Microscopic environment and molecular orientation of guest molecules within polyfluorinated surfactant and clay hybrids: Photochemical studies of stilbazolium derivatives

Abstract

Three kinds of cationic aromatic alkenes, trans-stilbazolium derivatives: trans-4-[2-[4-phenyl]ethenyl]-1-pyridinium chloride (1a), trans-4-[2-[4-phenyl]ethenyl]-1-methylpyridinium bromide (1b), and trans-4-[2-[4-(4′-hexadecyloxy)phenyl]ethenyl]-1-methylpyridinium bromide (1c), were each cointercalated with a cationic polyfluorinated surfactant (C3F-S) within the interlayer of cation-exchangeable clay (SSA) to form hybrid compounds (C3F-S/SSA hybrids). The molecular orientations of the intercalated cationic molecules within the C3F-S/SSA hybrids were investigated photochemically using the stilbazolium derivatives as probes. Compound 1a exhibited selective excimer emission, indicating that at least two 1a molecules take on proximal orientations. However, the major photoproduct from 1a was the cis-isomer, and photodimer production was negligible. Absorption spectroscopic analysis showed that cointercalated 1a molecules were deprotonated within the C3F-S/SSA hybrid. These results suggested that deprotonated 1a was situated in a hydrophobic environment formed by the alkyl chains of C3F-S molecules. Compound 1b also exhibited selective excimer emission and both syn-head-to-head and syn-head-to-tail cyclodimers were obtained, suggesting that 1b could be competitively coadsorbed with C3F-S up to the amount of 100% vs CEC of SSA to form parallel conformational aggregates on the same side of the SSA surface. The amount of 1b that was directly adsorbed on the SSA surface was proportional to the sum of all cationic molecules present (1b and C3F-S). The remaining 1b molecules were solubilized as aggregates in the long alkyl chain “forest” of C3F-S in an antiparallel manner. Compound 1c exhibited predominantly monomer emission, and the cis-isomer was observed as the major photoproduct. These results indicated that cationic 1c was adsorbed as isolated monomers in the C3F-S/SSA hybrids because of the formation of hydrophobic interactions with the substituted long alkyl chains of 1c and C3F-S molecules. Thus, it was revealed that C3F-S/SSA provides a unique microenvironment for guest molecules, such as cationic stilbazolium derivatives, through a delicate balance among (1) electrostatic interactions exerted by the negatively charged SSA surface, (2) hydrophobic interactions from the long alkyl chain of C3F-S, and (3) both hydrophobic and lipophobic interactions from the perfluoropropyl chains of C3F-S that form a polyfluorinated “wall”.