Unprecedented Near-Infrared (NIR) Emission in Diplatinum(III) (d7−d7) Complexes at Room Temperature

Abstract

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-C(6)H(3)-5-R-2-AsPh(2))(4)X(2)] (R = Me or CHMe(2); X = Cl, Br or I), together with that of their diplatinum(II) (5d(8)-5d(8)) precursors ([Pt(2)(mu-C(6)H(3)-5-R-2-AsPh(2))(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 C(6)H(3)-5-CHMe(2)-2-AsPh(2) ligands (1b) that exists in two crystalline modifications [d(Pt...Pt) = 3.4298(2) A and 4.3843(2) A]; the other is a full-lantern or paddle-wheel structure having four bridging C(6)H(3)-5-CHMe(2)-2-AsPh(2) ligands (2b) [d(Pt...Pt) = 2.94795(12) A]. 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) A] < Br (4b) [2.7526(3) A] < I (5b) [2.7927(7) A] approximately CN (6b) [2.7823(2), 2.7924(2) A 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, (P(2)O(5)H(2))(2-)], the diplatinum(II) complexes [Pt(2)(mu-C(6)H(3)-5-R-2-AsPh(2))(4)] [R = Me (2a), CHMe(2) (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.