Journal of Nuclear and Radiochemical Sciences, Vol. 8, No.1, pp. 1-3, 2007

Articles57

Fe Mössbauer, UV-visible, and FT-IR Study on Photoinduced Spin Transition ofFeII-Triazole Complex
Tadashi Mitsuoka,a Masaru Nakagawa,b Tomokazu Iyoda,b and Yasuaki Einaga*, aa

Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Yokohama
223-8522, Japanb
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226-8503, Japan

Received: January 23, 2007; In Final Form: February 25, 2007
We have synthesized a novel spin crossover FeII-triazole complex Fe(AZ-trz), where AZ-trz denotes 4-phenylazoN-[1,2,4]triazole-4-yl-benzamide. 57Fe Mössbauer spectra and magnetic susceptibility measurements of thiscompound exhibited a gradual spin crossover behavior by thermal change and also a light-induced excited spinstate trapping (LIESST) effect. Both UV-visible absorption spectra and IR spectra confirmed the phenomena.
The FeII high-spin, HS, state was induced by exciting a d–d electronic transition band of the FeII low-spin statespecies. Furthermore, the measurements revealed the lifetime of excited FeII HS state to be 1 min.

1. Introduction
The developments of new magnetic materials whose magnetic properties can be controlled by photoillumination havebeen attracting great interest.1 Metal complexes with the electron configuration d4 to d7 can take two different spin state, i.e.,the high-spin (HS) and low-spin (LS) states and they have been
investigated because some of them can exhibit the spin crossover phenomena.2 A long-lived photoinduced spin transition
phenomenon is called a light-induced excited spin state trapping (LIESST) effect and attracting extensively attention fromthe viewpoint of the development of optically switchable magnetic materials because of their possible applications for opticalmemories or other switch devices.
Triazole complexes is one of the most well-known spincrossover complexes and general formula is given as below,[FeII(R-trz)3]A2·nH2O, where R-trz is 4-R-substituted-1,2,4-triazole and A is anion. The structure consists of linear chains inwhich the neighboring Fe2+ ions are triply bridged by triazole
R-trz ligands through the N atoms occupying the 1- and 2positions and the FeN6 core is close to a regular octahedron 3(Figure 1). So far, a lot of triazole complexes which exhibit
spin crossover phenomena have been designed and synthesizedby modifying side chain R- or changing counter anion A.
Recently, some researches in an attempt to finding other functions such as gel-like networks and self-assembled macromolecules by using the advantage of polymeric coordination oftriazole complexes have also been reported.4 However, theFeRNN N

N NIINNNRRFeN NNNNFeIIligandRNRNFeIIFeFeIIFeFeNN
N N

N N

NRNRNRN NFeNRFeFeFeR=NH

N NOdFigure 1. Structure of Fe(AZ-trz). Left shows the molecular structure, while right shows the image of intermolecular structure.
*Corresponding author. E-mail: einaga@chem.keio.ac.jp. Fax:
+81-45-566-1697

number of the studies on photoinduced spin transition of triazole complexes or combining spin crossover of triazole complex with photochemistry are very few.5 In the present work,
we designed a novel triazole complex Fe(AZ-trz) for the purpose of realizing LIESST effects. As a result, we could confirm the LIESST effect of FeII in Fe(AZ-trz) complex by meansof 57Fe Mössbauer, FT-IR, and UV-visible absorption spectroscopies. This may open the possibility to the development oftriazole complexes for the purpose of the LIESST materials.
2. Experimental
2.1. Synthesis of Azobenzene Ligand. 4a, 4b Triazole ligand
AZ-trz (4-phenylazo-N-[1,2,4]triazole-4-yl-benzamide) wassynthesized as follows. To a THF (200 mL) solution of 4phenylazo benzoic acid (508 mg), triethylamine (500 µL), anddiphenyl(2,3-dihydro-2-thioxo-3-benzoxazolyl)phosphonate(DBOP, 1.20 g), 4-amino-1,2,4-triazole (205 mg) was added atroom temperature. The mixture was refluxed for 2 hours andallowed to cool to room temperature. After the reaction mixture was evaporated to dryness, the residue was washed bychloroform and recrystalized by ethanol to give an orange
product (407 mg, 61% yield). 1H NMR (400 MHz, CDCl3) δ(ppm) 7.64 (m, 3H, ArH), 7.95 (d, 2H, ArH), 8.05 (d, 2H, ArH),
8.16 (d, 2H, ArH), 8.81 (s, 2H, triazole-H); FT-IR (KBr; cm-1)
ν (C=O) 1685, ν (trz-ring) 1562, 1320, ν (N-N) 1155, δ (trzring) 618.
2.2. Synthesis of FeII-triazole complex.3b,3e,4a,4b To a THF(50 mL) solution of AZ-trz (198 mg) was added a THF (10 mL)
solution of Fe(ClO4)2·6H2O (98 mg) containing a small amountof ascorbic acid to prevent the partial oxidation of FeII, and thereaction mixture was refluxed for 10 minutes under nitrogen
atmosphere. Then, a pale orange precipitate was obtained
immediately and collected by filtration, washed by methanol
and THF, and dried under reduced pressure to give an orange
powder (193 mg, 61% yield). FT-IR (KBr; cm-1) ν (C=O) 1688,ν (trz-ring) 1553, 1339, ν (N-N) 1183, δ (trz-ring) 623; Anal.
Calcd for C45H36N18O11Cl2Fe·2H2O: C, 46.29; H, 3.45; N, 21.59.
Found C, 45.74; H, 3.61; N, 21.05.
2.3. Instruments. FT-IR and UV-visible absorption spectra
were recorded on an FT/IR-660 spectrometer (JASCO) and aV-560 spectrometer (JASCO), respectively. The magnetic
properties were investigated with the SQUID magnetometer(MPMS-XL, Quantum Design). Visible light illumination (fil-© 2007 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 05/10/2007

