Photo-induced reverse valence tautomerism in a metastableCo compound

Osamu Sato a,*, Shinya Hayami a, Zhong-ze Gu a, Kazuyuki Takahashi a,Rie Nakajima a, Akira Fujishima b

a Special Research Laboratory for Optical Science, Kanagawa Academy of Science and Technology, KSP Bldg. East 412, 3-2-1 Sakado,

Takatsu-ku, Kawasaki-shi, Kanagawa 213-0012, Japanb Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

Received 3 August 2001; in final form 17 December 2001

Abstract

In our preceding paper, we have reported that a Co complex, ½CoIII-LSðtmedaÞð3; 5-DBSQÞð3; 5-DBCatÞ� �0:5C6H5CH3, exhibits a photo-induced valence tautomerism. The photo-induced metastable state, ½CoII-HSðtmedaÞð3; 5-DBSQÞ2� � 0:5C6H5CH3, has a back electron transfer band from CoII-HS to 3,5-DBSQ at around 800 nm, indicating

that the valence tautomerism should be photo-reversible. In fact, we have found that the metastable state could be

photo-pumped back to the original state by exciting the back electron transfer band. The process can be expressed as

½CoII-HSðtmedaÞð3; 5-DBSQÞ2� � 0:5C6H5CH3ðmetastable stateÞ ! ½CoIII-LSðtmedaÞð3; 5-DBSQÞð3; 5-DBCatÞ� � 0:5C6H5

CH3 (ground state). � 2002 Published by Elsevier Science B.V.

1. Introduction

The design of molecules that could be utilizedfor information storage is one of the main chal-lenges in molecular materials science. To achievethis aim, molecules that exhibit bistability are ac-tively being investigated. Bistability may be de-fined as a property of a molecular system thatallows it to exist in two different electronic states.Typical examples of such molecular species are

spin-crossover complexes and valence tautomericcompounds [1–12]. Usually, the spin crossover andvalence tautomeric phenomena can be induced bya variation of temperature or of pressure. On theother hand, Decurtins et al. [5] have reported thatthe spin transition can be induced by illumination.This phenomenon is called light induced excitedspin state trapping (LIESST). Furthermore,Hauser [6] have reported reverse-LIESST can beinduced with 752.7 nm light. Recently, it hasbeen reported that several Co complexes exhibitlong-lived intramolecular electron transfer by ex-citing the ligand-to-metal charge transfer (LMCT)band [13–17]. The photo-process in a Co com-pound,½CoIII-LSðtmedaÞð3; 5-DBSQÞð3; 5-DBCatÞ� �0:5C6H5CH3, can be expressed as

25 March 2002

Chemical Physics Letters 355 (2002) 169–174

www.elsevier.com/locate/cplett

* Corresponding author. Fax: +81-44-819-2070.

E-mail address: sato@fchem.chem.t.u-tokyo.ac.jp (O. Sato).

0009-2614/02/$ - see front matter � 2002 Published by Elsevier Science B.V.

PII: S0009-2614 (02 )00205-1

½CoIII-LSðtmedaÞð3; 5-DBSQÞð3; 5-DBCatÞ�� 0:5C6H5CH3 ðground stateÞ! ½CoII-HSðtmedaÞð3; 5-DBSQÞ2�

� 0:5C6H5CH3 ðmetastable stateÞ;

where LS, HS, tmeda, 3,5-DBSQ and 3,5-DBCatare low-spin, high-spin, N ;N ;N 0;N 0-tetramethyl-ethylenediamine, 3,5-di-tert-butyl-1,2-semiquino-nate and 3,5-di-tert-butyl-1,2-catecholate, res-pectively. The lifetime of the metastable stateat 5 K was 175 min [17]. The metastable½CoII-HSðtmedaÞð3; 5-DBSQÞ2� � 0:5C6H5CH3 statehas a charge transfer (CT) band from CoII-HS to3,5-DBSQ at around 800 nm. The presence of themetal-to-ligand charge transfer (MLCT) band in-dicates that the photo-induced valence tautomer-ism should be reversible. That is, by selectivelyilluminating the MLCT band, it might be possibleto convert the ½CoII-HSðtmedaÞð3; 5-DBSQÞ2� � 0:5C6H5CH3 state back to the ground state, i.e.½CoIII-LSðtmedaÞð3; 5-DBSQÞð3; 5-DBCatÞ� � 0:5C6

H5CH3. In fact, we have found that back electrontransfer can be induced by exciting the MLCTband. Here, we describe the phenomenon of thereverse valence tautomerism. It should be notedthat the photo-induced valence tautomerism wasdescribed in a separate manuscript [17].

