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

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 consid erably shorter Rh-Rh distance (approximately 3.2 angstrom). Several te chniques (absorption, single-crystal polarized absorption, emission, p olarized excitation, resonance Raman, fluorescence, phosphorescence li fetime data, and MM2 calculations) help describe the ground- and lowes t excited-state potential energy surfaces of this complex. The absorpt ion corresponding to dsigma --> psigma singlet-singlet excitation (1A 1g --> 1A2u) is markedly asymmetric, with a sharp maximum (420-440 nm) and a long tailing shoulder (approximately 480 nm). A similarly asymm etric dsigma --> psigma singlet-triplet feature (1A1g --> 3A2u) is ob served approximately 3000 cm-1 below the singlet-singlet band in the p olarized single-crystal spectra of the B(C6H5)4- salt. In contrast, th e corresponding emission bands (545-600 nm, fluorescence; 660-714 nm, phosphorescence) are nearly symmetric. The spectroscopic properties ar e interpreted in terms of ground- and excited-state potential surfaces that combine parameters derived from previous work on more convention al (Rh(I)2 systems with the results of MM2 calculations of dimen defor mations. The calculated ground-state surface is very shallow, with a m inimum 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 lon ger Rh-Rh distances. For Rh-Rh distances greater than approximately 4 angstrom, the calculations predict that the individual Rh(I) square pl anes of Rh2(dimen)4(2+) will be eclipsed; at shorter distances, these units are expected to twist (at a dihedral angle inversely proportiona l to the Rh-Rh separation) to relieve the substantial strain energy in volved in distorting Rh2(dimen)4(2+) along the square planar, a2u bend ing coordinate. These predictions are consistent with the crystal stru ctures of [Rh2(dimen)4][PF6]2 (Rh-Rh = 4.48 angstrom; dihedral twist a ngle = 0-degrees) and [Rh2(dimen)4] [B(C6H5)4]2 (Rh-Rh = 3.861 angstro m; dihedral twist angle = 16.2-degrees) and provide an explanation for the dramatic rigidochromic effect (1000-cm-1 blue shift) on the emiss ion as the temperature is lowered through the glass transition of solu tions.