Valence Tautomeric Thin-Films Sérgio Mendes, Daniel Ruiz-Molina and Emilia Evangelio Institut de Ciència dels Materials de Barcelona (CSIC),Esfera UAB, 08193, Cerdanyola, Catalonia, Spain Abstract The discovery of molecular materials exhibiting magnetic bistability, some of them even magnetization relaxation phenomena characteristic of nanodomain particles, has attracted great deal of interest in the last few years. The current work was conducted at Institut Català de Nanotecnologia (ICN) in the NanoSFun group particularly in valence tautomeric compounds. Besides the bistability capacity, these molecular materials are normally soluble in common solvents providing advantages in the processing for potential application. Homogeneous, readily amenable to variations in their chemical and topological structure. These complexes show high potential to be used as molecular magnets to store information data as switches or thermochromic devices. One of the first descriptions of valence tautomerism, based on quinone ligands, and an excellent example of charge distribution sensitivity, to temperature or pressure for III example, is exhibited by the cobalt bis(quinone) complex [Co (3,5-DTBCat)(3,5-DTBSQ)(bpy)]. However, the Valence Tautomeric (VT) complex used to integrate a polymeric matrix in a thin film form was III [Co (Cat-N-SQ)(Cat-N-BQ]. This complex was incorporated in poly(bisphenol A carbonate) (PBC), poly(methyl methacrylate) (PMMA) and poly(vinyl acetate) (PVA), in different concentrations (2%, 4% and 8% wt). The thin films were prepared by drop casting and were characterized by optical microscopy, UV– Vis, X-ray, optical microscopy with polarized light and confocal microscopy. Only in this way by systematic studies of the characterization we can understand the valence tautomeric material behaviour and how can we manage, to control and switch their properties on the transition from a crystal to a surface. Keywords: Valence tautomerism / Bistability / Thermochromism / Thin films Introduction There is currently active interest in the development of molecular electronic devices that can be used for optical and/or magnetic data storage. Compounds of specific interest are bistable molecular materials having two nearly degenerated states with different optical and/or i magnetic properties [ ]. These complexes are quite sensitive to the environment so an external perturbation, such as temperature or irradiation, may lead to an interconversion between the two degenerated electronic states, settling down the conceptual basis of a molecular device. Examples of electronic labile complexes are ii iii mixed-valence,[ ] spin-crossover [ ] and valence iv tautomeric complexes [ ]. The comprehension and understanding of the characteristic features for all these complexes have been developed in parallel over the last decades, rising out a large interest in the scientific community as demonstrated by the large amount of examples developed for each case. Valence tautomeric (VT) metal complexes with at least two redox-active centres, the metal ion and an electro-active ligand, are characterized by the existence of two electronic isomers (valence tautomers) with different charge distributions, and consequently, different optical, v electric and magnetic properties [ ]. The interconversion between the different electronic isomers is accomplished by a reversible intramolecular electron transfer involving the metal ion and the redox active ligand. One of the first descriptions of valence tautomerism and an excellent example of the above mentioned charge distribution sensitivity is exhibited by the III cobalt bis(quinone) complex [Co (3,51 DTBCat)(3,5-DTBSQ)(bpy)],[ ] where 3,52DTBCat and 3,5-DTBSQ refer, respectively, to 2the catecholate (DTBCat ) and semiquinonate (DTBSQ ) forms of 3,5-di-tert-butyl-o-quinone, and bpy is 2,2’-bipyridine. In solution, the equilibrium in Figure 1 can be shifted by variations of temperature and monitored by magnetic measurements and spectroscopic techniques such as UV–VIS, NMR and EPR. 1 Figure 1: Valence Tautomerism of [CoIII(3,5-DTBCat)(3,5-DTBSQ)(bpy)]. Moreover, since valence tautomeric complexes are electronically labile, they exhibit significant vibronic interactions and therefore an appreciable sensitivity to the environment. As a consequence, intramolecular