Indole‐ and Carbazole‐Functionalized Dicyanomethyleneindan Push‐Pull Dyes. Part 1: Synthesis, Cyanide Sensing, Photophysical and NLO Properties

ABSTRACT

The 1‐Indanylidenemalononitrile‐based donor‐π‐acceptor chromophores are attractive platforms for the design of optical materials, cyanide‐responsive probes, and second‐order nonlinear optical systems. Herein, we compare indole and carbazole donor units on a common dicyanomethyleneindan acceptor scaffold and evaluate their photophysical behavior, cyanide sensing, cellulose‐supported solid‐state sensing performance, and second‐order nonlinear optical (NLO) properties. The chromophores were synthesized by Knoevenagel‐type condensation, characterized by Fourier‐transform infrared spectroscopy (FT–IR), nuclear magnetic resonance (NMR) spectroscopy, and high‐resolution mass spectrometry (HRMS), and investigated by ultraviolet‐visible (UV–vis) absorption spectroscopy, fluorescence spectroscopy, tetrabutylammonium cyanide (TBACN) titration, electric‐field‐induced second‐harmonic generation (EFISH) measurements, density functional theory (DFT), and time‐dependent density functional theory (TD‐DFT) calculations. Both compounds exhibited solvent‐sensitive ICT behavior and selective responses toward CN ⁻ in DMSO, with LOD values of 0.21 and 0.24 µM for compounds 2 and 3 , respectively. The cyanide‐induced optical response, Job's plot analysis, and ¹ H NMR titration were consistent with a 1:1 stoichiometry for a Michael‐type nucleophilic addition process that disrupts the donor‐acceptor conjugated pathway. Compound 2 was further transferred to a cellulose‐based textile substrate and produced visible colorimetric and fluorescence responses toward CN ⁻ in the solid state. EFISH measurements and theoretical calculations confirmed measurable second‐order NLO activity, with the carbazole derivative showing the highest response. Overall, the results demonstrate that donor variation on the dicyanomethyleneindan scaffold is an effective strategy for ICT tuning, cyanide reactivity, textile applicability, and NLO performance within a single push‐pull chromophore family.