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

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posted on 26.05.2010, 00:00 by Martin A. Bennett, Suresh K. Bhargava, Eddie Chung-Chin Cheng, Wai Han Lam, Terence Kwok-Ming Lee, Steven H. Privér, Jörg Wagler, Anthony C. Willis, Vivian Wing-Wah Yam
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-AsPh2)4X2] (R = Me or CHMe2; X = Cl, Br or I), together with that of their diplatinum(II) (5d8−5d8) precursors ([Pt2(μ-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···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 3a6a (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 [Pt2(μ-pop)4]4− [pop = pyrophosphite, (P2O5H2)2−], the diplatinum(II) complexes [Pt2(μ-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 [Pt2(μ-pop)4X2]4− that exhibit red luminescence only at 77 K in a glass or as a solid, complexes 3a6a and 3b6b 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,b5a,b involve a moderate to extensive mixing of XMMCT character and MC [dσ−dσ*] character, whereas the cyano complexes 6a and 6b are thought to involve relatively less mixing of the XMMCT character into the MC [dσ−dσ*] state.