Excited-state proton transfer of weak photoacids adsorbed on biomaterials: proton transfer to glucosamine of chitosan.

UV-vis steady-state and time-resolved techniques were employed to study the excited-state proton-transfer process from two weak photoacids positioned next to the surface of chitosan and cellulose. Both chitosan and cellulose are linear polysaccharides; chitosan is composed mainly of d-glucosamine units. In order to overcome the problem of the high basicity of the glucosamine, we chose 2-naphthol (pKa* ≈ 2.7) and 2-naphthol-6-sulfonate (pKa* ≈ 1.7) as the proton emitters because of their ground state pKa (≈9). Next to the 1:1 cellulose:water weight ratio, the ESPT rate of these photoacids is comparable to that of bulk water. We found that the ESPT rate of 2-naphthol (2NP) and 2-naphthol-6-sulfonate (2N6S) next to chitosan in water (1:1) weight ratio samples is higher than in bulk water by a factor of about 5 and 2, respectively. We also found an efficient ESPT process that takes place from these photoacids in the methanol environment next to the chitosan scaffold, whereas ESPT is not observed in methanolic bulk solutions of these photoacids. We therefore conclude that ESPT occurs from these photoacids to the d-glucosamine units that make up chitosan.