Ethyne-bridged (porphinato)zinc(II)-(porphinato)iron(III) complexes: Phenomenological dependence of excited-state dynamics upon (porphinato) iron electronic structure

We report the synthesis, spectroscopy, potentiometric properties, and excited-state dynamical studies of 5-[(10,20-di-((4-ethyl ester)methylene-oxy)phenyl)porphinato]zinc(II)-[5'-[(10',20'-di-((4-ethyl ester)methylene-oxy)phenyl)porphinato]iron(III)-chloride]ethyne (PZn-PFe-Cl), along with a series of related supermolecules ([PZn-PFe-(L)(1,2)](+) species) that possess a range of metal axial ligation environments (L = pyridine, 4-cyanopyridine, 2,4,6-trimethylpyridine (collidine), and 2,6-dimethylpyridine (2,6-lutidine)). Relevant monomeric [(porphinato)iron-(ligand)(1,2)](+) ([PFe(L)(1,2)](+)) benchmarks have also been synthesized and fully characterized. Ultrafast pump-probe transient absorption spectroscopic experiments that interrogate the initially prepared electronically excited states of [PFe(L)(1,2)](+) species bearing nonhindered axial ligands demonstrated subpicosecond-to-picosecond relaxation dynamics to the ground electronic state. Comparative pump-probe transient absorption experiments that interrogate the initially prepared excited states of PZn-PFe-Cl, [PZn-PFe-(py)(2)](+), [PZn-PFe-(4-CN-py)(2)](+), [PZn-PFe-(collidine)](+), and [PZn-PFe-(2,6-lutidine)](+) demonstrate that the spectra of all these species are dominated by a broad, intense NIR S-1 -> S-n transient absorption manifold. While PZn-PFe-Cl*, [PZn-PFe-(py)(2)](+)*, and [PZn-PFe-(4-CN-py)(2)](+)* evince subpicosecond and picosecond time-scale relaxation of their respective initially prepared electronically excited states to the ground state, the excited-state dynamics observed for [PZn-PFe-(2,6-lutidine)](+)* and [PZn-PFe-(collidine)](+)* show fast relaxation to a [PZn+-PFe(II)] charge-separated state having a lifetime of nearly 1 ns. Potentiometric data indicate that while Delta G(CS) for [PZn-PFe-(L)(1,2)](+)* species is strongly influenced by the PFe+ ligation state [ligand (Delta G(CS)): 4-cyanopyridine (-0.79 eV) < pyridine (-1.04 eV) < collidine (-1.35 eV) < chloride (-1.40 eV); solvent = CH2Cl2], the pump-probe transient absorption dynamical data demonstrate that the nature of the dominant excited-state decay pathway is not correlated with the thermodynamic driving force for photoinduced charge separation, but depends on the ferric ion ligation mode. These data indicate that sterically bulky axial ligands that drive a pentacoordinate PFe center and a weak metal axial ligand interaction serve to sufficiently suppress the normally large magnitude nonradiative decay rate constants characteristic of (porphinato)iron(III) complexes, and thus make electron transfer a competitive excited-state deactivation pathway.