Anomalous photoluminescence properties of emissive polymer originating from modulated chemical structures affected by antioxidant effect of natural β-carotene

Abstract

Abstract We investigated the antioxidant effect of natural β-carotene on the photoluminescence (PL) properties of emissive polymers and extracted the β-carotene from spinach using column chromatography. The follow-up observation of the PL spectra of β-carotene-doped poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) revealed anomalous properties; the PL intensity gradually increased. In addition, the PL peaking-wavelength was blue-shifted. To evaluate this phenomenon, we performed nuclear magnetic resonance (NMR) and PL measurements of other two emissive polymers whose chemical structures resemble those of MDMO-PPV. MDMO-PPV and β-carotene were mixed in the solution for the NMR measurement and irradiated by UV to accelerate the chemical reactions. NMR measurement clearly shows that the chemical structure of MDMO-PPV was changed by β-carotene-doping. In particular, the conjugated double-bond structure of the main chain backbone was shortened by the cleavage of the alkene, breaking the side chains of the methoxy groups. Thus, the origin of the blue-shifted PL spectra can be attributed to these changes in the MDMO-PPV’s chemical structure. To confirm this, the PL properties of MDMO-PPV and two other β-carotene-doped emissive polymers in oxygen gas were evaluated. Their PL intensities exhibited similar increases, but their peaking-wavelengths were not blue-shifted. This indicates that although β-carotene can improve the PL lifetime of emissive polymers, the chemical structures of some polymers can be easily destroyed.