A First TDDFT Study of Metallocorrole Electronic Spectra: Copper meso-Triarylcorroles Exhibit Hyper Spectra

The Soret maxima of a number of metallotriarylcorrole derivatives, notably the copper corroles, are exquisitely sensitive to substitutions on the meso phenyl groups. In contrast, the Soret maxima of silver triarylcorroles are essentially invariant with respect to phenyl-group substituents. TDDFT calculations indicate that the substituent sensitivity in the copper case results from significant phenyl-to-metal charge-transfer character in the main peaks of the Soret region. In other words, copper triarylcorroles exhibit so-called hyper spectra. By contrast, the Soret region in the silver case is largely pi-pi* in nature. DFT calculations suggest that the difference in nature of the Soret bands reflects the energy difference between the LUMOs of the copper and silver complexes, which, in both cases, may be described as an antibonding metal(d(x2-y2))-corrole(pi)-based MO. Strong support for this picture comes from electrochemistry: the reduction potentials of silver triarylcorroles are about 0.7-0.8 V more negative than for the analogous copper complexes. In the same vein, the electrochemical HOMO-LUMO gaps are ca. 1 eV for the silver corroles and only 0.8 eV for their copper analogues.