Access to NLO-active donor–acceptor-type spiro compounds via cascade reactions of 3-alkynylindoles and DDQ

In this study, five homoconjugated NLOphore candidates decorated with indole donor groups were synthesized in 55–85% yields via click-type formal [2 + 2] cycloaddition reactions. The obtained compounds were subse- quently transformed into spirocyclic push–pull-type chromophores through thermal rearrangements, affording the corresponding spiro products in 54–80% yields. During the formation of the spiro derivatives, the commonly reported low-yield issue associated with aniline-containing substrates in the literature was successfully overcome through the incorporation of indole donor groups, enabling the efficient formation of the target structures. All target compounds were investigated by UV/vis spectroscopy, thermogravimetric analysis (TGA), theoretical calculations, and custom-made Z-scan experiments. The homoconjugated structures exhibit intramolecular charge-transfer (ICT) absorption bands between 496 and 619 nm, whereas the spiro derivatives display λmax values in the range of 464–471 nm. TGA analysis revealed that the rigid spiro framework contributes to enhanced thermal stability compared to the homoconjugated chromophores. The ICT characteristics of the target molecules were examined in detail through frontier molecular orbital depictions, electrostatic potential maps, and time- dependent density functional theory (TD-DFT) calculations. In addition, the calculated first hyperpolarizability values indicate that all chromophores possess significant potential as nonlinear optical (NLO) materials. In this context, the NLO properties were experimentally investigated using a custom-designed Z-scan system. The homoconjugated chromophore series exhibited nonlinear refractive index values (n2) ranging from − 4.20 x 10− 7 to − 22.30 x 10− 7 cm2 W− 1. A similar, yet even more pronounced trend was observed in the spirocyclic chro- mophore series, with n2 values ranging from − 2.91 x 10− 7 cm2 W− 1 to − 46.21 x 10− 7 cm2 W− 1. Overall, the theoretical and experimental results show good agreement, supporting the potential of these chromophores as promising NLOphore candidates.