Tuning the Excited-State Dynamics of Acetophenone Using Metal Ions in Solution

The effects of dissolved metal salts on the excited-state dynamics of acetophenone in solution have been explored using ultrafast transient absorption spectroscopy at two UV excitation wavelengths. In the absence of metal ions, the S₁(nπ*) transition of acetophenone is excited at 320 nm, with intersystem crossing (ISC) occurring with a time constant τ_ISC = 5.95 ± 0.47 ps in acetonitrile solution. Excitation at 280 nm accesses the S₂(ππ*) state, which internally converts (<0.2 ps) to S₁ before undergoing ISC with τ_ISC = 4.36 ± 0.14 ps. Coordination to Mg²⁺ ions makes the S₂ state accessible to excitation at 320 nm, with the rate of S₂ → S₁ internal conversion reducing 3-fold, but the ISC rate increasing. These changes to the excited-state energies and dynamics of this model photosensitizer indicate that dissolved metal salts could modify the photochemistry of synthetically useful homogenous photocatalytic systems.