Population and decay of keto states in conjugated polymers

Detailed Photophysical Studies of Poly(9,9-di-(2-ethylhexyl)-9-H-fluorene-2,7-vinylene) in Liquid Media and Thin Films

For the present work, solvent (toluene, o-xylene, chloroform, chlorobenzene, and 1,2-dichloroethane) and concentration-dependent (∼10^-6-10^-3 g ml^-1) photophysical properties of poly(9,9-di-(2-ethylhexyl)-9-H-fluorene-2,7-vinylene) (PEFV) were investigated in detail in liquid media as well as thin films. Also, temperature-dependent (3-60 ^oC) fluorescence emission measurements of PEFV were conducted in liquid media. The steady-state and time-resolved data indicate the existence of weak interchain interaction in liquid media at high concentration. However, both ground-state aggregation and interchain interaction are present for PEFV in thin film. The interchain interaction plays a dominant role on the fluorescence emission of PEFV in thin film on the red side of the spectra.

Photochemical Degradation of Various Bridge-Substituted Fluorene-Based Materials

Photochemical degradation is an important issue to be overcome in advancing the lifetime of fluorene-containing conjugated polymers. In order to optimize the inertness of the materials, a quantitative measure for the efficiency of degradation is needed. Here, we introduce a method to measure a relative quantum yield of the photochemical degradation by monitoring the kinetics of the process by means of UV/vis spectroscopy and liquid chromatography (LC) techniques. This method is employed to a set of differently substituted 2,7-diphenylfluorenes, serving as model compounds for polyfluorene materials. Our measurements show that the quantum yield changes by orders of magnitude upon varying the bridge substituents and that altered kinetics indicate changing degradation mechanisms.

Synergistic photoluminescence enhancement in conjugated polymer-di-ureasil organic-inorganic composites

Energy transfer between a hybrid di-ureasil host and conjugated polymer dopants results in a dramatic enhancement in the photoluminescence quantum yield due to exciton isolation at long-lived trap sites.

Poly(fluorene) conjugated polyelectrolyte (CPE)-di-ureasil organic–inorganic composites have been prepared using a versatile sol–gel processing method, which enables selective localisation of the CPE within the di-ureasil matrix. Introduction of the CPE during the sol–gel reaction leads to a homogeneous distribution of the CPE throughout the di-ureasil, whereas a post-synthesis solvent permeation route leads to the formation of a confined layer of the CPE at the di-ureasil surface. The CPE and the di-ureasil both function as photoactive components, contributing directly to, and enhancing the optical properties of their composite material. The bright blue photoluminescence exhibited by CPE-di-ureasils is reminiscent of the parent CPE; however the distinct contribution of the di-ureasil to the steady-state emission profile is also apparent. This is accompanied by a dramatic increase in the photoluminescence quantum yield to >50%, which is a direct consequence of the synergy between the two components. Picosecond time-correlated single photon counting measurements reveal that the di-ureasil effectively isolates the CPE chains, leading to emissive trap sites which have a high radiative probability. Moreover, intimate mixing of the CPE and the di-ureasil, coupled with their strong spectral overlap, results in efficient excitation energy transfer from the di-ureasil to these emissive traps. Given the simple, solution-based fabrication method and the structural tunability of the two components, this approach presents an efficient route to highly desirable CPE-hybrid materials whose optoelectronic properties may be enhanced and tailored for a targeted application.

Unraveling the Electronic Heterogeneity of Charge Traps in Conjugated Polymers by Single-Molecule Spectroscopy

Charge trapping is taken for granted in modeling the characteristics of organic semiconductor devices, but very few techniques actually exist to spectroscopically pinpoint trap states. For example, trap levels are often assumed to be discrete in energy. Using the well-known keto defect in polyfluorene as a model, we demonstrate how single-molecule spectroscopy can directly track the formation of charge and exciton traps in conjugated polymers in real time, providing crucial information on the energetic distribution of trap sites relative to the polymer optical gap. Charge traps with universal spectral fingerprints scatter by almost 1 eV in depth, implying that substantial heterogeneity must be taken into account when modeling devices.

