Thermal activation (250 ºC) has been previously shown to induce the generation of coordinatively unsaturated iron sites (CUS) with mixed valence Fe II /Fe III trimers in MIL-100(Fe), manifested by a shifting of the νas(Fe3O) band located at 618 cm -1 to 597 cm -1 after thermal treatment in IR spectroscopy studies. In a similar study, we limited this high temperature activation to 190 ºC in order to remain below the carvacrol boiling point. Exposing MIL-100(Fe) control to this thermal activation leads to a reduced shifting in the νas(Fe3O) from 618 to 611 cm -1 recovering the original value upon 1 min of air exposure (Figure ). This band evolution reflects a reversible partial loss of coordinated water molecules as compared to the reported material, as may be expected for lower activation temperature and lack of sample isolation from ambient conditions. In case of carvacrol@MIL-100(Fe) treated at 190 ºC under vacuum, a shifting in the νas(Fe3O) band from 628 to 605 cm -1 is observed, and the original value is recovered after 1 min of air exposure (Figure ). The large shifting of the νas(Fe3O) band observed in the case of the activated composite is in good agreement with the displacement reported in the mixed valence Fe II /Fe III MIL-100(Fe) spectrum (23 vs 21 cm -1 , respectively). This phenomenon is accompanied by a drastic colour change from brown to an intense black carbon-like colour, this colour change being reversible to some extent upon exposure to air. It is worth mentioning that crystallinity was preserved during this process, as denoted by maintenance of the XRPD patterns in the material (Figure ). Mössbauer and EPR spectroscopies. In order to evaluate the effect of carvacrol loading in the MIL-100(Fe) electronic structure, Mössbauer spectroscopy experiments were carried out. All spectra were collected at 80K. Control experiments in the as synthesized MIL-100(Fe) and carvacrol@MIL-100(Fe) loaded materials revealed spectra with two broad peaks fitted to distributions of quadrupole doublets (Figure ). Table summarises the calculated isomer shift (IS) values as well as the temperature independent quadrupole splitting (QS), which are consistent with high-spin Fe III (S = 5/2) in both samples. In case of the carvacrol@MIL-100(Fe) composite, a lower QS of Fe III can be detected, which suggests a different Fe III environment in the composite. This difference may arise from the replacement of one of the water molecules coordinated to the Fe III centres by one molecule of carvacrol, in agreement with the shifting of the νas(Fe3O) band observed in IR measurements. Studies on the activated compounds were then performed, after exposing MIL-100(Fe) and carvacrol@MIL-100(Fe) samples to thermal treatment (190 °C) under vacuum for 6 hours and quenching the samples in liquid nitrogen for analysis. As expected, activated MIL-100(Fe) spectrum obtained at 80 K (Figure ) showed significantly higher average QS as compared to the pristine sample. This is consistent with a more distorted Fe III environment becoming 5coordinated due to the loss of coordinated water molecules. Once the sample was brought back to room temperature in air, the spectrum of the untreated sample was recovered, confirming the reversibility of the desolvation process. Similar analysis performed on activated carvacrol@MIL-100(Fe) spectrum (Figure ) indicates that approximately 24% of the Fe is reduced to high-spin Fe II , with the estimated IS and QS suggesting that both Fe II and Fe III are 6-coordinated. Then, the initial carvacrol@MIL-100(Fe) spectrum is recovered after exposing the activated sample in air and at room temperature for a few hours. These observations are in agreement with IR experiments and denote the effective reversible formation of the mixed valence trimer in the composite material with retention of the carvacrol molecules coordinated to the Fe III centers. To further investigate the coordination between carvacrol and MIL-100(Fe), EPR experiments were conducted. Spectra recorded at room temperature revealed similar signals for the as-synthesized materials before and after encapsulation (Figure ), whereas the appearance of a characteristic radical signal (g = 2.005) was observed upon activation of the composite (190 °C under vacuum for 2h), suggesting a radicalmediated mechanism for the carvacrol coordination to the Fe III centres. Interestingly, this radical remains stable with time, as long as the material is isolated from air.