Isostructural M2L4 Molecular Capsules with Anthracene Shells: Synthesis, Crystal Structures, and Fluorescent Properties

An isostructural series of M2L4 molecular capsules quantitatively self-assembled from two MII ions (M=Zn, Cu, Pt, Pd, Ni, Co, and Mn) and four bent ligands with embedded anthracene fluorophores. X-ray crystallographic analysis (for M=Zn, Cu, Ni, and Pd) confirmed the formation of closed-shell structures in which the large interior cavities inside the molecular capsules (about 1 nm) were shielded by eight anthracene panels. Analysis of the ZnII and CuII structures showed the inclusion of an unusual triad guest cluster; four MeCN molecules, one water molecule, and one CF3SO3- ion were located inside the cavities. Full characterization by NMR spectroscopy and MS (ESI-TOF) demonstrated that the molecular capsules were quite stable and persist in solution. The fluorescence properties of the isostructural capsules were strongly dependent on the identity of the metal species: the ZnII capsule emitted strong blue fluorescence with a high quantum yield (F=0.8), in sharp contrast to the weakly emissive NiII and MnII capsules and the completely non-emissive PdII, PtII, and CoII capsules. On the other hand, the CuII capsule exhibited solvatochromism and solvent-dependent emission behavior; blue emission of the capsule was on in DMSO but off in MeCN.