Use of the compound [iron(II) (hydrotris(3,5-dimethyl-1-pyrazolyl)borate) 2] as a temperature sensor

Abstract

The present invention relates to the use of the spin-transition molecular compound [iron(II) (hydrotris(3,5-dimethyl-1-pyrazolyl)borate)2] as a temperature detector by measuring a colour change in the molecular compound; to a temperature sensor comprising the compound, provided in the form of a film; and to a temperature detection system comprising the sensor and an optical detector.

Description

[0001] USE OF THE COMPOUND [IRON (II) (HYDROTRIS(3,5-DIMETHYL-1-PYRAZOLYL)BORATE)2] AS A TEMPERATURE SENSOR

[0002] FIELD OF INVENTION

[0003] The present invention falls within the field of preparing new materials for applications such as optical temperature sensors.

[0004] BACKGROUND OF THE INVENTION

[0005] Spin transition compounds (SCOs) are coordination complexes that have the ability to switch between two states of different electronic configuration, called high-spin and low-spin (HS and LS), depending on external conditions (i.e. temperature, pressure, light, etc.).

[0006] The greatest interest in spin transition compounds lies in these two characteristics: i) the spin transition is usually accompanied by important changes in other properties of the material (conductivity, transmittance, heat capacity, etc.);

[0007] i) The phase diagram of the transition depends on the presence of external stimuli (light, pressure, temperature, etc.).

[0008] Feature i) is of interest for the design of reconfigurable devices, i.e. devices whose response can be tuned (controlled) through the transition.

[0009] Control over the characteristic ¡i) would allow, among others, the application of these compounds as sensors of the external stimuli mentioned above.

[0010] The most commonly used stimulus to induce the SCO phenomenon is temperature. Its effect is characterized specifically by the parameter T1 / 2 (the temperature of the transition center). This parameter varies greatly within the family of SCO complexes, with some cases presenting values ​​below 100 K and others above 400 K. In the specific case of Fe(II)-based SCO compounds, the HS state configuration is paramagnetic, having a total spin moment (S) of 2 as it contains four unpaired electrons, while the LS state configuration is diamagnetic, as it contains no unpaired electrons (S = 0). Therefore, the value of T1 / 2 has generally been determined by magnetic susceptibility measurements due to the change in this property in these complexes between the paramagnetic (HS) and diamagnetic (LS) states.In this sense and by convention, T1 / 2 is defined as the temperature at which the magnetic susceptibility reaches the mean value between the two states when the temperature changes at a rate of 1 K / min.

[0011] To truly utilize the properties of spin-transition compounds, two requirements must be met: they must be processable and have robust and reproducible spin-transition reversibility. These factors severely limit the available materials.

[0012] Most spin transition compounds are not suitable for use in devices due to durability and stability issues, so the devices described in thermo-optical switching use different materials. Furthermore, the advantage of thin films made of SCO materials is their applicability in nanodevices, requiring very little material while providing sufficient optical contrast even at thicknesses around 10 nm.

[0013] So far, these types of films have been integrated into electronic devices of different configurations to incorporate the temperature- or light-induced bistability of the SCO into the electronic properties of the device, thus giving it the ability to switch these properties thermally or luminously [1—4]. On the contrary, its use as a thermometer has been much less exploited given the lower number of sublimate materials that present the transition around room temperature (T amb), as would be of greater interest for real applications. In this sense, the main work published in this field took advantage of a material with an abrupt SCO thermal transition centered at approximately 65 ° C, to develop a functional thermometer in a temperature range between 20 and 100 ° C and especially operational in the temperature range where the change of spin state is more abrupt (between 60 and 70 ° C with a resolution of only 1 ° C) [5], For this reason, the development of a system with much higher resolution and that can work at somewhat lower temperatures, while fully maintaining the reversibility of the change in its properties, is still necessary and of great interest.What differentiates the Fe(II)-based material of the present invention ([iron (II)(hydrotris(3,5-dimethyl-1-pyrazolyl)borate)2], SCO4 hereinafter) from other spin transition compounds is that the optical properties of this material when it is in a high spin state vary as a function of temperature, making it possible to use it in atmospheric contact since, on the one hand, it is a material resistant to oxidation and, on the other hand, it exhibits a large variation in optical contrast in the temperature range between -70°C and 50°C. Particularly, since in this temperature range it is very far from the center of the transition (T1 / 2 « -110°C or 160 K), the material changes its spin state (which is practically pure HS) very slightly and progressively, as at the limits of a distribution of populations of states according to the Boltzmann distribution equation.Therefore, this change in optical contrast follows a linear trend with temperature, allowing its use as an optical thermometer in the aforementioned range. Furthermore, since the material undergoes a very small state change—that is, the spin transition is partial—no irreversible structural changes occur, thus allowing its robust and reproducible use over a large number of thermal cycles.

