Chromophore-labelled, luminescent platinum complexes: syntheses, structures, and spectroscopic properties

Abstract

Ligands based upon 4-carboxamide-2-phenylquinoline derivatives have been synthesised with solubilising octyl hydrocarbon chains and tethered aromatic chromophores to give naphthyl (HL²), anthracenyl (HL³) and pyrenyl (HL⁴) ligand variants, together with a non-chromophoric analogue (HL¹) for comparison. ¹H NMR spectroscopic studies of the ligands showed that two non-interchangeable isomers exist for HL² and HL⁴ while only one isomer exists for HL¹ and HL³. Supporting DFT calculations on HL⁴ suggest that the two isomers may be closely isoenergetic with a relatively high barrier to exchange of ca. 100 kJ mol⁻¹. These new ligands were cyclometalated with Pt(II) to give complexes [Pt(L^1–4)(acac)] (acac = acetylacetonate). The spectroscopically characterised complexes were studied using multinuclear NMR spectroscopy including ¹⁹⁵Pt{¹H} NMR studies which revealed δ_Ptca. −2785 ppm for [Pt(L^1–4)(acac)]. X-ray crystallographic studies were undertaken on [Pt(L³)(acac)] and [Pt(L⁴)(acac)], each showing the weakly distorted square planar geometry at Pt(II); the structure of [Pt(L³)(acac)] showed evidence for intermolecular Pt–Pt interactions. The UV-vis. absorption studies show that the spectral profiles for [Pt(L^2–4)(acac)] are a composite of the organic chromophore centred bands and a broad ¹MLCT (5d → π*) band (ca. 440 nm) associated with the complex. Luminescence studies showed that complexes [Pt(L^2–4)(acac)] are dual emissive with fluorescence characteristic of the tethered fluorophore and long-lived phosphorescence attributed to ³MLCT emission. In the case of the pyrenyl derivative, [Pt(L⁴)(acac)], the close energetic matching of the ³MLCT and ³LC_pyr excited states led to an elongation of the ³MLCT emission lifetime (τ = 42 μs) under degassed solvent conditions, suggestive of energy transfer processes between the two states.