Luminescent Cycloplatinated Complexes Containing Poly(pyrazolyl)-borate and -methane Ligands

Cycloplatinated neutral [Pt(C∧N){H2B(pz)2}] (1–3) [C∧N = benzoquinolate (bzq), 2-phenylpyridinate (ppy), and 2-phenylquinolate (pq)] and [Pt(pq){HB(pz)3}] 10 and cationic [Pt(C∧N){H2C(pz)2}]+ (4–6) and [Pt(C∧N){HC(pz)3}]+ (7–9) complexes were synthesized by the reaction of the corresponding precursors [Pt(C∧N)(μ-Cl)]2 with the adequate poly(pyrazolyl)-borate or -methane ligand. However, the reactions of [Pt(C∧N)(μ-Cl)]2 (C∧N = bzq, ppy) with [HB(pz)3]− evolve with B–N bond cleavage, yielding the binuclear systems [Pt(C∧N)(μ-pz)]2 as a mixture of cis and trans isomers. Complexes were characterized in solution by multinuclear and multidimensional NMR spectroscopy. The solid-state structures of 1, 3, 6, 7–9, and [Pt(bzq)(μ-pz)]2 were confirmed by X-ray single-crystal studies. The absorption, emission, and electrochemical properties of these complexes are mainly dominated by the nature of the cyclometalated ligand and the charge of the complex. On the basis of TD-DFT calculations (1, 7–9), the lowest-energy absorption for neutral 1 has been ascribed to a mixed 1ILCT/1MLCT transition, whereas for the cationic 7–9, it is mainly attributed to 1ILCT combined with some CT to both ligands in 9 (1MLCT/ML′CT 9) or to the HC(pz)3 in 7 and 8 (1ML′CT). These compounds are emissive in all media (except 4 and 10 in the solid state at 298 K). In the solid state at 298 K and at 77 K, these complexes display intense phosphorescence, which is typical of monomers. In deoxygenated CH3CN solutions at 298 K, phosphorescence is accompanied by higher-energy fluorescence in complexes 1, 4, and 8, which disappears at concentrated solutions and at 77 K. Complex 7 displays a special behavior, observing fluorescence and/or excimer fluorescence only at 298 K and excimeric emission (diluted glasses) and emission from aggregates in concentrated glasses. TD-DFT of the lowest-lying excited states responsible for the phosphorescence of 1 and 7–9 reveals a 3ILCT origin with a mixed 3MLCT character for 1 and, in the case of the cationic 7–9, a 3ILCT transition mixed with 3ML′CT (especially in 8) and with some 3MLCT in 9.