Singlet Generation from Triplet Excitons in Fluorescent Organic Light-Emitting Diodes

A potential major drawback with organic light-emitting devices, (OLEDs) is the limit of 25% singlet exciton production through spin-dependent charge recombination. Recent device results, however, show that this limit does not hold and far higher efficiencies can be achieved in purely fluorescent-based systems (Wohlgenannt et al. (2001), Dhoot et al. (2002), Lin et al. (2003), Wilson et al. (2001), Cao et al. (1999), Baldo et al. (1999), and Kim et al. (2000)). Thus, the question arises; is recombination spin dependent (Tandon et al. (2003)) or are singlet excitons generated in secondary processes? Direct measurement of the singlet generation rate in working devices of 44% has been shown (Rothe et al. (2006)), which have been verified as being part due to direct singlets formed on recombination and part from triplet fusion, singlets produced during triplet annihilation (Kondakov et al. (2009), King et al. (2011), and Zhang and Forrest (2012)). Here, the various routes by which triplet excitons can generate singlet states are discussed and their relative contributions to the overall electroluminescence yield are given. The materials requirements to obtain maximum singlet production from triplet states are discussed. These triplet contributions can give very high device yields for fluorescent emitters, which in the case of blue devices can be highly advantageous. Further, new devices architectures open up which are simple and have intrinsically low turn on voltages, ideal for large-area OLED lighting applications.

Recent advances in white organic light-emitting materials and devices (WOLEDs)

WOLEDs offer new design opportunities in practical solid-state lighting and could play a significant role in reducing global energy consumption. Obtaining white light from organic LEDs is a considerable challenge. Alongside the development of new materials with improved color stability and balanced charge transport properties, major issues involve the fabrication of large-area devices and the development of low-cost manufacturing technology. This Review will describe the types of materials (small molecules and polymers) that have been used to fabricate WOLEDs. A range of device architectures are presented and appraised.

Control over phase behavior and solution structure of hairy-rod polyfluorene by means of side-chain length and branching

We present guidelines on how the solution structure of pi -conjugated hairy-rod polyfluorenes is controlled by the side-chain length and branching. First, the semiquantitative mean-field theory is formulated to predict the phase behavior of the system as a function of side-chain beads (N). The phase transition at N=N{ *} separates a lyotropic phase with solvent coexistence (N<N{ *}) and a metastable membrane phase (N>N{ *}). The membrane phase transforms into the isotropic phase of dissolved rodlike polymers at the temperature T_{mem}{ *}(N), which decreases both with N and with the degree of side-chain branching. This picture is complemented by polymer demixing with the transition temperature T_{IN}{ *}(N), which decreases with N . For N<N{ *}, the lyotropic phase turns isotropic with increasing T at T_{IN}{ *} . For N>N{ *}, stable membranes are predicted for T_{IN}{ *}<T<T_{mem}{ *} and metastable membranes with nematic coexistence for T<T_{IN}{ *}. Second, in experiment, samples of poly(9,9-dialkylfluorene) with N=6-10 were mixed in methylcyclohexane. For N=8 the side-chain branching was controlled by (9,9-dioctylfluorene)/(9,9-bis(2-ethylhexyl)fluorene) (F8/F2/6) random copolymers. The proportion of F8 to F2/6 repeat units was 100:0, 95:5, 90:10, 50:50, and 0:100. In accordance with the theory, lyotropic, membrane, and isotropic phases with the corresponding phase transitions were observed. For N<N{ *} approximately 6 only the lyotropic phase is present for attainable temperatures. The membrane and isotropic phases are present for N>N{ *}. T_{mem}{ *}(N) decreases from 340 K to 280 K for N > or = 8 . For copolymers, the membrane phase is found when the fraction of F8 units is at least 90%, T_{mem}{ *} decreasing with this fraction. The membrane phase contains three material types: loose sheets of two polymer layers, a better packed beta phase, and dissolved polymer. For N > or = 7 and T<T_{mem}{ *} the tendency for membrane formation becomes stronger with increasing temperature.

