Unique Optical Properties of Cellulosic Materials

Our natural environment inspires much of our innovation in all fields of research and application. In particular, unique optical properties are observed in natural systems such as bioluminescence and structural colors generated by bioengineering specific nanostructures. Cellulose is one such naturally occurring material that has been particularly surprising and impactful. Cellulose is one of the most abundant biopolymers with incredible versatility and distinct optical properties. Cellulose nanomaterials can readily self-assemble into chiral nematic phases which induces birefringence resulting in unique optical properties as well as causing incident irradiation to be circularly polarized. These properties unlock possibilities for cellulose materials to be used in encryption and sensing applications to name a few. Thus, cellulose materials have been used extensively as chiral scaffolds in composites but not as luminophores themselves in circularly polarized luminescent (CPL) materials. Recent discovery of the intrinsic luminescence of cellulose has expanded the use of cellulose materials in optical applications.

In addition to structural colors, the study of luminescent properties of cellulose is a perfect example of the scientific method. For many years it was presumed that such materials would only emit if they were contaminated with other luminophores. Researchers stress tested this hypothesis and found that not just cellulose but many everyday, biological and even very structurally simple molecules emit UV and visible light via a clustering triggered emission (CTE) mechanism. This phenomenon employs through-space conjugation of heteroatoms wherein the electron clouds of nearby electron moieties can overlap. CTE combines aspects of aggregation-induced emission (AIE) and crystallization-induced phosphorescence (CIP). There are several characteristic features observed in materials demonstrating CTE. These include concentration-dependent emission, excitation wavelength-dependent and multicolor emission, and room temperature phosphorescence.

The optical properties of cellulose are found to be particularly sensitive to environmental stimuli such as pH, humidity, temperature, etc. thus making them ideal for luminescent sensing applications. Cellulose-derived materials have also been used in a broad spectrum of other applications including encryption, bioimaging, and analytical tools. However, there are several aspects of the field that have yet to be explored. Arguably, the most important of these is the lack of specificity in the CTE mechanism. It is currently unknown what the specific requirements for cluster sizes are and what the maximum spacer length is in order for the cellulose materials to still enable effective heteroatom electronic overlap. Additionally, the materials can often suffer from low quantum yields.

This account includes a brief overview of some of the most impactful optical properties of cellulose materials, including birefringence, CPL and CTE, as well as their applications and perspectives on future research opportunities.