Luminescence properties of salts of the [Pt(4′Ph-terpy)Cl]⁺ chromophore: crystal structure of the red form of [Pt(4′Ph-terpy)Cl]BF₄ (4′Ph-terpy = 4′-phenyl-2,2′∶6′,2″-terpyridine)

Abstract

The complexes [Pt(4′Ph-terpy)Cl]A (4′Ph-terpy = 4′-phenyl-2,2′:6′,2″-terpyridine; A = SbF₆, CF₃SO₃, or BF₄) have been prepared by reaction of [Pt(PhCN)₂Cl₂] with the appropriate silver salt followed by addition of the 4′-phenyl-2,2′:6′,2″-terpyridyl ligand. The hexafluoroantimonate salt is yellow but, depending on the method of crystallisation, the triflate and tetrafluoroborate salts can be isolated in two forms, one yellow and the other red, the red forms only being stable at low temperatures. The crystal structure of red [Pt(4′Ph-terpy)Cl]BF₄·CH₃CN has been determined at 153 K by X-ray diffraction methods. The [Pt(4′Ph-terpy)Cl]⁺ cations are stacked face-to-face in an extended chain of stepped tetramers, with essentially equal Pt  · · ·  Pt distances of ca. 3.3 Å within a tetramer and a Pt  · · ·  Pt distance of 4.680 Å between successive tetramers. The spectroscopic and solid state emission properties of the salts have been recorded. The yellow salts are characterised by emission from an isolated chromophore in an excited state that reflects the admixture of ³MLCT (MLCT = metal-to-ligand charge-transfer) and ³IL (IL = intraligand) character. This assignment is supported by the presence of vibrational structure in the emission band as well as by a lifetime of ca. 1 µs for the emission. The red [Pt(4′Ph-terpy)Cl]BF₄·CH₃CN salt, in contrast, exhibits emission from a ³MMLCT (MMLCT = metal–metal–ligand charge-transfer) state as a consequence of the strong platinum d_z²–d_z² orbital interactions. This assignment is consistent with the observation of a narrow, structureless and asymmetric band as well as an emission lifetime of ca. 0.1 µs. There is a systematic and substantial red-shift of 75 nm in the emission maximum on cooling the red salt from 280 to 80 K, an unexpected result since bathochromic shifts of this kind are normally associated with stacked structures with a uniform Pt  · · ·  Pt separation. The above assignments are further supported by measurements of the emission spectra of solutions of varying concentrations of the tetrafluoroborate salt in a dimethylformamide–methanol–ethanol glass at 77 K and by lifetime measurements. Interestingly, pressure in the form of grinding the salts modifies their luminescent properties. Thus, crushed samples of the yellow hexafluoroantimonate salt exhibit multiple emission at 80 K from both the ³MMLCT and the mixed parentage ³MLCT/³IL excited states, whereas at room temperature the emission spectrum is dominated by a broad band centred at 644 nm associated entirely with the ³MMLCT emission.

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