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Optically switchable behaviour of a dioxolene adduct of a cobalt-macrocycle complex
Chemical Physics Letters 396 (2004) 198–201 www.elsevier.com/locate/cplett
Optically switchable behaviour of a dioxolene adduct of a cobalt-macrocycle complex Chiara Carbonera a, Andrea Dei
b,*
, Claudio Sangregorio b, Jean-Franc¸ois Le´tard
a,*
a
b
Groupe des Sciences Mole´culaires, Institut de Chimie de la Matie`re Condense´e de Bordeaux, UPR CNRS No. 9048, 87 Av. du Doc. A. Schweitzer, Universite´ Bordeaux 1, F-33608 Pessac, France Dipartimento di Chimica, INSTM Research Unit and LAMM, Universita` di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino (Firenze), Italy Received 21 April 2004; in final form 28 July 2004
Abstract Reflectivity and magnetic measurements show that Co(CTH)(Phendiox)]PF6 Æ H2O (CTH = DL -5,7,7,12,14,14-9,10-hexamethyl1,4,8,11-tetraazacyclotetradecane and Phendiox = semiquinonato or catecholato forms of 9,10-dioxophenantherene) undergoes valence tautomeric interconversion cobalt(III)–catecholato to cobalt(II)–semiquinonato upon light irradiation. This compound is the simplest cobalt-o-dioxolene complex undergoing this transition. The critical temperatures of the photoinduced transition and of the entropy driven transition seem to be related by the same empirical relationships observed for iron(II) spin crossover compounds. 2004 Elsevier B.V. All rights reserved.
The interest of the scientific community towards molecular materials showing properties of potential application as electronic devices is continuously growing. Particular attention is devoted to molecules exhibiting electronic bistability, i.e., molecules which may exist in two different electronic states depending on the external parameters [1]. As a consequence the possibility of inducing a reversible change of the electronic state by varying an appropriate parameter, like temperature, pressure, etc., implies the possibility of information storage at molecular level [2]. Among the different classes of bistable molecules to date investigated, there is no doubt that those showing electronic states with different magnetic properties are the most attractive. They are the spin crossover metal complexes [2,3] and the cobalt–
* Corresponding authors. Fax: +39 05 545 73372 (A. Dei), Fax: +33 0 540 002649 (J.-F. Le´tard). E-mail addresses: andrea.dei@unifi.it (A. Dei), claudio.sangregorio @unifi.it (C. Sangregorio), [email protected] (J.-F. Le´tard).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.08.026
dioxolene complexes undergoing valence tautomerism [4–6]. Photoinduced spin state interconversion in spin crossover iron(II) complexes has been described since many years [7–9]. Transient interconversions were also observed at room temperature for valence tautomeric complexes [10–12], but recently it was also found that ls-CoIII(N-N)(Cat)(SQ) complexes (N-N = diimine ligand, Cat and SQ = catecholato and semiquinonato forms of 3,5-di-tert-butyl-catechol) undergoes transitions to the hs-CoII(N-N)(SQ)2 isomer after illumination at low temperature [13–16]. We have recently observed that the same phenomenon occurs for a dinuclear [{Co(CTH)}2(DHBQ)](PF6)3 complex (CTH = DL -5, 7,7,12,14,14-9,10-hexamethyl-1,4,8,11-tetraazacyclotetradecane and DHBQ = deprotonated 2,5-di-hydroxy1,4-benzoquinone) [17]. In this work, our interest is focussed on the 1:1 cobalt-odioxolene complexes [Co(CTH)(Phendiox)]PF6 Æ (x Æ solvent) (Phendiox = semiquinonato (PhenSQ) or catecholato (PhenCat) forms of 9,10-dioxophenantherene, x Æ solvent = H2O, 1.5 CH2Cl2, 0.5 C6H5CH3), which have
C. Carbonera et al. / Chemical Physics Letters 396 (2004) 198–201
