Solid-state supramolecular structures and excellent photothermal activities of dimeric zinc(II) phthalocyanines axially bridged with bipyridine derivatives

Abstract

Metal-ligand axial coordination of zinc(II) phthalocyanines is an efficient strategy for regulation of structures and properties. In this work, we designed and prepared four unsubstituted zinc(II) phthalocyanine (ZnPc) dimer-based supramolecular complexes (1–4) via axtial coordination with bipyridine derivatives. Single crystal X-ray analysis revealed that the four novel complexes were dimeric (i.e. ZnPc-ligand-ZnPc), with H-shape for 1, saddle-like for 2, and Z-shapes for 3 and 4. It's interesting that a pair of saddle-like dimers of 2 crossed each other to form an interlocked tetramer structure. Furthermore, 3 and 4 were dense stacking because of their semirigid ligands, which significantly enhanced the intermolecular interactions. Intriguingly, in the crystal structure of 4, the naphthalimide ring of the ligand embedded into two neighboring phthalocyanine macrocycles, resulting in a slipped sandwich structure of ZnPc-naphthalimide-ZnPc with double π···π interactions. More importantly, the strong intermolecular interactions led to very broad absorption spectra in vis-NIR region, quenched fluorescence, and highly efficient photothermal effect. Upon irradiation, 3 and 4 could reach up to 85.7 °C and 89.2 °C, respectively, enabling them to serve as outstanding photothermal conversion materials used for photothermal imaging and therapy.