Synthesis of hyperpolarizable biomaterials at molecular level based on pyridinium–chitosan complexes

Abstract

Potential NLO-phore materials based on chitosan are described for the first time in this study. A series of fluorescent and quaternized pyridinium–chitosan derivatives have been synthesized by reaction of this polymer with easily available tunable pyrylium tetrafluoroborate salts. Among other spectroscopic techniques, ¹⁹F NMR, ¹³C CPMAS NMR and 2D diffusion experiments, were used to confirm the structures of the new pyridinium–chitosan complexes that show high fluorescence intensity. Degrees of N-substitution were achieved lower than 4.3%, allowing the original physicochemical properties of the biopolymer to be preserved. DFT calculations have been performed to investigate the molecular features related to the NLO properties in these compounds. NLO behavior was found to be clearly dependent on the nature and location of the substituent into the pyridinium core. Theoretical data reveal a large permanent dipolar moment, polarizabilities and hyperpolarizabilities making these molecules promising candidates as supramolecular devices exhibiting NLO properties with potentially enhanced solvatochromic properties.