Excited state dynamics of protonated keto uracil: intersystem crossing pathways in competition

Abstract

The relaxation dynamics of protonated keto uracil has been investigated through cryogenic UV photodissociation spectroscopy. Steady-state spectroscopy and time-resolved photochemistry, including pump-probe photodissociation and kinetics of appearance of photofragments, are monitored over 10 orders of magnitude as a function of excess energy imparted in the bright $$^{\mathrm {1}}\uppi \uppi $$ * state. Although photofragments are produced in the ground electronic state after internal conversion, the non-radiative decay mechanism abruptly changes with a slight increase of excess energy in the $$^{\mathrm {1}}\uppi \uppi $$ * state. At the band origin, a three-step decay involving electronic couplings to the charge transfer $$^{\mathrm {1}}\hbox {n}_{\text {o}}\uppi $$ * state and the triplet $$^{\mathrm {3}}\uppi \uppi $$ * state with lifetimes in the range of $$10\,{\upmu }\hbox {s}$$ and 2 ms, respectively, is proposed. However, the pathway through the charge transfer state closes a few hundreds of wavenumbers above the band origin. From this excess energy, the excited state population is transferred through a low energy barrier towards a region of the $$^{\mathrm {1}}\uppi \uppi $$ * potential energy surface where a triple crossing with the $$^{\mathrm {3}}\uppi \uppi $$ * state and the ground state is located. The experimental results are assigned with the help of ab initio calculations at the spin-component scaled coupled-cluster level.