Thermoluminescence and electroluminescence of annealed polyfluorene layers

Thermal Degradation of Photoluminescence Poly(9,9-dioctylfluorene) Solvent-Tuned Aggregate Films

The progression of the green emission spectrum during the decomposition of polyfluorenes (PFs) has impeded the development and commercialization of the materials. Herein, we constructed a solvent-tuned aggregated PFO film with the aim of retarding the material’s thermal degradation behavior which causes a significant decline in optical properties as a result of phase transformation. The tuning of the aggregate amount and distribution was executed by applying a poor alcohol-based solvent in chloroform. It emerges that at a lower boiling point methanol evaporates quickly, limiting the aggregate propagation in the film which gives rise to a more transparent film. Furthermore, because of the modulated β-phase conformation, the absorption spectra of PFO films were red-shifted and broadened. The increase in methanol percentage also led to a rise in β-phase percentage. As for the thermal degradation reactions, both pristine and aggregated PFO films exhibited apparent changes in the UV-Vis spectra and PL spectra. In addition, a 97:3 (chloroform:methanol) aggregated PFO film showed a more defined emission spectrum, which demonstrates that the existence of β-phase is able to suppress the unwanted green emission.

Suppression of Polyfluorene Photo-Oxidative Degradation via Encapsulation of Single-Walled Carbon Nanotubes

Polyfluorenes have achieved noteworthy performance in organic electronic devices but exhibit undesired green band emission under photo-oxidative conditions that have limited their broad utility in optoelectronic applications. In addition, polyfluorenes are well-known dispersants of single-walled carbon nanotubes (SWCNTs), although the influence of SWCNTs on polyfluorene photo-oxidative stability has not yet been defined. Here we quantitatively explore the photophysical properties of poly[(9,9-bis(3^'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) under photo-oxidative conditions when it is in van der Waals contact with SWCNTs. Photoluminescence spectroscopy tracks the spectral evolution of the polymer emission following ambient ultraviolet (UV) exposure, confirming that PFN exhibits green band emission. In marked contrast, PFN-wrapped SWCNTs possess high spectral stability without green band emission under the same ambient UV exposure conditions. By investigating a series of PFN thin films as a function of SWCNT content, it is shown that SWCNT loadings as low as ∼23 wt % suppress photo-oxidative degradation. These findings suggest that PFN-SWCNT composites provide an effective pathway toward utilizing polyfluorenes in organic optoelectronics.

Deep-blue Electroluminescence of 9,9-Dialkyl-substituted Polyfluorene Copolymers Incorporating Diphenylsilane Units

Bis(7-bromo-9,9-dialkylfluorenyl)diphenylsilanes with different alkyl chain lengths ( Cn = 4 and 8) were synthesized in this work. Conventional 9,9-dialkyl-substituted polyfluorenes (PFs) were modified through copolymerization of the diphenylsilane-containing comonomers by the Suzuki coupling method to improve electroluminescence (EL) color purity. The PF copolymers possessed high molecular weights that were enough for film-forming ability and good solubility in various common organic solvents such as toluene. The incorporation of the diphenylsilane units suppressed effectively the PF-inherent green PL, which is undesirable for the present purpose, with a slight blue-shift of the interesting blue emission band. The results are closely related to a bent and bulky structure at the diphenylsilane units which disturb interchain stacking and a controlled conjugation length along the PF main chains. This effect was observed much more prominently in the EL spectra. Even in the di- n-butyl-substituted PF copolymer which tends to show a strong green excimer emission, the incorporation of the diphenylsilane units completely erased the green EL band. The results suggest that the PF copolymers can be a good candidate as a novel deep-blue emission layer material in EL devices.

A Deep-Blue Electroluminescence from 2,5-Diphenyl-1,3,4-oxadiazole and Polyfluorene Copolymer End-capped with Triphenylamine

An electron-transporting 2,5-bis(4-bromophenyl)-1,3,5-oxadiazole (OXD) was incorporated by copolymerization into polyfluorene (PF) end-capped with a 4-bromo-4',4"-dimethyltriphenylamine (TPA) main chains on the Yamamoto coupling method to improve the initial electroluminescence (EL) color stability against applied voltage and the device operation lifetime. The copolymer, PF/OXD-TPA, possessed a high molecular weight enough for film-forming ability and excellent solution-processability, that is, good solubility in common organic solvents such as toluene. An EL device fabricated using this copolymer as a polymeric emission layer allowed the generation of a strong deep-blue EL peaking at 424 nm with a comparatively high EL efficiency of 0.47 cd A -1, and its color was extremely stable against applied voltage within 527 V. The incorporation of OXD units into the PF-TPA system also improved the operation lifetime, although it is not a practical level for displays. The results are probably attributed to the great roles of the incorporated OXD units for disturbing the formation of fluorene excimer sites and for tuning the hole/electron mobility balance..

