Substrate-Controlled Divergent Reactivity of 2-Alkynylindoles: [2 + 2] Cycloaddition-Retroelectrocyclization versus...

This study provides insight into the donor characteristics of the indole framework and allows a comparison between its C-2 and C-3 positions. Ten electron-rich alkynes incorporating an indole core were synthesized via the Sonogashira cross-coupling reaction. Structural variations within these substrates led to two distinct reaction pathways for the formation of cyano-containing conjugated systems, yielding eight tricyanovinylation and two [2 + 2] cycloaddition-retroelectrocyclization (CA-RE) products. The tricyanovinylation products formed with tetracyanoethylene (TCNE) were obtained in 40-82% yields, whereas the chromophores produced through the [2 + 2] CA-RE pathway arising from alkyne activation were isolated in 30-63% yields. They display pronounced intramolecular charge transfer (ICT), with λ_max values ranging from 494 to 504 nm for the tricyanovinylation products, while the two [2 + 2] CA-RE chromophores absorb at 471 and 495 nm. The observed ICT bands are supported by UV/vis studies by positive solvatochromism and protonation experiments. To clarify the relationship between NLO response and ICT properties, the dipole moment, band gap, electronegativity, average global hardness-softness, average polarizability, and first hyperpolarizability parameters were evaluated using computational methods. In addition to theoretical DFT calculations, the EFISHG technique was employed to investigate the NLO properties. The experimental μβ values of the selected molecules range from 150 to 560 × 10^-48 esu.