Molecular design of efficient white‐light‐emitting fluorene‐based copolymers by mixing singlet and triplet emission

A new strategy to realize efficient white‐light emission from a binary fluorene‐based copolymer (PF‐Phq) with the fluorene segment as a blue emitter and the iridium complex, 9‐iridium(III)bis(2‐(2‐phenyl‐quinoline‐N,C³′)(11,13‐tetradecanedionate))‐3,6‐carbazole (Phq), as a red emitter has been proposed and demonstrated. The photo‐ and electroluminescence properties of the PF‐Phq copolymers were investigated. White‐light emission with two bands of blue and red was achieved from the binary copolymers. The efficiency increased with increasing concentration of iridium complex, which resulted from its efficient phosphorescence emission and the weak phosphorescent quenching due to its lower triplet energy level than that of polyfluorene. In comparison with the binary copolymer, the efficiency and color purity of the ternary copolymers (PF‐Phq‐BT) were improved by introducing fluorescent green benzothiadiazole (BT) unit into polyfluorene backbone. This was ascribed to the exciton confinement of the benzothiadiazole unit, which allowed efficient singlet energy transfer from fluorene segment to BT unit and avoided the triplet quenching resulted from the higher triplet energy levels of phosphorescent green emitters than that of polyfluorene. The phosphorescence quenching is a key factor in the design of white light‐emitting polyfluorene with triplet emitter. It is shown that using singlet green and triplet red emitters is an efficient approach to reduce and even avoid the phosphorescence quenching in the fluorene‐based copolymers. The strategy to incorporate singlet green emitter to polyfluorene backbone and to attach triplet red species to the side chain is promising for white polymer light‐emitting diodes. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 453–463, 2008     

Expanding Anti‐Stokes Shifting in Triplet–Triplet Annihilation Upconversion for In Vivo Anticancer Prodrug Activation

A strategy to expand anti‐Stokes shifting from the far‐red to deep‐blue region in metal‐free triplet–triplet annihilation upconversion (TTA‐UC) is presented. The method is demonstrated by in vivo titration of the photorelease of an anticancer prodrug. This new TTA system has robust brightness and the longest anti‐Stokes shift of any reported TTA system. TTA core–shell‐structured prodrug delivery capsules that benefit from these properties were developed; they can operate with low‐power density far‐red light‐emitting diode light. These capsules contain mesoporous silica nanoparticles preloaded with TTA molecules as the core, and amphiphilic polymers encapsulating anticancer prodrug molecules as the shell. When stimulated by far‐red light, the intense TTA upconversion blue emission in the system activates the anticancer prodrug molecules and shows effective tumor growth inhibition in vivo. This work paves the way to new organic TTA upconversion techniques that are applicable to in vivo photocontrollable drug release and other biophotonic applications.     

A Bipolar and Small Singlet‐Triplet Splitting Energy Host with Triplet Energy Lower Than a Blue Phosphor for Phosphorescent OLEDs in Panchromatic Range

A bipolar host material, 2,4,6‐tris(3‐(carbazol‐9‐yl)phenyl)‐triazine (TCPZ), was synthesized according to reported method. Due to the higher triplet energy compared to green and red phosphors, TCPZ is suitable to host them in phosphorescent organic light‐emitting diodes (PhOLEDs). Although the triplet of TCPZ is slightly lower than a common blue phosphor, good blue PhOLEDs using TCPZ as the host were successfully demonstrated in this work. By low temperature emission measurement, it was found that the energy splitting between the singlet and triplet of TCPZ is as small as 0.24 eV. Therefore, thermal activated energy transfer from triplet to singlet in the host TCPZ is expected to occur, which can be afterwards efficiently transferred to the blue phosphor, hence enabling it to host blue phosphor. As a result, TCPZ can be used as host for phosphors in panchromatic range. Additionally, single‐carrier devices clearly prove its good bipolar transport feature, beneficial to device performance. By using TCPZ as a host, high performance deep‐red, green and blue PhOLEDs have been achieved, with maximum efficiencies of 9.3 cd·A^?1 (13.2%), 81.3 cd·A^?1 (23.1%) and 17.03 cd·A^?1 (10.4%), respectively.     

