The authors demonstrate that the reduction of quantum efficiency with increasing current density in phosphorescent light emitting diodes ͑PhOLEDs͒ is related to the formation of excitons in hole transporting layer based on the analysis of emission spectra and exciton formation zone. Low roll-off of efficiency in a PhOLED was achieved using dual emitting layers ͑D-EMLs͒ by confining the exciton formation near the interface between the emitting layers. The external quantum efficiency was maintained almost constant up to 22 mA/ cm 2 ͑10 000 cd/ m 2 ͒ by adopting the D-EMLs in Ir͑ppy͒ 3 based PhOLEDs, resulting in high external quantum efficiency ͑ ext = 13.1% ͒ at high luminance. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2745224͔ Phosphorescent organic lighting emitting diodes ͑PhOLEDs͒ have received considerable attention due to their ability of highly efficient emission compared with fluorescent OLEDs.1-4 Through harvesting of both singlet and triplet excitons, the external quantum efficiency ͑ ext ͒ of PhOLEDs has reached above 20% by using the optimized material systems, 5-7 p-i-n structures, 8,9 and microcavity structures.10,11 However, the efficiency roll-off ͑the decrease of efficiency with increasing current density͒ occurs at much lower luminance than required in displays or solid-state lighting. The roll-off of the quantum efficiency is one of the most significant problems facing electrophosphorescent devices and its origin was attributed to the triplet-triplet annihilation coming from long lifetime of triplet excitons, 12,13 electric field induced dissociation of excitons, 14 and tripletpolaron annihilation. 12,13 In this letter, we report that the roll-off of the quantum efficiency with an increasing current density is related to the exciton formation in the hole transporting layer ͑HTL͒. Analysis of the steady state emission spectra indicated that the significant portion of the efficiency reduction is originated from the more and more exciton formation in the HTL with increasing current density. Based on the results, we fabricated devices with double emitting layers ͑D-EMLs͒ in order to confine the exciton formation in the emitting layers. The external quantum efficiency ͑ ext ͒ was maintained constant up to 22 mA/ cm 2 ͑10 000 cd/ m 2 ͒ by adopting the D-EMLs in Ir͑ppy͒ 3 based PhOLEDs, resulting in high external quantum efficiency at high luminescence compared to the devices with single emitting layer ͑S-EML͒. The OLEDs with D-EMLs show significantly lower roll-off of efficiency ͓ ext = 13.1% at 10 000 cd/ m 2 ͑22 mA/ cm 2 ͔͒ than conventional S-EML OLEDs ͓ ext = 7.8% at 5400 cd/ m 2 ͑20 mA/ cm 2 ͒, and 6.9% at 10 000 cd/ m 2 ͑40 mA/ cm 2 ͔͒. The OLEDs were fabricated by thermal evaporation onto a cleaned glass substrate precoated with indium tin oxide ͑ITO͒ without breaking the vacuum. Prior to organic layer deposition, ITO substrates were exposed to UV-ozone flux for 10 min following degreasing in aceton and isoprophyl alcohol. All organic layers were grown by thermal evaporation at the...

Highly efficient deep‐blue fluorescent materials based on phenylquinoline–carbazole derivatives (PhQ‐CVz, MeO‐PhQ‐CVz, and CN‐PhQ‐CVz) are synthesized for organic light‐emitting diodes (OLEDs). The materials form high‐quality amorphous thin films by thermal evaporation and the energy levels can be easily adjusted by the introduction of different electron‐donating and electron‐withdrawing groups on carbazoylphenylquinoline. Non‐doped deep‐blue OLEDs that use PhQ‐CVz as the emitter show bright emission (Commission Internationale de L'Éclairage (CIE) coordinates, x = 0.156, y = 0.093) with an external quantum efficiency of 2.45%. Furthermore, the material works as an excellent host material for 4,4′‐bis(9‐ethyl‐3‐carbazovinylene)‐1,1′‐biphenyl dopant to get high‐performance OLEDs with excellent deep‐blue CIE coordinates (x = 0.155, y = 0.157), high power efficiency (5.98 lm W−1), and high external quantum efficiency (5.22%).

