Crystal engineering of molecular solids: "light" interactions for interactions with light.

Molecular crystal engineering, i.e. the design and synthesis of solid state molecular materials with predefined properties, is used here to exploit non-covalent interactions to pre-arrange buiding blocks in their crystalline state for properties modification and specific applications, all involving the absorption of UV radiation. Depending on the chemical complexity and/or application, crystals of appropriate size and purity are required for characterization, chemical reactivity and applicative processes. Three cases will briefly be presented here: (i) single crystal to single crystal photodimerization of cinnamic acid derivatives and the effect of crystal size on the solid-state reactivity; (ii) cocrystallization as a tool to switch from fluorescence to phosphorescence in the solid state, and the necessity of obtaining single crystals following a mechanochemical synthesis; (iii) molecular crystals for dye-sensitized solar cells applications, and the single crystal vs. polycrystalline material requirements on passing from characterization to photophysical measurements and to thin-film deposition.