Elucidating the polymorphic behavior of curcumin during antisolvent crystallization: Insights from Raman spectroscopy and molecular modeling

The focus of this work is to understand the polymorphic behavior of curcumin crystallized from two different organic solvents by liquid antisolvent (LAS) precipitation carried out in a semibatch mode. LAS precipitation of curcumin from ethanol yielded the metastable (orthorhombic) Form 3, while acetonitrile led to the precipitation of stable (monoclinic) Form 1. These polymorphic crystal structures mirrored clustering of curcumin molecules in the respective organic solutions determined using in situ Raman spectroscopy. The presence of strong CH···O bonds in acetonitrile solution indicated interactions between curcumin molecules through the phenolic end groups. On the other hand, parallel stacking of curcumin molecules was favored in ethanol solution due to stronger solute-solvent interactions at the polar end groups. These observations are consistent with the results obtained from density function theory (DFT) calculations and molecular dynamics (MD) simulations. Accordingly, with acetonitrile interacting weakly with the solute molecules, curcumin-curcumin interactions through the phenolic ends were promoted, in turn, favoring formation of Form 1. Stronger interaction between ethanol and curcumin led to relaxation of solute molecules into parallel stacked clusters, thereby favoring Form 3. Furthermore, the solvatochromic effect of curcumin exhibited in these solvents was related to solvent-induced conformations and, in turn, nucleation of curcumin crystal polymorphs.