PurposeChronic use of topical hypotensive agents induces several side effects caused by preservatives. The purpose of this study was to evaluate the effects of prostaglandin analogs with varying concentrations of benzalkonium chloride (BAC), preservative-free (PF), and alternative preservatives on mouse corneal tissue.MethodsThirty-five, 8- to 10-week-old female C57BL/6 mice (five mice for each group) were used for this study. To the control group, we applied normal saline, and to each drug-treated group we applied 0.02% BAC, bimatoprost 0.01% (with BAC 0.02%), latanoprost 0.005% (with BAC 0.02%), travoprost 0.004% (with 0.001% polyquad) or tafluprost 0.0015% with/without 0.001% BAC, once a day (9 p.m.) for 4 weeks. Corneal fluorescein staining was evaluated in all groups. After harvest, the corneal tissues were embedded in paraffin and then Hematoxylin-Eosin stain was performed for histopathological examination. Immunofluorescence staining was done against TNF-α, IL-6, HLA DR, pJNK, and pAkt.ResultsIn corneal fluorescein staining, severe punctate epithelial keratitis was seen in the groups of 0.02% BAC, 0.02% BAC containing bimatoprost 0.01% and latanoprost 0.005%. The surface desquamation, irregular surface, loss of cell borders, anisocytosis and stromal shrinkage were observed in the groups of BAC-containing eye drops. Moreover, the groups treated with BAC-containing eye drops have high inflammatory markers, significantly decreased cell viability-related signal, pAkt, and higher apoptosis-inducing signal, pJNK, than the control group. On the other hand, travoprost 0.004% and PF tafluprost 0.0015% have less cellular morphologic changes, lower inflammation, and higher cellular viability than BAC-containing formulations.ConclusionsCorneal damage, increased inflammation and apoptosis and low cell viability were observed in BAC-containing groups. PF or alternatively preserved glaucoma medications seem to be a reasonable and viable alternative to those preserved with BAC.

Iridium complexes containing cyclometalated 2‐cyclohexenylpyridine derivatives with rigid and bulky cyclohexene units are synthesized, and found to be highly efficient materials in EL devices (see figure). Devices based on these iridium complexes emit yellowish‐ green light with the very high external quantum efficiency of 18.7%.

Deguelin is a nature-derived chemopreventive drug. Endothelial progenitor cells (EPCs) are bone-marrow (BM)-derived key components to induce new blood vessels in early tumorigenesis and metastasis. Here we determined whether deguelin inhibits EPC function in vitro and in vivo at doses not affecting cancer cell apoptosis. Deguelin significantly reduced the number of EPC colony forming units of BM-derived c-kit+/sca-1+ mononuclear cells (MNCs), proliferation, migration, and adhesion to endothelial cell monolayers, and suppressed incorporation of EPC into tube-like vessel networks when co-cultured with endothelial cells. Deguelin caused cell cycle arrest at G1 without induction of apoptosis in EPC. In a mouse tumor xenograft model, tumor growth, lung metastasis and tumor-induced circulating EPCs were supressed by deguelin treatment (2 mg/kg). In mice tranplanted with GFP-expressing BM-MNCs, deguelin reduced the co-localization of CD31 and GFP, suggesting suppression of BM-derived EPC incoporation into tumor vessels. Interestingly, focal adhesion kinase (FAK)-integrin-linked kinase (ILK) activation and actin polymerization were repressed by deguelin. Decreased number of focal adhesions and a depolarized morphology was found in deguelin-treated EPCs. Taken together, our results suggest that the deguelin inhibits tumorigenesis and metastasis via EPC suppression and that suppression of focal adhesion by FAK-integrin-ILK-dependent actin remodeling is a key underlying molecular mechanism.