[0014] The present invention demonstrates that films with thicknesses as small as 10 nm can be grown, in which the spin transition can be observed by monitoring the transmittance or reflectivity of these films, thanks to the respective color change observable by both types of measurements during this transition. These films can be grown on any type of substrate, including flexible substrates (e.g., Mylar), transparent ones (e.g., quartz), or even on fabrics (e.g., clothing fabrics or nylon). This offers, on the one hand, a new technological platform for, taking advantage of the spin transition property, developing optical modulators where the optical contrast between the spin states (and therefore the film thickness) will determine the modulation amplitude.On the other hand, this same platform can be used to design temperature sensors by taking advantage of the phase diagram of the transition that depends on the presence of external stimuli.

[0015] In this way, the present invention solves the problem of the structural stability of SCO compounds in film form when undergoing temperature changes within the range where the center of the spin transition occurs. In this range, most films lose their initial structure after a few temperature cycles, presenting very short lifetimes for their applicability in devices. Secondly, the material of the present invention takes advantage of the transition of a very small part of the film in a temperature range very far from the center of the transition (between 40 and 160 °C above) to achieve, on the one hand, sufficient optical contrast defined by a stable and linear change in transmittance and, on the other hand, a wide temperature range that is of interest to both industry and everyday use.In short, it is possible to solve the problems that most of these materials present in devices, such as their lifespan, their robustness, and also the complexity of the device, which in the case of the present invention only requires the material itself and a simple camera that captures the light through the film or the light reflected by it.

[0016] Another typical limitation of spin transition compounds is the time required to induce the transition and subsequently reverse it, as they may exhibit a certain inertia in the process. This is usually related to the thermal hysteresis observed when comparing the heating and cooling curves in magnetic susceptibility measurements to characterize the transition phenomenon. Furthermore, a certain irreversibility can typically be observed in the process. As previously described, the main use of the material of the present invention avoids this limitation by maintaining the operating temperature regime far from and above the center of the transition, where the small spin state change that occurs is rapid and completely reversible, remaining mostly in the HS state. That is, this change is exclusively reversible when working above the transition temperature.

[0017] A particular use of the material takes advantage of the certain irreversibility it exhibits when using a temperature range that approaches or completely crosses the center temperature of the transition (occurring around -110°C). This is evidenced by the fact that, when the material is heated after cooling below the center temperature of the transition, the initial state is not recovered until several hours have passed (in the case of thicker films, over 100 nm thick). Therefore, this material according to the invention can also be used as proof that a product has not reached a certain temperature (very low, around -100°C) or, conversely, as proof that a product that should be maintained at that temperature has been heated at some point.In both cases, the limitation of structural irreversibility and spin state that occurs to the material when it crosses the temperature center of the transition is exploited. The material can also be used as an indicator for various products, such as a vaccine that must be transported at cryogenic temperatures, thus ensuring that the temperature chain is not lost.

[0018] According to this potential use as a temperature sensor, the temperature is not read in the temperature range around the transition center. It can simply be observed whether a product has cooled excessively, or whether, on the contrary, it should have been very cold and has heated up. In other words, the Fe(II) complex acts as a delayed alarm, since an irreversible change occurs in the film comprising the Fe(II) compound.

