ROTATIONAL ISOMERIZATION OF PHOTOTAUTOMER PRODUCED IN THE EXCITED-STATE PROTON-TRANSFER OF 2,2'-BIPYRIDIN-3-OL

Significantly Stokes-shifted fluorescence induced by the UV photoexcit ation of 2,2'-bipyridin-3-ol in 3-methylpentane exhibits a dramatic te mperature dependence such that dual fluorescence spectra (lambda(max) = 560 and 585 nm) at room temperature are replaced by a single fluores cence spectrum (lambda(max) = 560 nm) below 200 K. The respective emit ting species responsible for the short- and long-wavelength bands may be ascribed to the excited-state primary phototautomer (S-1') and its rotamer (S-1 ''), which is generated by, the rotational isomerization of S-1' with an activation barrier.