Solid-state Photoluminescence Quantum Yields Research Articles

The ‘double dendron’ approach to host free phosphorescent poly(dendrimer) OLEDs

The addition of dendrons to iridium(III) complexes attached to a poly(styrene) backbone is shown to improve the physical and optoelectronic properties of the phosphorescent materials. The iridium(III) complexes have two 2-phenylpyridyl ligands and one phenyltriazolyl ligand, with the latter providing the attachment point to the polymer backbone. It was found that by increasing the number of dendrons (from zero to two) per 2-phenylpyridyl ligand, the intra- and interchain interactions could be more effectively controlled. The poly(dendrimer) with two dendrons per ligand had solution and solid-state photoluminescence quantum yields of 67% and 47%, respectively. Organic light-emitting diodes containing the doubly dendronised (two dendrons per ligand) poly(dendrimer) had a low turn on voltage of 3.6 V (>1.0 cd m−2), a maximum luminance of 6 700 cd m−2 (at 12.2 V), and 100 cd m−2 was achieved at 6.0 V with an external quantum efficiency (EQE) of 9.2% (28.1 cd A−1) and power efficiency of 14.7 lm/W. A solution of the doubly dendronised poly(dendrimer) in N-methyl-2-pyrolidinone was found to have a viscosity of 4.6 mPa s, which falls in the range of solutions that can be inkjet printed.

A Phosphorescent Poly(dendrimer) Containing Iridium(III) Complexes: Synthesis and Light-Emitting Properties

A poly(styrene) with pendant dendronized iridium(III) complexes attached to every repeat unit was prepared in good yields using a free radical polymerization of a “macromonomer”. The dendronized pendant groups were heteroleptic iridium(III) complexes comprised of two 2-phenylpydridyl ligands, to which first generation biphenyl dendrons with 2-ethylhexyloxy surface groups were attached, and a phenyltriazolyl coligand that formed the attachment point to the polymer backbone. Dendronization of the pendant iridium(III) complexes was found to improve thermal stability, solubility, and solution (61%) and solid-state (13%) photoluminescence quantum yields (PLQYs) relative to the nondendronized homopolymer. Viscosity under normal solution processing conditions of 25 mg/mL was found to be 1.23 cSt. Importantly, although the phosphorescent iridium(III) chromophores are held closely along the polymer backbone, they do not significantly reduce the PLQY in solution by intrachain chromophore interactions. Simple bilaye...

A new benzimidazole/carbazole hybrid bipolar material for highly efficient deep-blue electrofluorescence, yellow–green electrophosphorescence, and two-color-based white OLEDs

The bipolar molecule CPhBzIm exhibits an excellent solid state photoluminescence quantum yield (ΦPL = 69%), triplet energy (ET = 2.48 eV), and bipolar charge transport ability (μh ≈ μe ≈ 10−6–10−5 cm2 V−1 s−1). We have used it to fabricate a non-doped deep-blue organic light emitting diode (OLED) exhibiting promising performance [ηext = 3%; CIE = (0.16, 0.05)] and to serve as host material for a yellow–green phosphorescent OLED [ηext = 19.2%; CIE = (0.42, 0.56)]. Exploiting these dual roles, we used CPhBzIm in a simple singly doped, two-color-based white OLED (ηext = 7%; CIE = 0.31, 0.33).

High-luminescence non-doped green OLEDs based on a 9,9-diarylfluorene-terminated 2,1,3-benzothiadiazole derivative

We have prepared a highly efficient non-doped green organic light-emitting diode (OLED) incorporating a novel 9,9-diarylfluorene-terminated 2,1,3-benzothiadiazole green emitter (DFBTA), which exhibits an excellent solid state photoluminescence quantum yield (81%), and new triaryldiamines (DPAInT2, DPAInF) with high hole mobility derived from rigid and coplanar cores. The optimal device: ITO/DPAInT2/DPAInF/TCTA/DFBTA/Alq3/LiF/Al displayed impressive device characteristics, including a maximum external quantum efficiency (ηext) of 3.7% (12.9 cd A−1) and a maximum brightness at 168 000 cd m−2.

