Excitation energy transfer between inequivalent chromium(III) dimers in [(NH3)5CrOHCr(NH3)5]Cl5·H2O

Single crystals of the title compound were studied by optical absorption and luminescence spectroscopy in the region of 4A2 → 2E excitations. Zeeman, MCD and MCPL spectra were also recorded. Site-selective and time-resolved luminescence and excitation spectra provided information about the transfer of excitation energy. Exchange splitting patterns in the ground and excited state were obtained separately for the two inequivalent sets of dimers A and B in the crystal, and the corresponding exchange parameters were determined. The molecular orientations of the dimer units were derived from the spectroscopic data. The N-H stretching frequencies were found to play the major role as accepting modes in a multiphonon relaxation process. Numerical values were obtained for the rates of energy transfer between the sets A and B in the temperature range 1·5–100 K. They could be quantitatively accounted for by a one-phonon assisted electric dipole-dipole mechanism involving the 5A1 ↔ 5B1 dimer transitions.