MODULATION OF VALENCE ORBITAL LEVELS OF METALLOPORPHYRINS BY BETA-SUBSTITUTION - EVIDENCE FROM SPECTROSCOPIC AND ELECTROCHEMICAL STUDIES OF 2-SUBSTITUTED METALLO-5,10,15,20-TETRAPHENYLPORPHYRINS

A new approach for obtaining information about the electronic structure and, in particular, the relative ordering of the frontier and subfrontier orbitals in metalloporphyrin systems is presented. This treatment involves a combination of electrochemical and spectral data. A series of 2-substituted copper(II) 5,10,15,20-tetraphenylporphyrins (2-11), in which the electronic nature of the substituent has been significantly varied, have been studied. The substituent has a considerable effect on the energies of the two highest occupied molecular orbitals, the a2u and the a1u orbitals, and can even cause the relative order of these orbitals to change. Substantial modulation of the a2u/a1u orbital energies is achieved by variation of the nature of a pyrrolic beta-substituent. The effect is, as expected, felt much more strongly on the a1u orbital, which has significant electron density associated with the pyrrolic beta-position of metalloporphyrins. Indeed, the relative energy of the a1u orbital in the nitroporphyrin 2 and the aminoporphyrin 11 differs by 0.71 eV; the corresponding difference in the energies of the a2u orbitals is 0.14 eV. In cases where there is a good electron-donating 2-substituent (NH2, OCH3, SPh), the "normal" ordering a2u > a1u is reversed. These observations have important consequences in metalloporphyrin systems as their patterns of reactivity, influenced by the electron distribution in the highest filled molecular orbital, will be particularly sensitive to the relative a1u/a2u separation and ordering. A role for pyrrolic beta-substituents in the fine tuning of energy levels in porphyrin-based molecular electronic logic and memory devices is suggested.