Photophysics of a fluorene co-polymer in solution and films

Investigation of the Origin of High Photoluminescence Quantum Yield in Thienyl-S,S-dioxide AIEgens Oligomers by Temperature Dependent Optical Spectroscopy

The development of organic molecules showing high photoluminescence quantum yield (PLQY) in solid state is a fundamental step for the implementation of efficient light emitting devices. In this work the origin of the high PLQY of two trimers and two pentamers having one central thiophene-S,S-dioxide unit and two and four lateral thiophene or phenyl groups, respectively, is investigated by temperature dependent photoluminescence and time resolved photoluminescence measurements. The experimental results demonstrate that the molecules with lateral phenyl rings show higher PLQY due to a weaker coupling with intramolecular vibrations-related to variations in the radiative and non-radiative decay rates-and indicate different molecular rigidity as the main factors affecting the PLQY of this class of molecules.

Synthesis and characterization of conductive polyaniline nanocomposite containing fluorene

Fluorene, an excellent electroluminescent material, has been used as a core to synthesize a core–shell fluorene/polyaniline (fluorene/PANI) composite using ammonium persulfate as an oxidant. The influence of the fluorene–aniline molar ratio and the oxidant–aniline molar ratio on the resistance of the composite was investigated, and the crystallinity, structure, and morphology of the fluorene/PANI composite were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. The results indicate that the fluorene/PANI composite has a core–shell structure and exhibits good conductivity. A more planar conformation of PANI could be formed because of strong π–π interactions between the fluorene core and the PANI shell.

Cellulose derivatives carrying triphenylamine (TPA) moieties: synthesis and electro-optical properties

A novel ethyl cellulose derivative [poly(1)] that carries triphenylamine moieties is synthesized with a moderate number-average molecular weight up to 78,200 in 85% yield by the reaction of 4-(diphenylamino)benzoic acid with the residual hydroxy group of ethyl cellulose. Poly(1) is soluble in common organic solvents including toluene, CHCl3, CH2Cl2, and tetrahydrofuran while insoluble in hexane, diethyl ether, and methanol. The polymer emits blue-green fluorescence with quantum yields up to 65% in CHCl3 and displays unique solvatochromism. The cyclic voltammograms of poly(1) indicate that the polymer carrying TPA moieties is electrochemically redox active. The onset temperature of weight loss of the poly(1) is about 177 degrees C according to thermogravimetric analysis in air.

Energy up-conversion in dilute polyfluorene solutions

Poly(9,9-dioctylfluorene) (PFO) shows highly efficient blue emission with photo excitation occurring between 340–400 nm. Here we show that PFO can in dilute solution emit at a wavelength well below that at which it is being exited. This, we propose is related to an energy transfer from conjugated parts of the polymer chain into more localised states which then emit at a lower wavelength. These localised states can be considered as defects in the conjugation of the polymer or as chain ends. These may produce quasi monomer or quasi dimer species within the chain, which will have a HOMO-LUMO gap of higher energy than the conjugated polymer. These then fluoresce at the lower wavelength; essentially causing, by energy transfer, a process of energy up-conversion.

Productive Synthesis and Properties of Polydiaminoanthraquinone and Its Pure Self-Stabilized Nanoparticles with Widely Adjustable Electroconductivity

Wholly aromatic poly(1,5-diaminoanthraquinone) (PDAA) particles were productively synthesized for the first time by chemically oxidative polymerization of 1,5-diaminoanthraquinone (DAA) by using CrO3, K2Cr2O7, K2CrO4, or KMnO4 as oxidants in acidic DMF. The effects of the oxidant species, oxidant/DAA ratio, polymerization temperature, and medium on the polymerization yield, macromolecular structure, size, electroconductivity, solubility, solvatochromism, thermostability, photoluminescence, and silver-ion sorption of the PDAA particles were systematically studied by IR, UV/Vis, fluorescence, and solid-state 13C NMR spectroscopies, wide-angle X-ray diffraction, scanning and transmission electron microscopies, and laser particle-size, differential scanning calorimetry (DSC), and thermal gravimetric (TG) analyses. It seems that the DAA is oxidatively polymerized at the 1,4- and 5,8-positions. Surprisingly, the chemical oxidative polymerization of DAA with CrO3 at 0 degrees C in H2SO4/DMF in the absence of external stabilizer simply affords novel PDAA nanoparticles of around 30 nm in diameter with high purity, clean surfaces, inherent semiconductivity, and self-stability that can be ascribed to the presence of a large number of 1,4-benzoquinone units that are negatively charged on their macromolecular chains. The polymers exhibit a high thermal stability at temperatures below 400 degrees C. Two unique nanoeffects were found, namely the strongest silver-ion adsorbability and photoluminescence of the PDAA nanoparticles. This gives a facile and general route for the application of the versatilities of PDAA nanomaterials. The PDAA particles are good semiconductors with a widely adjustable conductivity that moves across seven orders of magnitude through simple HClO4 redoping or Ag+ sorption, as expected.

