Dual Mode of Action of Organic Crystal Growth Inhibitors
Rajshree Chakrabarti, Peter G. Vekilov
Abstract
Crystallization often proceeds in media with rich chemical compositions. In pursuit of insight into how foreign compounds interact with the structures and dynamics that comprise crystal growth, we explore the mechanisms employed by protoporphyrin IX to inhibit the growth of etioporphyrin I crystals. Crystals of porphyrins exhibit potential for use as organic semiconductors, solar cells, and field-effect transistors. We employ solutions in 1-octanol and combine time-resolved in-situ atomic force microscopy (AFM) with scanning electron microscopy and absorption spectroscopy. We demonstrate that protoporphyrin IX does not sequester etioporphyrin I in soluble complexes, which would lower the amount of growth-competent etioporphyrin I. We show that the microscopic and mesoscopic structures on the crystal surface and their dynamics during growth unequivocally report on the interactions of the modifier with the crystal surface. AFM observations eliminate lattice strain caused by PPIX molecules, embedded in the top crystal layer or merely bound to it, as a factor for step inhibition. The lack of long-lasting strain allows steps to revert to uninhibited growth after the inhibitor is purged. We show that an inhibitor may bind to both kinks and crystal surfaces and thus exert a dual mode of suppression. We develop an analytical model for step growth in the presence of a dual-action modifier that highlights an antagonistic element of the cooperativity of the two modes of inhibition enforced by the decline of step edge free energy after the modifier associates to kinks. Our results demonstrate that dual-action inhibitors are superior regulators of crystal growth that afford the opportunity to fine-tune the degree of inhibition required to optimize the growth and shape of crystals of interest.