Grafting of perylene and naphthalene fluorophores onto chitosan for improved thermal, optical and electrical properties

The grafting of N-(4-hydroxyphenyl)-3,4,9,10-perylenetetracarboxylic-3,4-anhydride-9,10-imide (4) and N-(4-hydroxyphenyl)-1,4,5,8-naphthalenetetracarboxylic-1,8-anhydride-4,5-imide (8) onto low-molecular-weight chitosan (5) was performed. The fluorescence, stability, electroactivity, conductivity, and solubility properties of the grafted chitosan polymer 9 were highly enhanced compared to the original chitosan. The weight-average molecular weight (M_w) of 19,800 g/mol was obtained. Grafted chitosan has even more excellent thermal stability with a higher initial decomposition temperature of 285 °C and char yield at 900 °C up to 73%. The fluorescence quantum yield efficiencies for polymer 9 are very high in all studied solvents (70% in CH₃CN). The polymer showed five stepwise, fast, reversible one-electron reductions in electrochemical investigations due to the conductive fluorophores 4 and 8. The HOMO/LUMO levels were calculated as − 5.56 and − 4.14 eV, corresponding to the low band gap of 1.42 eV. The spectroelectrochemistry investigations confirmed the nature of the electron transfers. The morphological characterization using AFM, SEM, and TEM methods indicated a highly crystalline character of the grafted chitosan. The modified chitosan has the potential to be applied in various organic photonics and as a significant substrate material.

Graphical abstract

Grafted chitosan polymers combining high crystalline character and excellent fluorescence, electroactivity and conductivity are promising candidates for photonic applications