Stimuli-Responsive Lanthanide-Dianthracene Frameworks: Lanthanide-Dependent Photocycloaddition Reaction and Photophysical Properties

Metal-organic frameworks (MOFs) showing stimuli-responsive optical properties are very attractive due to their potential applications in information storage, molecular switches, and devices. In this work, we report a series of isomorphic lanthanide-dianthracene phosphonates with layered framework structures, namely, [Ln(NO3)3(depma2)1.5]<middle dot>(depma2)0.5 (1Ln, Ln = Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb), where depma2 stands for prephotodimerized 9-diethyl-phosphonomethylanthracene (depma). 1Ln experiences a thermally induced single-crystal-to-single-crystal (SC-SC) structural transformation forming a chain compound [Ln(NO3)3(depma)(depma2)]<middle dot>(depma2)0.5 (2Ln) in which one depma2 is dissociated into an anthracene pair. Interestingly, 2Ln (Ln = Sm, Gd, Tb, Tm, Yb) exhibit excimer emission and undergo [4 + 4] photocycloaddition of anthracene in an SC-SC fashion to form the pristine 1Ln, while the others either display a near-infrared (NIR) emission (for 2Nd, 2Er, 2Ho) or nonemissive (for 2Eu) and do not undergo photocycloaddition reaction. Obviously, reversible photoresponsive luminescence properties can be achieved for 2Ln (Ln = Sm, Gd, Tb, Tm, Yb) but not for 2Ln (Ln = Eu, Nd, Er, Ho).