[0019] The main use according to the present invention is in the working range above the transition and where the small change of spin state that occurs is fully reversible and uniquely related to the exact temperature in said range.

[0020] While devices based on spin transition materials are typically designed to exploit bistability when the transition occurs completely, the primary application of the material of the present invention allows for operation relatively close to the transition temperature, but without achieving a quantitative change in the material's spin state. This increases speed, ensures reproducibility, and overcomes the problem of irreversibility encountered so far in most devices previously manufactured with spin transition compounds that utilize the complete transition. Thus, the working range can be broadened, improving the sensitivity of transmittance measurements and establishing a one-to-one relationship between transmittance and temperature (something that thermal hysteresis prevents).The result is improved temperature calibration that allows temperatures to be measured with accuracy to hundredths of a degree Kelvin.

[0021] In previous proposals prior to this invention, the temperature range detected by the sensor was limited by the abrupt optical contrast between the spin states of the SCO material. In contrast, with the present invention, the detection range can be adjusted to the optical contrast across the entire working temperature range. In this way, we can improve sensitivity by taking advantage of the fact that we are working far from the transition temperature, where the change that occurs is progressive, reversible, and biunivocal. This has allowed us to improve transmittance measurements for ultrathin films, even when illuminating at wavelengths far removed from the material's absorbance peaks.

[0022] Notably, although the characteristics of the thermal transition are completely contrary to what would apparently be desirable to prepare an operational temperature sensor around T am b, surprisingly, the present invention has succeeded in exploiting the SCO phenomenon in the opposite way to how it has been used until now. To do this, it has been exploited that T1 / 2 for the Fe(II) compound is much lower than T am b. First, following a Boltzmann distribution, a T amb The material containing the Fe(II) compound continues to change its spin state very slightly despite being far from the center of the transition. This fact can be followed optically through reflectance or transmittance measurements. Furthermore, as occurs at the limits of a Boltzmann distribution, this small change in spin state occurs in a linear and very progressive manner with complete reversibility and reproducibility. Secondly, since there are no abrupt contrast changes that saturate the detection range of the measuring device within a few degrees, as occurs at the center of the transition, there is greater resolution over a very wide temperature range as a much milder and more progressive change in spin state occurs.Finally, also surprisingly and contrary to what is generally applied in the field, the film of the present invention uses light that is non-resonant to the material, i.e., with an energy lower than its absorption band. This avoids, on the one hand, possible light degradation of the films and, on the other hand, allows the use of a higher beam power, achieving greater resolution in the reflectance signal measured at the detector.

[0023] References

[0024] [1] KS Kumar, M. Ruben, Angew. Chemie - Int. Ed. 2021, 60, 7502. [2] C. Lefter, S. Rat, JS Costa, MD Manrique-Juárez, CM Quintero, L. Salmon, I. Séguy, T. Leichle, L. Nicu, P. Demont, A. Rotaru, G. Molnár, A. Bousseksou, Adv. Mater. 2016, 28, 7508.

[0025] [3] N. Konstantinov, A. Tauzin, II. N. Noumbé, D. Dragoe, B. Kundys, H. Majjad, A. Brosseau, M. Lenertz, A. Singh, S. Berciaud, M.-L. Boillot, B. Doudin, T. Mallah, J.-F. Dayen, J. Mater. Chem. C 2021, 9, 2712.

[0026] [4] M. Gavara-Edo, R. Córdoba, FJ Valverde-Muñoz, J. Herrero-Martin, JA Real, E. Coronado, Adv. Mater. 2022, 34, 2202551.

[0027] [5] K. Ridier, A.-C. Bas, Y. Zhang, L. Routaboul, L. Salmon, G. Molnár, C. Bergaud, A. Bousseksou, Nat. Comm. 2020, 11, 3611.

[0028] DESCRIPTION OF THE INVENTION

[0029] The present invention refers to the use of the molecular spin transition compound [hierro (II) (hydrotris(3,5-dimethyl-1 -pyrazolyl)borate)2], (SCO4), as a temperature detector, preferably arranged in the form of film.

