Host-Guest Interactions of Phosphorescent Molecular Tweezers Based on an Alkynylplatinum(II) Terpyridine System with Polyaromatic Hydrocarbons

Host-guest interactions of a molecular tweezer complex 1 with various planar organic molecules including polyaromatic hydrocarbons (PAHs) were investigated by 1D and 2D H-1 NMR spectroscopy, UV/Vis absorption and emission titration studies. 2D and DOSY NMR spectroscopies support the sandwiched binding mode based on 1: 1 host-guest interactions. The binding constants (K-S) of complex 1 for various PAHs were determined by NMR titration studies and the values were found to span up to an order of 10(4) M-1 for coronene to no observable interaction for benzene, indicating that the pi-surface area is important for such host-guest interactions. The substituent effect on the host-guest interaction based on the guest series of 9-substituted anthracenes was also studied. In general, a stronger interaction was observed for the anthracene guest with electron-donating groups, although steric and pi-conjugation factors cannot be completely excluded. The photophysical responses of complex 1 upon addition of various PAHs were measured by UV/Vis and emission titration studies. The UV/Vis absorption spectra were found to show a drop in absorbance of the metal-to-ligand charge-transfer (MLCT) and ligand-to-ligand charge-transfer (LLCT) admixture band upon addition of various guest molecules to 1, whereas the emission behavior was found to change differently depending on the guest molecules, showing emission enhancement and/or quenching. It was found that emission quenching occurred either via energy transfer or electron transfer pathway or both, while emission enhancement was caused by the increase in rigidity of complex 1 as a result of host-guest interaction.