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Yong-Sung Koo and José Ramón Galán-Mascarós*

The use of molecules for the construction of multifunctional hybrids offers unparalleled opportunities in the development of novel materials. The possibility to design the chemical and physical properties of a hybrid from tailor-made building blocks allows to deliver materials not commonly found in nature.[1–3] This approach has yielded, for example, photoactive magnets,[4–6] porous magnets,[7,8] magnetic superconductors,[9] ferromagnetic organic metals,[10] and their optically active analogs.[11] Spin crossover (SCO) compounds are a paradigmatic example of bistability at the molecular level, where the electronic state of a metal complex can be switched from a low-spin (LS) to a low-lying metastable high-spin (HS) configuration through external stimuli: thermally, under light irradiation or pressure.[12–14] In such compounds, cooperativity and memory effect may appear in bulk, when a structural phase transition is associated to the electronic transition. This arises from a dramatic change in the molecular geometry due to the greater metal to ligand distances in HS coordination, that can get over 10% longer than in LS, as observed in FeII complexes.[15,16] SCO materials with wide thermal hysteresis at and above room temperature have been proposed for applications in optical displays or magnetic devices.[17,18] The effect of the spin transition in the transport properties has been recently investigated, with particular emphasis in single molecule experiments.[19–24] The possible exploitation of the SCO switching in electronic devices, however, is difficult due to their intrinsic insulating character.[25–28] Thus, it is a dream goal to combine SCO with a conducting network. Hypothetically, the spin transition triggered by external stimuli could induce a significant change in electrical conductivity due to the associated structural transition. This hypothesis was tested in crystalline organic conductors by incorporating SCO complexes as building blocks.[29–34] Unfortunately, no synergic memory effect has been found in these materials so far. Here we report a successful alternative strategy to obtain bistable multifunctional films by the incorporation of SCO FeII compounds into a highly conducting organic polymer matrix. Dr. Y.-S. Koo, Prof. J. R. Galán-Mascarós Institute of Chemical Research of Catalonia (ICIQ) Av. Paisos Catalans 16 E-43007, Tarragona, Spain E-mail: [email protected] Prof. J. R. Galán-Mascarós Catalan Institution for Research and Advanced Studies (ICREA) Passeig Lluis Companys, 23 E-08010, Barcelona, Spain

DOI: 10.1002/adma.201402579

Adv. Mater. 2014, 26, 6785–6789

Organic conducting polymers (CPs) have reached a tremendous scientific and technological impact since their discovery in 1977.[35,36] The combination of electrical conductivity with the flexibility, light-weight, easy processing and low-cost of plastics has been very useful for their incorporation in a variety of devices, from transistors to solar cells.[37–45] Their easy preparation by chemical or electrochemical redox processes[46] has also been exploited for the fabrication of composite multifunctional materials,[47] with a secondary component that modulates or creates function, in an artificial symbiosis. This has been exploited, for example, in the development of chemical sensors,[43,48] by incorporation of a chemical receptor, where the electronic conductivity is modulated by interaction with the analyte. Regarding resistive switching,[49–51] this has been achieved by the incorporation of donor/acceptor entities in the polymer matrix, delivering films with very high intrinsic resistivity.[52–56] Polypyrrole (ppy) is probably the best studied CP due to its high stability, low-cost and commercial availability of its monomer.[43,57] As in all CPs, the transport properties in ppy depend on the degree of doping, but also on preparation, since the arrangement of the ppy fibers in space, and their interconnectivity, will determine transport.[58] Conductivity can be essentially modeled with a variable range hopping mechanism, and a short carrier mean free path.[59,60] Due to its flexibility, ppy conductivity shows a high pressure dependence. Under pressure, the distance between chains decreases, reducing the energy barriers and increasing the density of states at the Fermi level (N(EF )).[61] Typically, ppy conductivity reaches above a 40% increase under an isostatic pressure of 1.2 GPa.[62,63] Because of this, if SCO compounds are incorporated into a ppy film, the SCO volume change during the LS → HS expansion should change the internal pressure and, therefore, the conducting regime. Here we disclose a simple strategy for the preparation of SCO/ppy composite films as experimental evidence for the appearance of this synergy, where electrical switching can be triggered by a spin transition. SCO/ppy composite films with the SCO coordination polymer [Fe(trz)(Htrz)2][BF4] (1) can be prepared by a simple solution procedure.[17] Chemical oxidation of the monomer with sodium persulfate in a suspension containing 1 as a polycrystalline powder (particle size 450 ± 50 nm diameter) with a 2.5 SCO/pyrrole ratio (in weight) yields an amorphous shinny precipitate of doped ppy-covered SCO particles, that can be sintered to a thin film under isostatic pressure. The applied pressure controls the thickness of the film. Rugged ≈ 60 µm thick films are obtained applying 0.2 GPa as shown in Figure 1a. The magnetic behavior of these thin films shows identical magnetic bistability to bulk, and confirms that the SCO phenomena is preserved. We find a LS-HS transition at T1/2(LS →

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Spin Crossover Probes Confer Multistability to Organic Conducting Polymers

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Figure 1. Characterization of 1/ppy films: (a) TEM image of the film thickness; (b) Magnetic thermal hysteresis; (c) Thermal hysteresis in the conductivity data at 1 K min−1 scan rate, showing the two conducting regimes (empty circles) and the transition processes (full circles).

