Detecting Fe(II) Spin-Crossover by Modulation of Appended Eu(III) Luminescence in a Single Molecule

Abstract

Multifunctionality in spin-crossover (SCO) devices is limited to macroscopic or nanoscopic materials because of the need for long-range effects for inducing favorable cooperativity, efficient energy migration processes, and detectable magnetization transfer. The difficult reproducibility, control, and rational design of doped materials offer some place to SCO processes, modulating the optical properties of neighboring luminescent probes in single molecules. We report here on the combination of a [FeN6] chromophore, the SCO temperature and absorption spectra of which have been tuned to induce unprecedented room-temperature modulation of Eu(III)-based line-like luminescence in the molecular triple-helical [EuFe(L2)3]5+ complex in solution.

Unravelling Kinetics of Intramolecular NdIII → FeII Energy Transfer in Spin Crossover Single Molecules: Dotting the i’s and Crossing the t’s

“… The estimated radiative rate constants k Nd* rad ( 44 F 3/2 ↔ 4 I 11/2 ) = 1817(88) s –1 at λ ∼ 1056 nm and k Nd* rad ( 4 F 3/2 ↔ 4 I 13/2 ) = 375(18) s –1 at λ ∼ 1332 nm can be combined with k Nd* rad ( 4 F 3/2 ↔ 4 I 9/2 ) = 1145(52) s –1 to give the total radiative rate constant k Nd* rad ( 4 F 3/2 ) = 3337(103) s –1 (Tables and A2-1). It finally translates into τ Nd* rad ( 4 F 3/2 ) = 0.30(1) ms for [NdZn( L1 ) 3 ] 5+ in acetonitrile at 293 K, in good accord with previously reported values for Nd III emitters bound to N-donor ligands. , Compared to τ Eu* rad ( 5 D 0 ) = 3.50(1) ms reported for [EuZn( L1 ) 3 ] 5+ [i.e., k Eu* rad ( 5 D 0 ) = 286(1) s –1 ], the increase of k Ln* rad by 1 order of magnitude in [NdZn( L1 ) 3 ] 5+ confirms the partial respect of the spin rule for 4f–4f electronic transitions operating in light lanthanides. This compensates for the opposite cubic dependence on the energy gap k Ln* rad ∝ ( ν̅ ) 3 , which is indeed reduced from ν̅ = 17,240 cm –1 for Eu III (red emission) to 11,520 cm –1 for Nd III (NIR emission).…”

“…Reasonably considering a negligible hyperbolic cotangent dependence of the radiative rate constants on the average phonon bath found in lanthanide coordination complexes ( h ν ≈ 2000 cm –1 ), , the decay rate constants of the excited Nd( 4 F 3/2 ) spectroscopic level can be partitioned between a uniform radiative ( k Nd* rad ) and a thermally activated nonradiative contribution ( k Nd* nonrad ) modeled with an Arrhenius-type phonon-assisted deactivation mechanism summarized in eq ( k Nd* nonrad,0 stands for the nonradiative relaxation at T → ∞, and E nonrad is the activation energy of the phonon-assisted relaxation pathway, and the low-temperature tunneling, being much smaller than the radiative decay, is neglected). ,, k normalN normald * normalr normale normall normala normalx = k normalN normald * normalr normala normald + k normalN normald * n o n r a d , 0 · exp ( − E normaln normalo normaln normalr normala normald R T ) …”

“…Besides the radiative ( k Nd* rad ) and nonradiative ( k Nd* nonrad ) relaxation mechanisms affecting the Nd( 4 F 3/2 ) emissive level, which can be reasonably considered similar for [NdFe( L1 ) 3 ] 5+ and [NdZn( L1 ) 3 ] 5+ (eq ), the two intramolecular ETs Nd III → LS-Fe II ( k LS q , eq ) and Nd III → HS-Fe II ( k HS q , eq ) further contribute to quench the lanthanide-based luminescence (Figure ). , k normalL normalS q = k normalL normalS normalq , 0 · exp ( − E normalL normalS q R T ) k normalH normalS q = k normalH normalS normalq , 0 · exp ( − E normalH normalS q R T ) …”

“…(a) Kinetic scheme with dashed upward arrows = excitation, full downward arrows = relaxation, and straight diagonal arrows = LS ↔ HS transformations and (b) associated rate constants for the luminescent monitoring of the Fe II spin state using Ln III = Eu III as luminophore in the dinuclear [EuFe( L1 ) 3 ] 5+ complex ( k normalE normalu * normalr normale normall normala normalx = k normalE normalu * normalr normala normald + k normalE normalu * normaln normalo normaln normalr normala normald ) …”

Optical Phenomena in Molecule-Based Magnetic Materials

“…Very recently, an interesting example of a luminescent SCO Fe­(II) molecule-based system was reported, which utilizes the variation of emission from the attached Eu 3+ ions . The obtained complexes, [Eu III Fe II (L2) 3 ] 5+ (L2 = segmental bidentate-tridentate ligand), upon irradiation with the UV light, reveals typical Eu­(III) emission of the f–f electronic transitions’ origin in the MeCN solution.…”