[0030] In this specification, reference is made to the compound [iron (II)(hydrotris(3,5-dimethyl-1-pyrazolyl)borate)2] also with the expressions SCO4, Fe(II) compound of the invention, spin transition compound, spin transition molecular compound or iron compound of the invention.

[0031] The Fe (II) compound of the invention allows temperature changes to be detected with a change in its color, that is, in its absorption spectrum, and directly related to the specific value of said property.

[0032] The temperature at which biunivocal changes are detected in the Fe(ll) compound of the invention may be between -70 and 50°C, this interval being established as the working interval.

[0033] The term "biunique" means that the changes are directly related to temperature, i.e., there cannot be two transmittance values ​​for the same temperature and vice versa. The Fe(II) compound of the invention makes it possible to detect a temperature change of less than a tenth of a degree, which can be as little as several hundredths of a degree. Specifically, by illuminating with green light at 532 nm (very far from the material's absorption maximum at 320 nm), a sensitivity of 0.01 K was obtained in the range between 0 and 26°C.

[0034] Preferably, the spin transition molecular compound is arranged in the form of a film and with a thickness between 10 and 500 nm.

[0035] Taken together, the temperature-sensitive spin transition compound [iron(II) (hydrotris(3,5-dimethyl-1-pyrazolyl)borate)2] (SCO4) with high optical contrast in film form can be used in devices as an optical temperature sensor over a temperature range of -70 to 50 °C. The compound exhibits sufficient absorbance from 350 nm to 600 nm to detect optical contrast with good resolution in films with a thickness of 10 to 500 nm over this temperature range.

[0036] Specifically, it has been determined that the color contrast changes reversibly and stably over different temperature cycles within the effective range of -70 to 50°C, which is above the center temperature of the transition, enabling its applicability in nanodevices. Furthermore, even though the material's absorption maximum is around 330 nm, it can be used as a temperature sensor in both transmission and reflection using light sources with other wavelengths thanks to the material's broad spectral response. Specifically, the response across the entire spectrum is correlated with temperature, thus allowing calibration for use of the material film as a temperature sensor.

[0037] The present invention further relates to a temperature sensor comprising the spin transition molecular compound [iron (II) (hydrotris (3,5-dimethyl-1 - pyrazolyl) borate) 2], (SCO 4), arranged in the form of a film.

[0038] From here on, the film will be referred to as the invention film, or the spin transition compound film.

[0039] The spin transition compound film has a thickness of 10 to 500 nm. The spin transition compound film [iron(II) (hydrotris(3,5-dimethyl-1-pyrazolyl)borate)2] can be prepared on a substrate, which can be of various materials, such as graphene, silica, a fabric (such as clothing fabrics or nylon), glass, mylar, quartz, plastics (such as polystyrene terephthalate (PET) or polyvinyl alcohol (PVA)) or combinations thereof, thereby forming a label.

[0040] The present invention also relates to a method for indirect measurement of temperature, which comprises detecting a change in colour, i.e. in the absorption spectrum, produced by a partial spin state transition of the spin transition molecular compound [iron (I l)(hydrotris(3,5-dimethyl-1-pyrazolyl)borate)2] (SCO4) arranged in the form of a film.

[0041] The partial spin transition of the material is thermally induced within the aforementioned working temperature range, hence its use as a temperature sensor, and is detected by irradiation with a light source preferably of an energy lower than the absorption band energy of [iron (I l)(hydrotris(3,5-dimethyl-1 - pyrazolyl)borate)2], (SCO4), of the film.

[0042] For sensing, any light source between approximately 350 and 600 nm can be used.

[0043] The defined procedure may comprise measuring the response of the Fe(II) complex film in transmission or reflection.

[0044] In the case of reflection, the compound can be prepared in the form of a film on a label using a diffusing substrate, which could be, for example, a conventional acrylic fabric or PET films, such as those used in photography, or even more rigid substrates such as silica or graphene-coated silica. These types of substrates thus redirect the light beam to the detector.

