Establishing Photoexcited Nickel Porphyrins as Hot Charge Transfer Chromophores

In charge-transfer (CT) chromophores built around transition metal ions, energy losses caused by relaxation from the absorbing high-energy excited-state (HE-ES) to the metastable lowest-energy excited-state (LE-ES) inherently limits their functionalities. With emerging hot CT chromophores, intramolecular charge delocalization is utilized before complete relaxation to the LE-ES. However, rationalizing their design is largely contingent upon understanding their ultrafast dynamics in multidimensional energy landscapes. Here, transient optical absorption and X-ray emission spectroscopies are combined to probe the photoinduced dynamics in a prototypical Ni(II) porphyrin with electronic and spin sensitivities on the femtosecond timescale. Measurements across the HE-ES → LE-ES relaxation resolve an intermediate CT step involving Ni(II) and the porphyrin ring. Full-dimensional trajectory surface hopping dynamics simulations unveil clear correlations between CT and bifurcations in the excited-state landscape. These results establish Ni(II) porphyrins and other open-shell metalloporphyrins as a rich platform for developing hot CT chromophores to target challenging endergonic photoreactions.