Studies of Green Emission in Polyfluorenes Using a Model Polymer

He, Benqiao; Li, Jing; Bo, Zhishan; Huang, Yong

doi:10.1295/polymj.pj2007041

Cited by 11 publications

(8 citation statements)

References 36 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Therefore, it was concluded that the cause of the g-band was oxidation of a fluorene unit and formation of a keto defect. In a copolymer of fluorene and fluorenone (species formed by oxidation of fluorene), increasing fraction of the fluorenone caused increase of the intensity of the g-band. − This result directly supported the hypothesis of oxidation of fluorene monomers at the 9-position as the cause of the g-band. In addition, it was later found that the intensity of the g-band also increased by increasing the concentration of this copolymer in solution .…”

Section: Introductionsupporting

confidence: 52%

Intrachain Aggregates as the Origin of Green Emission in Polyfluorene Studied on Ensemble and Single-Chain Level

et al. 2018

View full text Add to dashboard Cite

Polyfluorenes are conjugated polymers that show strong blue emission and as such have been explored for potential applications in light-emitting devices. However, heat treatment, prolonged exposure to air, or extended operation in electroluminescent devices can lead to an appearance of parasitic green emission that degrades the material performance. This phenomenon has been extensively studied over the past two decades, and two main and conflicting explanations, i.e., oxidation and formation of fluorenone species on the one hand and inter-or intrachain aggregation on the other, have been put forward. There is abundant experimental evidence to support either of these theories, and the question is far from settled. Here, we aim at getting deeper insight into the problem of the green emission origin using single-molecule spectroscopy performed on individual chains of poly(9,9-di-n-octylfluorene) (PFO) to resolve the green emission band and reveal its spectral and temporal heterogeneity. We disperse single PFO chains in solid thin-film matrices of polystyrene (PS) and poly(methyl methacrylate), as well as in solutions of cyclohexane, toluene, or PS/toluene, to simulate good and poor-solvent environments and environments with different permeabilities and diffusions of oxygen, to systematically study the effects of intrachain aggregation as well as oxidation on the appearance and characteristics of the green band. The studies are complemented by direct measurement of individual chain conformation by atomic force microscopy and by bulk measurements of photoluminescence (PL) lifetimes and quantum yield. The single-molecule results reveal two PL spectral forms in the region of the green emission, a vibrationally resolved type located around 500 nm and broad structureless type located toward lower energies, none of them sensitive to the presence of oxygen. These two types are characterized by different lifetimes of 1.4 and 5.1 ns, respectively, and their oscillator strengths are 2 orders of magnitude smaller compared to those of the blue emission band. These results point to two different optical transitions comprising the green band, and these have been assigned to the emission of H-aggregates and charge transfer or indirectly excited excimer states, respectively.

show abstract

Section: Introductionsupporting

confidence: 52%

Intrachain Aggregates as the Origin of Green Emission in Polyfluorene Studied on Ensemble and Single-Chain Level

et al. 2018

View full text Add to dashboard Cite

show abstract

“…Polyfluorene polymers are well-known for their blue light emitting properties , however, undesired green-yellow band emissions are typically observed under photo-oxidative conditions and thermal annealing as a result of formation of excimers and aggregates . This yellow emission is attributed to the presence of either carbonyl (CO) or H-bonding groups in the polymer backbone. These films also become insoluble in solvents due to potential cross-linking during thermal annealing.…”

