A Correlation Study between Dendritic Structure and Macroscopic Nonlinearity for Second-Order Nonlinear Optical Materials
Xiaobo Zang, Guangchao Liu, Qianqian Li, Zhong’an Li, Zhen Li
Abstract
To study the effect of dendritic structure on the macroscopic nonlinearities, four types of dendritic nonlinear optical (NLO) materials containing the same isophorone-bridged chromophores are prepared, including AB2-type dendrimer (D1), AB2-type hyperbranched polymer (HBP, HP1), “A3+B2”-type HBP (HP2), and “A3+AB2”-type HBP (HP3). The detailed 1H NMR spectra analysis manifests that through suitable end-capping modifications, HP1 exhibits a relatively perfect dendritic structure like dendrimer with the degree of branching close to 1 while both HP2 and HP3 still contain a lot of linear structural defects. As a result, the measured second-harmonic generation (SHG) coefficients (d33) of D1 and HP1 can be up to 71.3 and 77.6 pm/V, respectively, both of which are much larger than those of HP2 (32.2 pm/V) and HP3 (55.4 pm/V). Our results strongly suggest that AB2-type self-polymerization with suitable end-capping modifications is a feasible methodology to achieve large NLO effects, while simultaneously maintaining merits of facile one-pot synthesis and high thermal stability of NLO effects. In addition, the higher d33 value for HP3 than HP2 could be due to the optimization of dendritic structure derived from the particular “A3+AB2”-type polymerization, providing a new way to design hyperbranched NLO polymers.