electron transfer (IET) can be induced not only by temperature variations but also by irradiation or pressure. For instance, Hendrickson et al. have reported results of the first picosecond time-resolved optical experiments on valence tautomeric vi complexes in solution [ ] and in the solid state vii viii ix after illumination at low temperatures [ , , ]. Light-induced VT has also been observed in polynuclear complexes. Examples of pressureinduced VT have also been reported although in less extent than thermally-driven or light-induced transitions, mainly due to experimental x difficulties [ ]. In these experiments, the increase of the molecular size on passing from the low to the high-spin isomer due to the population of antibonding orbitals is used to favour the lowspin isomer after application of pressure as an external stimulus. In summary, the interest for studying this family of complexes is considerable. First, they are unique model systems which provide insight into the basic factors affecting intramolecular electron transfer in coordination complexes. And second, from an applied perspective, the large changes in the optical, structural, and magnetic properties that often accompany the valence tautomeric interconversion have potential applications in bistable molecular materials and devices. By choosing the appropriate solvent and by controlling the temperature of the solution, the tautomeric equilibrium of this family of complexes can be modulated and one of the two isomers can be made to be the dominant species in solution. Moreover, there is no a direct correlation between the Tc ,critical temperature, values obtained experimentally for each solvent and their corresponding dielectric constant values. These complexes can interconvert between the VT isomers also in the solid state though with significant differences. Indeed, whereas in solution the interconversion takes place over a large temperature range, in a crystalline matrix they can interconvert cooperatively on a narrow temperature range. III interconversion in solution, the number of examples exhibiting a valence-tautomeric interconversion in the solid state is rather limited. The spin state degeneracy is not expected to vary on the different solvents under study. However, the variations on the metal-ligand bond lengths, i.e. the density of vibrational states, are expected to be strongly dependent on the surrounding matrix. The VT complex, [Co (Cat-N-SQ)(Cat-N-BQ)] (1), (see Figure 2)was selected to be incorporated in a polymeric matrix, and using temperature as external stimulus.The variations found for the behaviour of these complexes in solution and in solid state are not atypical. In fact, it is important to emphasize that whereas most of the valence tautomeric complexes thus far reported exhibit a temperature-dependent 2 Figure 2: VT equilibrium of complex 1 induced by temperature. The dispersion of the valence tautomeric complex 1, shown in Figure 2, in a polymeric matrix may lead to a dependence of the glass transition temperature, Tg, of the polymer. The increase of the structural disorder above Tg contributes to a greater volume occupied by the molecules involving volume entropy and enthalpy decrease. So, the best way to determine if the Tg is relevant for the VT phenomenon is test different polymers, with different Tg values. In this work we used three polymers as matrixes: the first one is the poly(Bisphenol A carbonate) (PBC), a thermoplastic and hydrophobic polymer, (see Figure 3) and it has a Tg about 155ºC. The second polymer is the poly(methyl methacrylate) (PMMA), a transparent thermoplastic, which has a Tg between 96-104ºC. The third polymer, PVA, poly(vinyl acetate), is a rubbery synthetic polymer, with the lower Tg value among the three polymers used in this work (29ºC). This Tg could influence the valence tautomerism (VT) phenomenon and as temperature is an external stimuli, the VT could be associated to a change in colour, known as thermochromism. There are previous examples of stimuli-sensitive polymers [xii] in thin films , though the observation of thermochromic valence tautomeric complexes in thin films is rare. This work is intended to increase the understanding of this subject. Figure 3: Repeat units of 1) PBC, 2) PVA and 3) PMMA. 2. Experimental Section 2.1. Materials