Excimer-Induced Low-Energy Emission in Spirobifluorene-Based Polymer: The Role of Meta-Linkage

Abstract

The behavior of intrinsic trace aggregate species based on π–π stacking in conjugated polymer and its influencing factors are significantly important for the color purity and stability of polymer light-emitting diodes. In this work, low-energy emission has been observed in the aggregate state of meta-linked polymer poly(9,9′-spirobifluorene-3,6-diyl). The origin of low-energy emission has been testified as an excimer emission because the absorption spectra for a well-dissolved dilute solution and that of the aggregated state are quite similar in shape, and the concentration/solvent dependence of low-energy emission in emission spectra is well consistent with character of an excimer. The excimer emission is only observed in meta-linked polymers but not para-linked ones, which indicates the importance of meta-linkage and the corresponding conformation in the formation of an excimer. The meta-linkage effectively broke the conjugation, which allows a fluorene segment as the excimer building block in the polymer, and induces chain distortion, where interchain interaction is not restrained by an adjacent fluorene segment. Especially when the adjacent 9-positions of fluorenes are in the same side of the backbone (defined as cis-conformation), local big chain distortion tends to induce a more effective segment interaction.