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 C
N 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 C
N 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 C
N double bond during the imine condensation and the reactivity of the CMMI-linked anthracene dimer towards fluoride anions were examined and discussed.