Ferromagnetism and its photo-induced effect in 2D iron mixed-valence plex coupled with photochromic spiropyran*
I. Kashima M. Okubo Y. Qno M. Itoi N. Kida M. Hikita M. Enomoto N. Kojima*
Gradate School of Arts and Scrences The U/niversity of Tokgo Komabe 3-8-7 Meguro-ka Tokyo /53-8902 Japan
Abstract
Iron mixed-valence plex (n-C;H)N[FeFe(dto)s] (dto = COS) shows a spin-entropy driven phase transition called charge transferphase transition around 120 K and a ferromagnetic transition at 6.5 K. These phase transitions remarkably depend on the hexagonal ring size in two-dimensional honeyb network structure of [FeF(dto)] In order to cotrol the magnetic properties and the electronic state in the(SP)[FeFe(dto) ] (SP=spiropyram) and investigated the photo-induced effect on the magnetic peoperties by the mesurements of magnetic do-bridged iron mixed-valence system by means of photo-irradiation we have synthesized a photo-sensitive organicinorganic hybrid system suscepibility ESR IR UVvis andFe Misbar stra. Upon UV iadiation at 337 nm abroad aboption band betwen 50 nm and 600 nm appears and continuously increases with the photo-iradiation time which implies that the UV iradiation changes the structure of spiropyran fromclsed fom to pen one in solid state. Te photohromism in spiropyran changes the femagnetic transition tmperature and the coercive fre. 2005 Elsevier B.V. All rights reserved.
Keyonds: Magnetic measurements; Magnetic phase transition
1. Introduction
nated by six S atoms and six O atoms respectively. In theFellI(S = 5/2) and Fel(S = 0) sites are coordinated by six O atoms lower temperature phase (T H)N* cation layers are intercalated between twon= 3 and 4 is diamagnetic low spin state. In the case of n=4 two ferromagnetic phases with Tc =6 K and Tc = 13 K coexist which correspond to the ferromagnetic phases in the lower andhigher temperature phases respectively.
Transition metal plexes with d*d? configuration havea possibility of spin transition berween a low-spin state and ahigh-spin state. In the case of mixed-valence plexes whose undergoes the charge transfer phase transition between 200 Kthat new types of conjugated phenomena coupled with spin andsitiondoesnot take place for (n-CH1)N[FF(dto)] charge take place between neighboring metal ions in order tominimize the free energy in the whole system [1].
Recently wehave discovered a new type of phase transitionaround 120 K for (n-CH）)N[FellFe(dto)s] (dto= COS2) where the thermally induced charge transfer between Fell and[FelFe(dto)3] layers [3]. In the higher temperature phase (T > 120 K) the Fe(S = 1/2) and Fe(S= 2) sites are coordi-
depend on the size of intercalated cation which implies a pos- As mentioned above these phase transitions remarkablyhoneyb network structure of [FeFe(dto)s]c by means sibility to control the magnetic properties of two-dimensionalof the isomerization of intercalated cation From this viewpoint we have synthesized a photo-sensitive organicinorganic hybrid
Scheme 1. Photochromism of cationic spiropyran (SP-R).
system (SP)[FelFe(dto)1 (SP =spiropyran) and investigatedmedium of photo-isomerization of spiropyran. In connection the photo-induced effect on the magnetic properties through thephotochromic properties of SP have been largely investi- with this the following should be mentioned. Althoughcationic pyrido-spiropyrans were found to show photochromism gated photochromic SP in solid state is very rare. Recently twoin the crystalline state [5]. Soon after an organicinorganichybrid plex (SP)MnCr(ox)3-HO was synthesized and was found to show photochromism in the crystalline state[6].
In this paper we report the synthesis of photo-sensitiveorganicinorganic hybrid plexes (SP-R)[FeFe(dto)s](Me) ethyl (Et) propyl (Pr) the photo-isomerization of SP-R (SP-R =cationic spiropyran shown in Scheme 1 R=methylproperties by the measurements ofmagnetic susceptibility ESR in the solid state and the photo-induced effect on the magneticIR UVvis and 7Fe Mossbauer sectra.
