Hiperpolarizabilidades Semi-EmpÃricas de Sistemas OrgÃnicos Doador-Receptor

AUTOR(ES)
DATA DE PUBLICAÇÃO

2001

RESUMO

The research on new nonlinear materials finds in Quantum Chemistry the possibility of designing organic molecules with optimized performance. It is performed by matching adequate components of structure, that is, a bridge (B) and an efficient donor(D)/acceptor(A) pair. Hence, some classes of D/A organic molecules were designed and theoretically observed to have potential for use in nonlinear optics. These systems display large magnitude of both β e γ hyperpolarizabilities that were calculated through the AM1/TDHF methodology. Firstly, D-A, D-D and A-A series were designed containing polyenic bridges. The more extended D/A system, which have eight alternated double bonds, shows the best performance. Subsequently, the mesoionic rings were inserted into the polyenic bridges. These systems containing mixed bridges produce even larger values of both β and γ nonlinear responses, therefore showing a promising perspective of practical applications. In addition, the AM1/TDHF semiempirical methodology was evaluated in relation to the first and second hyperpolarizabilities by the calculation of polyenic derivatives fully characterized, as described in the literature. The results indicate that the methodology is appropriate for the design and selection of organic molecules with optimized nonlinear responses. The selection of the more efficient D/A pair involved the investigation of the strength of donor and acceptor groups cited in the literature. Furthermore, the role of the N heteroatom of the phenylamine donor was studied. The new finding of the present work is the linking of this group to the bridge via the heteroatom, in contrast to the examples presented in the literature with the binding by the aromatic ring. This functionality associated with a strong acceptor dicyanomethylene, linked at the opposite ends of conjugated bridges, results in improved performance of these systems. Donor-acceptor organic molecules with saturated bridge of derivatives narylpiperidone and n-aryltropanone display the β values of the same order of magnitude of observed in 4-nitroaniline. To assess the effect caused in the magnitude of hyperpolarizabilities by increasing the bridge, we design systems with two fused rings. The results at the AM1/TDHF level of theory show that these systems have a behavior which is different from that of conjugated bridges. That is, the growth of the bridge did not contribute to the optimization of the β and γ nonlinear responses. Further, oligomers of polyaniline, which is an extremely versatile material, were studied. Model oligomers containing three and five rings were investigated, considering the three neutral forms of the polyaniline (leucoemeraldine, emeraldine and pernigraniline). In addition, the functionalization of the oligomers with D/A groups shows a perspective of obtaining aniline derivatives with high first hyperpolarizability. The theoretical results for the model trimers of the static γ present the observed experimental trends

ASSUNTO(S)

am1/tdhf semiempirical methodology nonlinear organic materials metodologia semiempÃrica am1/tdhf novos materiais hyperpolarizabilities β and γ e γ hiperpolarizabilidades β quimica

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