Two X-type chromophores

2-[4-(4

5-di(4-nitrophenyl) imidazolyl) phenyl]-4

5-di(4-methoxyphenyl)-imidazole(DNPIPDMOPI)

2-[4-(4

5-di(4-nitrophenyl)-imidazolyl) phenyl]-4

5-di(4-aminophenyl)-imidazole (DNPIPDAPI)

were synthesized and characterized. The results show that they possess good nonlinearity

considerable blue-shifted absorption (385 nm and 379 nm in THF) and high decomposition temperature (377 ℃ and 405 ℃). These mean that the X-type chromophores possess a rather good nonlinearity-transparency-thermal stability trade-off. The multi-step corona-poling technique at elevated temperature and in-situ SHG measurements were used to obtain and evaluate the poled films of these chromophores doped in PMMA. The largest SHG signals appeared at 110-120℃

which are 12.5 pm/V and 16.7 pm/V respectively. The dependence of poling induced orientation stability on temperature was measured by depoling experiments and the results indicate that the poling-induced orientation of the films is stable at about 100℃. Theoretic analyses imply that better orientation stability arises from the X-type structure of chromophore. The X-type chromophore has two crossed intramolecular CT

both βxxx and βxyy can contribute to the second-order susceptibility

and the ratio of the tensorial components (γ= βxyy/βxxx) is about 1/3

so the orientation decay of the films induced by rising temperature will provide a certain compensation for the contribution of βxyy of chromophores.