Investigating surface vs. bulk kinetics in the formation of a molecular complex via solid-state reaction by simultaneous Raman/X-Ray powder diffraction

The charge-transfer molecular complex fluorene/7,7,8,8-tetracyanoquinodimethane (TCNQ) was obtained by an unprecedented in situ solid-state direct synthesis. The reaction advancement was monitored with Raman spectroscopy and high- resolution X-ray powder diffraction (XRPD). The experiments were performed with a novel setup (SNBL, Grenoble) capable of combining both techniques simultaneously at in situ conditions. Raman spectroscopy allowed the characterization of the reaction kinetics during the initial steps at the surface of the TCNQ crystallites, when no or very little reaction is observed by the XRPD probe. From Rietveld refinement of the synchrotron XRPD data, the time evolution of the amounts of reactants and product was obtained for the reaction proceeding into the bulk of the TCNQ crystals, together with the crystal size analysis. These data were analyzed exploiting the Avrami equation to obtain rate constants, reaction orders, and activation energies. The reaction, after starting at the surface of the TCNQ platelet-like crystals, as was also confirmed by an hot-stage microscopy experiment, proceeds toward the inner part of the crystals with a decreasing rate, because diffusion is more critical into the bulk. A full comprehension of the kinetic features of the crystal growth with a multiscale approach from the nano- to micrometer level was thus obtained.