N-Cyanomethylmethanimine tethered anthracene dimer: concise synthesis, conformational properties and photoinduced configurational isomerization

Abstract

A new type of anthracene dimer was synthesized through an efficient approach involving the Strecker reaction and self-condensation of α-aminonitrile. This anthracene dimer contains a flexible N-cyanomethylmethanimine (CMMI) bridge, in which the imine CN double bond may adopt either a cis or a trans configuration. The imine condensation reaction yielded a trans product, which has been proven by single crystal X-ray diffraction (XRD) analysis. Solution-phase NMR analysis in conjunction with density functional theory (DFT) calculations has confirmed that configurational isomerization of the CN bond in this anthracene dimer needs to overcome relatively high energy barriers under thermal conditions and thus is difficult to achieve. Photoirradiation, on the other hand, can facilitate the trans-to-cis isomerization of the CN bond, and the resulting cis isomer shows closely engaged intramolecular π–π-stacking between the two anthracene moieties. Detailed photophysical properties of the anthracene dimer as well as related phenylene-CMMI-anthracene analogues were investigated by UV-Vis absorption and fluorescence spectroscopic analyses. The mechanisms for the selective formation of the trans CN double bond during the imine condensation and the reactivity of the CMMI-linked anthracene dimer towards fluoride anions were examined and discussed.