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 photoexcitation of 2,2'-bipyridin-3-ol in 3-methylpentane exhibits a dramatic temperature dependence such that dual fluorescence spectra (lambda(max) = 560 and 585 nm) at room temperature are replaced by a single fluorescence spectrum (lambda(max) = 560 nm) below 200 K. The respective emitting 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.