[0045] Alternatively, for the transmission configuration, a transparent substrate can be used, which allows light to be transmitted through the Fe(II) complex film and subsequently collected by the detector positioned behind the substrate on which it is arranged. An example of a transparent substrate could be, for example, a polymer (plastic) such as PVA or Mylar, or even more rigid substrates such as glass or quartz.

[0046] The method may comprise measuring temperatures between -70 and 50°C. The film of the present invention may be used as:

[0047] - nanometric temperature sensor (e.g., detection of loss of cold chain, sensing in vacuum systems, sensing applications in nanotechnology) for:

[0048] - Aeronautics.

[0049] - Automotive

[0050] - Home automation.

[0051] - Scientific instruments

[0052] - Temperature sensors in general: resistors and thermistors.

[0053] - Thermometers for remote measurements, where the label containing the SCO4 film is in contact with the object to be measured, while the detector is used remotely: Thermal cameras, for temperature measurements on surfaces or for measurements inside a vacuum chamber.

[0054] - Photonics in general.

[0055] The film according to the present invention can be used as a label (indicator) because its color varies depending on whether it is above, near, or below the transition temperature. Therefore, the present invention relates to products or objects bearing such a label.

[0056] The invention also relates to a temperature measurement system comprising a sensor as defined above and an optical detector.

[0057] The present invention also relates to a system for detecting the absorption / reflectivity of the film, which is biuniquely related to the temperature value of the film. That is, it comprises an optical detector, as defined above, capable of capturing the light that is reflected or transmitted through the film of the Fe(II) spin transition compound.

[0058] Therefore, the complete system of the invention (temperature sensor) comprises the label with the SCO4 material and a reflectance or transmittance device that includes a preferably modulated light source (lamp, diode or laser), a mirror or "beamsplitter" in the case of measuring reflectance, and a detector to detect the light transmitted or reflected in the sample. The detector can be a conventional silicon photodiode or a multi-channel detector or even a camera, such as that of a smartphone. For the latter case, a temperature detection system in which an application analyzes the photos taken with a camera (such as a mobile phone) can simplify conventional measurements that require a light source and a conventional / multi-channel detector, that is, together, a spectrometer.

[0059] The present invention also relates to a method for optical temperature measurement comprising detecting a color change produced by a partial spin state transition of the spin transition molecular compound [iron (I l)(hydrotris(3,5-dimethyl-1-pyrazolyl)borate)2] (SCO4) arranged in the form of a film.

[0060] The main advantage of the present invention, with respect to other devices manufactured until now, is that the material does not change spin completely, which results in total reproducibility by not passing to the low spin state completely, avoiding, for example, irreversible volume and structural changes in the film.

[0061] The experiment was carried out under experimental conditions contrary to those expected, and surprisingly, a qualitative improvement was found in the temperature measurement method, in terms of resolution and temperature range, using [iron (I l)(hydrotris(3,5-dimethyl-1-pyrazolyl)borate)2] (SCO4) as a sensor for this property. Specifically, SCO4 was used because it presents the center of the spin transition at a temperature very far (very low temperatures) from the temperature range of interest (around room temperature). This overcomes the limitation of having to search for or design a material whose center of the transition is precisely contained in said temperature range. In addition, its deterioration is also avoided, as usually occurs when the center of the transition is used in other materials, and the reproducibility of the measurements is ensured. Consequently, the material was characterized by changes in transmittance far from the center of the transition.Furthermore, these changes in transmittance were monitored using blue light from a standard laser pointer, avoiding the need to adapt the excitation to the absorption properties of the material.

[0062] Using the sensors obtained from the iron(II) complex of the present invention, a series of reflectance measurements were taken on various films as a function of temperature to produce calibration lines. Subsequent measurements of specific reflectance values ​​were made, and using these lines, they were related to a temperature value of the film, successfully corroborating the known temperature values ​​(measured by contact).

