We report on thermally evaporated, crystalline films of [Fe(HB(tz)3)2] (tz = triazolyl), which display spin transition around 338 K.

Using ultrafast optical absorption spectroscopy, we have studied the room-temperature spinstate switching dynamics induced by a femtosecond laser pulse in high-quality thin films of the molecular spin-crossover complex [Fe(HB(tz)3)2] (tz = 1,2,4-triazol-1-yl). These measurements reveal that the early, sub-picosecond, low-spin to high-spin photoswitching event, with linear response to the laser pulse energy, can be followed under certain conditions by a second switching process occurring on a tens of nanoseconds timescale, enabling nonlinear amplification. Such out-of-equilibrium dynamics is discussed in light of characteristic timescales associated with the different switching mechanisms, i.e. the electronic and structural rearrangements of photo-excited molecules, the propagation of strain waves at the material scale and the thermal activation above the molecular energy barrier. Importantly, size reduction effects are evidenced on the second switching step. Notably, this nonlinear process appears to be completely suppressed in the thinnest (50 nm) film due to the efficient heat transfer to the substrate, allowing the system to retrieve the thermal equilibrium state on the 100-ns timescale.These results provide a first milestone towards the assessment of the physical parameters that

Molecular spin crossover complexes are promising candidates for mechanical actuation purposes. The relationships between their crystal structure and mechanical properties remain, however, not well understood. In this study, combining high pressure synchrotron X-ray diffraction, nuclear inelastic scattering, and micromechanical measurements, we assessed the effective macroscopic bulk modulus ( B = 11.5 ± 1.5 GPa), Young's modulus ( Y = 10.9 ± 1.0 GPa), and Poisson's ratio (ν = 0.34 ± 0.04) of the spin crossover complex [Fe(HB(tz))] (tz = 1,2,4-triazol-1-yl). Crystal structure analysis revealed a pronounced anisotropy of the lattice compressibility, which was correlated with the difference in spacing between the molecules as well as by the distribution of the stiffest C-H···N interactions in different crystallographic directions. Switching the molecules from the low spin to the high spin state leads to a remarkable drop of the Young's modulus to 7.1 ± 0.5 GPa both in bulk and thin film samples. The results highlight the application potential of these films in terms of strain (ε = -0.17 ± 0.05%), recoverable stress (σ = -21 ± 1 MPa), and work density ( W/V = 15 ± 6 mJ/cm).

Temperature measurement at the nanoscale is of paramount importance in the fields of nanoscience and nanotechnology, and calls for the development of versatile, high-resolution thermometry techniques. Here, the working principle and quantitative performance of a costeffective nanothermometer are experimentally demonstrated, using a molecular spincrossover thin film as a surface temperature sensor, probed optically. We evidence highly reliable thermometric performance (diffraction-limited sub-µm spatial, µs temporal and 1°C thermal resolution), which stems to a large extent from the unprecedented quality of the vacuum-deposited thin films of the molecular complex [Fe(HB(1,2,4-triazol-1-yl) 3) 2 ] used in this work, in terms of fabrication and switching endurance (>10 7 thermal cycles in ambient air). As such, our results not only afford for a fully-fledged nanothermometry method, but set also a forthcoming stage in spin-crossover research, which has awaited, since the visionary ideas of Olivier Kahn in the 90's, a real-world, technological application.

An unexpected upshift of the spin transition temperature by ca. 3 K is observed in thermally evaporated films of the [Fe II (HB(tz) 3 ) 2 ] (tz = 1,2,4-triazol-1-yl) complex when reducing the film thickness from ca. 200 to 45 nm. Fitting the experimental data to continuum mechanics and thermodynamical models allows us to propose an explanation based on the anisotropy of the transformation strain leading to ∼5 mJ/m 2 higher 00l surface energy in the high-spin phase.

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We report on the vacuum thermal deposition of bilayer thin films of the luminescent complex Ir(ppy)3, tris[2-phenylpyridinato-C2,N]iridium(III), and the spin crossover complex [Fe(HB(tz)3)2], bis[hydrotris(1,2,4-triazol-1-yl)borate]iron(II). Switching the spin state of iron ions from the low spin to the high spin state around 337 K leads to a reversible jump of the luminescence intensity, while the spectrum shape and the luminescence lifetime remain unchanged. The luminescence modulation occurs due to the different UV light absorption properties of the iron complex in the two spin states and its magnitude can therefore be precisely adjusted by varying the film thickness. These multilayer luminescence switches hold potential for micro- and nanoscale thermal sensing and imaging applications.

Thin films of the molecular spin‐crossover complex [Fe(HB(1,2,4‐triazol‐1‐yl)3)2] undergo spin transition above room temperature, which can be exploited in sensors, actuators, and information processing devices. Variable temperature viscoelastic mapping of the films by atomic force microscopy reveals a pronounced decrease of the elastic modulus when going from the low spin (5.2 ± 0.4 GPa) to the high spin (3.6 ± 0.2 GPa) state, which is also accompanied by increasing energy dissipation. This technique allows imaging, with high spatial resolution, of the formation of high spin puddles around film defects, which is ascribed to local strain relaxation. On the other hand, no clustering process due to cooperative phenomena was observed. This experimental approach sets the stage for the investigation of spin transition at the nanoscale, including phase nucleation and evolution as well as local strain effects.