Sub-picosecond and picosecond dynamics in the S1 state of [2,2′-bipyridyl]-3,3′-diol investigated by UV–visible transient absorption spectroscopy

S1 excited state relaxation processes of [2,2′-bipyridyl]-3,3′-diol are investigated at room temperature in solution. After excitation at 355–360 nm, the transient UV–visible spectra are investigated in the 350–650 nm region where both transient absorption and stimulated emission occur. The signal is measured at various probe wavelengths as a function of time up to delays of 150 ps. The spectral features appear with times shorter than the instrumental resolution (<200 fs) and evolve with slower dynamics in the 10–20 ps range. A single-exponential on top of the instantaneous rise of the stimulated emission is observed with time constants of 8–10 ps depending on the solvent. This confirms the coexistence of two proton-transfer mechanisms, one directly forming the di-keto, the other involving the formation of the transient mono-keto tautomer. The transient absorption spectrum intensity decreases following a biexponential law, the fast component coinciding with that of the stimulated emission increase. The slow component is attributed to the decay of the (n, π∗) formed because of its proximity to the (π, π∗) state.