葡聚糖的光致发光性质及机理研究

Photoluminescence and Luminescent Mechanism of Dextran

天然产物具有良好的生物相容性和环境友好性,研究此类非典型发光化合物对发光机理的理解以及绿色环保型发光材料的开发具有重要意义.基于此,本文研究了天然产物葡聚糖的光致发光性质与发光机理.结果表明,葡聚糖发光具有浓度增强、激发波长依赖性、分子量依赖性及聚集诱导发光等特性,其浓溶液在低温下表现出明显的激发波长依赖性的余辉,并且高分子量葡聚糖粉末还具有超长寿命室温磷光(p-RTP)发射.这些现象可用簇聚诱导发光(CTE)机理进行合理解释.随着葡聚糖浓度的增加或形成聚集体,氧原子簇聚形成不同的离域扩展的氧簇,其在O···O相互作用、氢键、分子链缠结等协同作用下构象刚硬化,因此能够受激发射.经压片后,葡聚糖片呈现更明显的长余辉,这是由于压片后分子堆积更为紧密,分子间相互作用增强,从而减少了分子运动引起的非辐射跃迁及空气中氧气和水分子对三线态激子的猝灭.

Natural products have good biocompatibility and environmental friendliness.Herein,the photoluminescence(PL)of dextran aqueous solutions at different temperatures as well as that of their solid powders with different molecular weights is systematically investigated.It is found that dextran demonstrates the characteristics of concentration-enhanced emission and aggregation-induced emission(AIE).In addition,strong blue and green phosphorescence are observed after ceasing the 312 and 365 nm UV light at 77 K in concentrated solutions,respectively,featuring excitation-dependent characteristics.With the increment of the molecular weights(20 kDa,70 kDa,150 kDa,500 kDa),the quantum efficiency of dextran powders is progressively increased(1.6%,2.4%,3.2%,3.6%).Furthermore,the dextran solid powders with high molecular weights(150 kDa,500 kDa)even exhibit distinct persistent room temperature phosphorescence(p-RTP).Dextran tablets readily offer much more conspicuous afterglow with comparison to those of their powder counterparts.These phenomena can be well rationalized by the clustering-triggered emission(CTE)mechanism.Namely,in dilute solutions,polymer chains are well stretched and difficult to be excited owing to their short absorption.However,with the increasing concentration of dextran or the formation of aggregates,the clustering of oxygen atoms results in diverse emissive clusters with extended delocalization,which can get excited to emit with rigidified conformations induced by synergistic effect of O···O contacts,hydrogen bonds,chain entanglements,etc.The presence of varying emissive species with diverse electronic conjugation results in the excitation-dependent emission.Moreover,triplets are stabilized on account of the impeded molecular motions and decreased oxygen and moisture quenching,thus favoring p-RTP emissions.The study of dextran is conducive to gaining deeper insights into CTE mechanism and to develop environment-friendly luminescent materials.