Estrutura eletronica e polarizabilidades de betainas
AUTOR(ES)
Paulo Henrique Ribeiro Peixoto
DATA DE PUBLICAÇÃO
2007
RESUMO
This thesis is dedicated to a theoretical study of the electronic structure and optical properties of betaine molecules carried at the semiempirical and ab initio levels. The charge distribution, the dipole moment and the linear (a) and first nonlinear (Ã) polarizabilities of a total of 66 betaine molecules belonging to 6 different families were calculated. Within a given family, all molecules possess a same pair of standard electron donor and acceptor groups connected by conjugated bridges of increasing sizes. An initial study of the space distribution of the frontier molecular orbitals (HOMO and LUMO) in these betaines reveal that, as it could be expected, in the smaller members of each family the HOMO [LUMO] is more concentrated in the extremity that contains the donor [acceptor] group. However, we have verified that once the size of the conjugated bridge is increased beyond a certain critical value a definite and unexpected inversion in the spatial distribution of these orbitals takes place. We show that this peculiar behavior, which has not been previously described for any DA system, accounts for an inversion of the direction of the characteristic photoinduced intramolecular electron transfer of the betaines. As a consequence, we have been able to show that the donor or acceptor character of the electron transfer groups in the molecule deviates from its standard behavior once the HOMO-LUMO inversion takes place. However, our results also indicate that the observed HOMO-LUMO inversion is not directly reflected in the corresponding values of the polarizabilities a and b. Even so, we have also been able to show that, contrary to what it is observed in other conjugated systems of increasing sizes, the absolute value of the first hyperpolarizability à of the molecules of a given family, which initially grows very fast with the size of the conjugated bridge, soon tends to an apparent saturation. Therefore, amongst these betaine compounds the molecules with most promising nonlinear second order optical properties are those of intermediate sizes. Finally, we present an initial investigation of other systems that could be candidates to exhibit a similar HOMO-LUMO inversion, where we show that the nature of the connecting bridge between the electron transfer groups plays a crucial role for the occurrence of this phenomenon
ASSUNTO(S)
eletrÃnica molecular polarizabilidades estrutura eletrÃnica doador-aceitador betaÃnas fisica