[0063] The thermal transmittance response of an iron(II) complex film was successfully calibrated to small temperature changes monitored using a cryostat or Peltier. Finally, it is worth noting that, unlike what occurs around the center of the spin transition, the response to temperature changes is quasilinear when working in a temperature range far from it. The practical consequences of this fact were an improvement in the detector response, since the dynamic detection range was better utilized by adjusting it to the optical contrast of the material in the temperature range of interest. Specifically, by measuring the film's transmissivity or reflectivity response at different temperatures, it was possible to calibrate the film's response in the effective temperature range.

[0064] In summary, by exploiting this new and unexplored concept in the field of SCO materials working in a temperature range far from the center of the transition, and applying them to SCO4 sublimation-processed films with a thickness of a few nanometers (between 10 and 500 nm), it has been possible to use them as operational optical temperature sensors in a temperature range of high interest for realistic applications (-70 and 50 °C), with full reversibility and reproducibility, and, most importantly, with an unprecedented resolution around one hundredth of a degree.

[0065] The film of the present invention has advantages such as:

[0066] - Possibility of miniaturizing optical devices and sensors by using films around 10 nm thick.

[0067] - Minimum thermal inertia.

[0068] - Simple device design: the material is deposited directly onto a photodetector window, avoiding the use of complex optical and electronic components that increase the cost, complexity, calibration, and robustness of the device.

[0069] - Possibility of taking measurements remotely, in closed environments and without cables in the sensor.

[0070] - Longer lifespan than most similar devices. Brief description of the figures

[0071] Figure 1: Examples of methods for applying Fe(II) material as a temperature sensor in different configurations. Specifically, a) shows the use of a white light source transmitted through the SCO4 film and reaching a photodetector, as would be the case for a transmission mode measurement setup. b) indicates that, instead of white light as the light source, a collimated monochromatic laser can also be used, allowing the light to pass directly through the film to the detector. Finally, c) represents the reflection measurement mode, where the beam is reflected by the diffusing base (reflective material) on which the SCO4 film is arranged and redirected towards the detector.These examples allow us to schematically visualize how the color change of the Fe(ll) compound film (SCO4) can be measured optically by reflectance or transmittance with high sensitivity and can thus be used as a temperature sensor.

[0072] Figure 2: Transmittance response of a 10 nm SCO4 thin film using a photodiode as detector and a 532 nm laser as illumination. It can be observed that the material's response is stable within the effective temperature range exemplified in a) between room temperature (22°C) and -16°C and in b) between room temperature (22°C) and -43°C. Furthermore, these measurements show that the material's response speed to temperature changes is rapid, being more than sufficient for the cryostat's cooling / heating speed, which is low (minutes) when a large volume has to be cooled by pumping helium. For example, using a Peltier would obtain the same result, but much faster, since with this device, cooling and heating the sample in this temperature range can be done in a matter of seconds.

[0073] Figure 3 shows the changes in transmittance of a 50 nm thick film of the Fe(II) compound, measured using a white light source in a cryostat. Figure a) shows the tracking of the film's transmittance value at the absorption maximum (320 nm) throughout a complete thermal cycle between 300 and 60 K (cooling and heating). It is observed that this parameter is irreversible when the temperature of the center of the spin transition is exceeded, but when the temperature remains above this temperature, this is not the case (highlighted in a box as a reproducible area). Images b) show the irreversible changes that occur in the film with the naked eye during the complete thermal cycle shown in a) and in which the spin transition temperature of the material is exceeded. In these images, the film is placed on a circular copper support that allows the passage of light.The film turns blue as the temperature decreases, with the color change being most intense in the spin transition region. The transmittance spectra of the first three images taken in Figure b) are shown in c), and the transmittance spectra of the last three images taken in Figure b) are shown in d).

[0074] EXAMPLES

[0075] Example 1

[0076] Material processing

[0077] The [iron(II)(hydrotris(3,5-dimethyl-1-pyrazolyl)borate)2] powder, precursor of the films, was synthesized following the method described in [4]. Using this material, the films were grown on quartz substrates (ideal for optical measurements as a function of temperature) and following the method for the growth of the films described in that publication: precursor heated to 145 °C in a high vacuum of 5 x 10" 8mbar reaching an average deposition rate of 0.4 Á / s, monitored in real time with a quartz micro-balance, and cooling the substrate to a temperature of -90 °C during deposition [4]. Under these conditions, films of different thicknesses were prepared in a range between 10 and 500 nm (verified with profilometry with nanometric resolution).