Luminous Butterflies: Efficient Exciton Harvesting by Benzophenone Derivatives for Full‐Color Delayed Fluorescence OLEDs

Butterfly‐shaped luminescent benzophenone derivatives with small energy gaps between their singlet and triplet excited states are used to achieve efficient full‐color delayed fluorescence. Organic light‐emitting diodes (OLEDs) with these benzophenone derivatives doped in the emissive layer can generate electroluminescence ranging from blue to orange–red and white, with maximum external quantum efficiencies of up to 14.3 %. Triplet excitons are efficiently harvested through delayed fluorescence channels.     

Site modification of K atoms in solid Ar by X-ray and light irradiation

The influence of X-ray irradiation on the optical absorption of K-atoms in Ar-matrices was studied as a function of time and temperature. Exponential decrease of the so-called red and blue triplet absorptions was observed. Simultaneously a new absorption in the range of the red triplet was built up. The new absorption could alternatively be produced by light irradiation into the red or the blue triplet, as was shown in a second experiment studying correlated optical and ESR-absorption. After annealing at 15 K the new absorption disappeared, and the original absorptions were largely restored. All experimental results can well be understood in terms of exciton production, self-trapping, and deexcitation, and are described by a master equation system.     

Highly Efficient Organic Blue Electroluminescent Materials and Devices with Mesoscopic Structures

Due to the difficulty in achieving high efficiency and high color purity simultaneously, blue emission is the limiting factor for the performance and stability of OLEDs. Since 2003, we have been working on organic light‐emitting diodes (OLEDs), especially on blue light. After a series of molecular designs, novel strategies have been proposed from different aspects. At first, highly efficient deep blue emission could be achieved through molecular design with highly twisted structure to suppress fluorescence quenching and redshift. Deep blue emitters with high efficiency in solid state, a twisted structure with aggregation induced emission (AIE) characteristics was incorporated to inhibit molecular aggregation, and triplet‐triplet fusion (TTF) and hybridized localized charge transfer (HLCT) were adopted to increase the ratio of triplet exciton used. Secondly, a highly efficient blue OLED could be achieved through improving charge transport. New electron transport materials (ETMs) with wide band gap were developed to control charge transport balance in devices. Thirdly, a highly efficient deep blue emission could be achieved through a mesoscopic structure of out‐coupling layer. A mesoscopic photonic structured organic thin film was fabricated on the top of metal electrode by self‐aggregation in order to improve the light out‐coupling efficiency.     

The Preparation of (8‐Hydroxyquinolinato)Bis(2‐Phenylpyridyl)Iridium Complexes and Their Photophysical Properties

Four derivatives of the titled compounds, (8‐hydroxyquinoline)bis(2‐phenylpyridyl)iridium ( IrQ(ppy)₂ ), were prepared. Two of them were confirmed by single crystal X‐ray diffraction analyses, in which solvent molecules were found to be incorporated in the crystal lattices. Their emission spectra display separated dual bands in de‐aerated solutions at about 515 and 645 nm upon excitation. These green and red emissions are attributed to the triplet metal‐to‐ligand charge transfer (³MLCT) and triplet ligand centered (³LC) transitions in Ir(ppy)₂ and IrQ, respectively. It is suggested that such a multiple emission is feasible by nearly orthogonal orientation between the ppy and quinoline ligands in the mixed‐ligand Ir‐compounds which prohibits energy transfer between the two different ligands. The electroluminescence (EL) of these compounds was examined by the fabrication of light‐emitting diodes (LEDs). Unlike the spectra in solutions, their EL spectra displayed only the red emission band. Devices displaying white light can be obtained by mixing the red emission of IrQ(ppy)₂ with a compatible blue emitter (NPB) in separated layers.     