R*: Scheme 1) and the two-dimensional honeyb network (SP-R)[FelF(dto) ] consists of acationic spiropyran (SP.structure of [FelFel(dto)3] (Scheme 2). In general the cationic spiropyran is converted from the yellow colored closedform (CF) to the red colored open form (OF) upon the irradia-tion of UV-light (330370 nm) at room temperature. The open form is usually less stable and returns to the closed form boththermally and photochemically (500600 nm) in solution. How- ever as mentioned later the open form of SP-R is stable in(SP-R)(FEllFell(dto)s)]
Scheme 2 Schematic representation of [FEeFel(dto)s]
2. Experimental
3. Results and discussion
(SP-R)[FellFelI(dto)3] was synthesized as fllows. (SP-R)Iwas obtained as yellow powdered crystals according to Ref. [5]. A solution FeCl-4HO and (SP-R)I in a methanolwatermixture of 3:2 ratio was stirred. To this a solution of KBa[Fe(dto)3 ]3HO in a methanolwater mixture was added.In this way (SP-R)FeFe(dto)s] was obtained as black col-ored precipitate.
The static magnetic susceptibility was measured by a Quan-zero field could magnetization (ZFCM) and fields could mag- tum Design MPMS5 SQUID susceptometer under 5000 Oe. Thenetization FCM) were also measured for the investigation of the ferromagnetic phase in the temperature range of 235 Kunder 30 G. The remnant magnetization (RM) was measured10 mg was wrapped in polyethylene film and held in a plastic in the same temperature under zero field. Powdered sample ofstraw. The magnetic susceptibility obtained was corrected for the background and the core diamagnetism estimated from Pascal’sconstants.
In the case of 57Fe Mossbauer sectroscopic measuremet 5CoinRhwasusedat29KasaMosbauer urcehspetawere calibrated by using the six lines of a body-centered cubic iron foil (α-Fe) the center of which was taken as zero isomershift. The hyperfine parameters were obtained by least-squaresfitting to Lorentzian line shapes.
The ESR spectra were measured by a JEOL JES-TES-300spectrometer in the temperature range of 4300 K. The field was swept in the range of G.
(SP-Et)[FelFelI(dto)3] upon UV inradiation (337 nm). When Fig. 1 shows the electronic absorption spectral change ofthe UV light isiradiated for SP-Et)[FellFell(dto)s] in KBr
Fig. 1. Poto-inuced aortion spetrum of (SPE) FFl(do) in KBrpellet at 298 K upon UV irradiation (337 nm). The power of UV iradiation is 4 mW.
appears around 550 nm which corresponds to the = tran- pellet at 298 K a new absorption spectrum with wide half-widthintensity of the absorption spectrum around 550 nm increases sition in the open form of SP-R. As shown in Fig. 1 thewith increasing photo-irradiation time which implies that thephoto-isomerization of SP-Er takes place at room tempera- ture in solid stateof (SP-Et)[Fe(d)The poto-inducedabsorption spectrum around 550 nm does not decrease thermallytakes place for the crystalline state of (SP-R)[Fee(dto)s] and photochemically. The photo-isomerization of SP-R* also(R = Me and Pr).
In the case of SP-Pr the spin configurations of Fe and Felperatures between 300 K and 2 K which was confirmed by 57Fe are S= 1/2 and S= 2 respectively at the whole measuring tem-Mossbauer spectroscopy. The spin configuration of Fel(S = 1/2)(-CH)N[FelFedto)] and undergoes the feromagnetic and Fel(S =2) corresponds to the higher temperature phase oftransition at8 K. The coercive force is 4550 Gat2 K. In this crys- tal any photo-induced effect on the magnetic properties was notobserved which implies that the higher temperature phase isquite stable.
In the case of SP-Et the remnant magnetization curve (RM)decreases stepwise at 8 K with increasing temperature and then it disappears at 2 K. The zero field magnetization curve (ZFCM) on the other hand has two maxima at 7K and 20K. These netic phases. From the analysis of Fe Mossbauer spectra it iselucidated thathehighertemperature phase withF(S=1/2) and Fe(S= 2) and the lower temperature one with F(S=5/2)phases in (SP-Ex)[FeFe(dto)] undergo the feromagnetic and Fe(S = 0) coexist. The higher and the lower temperaturephase transitions with Tci = 22 K and Tc2 = 8 K respectively.while Tc2 changes from 8 K to 12K. The coercive force at 2 K After the UV irradiation (337 nm) Tc1 remains unchanged changes from 3800 G to 8400 G after the UV irradiation whichis shown in Fig. 2.
