Steady-state and time-resolved spectroscopy of 2, 2′-bipyridine-3, 3′-diol in solvents and cyclodextrins: Polarity and nanoconfinement effects on tautomerization

The ground- and excited-state tautomerization of the 2, 2′-bipyridine-3, 3′-diol molecule (BP(OH)₂) was studied in different solvents and in confined nanocavities of cyclodextrins (CDs) using steady-state and lifetime spectroscopic measurements. In all solvents, a dizwitterion (DZ) tautomer is produced in the excited state after intramolecular double-proton transfer. This tautomer is stabilized in the ground state in water only and produces two unique absorption peaks in the region of 400-450 nm. The DZ tautomer fluoresces in the green and as the solvent polarity increases, the fluorescence peak is blue-shifted (498 nm in cyclohexane versus 462 nm in water), and the fluorescence lifetime gets shorter (3.10 ns in cyclohexane versus 0.65 ns in water). The results indicate the sensitivity of this tautomer to solvent polarity, particularly the solvent's hydrogen-bonding capability. In water, another photoinduced tautomerization mechanism takes place via a water network solvating each of the two hydrogen-bonding centers of the molecule. The second tautomer is detected as a small shoulder in the blue side of the fluorescence peak and has a lifetime of 5.40 ns. Using BP(OH)₂ to probe the nanocavities of aqueous CDs reveals the degree of hydrophobicity of the cavities and the different mechanisms of probe encapsulation. As the cavity size decreases in the order y-CD to β-CD to α-CD, the cavity is more hydrophobic, which is reflected in an intensity decrease of the absorbance of the DZ tautomer and a red shift in its fluorescence peak. The measured lifetimes show the same trend and reveal how the probe interacts with the CD moiety. In y-CD, the probe is located near the secondary rim of the CD annulus, whereas in α-CD, the probe is completely sequestered between two CDs, and the hydrophobicity is close to that observed in cyclohexane. In β-CD and its derivatives, the spectral changes and the measured lifetimes indicate that the CD cavity gets more hydrophobic as a result of methyl substitution of the primary and secondary hydroxyls of the β-CD rims. In the fully methylated 2, 3, 6-tri-0-methyl-β-CD, the probe is exposed to water near the secondary rim due to the steric effect at the entrance rim that prevents the probe from full encapsulation.