Unprecedented near-infrared (NIR) emission in diplatinum(III) (d ⁷-d⁷) complexes at room temperature

The synthesis and single-crystal X-ray structures of the first family of efficient NIR emitters with tunable emission energy based on dihalodiplatinum(III) (5d⁷-5d⁷) complexes of general formulae [Pt₂(μ-C₆H₃-5-R-2-AsPh ₂)₄X₂] (R = Me or CHMe₂; X = Cl, Br or I), together with that of their diplatinum(II) (5d⁸-5d ⁸) precursors ([Pt₂(μ-C₆H ₃-5-R-2-AsPh₂)₄]) and cyano counterparts (X = CN), are reported. The diplatinum(II) complexes with isopropyl groups are isolated initially as a mixture of two species, one being a half-lantern structure containing two bridging and two chelate C₆H ₃-5-CHMe₂-2-AsPh₂ ligands (1b) that exists in two crystalline modifications [d(Pt⋯Pt) = 3.4298(2) Å and 4.3843(2) Å]; the other is a full-lantern or paddle-wheel structure having four bridging C₆H₃-5-CHMe₂-2-AsPh₂ ligands (2b) [d(Pt⋯Pt) = 2.94795(12) Å]. Complete conversion of the isomers into 2b occurs in hot toluene. The Pt-Pt bond distances in the diplatinum(III) complexes are less than that in 2b and increase in the order X = Cl (3b) [2.6896(2) Å] < Br (4b) [2.7526(3) Å] < I (5b) [2.7927(7) Å] ∼ CN (6b) [2.7823(2), 2.7924(2) Å for two independent molecules]. Comparison with the corresponding data for our previously reported series of complexes 3a-6a (R = Me) indicates that the Pt-Pt bond lengths obtained from single-crystal X-ray analysis are influenced both by the axial ligand and by intermolecular lattice effects. Like [Pt ₂(μ-pop)₄]^4- [pop = pyrophosphite, (P ₂O₅H₂)^2-], the diplatinum(II) complexes [Pt₂(μ-C₆H₃-5-R-2-AsPh ₂)₄] [R = Me (2a), CHMe₂ (2b)] display intense green phosphorescence, both as solids and in solution, and at room temperature and 77 K, with the emission maxima in the range 501-532 nm. In contrast to the reported dihalodiplatinum(III) complexes [Pt₂(μ-pop) ₄X₂]^4- that exhibit red luminescence only at 77 K in a glass or as a solid, complexes 3a-6a and 3b-6b are phosphorescent in the visible to near-infrared region at both room and low temperatures. The electronic spectra and photoemissive behavior are discussed on the basis of time-dependent density functional theory (TDDFT) calculations at the B3YLP level. The photoemissive states for the halide analogues 3a,b-5a,b involve a moderate to extensive mixing of XMMCT character and MC [ds-ds*] character, whereas the cyano complexes 6a and 6b are thought to involve relatively less mixing of the XMMCT character into the MC [dδ-dδ*] state.