Self-assembly-induced optical properties of carbohydrate derivatives
by Cheng Li
Date of Examination:2025-09-03
Date of issue:2025-10-16
Advisor:Prof. Dr. Kai Zhang
Referee:Prof. Dr. Kai Zhang
Referee:Prof. Dr. Carsten Mai
Persistent Address:
http://dx.doi.org/10.53846/goediss-11572
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Abstract
English
Structural color is a fascinating form of coloration generating from the interaction between incident light and periodic structures at a length scale comparable to the optical wavelength. Unlike the mechanism of structural color formation, fluorescent color is produced by the emission of light from electronically excited states of certain chemicals after they absorb light. Both structural and fluorescent color of materials hold great promise for a wide range of applications, including imaging, sensing, coding, anti-counterfeiting, and encryption. In this thesis, I first prepared the solution-derived spherulites with unique interconnected structural and fluorescent colors, self-assembled from stearoylated monosaccharides at room temperature. D-galactose stearoyl esters self-assemble into banded spherulites, containing twisted nanoplates and interconnected simultaneously changing structural and fluorescent colors. In comparison, D-mannose stearoyl esters can only form non-banded spherulites, which contain oriented nanoplates and uniform structural and fluorescent colors. Such materials revealed a novel negative correlation between fluorescence and birefringence, termed as alignment-promoted quenching propensity. Remarkably, the solid-state fluorescence quantum yields (FLQYs) of galactose and mannose-derived spherulites are as high as 49 ± 2% and 51 ± 2% respectively, approximately ten times higher than those of unmodified monosaccharides. These quantum yield values are even comparable to those of conventional aromatic chromophores. Moreover, these spherulites manifested an unexpected excitation-dependent multicolor photoluminescence with a broad-spectrum emission (410−620 nm). They show multiple peaks in the photoluminescent emission spectra and broad fluorescence lifetime distributions, which should be attributed to the clustering of a variety of oxygen-containing functional groups as the emissive moieties. Then, inspired by the intriguing optical properties of the monosaccharide-based spherulites, surface-stearoylated/lauroylated cellulose nanocrystals (CNCs) and polymeric cellulose stearoyl/lauroyl esters were prepared and allowed to construct macroscale helices (MHs) and porous bowl-shaped microparticles (BSMPs). Surprisingly, the two superstructures are highly emissive, with unanticipated high solid-state fluorescence quantum yields (FLQYs) of 86% and 91%, which are the highest among reported biomass-based systems. Specifically, surface-stearoylated CNCs and cellulose stearoyl esters co-assemble into MHs (FLQY: 86%) with diameters of 32−104 μm, consisting of layered nanoplates with thicknesses of 44−94 nm. Meanwhile, surface-lauroylated CNCs and cellulose lauroyl esters co-assemble into porous BSMPs (FLQY: 91%) with diameters of 8−19 μm, consisting of nanosized flaky structures with thicknesses of 70−120 nm. The two superstructures show excitation-dependent fluorescence in a broad wavelength range. The outstanding FLQYs are ascribed to the synergism of the dense oxygen clusters and abundant van der Waals interactions and hydrogen bonds between side stearoyl or lauroyl groups, which can promote through-space electron delocalization, ultimately improving fluorescence performance. These results were rationalized by theoretical calculations. Such superstructures exhibited great potential for stable anti-counterfeiting materials due to the excellent regeneration ability as well as structural stability of the oxygen clusters. In addition to the stearoylated and lauroylated carbohydrates mentioned above, D-glucose 11-octadecylthioundecanoate (D-Glc-C11S18E) was prepared as well. D-Glc-C11S18E can self-assemble into microspheres with diameters of 3−6 μm, composed of nanospheres with diameters of 100−300 nm. Within these nanospheres, a lamellar structure formed by octadecylthioundecanoyl groups (C29 lamella) developed in the interior, while a lamella formed by octadecyl groups (C18 lamella) appeared at the interfaces between adjacent nanospheres. Upon heating to 50 °C, the C18 lamella dissociated into an irregular structure, whereas the C29 lamella remained intact. When cooled to 20 °C, the C18 lamella reformed, demonstrating a selectively reversible self-assembly behavior for the two lamellar structures. When dispersed in methanol, the clustered microspheres dissociated into individual nanospheres at 50 °C, and individual ones reassembled into clustered ones at 20 °C, showing a reversible morphological nanostructure organization. This reversible structural transition was accompanied by fluorescence modulation, as the C18 and C29 lamellae induced distinct oxygen cluster environments. Notably, both the nanostructural and fluorescent modulations were reversible over at least ten heating−cooling cycles without fatigue. Benefiting from the reversible fluorescence, a fluorescent logic gate was constructed. In addition to providing fresh ideas for fundamental self-assembly study, these findings offer inspirations for developing smart optoelectronic devices based on reversible self-assembly. This thesis is a cumulative work including 3 publications. Two were already published and one was submitted to peer-reviewed journals. The background, objective of the study, results and discussion, and general conclusion and perspectives are presented in Section 1-4.
Keywords: self-assembly; carbohydrate; fluorescence; structural color
