Ligand-field dependence of the excited state dynamics of Hangman bisporphyrin dyad complexes.

A new Hangman porphyrin architecture has been developed to interrogate the ligand-field dependence of photoinduced PCET versus excitation energy transfer and intersystem crossing in PZn(II)-PFe(III)-OH dyads (P = porphyrin). In this design, a hanging carboxylic acid group establishes a hydrogen-bonding network to anchor the weak-field OH- ligand in the distal site of the PFe(III)-OH acceptor, whereas the proximal site is left available to accept strong-field imidazole ligands. Thus, controlling the tertiary coordination environment gives access to the first synthetic example of a porphyrin dyad with a biologically relevant weak-field/strong-field configuration of axial ligands at the heme. Transient absorption spectroscopy has been employed to probe the fate of the initial PZn(II)-based S1 excited state, revealing rapid S1 quenching for all dyads in the presence and absence of strong-field imidazole ligands (tau = 6-50 ps). The absence of a (P*+)Zn(II) signal that would complement photoinduced PCET at the PFe(III)-OH subunit (i.e., PFe(III)-OH --> PFe(II)-OH2) shows that excitation energy transfer and intersystem crossing channels dominate the quenching, regardless of whether proximal strong field ligands are present. Moreover, this photophysical assignment is independent of the solvent dielectric constant and whether a phenylene or biphenylene spacer is used to span the two porphyrin subunits. Electronic structure calculations suggest that the structural reorganization attendant to reductive PCET at the high-spin Fe(III)-OH center imposes a severe kinetic cost that can only be alleviated by inducing a low-spin electronic configuration with two strong-field axial ligands.