Observation of low-energy metal-metal-to-ligand charge transfer absorption and emission: electronic spectroscopy of cyclometalated platinum(II) complexes with isocyanide ligands

Abstract

A series of luminescent mononuclear [(CNN)Pt(CNR)]X (HCNN = 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 [(CNN)Pt(CO)]CF3SO3 (6(CF3SO3)) and binuclear [{(CNN)Pt}2(μ-CN(CH2)3NC)](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 [CNN] 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*(CN)] absorption of a dimeric species via 2[Pt(CNN)(CNnBu)]+ [(CNN)Pt(CNnBu)]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 [(CNC)Pt(L)] (HCNCH = 2,6-diphenylpyridine; L = CO and CNAr‘) solids are compared, and no MMLCT emission is evident.