2Mitsuoka

J. Nucl. Radiochem. Sci., Vol. 8, No. 1, 2007

tered light, λmax = 450 nm) was carried out using a xenon lightsource (BA-X500, USHIO). XRD patterns were recorded on
R I NT R AD-RC (R iga ku) with Cu K α radiation. 5 7 Fe
Mössbauer spectra were measured by using a TopologicSystems model 222 constant-acceleration spectrometer with a57
Co/Rh source in transmission mode. When we measured thespectra at low temperature, a closed-cycle helium refrigerator(Nagase Electronic Equipments Service Co., Ltd.) was used.
3. Results and Discussion
The azobenzene ligand AZ-trz and FeII -triazole complex
Fe(AZ-trz) (Figure 1) were newly synthesized in the method
similar to previous reports.4a, 4b They were characterized bymeans of 1H NMR, FT-IR, and elemental analysis. In FT-IR
spectroscopy, Fe(AZ-trz) displayed shifts of vibrational bandsof C=O (1688 cm-1) and triazole ring (1553, 1339, 1183 cm-1) by
reference to AZ-trz, indicating that Fe2+ ion coordinated with
3.5

χMT / cm3 K mol -1

32.5

LS21.5HS1heating

0.5cooling

0050

100

150200250300

T/K

Figure 2. χMT vs. T plots for Fe(AZ-trz). The sample was warmed
from 5 to 300 K and then cooled from 300 to 5 K at a rate of 10 K
min-1.

HSLS(a)100999897

HS>95%
I.S.=0.89 mm/s
Q.S.=2.8 mm/s

Relative Intensity / %100999897(b)LS=26%
I.S.=0.31
Q.S.=0.30

HS=74%
I.S.=0.97
Q.S.=3.2100999897(c)HS=53%
I.S.=1.0
Q.S.=3.3

LS=47%
I.S.=0.39
Q.S.=0.36

100999897-6(d)LS=21%
I.S.=0.31
Q.S.=0.39

HS=79%
I.S.=0.94
Q.S.=3.1-30Velocity / mm s

36-1Figure 3. 57Fe Mössbauer spectra of Fe(AZ-trz) at room temperature(a), 150 K (b), 10 K (c), and under visible light illumination at 10 K(d).