been shown to exhibit in solid-state a temperature and a pressure driven valence tautomeric transitions assisted by an intramolecular electron transfer process from the ligand to the metal ion [18,19]. These complexes are the simplest cobalt-o-dioxolene complexes undergoing valence tautomerism. The question now is to see if the same effect can be light-induced at low temperature. To reach this goal we have selected the [Co(CTH)(Phendiox)]PF6 Æ H2O complex whose formula is depicted in Fig. 1. The magnetic susceptibility measurements of this complex were carried out by using a SQUID magnetometer operating at 2 T in the 10–350 K temperature range and is also reported in Fig. 1 as high spin fraction, cHS vs. temperature curve. As previously reported [18], a gradual thermal transition without hysteresis occurs at T1/2 = 253 K. At low temperature the system is diamagnetic while at 340 K the magnetic response is paramagnetic with a vMT product close to 2.5 cm3 mol1 K. These values are, in fact, consistent with a change of charge distribution of the [Co(CTH)(Phendiox)]+ cation from a 3d(p)6–(p*)2 characterising the low-spin cobalt(III)–catecholato complex to the 3d(p)5(r*)2–(p*)1 of the high-spin cobalt(II)– semiquinonato one. This physical switching process can be also followed by recording the reflectivity signal vs. the temperature, as illustrated on Fig. 1. The analysis has been performed using directly the powdered sample without any dispersion in a matrix. The reflectivity signal was recorded at 900 ± 5 nm using our home-build set-up which allows to collect both the reflectivity spectra in the 400–1000 nm range at a given temperature and to follow the tem-
0.9
O
HS Fraction
N N 0.6
N
Co N
199
perature dependence of the signal in the 5–290 K range at a selected wavelength (±5 nm). The recorded signal perfectly follows the thermal transition observed with magnetic measurements. Along this line, Fig. 2 presents some selected absorption spectra in the 550–950 nm region recorded on powdered sample. The UV region of the spectrum is not showed as the signal is totally saturated by the strong absorptions of both the catecholato and semiquinonato forms of the dioxolene ligand. Interestingly, in the visible range the spectral features of the two spin state isomers are significantly different. At room temperature, the spectrum is mainly constituted by a broad band at ca. 700 nm which can be assigned to MLCT transitions of the cobalt(II)–semiquinonato chromophore [20] (Fig. 2). The intensity of this transition decreases as the temperature decreases and at 150 K only a weak broad band at ca. 880 nm remains. This band can be assigned to the first LMCT transition characterising the cobalt(III)–catecholato chromophore [21]. Its broadness and low intensity are consistent with the proposed symmetry forbidden charge transfer from the p* (HOMO) ligand orbital to the empty 3d(r*) orbital of the metal ion. In regard to the weak LMCT transition found at low temperature, we firstly investigated the existence of a photoinduced phenomenon by using a surface detection, with our reflectivity set-up. Fig. 1 also reports the temperature dependence of the high spin fraction derived from the reflectivity signal at 900 nm. Some selected spectra under irradiation are also presented in Fig. 2. Clearly, the spectrum found at 10 K under irradiation is very similar to the one obtained at room temperature, suggesting an almost quantitative photoexcitation at the sample surface. Thus, as usually observed in all the systems undergoing LIESST effect [3], the efficiency of the photoinduction process decreases on increasing temperature, as the activated region is reached, and in the practice vanishes around 70 K.
O 1.0
0.3
0.0 0
50
100
150
200
250
300
350
Temperature / K Fig. 1. Schematic view of the cationic part of complex 2 and of the temperature dependence of the high spin fraction derived from the vMT vs. T curves measured before (full triangles) and after (empty circles) irradiation at 406–415 nm. Empty triangles represent the high spin fraction increase with time while irradiating the sample at the lowest temperature. The small bump at 45 K on the irradiated curve is due to a small amount of oxygen. Empty squares represent the high spin fraction vs. temperature curve obtained from the reflectivity measurement at 900 ± 5 nm (see text for details).
Absorbance / u.a.
0.8
280K
0.6
10K
240K 20K
220K
0.4
200K 40K
0.2 160K
80K
0.0 550
600
650
700
750
800
850
900
950
Wavelenghth/ nm
Fig. 2. Reflectance spectra of 2 recorded at different temperatures between 10 and 280 K.
