Clustering-Triggered Emission and Luminescence Regulation by Molecular Arrangement of Nonaromatic Polyamide-6

Photoluminescent polymers with merely nonaromatic chromophores have attracted rapidly growing attention owing to their importance in the significant fundamental, encryption, and anticounterfeiting fields. Based on the clustering-triggered emission mechanism, through-space conjugation and conformational rigidification of nonaromatic chromophores are crucial to photoluminescence, which are also dependent on molecular arrangement. Herein, polyamide-6 (PA-6) with well-defined molecular arrangements was thus studied. The luminescence from the PA-6 solution is enhanced upon aggregation from solution to amorphous solid and further boosted with the formation of highly regular molecular arrangement. More importantly, both blue and green fluorescence from PA-6/formic acid (FA) solutions were observed because of the variable clusters formed among PA-6 and FA. To make clear of this, PA-6 cast film (PCF) and PA-6 electrospun film (PEF) were prepared and belonged to α- (antiparallel molecular arrangement) and γ (parallel molecular arrangement)-form crystals, as confirmed by Fourier transform infrared, X-ray diffraction, and Raman measurements. The relationship between molecular arrangement and luminescence of PA-6 molecules was clarified by their photophysical properties in solids and solutions. Notably, color-tunable cryogenic phosphorescence of PA-6 solids was also detected. Such aggregation-enhanced emission and tunable phosphorescence of PA-6 solids are ascribed to the formation of diversified amide clusters together with remarkably rigidified molecular conformations owing to the highly regular molecular arrangement in the aggregated states.