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Fluorescent labeling of a bisurea-based supramolecular polymer.

Bisurea-based supramolecular polymer 2-ethylhexyl-3-[3-(3-(2-ethylhexyl)ureido)-4-methyl-phenyl]urea (EHUT) has been shown previously to self-assemble through hydrogen bonding into high-molecular-weight structures. The present publication reports the study of the thermodynamics of these tubular structures by time-resolved fluorescence spectroscopy, with the help of a tetrazine labeled monomer. Results of calorimetry and time-resolved fluorescence spectroscopy show that the as-modified monomer EHUTz does not interfere with the formation of the supramolecular assembly. When incorporated, these labeled monomers exhibit a longer fluorescence lifetime due to the electron-rich tolyl group buried in the structure. Dilution experiments allowed us to measure their partition coefficient, and to compare it with the critical aggregation concentration of EHUT. Measurements at higher dye loads, where interactions between neighboring tetrazines occur, show that EHUTz is uniformly dissolved in the supramolecular polymer. Tetrazine dye is a good reporter of events occurring in solution, such as disruption of the assembly upon heating. Our results confirm the pseudophase diagram for EHUT solution in toluene obtained previously with other techniques such as infrared spectroscopy and calorimetry.