The effects of conformation and coordination to zinc(II) ions on the luminescence properties of 2,2′-bipyridine, methyl-substituted 2,2′-bipyridines and 2,2′-biquinoline

Abstract

Using a 2-propanol-water (i-PrOH-H2O) mixture as a host, luminescence and electron paramagnetic resonance spectra have been observed for the lowest excited states of the trans and metal-free cis conformers of 2,2′-bipyridine (bpy), 4-, 5- and 6-methyl-2,2′-bipyridine (4-,5- and 6-Mebpy), 4,4′-, 5,5′- and 6,6′-dimethyl-2,2′-bipyridine (4,4′ – 5,5′ – and 6,6′-Me2bpy) and 2,2′-biquinoline (bq) at 77 K. Both trans and cis conformers are non-fluorescent or very weakly fluorescent for bpy. The trans conformer is non-fluorescent or weakly fluorescent in i-PrOH, while the cis conformer is fluorescent in i-PrOH-H2O (33 wt.% i-PrOH) for Mebpys, Me2bpys and bq. The observed fluorescence properties are expected considering that the main character of the lowest excited singlet state changes from 1nπ* in trans-bq to 1ππ* in cis-bq, while it is 1nπ* in both trans and cis conformers of bpy. The Zn2+ complexes with bpy, Mebpys, Mebpys and bq are fluorescent.