Local viscosity and solvent relaxation experienced by rod-like fluorophores in AOT/4-chlorophenol/m-xylene organogels

• AOT-based organogels consist of rigid fiber bundles with solvent trapped in interstices.
• Rotational mobility of rod-like fluorophores inside the gels is extremely fast.
• Hence low-microviscosity interstitial solvent domains exist far from the rigid fibers.
• Solvent relaxation in these interstitial solvent domains is slower than that in neat solvent.
• Solvent trapped in interstitial domains may be described to be in a dynamically “bound” state.

Organogels prepared from AOT/4-chlorophenol/m-xylene are immobile in the macroscopic sense, with a well-characterized internal structure. However, the molecular level dynamics inside the gels is not too clear, although a very slow structural relaxation has been reported previously. Using a set of rod-like fluorophores, we find that the rotational mobility of a small guest molecule inside the gel can be extremely fast, indicating presence of sufficiently low-microviscosity domains. These domains consist of m-xylene solvent molecules trapped in the interstices of fiber bundles comprising columnar stacks of 4-chlorophenol surrounded by AOT molecules. However, interstitial trapping of m-xylene does retard its own dynamics, which explains the slow solvent relaxation inside the gels. Hence, the state of m-xylene in the organogel may be characterized as “bound”, in contrast to the “free” state in neat m-xylene.

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