Optical spectroscopic studies of square planar d8 dimers

Abstract

The single crystal polarized absorption spectra of rhodium(I) diisocyanide dimers presented confirm the assignment of Mann and Gray for the intense visible absorption band as 1A1g → 1A2u. The structured 1A1g → 3A2u absorption band in RH2(b)4(Bph4)2 shows the 3A2u excited state to have a metal-metal vibrational frequency of 149 cm-1. This is compared to the 85 cm-1 ground state frequency which results in the orbital assignment of this transition as d z2σ* → pzσ. The excited state has an internuclear bond distance .31 A shorter than that of the ground state. The bonding description obtained through the analysis of the spectra depicts a ground state that is formally nonbonding stabilized through configurational interaction with a low lying, strongly bonded excited state. The 3A2u state in these dimers has a pronounced phosphorescence with a 77K lifetime of 20 μsec. The quantum yields and lifetimes are very temperature sensitive with several 300K lifetimes of about 10 nsec. This temperature dependence, characteristic of a strong coupling limit, is interpreted in terms of a nonradiative pathway depopulating the 3A2u through another excited state associated with a dz2 → dx2-y2 transition. A compound isoelectronic with these rhodium diisocyanides, Pt2(H2P2O5)4-4, is shown to have a similar metal-metal interaction in both the ground and lowest excited states. Recent results regarding the low temperature photophysics of this compound have been verified and extended through an analysis of the vibronically structured 3A2u → 1A1g luminescence. The 3A2u state is demonstrated to have an internuclear distance of 2.71 A, .21 A shorter than that of the ground state. The metal-metal vibrational frequency in the 3A2u is 155 cm-1, distinctly higher than the 110 cm-1 of the ground state.