2. Experimental

TheCo complex, ½CoII-HSðtmedaÞð3; 5-DBSQÞ2� �0:5C6H5CH3, was prepared by adding a 15 mltoluene solution of tmeda (0.046 g) to a 50 mltoluene solution with ½Coð3; 5-DBSQÞ2�4 (0.20 g)in suspension, followed by slow evaporationunder Ar gas flow. Anal. Calcd (found) for½CoC34H56N2O4� � 0:5C6H5CH3: C 68.60 (67.93); H9.30 (9.09); N 4.10 (4.18); Co 8.63 (8.73). X-raystructural analyses were done using a RigakuRAXIS-RAPID Imaging Plate diffractometer withgraphite-monochromated Mo-Ka radiation (Fig.1). Out of the 35 998 reflections that were collected,17 313 were unique (Rint ¼ 0:088) and 9284 withI > 3rðIÞ were used to determine the structurewith SIR92. The final R values gave R1 ¼ 0:124for I > 3rðIÞ, R ¼ 0:160 and Rw ¼ 0:228 for

all data, with a linear absorption coefficientlðMo-KaÞ ¼ 18:35 cm1. All non-hydrogen atomsexcept disordered solvent molecules were refinedanisotropically. Hydrogen atoms were calcu-lated, but not refined. X-ray crystallographicdata at room temperature were as follows:F.W.¼ 1323.66, green needle-like crystal(0:5 0:1 0:1), triclinic, space group P1,a ¼ 15:0805ð5Þ, b ¼ 15:9615ð2Þ, c ¼ 18:4550ð3Þ �AA,a ¼ 67:655ð2Þ, b ¼ 75:268ð2Þ, c ¼ 73:783ð3Þ,V ¼ 3890:0ð2Þ �AA

3, Z ¼ 2, Dcalcd ¼ 1:130 g cm3.

UV–VIS absorption spectra of the polystyrenefilm, in which the Co complex was embedded, wererecorded on a Shimadzu model UV-3100PCspectrophotometer. The polystyrene film wasprepared by dissolving the Co complex and poly-styrene in a toluene solution and then solution-casting on glass slides. Infra-red (IR) spectrameasured using the KBr method were obtained ona Bio-RAD model FTS-40A spectrophotometer.The UV–VIS and IR measurements at low tem-perature were performed using a helium-flow typerefrigerator (Helitran LT-3-110). The magneticproperties were investigated with a superconduct-ing quantum interference device (SQUID) mag-

Fig. 1. View of ½CoII-HSðtmedaÞð3; 5-DBSQÞ2�.

170 O. Sato et al. / Chemical Physics Letters 355 (2002) 169–174

netometer (model MPMS-5S, Quantum Design).Light was guided by a quartz optical fiber to illu-minate the sample in the SQUID magnetometer.A powder sample was supported on commer-cial transparent adhesive tape and placed on theedge of the optical fiber. A laser-diode with awavelength of 830 nm, a laser-diode pumpedNd:YAG laser (Crystal Laser GCL-150-M) with awavelength of 532 nm and a wavelength-variablepulse laser with a pulse-width of ca. 6 ns (Con-tinuum Surelite OPO) were used as the lightsources. Note that the Co complexes were dis-persed randomly in the polystyrene film for theUV–VIS measurements and hence no cooperativeinteraction occurred. This means that the samplecondition for the UV–VIS measurements was dif-ferent from that for the IR and magnetizationmeasurements.

3. Results and discussion

When the LMCT band in the Co complex wasexcited with 532 nm light below 50 K, the groundstate, ½CoIII-LSðtmedaÞð3; 5-DBSQÞð3; 5-DBCatÞ� �0:5C6H5CH3, was transformed into the metasta-ble state, ½CoII-HSðtmedaÞð3; 5-DBSQÞ2� � 0:5C6H5

CH3. The electronic structure of this metastablestate is identical with the high temperature phase.Hence, after illumination, the MLCT band fromCoII-HS to 3,5-DBSQ at around 800 nm in themetastable state increased and the LMCT bandfrom 3,5-DBCat to 3,5-DBSQ at 2500 nm inthe ground state, ½CoIII-LSðtmedaÞð3; 5-DBSQÞð3; 5-DBCatÞ� � 0:5C6H5CH3, decreased (Fig. 2)[18]. On the other hand, when the metastable statewas illuminated with 830 nm light in order toexcite the reverse CT band from CoII-HS to 3,5-DBSQ, the absorbance at around 800 nm, char-acteristic of the metastable state, decreased.Furthermore, it was found that the absorptionband at 2500 nm, i.e. the ligand-to-ligand chargetransfer (LLCT) band of the ½CoIII-LSðtmedaÞð3; 5-DBSQÞð3; 5-DBCatÞ� � 0:5C6 H5CH3 state,increased. This suggests that CT from CoII-HS to3,5-DBSQ was induced in the metastable state bythe 830 nm light. Hence, the photo-process in-duced by 830 nm light can be expressed as

