NanoDMA and Nanoscratch Study of (Visco)-Elastic and Plastic Behaviors of a Thin Film of a Molecular Spin Crossover Complex.

We report a nanomechanical study of ca. 1 µm thick crystalline films of the molecular spin crossover (SCO) complex [Fe(HB(1,2,4-triazol-1-yl)3)2] across its thermal spin transition. Using quasi-static and dynamic (1-100 Hz) indentation techniques, a step-wise decrease in the modulus, E (from 9 ± 1 to 4.0 ± 0.5 GPa) and hardness, H (from 320 ± 40 to 170 ± 20 MPa) is observed when going from the low-spin (LS) to the high-spin (HS) state, which can be attributed to the reduced mass density of the HS phase. The significantly higher H/E2 ratio in the HS state indicates its better resistance to wear and plastic deformation. On the other hand, only a modest modulus relaxation and no clear loss peak are observed at the transition point (330 K), likely due to ensemble averaging effects in nano-sized domains as well as due to the gradual nature of the SCO in the films. Complementary nanoscratch experiments further confirm the impact of SCO on mechanical compliance, revealing an abrupt increase in scratch depth in the HS state. This comprehensive mechanical analysis helps to understand the coupling between spin state and mechanical properties and to optimize SCO materials for technological applications.