and Sample Preparation. PBC, PMMA and PVA were purchased from SigmAldrich. PBC (Mw = 64000 g/mol, PDI=1.45), PMMA (Mw = 350000, PDI= 1.10, white solid) and PVA (Mw ~100,000, PDI=1.18, transparent solid). Complex 1 was obtained from the synthetic procedure. Before carrying out the optical, x-ray and confocal experiments, each sample( for each complex 1 concentration) was heated up to 140ºC(below the glass transition temperature, Tg = 155ºC). To characterize complex 1, UV–Vis was carried out in DCM (this complex has high solubility in this solvent). Complex 1 is a purple solid at room temperature and PBC is a transparent solid. Both materials were used without further purification. To prepare the thin films, in a first step, PBC was dissolved in 10 mL dichloromethane (DCM) with a concentration of 15 wt% (0,150g). In a second step, complex 1 was dissolved with the selected concentrations (2%, 4% and 8%) wt in the PBC/DCM solution. After 10 minutes testing period, the mixtures were cast onto a glass substrate. During the drop casting the glass plate was placed inside a closed chamber in order to control the initial evaporation rate of the solvent from the prepared film. After solvent evaporation, the films where characterized without being removed from the substrate. Samples with relative large plane surfaces, homogeneous and without pinholes are required. The thicknesses of the films are in the range from 50 to 100 µm. While the samples with 2% wt of complex 1 are light ciano/green, those of higher concentrations (4% and 8% wt) are increasingly purple. For the other polymers, PMMA and PVA the procedure was the same described above only with the following differences: PMMA samples with 2% wt of complex 1 are light brown and PVA samples 3 with 2% wt of complex 1 are light purple. PBC, PMMA and PVA units are shown in the Figure 3. Figure 4: (a) Absorption spectrum for complex 1 in solution during heating in the range 25 – 55ºC. (b) Colour change of complex 1 in DCM. 2.2. Methods. Electronic absorption spectra were recorded on a Perkin Elmer Lambda 35 spectrophotometer in the Laboratoire de Chimie de la Coordination (LCC-CNRS) of Toulouse. The instrument was equipped with a thermostatic cell holder that can operate between 180 and 370 K. Temperature stability was better than ±5K. Spectra were collected after the sample had been allowed to thermally equilibrate at each temperature for 10 min. In some specific experiments, Tc, the temperature at which the isomer ratio is 1:1, was deduced from the temperature at which the peaks of the low-spin and the high-spin exhibit similar intensities. The N2 was the carrier gas used in the cooling and heating programmes. X-ray powder diffraction data have been recorded on an INEL Diffractometer (DebyeScherrer geometry and CPS120 curved detector) at the Institut de Ciència dels Materials de Barcelona (ICMAB-CSIC) which collects 120º of the diffraction circle using the Kα radiation of copper. The sample was introduced in the diffractometer in 0.3 mm capillars. Optical microscopy (Zeiss Observer.z1m) was applied to investigate the structure of the complex in the different polymeric matrixes. This method is sensitive and the sample for the optical microscopy measurement is placed in the microscope plate and observed with different magnification objectives (5, 10, 20, 50 and 100x). Optical microscopy with polarized light (applied to investigate the structure and the presence of crystals) was carried out with an Optical Microscope Leica DMRB.To see with more detail the Stereoscopical LUPA Leica MZFLIII was used to. The images where obtained with the 40x objective and the samples were covered with oil before the measurement to enhance the performance of the equipment. Confocal microscopy technique used to investigate the morphology and topology of the crystal structures and the data was obtained from the Confocal Leica Systems – Laser Confocal Optical Microscope TCS SP2 AOBS and Confocal Microscope Olympus. The samples were analyzed with a 40 x objective. Figure 5: (a) Non reversible change in the visible spectrum of 2% wt complex 1 PMMA thin film. The insert indicates the appearance of complex 1 at -110 (brown) and 140 ºC (green). (b) Change in the colour of complex 1 at room temperature from brown (LS) to green at 140ºC (HS) during 1 hour, embedded in poly(methyl methacrylate) matrix. 