High second-order nonlinear optical effect achieved by gradually decreased rotational energy barriers†

With the aim of efficiently converting the microscopic second-order nonlinear optical (NLO) effect of chromophore moieties into macroscopic NLO performance as high as possible, this work focused on the connection groups between the chromophore moieties of NLO polymers, in which alkoxy chains with different lengths and positions were systematically incorporated. The ignorable difference of the alkoxy chain from the normally utilized alkyl one directly resulted in improved macroscopic NLO performance, and d₃₃ values increased gradually from 105 to 131/157, then to 165 pm V⁻¹ with increasing contents of alkoxy chains, and further reached up to 178 pm V⁻¹ with the prolonged lengths of alkoxy chains. This was mainly due to the lower rotational barriers of ether bonds than those of the commonly used alkyl chains with carbon–carbon bonds, and the isolated effect of alkoxy chains with larger sizes. This work provides a new way to achieve a high second-order NLO effect from efficient modulation of chromophore orientations by adjustment of energy barriers.