Narrow spectral holes in a concentrated binuclear chromium(III) compound: studies of the 2E4A2←4A24A2 transition in single crystals of [LCr(III)(μ-OH)3Cr(III)L](ClO4)3 · H2O (L=1,4,7-trimethyl-1,4,7-triazacyclononane)

Properties of non-photochemical spectral holes are reported for the 2E4A2← 4A24A2 transitions in single crystals of perprotonated and partially deuterated (lattice water and hydroxo bridges) binuclear [LCr(III)(μ-OH)3Cr(III)L](ClO4)3 · H2O (L=1,4,7-trimethyl-1,4,7-triazacyclononane). The occurrence of relatively narrow holes of Γ ≈ 80 MHz is facilitated by the anti-ferromagnetic coupling in the 4A24A2 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.