Excited state dynamics and electron transfer in a phosphorus(V) porphyrin – TEMPO conjugate

The synthesis and initial spectroscopic characterization of a phosphorus(V) octaethylporphyrin-nitroxide adduct is reported. The nitroxide (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) is axially and covalently bound to the phosphorus porphyrin (PPor⁺) to give the studied PPor⁺-TEMPO conjugate. The complex allows the influence of the type of bonding and geometry of the central element in the porphyrin on the excited state dynamics to be investigated. In addition, the high oxidation potential of the PPor⁺ 1.35 V vs SCE means that it is a strong oxidant in its excited state and is energetically well-placed to allow charge transfer (CT) from TEMPO to PPor⁺ to occur. Cyclic voltammetry data and DFT computations show that the CT state is the lowest excited state, lying well below the excited sing-doublet and trip-quartet states of the complex. Time-resolved optical data show that the presence of the bound nitroxide leads to a large decrease in the excited state lifetime in acetonitrile, suggesting that rapid charge separation and recombination occurs. However, transient EPR (TREPR) data reveal that the trip-quartet state is also populated and that emissive spin polarization is generated in the ground state at room temperature, consistent with radical-quartet pair interactions analogous to the radical-triplet pair mechanism. At low temperature, the TREPR data show that the trip-quartet state is formed with strong absorptive net polarization. The shape of the spectrum provides evidence of significant dynamics in the excited state probably due to reversible transitions between the thermally accessible quartet and doublet states. Graphical abstract: [Figure not available: see fulltext.] Synopsis: Spectroscopic studies of a phosphorus (V) porphyrin with an axial covalently bound TEMPO radical reveal different relaxation pathways following light excitation. Rapid deactivation of the excited sing-doublet state populates the trip-doublet and trip-quartet states as well as the CT state generated by oxidation of the radical by the excited porphyrin.