Light‐Induced Bistability in the 2 D Coordination Network {[Fe(bbtr)₃][BF₄]₂}_∞: Wavelength‐Selective Addressing of Molecular Spin States

Abstract

Whereas the neat polymeric Fe(II) compound {[Fe(bbtr)3 ][ClO4 ]2 }∞ (bbtr=1,4-di(1,2,3-triazol-1-yl)butane) shows an abrupt spin transition centered at 107 K facilitated by a crystallographic symmetry breaking, in the covalently linked 2D coordination network of {[Fe(bbtr)3 ][BF4 ]2 }∞ , Fe(II) stays in the high-spin state down to 10 K. However, strong cooperative effects of elastic origin result in reversible, persistent, and wavelength-selective photoswitching between the low-spin and high-spin manifolds. This compound thus shows true light-induced bistability below 100 K. The persistent bidirectional optical switching behavior is discussed as a function of temperature, irradiation time, and intensity. Crystallographic studies reveal a photoinduced symmetry breaking and serve to establish the correlation between structure and cooperative effects. The static and kinetic behavior is explicated within the framework of the mean-field approximation.

Light-induced spin-state switching in the mixed crystal series of the 2D coordination network {[Zn1−xFex(bbtr)3](BF4)2}∞: optical spectroscopy and cooperative effects

“…As previously discussed, γ crit LS also depends on temperature. 40 In summary, the mixed crystal system, {[Zn 1−x Fe x (bbtr) 3 ]-(BF 4 ) 2 } ∞ , shows a light-induced persistent bistability for x > 0.5 below a given temperature, which depends on x. As shown in ref.…”

“…This indicates that in the highly concentrated and neat compound, the strong cooperative effects of elastic origin result in a light-induced bistability. 37,[40][41][42][43][44][45][46] Fourthly, for these higher initial LS fractions the return to the HS state is very abrupt, occurs at 88 and 100 K for x = 0.87 and 1, respectively, does not depend on the initial LS fraction and coincides with the jump in the baseline of the absorption spectra due to the above mentioned crystallographic phase transition. This temperature is therefore not a dynamic relaxation temperature as expressed by T r-LIESST anymore, but rather the thermodynamic temperature of the phase transition in the heating direction T ↑ c as indicated by the jump in the corresponding curves shown in Fig.…”

“…The light-induced LS state is but a kinetically trapped, metastable state and relaxes back to the HS state at temperatures above 50 K. At higher iron(II) concentrations this is no longer necessarily the case. On the light-induced HS → LS conversion, at a certain light-induced LS fraction a crystallographic phase transition from P3 ˉto P1 ˉwith a doubling of the unit cell along the c-axis is triggered, 37,40 in analogy to the symmetry lowering, but without the doubling, also observed during the thermal spin transition to the LS state in the cooling branch of the perchlorate analogues. 18,36 Above this critical light-induced LS fraction, γ crit LS , the LS state becomes the quantum mechanical ground state due to the cooperative effects, which tend to stabilise the majority species.…”

“…As shown in ref. 40, for the neat compound this is equivalent to saying that {[Fe(bbtr) 3 ](BF 4 ) 2 } ∞ is a spin-crossover compound with a very wide hysteresis such that T ↑ c = 100 K and T ↓ c is below 30 K. The latter could even be formally negative. Dilution with Zn reduces the cooperative effects; thus the width of the hysteresis is reduced, and as is common for Zn as the host, the transition temperature is lowered.…”

Slow Dynamics of the Spin‐Crossover Process in an Apparent High‐Spin Mononuclear FeII Complex

“…These studies have also been completed for the [Fe(etz) 6 ] 2+ complex (etz: ethyltetrazole), which shows a remarkable light‐induced bistability of the HS sites. More recently, comparable bistability has been reported in a two‐dimensional (2D) coordination polymer, {[Fe HS (bbtr) 3 ](BF 4 ) 2 } ∞ (bbtr: 1,4‐di(1,2,3‐triazol‐1‐yl)butane), as well as in 2D HS:LS Hofmann‐like frameworks and an Fe III SCO system …”