Optimizing the vectorial component of first hyperpolarizabilities of push-pull chromophores to boost the electro-optic activities of poled polymers over broad telecom wavelength bands

The use of thiophene as a conjugation unit has long been studied for the design and synthesis of thermally stable donor-π-bridge-acceptor (D-π-A) chromophores for second-order nonlinear optics (NLO). Herein we report the rational molecular design of thiophene-bridge-based push-pull chromophores by alternating the sequence of π-conjugation units to boost the EO activities of poled polymers. Through facile synthesis, a thienyl group is directly attached to the 4-position of the N,N-dibutylaminophenyl donor to furnish a new 4-[5-(4-dibutylamino-phenyl)-thiophen-2-yl]-buta-1,3-dienyl donor-bridge component (APTBD) with strong tricyanovinyldihydrofuran (TCF) and CF₃-TCF acceptors. Two site-isolators of 4′-methoxyphenyl (MeOPh) and 4′-dimethylaminophenyl (DMA) groups were orthometrically attached to the middle of the primary D-π-A framework to study their role in changing the solubility and properties of chromophores. Through a systematic study of the linear and EO properties of this series of new chromophores and with the assist of DFT calculations, we find that alternating the sequence of π-conjugation units of thiophene-bridge-based chromophores can significantly affect the shape and dipole moment components of molecules, which are the key molecular parameters in determining the bulk NLO performance of this series of chromophores. We also find unexpected results of orthometric site-isolators in affecting the properties of these chromophores in terms of solubility, NLO property, and geometric isomerism. Our studies lead to remarkable performance improvement of these chromophores in their molecular hyperpolarizabilities and effective translation to large r₃₃ values up to 106.1 pm V^-1 at 1304 nm and 88.7 pm V^-1 at 1541 nm in poled polymers.