Unprecedented Near-Infrared (NIR) Emission in Diplatinum(III) (d7-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) (5d(7)-5d(7)) complexes of general formulae [Pt-2(mu-C6H3-5-R-2-AsPh2)(4)X-2] (R = Me or CHMe2; X = Cl, Br or l), together with that of their diplatinum(II) (5d(8)-5d(8)) precursors ([Pt-2(mu-C6H3-5-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 C6H3-5-CHMe2-2-AsPh2 ligands (1b) that exists in two crystalline modifications [d(Pt center dot center dot center dot Pt) = 3.4298(2) angstrom and 4.3843(2) angstrom]; the other is a full-lantern or paddle-wheel structure having four bridging C6H3-5-CHMe2-2-AsPh2 ligands (2b) [d(Pt center dot center dot center dot Pt) = 2.94795(12) angstrom]. 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) angstrom] < Br (4b) [2.7526(3) angstrom] < 1 (5b) [2.7927(7) angstrom] similar to CN (6b) [2.7823(2), 2.7924(2) angstrom 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(mu-pop)(4)](4-) [pop = pyrophosphite, (P2O5H2)(2-)], the diplatinum(II) complexes [Pt-2(mu-C6H3-5-R-2-AsPh2)(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 [Pt-2(mu-pop)(4)X-2](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 [d sigma-d sigma*] character, whereas the cyano complexes 6a and 6b are thought to involve relatively less mixing of the XMMCT character into the MC [d sigma-d sigma*] state.