½CoII-HSðtmedaÞð3; 5-DBSQÞ2� � 0:5C6H5CH3

ðmetastable stateÞ! ½CoIII-LSðtmedaÞð3; 5-DBSQÞð3; 5-DBCatÞ�� 0:5C6H5CH3 ðground stateÞ

Fig. 3 shows the change in magnetization beforeand after illumination. Before illumination, themagnetization value is ca. 1:7 lB. On the otherhand, after the LMCT band in the ground statewas excited with 532 nm light (ca. 70 mW=cm2),the magnetization value was increased to ca.2:3 lB. By contrast, when the metastable complex,½CoII-HSðtmedaÞð3; 5-DBSQÞ2� � 0:5C6H5CH3, wasilluminated at 5 K with 830 nm light (ca.30 mW=cm2), the magnetization value decreased.As shown in the figure, the magnetization valueafter excitation of the MLCT band was ca. 2:2 lB.This means that the back electron transfer fromthe CoII-HS to 3,5-DBSQ was induced in themetastable state by light, which is consistent withthe results of the absorption spectra describedabove. The reversible change in magnetization in-duced by alternate illumination with 532 and 830nm light could be repeated several times (Fig. 3).

Fig. 2. UV–VIS spectra at 19 K. The ½CoIII-LSðtmedaÞð3; 5-DBSQÞð3; 5-DBCatÞ� � 0:5C6H5CH3 complex has a CT

band from 3,5-DBCat to 3,5-DBSQ at around 2500 nm. Ad-

ditionally, it has an absorption band at around 650 nm. In

analogy with the assignment of the UV–VIS spectrum of

[CoII-HS(1,10-phenanthrolin)ð3; 5-DBSQÞ2� � C6H5CH3, it can

be concluded that the absorption is essentially ligand field in

nature, but does contain some charge transfer from 3,5-DBCat

to CoIII-LS. On the other hand, [CoII-HSðtmedaÞð3; 5-DBSQÞ2� � 0:5C6H5CH3 induced by the 532 nm light has an

absorption band at around 800 nm, which is ascribable to a CT

band from CoII-HS to 3,6-DBSQ [18].

O. Sato et al. / Chemical Physics Letters 355 (2002) 169–174 171

Fig. 3 shows the IR spectra before andafter illumination. The C–O stretch vibration of3,5-DBCat in ½CoIII-LSðtmedaÞð3; 5-DBSQÞð3; 5-DBCatÞ� � 0:5C6H5CH3 was observed at1280 cm1 [16]. The C–O stretch peak is reducedby excitation of the LMCT band with 532 nmlight, because it converts to the metastable½CoII-HSðtmedaÞð3; 5-DBSQÞ2� � 0:5C6H5CH3 state.On the other hand, the peak clearly increases afterexcitation of the MLCT band in the metastablestate with 830 nm light. This result supports theidea that 830 nm light induced the back electrontransfer from CoII-HS to 3,5-DBSQ. It should benoted that the photo-induced metastable fractionestimated from the C–O stretching peak at1280 cm1 was 50� 10%, while the fraction esti-

mated from the absorption band at 2500 nm in theUV–VIS spectra was less than 10%. The discrep-ancy arises because of the different sample condi-tions, i.e. the polystyrene film for the UV–VISmeasurements and the polycrystalline sample forthe IR measurements. In our opinion, this meansthat the cooperativity due to the intermolecularinteraction plays a key role in achieving the long-lived metastable state and hence a larger photo-conversion was observed for the polycrystallinesample [17].

An important characteristic of the back elec-tron transfer in the metastable state is that, evenunder more intense light illumination, the magne-tization value does not reach the original levelobserved for the pure ½CoIII-LSðtmedaÞð3; 5-DBSQÞð3; 5-DBCatÞ� � 0:5C6H5CH3 state at 5 K, i.e.1:7 lB. As described above, the magnetizationvalue decreased from ca. 2.3 to 2:2 lB after illu-mination with 830 nm light. This suggests that70% of the moieties, whose electronic statewas changed from ½CoIII-LSðtmedaÞð3; 5-DBSQÞð3; 5-DBCatÞ� � 0:5C6H5CH3 to ½CoII-HSðtmedaÞð3; 5-DBSQÞ2� � 0:5C6H5CH3 by illumination with532 nm light, remained unchanged after illumina-tion with 830 nm light. Furthermore, it was foundthat when the complex with the electronic state½CoIII-LSðtmedaÞð3; 5-DBSQÞð3; 5-DBCatÞ� � 0:5C6

