Modulations of a Metal–Ligand Interaction and Photophysical Behaviors by Hückel–Möbius Aromatic Switching

Abstract

In organometallic complexes containing π-conjugated macrocyclic chelate ligands, conformational change significantly affects metal–ligand electronic interactions, hence tuning properties of the complexes. In this regard, we investigated the metal–ligand interactions in hexaphyrin mono-Pd(II) complexes Pd[28]M and Pd[26]H, which exhibit a redox-induced switching of Hückel–Möbius aromaticity and subsequent molecular conformation, and their effect on the electronic structure and photophysical behaviors. In Möbius aromatic Pd[28]M, the weak metal–ligand interaction leads to the π electronic structure of the hexaphyrin ligand remaining almost intact, which undergoes efficient intersystem crossing (ISC) assisted by the heavy-atom effect of the Pd metal. In Hückel aromatic Pd[26]H, the significant metal–ligand interaction results in ligand-to-metal charge-transfer (LMCT) in the excited-state dynamics. These contrasting metal–ligand electronic interactions have been revealed by time-resolved electronic and vibrational spectroscopies and time-dependent DFT calculations. This work indicates that the conspicuous modulation of metal–ligand interaction by Hückel–Möbius aromaticity switching is an appealing approach to manipulate molecular properties of metal complexes, further enabling the fine-tuning of metal–ligand interactions and the novel design of functional organometallic materials.