Spectroscopy and Photophysics of Rh2(dimen)42+ (dimen =1,8-Diisocyanomenthane). Exceptional Metal-Metal Bond Shortening in the Lowest Electronic Excited States

Vincent M. Miskowski, Steven F. Rice, Harry B. Gray, Richard F. Dallinger, Steven J. Milder, Michael G. Hill, Christopher L. Exstrom, Kent R. Mann

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Abstract

Rh2(dimen)42+ (dimen = 1,8-diisocyanomenthane) exhibits very long Rh-Rh bond distances in the solid state (varying from 4.48 Å for the PF6- salt to 3.861 Å for the B(C6H5)4- salt), but the lowest dσ* → pσ excitation produces an excited state with a considerably shorter Rh-Rh distance (~3.2 Å). Several techniques (absorption, singlecrystal 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 dσ* → pσ singlet-singlet excitation (1Ag1g3A2u) is markedly asymmetric, with a sharp maximum (420-440 nm) and a long tailing shoulder (~480 nm). A similarly asymmetric dσ* → pσ singlet-triplet feature (1Ag1g3A2u) is observed ~3000 cm-1 below the singlet-singlet band in the polarized singlecrystal 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 (Rh1)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 A, but nearly harmonic in the range of interest. The calculated excited-state surface is extremely anharmonic: after a well-defined minimum at 3.35 Å, it is broad and shallow at longer Rh-Rh distances. For Rh-Rh distances greater than ~4 Å, the calculations predict that the individual Rh(I) square planes of Rh2(dimen)42+ 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)42+ 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 Å; dihedral twist angle = 0°) and [Rh2(dimen)4] [Bi(C6H5)4]2 (Rh-Rh = 3.861 Å; dihedral twist angle = 16.2°) 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.

Original languageEnglish (US)
Pages (from-to)2799-2807
Number of pages9
JournalInorganic Chemistry
Volume33
Issue number13
DOIs
StatePublished - Jun 1 1994

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ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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