Synthesis and characterization of a polyfluorene containing carbazole and oxadiazole dipolar pendent groups and its application to electroluminescent devices

We have synthesized a blue‐light‐emitting polyfluorene (PF) derivative ( PF‐CBZ‐OXD ) that presents bulky hole‐transporting carbazole and electron‐transporting oxadiazole pendent groups functionalized at the C‐9 positions of alternating fluorene units. The results from photoluminescence and electrochemical measurements indicate that both the side chains and the PF main chain retain their own electronic characteristics in the copolymer. An electroluminescent device incorporating this polymer as the emitting layer was turned on at 4.5 V; it exhibited a stable blue emission with a maximum external quantum efficiency of 1.1%. Moreover, we doped PF‐CBZ‐OXD and its analogue PF‐TPA‐OXD with a red‐light‐emitting iridium phosphor for use as components of phosphorescent red‐light emitters to investigate the effect of the host's HOMO energy level on the degree of charge trapping and on the electrophosphorescent efficiency. We found that spectral overlap and individual energy level matching between the host and guest were both crucial features affecting the performance of the electroluminescence devices. Atomic force microscopy measurements indicated that the dipolar nature of PF‐CBZ‐OXD , in contrast to the general nonpolarity of polydialkylfluorenes, provided a stabilizing environment that allowed homogeneous dispersion of the polar iridium triplet dopant. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2925–2937, 2007     

Rational Engineering of Photoconvertible Fluorescent Proteins for Dual‐Color Fluorescence Nanoscopy Enabled by a Triplet‐State Mechanism of Primed Conversion

Green‐to‐red photoconvertible fluorescent proteins (pcFPs) are powerful tools for super‐resolution localization microscopy and protein tagging. Recently, they have been found to undergo efficient photoconversion not only by the traditional 400‐nm illumination but also by an alternative method termed primed conversion, employing dual wavelength illumination with blue and far‐red/near‐infrared light. Primed conversion has been reported only for Dendra2 and its mechanism has remained elusive. Here, we uncover the molecular mechanism of primed conversion by reporting the intermediate “primed” state to be a triplet dark state formed by intersystem crossing. We show that formation of this state can be influenced by the introduction of serine or threonine at sequence position 69 (Eos notation) and use this knowledge to create “pr”‐ (for primed convertible) variants of most known green‐to‐red pcFPs.     

Benzimidazobenzothiazole‐Based Bipolar Hosts to Harvest Nearly All of the Excitons from Blue Delayed Fluorescence and Phosphorescent Organic Light‐Emitting Diodes

Much effort has been devoted to developing highly efficient organic light‐emitting diodes (OLEDs) that function through phosphorescence or thermally activated delayed fluorescence (TADF). However, efficient host materials for blue TADF and phosphorescent guest emitters are limited because of their requirement of high triplet energy levels. Herein, we report the rigid acceptor unit benzimidazobenzothiazole (BID‐BT), which is suitable for use in bipolar hosts in blue OLEDs. The designed host materials, based on BID‐BT, possess high triplet energy and bipolar carrier transport ability. Both blue TADF and phosphorescent OLEDs containing BID‐BT‐based derivatives exhibit external quantum efficiencies as high as 20 %, indicating that these hosts allow efficient triplet exciton confinement appropriate for blue TADF and phosphorescent guest emitters.     

HYDRODYNAMIC EFFECTS IN THE PHOTOSENSITIZED LYSIS OF LIPOSOMES*

Abstract— The photosensitized lysis of phosphatidylcholine liposomes incorporating methylene blue in the membrane or in the presence of external methylene blue is promoted by hydrodynamic agitation concurrent with or subsequent to irradiation with red light. The results implicate the attack of singlet oxygen on an unsaturated lipid component as the key photochemical step, which is followed by additional membrane damage induced by hydrodynamic action.     

