Photoresponsive gadolinium–anthracene complexes: tuning the orientation and π–π stacking of anthracene groups via alkyl ester substituents of phosphonate ligands

Abstract

Controlled assembly of metal–anthracene complexes is important for the development of smart photoresponsive materials based on [4 + 4] anthracene photocycloaddition reactions. Here, we report three mononuclear complexes containing two anthracene units, namely, [Gd(NO₃)₃(L^R)₂(dmpu)] [R = Me (1), Et (2), i-Pr (3)], where L^R stands for 9-dialkyl phosphonomethylanthracene, and dmpu is N,N′-dimethylpropyleneurea. For compounds 1 and 3, the anthracene moieties of phosphonate ligands are joined head-to-tail by π–π interactions to form a linear supramolecular chain. Meanwhile for 2, they are almost vertically aligned to create a zigzag supramolecular chain. Both 1 and 2 underwent photodimerization reaction upon 395 nm UV light irradiation and showed a photochromic behaviour, but 3 could not. The reaction rate of 2 is about 40 times that of 1. These results indicate that the alkyl ester group of anthracene–phosphonate ligands can finely regulate the orientation and stacking of anthracene groups, and the onset of the photocycloaddition reaction depends not only on the stacking geometry of the anthracene groups but also on steric hindrance. In addition, thermally induced phase transition was observed for 2 due to the order–disorder transition of nitrate anions, associated with a shift of the emission peak. This work may shed light on the design and synthesis of photoresponsive metal–organic systems containing anthracene ligands.