respectively, where the superalkali doping are muchsuperior to the alkali in enhancing the β0of the combined dyads (M3O-BC59).Clearly, through interacting superalkali with boron-heterofullerene to constructformal Donor-Acceptor (DA) chromophores, which can exhibit not only excellentstability but also large nonlinear optical response; therefore, they are highlyexpected to be potential candidates for excellent second-order NLO materials.