Thermal and Light‐Activated Spin Crossover in Iron(III) qnal Complexes

Abstract

Three iron(III) complexes, [Fe(qnal) 2 ]Y {qnal = 1-[(8-quinolinylimino)methyl]-2-naphthalenolate; Y = NO 3 1, BPh 4 ·CH 2 Cl 2 2, NCS 3} have been prepared to explore anion effects in spin crossover systems. Structural studies on 2 reveal supramolecular chains formed by π-π and C-H···π interactions between the [Fe(qnal) 2 ] + cations. Magnetic susceptibility measurements show the onset of spin crossover in 1 above 200 K, [a]

Room temperature conductance switching in a molecular iron(iii) spin crossover junction

“…Both L 2 (720 eV) and L 3 (707 eV) edges shift to higher binding energy with decreasing temperature from 340 K to 120 K, which confirms the HS to LS transition of the molecule occurs at lower temperature in the powder form (see Fig. S3B† for T dependent magnetic hysteresis loop) as compared to that of the molecule on the graphene surface in agreement with a previously reported transition of the SCO molecule in powder form where the transition temperature T 1/2 for cooling is 248 K and T 1/2 for heating is 278 K. 26,39…”

“…Both L 2 (720 eV) and L 3 (707 eV) edges shift to higher binding energy with decreasing temperature from 340 K to 120 K, which confirms the HS to LS transition of the molecule occurs at lower temperature in the powder form (see Fig. S3B for T dependent magnetic hysteresis loop) as compared to that of the molecule on the graphene surface in agreement with a previous report Please do not adjust margins Please do not adjust margins the transition of the SCO molecule in powder form where the transition temperature T ½ for cooling is 248 K and T ½ for heating is 278 K. 26,39 To further confirm the spin transition of the monolayers on graphene, we recorded the X-ray magnetic circular dichroism (XMCD) spectra in total electron yield mode with a magnetic field applied to the sample, which is parallel to the circularly polarized photoelectron propagation vector. All the spectra are reported with a magnetic field of ±1.5 T at different temperatures.…”

Halogen-Substituted Spin-Crossover Fe(III) Compounds with Photoresponsive Properties

“…The LIESST effect has been observed in many SCO compounds; however, the reverse-LIESST effect that the HS species is photoexcited to the metastable LS state has been relatively less investigated. , We tried to study the reverse-LIESST effect on the three HS compounds 1 , 4 ·MeOH, and 4 , and found that compound 4 ·MeOH did show a reverse-LIESST effect upon exposure to a 1064 nm laser (13 mW·cm –2 , Figure S20a, Supporting Information). Taking into account the photothermal effect of near-infrared light on the reverse-LIESST effect, the χ M T values were recorded under light irradiation for approximately 2160 min (Figure S20b, Supporting Information), which showed a slow decrease from 4.52 to 3.40 cm 3 K mol –1 and remained invariant after the light was switched off, demonstrating that the photothermal effect could be ruled out.…”

“…These data show the occurrence of SCO in the Fe1 center is accompanied by a larger structural distortion of the coordination sphere. The capability of compounds 2 and 3 to present reversible LIESST effects may be attributed to variable coordination-sphere environments of Fe­(III) ions upon spin-state changes because large changes in the distortion parameters and the Fe-ligand bond distances during HS ⇌ LS transition have been suggested to favor the occurrence of the LIESST effect. ,,,, …”

“…On the other hand, the relatively smaller changes in the average Fe–N/O bond distances between the high-spin (HS) and low-spin (LS) states in SCO Fe­(III) compounds, Δ r HL (= | r HS – r LS |), are thought to prohibit the light-induced excited spin-state trapping (LIESST) effect because a small Δ r HL value (∼0.13 Å for Fe III versus ∼0.20 Å for Fe II ) , promotes fast spin relaxation from the metastable HS state to the LS state of the Fe­(III) ion. − So, the LIESST effects in SCO Fe­(III) compounds previously did not attract much attention. But since the discovery of the first LIESST effect in the SCO Fe­(III) compound [Fe­(pap) 2 ]­ClO 4 ·H 2 O, it is suggested that the structural distortion parameters of the Fe­(III) coordination sphere, Σ (the sum of the deviation from 90° of the 12 cis -(N/O–Fe–N/O) angles about the iron atom) and Δ∠ HL,total (the sum of the bond angle differences between the different spin states; this parameter is calculated by using 15 cis and trans (N/O–Fe–N/O) bond angles), instead of the Δ r HL value determine the activation energy for metastable HS-to-LS relaxation, so this discovery has stimulated researchers’ enthusiasm in the study of LIESST effects in SCO Fe­(III) compounds. − Strong intermolecular interactions favoring cooperative SCO might contribute to large structural distortion responsible for achieving the LIESST effect but are not necessary. , Reversible LIESST effects in Fe­(II) SCO compounds have been examined systematically since 1986, which have opened up another way for accurate and reversible regulation of the spin states and therefore physical properties of SCO compounds. ,− Despite these achievements, the design of a bidirectional photocontrolled magnetic switching in the Fe­(III) SCO system remains a daunting challenge. ,,− …”