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Reactivity trends for molecular solids cannot be explained exclusively through the topochemical phenomenon ( i.e. diffusivity, reaction cavities) or electronic structure of the molecules. As an example of this class, Diels–Alder reactions of small molecules with pentacene thin films are examined to elucidate the importance of surface phenomena to reactivity. Polarization modulation-infrared reflection–absorption spectroscopy (PM-IRRAS) has revealed that vapors from the small molecules condense on the surface, in a non-covalent manner, to form a coating 2–3 molecules thick. The phase of this layer can provide increased surface diffusion (both reactant and product) which rapidly accelerates the reaction rate. Kinetic studies of pentacene thin film reactions demonstrate the importance of this condensed state to trends in reactivity, with layers in a quasi-liquid state showing a rate acceleration of 13–30 times compared to those in a quasi-solid state. Scanning electron microscopy provides further evidence of this phase behavior, while solid-state UV-vis confirms the kinetic results. 

Anhydride terminated acene thin films were chemically transformed to thiol or carboxylic acid functionalities, groups heretofore incompatible with monolayer reactions. The molecular surface imparts large rate acceleration when imides are formed, while disfavored disulfides can be formed from the thiols. The modified surface imparts improved adhesion to top metal contacts in flexible/bendable applications. 

Morphology plays a critical role in determining the properties of solid-state molecular materials, yet fluctuates wildly as these materials undergo reaction. A prototypical system, a vapor–solid Diels–Alder reaction of tetracene and pentacene thin-films, is used to observe the evolution of morphology features as the reaction transitions from surface to bulk. The initial stages of reaction display little topographical change as measured by atomic force microscopy (AFM) and scanning electron microscopy (SEM), and substrates are coated with a uniform layer of product 1–2 molecules thick, as determined by energy-dispersive X-ray (EDX) spectroscopy. The highly textured surfaces of late stage reactions are a result of aggregated products, as identified via EDX spectroscopy and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS); areas of the surface in between product aggregates resemble the initial stages. The mechanism by which products aggregate into surface asperities requires the assistance of a facilitating media – in this case condensed vapor; simple thermally assisted surface diffusion was unable to generate these morphology changes. The combined data indicate that reactions of molecular solids, could be confined to the surface in the absence of condensate of the vapor phase reactant.