200
C. Carbonera et al. / Chemical Physics Letters 396 (2004) 198–201
The possibility of observing a light-induced transition in the bulk material was also investigated. For this purpose, the sample was placed inside the coil of a SQUID magnetometer and then irradiated by using an optical fibre connected to an optical source. The wavelengths used were those of a Krypton Laser (498, 647.1–676.4, 752.5–799.3 nm) and of a Diode Laser (830, 980 nm). In order to optimise the light absorption, a thin layered sample of the compound was used and its weight was obtained by scaling its thermal valence tautomeric transition curve measured without irradiation with that obtained using a heavier and accurately weighted sample of the same compound. For each selected wavelength, the level of photoexcitation was carefully investigated as a function of the irradiation time necessary to reach the photostationary limit, (usually less than 1–2 h) and/or the power intensity (up to 10 mW cm2). As expected from the absorption spectra, the highest photoinduced effect in the investigated range was observed by irradiating the sample at 980 nm. The level of photoexcitation, according to the ls-CoIII-(CTH)(phenCat) ! hs-CoII-(CTH)(phenSQ) process, in bulk was estimated at around 10%. Such a nonquantitative photoexcitation in bulk is not unusual. There exist in fact many reasons for explaining this finding. It is often the consequence of a strong opacity of the sample, which prevents the penetration of light. In our case we think that the low intensity of the charge transfer band around 980 nm may firstly play a primary role in determining this low efficiency. To test this idea, we performed an irradiation at 406.7–415.4 nm where more intense LMCT transitions of the cobalt(III)–catecholato complex are expected [21]. Interestingly, in agreement with our expectations, the photoexcitation was found to increase, and reached a value up to 19%. A second important factor to consider, with respect to the incomplete level of photoexcitation, is the relaxation behaviour. Fig. 3 reports the kinetic curve recorded at 10 K. The 0.20 β
γHS(t)= γHS(0)exp(-(t/τ) ) τ = 19975 ± 226 s β = 0.415 ± 0.004
HS fraction
0.15
2
χ = 5.4274e-6 0.10
0.05
0.00 0
10000
20000
30000
time / s Fig. 3. Relaxation curve after illumination at 10 K. The best fit is obtained with a stretched exponential decay (see text for details).
shape of the curve is typical to a stretched exponential decay with a fast component at earlier times and a long decay process at infinite times. Such a stretched exponential behaviour has been already reported for disordered systems such as spin glasses [22], for an iron(II) spin crossover complex dispersed into a polymer matrix [23] or organised into a surface by a Langmuir–Blodgett technique [24], and also, very recently, for a valence tautomeric Co compound [25]. In our fitting procedure, cHS(t) = cHS(0)exp[(t/s)b], the average relaxation time is given by s and b is related to the width of the distribution. From the best parameters found at 10 K, s 2 · 105 s and b 0.4, it is somewhat clear that the distribution of relaxation rates is very large. The existence of this large amount of short relaxation rates may explain the limit reached at the photostationary point, which is an equilibrium between the photoexcitation and the relaxation processes. Fig. 1 displays also the temperature evolution of the high spin fraction deduced from the measurement of magnetic susceptibility after irradiation of the sample at 10 K until saturation was reached. The light was then switched off and the temperature was slowly increased. This procedure was recently introduced as an easy way to test the possibility for a material of retaining the photoinduced excited state. Indeed the procedure allows the evaluation of the critical temperature TLIESST, i.e., the temperature at which the half of the molecules which have been excited upon irradiation remain in the metastable state [26]. The investigation of the photoinduced properties of more than 60 compounds led Le´tard et al. [27,28] to observe a linear correlation between TLIESST and T1/2, TLIESST = T0 0.3T1/2, where T1/2 is the critical temperature characterising the entropy driven transition. In the case of our compound, the TLIESST value is estimated around 35 K, and since the temperature of the valence tautomeric equilibrium is 253 K, it means that the compound lies on the T0 = 100 K line [28]. Interestingly, the dinuclear [{Co(CTH)}2(DHBQ)](PF6)3 complex, characterised by a TLIESST of around 40 K and a temperature of the valence tautomeric equilibrium of 253 K, is also around the same T0 = 100 K line. Similarly, the Co complexes previous described by Sato et al. [25] and Varret et al. [16] are also around this line. We will have in the future to understand this interesting tendency. This may give some important information for identifying the key factors which allow to stabilise the lifetime of the photoinduced metastable state. This open challenge is essential for the future development of molecular optical switches. In conclusion, this work shows that also the simplest cobalt-o-dioxolene undergoing valence tautomeric transition show a photomagnetic effect which involves an intramolecular one-electron transfer process between the catecholato ligand and the metal ion acceptor with a spontaneous change in spin state from the diamagnetic cobalt(III) (S = 0) to high-spin cobalt(II) (S = 3/2).
