Novel electro-optic chromophores based on substituted benzo[1,2-b:4,5-b′] dithiophene π-conjugated bridges

Two novel non-linear optical (NLO) chromophores were designed and synthesized based on the substituted benzo[1,2-b:4,5-b'] dithiophene unit (BDT), tricyanofuran (TCF) electron acceptor and two different electron donors. These new chromophores, which exhibited good thermal stability and solubility in common organic solvents, were systematically characterized by thermogravimetric analysis, UV-Vis spectra, density functional theory (DFT) calculations and measurements of the electro-optic (EO) coefficients. Compared with the dodecyl group in chromophore BDT1 that we have previously reported, the isooctane group in BDT2 could act as a suitable isolation group, and as a result, the guest-host system containing 30% of BDT2 in amorphous polycarbonate (APC) displayed the largest EO coefficient of 102 pm V-1, which was greatly improved over BDT1 and the analogous thiophene-based chromophore with the same electron donor and acceptor. The solvatochromic analysis and DFT study demonstrated that the julolidine group in BDT3 possessed a stronger donating ability. However, the 30% BDT3/APC exhibited an EO coefficient of only 82 pm V-1, which was probably caused by the diminishment of the hyperpolarizability value of BDT3 in the polar polymer matrix. These results indicate the potential of the isooctane-substituted benzo[1,2-b: 4,5-b'] dithiophene pi-conjugated bridge in chromophore design and NLO materials, and further illustrate the critical role of suitable isolation groups and fine-tuning of the donor's electron-donating strength in optimizing the non-linear properties of NLO chromophores.