Reversible chemistry of CO2 in the preparation of fluorescent supramolecular polymers

The chemistry between CO2 and primary amines was used to construct novel types of supramolecular polymers and networks. Fluorescent self-assembling gel 2 was prepared, which employs both hydrogen bonding and dynamic, thermally reversible carbamate bonds. As precursors, biscalixarenes 1, 3, 4, 6, and 7 were synthesized, which strongly aggregate (K-D greater than or equal to 10(6) M-1 per capsule) in apolar solution with the formation of linear self-assembling polymers. Polymer 1(n) possesses CO2-philic primary amino groups on the periphery. CO2 rapidly reacts with chains 1(n) in apolar solvents and cross-links them with the formation of multiple carbamate salt bridges. Three-dimensional polymeric network 2 was characterized by C-13 NMR spectroscopy and SEM. Addition of competitive solvent breaks hydrogen bonding in 2 but does not influence the carbamate linkers. Carbamate salt 9 was obtained. On the other hand, thermal release of CO2 from 2 and 9 was easily accomplished (1 h, 100 degreesC) with retaining the hydrogen-bonding capsules. Thus, three-dimensional polymeric networks 2 were transformed back to linear polymeric chains 1(n) without their breakup. Multiple pyrene fluorophores, attached on the periphery of 2, cause strong fluorescence of the gel with benzene. When similar to5% nitrobenzene was gelated together with benzene, fluorescence strongly decreases due to the energy transfer from the pyrene donors in gel 2 to trapped nitrobenzene molecules. This opens a way to switchable fluorescent materials.