Conspicuous absorption and fluorescence spectroscopic properties of 3,3′-dihydroxy-2,2′-bipyridines in solution

Abstract

The spectroscopic and photophysical properties of 3,3′-dihydroxy-2,2′-bipyridine and 3,3′-dihydroxy-6,6′-dimethyl-2,2′-bipyridine have been studied. The strong electronic transition [ε(343 nm)= 17 000 ± 200 dm³ mol^–1 cm^–1 in CH₂Cl₂] is rather broad (HBW = 50 cm) with a double maximum. In water–alcohol (ethanol, propane-1,2-diol, glycerol) mixtures, a new broad transition appears at about 440 nm. The intensity of this absorption band increases with increasing water content. The 440 band is also present in organic solvents, but strongly suppressed. By raising the temperature the strength of the 440 and 340 bands decreases and increases, respectively. The bandshape of the fluorescence spectrum (centred at ca. 500 nm), the fluorescence anisotropy, as well as the photophysics are independent of the excitation wavelength. Surprisingly, the absorption and fluorescence anisotropy experiments reveal the same magnitude and direction of the electronic transition dipoles of the 340 and 440 bands. Experimental data are compatible with one chromophore whose electronic ground state contains two energy minima. The mutual levels of these minima depend strongly on solute–solvent interactions, which qualitatively explain observed variations of the relative intensity of the 340 and 440 bands.