Observation of exceptionally low-lying π-π* excited states in oxidized forms of quadruple-decker phthalocyanine complexes.

Spectroscopic investigations have been performed for the oxidized forms of two quadruple-decker phthalocyanine complexes in order to clarify the electronic structures of multiply stacked π-systems. Up to three-electron-oxidized species were isolated by using phenoxathiin hexachloroantimonate as the oxidant. As the oxidations proceed, the Q-bands in the visible region shift bathochromically along with the clear isosbestic points. The one- and three-electron-oxidized species exhibited typical π-radical signals in the ESR spectra, while the neutral and two-electron oxidized species gave no indication of the presence of π-radicals. The electronic transitions observed for the oxidized species reach even into the so-called fingerprint region in IR spectroscopy (∼1000 cm–1). With the aid of theoretical calculations, these bands can be assigned to the π–π* transitions. Our results provide new insights into π-electronic systems having exceptionally small MO energy gaps.