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Abstract
Single crystals of the title compound were studied by optical absorption, luminescence, Zeeman and MCD spectroscopy in the region of 4 A 2 4 A 2 → 2 E 4 A 2, 2 T 1 4 A 2 transitions. The ground state exchange parameters J, j could be accurately determined from the luminescence spectra. From a complete analysis of the observed electronic transitions to the singly excited pair states 2 E 4 A 2 and 2 T 1 4 A 2, the trigonal orbital exchange parameters Ja and Je could be deduced. Ja was found to represent the dominant exchange pathway. This was rationalized by an Extended Hückel calculation. The exchange interaction leads to a strong mixing of the 2 E 4 A 2 and 2 T 1a± 4 A 2 pair states. This is the main reason for the unusually large g ‖c values found in the lowest-energy excited states. The O-H stretching frequencies were found to play the major role as accepting modes in a multiphonon relaxation process.