4,4,5,5,5-Pentafluoro-1-(9H-fluoren-2-yl)-1,3-pentanedione complex of Eu3+ with 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene oxide as a promising light-conversion molecular device

A novel beta-diketone ligand, 4,4,5,5,5-pentafluoro-1-(9H-fluoren-2-yl)-1,3-pentanedione (Hpffpd), which contains a polyfluorinated alkyl group, as well as a long conjugated fluorene unit, and a chelate phosphine oxide ligand, 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene oxide (DDXPO) were synthesized and utilized for the synthesis of two new europium complexes [Eu(pffpd)3(C2H5OH)(H2O)] 1 and [Eu(pffpd)3(DDXPO)] 2. For comparison, the [Eu(pffpd)3(DPEPO)] 3 complex was also synthesized involving a known chelate phosphine oxide, bis(2-(diphenylphosphino)phenyl) ether oxide (DPEPO). The synthesized complexes have been characterized by various spectroscopic techniques and their solid-state photophysical properties were investigated. The single-crystal X-ray diffraction analyses of 2 and 3 revealed that these complexes are mononuclear, and that the central Eu3+ ion is surrounded by eight oxygen atoms, six of which are from the three bidentate fluorinated beta-diketonates, and the other two oxygen atoms from the chelate phosphine oxide. The coordination polyhedra can be described as distorted square antiprism. Compound 2 has a solid-state photoluminescence quantum yield of 48%, which is about two times higher than that of compound 3 (28%). This may be due to the fact that DDXPO in 1 has the mezzo first triplet excited state energy level (T1) between the first singlet excited energy level (S1) and T1 of Hpffpd, which may support one more additional energy transfer from the T1 energy level of DDXPO to that of Hpffpd, and consequently improves the energy transfer in the Eu3+ complex. Furthermore, DDXPO (average Eu-O = 2.34 A) in complex 2 coordinates more strongly with the central Eu3+ as compared to DPEPO in complex 3 (average Eu-O = 2.38 A) which can improve the energy transfer between the ligands and central metal ion, and consequently enhances the photoluminescence efficiency of the corresponding Eu3+ complex.

Asymmetric Pentacene Derivatives for Organic Light-Emitting Diodes

Two aryl-substituted red emitting pentacenes, 5,6,13,14-tetraphenylpentacene (asym-TPP) and 5,14-bis(2,6-dimethylphenyl)-6,13-diphenylpentacene (DMPDPP), have been prepared and spectroscopically characterized. Guest−host films of pentacene derivatives dispersed in tris(quinolin-8-olato)aluminum(III) (Alq3) exhibit narrow red emission (λmax = 663−680 nm), indicative of efficient Forster energy transfer from the Alq3 host to the guest molecules. Solid-state absolute photoluminescence quantum yields of the films were measured as a function of guest molecule concentration (ΦPL ∼20% at 0.20−0.50 mol %). Red light-emitting diodes based on the fluorescent DMPDPP yield external electroluminescence quantum efficiency (ηEL ∼1%) close to the theoretical limit.

Synthesis of Conjugated Polyrotaxanes.

A series of conjugated polyrotaxane insulated molecular wires are synthesised by aqueous Suzuki polymerisation, using hydrophobic binding to promote threading of the cyclodextrin units. These polyrotaxanes have conjugated polymer cores based on poly(para-phenylene), polyfluorene, and poly(diphenylene-vinylene), threaded through 0.9-1.6 cyclodextrins per repeat unit. Bulky naphthalene-3,6-disulfonate endgroups prevent the macrocycles from slipping off the conjugated polymer chains. Dialysis experiments show that the cyclodextrins become unthreaded only if smaller stoppers are used. MALDI TOF mass spectra detect oligomers with up to ten threaded cyclodextrins, and reveal the presence of some defects that result for oxidative homo-coupling of boronic acids. Weight-average molecular weights were determined by analytical ultracentrifugation, demonstrating that step-growth polymerisation is efficient enough to achieve degrees of polymerisation up to approximately 20 repeat units (84 para-phenylenes). The fluorescence spectra of these polyrotaxanes indicate that the presence of the threaded cyclodextrin macrocycles reduces the flexibility of the conjugated polymer pi-systems. Both the solution and the solid-state photoluminescence quantum yields are enhanced upon threading of the conjugated polyaromatic cores through alpha- or beta-cyclodextrins, and the emission spectra of the polyrotaxanes are blue-shifted compared to the corresponding unthreaded polymers. The greater weight of the 0-0 transition in the emission spectra, as well as the smaller Stokes shift, indicate that the polyrotaxanes are more rigid than the unthreaded polymers.

