Preferential Solvation Unveiled by Anomalous Conformational Equilibration of Porphyrin Dimers: Nucleation Growth of Solvent-Solvent Segregation.

Preferential solvation was explored using ethynylene- or butadiynylene-linked porphyrin dimers bearing 3,4,5-tri(( S)-3,7-dimethyloctyloxy)phenyl groups at the meso positions in binary hexafluorobenzene (C6F6) and cyclohexane (C6H12) mixture, expecting contrasting solvent affinity of the porphyrin core and the alkyl side chains toward the individual solvent component. Although the solvent polarity remained nearly constant along with the continuous variation of the solvent composition, the porphyrin dimer showed dramatic change in spectroscopic signatures, indicating the occurrence of preferential solvation. Because of small rotational barrier around the ethynylene and butadiynylene linkage, the torsional conformations of the porphyrin dimers varied from orthogonal to planar due to continuous variation of molar fraction of C6H12-C6F6 mixture. Thorough thermodynamic analyses inferred that nucleation as the enthalpic component and phase segregation as the entropic component operated preferential solvation. The porphyrin dimer nucleated the C6H12-C6F6 segregation, and the torsional conformation was diagnostic of the inversion of the interfacial curvature of the solvent segregation along with the continuous variation.