Recent applications of MCD spectroscopy to porphyrinoids

Over the last 40 years, magnetic circular dichroism (MCD) spectroscopy has provided many key insights into the optical spectroscopy and electronic structure of porphyrinoid compounds [1, 2]. In this chapter, the use of the technique, since the start of the new millennium, to assign the optical spectra of newly synthesized porphyrinoid ligands and to elucidate their electronic structures is described in depth. The recent emergence of DFT theoretical treatments in commercially available software packages has made the calculation of detailed descriptions of the electronic structures of newly synthesized compounds routine. MCD spectroscopy provides key additional information about band polarization and state degeneracies, which can be used to validate time-dependent density functional theory (TD-DFT) calculations and to test the accuracy of density functional theory (DFT) descriptions of the electronic structure. In this chapter, the theoretical background to MCD spectroscopy and recent applications of the technique to radially symmetric and low-symmetry synthetic porphyrinoids and to bioinorganic complexes of transition metals, such as heme proteins, are described in detail. The radially symmetric synthetic porphyrinoids dealt with in this chapter include deeply saddled compounds such as tetraphenyltetraacenaphthoporphyrins and α-octaphenylphthalocyanines, ring-contracted subporphyrin and [14]triphyrin(2.1.1) compounds, and expanded porphyrinoids with both 4N and 4N+2 π-electron systems such as gold hexaphyrins. The low-symmetry compounds covered include core-modified tetrabenzoporphyrins, corrolazines, fused-ring-expanded phthalocyanines, tetraazachlorins, azulene-fused porphyrins, azulenocyanines, and benzoporphycenes.