Role of Boron in the Polymer Chemistry and Photophysical Properties of Difluoroboron-Dibenzoylmethane Polylactide

Difluoroboron beta-diketonate-polymer conjugates have remarkable solid-state luminescent properties that are useful in a variety of fields including multipholon microscopy, cell biology, and tumor hypoxia imaging. Despite the successful applications of these systems, the role of boron in these polymeric materials has not been thoroughly investigated. Here we explore a boron-free model system with dibenzoylmethane chromophores in poly(lactic acid) (PLA) for comparison. The hydroxyl-functionalized aromatic diketone, dibenzoylmethane (dbmOH), is weakly fluorescent in the solid state and nonfluorescent in solution while its difluoroboron complex (BF(2)dbmOH) is highly emissive in both states. Using dbmOH and BF(2)dbmOH as initiators, well-defined end-functionalized polylactides, dbmPLA and BF(2)dbmPLA, were obtained via tin-catalyzed controlled ring-opening polymerization. Boronation of the dbmOH initiator affects the polymerization kinetics and the photophysical properties of the resulting BF(2)dbmPLA material. Both dbmPLA and BF2dbmPLA are dual emissive in the solid state, exhibiting both fluorescence and room-temperature phosphorescence (RTP), whereas only BF,dbmPLA is luminescent in solution. These results suggest that boron plays two roles: (1) as a protecting group in the polymerization and (2) as an emission enhancer. Finally, the presence of dual emission for both polymers indicates that it may be the diketone core structure rather than the difluoroboron that is essential for RTP in a rigid PLA matrix,