SPECTROSCOPY AND PHOTOPHYSICS OF RH2(DIMEN)4(2+) (DIMEN = 1,8-DIISOCYANOMENTHANE) - EXCEPTIONAL METAL-METAL BOND SHORTENING IN THE LOWEST ELECTRONIC EXCITED-STATES

Rh2(dimen)4(2+) (dimen = 1,8-diisocyanomenthane) exhibits very long Rh-Rh bond distances in the solid state (varying from 4.48 angstrom for the PF6- salt to 3.861 angstrom for the B(C6H5)4-salt), but the lowest dsigma* --> psigma excitation produces an excited state with a considerably shorter Rh-Rh distance (approximately 3.2 angstrom). Several techniques (absorption, single-crystal polarized absorption, emission, polarized excitation, resonance Raman, fluorescence, phosphorescence lifetime data, and MM2 calculations) help describe the ground- and lowest excited-state potential energy surfaces of this complex. The absorption corresponding to dsigma* --> psigma singlet-singlet excitation (1A1g --> 1A2u) is markedly asymmetric, with a sharp maximum (420-440 nm) and a long tailing shoulder (approximately 480 nm). A similarly asymmetric dsigma* --> psigma singlet-triplet feature (1A1g --> 3A2u) is observed approximately 3000 cm-1 below the singlet-singlet band in the polarized single-crystal spectra of the B(C6H5)4- salt. In contrast, the corresponding emission bands (545-600 nm, fluorescence; 660-714 nm, phosphorescence) are nearly symmetric. The spectroscopic properties are interpreted in terms of ground- and excited-state potential surfaces that combine parameters derived from previous work on more conventional (Rh(I)2 systems with the results of MM2 calculations of dimen deformations. The calculated ground-state surface is very shallow, with a minimum at 4.75 angstrom, but nearly harmonic in the range of interest. The calculated excited-state surface is extremely anharmonic: after a well-defined minimum at 3.35 angstrom, it is broad and shallow at longer Rh-Rh distances. For Rh-Rh distances greater than approximately 4 angstrom, the calculations predict that the individual Rh(I) square planes of Rh2(dimen)4(2+) will be eclipsed; at shorter distances, these units are expected to twist (at a dihedral angle inversely proportional to the Rh-Rh separation) to relieve the substantial strain energy involved in distorting Rh2(dimen)4(2+) along the square planar, a2u bending coordinate. These predictions are consistent with the crystal structures of [Rh2(dimen)4][PF6]2 (Rh-Rh = 4.48 angstrom; dihedral twist angle = 0-degrees) and [Rh2(dimen)4] [B(C6H5)4]2 (Rh-Rh = 3.861 angstrom; dihedral twist angle = 16.2-degrees) and provide an explanation for the dramatic rigidochromic effect (1000-cm-1 blue shift) on the emission as the temperature is lowered through the glass transition of solutions.