HS) = 396 K and T1/2(HS → LS) = 343 K, with a 53 K wide hysteresis (Figure 1b), in good agreement with reported data.[17] 1/4 σ = σ 0 × exp − ⎡⎣(T0 / T ) ⎤⎦

(1)

Transport properties were determined with the 4-point probe method (Figure 1c). The room temperature conductivity (σ) of these films is relatively high, σ = 3.2 × 10−2 S cm−1, taking into account the intergrain boundaries. When the temperature is increased, σ follows a typical semiconducting behavior. It can be modeled to a Mott variable-range hopping (VRH) mechanism (Equation (1)), as in classic charge-transport CPs, with σ0 = 1.32 S cm−1 and a Mott parameter T0 = 7.15 × 104 K. Above 387 K, σ shows an abrupt transition towards a new highly conducting phase, as result of the synergy with the spin transition. σHS reaches 5.6 × 10−2 S cm−1 at 400 K. When the sample is cooled down the films stays in the highly conducting state

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Figure 2. Characterization of 1/ppy films: (a) Difference in conductivity during a cooling (σ↓) and heating (σ↑) cycle; (b) conductivity thermal hysteresis at different scan rates.

following a VRH behavior with a distinct σ0 = 4.48 S cm−1 and T0 = 1.42 × 105 K. Below 353 K, σ reverts to the original regime. The average difference in conductivity between the heating and cooling cycles reaches over 60% (Figure 2a), which is comparable, for example, to that observed in giant magnetoresistance devices[64] although, in this case, triggered by temperature and occurring in a single composite material. This σHS/σLS ratio is also remarkable when compared with the effect of isostatic pressure which induces about a 40% increase in σ at 1.2 GPa.[61] In our 1/ppy films the electrical hysteresis loop is 43 K wide, defined by T1/2(↑) = 390 K and T1/2(↓) = 347 K. The films are robust without significant appearance of fatigue after multiple cycles. The HS and LS regimes are reliable, maintaining constant resistance at very long times (Figure S1 in the Supporting Information). We found a dynamic widening of the hysteresis

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Adv. Mater. 2014, 26, 6785–6789

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Adv. Mater. 2014, 26, 6785–6789

average 100 % between the three different phases in the cooling and heating cycles (Figure 4b). The two hysteresis cycles are 40 K (397-357 K) and 18 K (367–345 K) wide. This film exhibits slightly wider hysteresis cycles and, more importantly, significantly larger

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loop (Figure 2b and Figure 2S in the Supporting Information) at faster scan rates. A dynamic 55 K hysteresis appears with a 9 K min−1 scan rate, with T1/2(↑) = 398 K and T1/2(↓) = 343 K, which should be due to a slow thermal equilibrium between film and the temperature controlled nitrogen stream. In al cases the electronic transition is fast, being completed in just a few seconds when the film of several square centimeters reaches thermal equilibrium. Although different transport properties are expected in HS or LS configuration,[65] this behavior cannot be attributed to a direct contribution of the SCO component. 1 is an insulator, with a conductivity below 10−7 S cm−1 at 400 K when measured in pressed pellets (not shown here). Thus, the abrupt jump in σ right at the spin crossover transition needs to be related to the interplay between SCO and ppy transport. The most plausible hypothesis is that, as we expected, the volume expansion in the SCO particles during the LS → HS transition generates a local pressure upon the ppy fibers, constraining the ppy molecular arrangement, and facilitating the hopping mechanism. This is reversed to the original regime during the HS → LS process, when SCO particles shrink, and the ppy fibers are able to relax to the initial state. The synergy between ppy and SCO depends on several factors. Higher loadings yield larger σHS/σLS ratios (vide infra), although the films may become brittle: an excess of 1 severely affects the mechanical strength of the composite. The relative change in σ can also be tuned with the pressure applied during sintering. Films obtained under lower pressure (0.01 GP) are thicker (>0.5 mm) and show smaller σHS/σLS ratios. At higher pressures (0.4 GPa) the film becomes thinner (

Spin crossover probes confer multistability to organic conducting polymers.

Switchable organic conductors can be readily obtained by combining organic conducting polymers (CPs), with the unparalleled bistability of spin crosso...
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