Synthesis, characterization and electrochemiluminescent properties of cyclometalated platinum(ii) complexes with substituted 2-phenylpyridine ligands

Abstract

Two neutral cyclometalated platinum(II) complexes, Pt(DPP)(acac) and Pt(BPP)(acac) (DPP = 2,4-diphenylpyridine, BPP = 2-(4-tert-butylphenyl)-4-phenylpyridine, acac = acetylacetone), have been synthesized and characterized by ¹H NMR spectroscopy, mass spectrometry, elemental analyses and by X-ray crystallography for Pt(DPP)(acac). Electrogenerated chemiluminescence (ECL) of the two complexes in the absence or presence of coreactant tri-n-propylamine (TPrA) in different solvents (CH₃CN, CH₂Cl₂, DMF, CH₃CN/H₂O (V, 50 : 50)) has been studied. The ECL spectra are identical to their own PL spectra, indicating that ECL processes lead to the same metal-to-ligand charge-transfer (³MLCT) excited state that is generated by light excitation. The ECL potentials of Pt(DPP)(acac) and Pt(BPP)(acac)/TPrA in CH₃CN and CH₃CN/H₂O solution were at ∼0.75 V vs. SCE, and significantly negatively shifted by about 0.6 V compared to that of the Ru(bpy)₃²⁺/TPrA system. The ECL quantum efficiencies of the complexes are comparable to that of the Ru(bpy)₃²⁺/TPrA system. The significant increase of the ECL signal in the coreactant system is due to the formation of the strongly reducing intermediate TPrA˙. It is noteworthy that the ECL efficiencies of the synthesized compounds are much higher than that of the tridentate polypyridyl ligands.