On‐Chain Fluorenone Defect Emission from Single Polyfluorene Molecules in the Absence of Intermolecular Interactions

Abstract

The emission of semiconducting polyfluorenes is often accompanied by an undesired feature in the green spectral region. Whereas a number of previous investigations have argued in favor of a monomolecular origin of the emission species based on ketonic defects, recent experimental results suggested the necessity of excimer formation between individual fluorenone units. We provide a range of new evidence supporting the monomolecular origin of green band emission in polyfluorenes. Most importantly, we succeed in performing single‐molecule spectroscopy on fluorenone‐containing polyfluorene model compounds. Whereas most fluorenone‐containing molecules exhibit both blue backbone and green fluorenone emission independent of fluorenone concentration, it is the relative intensities of the two species which correlate strongly with the fluorenone concentration on the single‐molecule level. Furthermore, we consider a novel model compound with a bifacial arrangement of two fluorenone units. This compound does not provide any signatures of enhanced intramolecular excimer formation but does strongly indicate that concentration quenching effects occur once fluorenone units can interact electronically. The ability to detect on‐chain defect emission in a single polymer molecule demonstrates that photochemical reactions in conjugated polymers can be monitored by fluorescence spectroscopy down to the level of a few atoms, constituting an unprecedented degree of materials characterization.

Single-molecule electroluminescence and photoluminescence of polyfluorene unveils the photophysics behind the green emission band

“…Some studies indicated that interchain interactions between fluorenones and formation of excimers are required for the appearance of the green band 24 . These claims have later been disputed by single-chain spectroscopic experiments 25 . The prevalence of the green band in EL can be explained by assuming that charges are preferentially trapped on low-energy sites in the film, the fluorenone defects.…”

Photophysics of Fluorene Copolymers: Control of Fluorescence and Charge Separation by the Presence of Carbazole, Oxadiazole, or Biphenyl Units

“…It was easily converted to the dicarboxylic acid by lithiation followed by reacting with CO 2 . This reaction is almost quantitative, and the 19 F NMR showed no impurities in the crude product. The purified dicarboxylic acid was then reacted with the tetrazole compound to form the final monomer through a two-step reaction without purifying the intermediate.…”

“…These photoluminescence spectra of P(F x y Cz) polymers only show a very weak emission tail expanding to the green-orange region. This tail is observed for the fluorene homopolymer and based on the literature can be attributed to the contribution of two different phenomena: on the one hand to a dipole transition in the keto-defect units 19,20 and on the other to an increase of the interchain interactions. 21 It has been reported that oxidized chains of fluorene homoplymer, due to the spatial distribution of keto-defects units, have an absorption band extending into the green spectral region.…”

Deep-red electrophosphorescence from a platinum(II)–porphyrin complex copolymerised with polyfluorene for efficient energy transfer and triplet harvesting

“…We also note a weak fluorescence emission band between 500 and 600 nm that can be related to the presence of fluorenone defects or aggregates/excimers whose presence is further validated by the TCSPC data. [23,24] In fact, the radiative lifetime of PFO taken at 550 nm (Fig. S2) can be fit with a bi-exponential function yielding time constants of ∼ 400 ps and 6.6 ns that are consistent with PFO exciton and fluorenone excimer respectively.…”