Optically transparent and thermally stable nonlinear optic chromophores featuring a thieno[2,3-b]thiophene donor

A thieno[2,3-b]thiophene core has been utilized as a \pi-donor component to design two series of push–pull thienothiophenes by introducing various acceptor groups either via olefinic or aza-spacers. The molecules show a UV–visible cut-off wavelength below the second harmonic generation (SHG) at $ \lambda /2$ of 532 nm, thereby conforming to the nonlinearity–transparency trade-off. Second order molecular nonlinearity, \beta measured by Hyper-Rayleigh scattering technique was found in the range of $9.58–47.66 X 10^{-30} esu$, while the Kurtz powder technique produced signals of the order of 0.43–1.02 U. Thermal decomposition temperatures measured by differential scanning calorimetry revealed decomposition temperatures \geq 275° C, indicating high thermal stability