4 3. Results and Discussion III In solution, the valence tautomerism of ls-Co (Cat-N-SQ)(Cat-N-BQ) is identified by a colour change. At 25ºC the solution is purple and with the heating up to 55ºC, the colour changes to II blue corresponding to hs-Co (Cat-N-SQ)2. At room temperature, the UV-Vis spectra of complex 1 show bands at 391 nm, 439 nm and 533 nm characteristic of the ls-Co(III) (S=1/2) tautomer. An increase of the solution temperature to 55ºC, promotes an intramolecular electron transfer from the ligand to the metal ion. In consequence of this electron transfer above 55ºC, the intensity of such bands decreases and bands at 721 nm and 797 nm characteristic of the hs-Co(II) tautomer increase in intensity (Figure 4). In solution, two isosbestic points at 590 and 856 nm are identified, demonstrating that at least two species are interconverting within the studied temperature range. The different bands aren’t attributed to any ligand-metal electronic transfer, but to electronic transitions within the ligands, modulated by the metal at large wave lengths. Even though there isn’t any band that could be associated to a ligand-metal electronic transfer. In the Figure 4, the band at 390 nm that is marked with a red symbol, is associated to the radical in the Cat-N-SQ ligand of the tautomer lsIII Co (Cat-N-SQ)(Cat-N-BQ), radical placed. This band decreases as the temperature increases, due to the tautomer conversion from ls-Co(III) to [xiii] hs-Co(II). Girgis et al. predicted that the major bands observed in the electronic spectra of complexes 1with general formula ML2, where L is Cat-N-BQ 2and/or Cat-N-SQ , are associated with electronic transitions within the ligands though modulated specially at higher wavelengths depending on the nature of the metal ion. One of them is the band centred around 390 nm, attributed to the radical character of the Cat-N2SQ ligand. This fact can provide information about the electronic distribution of the complex 1 at a specific temperature. Figure 5 shows the absorbance of the 2% wt complex 1 in PMMA III initially (T=25ºC) in the form of Co LS state, which is converted upon heating to 140ºC into II Co HS state, with well defined isosbestic points at the same wavelength 590 and 856 nm that appear in solution state (Figure 4). In addition, this thin film with 2% wt complex 1 exhibits thermochromism: with the increment of temperature from -110 to 140ºC the brown III coloured ls-Co is converted into a light green II species (Co ). The thermochromism is well demonstrated in the Figure 5.In sequence of these results, we conclude that, firstly the exhibition of the valence tautomerism, III associated to the conversion of the ls-Co into II the hs-Co with the influence of temperature, accompanied by a colour change from brown to light green. These properties are crucial for the development of new devices like thermosensors or memory devices. Figure 6: (left) X-ray diffraction patterns of PMMA with different complex 1 concentrations. (right) X-ray diffraction patterns of PBC with different complex 1 concentrations. Referring to x-ray studies the thin films with different concentrations of complex 1 were incorporated in two distinct polymeric matrixes, PBC and PMMA, were characterized to evaluate if the increase of complex 1 concentrations has any influence one the x-ray diffraction patterns (Figure 6). Some analogies were done, after looking to the Figure 6, first in PBC then in PMMA. 5 Figure 7: X-ray diffraction patterns of 50% wt complex 1 in PMMA in comparison with simulated X-ray powder diffraction pattern of complex 1 single crystal. A 50% wt complex 1 in PMMA sample was prepared to verify that the above X-ray data correspond to complex 1 crystals and not to any crystal impurity. The proof that the crystal structures match up with complex 1 crystals is in figure 7, were a association is made with the crystallographic data of complex 1 single crystal. Comparing the observed reflection peaks of figure 6 with the standard single crystal complex 1, Figure 7, confirms that the recoded pattern corresponds to some peaks, 8,42, 16,82 and 25,32º, from complex 1 single crystal data . The calculated parameters for the three peaks are [100], [200] and [300] (a,b and c are coordinates) shows a preferential growth in