Cyano-rich donor-acceptor-donor-type NLOphores containing dialkylated triazene and aniline groups

Abstract

Continuous progress on the synthesis of push-pull chromophores is related to their outstanding optoelectronic properties. Expanding scope is a challenge due to the instability of targets and long synthetic strategies. Herein, we report the synthesis of thirteen different triazene-substituted alkyne substrates using Sonogashira cross-coupling reactions. Although we have previously described successful syntheses of homoconjugated and π-conjugated push-pull systems starting from triazene-substituted terminal alkynes, symmetric and unsymmetric bis-triazenes that we reported in this study unexpectedly did not undergo [2 + 2] cycloaddition-retroelectrocyclizations (CA-RE) with well-known electron acceptors, TCNE and TCNQ. With the use of diethylaniline-substituted substrates in CA-RE, this issue was successfully circumvented. Two different groups of NLOphores were synthesized in very high yields via [2 + 2] CA-RE with TCNE (95–99%) and TCNQ (92–99%). All target NLOphores were investigated by UV/Vis spectroscopy, thermal gravimetric analysis, high-resolution mass spectrometry, theoretical studies, and EFISHG experiments. While different dialkyltriazene groups did not appear to have an important impact on ICT properties of the chromophores, modifications made in the electron withdrawing units significantly affected the ICT performance of the chromophores. Target push–pull-systems displayed λmax values for their CT bands ranging between 454 and 692 nm. The ICT properties of the chromophores were confirmed by computational studies (Time-dependent density functional theory (TD-DFT) studies, frontier orbital visualizations, electrostatic potential maps, and calculation of several parameters such as dipole moment, HOMO-LUMO gaps, electronegativity, global chemical hardness, and softness, average polarizability, first hyperpolarizability) as well as experimental UV/Vis studies. As computational studies indicate that synthesized chromophores are potential NLOphores, experimental NLO measurements were performed by the EFISHG technique. Target structures exhibited large NLO responses (μβ values ranging 850 × 10−48 esu to 6050 × 10−48 esu).