Lars-Olof Pålsson

Measurements of Solid‐State Photoluminescence Quantum Yields of Films Using a Fluorimeter

In this article we show that a commercial spectrofluorimeter can be used successfully in combination with an integrating sphere to measure solid-state photoluminescence quantum yields of films. This approach significantly simplifies the experimental method as the need for specialized equipment on the excitation and detection side is relaxed. Two different light-emitting polymer systems are examined with this approach, and the results agree with those that have been reported previously using different methods.

Absolute measurements of photoluminescence quantum yields of solutions using an integrating sphere.

We demonstrate that absolute measurements of the photoluminescence quantum yield of solutions can be made using an integrating sphere and a conventional fluorimeter. With this method the need for measurements against a luminescence standard is overcome. The sphere is mounted inside a commercial fluorimeter, which gives flexibility in excitation and emission wavelength ranges. A number of compounds have been investigated and the results are compared to literature values and data obtained using a comparative method.

The synthesis and one- and two-photon optical properties of dipolar, quadrupolar and octupolar donor–acceptor molecules containing dimesitylboryl groups

Two series of related donor-acceptor conjugated dipolar, pseudo-quadrupolar (V-shaped) and octupolar molecular systems based on the p-dimesitylborylphenylethynylaniline core, namely, 4-(4-dimesitylborylphenylethynyl)-N,N-dimethylaniline, 4-[4-(4-dimesitylborylphenylethynyl)phenylethynyl]-N,N-dimethylaniline, 3,6-bis(4-dimesitylborylphenylethynyl)-N-n-butylcarbazole and tris[4-(4-dimesitylborylphenylethynyl)phenyl]amine, and on the E-p-dimesitylborylethenylaniline motif, namely, E-4-dimesitylborylethenyl-N,N-di(4-tolyl)aniline, 3,6-bis(E-dimesitylborylethenyl)-N-n-butylcarbazole and tris(E-4-dimesitylborylethenylphenyl)amine have been synthesised by palladium-catalyzed cross-coupling and hydroboration routes, respectively. Their absorption and emission maxima, fluorescence lifetimes and quantum yields have been obtained and their two-photon absorption (TPA) spectra and TPA cross-sections have been examined. Of these systems, the octupolar compound tris(E-4-dimesitylborylethenylphenyl)amine has been shown to exhibit the largest TPA cross-section among the two series of approximately 1000 GM at 740 nm. Its TPA performance is comparable to those of other triphenylamine-based octupoles of similar size. The combination of such large TPA cross-sections and high emission quantum yields, up to 0.94, make these systems attractive for applications involving two-photon excited fluorescence (TPEF).

The time domain in co-stained cell imaging: time-resolved emission imaging microscopy using a protonatable luminescent iridium complex

The intense luminescence of the new complex Ir(ppy)(2)(pybz) (1) within the cytoplasm of live cells can be discriminated from the fluorescence of an organic stain, solely on the basis of the emission timescale {pybzH = 2-pyridyl-benzimidazole}. The protonated form of 1 displays red-shifted emission, and may be implicated in a superior uptake compared to Ir(ppy)(3).

3‐Hydroxypyridin‐2‐one Complexes of Near‐Infrared (NIR) Emitting Lanthanides: Sensitization of Holmium(III) and Praseodymium(III) in Aqueous Solution

There is a growing interest in Near Infra-Red (NIR) emission originating from organic complexes of LnIII cations.[1,2] As a major impetus, biological tissues are considerably more transparent at these low energy wavelengths when compared to visible radiation, which facilitates deeper penetration of incident and emitted light.[3] Furthermore, the long luminescence lifetimes of LnIII complexes (eg. YbIII, τrad ~ 1 ms) when compared to typical organic molecules can be utilized to vastly improve signal to noise ratios by employing time-gating techniques. While the improved quantum yield of YbIII complexes when compared to other NIR emitters favours their use for bioimaging applications, there has also been significant interest[4,5,6] in the sensitized emission from other 4f metals such as Ln = Nd, Ho, Pr and Er which have well recognised applications as solid state laser materials[7] (eg. Nd ~ 1.06 μm, Ho ~ 2.09 μm), and in telecommunications (eg. Er ~ 1.54 μm) where they can be used for amplification of optical signals.[8]

Investigations of excitation energy transfer and intramolecular interactions in a nitrogen corded distrylbenzene dendrimer system

The photophysics of an amino-styrylbenzene dendrimer (A-DSB) system is probed by time-resolved and steady state luminescence spectroscopy. For two different generations of this dendrimer, steady state absorption, emission, and photoluminescence excitation spectra are reported and show that the efficiency of energy transfer from the dendrons to the core is very close to 100%. Ultrafast time-resolved fluorescence measurements at a range of excitation and detection wavelengths suggest rapid (and hence efficient) energy transfer from the dendron to the core. Ultrafast fluorescence anisotropy decay for different dendrimer generations is described in order to probe the energy migration processes. A femtosecond time-scale fluorescence depolarization was observed with the zero and second generation dendrimers. Energy transfer process from the dendrons to the core can be described by a Forster mechanism (hopping dynamics) while the interbranch interaction in A-DSB core was found to be very strong indicating the crossover to exciton dynamics.

