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 by 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 τISC = 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 τISC = 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 homogeneous photocatalytic systems.