Hydrogen-bond assisted nonconventional photoluminescence of crystalline and amorphous cellulose

Cellulose is hierarchically arranged to form a sophisticated supramolecular structure. The structural complexity leads to a dilemma where the intrinsic luminescence properties of cellulose itself have not been well understood. In this study, crystalline and amorphous cellulose from tunicate was prepared to investigate the nonconventional photoluminescence property. In cellulose, the crystalline regions are in the form of cellulose nanocrystals (CNCs). After extraction from the original fibers, the CNCs are used in foams, in films made only of CNCs, and also in CNC/PVA films. In those different systems, there are different hydrogen bonding interactions among the hydroxy groups on the CNC surfaces. CNC foam has the strongest fluorescence emission intensity with the highest photoluminescence quantum yield (PLQY 10.94%), but lowest phosphorescence intensity with the shortest phosphorescence lifetime of 178.77 ms. The CNC/PVA films, on the other hand, have the lowest PLQY (5.15%) and longest phosphorescence lifetime (569.38 ms). The results indicate that hydrogen bonding interaction effectively inhibits the motion of hydroxyl groups on the surface of CNCs, which is favor for stronger phosphorescence emission and longer phosphorescence lifetime. Compared with the crystalline cellulose (pulp film), the highly amorphous cellulose (ReAC-Pulp film) exhibits stronger phosphorescence emission intensity and longer lifetime (808.33 ms), with the maximum emission wavelength significantly blue-shifted from 494 nm to 461 nm. This is attributed to the different intra- and intermolecular hydrogen bonding networks of amorphous cellulose and the lack of surface group motion on the nanocrystals. This hydrogen-bond assisted luminescence property may contribute to reveal the complicated supramolecular structure of cellulose materials.

Automated summary

In this study, crystalline and amorphous cellulose from tunicate were prepared to investigate the non-conventional photoluminescence property. The results showed that CNC foam had the strongest fluorescence emission intensity and shortest phosphorescence lifetime, while CNC/PVA films had the lowest photoluminescence quantum yield and longest phosphorescence lifetime. Highly amorphous cellulose exhibited stronger phosphorescence emission intensity and longer lifetime compared to crystalline cellulose, attributed to the different hydrogen bonding networks and surface group motion on nanocrystals.