Optically Ambidextrous Reflection and Luminescence in Self-Organized Left-Handed Chiral Nematic Cellulose Nanocrystal Films

Abstract

Defect-embedded chiral nematic organizations are attractive due to their light manipulation ability that is crucial for photonic applications. Self-assembly of cellulose nanocrystals (CNC) forms a left-handed chiral nematic structure, which makes simultaneous reflection of left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) light a challenging task. Herein, we present unprecedented evidence that self-organized CNC films with left-handed helical sense and photonic bandgaps (PBG) display ambidextrous CP light reflection with peak reflectivity up to 68% and transform spontaneous photoluminescence to RCP and LCP luminescence with respective |g lum | values up to 0.796 and 0.120. These ambidextrous chiroptical properties originate from a left-handed chiral nematic structure consisting of a nematic-like phase, acting as a half-wave retarder, sandwiched between two PBG layers. The latter are spontaneously intercalated into the left-handed helicoids by rapid gelation of CNC colloidal suspensions. We demonstrate that the ambidextrous chiroptical properties can be manipulated by changes in evaporation temperatures, concentrations, and characteristics of CNC suspensions. We showcase the potential of defect-embedded chiral nematic CNC films for the sustainable and scalable development of anti-counterfeiting optical labels.

Cellulose for Light Manipulation: Methods, Applications, and Prospects

“…These optical properties arise from the left-handed chiral nematic arrangement, which is produced by evaporation-induced self-assembly from CNC suspensions at a critical concentration. [61,62] Therefore, a maximum of 50% of left-handed circularly polarized light will be reflected, and the opposite handedness transmits through the film without loss of intensity. The structural colors that is the wavelength of the reflected light can be manipulated by changing the helical pitch size of the chiral nematic structure.…”

“…A large number of different methods, such as changing the ionic strength of the suspension, using ultrasound, changing the drying conditions, or using magnetic or electrical fields as well as the use of different substrates on which the helical structure can be created have already been described to specifically manipulate the optical properties. [2,52,57,[60][61][62][63][64][65][66][67][68][69][70][71] The distance of the helical pitch can be changed by creating a higher suspension concentration or by adding electrolytes. Increasing the electrolyte concentration, by adding HCl, NaCl, or even KCl, will reduce the electrostatic repulsion between the CNCs.…”

“…Exceeding a critical amount of electrolytes can lead to coagulation and thus to gelation or precipitation of the CNC. [57,61] The chiral nematic pitch of self-assembled CNCs in suspension can also be affected by the addition of polyethylene glycol (PEG). With increasing amount of PEG, a red-shift of the reflection peak is observed, which can be attributed to an increasing helical pitch (Figure 4).…”

Signal transmission encryption based on dye-doped chiral liquid crystals via tunable and efficient circularly polarized luminescence

“…Chiroptical materials with circularly polarized luminescence (CPL) provides an exceptional platform for optical displays, [1][2][3][4][5][6] data storage [7][8][9] and sensors 10,11 . Herein, to endow luminescent materials with an extra dimensional property of polarization plays an important role in these application fields.…”

Chiral transfer‐dictated self‐assembly of chiral block copolymers

“…[224,238] Beyond the chiral helical/spiral/network phase, it has been manifested that chiral liquid crystalline phases from small molecules displayed chiral optical and luminescent functionalities. [239,240] These characters might be carried by liquid crystalline BCPs* as well. [89,92]…”