Fig. 2. Hysteresis loops of (SP-Et[FelF(do)s] at 2 K before () and afer (O) UV irradiation (337 nm). The UV was irradiated at 298 K.
4. Conclusions
Acknowledgments
(RM) decreases stepwise at 5 K with increasing tempera- In the case of SP-Me the remnant magnetization curveture and then it disappears at 2 K. The zero field magne-ima at 5 K and 21 K. These magnetization curves imply the tization curve (ZFCM) on the other hand has two max-ofFe Mossbauer spectra it is elucidated that the higher coexistence of two ferromagnetic phases. From the analysistemperature phase with Fe(S= 1/2) and Fe(S=2) and the lower temperature one with Fe(S= 5/2) and Fe(S=0) coex-ist at the whole measuring temperatures between 300 K and2 K. The higher and the lower temperature phases in (SP- Et)[FF(dto)s] undergo the ferromagnetic phase transi-UV irradiation (337 nm). the peak of ZFCM at 5 K disap- tions with Tc =22 K and Tc2=5 K respectively. After thepears which means that the mixed ferromagnetic phases withthe higher temperature phase (Tci = 22 K) and the lower tem- perature phase (Tc2 =5 K) can be converted to the singleferromagnetic phase of Te=17K due to the higher tempera- ture phase through the medium of the photo-isomerization ofSP-Me*.
Iron mixed-valence plex (nCyH)N[FlFel(dto)](dto=COS2) shows a new type of phase transitiona ferromagnetic transition at 6.5K. These phase transi- called charge transfer phase transition around 120K andtions remarkably depend the size of cation intercalated between the two-dimensional honeyb network layers of [FelFe(dto)s]c. In order to control the magnetic proper-ties and the electronic state in the dto-bridged iron mixed- valence system by means of photo-irradiation we have syn-(SP)[FeFe(do)s] (SP=spiropyran) and investigated the thesized a photo-sensitive organicinoeganic hybrid system photo-induced effect on the magnetic properties through themedium of photo-isomerization of spiropyran. When the UV light (337nm) is iradiated for (SP-R)[FeFl(dto)](R=Me Et Pr) in KBr pellet at 298K a new absorp- tion spectrum appears around 550 nm. corresponding to theπ* transition in the open form of SP-R* which impliesthe open one takes place at room temperature in the crys- that photo-isomerization of SP-R from the closed form totalline state of (SP-Me)[FelFel(dto)s1 The photochromism in spiropyran changes the ferromagnetic transition tem-perature and the coercive force for (SP-R)[FellFel(dto)]mixed ferromagnetic phases with the higher temperature (R= Me Et). In the case of (SP-Me)[FellFel(dto)s]. thephase and the lower temperature phase is converted to the single ferromagnetic phase due to the higher temperaturephase through the medium of the photo-isomerization ofSP-Me*.
This work was supported by a Grant-in-Aid for ScientificResearch and a Grant-in-Aid from 21st Century COE (Center of Excellence) Program (Research Center for Integrated Science)
Technology Japan. from the Ministry of Education Culture Sports Science and
[3] M. Itoi A. Taira M Enomoto N. Matsushita N. Kojuma Y. Kobayashi K. Asai K. Koyama T. Nakano Y. Uwatoko JI. Yamaura Solid State[4] M. Itoi N. Kojima Y. Kobayashi K. Asai MoL. Ctyst. Liq. Cryst. 379 Commun. 130 (2004) 415420.[5] S. Benard P: Yu Adv. Mater. 12 (2000) 4850. (2002) 377382.[6] S. Benard E Rivtere P Yu K. Nakatani JF. Delouis Chem Mater. 13 (2001) 159162.
References
[1| T. Nakamoto Y. Miyazaki M. hoi Y. Ono N. Kojina M. Sorai Angew. Chem. Int. Ed. 40 (2001) .[2] N. Kojima W. Aoki M. Btoi Y. Ono M. Seto Y. Kobayashi Yu. Maeda. Solid Stale Commun. 120 (2001) 165170.
