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Observation of Low-Energy Metal−Metal-to-Ligand Charge Transfer Absorption and Emission: Electronic Spectroscopy of Cyclometalated Platinum(II) Complexes with Isocyanide Ligands

2001; American Chemical Society; Volume: 21; Issue: 1 Linguagem: Inglês

10.1021/om0106276

1520-6041

Siu‐Wai Lai, Hiu‐Wah Lam, Wei Lu, Kung‐Kai Cheung, Chi‐Ming Che,

Organometallic Complex Synthesis and Catalysis

A series of luminescent mononuclear [(C∧N∧N)Pt(C⋮NR)]X (HC∧N∧N = 6-phenyl-2,2'-bipyridine; R = tBu (1), nBu (2), iPr (3), cyclohexyl (Cy, 4), X = ClO4; R = 2,6-Me2C6H3 (Ar', 5), X = PF6) and [(C∧N∧N)Pt(CO)]CF3SO3 (6(CF3SO3)) and binuclear [{(C∧N∧N)Pt}2(μ-C⋮N(CH2)3N⋮C)](PF6)2 (7(PF6)2) complexes were synthesized, and their spectroscopic and photophysical properties have been investigated. The crystal lattices of 1(ClO4) and 5(PF6) reveal close [C∧N∧N] π−π intermolecular contacts (3.4−3.6 Å). Additional metal−metal interactions are evident in 5(PF6) with intermolecular Pt−Pt distances of 3.3831(9) Å. At complex concentration greater than 5 × 10-3 mol dm-3, the UV−vis absorption spectrum of 2 displays a weak shoulder at 511 nm (ε = 120 dm3 mol-1 cm-1), which does not obey Beer's law. A metal−metal-to-ligand charge transfer (MMLCT) [dσ*(Pt−Pt) → π*(C∧N∧)] absorption of a dimeric species via 2[Pt(C∧N∧N)(C⋮NnBu)]+ ⇋ [(C∧N∧N)Pt(C⋮NnBu)]22+ is proposed. The room-temperature emission spectrum of 4 for complex concentrations ≥ 7 × 10-3 mol dm-3 shows a low-energy band at 710 nm originating from a 3MMLCT excited state. A corresponding well-defined low-energy absorption at 500 nm in the excitation spectrum of 4 (λem 710 nm) is assigned to the 1MMLCT transition. The 77 K emissions of complexes 1−6 in glassy solutions are sensitive to complex concentration. Upon increasing concentrations from 5 × 10-5 to 2 × 10-3 mol dm-3 for 1−4, red emissions at 600−625 nm ascribed to π−π excimeric excited states develop at the expense of the vibronic 3MLCT emissions at λmax ∼502 nm. Complexes 2 and 4 show additional low-energy emissions at 739 and 710 nm, respectively, attributable to 3MMLCT states. Concentrated 77 K glassy solutions of complexes 5−7 exhibit 3MMLCT emissions (λmax 711−744 nm). The broad structureless solid-state luminescence of 2(ClO4), 3(ClO4), 5(PF6), 6(CF3SO3), and 7(PF6)2 at 298 K (λmax 701−748 nm) and 77 K (λmax 744−813 nm) are assigned to 3MMLCT excited states arising from intermolecular stacking interactions in the solid state. The spectroscopic properties of the related [(C∧N∧C)Pt(L)] (HC∧N∧CH = 2,6-diphenylpyridine; L = CO and C⋮NAr') solids are compared, and no MMLCT emission is evident.