Theory of non-Condon emission from the interchain exciton in conjugated polymer aggregates

The authors present here a simple analysis that explains the apparent strengthening of electron phonon interaction upon aggregation in conjugated polymer materials. The overall scheme is that of an intermolecular Herzberg-Teller effect whereby sidebands of a forbidden transition are activated by oppositely phased vibrations. The authors show that upon aggregation, the 0-0 emission becomes symmetry forbidden and the apparent redshift and remaining vibronic structure are due to sideband (0-1,0-2, etc.) emission. At higher temperatures, the 0-0 peak is due to thermal population in a higher lying even-parity vibronic state rather than direct emission from the odd-paritied lowest intermolecular vibronic state.

Influence of Aggregates and Solvent Aromaticity on the Emission of Conjugated Polymers

The influence of aggregates and solvent aromaticity on the photophysics and fluorescence dynamics of two conjugated polymers is studied. The two polymers are derivatives of poly(p-phenylene vinylene) (PPV) containing different kinked moieties along the main chain. The polymers contain 2,6-diphenylpyridine and m-terphenyl kinked moieties and they are abbreviated as PN and PC, respectively. The insertion of kinked segments along the main chain shifts the emission spectrum from the yellow-orange spectral region, common to PPV derivatives, to the blue-green spectral region. The results show that in dilute solutions the polymers decay monoexponentially, while in concentrated ones the fluorescence decays biexponentially, indicating fluorescence quenching. This is attributed to an energy transfer process from polymer chains to aggregates that occurs within a few tens of picoseconds. By comparing the photophysics and fluorescence dynamics of polymer PN in a nonaromatic and an aromatic solvent, we conclude that the polymer conformation adopted in the aromatic solvent leads to a higher fluorescence quantum yield and a longer fluorescence lifetime. Furthermore, the fluorescence quenching of PN because of aggregates is faster and more efficient in the aromatic than in the nonaromatic solvent. These results can be explained through a more extended chain conformation of PN in the aromatic solvent.

Polarized optical spectroscopy applied to investigate two poly(phenylene-vinylene) polymers with different side chain structures

Two related poly(phenylene-vinylene) (PPV) light-emitting polymers have been investigated by means of polarized optical spectroscopy. The purpose of the investigation was to investigate the nature of the interactions in thin films and to examine what impact the difference in side chain structure and molecular weight in poly(2'-methoxy-5-2-ethyl-hexoxy)-1,4-phenylene vinylene (MEH-PPV) and poly(2-(3',7'-dimethyloctyloxy)-5-methoxy-1,4-phenylene-vinylene) (OC1C10-PPV) has on the electronic and optical properties of the two polymers. Aligning the polymers by dispersing them in anisotropic solvents and stretched films shows that the side chains have an impact on the relative orientations of the transition dipole moments. In anisotropic solvents the linear dichroism is larger for MEH-PPV than for the related polymer OC1C10-PPV, while in stretched films the opposite situation prevails. A lower polarization of the luminescence from OC1C10-PPV, relative to MEH-PPV, was also obtained independent of alignment medium used. The data therefore suggest that while mechanical stretching may align the OC1C10-PPV to a greater degree, the emitting species is distinct from the absorbing species. The circular dichroism (CD) spectra of both polymers undergo dramatic changes when the liquid phase and the solid state (film) are compared. The solution CD spectra shows no evidence of interchain interactions; instead the spectra of both systems indicate a helical conformation of the polymers. The CD spectra of films are dramatically different with the strong Cotton effect being observed. This points to the formation of an aggregate in the film, with an associated ground state interaction, an interchain species such as a physical dimer, or a more complex higher aggregate.

Femtosecond Time Resolved Fluorescence Dynamics of a Cationic Water-Soluble Poly(fluorenevinylene-co-phenylenevinylene)

A recently synthesized cationic water-soluble poly(fluorenevinylene-co-phenylenevinylene) was studied by means of steady state and femtosecond time resolved upconversion spectroscopy in aqueous and EtOH solutions. Steady state spectroscopic measurements showed that the polymer emits at the blue-green spectral region and that aggregates are formed in concentrated polymer solutions. The fluorescence dynamics of the polymer in concentrated solutions, studied at a range of emission wavelengths, exhibited a wavelength dependent and multiexponential decay, indicating the existence of various decay mechanisms. Specifically, a rapid decay at short emission wavelengths and a slow rise at long wavelengths were observed. Both features reveal an energy transfer process from isolated to aggregated chains. The contribution of the energy transfer process as well as of the isolated chains and the aggregates on the overall fluorescence decay of the polymer was determined. The dependence of the energy transfer rate and efficiency on polymer concentration was also examined.

