Thermal and light-induced spincrossover in iron( II) complexes-new perspectives in optical storage

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1 Pure & Appl. Chem., Vol. 6, No., pp , 989. Printed in Great 989 IUPAC Thermal and light-induced spincrossover in iron( II) complexes-new perspectives in optical storage Philipp Gutlich and Andreas Hauser Institut fur Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universitat, D-600 Mainz/FRG Abstract - Recently, we have discovered a fascinating photophysical effect in spincrossover complexes of iron(i): Light-Induced Excited Spin State Trapping (LIESST). At sufficiently low temperatures, the low spin state ('Alg) can be (quantitatively) converted to the high spin state ( T ) by 2g irradiating the sample into the 'A4 T d-d absorption band (-40 nm). The metastable HS state has a practically infinite lifetime. It relaxes back to the LS state upon heating the sample. It can also be converted back to the LS state by irradiating into the T24 E band (-80 nm). The system behaves like an optical switch. Kinetic studies on metal-diluted crystals show that the thermal relaxation is cooperative in nature in the solid state. The relative positioning - horizontally and vertically - of the two spin state potential wells is crucial for the lifetime of the metastable HS state. INTRODUCTION Temperature dependent spin transition (spincrossover) is a well-established phenomenon in the coordination chemistry of first-row transition elements. Many examples are known, e.g. for iron(i) compounds, for which the low spin 'A (0 ), "LS"+high spin ( T (0 ), "HS") tran- g h 2g h sition has been extensively studied employing various physical techniques (refs. -3). In the course of our investigations of thermally driven spin transitions in solid state ferrous systems we have, recently and quite accidentally, observed a fascinating photophysi- cal effect: If the sample is irradiated at sufficiently low temperatures with green light into the A-b T ligand field absorption band, the thermodynamically stable LS state ( A ) can be converted quantitatively to the metastable HS state ( T2), where the system remains trapped with a practically infinite lifetime. We have called this phenomenon "Light-Induced Excited Spin State Trapping (LIESST)" (refs. 4,). The metastable HS state formed by LIESST in a solid can relax thermally to the stable LS state. It has also been demonstrated that a light-induced HS+LS reconversion can be effec- ted by irradiating the sample with red light ( X = 80 nm) into the T2* E band (ref. 6). 2 LIESST IN [Fe(l?t~)~lX~ (X=BF4, C04) CRYSTALS The first example where we have observed the LIESST effect is [Fe(ptz)6](BF4)2 (ptz = l-pro- pyltetrazole) (ref. 4). This complex compound is known to exhibit a thermally driven L SdHS transition with a hysteresis of ca. 7 K near 30 K (refs. 7,8). The spin transition is accom- panied by a dramatic colour change from white (HS) to purple (LS). At 273 K, there is only one absorption band at around 2.20 cm-l arising from the spin-allowed T2-+ E transition in the BS molecules (s. Fig. ). At 8 K, this absorption band has completely vanished and is 849

2 80 P. GUTLICH AND A. HAUSER Fig.. Single-crystal absorption spectra (unpolarized light propagating //c) of [Fe(ptz)6] (BF4)2 before bleaching (b.b.) at 273 K and 8 K, and after bleaching (a.b.) for 2 min with white light (Xe arc lamp) at 8 K (from ref. ). replaced by the two spin-allowed transitions 'Aid 'T (8.400 cm-l) and 'AdLT2 (26.60 cm-l) in the LS molecules (ref. ). After bleaching the crystal with white light (Xe arc lamp, 0 W, -2 min) at 8 K, the typical HS spectrum is again obtained. At around 0 K, the trapped metastable HS state does not decay noticeably within 24 hours. Relaxation back to the thermodynamically stable LS state occurs only if the temperature is raised to above 0 K. Fie have also followed (ref. ). If the sample is irradiated at T < 0 K, values for peff are obtained. typical for HS-Fe(I) As the lifetime of the metastable HS state is extremely long, the sequence of 7Fe Mossbauer spectra reproduced in Fig. 2 could easily be recorded. The Mossbauer spectra demonstrate again the light-induced LS+HS conversion at K (a+b), the thermal back relaxation HS-eLS at ca. K 30 K (e,f). r-7- loo 90 a 8ot \I I LIESST in [Fe(ptz)6] (BF4)2 by magnetic susceptibility measurements (c,d), and the well-established thermal LSpHS transition at around I I Fig. 2. Mossbauer spectra of [Fe(ptz)6] (BF~)~ (from ref.4) (a) Before bleaching (mea- suring temperature T = M K); (b) after bleaching at K (T = K); M (c) after heating to ca. K for a few minutes and subsequent cooling to T = K; M (d) after repeating cycle (c); (e) after heating to 97 K (TM = 97 K);.. v/mms-' v/mms-' (f) after heating to 48 K (TM = 48 K) The mechanism of LIESST can be explained on the basis of Fig. 3: Irradiating the cold sample induces spin-allowed transitions 'A -+ T and 'A +T (s. Fig. ). The excited spin- g g g singlet states are short-lived and can decay back to the '? ground state within nanoseconck. g There is, however, an alternative decay path, made possible by spin-orbit coupling, which leads to a population of the spin triplet states 3T and 3T (intersystem crossing). These g 2g