C. Carbonera et al. / Chemical Physics Letters 396 (2004) 198–201
Acknowledgements The authors thank the CNRS, the COST program (D14-WG 0011-01), the CNRS/DFG bilateral project, MOLNANOMAG HPRN-CT-1999-00012, MIUR for PRIN 2003 and FIRB 2001 projects, and QUEMOLNA, FP6-504880 for financial support. References [1] O. Kahn, J.P. Launay, Chemtronics 3 (1988) 140. [2] P. Gu¨tlich, A. Hauser, H. Spiering, Angew. Chem., Int. Ed. Engl. 33 (1994) 2024. [3] P. Gu¨tlich, Y. Garcia, T. Woike, Coord. Chem. Rev. 839 (2001) 219. [4] C.G. Pierpont, Coord. Chem. Rev. 99 (2001) 219. [5] P. Gu¨tlich, A. Dei, Angew. Chem., Int. Ed. Engl. 36 (1997) 2734. [6] D.A. Shultz, in: J.S. Miller, M. Drillon (Eds.), Magnetism: Molecules to Materials, Wiley–VCH, Weinheim, 2001, p. 281. [7] S. Decurtins, P. Gu¨tlich, C.P. Ko¨hler, H. Spiering, A. Hauser, Chem. Phys. Lett. 105 (1984) 1. [8] S. Decurtins, P. Gu¨tlich, K.M. Hasselbach, A. Hauser, Inorg. Chem. 24 (1985) 2174. [9] A. Hauser, Chem. Phys. Lett. 124 (1986) 543. [10] D.M. Adams, B. Li, J.D. Simon, D.N. Hendrickson, Angew. Chem., Int. Ed. Engl. 32 (1993) 880. [11] D.M. Adams, D.N. Hendrickson, J. Am. Chem. Soc. 118 (1996) 11515. [12] F.V.R. Neuwahl, R. Righini, A. Dei, Chem. Phys. Lett. 352 (2002) 408.
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[13] O. Sato, S. Hayami, Z.-Z. Gu, K. Takahashi, R. Nakajima, A. Fujishima, Chem. Phys. Lett. 355 (2002) 169. [14] O. Sato, S. Hayami, Z.-Z. Gu, R. Saki, R. Nakajima, A. Fujishima, Chem. Lett. (2001) 26. [15] O. Sato, S. Hayami, Y, Einaga, Z.-Z. Gu, Bull. Chem. Soc. Jpn. 76 (2003) 443. [16] F. Varret, M. Nogues, A. Goujon, in: J.S. Miller, M. Drillon (Eds.), Magnetism: Molecules to Materials, Wiley–VCH, Weinheim, 2001, p. 258. [17] C. Carbonera, A. Dei, J.-F. Le´tard, C. Sangregorio, L. Sorace, Angew. Chem., Int. Ed. Engl. 43 (2004) 3136. [18] A. Caneschi, A. Dei, F. Fabrizi de Biani, P. Gu¨tlich, V. Ksenofontov, G. Levchenko, A. Hoefer, F. Renz, Chem. Eur. J. 7 (2001) 3924. [19] O. Cador, A. Dei, C. Sangregorio, Chem. Commun. (2004) 625. [20] A. Caneschi, A. Dei, D. Gatteschi, V. Tangoulis, Inorg. Chem. 41 (2002) 3508. [21] C. Benelli, A. Dei, D. Gatteschi, L. Pardi, Inorg. Chim. Acta 163 (1989) 99. [22] R.V. Chamberlin, G. Mozurkewich, R. Orbach, Phys. Rev. Lett. 52 (1984) 867. [23] A. Hauser, J. Adler, P. Gu¨tlich, Chem. Phys. Lett. 152 (1988) 468. [24] J.-F. Le´tard, O. Nguyen, H. Soyer, C. Mingotaud, P. Delhae`s, O. Kahn, Inorg. Chem. 38 (1999) 3020. [25] O. Sato, S. Hayami, Z.-Z. Gu, K. Takahashi, R. Nakajima, K. Seki, A. Fujishima, J. Photochem. Photobiol. A: Chem. 149 (2002) 111. [26] J.-F. Le´tard, P. Guionneau, L. Rabardel, J.A.K. Howard, A.E. Goeta, D. Chasseau, O. Kahn, Inorg. Chem. 37 (1998) 4432. [27] J.-F. Le´tard, L. Capes, G. Chastanet, N. Moliner, S. Le´tard, J.-A. Real, O. Kahn, Chem. Phys. Lett. 313 (1999) 115. [28] S. Marcen, L. Lecren, L. Capes, H.A. Goodwin, J.-F. Le´tard, Chem. Phys. Lett. 358 (2002) 87.
