Mechanochromic and Single-Molecule Magnetic Properties of a Rhodamine 6G Dy(III) Complex

Mechanochromic
luminescent materials have attracted considerable
attention because of their interesting color switching behavior under
mechanical grinding or hydrostatic pressure. Here, the intriguing
Dy­(III) complex [Dy­(L-c)2(MeOH)2]­ClO4·Et2O·H2O·0.5MeOH (1) (HL-c = ring-closed form of rhodamine 6G salicylaldehyde hydrazone)
with mechanochromic luminescent behavior was synthesized using the
non-mechanochromic HL-c ligand. To the best of our knowledge, it is
the first example of a mechanochromic luminescent material where mechanochromism
is realized by formation of a lanthanide complex. Complex 1 shows a color change from yellow to red under gentle grinding, and
the color can be reversed under a methanol atmosphere. Moreover, the
color change can alternatively be triggered by hydrostatic force.
These behaviors indicate that the spirolactam C–N bond in the
rhodamine 6G moiety breaks under external stimuli to afford the fluorescent
ring-opened xanthene. Cleavage of the C–N bond in 1 is verified by density functional theory calculations. Complex 1 also shows slow relaxation of magnetization below 6 K, which
is typical of a field-induced single-molecule magnet.