Synthesis of conjugated polyrotaxanes.

A series of conjugated polyrotaxane insulated molecular wires are synthesised by aqueous Suzuki polymerisation, using hydrophobic binding to promote threading of the cyclodextrin units. These polyrotaxanes have conjugated polymer cores based on poly(para-phenylene), polyfluorene, and poly(diphenylene-vinylene), threaded through 0.9-1.6 cyclodextrins per repeat unit. Bulky naphthalene-3,6-disulfonate endgroups prevent the macrocycles from slipping off the conjugated polymer chains. Dialysis experiments show that the cyclodextrins become unthreaded only if smaller stoppers are used. MALDI TOF mass spectra detect oligomers with up to ten threaded cyclodextrins, and reveal the presence of some defects that result for oxidative homo-coupling of boronic acids. Weight-average molecular weights were determined by analytical ultracentrifugation, demonstrating that step-growth polymerisation is efficient enough to achieve degrees of polymerisation up to approximately 20 repeat units (84 para-phenylenes). The fluorescence spectra of these polyrotaxanes indicate that the presence of the threaded cyclodextrin macrocycles reduces the flexibility of the conjugated polymer pi-systems. Both the solution and the solid-state photoluminescence quantum yields are enhanced upon threading of the conjugated polyaromatic cores through alpha- or beta-cyclodextrins, and the emission spectra of the polyrotaxanes are blue-shifted compared to the corresponding unthreaded polymers. The greater weight of the 0-0 transition in the emission spectra, as well as the smaller Stokes shift, indicate that the polyrotaxanes are more rigid than the unthreaded polymers.

High efficiency molecular organic light-emitting diodes based on silole derivatives and their exciplexes

Abstract Here, we report the performance of molecular organic light-emitting diodes (MOLEDs) using novel fluorescent silole derivatives as highly efficient blue and green-emitting organic materials. Three silole derivatives, namely 2,5-di-(3-biphenyl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene (PPSPP), 9-silafluorene-9-spiro-1′-(2′,3′,4′,5′-tetraphenyl)-1′H-silacyclopentadiene (ASP) and 1,2-bis(1-methyl-2,3,4,5,-tetraphenylsilacyclopentadienyl)ethane (2PSP), with high solid-state photoluminescence (PL) quantum yields of 0.85, 0.87 and 0.94, respectively, were used as emissive materials. Another high electron mobility silole derivative, 2,5-bis(2′,2″-bipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene (PyPySPyPy), was used as the electron transport material. MOLEDs using these siloles as emitters and N,N′-diphenyl-N,N′-(2-napthyl)-(1,1′-phenyl)-4,4′-diamine (NPB) or N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′diamine (TPD) as the hole transport material show low operating voltages of 4–4.5 V at a luminance of 100 cd/m2 and high external electroluminescence (EL) quantum efficiencies of 3.4–4.1% at ∼100 A/m2. MOLEDs based on PPSPP and PyPySPyPy exhibit red-shifted EL spectra which are assigned to exciplexes formed at the interface between the hole transporting layer NPB or TPD and the PPSPP or PyPySPyPy light-emitting layer, respectively.

Highly efficient molecular organic light-emitting diodes based on exciplex emission

Highly efficient exciplex emission is observed from molecular organic light-emitting diodes (MOLEDs) based on silole derivatives as emissive and electron transport materials, and a hole transporting amine derivative. A silole derivative, 2,5-di-(3-biphenyl)-1,1-dimethyl- 3,4-diphenylsilacyclopentadiene (PPSPP), which shows blue fluorescence (476 nm) with a high solid-state photoluminescence quantum yield of 85% was used as the emitter. Another silole derivative, 2,5-bis-(2′,2″-bipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene which exhibits high electron mobility, was used as the electron transport material. MOLEDs using these two siloles and N,N′-diphenyl-N,N′-(2-napthyl)-(1,1′-phenyl)-4,4′-diamine (NPB) as the hole transporter show electroluminescence (EL) emission centered at 495 nm. This red-shifted EL band relative to the blue fluorescence of PPSPP is assigned to a NPB:PPSPP exciplex. An operating voltage of 4.5 V was measured at 100 cd/m2 and an EL quantum efficiency of 3.4% was achieved at 100 A/m2.

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.