3b,3e,4a,4b

triazole ligand.
The X-ray diffraction (XRD) pattern of Fe(AZ-trz) at roomtemperature showed a diffraction peak at 2θ = 3.74° with a dspacing 23.6 Å. This value indicates an interdigitation of theazobenzene side chain to give parallel-aligned polynuclear FeIIchains, whose center-to-center separation is the same as d spacing value.4a-c Moreover, the d spacing value also suggests anintermolecular π–π interaction of azobenzene moieties because
the d spacing value (23.6 Å) is larger than the length of theazobenzene side chain (about 15 Å) (Figure 1). UV-visible
absorption spectrum of the complex Fe(AZ-trz) in KBr pellet
showed a red shift of the π–π* transition band of trans-form
azobenzene compared with the ligand AZ-trz, which is due toπ–π interaction of azobenzene.
The χMT value, on heating process, gradually increased fromabout 0.6 cm3 K mol-1 at low temperature up to 2.9 cm3 K mol-1
above 250 K. That is, characteristic of the LS-to-HS transition
was observed (Figure 2). On the other hand, when the resulting sample in the HS state was cooled down, the χM T value
decreased along with heating process reversibly and hysterisis
was not observed. 57Fe Mössbauer spectra of Fe(AZ-trz) wereshown 6 in Figure 3. The temperature dependence of 57 Fe
Mössbauer spectra of Fe(AZ-trz) were consistent with magnetic data. Typically two doublet peaks were observed. Thevery narrow doublet peak is typical of the LS state of FeII with
t2g6 electron configuration, while the wide doublet peak is typical of the HS state of FeII with t 2g4 eg2 electron configuration.
Spectrum at 10 K showed that the LS fraction was 47% and therest was HS fraction (Figure 3c). When sample was heated,the LS fraction decreased to 26% and below 5%, at 150 K androom temperature, respectively (Figure 3a, b). The Mössbauer
spectra also indicated the spin crossover phenomena. The χMTvalue and the HS fraction of 57Fe Mössbauer spectrum at low
temperature before the LS-to-HS transition was not zero. It issuggested that the existence of HS fraction even at low temperature is due to the chain-end FeII with HS state, which are
hydrated.4b From the HS fraction (53%) in the Mössbauer
spectrum at 10 K (Figure 3c), we can know that this compound
have a lot of terminals. The reason for the small degree ofpolymerization in Fe(AZ-trz) was the large size of the AZ-trzligand, because it is known that the size of 4- position’s functional groups of triazole ligand influenced the polymerizationof the triazole complexes.4bA xenon lamp was used as the visible light source to investigate photoinduced spin transition. Mössbauer spectrum wasmeasured while being illuminated with the filtered light (λmax= 450 nm) (Figure 3d). The HS fraction of the FeII complex
under illumination drastically increased up to 79%. However,this increased value returned to the initial state by turning off
the illumination. That is, the LIESST effect was observed
although the lifetime of the photoinduced excited state (HS)was not long.
In order to estimate the lifetime of the HS state, the absorption band at 520 nm, corresponding to the d–d electronic transition from 1A1g to 1T1g levels in the FeII LS state was monitored
at 10 K (Figure 4). After visible light illumination for 3 min,the absorbance decreased immediately and recovered to theformer value after 1 min. These indicate that the visible lightillumination, which selectively excited the LS state, induces theLS-to-HS transition by LIESST effect and the relaxation process occurred within 1 min. Furthermore, the LIESST effectof Fe(AZ-trz) could be repeated several times.
The spin crossover phenomena were also monitored by FTIR spectra. Figure 5a illustrated thermal changes of the spectra
respectively. When the sample was heated, the peak at 1183
cm -1 exhibited the red shift to 1177 cm -1 and its intensity
decreased. In addition, the intensity of the peak at 1342 cm-1
increased, while that at 1327 cm-1 decreased relatively andthese two peaks slightly underwent red shift. On the other57

Fe Mössbauer, UV-visible, and FT-IR Study on Photoinduced Spin

hand, when the same sample was cooled down, spectrum
recovered to the initial one. The peak of 1183 cm-1 is assigned
to the vibrational band of triazole N–N bond, while the peaksof 1342 cm-1 and 1327 cm-1 is assigned to the vibrational bandof triazole ring. Generally, the distance in the metal-to-ligand
bond length of the HS state is longer than that of the LS state.
It means that the thermal change of the vibrational band of triazole N-N bond (1183 cm-1) and triazole ring (1324 cm-1, 1327
cm -1) might reflect the thermally induced spin transition.3b
That is, the vibrational band of the triazole ring coordinatedwith the HS state of FeII is less influenced by positive charge ofFeII than the one coordinated with the LS state because of thedifference of the distance between metal and ligand. In thiscase, some vibrational bands in the triazole ring exhibited theshift or the change of intensity by thermal LS-to-HS spin transition.
Moreover, FT-IR spectra were also consistent with theLIESST effects. Figure 5b showed that some vibrational bandsof ligands exhibited the reversible peak shifts and the changesof peak intensities by visible light illumination. It is also recognized that the FT-IR spectrum at 10 K soon after illumination is almost compatible with that at 300 K, indicating that thephotoinduced metastable state and the room temperature state
Absorbance at 520 nm / a. u.

beforeeafterHS050100150200

Time / sFigure 4. The absorption change of Fe(AZ-trz) at 520 nm after visible light illumination for 3 min at 10 K. (The minus area shows theabsorbance before illumination, while the plus area shows after illumination.)heat

Transmittance / a. u.

heat
300K
200K
150K
100K
50K
10K(a)

1400

1350

1300

1250

1200

1150

-1

Transmittance / a. u.

