Femtosecond nonlinear optical properties of push–pull chromopohores using Z-scan technique: experimental and computational studies

To shed the light about the effect of the linkers of the small organic molecules on their optical properties, we examine herein the linear, and nonlinear optical properties of the two easy synthesized push–pull compounds 1 and 2 bearing similar donor and acceptor moieties based on stilbene derivatives, but differ in the spacer group, namely 4-cyano-4′-(N-dihexylamino) stilbene (1), and 4-cyanobenzal-4′-(N,N-diethyl) aniline (2). The optical studies were initially examined using various spectroscopic techniques. Compounds 1 and 2 exhibited strong absorption bands in the visible spectra with maxima at 400 and 429 nm, respectively. The steady-state emission measurements of 1 with its π–π* transition showed a strong emission band at 500 nm with a fluorescence quantum yield (Φ_f) of 0.30, which is much higher than that of 2 with its n–π* transition (Φ_f = 0.004) reflecting the nature of singlet state on the optical emission properties. The cyclic voltammograms showed the oxidation potentials of 1 and 2 at 670 and 840 mV versus Ag/AgCl, respectively, confirm the effect of the inkers on their electrochemical properties. Unlike compound 2, the thin film of 1 exhibited a sharp absorption edge at 360 nm suggesting its usefulness as an excellent UV-cut optical filter. The nonlinear absorption coefficients of the examined compounds were measured using the Z-scan technique. Compounds 1 and 2 were both exposed to 100 fs laser pulses using different excitation wavelengths. The obtained positive nonlinear absorption coefficient values of 1 and 2, such positive values may be caused by reverse saturable absorption. The influence of the laser parameters (in particular, excitation wavelength, and incident peak intensity) on the nonlinear absorption coefficient of 1 and 2 has been examined. These observations suggest the significant effect of linkers on both the photophysical, linear, and nonlinear optical features of 1 (with C=C linker) compared to 2 (with C=N linker). Furthermore, the DFT/TD-DFT methods at 6-31+G(d,p) were performed to predict the geometric, electronic, and optical properties of 1 and 2.