Optically switchable behaviour of a dioxolene adduct of a cobalt-macrocycle complex Chiara Carbonera a, Andrea Dei
b,*
, Claudio Sangregorio b, Jean-Franc¸ois Le´tard
a,*
a
b
Groupe des Sciences Mole´culaires, Institut de Chimie de la Matie`re Condense´e de Bordeaux, UPR CNRS No. 9048, 87 Av. du Doc. A. Schweitzer, Universite´ Bordeaux 1, F-33608 Pessac, France Dipartimento di Chimica, INSTM Research Unit and LAMM, Universita` di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino (Firenze), Italy Received 21 April 2004; in final form 28 July 2004
Abstract Reflectivity and magnetic measurements show that Co(CTH)(Phendiox)]PF6 Æ H2O (CTH = DL -5,7,7,12,14,14-9,10-hexamethyl1,4,8,11-tetraazacyclotetradecane and Phendiox = semiquinonato or catecholato forms of 9,10-dioxophenantherene) undergoes valence tautomeric interconversion cobalt(III)–catecholato to cobalt(II)–semiquinonato upon light irradiation. This compound is the simplest cobalt-o-dioxolene complex undergoing this transition. The critical temperatures of the photoinduced transition and of the entropy driven transition seem to be related by the same empirical relationships observed for iron(II) spin crossover compounds. 2004 Elsevier B.V. All rights reserved.
The interest of the scientific community towards molecular materials showing properties of potential application as electronic devices is continuously growing. Particular attention is devoted to molecules exhibiting electronic bistability, i.e., molecules which may exist in two different electronic states depending on the external parameters [1]. As a consequence the possibility of inducing a reversible change of the electronic state by varying an appropriate parameter, like temperature, pressure, etc., implies the possibility of information storage at molecular level [2]. Among the different classes of bistable molecules to date investigated, there is no doubt that those showing electronic states with different magnetic properties are the most attractive. They are the spin crossover metal complexes [2,3] and the cobalt–
* Corresponding authors. Fax: +39 05 545 73372 (A. Dei), Fax: +33 0 540 002649 (J.-F. Le´tard). E-mail addresses: andrea.dei@unifi.it (A. Dei), claudio.sangregorio @unifi.it (C. Sangregorio), [email protected] (J.-F. Le´tard).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.08.026
dioxolene complexes undergoing valence tautomerism [4–6]. Photoinduced spin state interconversion in spin crossover iron(II) complexes has been described since many years [7–9]. Transient interconversions were also observed at room temperature for valence tautomeric complexes [10–12], but recently it was also found that ls-CoIII(N-N)(Cat)(SQ) complexes (N-N = diimine ligand, Cat and SQ = catecholato and semiquinonato forms of 3,5-di-tert-butyl-catechol) undergoes transitions to the hs-CoII(N-N)(SQ)2 isomer after illumination at low temperature [13–16]. We have recently observed that the same phenomenon occurs for a dinuclear [{Co(CTH)}2(DHBQ)](PF6)3 complex (CTH = DL -5, 7,7,12,14,14-9,10-hexamethyl-1,4,8,11-tetraazacyclotetradecane and DHBQ = deprotonated 2,5-di-hydroxy1,4-benzoquinone) [17]. In this work, our interest is focussed on the 1:1 cobalt-odioxolene complexes [Co(CTH)(Phendiox)]PF6 Æ (x Æ solvent) (Phendiox = semiquinonato (PhenSQ) or catecholato (PhenCat) forms of 9,10-dioxophenantherene, x Æ solvent = H2O, 1.5 CH2Cl2, 0.5 C6H5CH3), which have
C. Carbonera et al. / Chemical Physics Letters 396 (2004) 198–201
