Tuning solid emission by salts of the [Pt{4′-(o-CH3–Ph)trpy}Cl]+ and [Pt{4′-(o-CF3–Ph)trpy}Cl]+ luminophores: crystal structures of [Pt{4′-(o-CH3–Ph)trpy}Cl]A (A = BF4 or SbF6) and [Pt{4′-(o-CF3–Ph)trpy}Cl]SbF6 (trpy = 2,2′:6′,2″-terpyridine)

Abstract

The synthesis and characterisation of 4′-(2‴-methylphenyl)-2,2′:6′,2″-terpyridine [4′-(o-CH₃–Ph)trpy] and 4′-(2‴-trifluoromethylphenyl)-2,2′:6′,2″-terpyridine [4′-(o-CF₃–Ph)trpy] are described. Reaction of these ligands with [Pt(PhCN)₂Cl₂] in the presence of the appropriate silver salt afforded [Pt{4′-(o-CH₃–Ph)trpy}Cl]A and [Pt{4′-(o-CF₃–Ph)trpy}Cl]A (A = BF₄ or SbF₆). The crystal structure of the SbF₆⁻ salt of the [Pt{4′-(o-CH₃–Ph)trpy}Cl]⁺ cation consists of columns of cations and anions with the cations stacked parallel and head-to-tail and with a constant Pt⋯Pt distance [3.368(1) Å] and interplanar spacing (3.36 Å) along the stack. Crystals of the BF₄⁻ salt also contain columns of cations and anions with the cations stacked parallel and head-to-tail. However, the platinum atoms of successive cation pairs are offset to different extents with respect to a line drawn perpendicular to the stack. As a result the Pt⋯Pt distances along the stack alternate between 3.573(1) and 3.827(1) Å while the interplanar spacing is within experimental error, constant at an average value of 3.42 Å. The [Pt{4′-(o-CF₃–Ph)trpy}Cl]SbF₆ salt exhibits the same packing motif but with alternating Pt⋯Pt distances of 3.629(1) and 3.685(1) Å and an average interplanar spacing of 3.46 Å. The variable temperature emission spectra recorded on microcrystalline samples of these compounds reflect the environments of the cations in the crystals. The red [Pt{4′-(o-CH₃–Ph)trpy}Cl]SbF₆ salt exhibits ³MMLCT (MMLCT = metal–metal–ligand-charge-transfer) emission [λ(em)_max = 616 nm] that is distinguished by a large red-shift of ca. 60 nm in the emission maximum when the sample is cooled from room temperature to 80 K; such a red-shift is associated with strong platinum d_z²–d_z² orbital interactions perpendicular to the cation stack. The other three salts are yellow with broad featureless emission spectra that do not change position when the temperature is lowered but which are red-shifted when compared to the low temperature dilute glass spectrum recorded for [Pt{4′-(o-CH₃–Ph)trpy}Cl]SbF₆ in a dimethylformamide/methanol/ethanol (1 ∶ 5 ∶ 5) mixture; this is interpreted in terms of excimeric emission consistent with the presence of π–π interactions in the solid. The roles of the ortho-substituents (CH₃ and CF₃) and the anions (SbF₆⁻ and BF₄⁻) in determining the precise arrangements of the cations in the crystals and hence the photoluminescence properties of these materials are briefly discussed.