Chemical Physics Letters, Vol.383, No.5-6, 512-517, 2004
Narrow spectral holes in a concentrated binuclear chromium(III) compound: studies of the (2)E(4)A(2)<-(4)A(2)(4)A(2) transition in single crystals of [LCr(III)(mu-OH)(3)Cr(III)L](ClO4)(3)center dot H2O (L=1,4,7-trimethyl-1,4,7-triazacyclononane)
Properties of non-photochemical spectral holes are reported for the (2)E(4)A(2)<--(4)A(2)(4)A(2) transitions in single crystals of perprotonated 2 and partially deuterated (lattice water and hydroxo bridges) binuclear [LCr(III)(mu-OH)(3)Cr(III)L](ClO4)(3) . H2O (L = 1,4,7-trimethyl-1,4,7-triazacyclononane). The occurrence of relatively narrow holes of Gamma approximate to 80 MHz is facilitated by the anti-ferromagnetic coupling in the (4)A(2)(4)A(2) ground state: the S = 0 lowest-energy level is not cause of and subject to magnetic fluctuations. To the best of our knowledge this is the first report of spectral hole-burning in a fully concentrated Cr3+ compound and also in a binuclear Cr3+ complex. Very high hole-burning efficiencies of 1% and 4% are estimated for the partially deuterated and perprotonated system, respectively, at 2.5 K. At this temperature spontaneous hole-filling occurs on the minute and hour timescale for the perprotonated and partially deuterated samples, respectively, but the rates show a dramatic increase at 7-8 K. Possible hole-burning mechanisms, such as methyl group rotations and translational proton tunnelling, are discussed. (C) 2003 Elsevier B.V. All rights reserved.