We wondered whether this stark difference in sensitivity to oxygen quenching could explain the significant differences in the yields we reported previously for the atom transfer radical addition (ATRA) of sulfonyl chlorides to styrenes under aerated versus degassed conditions in solution using [Cu(dmp)(xantphos)]PF6 ([Cu1]) as the photocatalyst. Our previous study concluded that while the solution-state ATRA reaction was highly air sensitive, the transformation was very efficient using mechanophotocatalysis under aerobic conditions. We thus decided to determine the excited-state lifetime of [Cu1] in both MeCN and as a neat powder, Figure , to assess whether this parameter correlated with the trend in reaction yields. The behavior of the time-resolved PL decays of [Cu1] under degassed and aerated conditions broadly mirrors [Ir1]; when going from degassed to aerated MeCN, the lifetime of [Cu1] decreases significantly from 201 ns to 56.9 ns (lit. tPL of 170 ns in degassed MeCN 77 ), whereas the powder sample possesses a significantly longer tPL that is insensitive to the presence of O2, (tPL of 23 µs under both air and vacuum; lit. tPL of 30.2 µs with tetrakis(bis-3,5-trifluoromethylphenylborate) as the counter anion ). While we cannot be certain that this increased robustness to aerobic conditions is the origin of the improved performance of [Cu1] under mechanochemical conditions in the ATRA reaction, this provides a starting point for future mechanistic studies to assess the influence of solvent-minimized environments on photocatalyst reactivity.