Ultrafast Charge-Transfer Dynamics:  Studies of p-Nitroaniline in Water and Dioxane

The intersystem crossing, internal conversion, and vibrational relaxation of p -nitroanaline (PNA) in water and 1,4-dioxane have been studied using ultrafast transient absorption spectroscopy. Following the photoexcitation of PNA at 400 nm, the transient absorption dynamics were probed from 340 to 960 nm. The measurements were performed on a common, absolute absorption scale, permitting an accurate determination of the temporal evolution of the absorption spectrum. The data reveal that relaxation on the excited singlet state surface, followed by internal conversion to the ground state and intersystem crossing to the triplet state, is extremely rapid (<0.3 ps) in both solvents. The observed intersystem crossing efficiency is Φ isc ≈ 0.4 in dioxane and Φ isc ≈ 0.03 in water, indicating that the coupling between the excited singlet and triplet states depends strongly on the solvent polarity. With the estimated quantum yield for intersystem crossing in water and dioxane, we find a time constant for intersystem crossing of ≤10 ps in water and ≤0.8 ps in dioxane. The transient absorption features observed in the visible region are assigned to vibrationally excited PNA in the electronic ground state and three triplet−triplet absorption bands.