Aggregation induced emission-active dimethoxy phenyl conjugated cyanoacetates with significant second order-nonlinear optical properties: Experimental and theoretical calculations

Abstract

The methyl and ethyl 5,6-dimethoxy-1-indanylidene cyanoacetate compounds [(OCH3)2C6H2C3H4CCN, COOR {RCH3 (1), C2H5 (2)}] were synthesized and structurally analysed. Compound 1 was crystalized in a triclinic crystal system with a centrosymmetric space group (P21/m) by single crystal X-Ray diffraction studies. Additionally, the non-covalent interactions were observed in the crystal packing (C–H⋅⋅⋅⋅⋅π interaction, inter and intra-molecular H-bonding). The presence of the cyano vinylene group in both compounds diminishes the emission behaviour in the solution state due to the twisted intramolecular charge transfer (TICT) phenomena. The fluorescence was improved by aggregation induced emission (AIE) properties using THF/H2O mixture at 90% for both compounds 1 and 2, which follows the restriction of the intramolecular rotation (RIR) process. The aggregated state exhibits 9 times (1) and 1.07 times (2) improved quantum yield when compared to the solution state and the lifetime decay value reveal 3.2 ns for 1 and 4.3 ns for 2. The second harmonic generation (SHG) efficiency was studied by Kurtz and Perry powder technique and potassium dihydrogen phosphate (KDP) as a reference. The compounds 1 [58 mV (2.1 times)] and 2 [65 mV (2.3 times)] show higher SHG efficiency than the reference KDP (28 mV), attributes to the several non-covalent interactions (C–H⋅⋅⋅⋅⋅π interaction, inter and intra molecular H-bonding) in the crystal packing. In addition, density functional theory and time dependent density functional theory (DFT/TD-DFT) methods using various functionals (B3LYP, long-range corrected CAM-B3LYP and LC-BLYP) were performed to calculate the second order non-linear optical parameters for both the compounds. The B3LYP functional gives better results and its close correlation with the experimental results was due to incorrect long-range charge transfer between the donor and acceptor system.