Design and synthesis of heterocycle-based push-pull NLOphores: A comprehensive study of their linear and non-linear optical properties

Push-pull chromophores, with strong intramolecular charge transfer (ICT), exhibit high ground-state polarization, driving strong NLO responses. The donor and acceptor abilities of the groups in the investigated compounds influence ICT, so the nonlinear response of these compounds was analyzed in relation to their strength. Two different families of NLOphores were synthesized through [2 + 2] cycloaddition-retroelectrocyclizations of heterocycle-based electron-rich alkynes with tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ). The λ_max values of push-pull chromophores, particularly for charge transfer bands, fall within the range of 424–758 nm. The linear and nonlinear optical properties of these NLOphores were subsequently examined through a combination of experimental and theoretical methods. NLO measurements were conducted using a validated custom-made Z-scan device. The nonlinear absorption coefficients range from −1.44 × 10⁻⁴ cm.W⁻¹ to −9.90 × 10⁻⁴ cm.W⁻¹, while nonlinear refractive indices range from −1.59 × 10⁻⁷ cm². W⁻¹ to −2.26 × 10⁻⁶ cm². W⁻¹. As expected from their molecular structures, the TCNQ products displayed stronger nonlinear responses than the TCNE products.