Section: Resultsmentioning

confidence: 99%

Polymer-Free Electronic-Grade Aligned Semiconducting Carbon Nanotube Array

Joo

Brady

Kanimozhi

et al. 2017

ACS Appl. Mater. Interfaces

View full text Add to dashboard Cite

Conjugated polymers are used commonly to selectively sort semiconducting carbon nanotubes (S-CNTs) from their metallic counterparts in organic solvents. The polymer-wrapped S-CNTs can be easily processed from organic solvents into arrays of CNTs for scalable device fabrication. Though the conjugated polymers are essential for sorting and device fabrication, it is highly desirable to remove them completely as they limit the electronic properties of the device. Here, we use a commercially available polymer, namely, poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,6'-(2,2'-bipyridine))] (PFO-BPy), to sort large-diameter S-CNTs with ultrahigh selectivity and fabricate CNT-array-based field effect transistors (FETs) via a floating evaporative self-assembly (FESA) process. We report quantitative removal of the polymer wrapper from the FESA aligned S-CNT arrays using a metal-chelation-assisted polymer removal (McAPR) process. The implementation of this process on FESA films requires the selective thermal degradation of the polymer into oligomers, combined with optimization of the solvent type and temperature of the metal complexation reaction. Resulting S-CNT array FET devices show that the electronic properties of pristine CNT are preserved through this process. Optical microscopy, UV-vis spectroscopy, and X-ray photoelectron spectroscopy (XPS) were used to characterize the quantitative polymer removal. We quantitatively describe the FET devices to analyze the fundamental characteristics of FETs (mobility (μ), on-conductance (G), and contact resistance (2R)) by comparing before and after polymer removal. The ability to completely remove the polymer wrapper in aligned CNT arrays without adversely affecting the device properties opens up applications beyond FETs into photovoltaics and biosensing.

show abstract

“…However, polyfluorenes are not compatible for use in devices owing to their poor charge carrier properties. The high voltages required for operating polyfluorene based OLEDs cause defects resulting from over oxidation in addition to poor device performance and reduced device lifetime. − Additionally, polyfluorenes tend to aggregate in the solid state causing excimer formation and unwanted color contamination. , Similarly, undesired green emission arises from keto defects. − This unwanted color contamination and increased device performance can be overcome by incorporating vinylene linkages into the polymer backbone and by copolymerizing with charge carrier monomers. , Furthermore, color tuning of the fluorene emission to match a specific electronic application and modulating the highest-occupied molecular orbital (HOMO)/lowest-unoccupied molecular orbital (LUMO) energy gaps is possible by incorporating electronic donor and acceptor comonomers. , …”

Section: Introductionmentioning

confidence: 99%

Conjugated Fluorenes Prepared From Azomethines Connections-II: The Effect of Alternating Fluorenones and Fluorenes on the Spectroscopic and Electrochemical Properties

2009

View full text Add to dashboard Cite

The photophysics and electrochemistry of fluorene and fluorenone azomethine derivatives were examined in order to understand the deactivation pathways responsible for the quenched fluorescence of these compounds, which should otherwise be fluorescent. Steady-state fluorescence showed that the fluorene singlet excited state is quenched both by fluorenone (1) and a model aliphatic azomethine compound (14) with k(q) approximately 10(10) M(-1) s(-1). The quencher concentration required to deactivate 95% of the excited singlets formed was 8.4 mM for fluorenone and 34 mM for 14. Intramolecular photoinduced electron transfer (PET) from fluorene to both 1 and 14 was found as the principle deactivation mode of the fluorene's excited state. The high degree of conjugation of the azomethines promotes intersystem crossing to the triplet manifold by narrowing the singlet-triplet energy gap, which is also in part responsible for the reduced fluorescence observed for the fluorenone azomethine derivatives 5-11. Fluorescence quenching by PET was corroborated from the electrochemical and spectroscopic data by applying the Rehm-Weller equation. Meanwhile, the inherent fluorene fluorescence can be restored by protonating the azomethine, resulting from suppressed PET. Although PET is exergonically favorable (-242 kJ/mol for 1 and -96 kJ/mol for 14), intersystem crossing still occurs. The resulting fluorene triplet state is efficiently quenched by both 1 (k(q) = 7 x 10(9) M(-1) s(-1)) and 14 (k(q) = 5 x 10(9) M(-1) s(-1)) confirming that the absence of triplet signal by laser flash photolysis is a result of rapid intramolecular energy transfer to the two quencher sites. A concentration-dependent second emission was observed for the fluorenone containing azomethines assigned to the formation of excimers.

show abstract

Studies of Green Emission in Polyfluorenes Using a Model Polymer

Cited by 11 publications

References 36 publications

Intrachain Aggregates as the Origin of Green Emission in Polyfluorene Studied on Ensemble and Single-Chain Level

Intrachain Aggregates as the Origin of Green Emission in Polyfluorene Studied on Ensemble and Single-Chain Level

Polymer-Free Electronic-Grade Aligned Semiconducting Carbon Nanotube Array

Conjugated Fluorenes Prepared From Azomethines Connections-II: The Effect of Alternating Fluorenones and Fluorenes on the Spectroscopic and Electrochemical Properties

Contact Info

Product

Resources

About