Self-assembled binary multichromophore dendrimers with enhanced electro-optic coefficients and alignment stability

Abstract

Novel binary self-assembled electro-optic (EO) dendrimers were developed to maximize the acentric order of chromophores with large hyperpolarizabilities and strong dipole–dipole interactions. To achieve high EO coefficient, a multichromophore dendritic structure and supramolecular self-assembly strategy through π–π stacking of fluoroaromatics and aromatics were adopted to minimize dipole–dipole interaction of the chromophores at high loading density. In these dendrimers, three tetraene chromophores were substituted with an aromatic dendron (HD) or a fluoroaromatic dendron (FD) on the donor ends as the peripheral groups. The neat dendrimer films show excellent film-forming ability and a higher refractive index (n = 1.81 for FDSD and n = 1.86 for HDSD at 1310 nm) compared with guest–host EO polymers (n ≈ 1.6, 1310 nm). Higher n is beneficial for reducing the driving voltage for Mach–Zehnder modulators. By doping a secondary chromophore (HJD, 25 wt%) into neat dendrimers and blending two dendrimers together with 1 : 1 ratio (wt), three kinds of binary EO materials were prepared to afford better EO performance and larger index of refraction without phase separation. Due to supramolecular self-assembly through FD–HD π–π interactions, the poled binary film of 1 : 1 HDSD : FDSD exhibited a very large EO coefficient value up to 289 pm V⁻¹ together with a large refractive index (n = 2.01) at 1310 nm, which represents a very high n³r₃₃ figure-of-merit of 2347 pm V⁻¹ at 1310 nm. Meanwhile, a non-covalently crosslinked network was formed by the HD–FD π–π interactions which afforded an improved long-term alignment stability. After annealing at room temperature for 1000 h, about 97% of the initial r₃₃ values could be still maintained for the poled films of 1 : 1 HDSD : FDSD. The high poling efficiency and temporal EO stability combined with refractive index make them promising candidates for application in photonic devices.