H5CH3 was illuminated with 830 nm light at 5 K,an increase in the magnetization value from ca. 1.7to 2.2 lB was observed (Fig. 3). IR spectra showthat the peak of the C–O stretch vibration wassignificantly decreased after excitation (Fig. 3),which is consistent with the results of the magne-tization measurements. This means that the mag-netization value, leff ¼ ca: 2:2 lB, is observed as aresult of achieving a photo-stationary state underillumination with 830 nm light. That is, the exci-tation at a wavelength of 830 nm induces LMCTin the ½CoIII-LSðtmedaÞð3; 5-DBSQÞð3; 5-DBCatÞ��0:5C6H5CH3 (ground state) as well as MLCT inthe ½CoIIHSðtmedaÞð3; 5-DBSQÞ2� � 0:5C6H5CH3

(metastable state), because the edge of the LMCTband extends towards the wavelength of 830 nm.Consequently, the magnetization value does notreach the original level, i.e. 1.7 lB, after themetastable Co complex is illuminated with 830 nmlight.

Abs

orba

nce

(a.u

.)

1500 1400 1300 1200 1100 1000Wavenumber (cm

-1)

300K before illumination (20K) 532nm illumination (20K) 830nm illumination (20K) 830nm illumination

after 532 nm illumination (20K)

0.66

0.64

0.62

0.60

0.58

Abs

orba

nce

1310 1300 1290 1280 1270 1260Wavenumber (cm

-1)

830nm830nm (after 532nm)

532nm

300K

before illumination(20K)

2.4

2.3

2.2

2.1

2.0

1.9

1.8

1.7

µ eff (

µ B)

6543210Cycle index

2.2

2.0

1.8

µ eff (

µ B)

2.01.00.0Cycle index

532nm532nm532nm 532nm 532nm

830nm 830nm 830nm

∆830nm

Fig. 3. Top: Change in the magnetization by alternate illumi-

nation with 532 nm light and 830 nm light. Inset: Change in the

magnetization induced by illumination with 830 nm light and

by thermal treatment (D) at 60 K. Bottom: IR spectra before

and after illumination. Inset: Expanded spectra from 1310 to

1255 cm1. The peak at 1280 cm1 can be ascribed to the C–O

stretch vibration of 3,5-DBCat.

172 O. Sato et al. / Chemical Physics Letters 355 (2002) 169–174

The achievement of the photo-stationary stateleads us to predict that, by exciting the compoundwith light longer than 830 nm, a larger fraction ofthe metastable moieties might change to theground state, ½CoIII-LSðtmedaÞ ð3; 5-DBSQÞð3; 5-DBCatÞ� � 0:5C6H5CH3, because the absor-bance of the LMCT band decreases with increas-ing wavelength. Fig. 4 shows the wavelengthdependence of the change in magnetization re-sulting from the photo-induced back electrontransfer. As shown in the figure, the magnetizationvalue decreased from ca. 2.3 to 2.1 lB, when thecomplex was illuminated with 1100 nm light. Thisshows that the change in magnetization induced by1100 nm light is larger than the one induced by 830nm light, which is consistent with the above ex-pectation. Note that the back electron transfercould not be effectively induced by 1200 nm light,because the absorbance of the MLCT band in themetastable state is quite small at 1200 nm. Con-sequently, it was found that, because of the com-petition between MLCT and LMCT, the backelectron transfer in the metastable state could bemost efficiently induced by using light at around1100 nm.

The charge transfer process is schematically il-lustrated in Fig. 5. Shining laser light into the 800nm charge transfer absorption band of the meta-stable ½CoII-HSðtmedaÞð3; 5-DBSQÞ2� � 0:5C6H5CH3

state results in population of a MLCT excitedstate. After excitation, the MLCT excitedstate relaxes to the ½CoIII-LSðtmedaÞð3; 5-DBSQÞ

ð3; 5-DBCatÞ� � 0:5C6H5CH3 state via a spin for-bidden decay path due to spin–orbit coupling.Consequently, the original ground state, ½CoIIILSðtmedaÞð3; 5-DBSQÞð3; 5-DBCatÞ� � 0:5C6H5CH3,can be populated by illuminating with near-IRlight.

In summary, we have shown that exciting theMLCT band in the metastable ½CoII-HSðtmedaÞð3; 5-DBSQÞ2� � 0:5C6H5CH3 state, which is popu-lated by the light of the LMCT band in the com-plex ½CoIII-LSðtmedaÞð3; 5-DBSQÞð3; 5-DBCatÞ��0:5C6H5CH3, can induce a back electron transferfrom CoII-HS to 3,5-DBSQ. That is, upon illumi-nation, one electron in CoII-HS is transferred to3,5-DBSQ, yielding CoIII-LS and 3,5-DBCat. Theprocess can be expressed as ½CoII-HSðtmedaÞð3; 5-DBSQÞ2� � 0:5C6H5CH3 ! ½CoIII-LSðtmedaÞð3; 5-DBSQÞð3; 5-DBCatÞ� � 0:5C6H5CH3.

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