Structural, Spectroscopic, and Reactivity Comparison of Xanthene- and Dibenzofuran-Bridged Cofacial Bisporphyrins

A comparison of the structure, spectroscopy, and oxygen atom-transfer reactivity of cofacial bisporphyrins anchored
by xanthene (DPX) and dibenzofuran (DPD) pillars is presented. The synthesis and characterization of dicopper(II)
and dinickel(II) complexes of DPD completes a homologous series of homobimetallic zinc(II), copper(II), and nickel(II)
complexes for both cofacial platforms. X-ray crystallographic analysis of the parent free-base porphyrins H4DPX (1)
and H4DPD (5) confirms the face-to-face arrangement of the two porphyrin macrocycles with a large available
range of vertical pocket sizes:  1 (C80H92Cl2N8O), triclinic, space group P1̄, a = 13.5167(12) Å, b = 21.7008(18)
Å, c = 23.808(2) Å, α = 80.116(2)°, β = 76.832(2)°, γ = 80.4070(10)°, Z = 4; 5 (C80H83N8O2), monoclinic,
space group C2/c, a = 22.666(2) Å, b = 13.6749(14) Å, c = 42.084(4) Å, β = 94.554(2)°, Z = 8. EPR spectroscopy
of dicopper(II) derivatives Cu2DPX (3) and Cu2DPD (7) complements the crystallographic studies by probing
intramolecular metal−metal arrangements in frozen solution. Exciton interactions between the porphyrin subunits in
fluid solution are revealed by steady-state and time-resolved electronic absorption and emission spectroscopy. The
resulting compilation of structural and spectroscopic data provides a benchmark for the use of these and related
platforms for the activation of small-molecule substrates. A structure−function relation is developed for the
photoinduced oxygen atom-transfer reactions of bisiron(III) μ-oxo derivatives of DPX and DPD. The efficiency of
the photochemical process is markedly dependent (∼104-fold) on the vertical flexibility of cofacial architecture
provided by the spacer.