Triplet-State and Singlet Oxygen Formation in Fluorene-Based Alternating Copolymers

Data are reported on the triplet states of a series of fluorene-based A-alt-B type alternating copolymers based on pulse radiolysis-energy transfer and flash photolysis experiments. From the pulse radiolysis experiments, spectra are given for eight copolymers involving phenylene, thiophene, benzothiadiazole, and oligothienylenevinylene groups. Quantum yields for triplet-state formation (PhiT) have been obtained by flash photolysis following laser excitation and in one case by photoacoustic calorimetry. In addition, yields of sensitized formation of singlet oxygen have been determined by time-resolved phosphorescence and are, in general, in excellent agreement with the PhiT values. In all cases, the presence of thiophene units is seen to increase intersystem-crossing quantum yields, probably because of the presence of the heavy sulfur atom. However, with the poly[2,7-(9,9-bis(2'-ethylhexyl)fluorene)-alt-1,4-phenylene] (PFP), thiophene S,S-dioxide (PFTSO2) and benzothiadiazole (F8BT) copolymers, low yields of triplet formation are observed. With three of the copolymers, the energies of the triplet states have been determined. With PFP, the triplet energy is virtually identical to that of poly[2,7-(9,9-bis(2'-ethylhexyl)fluorene)]. In contrast, with fluorene-thiophene copolymers PFaT and PF3T, the triplet energies are closer to those of thiophene oligomers, indicating that there is significant conjugation between fluorene and thiophene units but also that there is a more localized triplet state than with the homopolymers.

Electron-Vibrational Dynamics of Photoexcited Polyfluorenes

The highly polarizable pi-electron system of conjugated molecules forms the basis for their unique electronic and photophysical properties, which play an important role in numerous biological phenomena and make them important materials for technological applications. We present a theoretical investigation of the dynamics and relaxation of photoexcited states in conjugated polyfluorenes, which are promising materials for display applications. Our analysis shows that both fast (approximately 20 fs) and slow (approximately 1 ps) nuclear motions couple to the electronic degrees of freedom during the excited-state dynamics. Delocalized excitations dominate the absorption, whereas emission comes from localized (self-trapped) excitons. This localization is attributed to an inherent nonlinear coupling among vibronic degrees of freedom which leads to lattice and torsional distortions and results in specific signatures in spectroscopic observables. Computed vertical absorption and fluorescence frequencies as well as photoluminescence band shapes show good agreement with experiment. Finally, we demonstrate that dimerization such as spiro-linking does not affect the emission properties of molecules because the excitation becomes confined on a single chain of the composite molecule.

New pyrimidine- and fluorene-containing oligo(arylene)s: synthesis, crystal structures, optoelectronic properties and a theoretical study

New pyrimidine containing oligo(arylene)s, notably the pyrimidine-fluorene hybrid systems 13-16, have been synthesised by Suzuki cross-coupling methodology. An efficient synthesis of the key reagent 9,9-dihexylfluorene-2,7-diboronic acid 10 from 2,7-dibromo-9,9-dihexylfluorene 9 is reported. Cross-coupling of 10 with two equivalents of 2-bromopyrimidine, 5-bromopyrimidine and 2,5-dibromopyrimidine gave 2,7-bis(2-pyrimidyl)-9,9-dihexylfluorene 13. 2,7-bis(5-pyrimidyl)-9,9-dihexylfluorene 14 and 2,7-bis(5-bromo-2-pyrimidyl)-9,9-dihexylfluorene 15 in 23-34% yields. A further two-fold Suzuki reaction of benzeneboronic acid with compound 15 gave 2,7-bis(5-phenyl-2-pyrimidyl)-9,9-dihexylfluorene 16 (35% yield). Ab initio calculations of the geometries and electronic structures at the Hartree Fock (HF) and density functional theory (DFT) levels of theory are reported for compounds 13, 14 and 16 (with ethyl substituents replacing hexyl) and for their dipyrazinyl and bistetraazenyl analogues, 17, 18, 20 and 21. The heterocyclic nitrogen atoms of 13 and 16 facilitate planarisation of the system, compared to 14, which is in agreement with X-ray structural data obtained for 5-bromo-2-phenylpyrimidine 6, 2,5-diphenylpyrimidine 7 and compound 15. Bistetrazenyl derivative 21 is calculated to be a fully planar system. The cyclic voltammogram (CV) of compound 16 in dichloromethane solution shows a quasi-reversible oxidation wave at E(1/2)0 = +1.36 V (vs. Ag/Ag+). Compound 13 is a poorer donor with an oxidation observed at Epa = +1.50 V which is in good agreement with the difference in the energies of their HOMO orbitals calculated at both HF and DFT levels of theory (0.11-0.12 eV). For compound 14 we were not able to measure an Eox potential which should lie at much more positive potentials. Compounds 15 and 16 are blue emitters in solution, with photoluminescence quantum yields (PLQY) of 25% and 85%, respectively. For thin films of 16 the PLQY is reduced to 21%. An OLED using compound 16 as the emissive layer has been fabricated in the configuration ITO/PEDOT/16/Ca/Al: blue-green light (lambda max 500 nm) most likely emanating primarily from excimer states is emitted at a high turn-on voltage.