the film in the (h00) x plan direction. In all polymeric matrixes the increase of the complex 1 concentration influence and increase the intensity of the peaks. In addition, we find no diffraction peaks for 2% wt complex 1, two peaks for 4% wt and three peaks for 8% wt at similar 2 Theta values. Figure 8: (a)Confocal microscope image from the PMMA thin film at room temperature: projection xy. (b)Optical microscope image with polarized light from the PMMA thin film at room temperature. The presence of crystals in the thin films is therefore confirmed not only by the optical microscope images but also by the results from X-ray data (Figures 6 and 7). To obtain additional confirmation of the presence of crystals, were used optical microscopy with polarized light. In this technique, the sample is illuminated with plane polarized light and its rotation at 0º, 90º, 180º and 270º reveals the presence of crystal structures. Thin PMMA films without the complex 1 were first characterized, to show that PMMA is an amorphous polymer. The different samples were illuminated and the polarizer rotated 0º and 270ºC to produce the results shown in Figure 9. The 2% wt complex 1 thin film in PMMA was characterized at room temperature. Like was said earlier, in the UV visible characterization, at 2% wt complex 1 there is a structure that seems like a crystal that I named as “pre-crystal” (see Figure 9). In this thin film this was the only structure detected, like the optical microscopy technique showed. The rest of the thin film there wasn’t any structures, the presence of the precrystal is a tentative of crystal formation in this case, and the complex 1 concentration isn’t enough for the crystal growth. 6 Figure 9: Confocal microscope images from the pure PMMA and PMMA with 2% wt complex 1 thin film at room temperature. Optical microscope images with polarized light from the PMMA thin film with 2% wt Complex 1 at room temperature: light polarization (left) 0º and (right) 270º. 7 Conclusion The valence tautomerism phenomenon induced by temperature only occurs with two complex 1 concentrations, 2 and 4% wt, and only in PMMA. In reality, for the 2% wt of complex 1 the increase of temperature, from room temperature to 120ºC, leads to the unambiguous observation of the thermochromism effect, associated to the valence tautomerism phenomenon. There’s an abrupt colour change, from brown to light green, corresponding, III respectively, to the LS state [Co (Cat-N-SQ)(Cat-NII BQ)] and the HS state [Co (Cat-N-SQ)2]. This is due to the fact that at the 2% wt of complex 1 in 15% wt PMMA the thin film behaves like a “solution” due to segregation phase phenomenon, rarefied of solid structures, the molecules are disorganized and there’s an entropy gain. The increase of crystal structures number and complex 1 concentration in the polymeric matrix blocks the valence tautomerism phenomenon. The crystal structures present in the 15% wt PMMA films are really complex 1 crystals that preferentially grow in the plan direction (h00). For complex 1 concentrations higher then 4% wt in 15% wt PMMA there’s no valence tautomeric phenomenon associated, as the thin film starts to behave like a solid. On other hand, the valence tautomerism phenomenon also occurs in PBC for 2% complex 1 however, less significant. One of the many differences in the two polymeric matrixes, PMMA (96-104ºC) and PBC (155ºC), that could determine the valence tautomerism occurrence is the Tg. Although, to reach to the conclusion that the Tg is the main and determinant factor for the valence tautomerism, SQUID, DSC or TGA measurements should be done. References 1 [ ] P. Gütlich, Topics in Current Chemistry, Spin Crossover in Transition Metal Complexes I, II, and III. ISBN: 978-3-540-40394-4 (I), 978-3-540-40396-8 (II), 978-3-540-40395-1 (III) and references cited therein 1 [ ] E. Evangelio, D. Ruiz–Molina, Eur. J. Inorg. Chem., 15 (2005) 2957 and references cited therein. 1 [ ] a) D. N. Hendrickson, C. G. Pierpont, Top. Curr. Chem. 234 (2004) 63. b) O. Sato, J. Tao, Y. –Z. Zhang, Angew. Che. Int. Ed. 46 (2007) 2152. c) D. A. Shultz in Magnetism: Molecules to Materials, Vol. II (Eds.: J. S. Miller, M. Drillon), Wiley-VCH, Weinheim, 2001, 81-306. 1 [ ] R. M. Buchanan, C. G. Pierpont, J. Am. Chem. 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