Colour tuning of blue electroluminescence using bipolar carbazole–oxadiazole molecules in single-active-layer organic light emitting devices (OLEDs)

A synthetically versatile strategy has been employed for luminescence colour tuning in a new series of bipolar carbazole–2,5-diaryl-1,3,4-oxadiazole hybrid molecules 1–7. Their syntheses, solution absorption and emission properties and cyclic voltammetric data are reported. Calculations using DFT (density functional theory) establish that they possess molecular orbitals which favour bipolar charge-transport. Single-active-layer organic light emitting devices (OLEDs) have been fabricated by thermal evaporation using the bipolar compounds as the emitters in the architecture ITO:PEDOT-PSS:X:Ca/Al (X = 1–7). The structure–property relationships within the series of compounds are assessed with emphasis on the OLED performance and emission colour. The HOMO–LUMO gap has been varied by systematic modifications of the molecular subunits of 1–7, allowing the colour of the electroluminescence to be tuned from deep blue (CIE x,y 0.157, 0.079) through to green (CIE x,y 0.151, 0.096). These materials are very attractive for further development due to the combination of good processability of the molecules, their bipolar structure, colour tunability and efficient performance of OLEDs using a simple device architecture.

Polymeric Alkoxy PBD [2-(4-Biphenylyl)-5-Phenyl-1,3,4-Oxadiazole] for Light-Emitting Diodes

The syntheses are reported of the title polymeric alkoxyPBD derivative 5 and the dipyridyl analogue 12 using Suzuki coupling reactions of 1,4-dialkoxybenzene-2,5-diboronic acid with 2,5-bis(4-bromophenyl)-1,3,5-oxadiazole, and its dipyridyl analogue, respectively. Thermal gravimetric analysis shows that polymers 5 and 12 are stable up to 370 C and 334 C, respectively. Films of polymer 5 spun from chloroform solution show an absorption at kmax = 367 nm, and a weaker band at 312 nm, and strong blue photoluminescence at kmax = 444 nm. The photoluminescence quantum yield (PLQY) was found to be 27 ‐ 3 %. For polymer 12, the absorption spectra reveal bands of equal intensity at kmax = 374 and 312 nm, with PL at kmax = 475 nm. Device studies using polymer 12 were hampered by its instability under illumination and/or electrical excitation. Polymer 5 is stable under these conditions and acts as an efficient electron-transporting/hole-blocking layer. For devices of configuration ITO/PEDOT/ MEH-PPV/polymer 5/Al an external quantum efficiency of 0.26 % and brightness of 800 cd/m 2 was readily achieved: orange emission was observed, identical to the MEH-PPV electroluminescence.

Syntheses, structures, two-photon absorption cross-sections and computed second hyperpolarisabilities of quadrupolar A–π–A systems containing E-dimesitylborylethenyl acceptors

A series of bis(E-dimesitylborylethenyl)-substituted arenes, namely arene = 1,4-benzene, 1,4-tetrafluorobenzene, 2,5-thiophene, 1,4-naphthalene, 9,10-anthracene, 4,4′-biphenyl, 2,7-fluorene, 4,4′-E-stilbene, 4,4′-tolan, 5,5′-(2,2′-bithiophene), 1,4-bis(4-phenylethynyl)benzene, 1,4-bis(4-phenylethynyl)tetrafluorobenzene and 5,5″-(2,2′:5′,2″-terthiophene), have been synthesised viahydroboration of the corresponding diethynylarenes with dimesitylborane. Their absorption and emission maxima, fluorescence lifetimes and quantum yields are reported along with the two-photon absorption (TPA) spectra and TPA cross-sections for the 5,5′-bis(E-dimesitylborylethenyl)-2,2′-bithiophene and 5,5′-bis(E-dimesitylborylethenyl)-2,2′:5′,2″-terthiophene derivatives. The TPA cross-section of the latter compound of ca. 1800 GM is the largest yet reported for a 3-coordinate boron compound and is in the range of the largest values measured for quadrupolar compounds with similar conjugation lengths. The X-ray crystal structures of 1,4-benzene, 2,5-thiophene, 4,4′-biphenyl and 5,5″-(2,2′:5′,2″-terthiophene) derivatives indicate π-conjugation along the BCC–arene–CCB chain. Theoretical studies show that the second molecular hyperpolarisabilities, γ, in each series of compounds are generally related to the HOMO energy, which itself increases with increasing donor strength of the spacer. A strong enhancement of γ is predicted as the number of thiophene rings in the spacer increases.

Efficient intramolecular charge transfer in oligoyne-linked donor–π–acceptor molecules.

Studies are reported on a series of triphenylamine-(C[triple bond]C)(n)-2,5-diphenyl-1,3,4-oxadiazole dyad molecules (n=1-4, 1, 2, 3 and 4, respectively) and the related triphenylamine-C(6)H(4)-(C[triple bond]C)(3)-oxadiazole dyad 5. The oligoyne-linked D-pi-A (D=electron donor, A=electron acceptor) dyad systems have been synthesised by palladium-catalysed cross-coupling of terminal alkynyl and butadiynyl synthons with the corresponding bromoalkynyl moieties. Cyclic voltammetric studies reveal a reduction in the HOMO-LUMO gap in the series of compounds 1-4 as the oligoyne chain length increases, which is consistent with extended conjugation through the elongated bridges. Photophysical studies provide new insights into conjugative effects in oligoyne molecular wires. In non-polar solvents the emission from these dyad systems has two different origins: a locally excited (LE) state, which is responsible for a pi*-->pi fluorescence, and an intramolecular charge transfer (ICT) state, which produces charge-transfer emission. In polar solvents the LE state emission vanishes and only ICT emission is observed. This emission displays strong solvatochromism and analysis according to the Lippert-Mataga-Oshika formalism shows significant ICT for all the luminescent compounds with high efficiency even for the longer more conjugated systems. The excited-state properties of the dyads in non-polar solvents vary with the extent of conjugation. For more conjugated systems a fast non-radiative route dominates the excited-state decay and follows the Engelman-Jortner energy gap law. The data suggest that the non-radiative decay is driven by the weak coupling limit.