been shown to exhibit in solid-state a temperature and a pressure driven valence tautomeric transitions assisted by an intramolecular electron transfer process from the ligand to the metal ion [18,19]. These complexes are the simplest cobalt-o-dioxolene complexes undergoing valence tautomerism. The question now is to see if the same effect can be light-induced at low temperature. To reach this goal we have selected the [Co(CTH)(Phendiox)]PF6 Æ H2O complex whose formula is depicted in Fig. 1. The magnetic susceptibility measurements of this complex were carried out by using a SQUID magnetometer operating at 2 T in the 10–350 K temperature range and is also reported in Fig. 1 as high spin fraction, cHS vs. temperature curve. As previously reported [18], a gradual thermal transition without hysteresis occurs at T1/2 = 253 K. At low temperature the system is diamagnetic while at 340 K the magnetic response is paramagnetic with a vMT product close to 2.5 cm3 mol1 K. These values are, in fact, consistent with a change of charge distribution of the [Co(CTH)(Phendiox)]+ cation from a 3d(p)6–(p*)2 characterising the low-spin cobalt(III)–catecholato complex to the 3d(p)5(r*)2–(p*)1 of the high-spin cobalt(II)– semiquinonato one. This physical switching process can be also followed by recording the reflectivity signal vs. the temperature, as illustrated on Fig. 1. The analysis has been performed using directly the powdered sample without any dispersion in a matrix. The reflectivity signal was recorded at 900 ± 5 nm using our home-build set-up which allows to collect both the reflectivity spectra in the 400–1000 nm range at a given temperature and to follow the tem-
0.9
O
HS Fraction
N N 0.6
N
Co N
199
perature dependence of the signal in the 5–290 K range at a selected wavelength (±5 nm). The recorded signal perfectly follows the thermal transition observed with magnetic measurements. Along this line, Fig. 2 presents some selected absorption spectra in the 550–950 nm region recorded on powdered sample. The UV region of the spectrum is not showed as the signal is totally saturated by the strong absorptions of both the catecholato and semiquinonato forms of the dioxolene ligand. Interestingly, in the visible range the spectral features of the two spin state isomers are significantly different. At room temperature, the spectrum is mainly constituted by a broad band at ca. 700 nm which can be assigned to MLCT transitions of the cobalt(II)–semiquinonato chromophore [20] (Fig. 2). The intensity of this transition decreases as the temperature decreases and at 150 K only a weak broad band at ca. 880 nm remains. This band can be assigned to the first LMCT transition characterising the cobalt(III)–catecholato chromophore [21]. Its broadness and low intensity are consistent with the proposed symmetry forbidden charge transfer from the p* (HOMO) ligand orbital to the empty 3d(r*) orbital of the metal ion. In regard to the weak LMCT transition found at low temperature, we firstly investigated the existence of a photoinduced phenomenon by using a surface detection, with our reflectivity set-up. Fig. 1 also reports the temperature dependence of the high spin fraction derived from the reflectivity signal at 900 nm. Some selected spectra under irradiation are also presented in Fig. 2. Clearly, the spectrum found at 10 K under irradiation is very similar to the one obtained at room temperature, suggesting an almost quantitative photoexcitation at the sample surface. Thus, as usually observed in all the systems undergoing LIESST effect [3], the efficiency of the photoinduction process decreases on increasing temperature, as the activated region is reached, and in the practice vanishes around 70 K.
O 1.0
0.3
0.0 0
50
100
150
200
250
300
350
Temperature / K Fig. 1. Schematic view of the cationic part of complex 2 and of the temperature dependence of the high spin fraction derived from the vMT vs. T curves measured before (full triangles) and after (empty circles) irradiation at 406–415 nm. Empty triangles represent the high spin fraction increase with time while irradiating the sample at the lowest temperature. The small bump at 45 K on the irradiated curve is due to a small amount of oxygen. Empty squares represent the high spin fraction vs. temperature curve obtained from the reflectivity measurement at 900 ± 5 nm (see text for details).
Absorbance / u.a.
0.8
280K
0.6
10K
240K 20K
220K
0.4
200K 40K
0.2 160K
80K
0.0 550
600
650
700
750
800
850
900
950
Wavelenghth/ nm
Fig. 2. Reflectance spectra of 2 recorded at different temperatures between 10 and 280 K.
