Photophysics and Excited State Dynamics of Cyclometalated [M(Phbpy)(CN)] (M = Ni, Pd, Pt) Complexes: A Theoretical and Experimental Study

• Published in: Inorganic chemistry 2021-06, Vol.60 (12), p.8777-8789
• Main Authors: Eskelinen, Toni, Buss, Stefan, Petrovskii, Stanislav K, Grachova, Elena V, Krause, Maren, Kletsch, Lukas, Klein, Axel, Strassert, Cristian A, Koshevoy, Igor O, Hirva, Pipsa
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.1c00680

Cyclometalated complexes [M­(Phbpy)­(CN)] (HPhbpy = 6-phenyl-2,2′-bipyridine) of the group 10 metals (Ni, Pd, and Pt) bearing a carbanionic –C∧N∧N pincer ligand were synthesized and studied in a combined experimental and computational DFT approach. All three complexes were crystallographically characterized showing closely packed dimers with head-to-tail stacking and short metal-metal contacts in the solid state. The computational models for geometries, excited states, and electronic transitions addressed both monomeric (Ni-mono, Pd-mono, and Pt-mono) and dimeric (Ni-dim, Pd-dim, and Pt-dim) entities. Photophysical properties and excited state dynamics of all title complexes were investigated in solution and in the solid at 298 and 77 K. [Ni­(Phbpy)­(CN)] and [Pd­(Phbpy)­(CN)] are virtually nonemissive in solution at 298 K, whereas [Pt­(Phbpy)­(CN)] shows phosphorescence in CH2Cl2 (DCM) solution (λ em = 562 nm) stemming from a mixed 3MLCT/ILCT (metal-to-ligand charge transfer/​intraligand charge transfer) state. At 77 K in a glassy frozen DCM:​MeOH matrix, [Pd­(Phbpy)­(CN)] shows a remarkable emission (λ em = 571 nm) with a photoluminescence quantum yield reaching almost unity, whereas [Ni­(Phbpy)­(CN)] is again nonemissive. Calculations on the monomeric models M-mono show that low-lying metal-centered states (MC, i.e., d-d* configuration) with dissociative character quench the photoluminescence. In the solid state, the complexes [M­(Phbpy)­(CN)] show defined photoluminescence bands (λ em = 561 nm for Pd and 701 nm for Pt). Calculations on the dimeric models M-dim shows that the axial M···M interactions alter the photophysical properties of Pd-dim and Pt-dim toward MMLCT (metal-metal-to-ligand charge transfer) excited states with Pd-dim showing temperature-dependent emission lifetimes, suggesting thermally activated delayed fluorescence, whereas Pt-dim displayed phosphorescence with excimeric character. The metal-metal interactions were analyzed in detail with the quantum theory of atoms in molecules approach.