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Abstract
Nearly a half-century after Gouterman classified the UV–vis–NIR spectra of porphyrin derivatives as normal, hyper, or hypso, we propose a heretofore unsuspected “mechanism” underlying hypso spectra. Hypsoporphyrins, which exhibit blueshifted optical spectra relative to normal porphyrins (such as Zn porphyrins), typically involve dn transition metal ions, where n > 6. The spectral blueshifts have been traditionally ascribed to elevated porphyrin eg LUMO (lowest unoccupied molecular orbital) energy levels as a result of antibonding interactions with metal dπ orbitals. Herein, we have found instead that the blueshifts reflect a lowering of the a2u HOMO (highest occupied molecular orbital) energy levels. Electronegative metals such as Pd and Pt transfer smaller quantities of electron density to the porphyrin nitrogens, compared to a more electropositive metal such as Zn. With large amplitudes at the porphyrin nitrogens, the a2u HOMOs of Pd(II) and Pt(II) porphyrins accordingly exhibit lower orbital energies than those of Zn(II) porphyrins, thus explaining the hypso effect. Hypso spectra are also observed for corroles: compared with six-coordinate Al(III) corroles, which may be thought of exhibiting normal spectra, Au(III) corroles, for instance, exhibit blueshifted or hypso spectra.
Highlights
The famous four-orbital model,[1,2] which explained the electronic absorption spectra of simple porphyrins, was devised (LUMOs) are energetically well-separated from all other occupied and virtual molecular orbitals (MOs)
We began by examining to what extent time-dependent density functional theory (TDDFT) calculations reproduce known trends in relative positions of the absorption maxima of the compounds studied
That systemic oxidation results in a lowering of the a2u highest occupied molecular orbitals (HOMOs), and an even greater lowering of the egLUMOs, which explains the lack of a hypsoporphyrin spectrum for PtIV[TPP]Cl2
Summary
The famous four-orbital model,[1,2] which explained the electronic absorption spectra of simple porphyrins, was devised (LUMOs) (eg) are energetically well-separated from all other occupied and virtual molecular orbitals (MOs). Normal porphyrins exhibit electronic absorption spectra that can be largely accounted for with the four-orbital model. Hypsoporphyrins exhibit blueshifted spectra, typical examples including dn metalloporphyrins for n > 6. Sensing and photodynamic therapy.[8−12] Gouterman and coworkers famously exploited platinum(II) porphyrins to devise pressure-sensitive paints for airplane wings.[13−16] More recently, 5d metallocorroles,[17−21] including ReO,[22−24] OsN,[25,26] Ir,− Pt,[30,31] and Au20,32−35 corroles, have been found to exhibit NIR phosphorescence under ambient conditions, raising the question whether they, or at least some of them, should be described as hypsocorroles. The present reinvestigation provided no support whatsoever for this long-held picture, suggesting instead an entirely different “mechanism” underlying hypso spectra
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