Luminescent properties of a novel fluorene organic material

The photo-physical properties of 6,6'-(9H-fluoren-9,9-diyl)bis(2,3-bis(9,9-dihexyl-9H-fluoren-2-yl)quinoxaline) (BFLBBFLYQ) and its blend doped with N'-biphenyl-N,N'-bis-(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) were investigated. The absorption, photoluminescence (PL) and electroluminescence (EL) properties of pristine BFLBBFLYQ and blend in solution and spin-coated film are outlined, including a discussion of charge-transfer (CT) exciplex emission of BFLBBFLYQ:TPD blend in the solid state. The luminescent properties of BFLBBFLYQ films using different deposition processes were studied. It was found that the low-energy emission bands at 530-570 nm appeared in the PL spectra of BFLBBFLYQ evaporated films in ultra-high vacuum. Also, the low-energy band was the exclusive emission in the EL spectra of BFLBBFLYQ films.

Modeling of molecular packing and conformation in oligofluorenes

We describe the application of molecular modeling to study problems related to the packing and conformation of oligofluorene molecules in the solid state. First of all, we describe an improved force field for oligofluorenes. The model is based on the MM3 force field for the intramolecular degrees of freedom, but it relies on ab initio calculations for the torsion potential between two monomers and the electrostatic interactions. We also report ab initio calculations of the interaction potentials between fluorene and fluorenone units. The force field has been tested on the crystal structures of a fluorene monomer, a dimer, and a pentamer containing a fluorenone at the center. It has then been employed to study conformational defects of the chains, both in vacuo and in the bulk. We find that certain modes of inversion from right-handed to left-handed helices are also possible within the constraining environment of the crystals. The effect of the presence of two different types of side chains has been also addressed. Finally, the possibility of having two fluorene units parallel and close to each other has been investigated as a model of a ground-state precursor of an excimer. Our simulations show that this configuration is sterically and energetically unfavorable so that formation of an excimer following optical excitation appears to be unlikely.

Thermally Stable Blue Emitting Terfluorene Block Copolymers

Spectroscopic and morphological studies on a series of rod-coil block copolymers containing terfluorene segments as the rigid blocks and polystyrene as the flexible parts demonstrate an increase in the photoluminescence intensity and the exciton lifetime as well as formation of isolated spheres as thin films upon thermal annealing in air (200 degrees C for 30 min). Moreover, no appearance of the low energy emission band centered at 520 nm was found after the same thermal treatment which permits these copolymers to emit pure blue light.

Monodisperse Oligofluorenes with Keto Defect as Models to Investigate the Origin of Green Emission From Polyfluorenes: Synthesis, Self-Assembly, and Photophysical Properties

Oligofluorenes (a trimer, pentamer, and heptamer) with one fluorenone unit in the center (OFnK: n=3, 5, or 7) were synthesized and used as models to understand the origin of the low-energy emission band in the photoluminescence and electroluminescence spectra of some polyfluorenes. All compounds form glasses with T(g) at 30 degrees C (OF3 K), 50 degrees C (OF5 K) and 57 degrees C (OF7 K). Oligomers OF5 K and OF7 K exhibit smectic liquid crystal phases that undergo transition to isotropic melts at 107 and 205 degrees C, respectively. Oligomer OF5 K could be obtained in form of single crystals. The X-ray structure analysis revealed the helical nature of the molecule and a helix reversal defect located at the central fluorenone unit. The packing pattern precludes formation of excimers. Electrochemical properties were investigated by cyclic voltammetry. The ionization potential (I(p)) and electron affinity (E(a)) were calculated from these data. Studies of the photophysical properties of OFnK in solution and thin film by steady-state and time-resolved fluorescence spectroscopic measurements suggest efficient funneling of excitation energy from the photoexcited fluorene segments to the low-energy fluorenone sites by both intra- and intermolecular hopping events whereby they give rise to green emission. Intermolecular energy transfer was investigated by using a model system composed of a highly defect free polyfluorene PF2/6 doped by OFnK. Förster-type energy transfer takes place from PF2/6 to OFnK. The energy-transfer efficiency increases predictably with increasing concentration of OFnK.