Structural Modulation of Fluorescent Rhodamine-Based Dysprosium(III) Single-Molecule Magnets

Abstract

Two rhodamine 6G-based mononuclear dysprosium complexes, [Dysalicylaldehyde rhodamine 6G hydrazone, L A = 2-pyridylcarboxaldehyde benzoyl hydrazone), are synthesized, aiming at improving the magnetic behavior by modulating their coordination environment. Both complexes own one exclusive short Dy− O phenoxy coordination bond as the predominant bond and exhibit singlemolecule magnet behavior under zero dc field with the energy barrier (U eff /k B ) of 90 K (1) and 320 K (2) and apparent hysteresis at 1.9 K. The ab initio calculations indicate that the short Dy−O phenoxy bond determines the direction of magnetic anisotropic axis for 1 and 2. The quantum tunneling of magnetization (QTM) between the ground Kramers doublets (KDs) in 1 cannot be neglected, leading to an experimental U eff /k B much lower than the calculated energy of the first excited state (318.2 K). For 2, the stronger magnetic anisotropy and negligible QTM between the ground KDs guarantees that the energy barrier is close to the calculated energy of first KDs (320.8 K). On the other hand, the presence of ring-opened xanthene moiety makes complexes 1 and 2 in the solid state emit red light with emission bands of 645 and 658 nm, respectively.

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

“…Therefore, we constructed a simplified molecular structure ( 1-X ) with one ring-closed HL ligand (Figure S13) and the ring-opened complex [Dy­(HL)­(H 2 O) 4 (MeCN)] 3+ ( 2-X ) (Figure S14). By optimizing the structures of 1-X and 2-X , the C–N bond energy was determined to be 60 kJ mol –1 , which was much smaller than that of HL. Thus, the C–N bond in complex 1 can be easily broken, even by gentle grinding.…”

Realizing Fluorescence-Valence-Tautomerism Synergy in Mononuclear Cobalt Compounds

“…To the best of our knowledge, this is the first time that coexisting VT and fluorescence phenomena have been realized in single compounds by covalently combining transition metals with fluorophores. Some fluorescent SCO materials have been reported to exhibit fluorescence enhancement or abnormal phenomena in their SCO temperature regions. − ,− ,, Nevertheless, these previous studies focused on changes in fluorescence intensity when the strongest changes in emission occur in a small range (5–30 nm). ,,,,, Compared with other fluorescent SCOs, the fluorescent VT phenomenon observed for 3 ·MeOH·H 2 O shows two primary characteristics: a large hypochromic shift and stronger emissions at high temperatures. The clear change in primary emission is derived from energy transfer between the ligand and the metal, which reveals that the physical properties of valence tautomeric complexes can be flexibly tuned.…”

“…Some fluorescent SCO materials have been reported to exhibit fluorescence enhancement or abnormal phenomena in their SCO temperature regions. [19][20][21][23][24][25][26]53,54 Nevertheless, these previous studies focused on changes in fluorescence intensity when the strongest changes in emission occur in a small range (5−30 nm). 19,20,23,25,26,53 Compared with other fluorescent SCOs, the fluorescent VT phenomenon observed for 3•MeOH•H 2 O shows two primary characteristics: a large hypochromic shift and stronger emissions at high temperatures.…”

“…However, combining fluorescence and VT within a system seems counterintuitive because metal ions, such as Co­(II) and Fe­(II), can easily quench fluorescence. However, spin-state switching of central metals may sufficiently perturb the environment of the fluorophore to facilitate a fluorescence response upon VT. , Strategies for constructing such multifunctional systems roughly fall into two groups: (i) those that covalently connect the fluorescent unit to a chelating ligand coordinated to the central metal and (ii) those that dope a fluorophore into a VT system . A few fluorescent SCO complexes have been prepared using these strategies. − In contrast, the introduction of fluorescence into a VT system has rarely been reported.…”

“…However, spin-state switching of central metals may sufficiently perturb the environment of the fluorophore to facilitate a fluorescence response upon VT. , Strategies for constructing such multifunctional systems roughly fall into two groups: (i) those that covalently connect the fluorescent unit to a chelating ligand coordinated to the central metal and (ii) those that dope a fluorophore into a VT system . A few fluorescent SCO complexes have been prepared using these strategies. − In contrast, the introduction of fluorescence into a VT system has rarely been reported. Sato et al connected naphthalene to Ni­(II) ions through methylene derivative ligands and prepared a fluorescent molecular thermometer based on the low-spin/high-spin interconversion of Ni­(II) .…”