Viscosity sensitive red shifted novel D-π-A carbazole chromophore with chlorine in π-spacer: Synthesis, photophysical properties, NLO study and DFT approach

Four novel D-pi-A extended styryl colorants containing carbazole unit were designed and modified synthetically to evaluate the effect of various acceptors on ICT (intramolecular charge transfer) characteristics and to investigate the effect of incorporation of an additional pi-bond along with the assistance of electron withdrawing chlorine group in the conjugation. Dye 4d with rhodanine-3-acetic acid and 4c with 2-(benzothiazol-2-yl) cyanomethylene showed the longer absorption and emission maxima than 4a and 4b in the polar solvents. The studied dyes exhibited positive solvatochromism and large Stokes shift due to twisted geometry. ICT characteristics of dyes were confirmed with the help of different solvent polarity plots to afford the validation of ICT character. The order of epsilon(max) (molar absorption coefficient) was found as, 4c > 4d > 4b > 4a. All the studied dyes demonstrated good fluorescence molecular rotor (FMR) properties. Dyes 4c and 4d showed better sensitivity towards the viscosity with x = 0.33 and x = 0.30 respectively. The experimental and the computational studies were opted to study the NLO (Non-linear optical) characters of the dyes. Smallest band gap in 4c and 4d amongst all the four dyes was evident from the higher values of "linear polarizability (alpha(0))", "first order hyperpolarizability (beta)" and " second order hyperpolarizability (gamma)". These dyes have showed better NLO properties than urea. DFT calculations [(B3LYP/6-31 + G(d)] and [CAM-B3LYP/6-31 + G(d)] were employed to understand the structural and electronic behaviour of 4a-4d which also have supported the experimental results. Thus, from the observed results of the solvatochromic and computational method, reported new carbazole styryls were found to be a good candidate of FMR and NLO material.