Crystal structures and photoluminescent properties of the orange and yellow forms of [Pt{4′-(o-ClC6H4)trpy}(CN)]SbF6: an example of concomitant polymorphism

Abstract

Vapour diffusion of diethyl ether into a concentrated solution of [Pt{4′-(o-ClC₆H₄)trpy}(CN)]SbF₆ (trpy = 2,2′:6′,2″-terpyridine) in acetonitrile induces the concomitant formation of orange and yellow single crystals that have been shown by means of X-ray crystal structure determinations at 200 K to be different polymorphs of the compound. The [Pt{4′-(o-ClC₆H₄)trpy}(CN)]⁺ cations in the orange polymorph are stacked parallel and head-to-tail in an extended chain of tetramers with Pt⋯Pt distances of < 3.5 Å within a tetramer and a Pt⋯Pt distance of > 3.6 Å linking the outer cations of successive tetramers. The [Pt{4′-(o-ClC₆H₄)trpy}(CN)]⁺ cations in the yellow polymorph form a “staircase” motif, comprising a continuous sequence of stepped cation pairs where the members of each pair are coplanar, and linked by C—H⋯N hydrogen bonds between the ortho- and meta-H atoms of an outer pyridine ring of one trpy moiety and the cyano ligand N atom of the other cation. Successive cation pairs (or dimers) are parallel to each other, and linked by π(trpy)–σ(trpy) interactions that follow from the offset geometry of the trpy overlap and a short perpendicular separation between the trpy planes of 3.30 Å. The emission spectra recorded on crystalline samples of the two polymorphs are very different and reflect the different crystal structures. That of the orange polymorph is typical of emission from a ³MMLCT (MMLCT = metal–metal bond-to-ligand charge transfer) excited state that derives from finite d_z²(Pt)–d_z²(Pt) orbital overlap within the tetramers: λ(em)_max = 650 nm at 200 K. On the other hand, a much broader emission band was recorded for the yellow polymorph that peaks at 583 nm and which has higher energy shoulders at ca. 545 and 505 nm. Deconvolution of this band suggests that it comprises two emission origins, most likely ³IL (IL = intraligand) emission peaking at 543 nm and ³MLCT (MLCT = metal-to-ligand charge transfer) emission with a maximum at 588 nm.