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.

Supramolecular Polymer–Molecule Complexes as Gain Media for Ultraviolet Lasers

“…As a concentration is increased, the spectral contribution at longer wavelengths noticeably rises. There is also, what appears to be, the appearance of an supramolecular aggregated emission peak at ~ 450 nm, (please note that an excimer may exist in the aggregation states 26 ). It is clearly visible for the highest concentrations ( Figure S10).…”

Aggregation enhanced pure violet emission of a spiral meta-polyfluorene by supramolecular control of excimer formation

“…1A. 22 The number-average molecular weight (M n ) of PsF36 is 14 000 (PDI = 1.9), where each polymer chain possesses B30-60 repeating units on average. In the case of meta-linked PF36, the 3-position of a fluorene unit is linked with the 6-position of an adjacent fluorene unit.…”

“…PsF36 exhibits low energy emission at high concentration of good solvents such as CHCl 3 , toluene and THF, which is due to excimer emission. 22 Fig. 2A and B show the absorption and PL spectra of PsF36 in THF solution and spin-coated film (single polymer chain and random aggregation), respectively.…”

“…21 In our previous work, we have determined the fluorene based excimer to be the origin of low energy emission in meta-linked polyspirobifluorenes. 22 Different from structural defects, excimer formation depends on the nature of the p-conjugated system, which is probably formed by the tight packing of p-molecules (or p-segments) with the distance smaller than 4.0 Å. 23 In previous studies, the reported effective strategy to suppress excimer emission focuses on increasing the intermolecular distance, by introducing large substituents and cycloalkane coating, [24][25][26][27] but it may have some negative effects on the comprehensive performance of materials for optoelectronic applications, such as lowered charge mobility and morphological stability as the result of weakened intermolecular interactions.…”

Novel violet emitting material synthesized by stepwise chemical reactions

“…This observation might be due to the aggregation effect as the intermolecular interactions in the thin lms were much stronger than those of the dilute solution (10 À5 M). 33,34 Especially for CzPySiSF, the emission peak at 402 nm became even stronger and a new shoulder was observed at 424 nm in the solid state.…”