Supramolecular photochemistry and photophysics. adducts of pt(bpy)(nh₃)₂²⁺ with aromatic crown ethers

We report results of an investigation concerning absorption spectra, emission spectra, luminescence lifetimes, electrochemical properties, and photochemical behavior of Pt(bpy)(NH₃)₂²(bpy = 2,2'-bipyridine) and its adducts with the following crown ethers in CH₂Cl₂ and CH₃CN solution: 18-crown-6 (18C6), dibenzo-18-crown-6 (DB18C6), dibenzo-24-crown-8 (DB24C8), dibenzo-30-crown-10 (DB30C10), dibenzo-36-crown-12 (DB36C12), bis(m-phenylene)-32-crown-10 (BMP32C10), bis(p-phenylene)-34-crown-10 (BPP34C10), and dinaphtho-30-crown-10 (DN30C10). Adduct formation with the aliphatic crown ether 18C6 does not change the photochemical and photophysical properties of the Pt(bpy)(NH₃)₂²⁺ complex. By contrast, adduct formation with the aromatic crown ethers causes (i) a strong decrease of the crown ether absorption band and of the ligand (bpy)-centered absorption bands of Pt(bpy)(NH₃)₂²⁺ in the 260-330-nm region, (ii) the appearance of a new broad absorption band in the 340-450-nm region, (iii) the complete or partial quenching of the crown ether fluorescence and of the ligand-centered phosphorescence of Pt(bpy)(NH₃)₂²⁺, (iv) the appearance of a new, broad, and short-lived luminescence band in the 550-630-nm region, (v) the quenching of the photoreaction of Pt(bpy)(NH₃)₂²⁺ in CH₂Cl₂, and (vi) a perturbation in the electrochemical reduction potentials. These results are attributed to an electronic interaction, in the ground and excited state, between the bpy ligand of the Pt complex and the aromatic rings of the crowns. The intensity of such electronic interaction depends on the size of the crown ring and on the nature and substitution positions of the aromatic rings present in the crown. The results obtained show that the assembly of a coordination compound into an appropriate supramolecular structure can protect the compound from photoreaction and can profoundly change its spectroscopic, photophysical, and electrochemical properties.