Inversion of Circularly Polarized Luminescence in the Left-Handed Chitosan-Templated Co-assemblies

Abstract

Circularly polarized luminescence (CPL) materials are attractive due to their unique applications in fields such as 3D displays, information encryption, and chiroptical switches. Natural biomolecules-based CPL materials are gaining plenty of attention due to their chiral diversity and sustainability. However, it is still challenging to construct CPL materials with opposite CPL signs from a single natural biomolecule due to its inherent chirality. Here, chiral assemblies with opposite CPL signs using chitosan oligosaccharide (COS) and achiral luminescent dyes are successfully prepared. It is found that COS can serve as a chiral template to induce the ordered assembly of the dyes along the polymer chain through electrostatic attraction interaction. It is demonstrated experimentally that the structural planarity of the dye molecules is crucial for the formation of chiral co-assemblies. Interestingly, the left-handed COS-templated co-assemblies can emit CPL with opposite handedness, which is controlled by the helicity degree of the co-assemblies. This study not only deepens the understanding of the complex assembly of natural biomacromolecules but also provides new insights into the design and construction of CPL materials.

Keywords: J‐aggregates; chiral assembly; chitosan; circularly polarized luminescence.

Scheme 1

Schematic illustration of COS‐templated assembly with dye molecules and the CPL inversion. The planar molecule CBS can assemble along the helical COS template to form left‐handed J‐aggregates, which emit opposite CPL signals depending on the equivalents of CBS and aging time.

Figure 1

Normalized CD and UV–vis spectra of CBS solution and COS/CBS mixtures upon mixing at different COS_unit/CBS molar ratios with different COS concentrations: a,b) [COS]_unit = 1 mm. c,d) [COS]_unit = 5 mm. e,f) [COS]_unit = 10 mm. g) Plots of CD intensity at 380 nm and 330 nm of COS/CBS suspensions as a function of CBS/COS_unit molar ratios. [COS]_unit = 5 mm. h) Schematic representation of the COS‐templated co‐assembly with CBS into a left‐handed helix upon mixing.

Figure 2

CD and UV–vis spectra of COS/CBS suspensions at COS_unit/CBS molar ratios of a,b) 1:0.2 and (c,d) 1:0.6 at different aging times. [COS]_unit = 5 mm.

Figure 3

a) Normalized fluorescence spectra of COS/CBS suspensions at different COS_unit/CBS molar ratios. λ_ex = 347 nm, [COS]_unit = 5 mm. b) Normalized CPL spectra of COS/CBS suspensions upon mixing at different COS_unit/CBS molar ratios. λ_ex = 347 nm. c) The g _lum values of COS/CBS suspensions upon mixing at COS_unit/CBS molar ratios of 1:0.2 and 1:0.6. [COS]_unit = 5 mm. d) Schematic representation of the CPL emission with the same handedness but different g _lum values from the left‐handed helical co‐assemblies at COS_unit/CBS molar ratios of 1:0.2 and 1:0.6 without aging.

Figure 4

a,b) Normalized CPL spectra of COS/CBS suspensions at COS_unit/CBS molar ratios of a) 1:0.2 and b) 1:0.6 at different aging times. [COS]_unit = 5 mm, λ_ex = 347 nm. c) Plots of CPL intensity for COS/CBS suspensions at COS_unit/CBS molar ratios of 1:0.2 (467 nm) and 1:0.6 (442 nm) as a function of aging time. d) The g _lum values of COS/CBS suspensions at COS_unit/CBS molar ratios of 1:0.2 and 1:0.6 aged for 1 h. e) Schematic representation of the CPL signal inversion induced by the helicity increasing of COS‐templated co‐assembly with the aging time. The COS_unit/CBS molar ratio is 1:0.6.