3 Thermal and light-induced spincrossover in iron(//) complexes 8 T2g IT9 3 T2g 3 Tls pj I NUCLEAR COORDINATE Fig. 3. Potential surface diagramm according to experimental and calculated energies of the ligand field states of [Fe(ptz)d (BF4)2 (from ref. ) again decay via intersystem crossing, either to the A g ground state or to the metastable T state. There is no *g radiative decay path from the T to the A state, and 2g g the T (HS) state remains trapped with practically infinite 2g lifetime as long as the temperature is sufficiently low so that the energy barrier between the T and the.alg po- 2g tential surfaces, which are well separated by the large difference in the metal-ligand bond length of ca (ref. 9), is not thermally overcome. The trapped HS state can be pumped back to the LS state by irradiating with red light (of ca. 80 nm) into the T + E absorption band 2g g (ref. 6). The thermal T -p A relaxation kinetics were examined 2 with neat [Fe(ptz)a (BF4)2 crystals as well as with mixed crystals ~FexZnl-x( ptz)d (BF4)2 using optical spectroscopy (ref. 0). It was found that (a) for diluted systems (x<o.l) a single-ion treatment of both the spin state equilibrium (withahhl = HHS-HLS = 0(2) cm -, 4SHL= SHS-SLs =.(2) cm- /K at T = 00 K) and the T2+A relaxation of the light-induced HS state (with Arrhenius activation energy E: = 80(30) cm-l and frequency factor A - IOs-l is appropriate, and (b) for concentrated systems (0. C x 4 ) cooperativeeffects become more and more important for both the LS+HS equilibrium and the thermal relaxation of the metastable HS state. Figures 4 and elucidate the importance of cooperative effects. These are of elastic rather than of electronic origin. Fig. 4. (a) Normalized T + A 2 relaxation curves for mixed crystals of [FexZnl-x(ptz)6](BF4)2 (x 0.) at various temperatures. (b) Arrhenius plot ln(k) vs. /T. An exponential fit (t) = -exp(-kt) describes well LS the relaxation curves in the dilute material (x cr 0.) with constant activation energy Eo = 80(30) cm-l and frequency factor a - A -0 s (from ref. 0) The "lattice expansion model" (ref. ) based on long-range elastic interactions arising from the drastic volume change AV = VHS-VLS(ref. 9) is well suited to explain the experimental

