Physical gelation process for cellulose whose hydroxyl groups are regioselectively substituted by fluorescent groups

Five cellulose derivatives whose hydroxyl groups are regioselectively substituted by benzyl ether and methyl ether groups were prepared, i.e. 6-O-benzylcellulose, 2,3-di-O-methyl-6-O-benzylcellulose, 2,3-di-O-benzylcellulose (23B6O), 2,3-di-O-benzyl-6-O-methylcellulose and 2,3,6-tri-O-benzylcellulose. The gelation did not take place in tetrahydrofuran solutions of the cellulose derivatives whose hydroxyl group at the 6-position was substituted to methyl ether or benzyl ether groups, but only the cellulose derivative having the 6-position hydroxyl group, i.e. 23B6O, was found to form gels. All the samples except 23B6O showed only usual fluorescence of benzyl group over the temperature range 200-310 K. In contrast, the fluorescence of 23B6O shifted to the red and the excimer fluorescence increased with an increase of interactions between the cellulose molecules. Thus, our fluorescent probe method could elucidate the gelation process with a change of temperature in terms of the molecular association involving a hydrogen bond. In addition, the new absorption corresponding to the red-shifted fluorescence peak was also confirmed. The new species were concluded to be a ground state dimer formed intermolecularly between benzyl groups, meaning that there exists a hydrophobic interaction between them. In conclusion, (1) the main cause for the gel formation in our system is the hydrogen bonding by means of the 6-position hydroxyl group, and (2) the hydrophobic interaction between benzyl groups also keeps 23B6O molecules associated with one another, as well as the hydrogen bonds, after it is aggregated.