The radical carvacrol species coordinated to MIL-100(Fe) is predicted to exhibit several low-lying singlet excited states of charge-transfer nature, with a moderate intensity, and extending over the full visible region of the absorption spectrum (Figure ). These results are in agreement with the black carbon-like colour displayed by carvacrol@MIL-100(Fe) upon thermal activation. Release kinetics of encapsulated carvacrol from films. Carvacrol liberation was studied after the incorporation of the composite in a Zein polymeric matrix simulating fresh food environmental conditions (see Materials and Methods section). Figure shows the results of carvacrol liberation from the compositecontaining film as compared to a control film containing free carvacrol in the same concentration. Control free carvacrol release profile is characterized by a direct continuous delivery with a maximum at 20 h followed by an exponential decay, which corresponds with a delivery governed by diffusion until agent depletion. A unique profile is observed in the composite-containing film, which is described with a significant carvacrol retention of ca. 5 hours occurring before the liberation starts. After 5 h, two clearly different delivery phases are observed that are ascribed to the occurrence of sequential fast and slow desorption processes reflected in a first maximum at 20 h, followed by an additional second maximum at 66 h. This behaviour can be explained by attributing each maximum to different host-guest interactions between the carvacrol molecules and the MIL-100(Fe) framework. The first release may correspond to weakly interacting carvacrol molecules, whereas up to two days of high relative humidity conditions are required in order to trigger the liberation of the chemisorbed molecules. This MOF-mediated two-step release represents a clear improvement as compared to the same amount of free molecule in the polymeric film, evidencing the unique MOF capability to deliver carvacrol at prolonged times. Microbiology studies were conducted in order to determine the efficiency of the obtained composite against bacteria. Three dishes were seeded with E. coli. The first one contained a film of the carvacrol@MIL-100(Fe) composite, the second contained a zein film with the same concentration of free carvacrol, and the third one was a control. Figure reveals a clear reduction of the grass density occurring on the samples with carvacrol, especially, with the film incorporated with carvacrol@MIL-100(Fe) composite corroborating the antimicrobial effect of the active films. Quantitative effects of the active films (see Table ) expose a clear reduction on the bacterial growth with respect control, with a 90% decrease in the number of colony-forming units (CFU) in the case of free carvacrol film, and 98.3% of CFU reduction for the carvacrol@MOF composite film. This translates into a 60% increase in the antimicrobial effect of carvacrol when encapsulated into the MIL-100(Fe) framework, compared to the addition of the same amount of free carvacrol in the film. Table . Antimicrobial activity of films (80 mm diameter surface) against E. coli. Data are colony forming units observed after incubation in Preti dishes without film (control), free carvacrol film and carvacrol@MOF composite.