Design of "parallel connection" chromophores could give a way of acquiring effective chromophores. The semiempirical method ZINDO was employed to study relationship between static first hyperpolarizabilities of "parallel connection" chromophores and the number of parallel nonconjugated N,N-dimethyl-4-nitroaniline (DMNA) units in the chromophore. The results show that the chromophore containing three parallel non-conjugated DMNA units exhibits the highest static first hyperpolarizability, which is 1.8 times that of chromophore DMNA. However, static first hyperpolarizabilities of the chromophores containing four or five DMNA units are very small. The absorption maximum wavelength (λ max ) of "parallel connection" chromophores is remarkably shorter (34.9 nm-38.1 nm) than that of 1DMNA. Therefore, the "parallel connection" chromophore containing three DMNA units would be an effective chromophore with a large first hyperpolarizability and a good optical transparency. It could give a useful suggestion for designing effective chromophores containing parallel non-conjugated D-π-A units.
H-shaped chromophores containing three parallel non-conjugated D-π-A units are effective chromophores with high hyperpolarizability and good optical transparency. The semi-empirical methods ZINDO, AM1, MNDO and PM3 were employed to study the effect of strength of acceptors and donors, and steric repulsion between substituents on static first hyperpolarizabilities (β0) and enhancements of β0 of the H-shaped chromophores. The results show that the H-shaped chromophore would exhibit the largest β0 and/or the largest enhancement of β0 of the chromophore when combination of a donor (D) and an acceptor (A) in a D-π-A unit is suitable.
The semi-empirical method ZINDO was employed to study relationship between macroscopic optical nonlinear parameter μβ/MW (where μ is the dipole moment, β is the first hyperpolarizability, and MW is molecular weight) and the number of parallel non-conjugated D-π-A units in a chromophore. The computational results show that macroscopic optical nonlinear parameter μβ/MW value increases remarkably from 1.64 to 2.53 with increasing the number of parallel and non-conjugated p-Nitroaniline (PNA) units in a chromophore from 1 to 3. Then the μβ/MW value decreases rapidly from 2.53 to 0.43 with increasing the number of PNA units in a chromophore from 3 to 5. It suggests that design of chromophores containing two or three parallel non-conjugated D-π-A units would be an effective strategy for increasing the first hyperpolarizability and macroscopic optical nonlinearity of designed NLO materials.
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