Wavenumber / cm

1400

After vis illumination

visvisinitial3min later

(b)
1350

1300

1250

3

have similar characteristic in the electronic state around theFeII site. The results demonstrated that the photoinduced LSto-HS transition induced the elongation of the distance between
FeII ion and N atom of triazole ring.
4. Conclusions
In summary, we have succeeded in synthesizing a novel spincrossover Fe I I -triazole complex Fe (AZ-trz). From 57 Fe
Mössbauer spectra, we could have directly observed the photoinduced spin transition of Fe(AZ-trz) by visible light illumination at low temperature. It was confirmed that the photoinduced
spin transition occurred by exciting a d–d electronic transition
band of the FeII LS state species and the LIESST effect continued for 1 min by monitoring the absorption change at 520 nm.
Moreover, the photoinduced spin transition phenomenon wasalso confirmed by monitoring the change of FT-IR spectra, andthe result indicated that the electronic state around FeII of thephotoinduced metastable form at low temperature was compatible with the electronic state of FeII at room temperature.
Acknowledgement. This work was supported by Grant-inAid for Scientific Research on Priority Areas (417) from theMinistry of Education, Culture, Sport, Science, and Technology(MEXT) of Japan, and by New Energy and Industrial
Technology Development Organization (NEDO) program(Japan).
References

LS-50J. Nucl. Radiochem. Sci., Vol. 8, No. 1, 2007

12001150-1

Wavenumber / cm

Figure 5. The FT-IR spectra changes of Fe(AZ-trz). (a) Thermalchange. (b) Photoinduced change at 10 K. (Spectra were obtained
before illumination, right after illumination, and 3 min after illumination.)(1) (a) O. Sato, T. Iyoda, A. Fujishima, and K. Hashimoto,Science 272, 704 (1996); (b) O. Sato, Acc. Chem. Res. 36,692 (2003); (c) R. Mikami, M. Taguchi, K. Yamada, K.
Suzuki, O. Sato, and Y. Einaga, Angew. Chem. Int. Ed. 43,6135 (2004); (d) T. Yamamoto, Y. Umemura, O. Sato, andY. Einaga, J. Am. Chem. Soc. 127, 16065 (2005); (e) M.
Taguchi, I. Yagi, M. Nakagawa, T. Iyoda, and Y. Einaga, J.
Am. Chem. Soc. 128, 10978 (2006).
(2) (a) P. Gütlich, Y. Garcia, and T. Woike, Coord. Chem. Rev.
839, 219 (2001); (b) J. A. Real, A. B. Gaspar, and M. C.
Muñoz, Dalton. Trans. 2062 (2005).
(3) (a) J. Kröber, E. Codjovi, O. Kahn, F. Grolière, and C. Jay,
J. Am. Chem. Soc. 115, 9810 (1993); (b) F. Armand, C.
Badoux, P. Bonville, A. R. Teixier, and O. Kahn, Langmuir
11, 3467 (1995); (c) O. Kahn and C. Jay, Science 279, 44(1998); (d) J. A. Real, A. B. Gaspar, V. Niel, and M. C.
Muñoz, Coord. Chem. Rev. 236, 121 (2002); (e) O.
Roubeau, B. Agricole, R. Clèrac, and S. Ravaine, J. Phys.
Chem. B 108, 15110 (2004).
(4) (a) T. Fujigaya, D.-L. Jiang, and T. Aida, J. Am. Chem.
Soc. 125, 14690 (2003); (b) T. Fujigaya, D.-L. Jiang, andT. Aida, J. Am. Chem. Soc. 127, 5484 (2005); (c) M.
Seredyuk, A. B. Gaspar, V. Ksenofontov, S. Reiman, Y.
Galyametdinov, W. Hasse, E. Rentschler, and P. Gütlich,CMater.
A. Takada, N. Nemoto, and N. Kimizuka, J. Am. Chem.
Soc. 126, 2016 (2004); (e) S. Kume, K. Kuroiwa, and N.
Kimizuka, N. Chem. Commun. 2442 (2006).
(5) (a) X. J. Liu, Y. Moritomo, A. Makamura, T. Hirao, S.
Toyazaki, and N. Kojima, J. Phys. Soc. Jpn. 70, 2521(2001); (b) A. Nakamoto, Y. Ono, N. Kojima, D.
Matsumura, T. Yokoyama, X. J. Liu, and Y. Morimoto,Synth. Met. 137, 1219 (2003); (c) Y. Garcia, V.
Ksenofontov, and P. Gütlich, Hyperfine Interac. 139/140,
543 (2002).
(6) 57Fe Mössbauer spectra were calibrated using the known
hyperfine splitting of α-iron.