4 82 P. GUTLICH AND A. HAUSER - experimental --- calc with Linear calc from experimental relax at ion Fig.. (a) Normalized T -b A relaxation cur- 2 ves for neat crystals of [Fe(ptz)6](BF4)2 at various temperatures. The sigmoidal (rather than exponential) form of the decay curves is due to cooperative effects. The activation energy Ea is now a function of (molar LS fraction of LS molecules) as shown in (b). The solid line in (b) was obtained from a si- multaneous fit of the five measured decay curves of Fig. a. The broken line results from a linear regression (slope E = 64 cm- ; intercept E: = 797 cm-') (from refa 0) yls I.OO findings. In particular, the model predicts the activation energy Ea(PLs) to be a linear function of the fraction of LS molecules. I Current investigations on the ligand-substituted system lfe (mtz)6](bf4)2 (m = methyl), in which the iron complex molecules occupy two different crystallographic sites (ref. 2), have revealed some surprising results. (i) Only one of the two iron sites (A) undergoes thermal spin transition; the transition temperature (T 06 K) is considerably lower than in the ptz system. The other iron site (B) remains in the HS state down to 4 K. (ii) Both lattice sites A and B can be pumped back and forth between the LS and the.hs states using light of appropriate wavelengths. Site A molecules, with the LS state as ground state at T < Tc "6 K, undergoe LIESST and "Reversed LIESST" in much the same way as the ptz system. Site B molecules, with the HS state as ground state at all temperatures, are converted to the mw metastable LS state upon irradiation with red light (-80 nm); this trapped LS state has again practically infinite lifetime at ca. 20 K (ref. 2). Preliminary results from current studies of the mixed crystal system [FexNil-x(mtz)6~(C0 ) 4 2 indicate the possibility of keeping the metastable states formed by LIESST trapped up to temperatures above 00 K (ref. 2). 3 LIESST IN OTHER IRON(I) SPINCROSSOVER COMPOUNDS LIESST is not unique to[fe(ptz)6j(bf4)2 and related systems, nor is it restricted to cry- stalline materials. Since the discovery of LIESST in 984, long-lived metastable HS states have been observed in a number of iron(i) spincrossover compounds: [Fe(pi~)~]X~. sol (X: C, Br; sol:e.leoh, EtOH) (ref. ), [Fe(phen)2(SCN)d (ref. 3), [Fe(bipy),(SCN)J(ref. 4), tfe(2-nephen)~i (C04)2 (ref. ), all in the solid state, and [Fe(2-nephen)~]~' doped into poly-vinyl-acetate and Nafion foils (ref. 6). In all of these compounds the relaxation rate of the metastable HS state is smaller than 0-4s- at 0 K. is even more general, albeit the rates for (thermal) HS+LS However, the phenomenon of LIESST relaxation are often quite con- siderably larger. McGarvey and Lawthers (ref. 7) were the first to notice a rapid depopula- tion of the LS state upon pulsed laser photolysis of an iron(i) spincrossover compound in so- lution. They obtained relaxation rates of the order of 07s-l at room temperature. Recently,

5 Thermal and light-induced spincrossover in iron(//) complexes 83 Fig. 6. HS+LS temperature, ln(k) vs. /T relaxation rate as a function of Ape(mepy)2(py)(tren)](PF6)2 in PSS (from ]Cl SMeOH from lineshape analysis of 3 2 Mossbauer spectra (from ref. 9) o[fexznl-x(pic)j C2* MeOH (x r 0.03%) from optical spectra (from ref. 2) /T [/K Xie and Hendrickson (ref. 8) investigated the excited state lifetimes of [Fe(mepy)2(py)(tren)](PF6)2 doped into a poly-styrene sulfonate (PSS) matrix. They found temperature independent relaxation rates of - 04s-l below -80 K, evidence for nuclear tunneling (s. Fig. 6). Above -00 K, an activated relaxation process became dominant. Recent work on [Fe(pi~)~]Cl~aMeOH (ref. 9) and [FexZnl-x(pic)JC2.MeOH (x zz 0.03%) (ref. 2) shows a similar high temperature region with an Arrhenius type relaxation behaviour and a more or -2 - less temperature independent relaxation rate of -0 s below -0 K (s. Fig. 6). 4 DISCUSSION The fact that LIESST is observed both in the dilute materials as well as in [Fe(2-mephen)j2' doped into amorphous matrices shows that it is basically a single ion phenomenon. Our in- termediate case of [Fe(pic)3]C2.MeOH with a relaxation rate of ~lo-~s-l at 0 K suggests that there is a smooth transition from the fast relaxation and tunnelling at low temperatures as observed by Xi and Hendrickson in [Fe(mepy)2(py)(tren)l(PF6)2 laxation with no apparent tunnelling at low temperatures in [Fe(ptz)6](BF4)2. and the extremely slow re- The question now is, what are the parameters respmble for the difference of more than eight orders of magnitude in the low temperature relaxation rates? The theory of HS+LS relaxation of Buhks et al. (ref. 20), although making a number of drastic assumptions, is a fully quantum mecha- nical approach based on non-adiabatic multiphonon relaxation. It predicts a temperature in- dependent relaxation rate, i.e. a tunnelling process, at low temperatures and an activated process at higher temperatures, The important parameters in the theory are the energy gap AEHL, the difference in metal-ligand bondlength& between the HS and the LS states, the force constant and the vibrational frequency along the reaction coordinate. A full dis- cussion at all this would be beyond the scope at this paper. Suffice it to say that Xie and Hendrickson found a set of reasonable parameters to suit their case, and that for values of AEHL = 400 cm-', Ar = 0.2 A, fa =.8x0 dyn/cm and'fiw, = 230 cm-l (values appropriate for [Fe(ptz)6](BF4)2) we estimate a low temperature relaxation rate according to Buhks et al. (ref. 20) of ~ lo-~s-~. This is in the range needed to explain our extremely long-lived ex- cited states at least semi-quantitatively. The photophysical effects described above show that such spincrossover systems behave like an optical switch. They may therefore have the potential of future applications in optical information storage.

