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

Abstract

Optoelectronic properties of polyfluorene, a blue light-emitting organic semiconductor, are often degraded by the presence of green emission that originates mainly from oxidation of the polymer. Here, we use single-molecule electroluminescence (EL) and photoluminescence (PL) spectroscopy on polyfluorene chains confined in vertical cylinders of a phase-separated block copolymer to spectrally resolve the green band and investigate in detail the photophysical processes responsible for its appearance. In both EL and PL, a substantial fraction of polyfluorene chains shows spectrally stable green emission which is ascribed to a keto defect. In addition, in EL, we observe a new type of vibrationally resolved spectra distributed over a wide range of frequencies and showing strong spectral dynamics. Based on quantum chemical calculations, this type is proposed to originate from charge-assisted formation and stabilization of ground-state aggregates. The results are expected to have broad implications in the fields of photophysics and material design of polyfluorene materials.

Single-Molecule Spectroscopy of Polyfluorene Chains Reveals β-Phase Content and Phase Reversibility in Organic Solvents

“…23,24 The use of single-molecule spectroscopy (SMS), with its unique ability to investigate polymer chains one by one, [25][26][27] has been particularly useful for the study of PFO because its intrinsic phase heterogeneity cannot be resolved by conventional ensemble-averaging spectroscopy. 25,[28][29][30] SMS studies of PFO molecules dispersed in a Zeonex matrix demonstrated the coexistence of aand b-phase chains at 5 K. 31 Considering the highly diluted nature of the samples used in these studies, it was concluded that b-phase formation does not need interchain interactions and that stress-induced backbone planarization could result from interactions with the host matrix. Similar SMS studies using matrix-embedded PFO oligomers have shown that b-phase formation is a molecular-length-dependent process and that nine or more repeat units are necessary to form a stable planarized structure, at least when prepared through the vapor-swelling procedure.…”

Highly Luminescent Organic Nanorods from Air Oxidation of para‐Substituted Anilines for Freestanding Deep‐Red Color Filters

“…The FWHM values for the ONs are ≈25 nm, which is comparable to those of semiconductor quantum dots (30–40 nm). The minor peaks detected at longer wavelengths (≈30 nm) are attributed to the vibronic transitions of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states, which are typically observed in organic fluorophores . Furthermore, for the F‐, C‐, and N‐ON samples, relatively small emission bands appear in the 400–500 nm region, presumably due to oxygen‐containing defects.…”

Optically switchable organic light-emitting transistors

“…Furthermore, we note the presence of an emission band at longer wavelengths, i.e. a green band, which is common in F8-based emissive devices due to the formation of inter-chain states and/or fluorenone defects [37][38][39][40] . Importantly, the emissive bands of the three OSOLETs cover well both the visible region (400-700 nm) and even stretch into the near infrared (NIR) up to 800 nm.…”