Synthesis and characterization of a novel second-order nonlinear optical chromophore based on a new julolidine donor

A new chromophore HK containing the cis,cis-1,7-diethoxy-3-isopropyljulolidine group as a novel electron-donor, thiophene as a π-conjugated bridge and a tricyanofuran (TCF) acceptor has been synthesized and systematically investigated in this paper. Its corresponding chromophore FTC using 4-(diethyl amino)benzyl as the electron donor group was also prepared for comparison. This is the first time that the cis,cis-1,7-diethoxy-3-isopropyljulolidine group was introduced into NLO materials. Density functional theory (DFT) was used to calculate the HOMO-LUMO energy gap and first-order hyperpolarizability (β) of the new chromophore. The HOMO-LUMO gap was also investigated by cyclic voltammetry (CV). Upon using the cis,cis-1,7-diethoxy-3-isopropyljulolidine group as the donor, a reduced energy gap of 1.007 eV was obtained compared with chromophore FTC (ΔE = 1.529 eV). The high molecular hyperpolarizability of the new chromophore can be effectively translated into large electro-optic (EO) coefficients (r33) in poled polymers. The doped films containing the new chromophore HK showed a value of 72 pm V(-1) at the concentration of 25 wt% at 1310 nm. This value is almost two times higher than the EO activity of the usually reported traditional (N,N-diethyl) aniline nonlinear optical (NLO) chromophore FTC. High r33 values indicated that the new julolidine donor can efficiently improve the electron-donating ability and reduce intermolecular electrostatic interactions, thus enhancing the macroscopic EO activity. These properties, together with good solubility, suggest the potential use of the new chromophore in advanced materials devices.