6 a4 P. GUTLICH AND A. HAUSER Acknowledgements We wish to thank our collaborators and students who have enthusiastically contributed to our spincrossover and LIESST research. P. Adler, T. Buchen, P. Poganiuch, A. Vef, and L. Wiehl have made available results prior to publication. Particularly thanks are extended to the Bundesministerium fur Forschung und Technologie, the Deutsche Forschungsgemeinschaft the Fonds der Chemischen Industrie, and the University of Maim for financial support. REFERENCES. H.A. Goodwin, Coord. Chem. Rev., 8, 293 (976). 2. P. Gutlich, Structure and Bonding, 44, 83 (98). 3. E. Konig, Progr. Inorg. Chem., 3, 27 (987) 4. S. Decurtins, P. Gutlich, C.P. Kohler, H. Spiering and A. Hauser, Chem. Phys. Lett., - 0, (984).. S. Decurtins, P. Gutlich, K.M. Hasselbach, A. Hauser and H. Spiering, Inorg. Chem., - 24, 274 (98). 6. A. Hauser, Chem. Phys. Lett., 24, 43 (986). 7. P.L. Franke, J.G. Haasnoot, A.P. Zuur, Inorg. Chim. Acta, 3, (982). 8. E.W. Muller, J. Ensling, H. Spiering and P. Gutlich, Inorg. Chem., 2, 074 (983). 9. M. Mikami, M. Konno and Y. Saito, Chem. Phys. Lett., 63, 66 (979). 0. A. Hauser, P. Gutlich and H. Spiering, Inorg. Chem., 2, 424 (986).. H. Spiering, E. Meissner, H. Koppen, E.W. Miiller and P. Giitlich, Chem. Phys., 68, 6 (982) to be published. 3. S. Decurtins, P. Gutlich, C.P. Kohler and P. Spiering, J. Chem. SOC. Chem. Commun., 430 (98). 4. R.H. Herber, Inorg. Chem., 26, 73 (987).. P. Poganiuch and P. Giitlich, Inorg. Chem., 6, 4 (987). 6. A. Hauser, J. Adler and P. Gutlich, submitted to Chem. Phys. Lett. 7. J.J. McGarvey and I. Lawthers, J. Chem. SOC. Chem. Commun., 906 (982). 8. C.-L. Xie and D.N. Hendrickson, J. Amer. Chem. SOC., 09, 698 (987). 9. P. Adler, Dissertation, University of Mainz, Chemistry Department, E. Buhks, G. Navon, M. Bixon and J. Jortner, J. Amer. Chem. SOC., 02, 298 (980).

Article. Reference. Quasi-static nature of the light induced thermal hysteresis in [Fe(ptz)6](BF4)2 spin-transition solid. JEFTIC, Jelena, et al.

Article. Reference. Quasi-static nature of the light induced thermal hysteresis in [Fe(ptz)6](BF4)2 spin-transition solid. JEFTIC, Jelena, et al. Article Quasi-static nature of the light induced thermal hysteresis in [Fe(ptz)6](BF4)2 spin-transition solid JEFTIC, Jelena, et al. Abstract The quasi-static nature of a light induced thermal hysteresis