Unprecedented intramolecular cyclization in strongly dipolar extended merocyanine dyes: A route to novel dyes with improved transparency, nonlinear optical properties and thermal stability

• Unprecedented intramolecular cyclization in strongly dipolar merocyanine dyes.
• New magenta dyes.
• Improved NLO-transparency-stability trade-off.
• Novel class of dyes for NLO materials.

Novel magenta dyes were obtained via an unprecedented base-promoted intramolecular cyclization of strongly dipolar elongated merocyanine dyes bearing a phthalimide substituted tricyanopropylidene terminal segment. These shorter diarylpolyene derivatives retain a dipolar character and show a typical intramolecular charge transfer (ICT) transition. The cyclization noticeably modifies the electronic structure inducing both onset of bond length alternation and reduction of dipole moment as well as a marked hypsochromic and hypochromic shift of the ICT absorption band compared to their merocyanine precursors. Yet the new dyes show improved quadratic hyperpolarizability and thermal stability as compared to the merocyanine dyes having the same number of conjugated double bonds in the polyenic linker. As such these derivatives hold promise as new class of dyes for NLO materials and SHG probes.