Enhanced Luminescence from Emissive Defects in Aggregated Conjugated Polymers

Degradation experiments and model studies suggested that the longer lived green fluorescence from an aggregated poly(p-phenylene ethynylene) (PPE) was due to the presence of highly emissive, low-energy, anthryl defect sites rather than the emissive conjugated polymer excimers proposed in a previous report. After elucidating the origin of the green fluorescence, additional anthryl units were purposely incorporated into the polymer to enhance the blue-to-green fluorescence color change that accompanied polymer aggregation. The improved color contrast from this anthryl-doped conjugated polymer led to the development of crude solution-state and solid-state sensors, which, upon exposure to water, exhibited a visually noticeable blue-to-green fluorescence color change.

Single-Molecule Studies of a Model Fluorenone

Single-molecule fluorescence measurements of 2,7-bis(3,4,5-trimethoxyphenylethenyl)fluorenone (OFOPV) reveal narrow emission spectra concentrated around 540 nm, with weak emission at longer wavelengths. The wide scattering of emission-maximum wavelengths is attributed to varying molecular environments, with dimers or higher-order aggregates contributing to the low-energy emission. This spectral distribution indicates that emission from monomers of this model fluorenone is mostly green, which is consistent with contaminant emission (g-bands) often observed in fluorene- and polyfluorene-based organic light emitting diode (OLED) devices. A histogram of center wavelengths from 118 single-molecule spectra shows good agreement with the green emission previously observed in thermally stressed 2,7-bis(3,4,5-trimethoxyphenylethenyl)-9,9-diethylfluorene (OFPV). Whereas bulk OFPV exhibits blue fluorescence at about 480 nm, OFOPV bulk thin film measurements reveal red luminescence shifted to 630 nm. This unexpected peak position for bulk OFOPV shifts to higher energies (ca. 540 nm) upon dilution in a solid-state matrix, suggesting that the bulk red emission finds its origins in interactions between fluorenone molecules. Explanations for this red emission include aggregate or excimer formation or intermolecular energy transfer between fluorenone molecules.

Regarding the origin of the delayed fluorescence of conjugated polymers

In the first part of this work we revisit and reevaluate the experimental data that lead to the assignment of the origin of the delayed fluorescence (DF) to triplet-triplet annihilation for polyfluorene and to geminate pair recombination in the case of the ladder-type polyparaphenylene (MeLPPP); the ambiguity of this classification is unveiled. Next, new data about the DF of MeLPPP under applied electric field are presented. Here, the DF intensity completely recovers once the field is turned off, which rules out geminate pairs as the origin of the DF and in turn provides clear evidence of the triplet-triplet annihilation picture. Finally, we show and discuss how recombination of space charge layers may also give rise to electric field induced delayed fluorescence, whereby the formation of these space charge layers strongly depends on device configuration and purity of the materials.

Influence of molecular weight on the phase behavior and structure formation of branched side-chain hairy-rod polyfluorene in bulk phase

