  Two-photon excitation is an attractive means for controlling chemistry in
both space and time. Isoenergetic one- and two-photon excitations (OPE and TPE)
in non-centrosymmetric molecules are often assumed to reach the same excited
state and, hence, to produce similar excited-state reactivity. We compare the
solvent-to-solute excited-state proton transfer of the super photobase FR0-SB
following isoenergetic OPE and TPE. We find up to 62 % increased reactivity
following TPE compared to OPE. From steady-state spectroscopy, we rule out the
involvement of different excited states and find that OPE and TPE spectra are
identical in non-polar solvents but not in polar ones. We propose that
differences in the matrix elements that contribute to the two-photon absorption
cross sections lead to the observed enhanced isoenergetic reactivity,
consistent with the predictions of our high-level coupled-cluster-based
computational protocol. We find that polar solvent configurations favor greater
dipole moment change between ground and excited states, which enters the
probability for two-photon excitations as the absolute value squared. This, in
turn, causes a difference in the Franck-Condon region reached via TPE compared
to OPE. We conclude that a new method has been found for controlling chemical
reactivity via the matrix elements that affect two-photon cross sections, which
may be of great utility for spatial and temporal precision chemistry.