[0078] Optical measurements

[0079] To optically measure the spin transition, methods such as absorption or transmittance measurements would normally be used. In this case, a xenon lamp was used, whose light is passed through the Fe(II) compound sample evaporated on a transparent substrate. In this way, the light is transmitted through the Fe(II) compound film and reaches a detector (typically a spectrometer) where the composition of the light is analyzed in different absorption and / or transmittance bands. Using this method, the spin transition can be followed using the transmittance spectra of different films, as observed in Figure 3 for 50 nm thick films. As seen in Figure 1 , using a light source, such as a laser or white light, together with a photodetector (which generates an electrical signal depending on the intensity of the light it receives) it is possible to work in a single channel with a specific fixed wavelength.In this way, we can optically measure the material in both reflection and transmission with greater sensitivity and with a set of simpler and cheaper equipment.

[0080] Following these experiments, it was observed that this material presents a change in contrast away from the center of the spin transition (-110 °C), making it possible to detect changes optically at higher temperatures, between -70 and 50 °C. Furthermore, if the photodetector is coupled to a signal amplifier, minimal color changes in the film can be detected, as observed in Figure 2, being able to obtain a greater sensitivity of the order of 10 mK. In this way, by calibrating this voltage response of the photodetector, it is possible to use this material as a robust, precise and contactless thermometer in the temperature range indicated above.

Claims

CLAIMS 1. Use of the spin transition molecular compound [iron (II)(hydrotris(3,5-dimethyl-1 - pyrazolyl)borate)2] as a temperature detector by measuring a color change of the spin transition molecular compound.

2. Use according to claim 1, wherein the temperature is between -70 and 50°C.

3. Use according to claim 1 or 2, wherein the detectable temperature change is less than one-tenth of a degree.

4. Use according to claim 1 to 3, wherein the spin transition molecular compound is arranged in the form of a film.

5. A temperature sensor comprising the spin transition molecular compound [iron (I l)(hydrotris(3,5-dimethyl-1 -pyrazolyl)borate)2], (SCO4), arranged in the form of a film.

6. A sensor according to claim 5, wherein the film of the spin transition compound has a thickness between 10 and 500 nm.

7. A sensor according to one of claims 5 to 6, wherein the film of the spin transition compound is disposed on a substrate, thereby forming a label.

8. A sensor according to claim 8, wherein the substrate is selected from graphene, silica, fabrics, glass, quartz, mylar, plastics or combinations thereof, thereby forming a label.

9. A temperature measurement system comprising a sensor as defined in one of claims 5 to 8 and an optical detector.

10. A system according to claim 9, wherein the optical detector is selected from a conventional silicon photodiode, a multi-channel detector, and a photographic camera.

11. A method of optical temperature measurement comprising detecting a color change produced by a partial spin state transition of the spin transition molecular compound [iron (I l)(hydrotris(3,5-dimethyl-1 -pyrazolyl)borate)2] (SCO4) arranged in the form of a film.

12. A method according to claim 11, wherein the partial spin transition of the material is thermally induced and optically detected by irradiating the SCO4 film with a light source of an energy lower than the energy of its absorption band maximum.

13. A method according to claim 12, comprising the use of a light source of wavelength between 350 nm and 600 nm.

14. Method according to any one of the preceding claims 11 to 13, comprising measuring the response of the SCO4 film in transmission or reflection.

15. Method according to one of the preceding claims 11 to 14, wherein the partial spin transition of SCO4 occurs at a temperature between -70 and 50°C.

16. Use of the sensor defined in one of claims 5 to 10, in the manufacture of devices.

17. Use according to claim 16, in devices for the aeronautical, automotive, home automation, scientific instrument, telecommunications, photonics, acoustic and optical industries.

Images