Micro-photoluminescence mapping of photonic heterogeneity in cellulose nanocrystal films

Cellulose nanocrystals self-assemble into cholesteric liquid-crystalline structures that exhibit photonic bandgaps in the visible range. While the morphology of cellulose nanocrystal films has been widely studied, the relationship between microscale domain structure and local photoluminescence modulation by photonic bandgap effects remains insufficiently understood. Here, Rhodamine 6G is incorporated into cellulose nanocrystal films and spatially resolved micro-photoluminescence mapping is used to probe this relationship. Films cast under controlled humidity form relatively larger domains with fewer boundaries and exhibit lower spontaneous photoluminescence intensity due to a redshift of the photonic bandgap away from the dye emission and increased scope for dye aggregation during slow drying. In contrast, films cast at room temperature under ambient conditions exhibit smaller domains with a higher density of boundaries and enhanced emission at low dye concentration (0.05 wt. %), where spectral matching between emission and reflection is strongest. Increasing the dye concentration up to 0.5 wt. % does not enhance emission, but instead contributes to spectral detuning and, together with concentration-dependent quenching, reduces photoluminescence. Quantitative spectral-overlap analysis confirms that emission intensity is governed primarily by reflection–emission proximity rather than dye concentration alone. This work establishes micro-photoluminescence mapping as a sensitive approach for linking microscale structural heterogeneity to local optical response in cellulose nanocrystal-based photonic materials.