Direct spectroscopic detection of a zwitterionic excited state.

Two electrons in two weakly coupled orbitals give rise to two states (diradical) with electrons residing in separate orbitals and two states (zwitterionic) with both electrons paired in one orbital or the other. This two-electron, two-orbital state manifold has eluded experimental confirmation because the zwitterionic states have been difficult to locate. Two-photon excitation of fluorescence from Mo(2)CI(4)(PMe(3))(4) (D2d) has been measured with linearly and circularly polarized light. From the polarization ratio and the energy of the observed transition, the 2(1)A(1) (delta*delta*) excited state has been located and characterized. In conjunction with the one-photon allowed (1)B(2) (deltadelta*) excited state, the zwitterionic state manifold for the quadruply bonded metal-metal class of compounds is thus established.