Third-order nonlinear optical properties and ultrafast excited-state dynamics of benzothiazolium salts: Transition in absorption and refraction under different time regimes

Two benzothiazolium salts of D-π-A structure, (E)-3-ethyl-2-(2-(pyren-1-yl) vinyl) benzo[d]thiazol-3-ium iodide (EPBI) and (E)-3-ethyl-2-(2-(9-ethyl-9H-carbazol-3-yl) vinyl) benzo[d]thiazol-3-ium iodide (ECBI) were synthesized and spectroscopically characterized. Z-scan measurements were performed at 532 nm with 190 femtosecond and 15 picosecond laser beam respectively. EPBI displays the efficient third-order nonlinear optical susceptibility χ⁽³⁾ of 1.5 × 10⁻¹¹ esu and molecular second-order hyperpolarizability γ of 2.8 × 10⁻²⁹ esu which are one order larger than those of ECBI, and EPBI exhibits reverse saturable absorption and self-defocusing refraction while ECBI exhibits saturable absorption and self-focusing refraction in 15 ps time mechanisms. In 190 fs time mechanisms, both of them exhibits saturable absorption. Further dynamics analysis for these transition in different time regimes and determination of key parameters were performed via DFT calculation and transient absorption experiment.