Ultrafast dynamics of the excited states of the uranyl ion in solutions

Abstract

We have investigated the relaxation dynamics of the higher excited states of the uranyl ion in aqueous and methanolic solutions following photoexcitation to the S₁(¹Φ_g) state using 400 nm light. Although the time-resolved spectra are significantly different in these two solvents, the temporal dynamics studied in the entire wavelength region clearly suggest the involvement of three excited state processes in both solvents. The S₁(¹Φ_g) state undergoes ultrafast intersystem crossing (τ_ISC ~<100 fs) to the higher vibrational levels of the T₂(³Δ_g) state, followed by the intramolecular vibrational relaxation (IVR) process in the later electronic state (τ_IVR~0.85 and 1 ps in aqueous and methanolic solutions, respectively). Subsequently, the T₂(³Δ_g) state undergoes an internal conversion (IC) process (t_IC∼ l.6 and 4.5 ps in aqueous and methanol solutions, respectively) to the long-lived T₁(³Φ_g) state, which is responsible for the luminescent properties of the uranyl ion. In neat methanol, because of stronger interaction between the excited triplet, T₁(³Φ_g), state and the solvent via solvent to uranyl charge transfer, the U(VI) ion undergoes partial reduction to U(V) and the energy level of this state possibly lies lower than that of (UO₂²⁺)^∗, which is the transient species existing in aqueous solution, and hence increasing the energy gap between the T₂ and T₁ states in methanol solution. These facts possibly explain different spectral characteristics of the transient species produced in methanol and aqueous solutions as well as the longer lifetime of the IC process in methanol solution.