We report on an experimental study of the self-organization and phase behavior of hairy-rod pi -conjugated branched side-chain polyfluorene, poly[9,9-bis(2-ethylhexyl)-fluorene-2,7-diyl]-i.e., poly[2,7-(9,9-bis(2-ethylhexyl)fluorene] (PF2/6) -as a function of molecular weight (M(n)) . The results have been compared to those of phenomenological theory. Samples for which M(n) =3-147 kg/mol were used. First, the stiffness of PF2/6 , the assumption of the theory, has been probed by small-angle neutron scattering in solution. Thermogravimetry has been used to show that PF2/6 is thermally stable over the conditions studied. Second, the existence of nematic and hexagonal phases has been phenomenologically identified for lower and higher M(n) (LMW, M(n) < M(*)(n) and HMW, M(n) > M(*)(n) ) regimes, respectively, based on free-energy argument of nematic and hexagonal hairy rods and found to correspond to the experimental x-ray diffraction (XRD) results for PF2/6 . By using the lattice parameters of PF2/6 as an experimental input, the nematic-hexagonal transition has been predicted in the vicinity of glassification temperature (T(g)) of PF2/6 . Then, by taking the orientation parts of the free energies into account the nematic-hexagonal transition has been calculated as a function of temperature and M(n) and a phase diagram has been formed. Below T(g) of 80 degrees C only (frozen) nematic phase is observed for M(n)< M(*)(n) = 10(4) g/mol and crystalline hexagonal phase for M(n) > M(*)(n) . The nematic-hexagonal transition upon heating is observed for the HMW regime depending weakly on M(n) , being at 140-165 degrees C for M(n) > M(*)(n). Third, the phase behavior and structure formation as a function of M(n) have been probed using powder and fiber XRD and differential scanning calorimetry and reasonable semiquantitative agreement with theory has been found for M(n) >or=3 kg/mol. Fourth, structural characteristics are widely discussed. The nematic phase of LMW materials has been observed to be denser than high-temperature nematic phase of HMW compounds. The hexagonal phase has been found to be paracrystalline in the (ab0) plane but a genuine crystal meridionally. We also find that all these materials including the shortest 10-mer possess the formerly observed rigid five-helix hairy-rod molecular structure.

Intramolecular fluorescence quenching in luminescent copolymers containing fluorenone and fluorene units: A direct measurement of intrachain exciton hopping rate

The quenching process of fluorescence emission in polyfluorene (PF) due to the presence of intramolecular 9-fluorenone (9 FL) moieties is studied in dilute toluene solution as a function of 9 FL content in eight copolymers containing both fluorene and fluorenone units (PF/FL(x)). The absorption spectrum of PF/FL(x) copolymers clearly shows a new absorption band, redshifted relatively to the PF and 9-fluorenone absorption, which increases in intensity when the fluorenone fraction increases and also decreases with solvent polarity. Fluorescence emission spectra of PF/FL(x) show that this redshifted and unstructured emission does not coincide with the 9-fluorenone emission and, with increasing solvent polarity, it further redshifts and decreases in intensity. An isoemissive point is clearly observed on the fluorescence emission spectra of PF/FL(x) as a function of fluorenone content, showing that the new emission band is formed at the expense of PF. We propose the formation of an intramolecular charge transfer complex (ICTC) between PF units and 9-fluorenone to explain the appearance of the new emission band. Global analysis of time resolved fluorescence decays collected at 415 nm (PF emission) and 580 nm (the ICTC emission) show that three exponentials are generally needed to achieve excellent fits. Two of the components (420 ps and 6.5 ns) are independent of 9-fluorenone fraction. A further fast component is strongly dependent on fluorenone fraction and ranges between 280 and 70 ps. This component appears as a decay time at 415 nm and as a rise time at 580 nm and is ascribed to the migration of exciton to quenching sites (formation of intramolecular CT complex or exciton ionization at CT complex). A kinetic mechanism involving three different kinetic species, quenched PF units kinetically coupled with the ICTC complex, and unquenched PF units is proposed to explain the experimental data and the quenching rate constant is obtained, k(1) congruent with 10(11) s(-1). This is an experimental measurement of the intrachain exciton hopping rate.

An investigation into the excitation migration in polyfluorene solutions via temperature dependent fluorescence anisotropy

Fluorescence anisotropy of dilute polyfluorene polymer solutions has been used to show that two processes, exciton migration and conformational relaxation (twisting of part of the chain), occur within polyfluorene polymers of 20 or more repeat units. The former process is dominant and temperature independent for high excitation energy but, as the chain length is decreased, exciton migration is eliminated and the conformational relaxation becomes the only mechanism by which excited state energy relaxation to the emission site can occur. When the polymers are excited in the absorption band tails, again no migration is observed but conformational relaxation is still present.