200
C. Carbonera et al. / Chemical Physics Letters 396 (2004) 198–201
The possibility of observing a light-induced transition in the bulk material was also investigated. For this purpose, the sample was placed inside the coil of a SQUID magnetometer and then irradiated by using an optical fibre connected to an optical source. The wavelengths used were those of a Krypton Laser (498, 647.1–676.4, 752.5–799.3 nm) and of a Diode Laser (830, 980 nm). In order to optimise the light absorption, a thin layered sample of the compound was used and its weight was obtained by scaling its thermal valence tautomeric transition curve measured without irradiation with that obtained using a heavier and accurately weighted sample of the same compound. For each selected wavelength, the level of photoexcitation was carefully investigated as a function of the irradiation time necessary to reach the photostationary limit, (usually less than 1–2 h) and/or the power intensity (up to 10 mW cm2). As expected from the absorption spectra, the highest photoinduced effect in the investigated range was observed by irradiating the sample at 980 nm. The level of photoexcitation, according to the ls-CoIII-(CTH)(phenCat) ! hs-CoII-(CTH)(phenSQ) process, in bulk was estimated at around 10%. Such a nonquantitative photoexcitation in bulk is not unusual. There exist in fact many reasons for explaining this finding. It is often the consequence of a strong opacity of the sample, which prevents the penetration of light. In our case we think that the low intensity of the charge transfer band around 980 nm may firstly play a primary role in determining this low efficiency. To test this idea, we performed an irradiation at 406.7–415.4 nm where more intense LMCT transitions of the cobalt(III)–catecholato complex are expected [21]. Interestingly, in agreement with our expectations, the photoexcitation was found to increase, and reached a value up to 19%. A second important factor to consider, with respect to the incomplete level of photoexcitation, is the relaxation behaviour. Fig. 3 reports the kinetic curve recorded at 10 K. The 0.20 β
γHS(t)= γHS(0)exp(-(t/τ) ) τ = 19975 ± 226 s β = 0.415 ± 0.004
HS fraction
0.15
2
χ = 5.4274e-6 0.10
0.05
0.00 0
10000
20000
30000
time / s Fig. 3. Relaxation curve after illumination at 10 K. The best fit is obtained with a stretched exponential decay (see text for details).
shape of the curve is typical to a stretched exponential decay with a fast component at earlier times and a long decay process at infinite times. Such a stretched exponential behaviour has been already reported for disordered systems such as spin glasses [22], for an iron(II) spin crossover complex dispersed into a polymer matrix [23] or organised into a surface by a Langmuir–Blodgett technique [24], and also, very recently, for a valence tautomeric Co compound [25]. In our fitting procedure, cHS(t) = cHS(0)exp[(t/s)b], the average relaxation time is given by s and b is related to the width of the distribution. From the best parameters found at 10 K, s 2 · 105 s and b 0.4, it is somewhat clear that the distribution of relaxation rates is very large. The existence of this large amount of short relaxation rates may explain the limit reached at the photostationary point, which is an equilibrium between the photoexcitation and the relaxation processes. Fig. 1 displays also the temperature evolution of the high spin fraction deduced from the measurement of magnetic susceptibility after irradiation of the sample at 10 K until saturation was reached. The light was then switched off and the temperature was slowly increased. This procedure was recently introduced as an easy way to test the possibility for a material of retaining the photoinduced excited state. Indeed the procedure allows the evaluation of the critical temperature TLIESST, i.e., the temperature at which the half of the molecules which have been excited upon irradiation remain in the metastable state [26]. The investigation of the photoinduced properties of more than 60 compounds led Le´tard et al. [27,28] to observe a linear correlation between TLIESST and T1/2, TLIESST = T0 0.3T1/2, where T1/2 is the critical temperature characterising the entropy driven transition. In the case of our compound, the TLIESST value is estimated around 35 K, and since the temperature of the valence tautomeric equilibrium is 253 K, it means that the compound lies on the T0 = 100 K line [28]. Interestingly, the dinuclear [{Co(CTH)}2(DHBQ)](PF6)3 complex, characterised by a TLIESST of around 40 K and a temperature of the valence tautomeric equilibrium of 253 K, is also around the same T0 = 100 K line. Similarly, the Co complexes previous described by Sato et al. [25] and Varret et al. [16] are also around this line. We will have in the future to understand this interesting tendency. This may give some important information for identifying the key factors which allow to stabilise the lifetime of the photoinduced metastable state. This open challenge is essential for the future development of molecular optical switches. In conclusion, this work shows that also the simplest cobalt-o-dioxolene undergoing valence tautomeric transition show a photomagnetic effect which involves an intramolecular one-electron transfer process between the catecholato ligand and the metal ion acceptor with a spontaneous change in spin state from the diamagnetic cobalt(III) (S = 0) to high-spin cobalt(II) (S = 3/2).
C. Carbonera et al. / Chemical Physics Letters 396 (2004) 198–201
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