Pyridinium salts are very useful synthetic building blocks to obtain substituted pyridines, dihydropyridines or piperidines. Furthermore, pyridinium dyes present applications on multiple fields including biological and optical responses due to their photophysical features, in particular, high fluorescence, charge transfer character and solvatochromic properties. Recently, pyridinium-chitosan derivatives have shown NLO behaviour at molecular level as well as intrinsic fluorescent features.
Different methods are available for the synthesis of pyridinium salts depending on the symmetry around of the cation. One of them involves the use of pyrylium salts as precursor. The synthesis of pyrylium tetrafluoroborate salts can be achieved by direct reaction of 4-substituted benzaldehydes with 4-substituted acetophenones or via the previously obtained chalcone of the less reactive ketone by using Lewis acid catalyst which mediate cyclisation step. This kind of compounds constitutes a promising class of tunable emission wavelength dyes for laser technology applications.
Herein, we report the synthesis and characterization of new pyridinium-chitosan derivatives by reaction of this bipolymer with electronically tunable 2,4,6-tri-arylpyrylium tetrafluoroborates in mild conditions to preserve the original physicochemical properties of the biopolymer. Fluorescence studies have been also performed on polymeric biomaterial in order to demonstrate the redox sensor behaviour.