Photoinduced spin crossover in a Fe-picolylamine complex: A far-infrared study on single crystals

Far-infrared spectroscopy has been performed on $\left[\mathrm{Fe}{\left(2\text{−}\text{picolylamine}\right)}_3\right]{\mathrm{Cl}}_2\mathrm{EtOH}$ (Fe-pic) single crystals to probe changes in the molecular vibrations upon the photoinduced and temperature-induced spin crossovers. Synchrotron radiation has been used as the far-infrared source to overcome the strong absorption and the small sizes of the samples. Absorption lines due to ${\mathrm{FeN}}_6$ cluster vibrations, observed below $400{\mathrm{cm}}^{-1}$, show strong intensity variations upon the crossover due to the compression/expansion of ${\mathrm{FeN}}_6$ between high-spin and low-spin states. However, they remain almost unchanged between the photo- and temperature-induced high-spin states. This is in sharp contrast to the lines at $500\text{–}700{\mathrm{cm}}^{-1}$ due to intramolecular vibrations of the picolylamine ligands, which show marked variations between the two high-spin states. It is concluded that the most important microscopic difference between the two high-spin states arises from the ligands, which are likely to reflect different states of intermolecular bonding.

Figure 1

(a) The structure of ${\left[\mathrm{Fe}{\left(2\text{−}\mathrm{pic}\right)}_3\right]}^{2+}$. (b) Electron configurations of ${\mathrm{Fe}}^{2+}$ in high-spin $(S=2)$ and low-spin $(S=0)$ states. The average $\mathrm{Fe}\text{−}\mathrm{N}$ bond length is larger in the $S=2$ state corresponding to the smaller crystal-field splitting.

Figure 2

Optical density of Fe-pic at several temperatures without photoexcitation. The labels a–g indicate the absorption bands and lines analyzed in Fig. 3.

Figure 3

Optical density (OD) of selected absorption lines and bands for Fe-pic, which are indicated in Fig. 2 by the labels a–f, as a function of temperature. The OD has been integrated over wave-number regions of (a) $187\text{–}224{\mathrm{cm}}^{-1}$, (b) $224\text{−}241{\mathrm{cm}}^{-1}$, (c) $322\text{−}340{\mathrm{cm}}^{-1}$, (d) $365\text{–}392{\mathrm{cm}}^{-1}$, (e) $462\text{–}471{\mathrm{cm}}^{-1}$, (f) $473\text{–}488{\mathrm{cm}}^{-1}$, and (g) $650\text{–}680{\mathrm{cm}}^{-1}$, then normalized by the difference of OD between 70 and 140 K.

Figure 4

Optical density spectra of Fe-pic in (a) LTLS at 9 K, (b) PIHS at 9 K, and (c) HTHS at 140 K. The thin curve in (c) is the same as the spectrum in (b), shown for comparison. The vertical bars in (a) and (b) show the calculated frequencies of molecular vibrations for ${\left[\mathrm{Fe}{\left(2\text{−}\mathrm{pic}\right)}_3\right]}^{2+}$. See text for the labels.