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 S(1)((1)Phi(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 S(1)((1)Phi(g)) state undergoes ultrafast intersystem crossing (tau(ISC) approximately <100 fs) to the higher vibrational levels of the T(2)((3)Delta(g)) state, followed by the intramolecular vibrational relaxation (IVR) process in the later electronic state (tau(IVR) approximately 0.85 and 1 ps in aqueous and methanolic solutions, respectively). Subsequently, the T(2)((3)Delta(g)) state undergoes an internal conversion (IC) process (tau(IC) approximately l.6 and 4.5 ps in aqueous and methanol solutions, respectively) to the long-lived T(1)((3)Phi(g)) state, which is responsible for the luminescent properties of the uranyl ion. In neat methanol, because of stronger interaction between the excited triplet, T(1)((3)Phi(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 (UO(2)(2+))*, which is the transient species existing in aqueous solution, and hence increasing the energy gap between the T(2) and T(1) 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.

Ultrafast Transient Absorption Spectroscopy of UO22+ and [UO2Cl]+

“…A direct transition from S 0 to T 2 was assumed in this model as any kinetics like absorption from the S 0 to higher states, vibronic relaxation and intersystem crossing (ISC) processes in between absorption and reaching the T 2 state were not resolved due to the time resolution of the experiment, as these excitation processes usually happen on the low femtosecond time scale. Transition rates of k P / k ISC = 2.15 ± 0.07 ps –1 and k IC = 0.354 ± 0.003 ps –1 were found and are in reasonable agreement with uranyl­(VI) in nitrate solution, for which rate constants of 1.18 and 0.625 ps –1 , respectively, were reported …”

“…The results for the ClO 4 – system are in good agreement with TAS measurements reported for uranyl­(VI) in nitrate solution. Here, in nitrate solution the broader and shorter-lived state was assumed to be the T 2 state of the uranyl and the change of the transient spectra was attributed to the transition from the T 2 to the luminescent T 1 state. Until now it is unclear whether the 3 Δ or the 3 Φ electronic state is the lowest electronic state in energy and corresponding to the luminescent state, as theoretical calculations suggest different results depending on the calculation method applied .…”

“…Transition rates of k P /k ISC = 2.15 ± 0.07 ps −1 and k IC = 0.354 ± 0.003 ps −1 were found and are in reasonable agreement with uranyl(VI) in nitrate solution, for which rate constants of 1.18 and 0.625 ps −1 , respectively, were reported. 20 By fitting the absorption and transient absorption data (Figure 1) with Gaussian peaks, under the condition that the peaks are equidistant due to the regular vibronic structure of the higher states, we were able to calculate the energy gaps between the different electronic states. The absorption band lowest in energy at 20 520 cm −1 corresponds to the energy difference between the S 0 and T 1 state, and the energy difference between the vibrational bands is the gap between the vibronic states of the T 1 state with an energy spacing of ΔE = 702 cm −1 (Table 1).…”

Photoinduced Generation of Superoxidants for the Oxidation of Substrates with High C−H Bond Dissociation Energies

“…Uranyl nitrate hexahydrate ([UO 2 ](NO 3 ) 2 .6H 2 O) is proven to be an efficient photocatalyst for activation of C−H bond in alkanes to form C−C bond . The photophysical and photochemical properties of uranyl cation [UO 2 ] 2+ with the uranium oxidation state of +6 have been reported . The triplet excited state of [UO 2 ] 2+ has a lifetime in microseconds with the reduction potential of +2.36 V vs. SCE .…”

“…The photophysical and photochemical properties of uranyl cation [UO 2 ] 2+ with the uranium oxidation state of +6 have been reported . The triplet excited state of [UO 2 ] 2+ has a lifetime in microseconds with the reduction potential of +2.36 V vs. SCE . The oxygen site of the excited state generates the carbon‐centered radical via homolytic cleavage of C−H bond in alcohols and arylaldehydes .…”