Highly Efficient Near‐Infrared Delayed Fluorescence Organic Light Emitting Diodes Using a Phenanthrene‐Based Charge‐Transfer Compound

Significant efforts have been made to develop high‐efficiency organic light‐emitting diodes (OLEDs) employing thermally activated delayed fluorescence (TADF) emitters with blue, green, yellow, and orange–red colors. However, efficient TADF materials with colors ranging from red, to deep‐red, to near‐infrared (NIR) have been rarely reported owing to the difficulty in molecular design. Herein, we report the first NIR TADF molecule TPA‐DCPP (TPA=triphenylamine; DCPP=2,3‐dicyanopyrazino phenanthrene) which has a small singlet–triplet splitting (ΔE_ST) of 0.13 eV. Its nondoped OLED device exhibits a maximum external quantum efficiency (EQE) of 2.1 % with a Commission International de L′Éclairage (CIE) coordinate of (0.70, 0.29). Moreover, an extremely high EQE of nearly 10 % with an emission band at λ=668 nm has been achieved in the doped device, which is comparable to the most‐efficient deep‐red/NIR phosphorescent OLEDs with similar electroluminescent spectra.     

Kinetics of spreading of photodamaged DOPC liposomes at the air-water interface

The kinetics of surface film formation from DOPC small unilamellar liposomes spread at the air-water interface was studied. A simple theoretical and experimental approach was used.It was found that the surface transformation process was accelerated under photodynamic lipid peroxidation in presence of methylene blue and red light illumination. A theoretical approach, coupling the photodynamic action and the process of spreading of liposomes was developed. The mechanisms of photomodification and destabilization of the liposomal bilayer structures was analyzed.     

Using an Organic Molecule with Low Triplet Energy as a Host in a Highly Efficient Blue Electrophosphorescent Device

To achieve high efficiencies in blue phosphorescent organic light‐emitting diodes (PhOLEDs), the triplet energies (T₁) of host materials are generally supposed to be higher than the blue phosphors. A small organic molecule with low singlet energy (S₁) of 2.80 eV and triplet energy of 2.71 eV can be used as the host material for the blue phosphor, [bis(4,6‐difluorophenylpyridinato‐N,C^2′)iridium(III)] tetrakis(1‐pyrazolyl)borate (FIr6; T₁=2.73 eV). In both the photo‐ and electro‐excited processes, the energy transfer from the host material to FIr6 was found to be efficient. In a three organic‐layer device, the maximum current efficiency of 37 cd A^?1 and power efficiency of 40 Lm W^?1 were achieved for the FIr6‐based blue PhOLEDs.     

Pd–Porphyrin Oligomers Sensitized for Green‐to‐Blue Photon Upconversion: The More the Better?

A series of directly meso‐meso‐linked Pd–porphyrin oligomers (PdDTP‐M, PdDTP‐D, and PdDTP‐T) have been prepared. The absorption region and the light‐harvesting ability of the Pd–porphyrin oligomers are broadened and enhanced by increasing the number of Pd–porphyrin units. Triplet–triplet annihilation upconversion (TTA‐UC) systems were constructed by utilizing the Pd–porphyrin oligomers as the sensitizer and 9,10‐diphenylanthracene (DPA) as the acceptor in deaerated toluene and green‐to‐blue photon upconversion was observed upon excitation with a 532 nm laser. The triplet–triplet annihilation upconversion quantum efficiencies were found to be 6.2 %, 10.5 %, and 1.6 % for the [PdDTP‐M]/DPA, [PdDTP‐D]/DPA, and [PdDTP‐T]/DPA systems, respectively, under an excitation power density of 500 mW cm^?2. The photophysical processes of the TTA‐UC systems have been investigated in detail. The higher triplet–triplet annihilation upconversion quantum efficiency observed in the [PdDTP‐D]/DPA system can be rationalized by the enhanced light‐harvesting ability of PdDTP‐D at 532 nm. Under the same experimental conditions, the [PdDTP‐D]/DPA system produces more ³DPA* than the other two TTA‐UC systems, benefiting the triplet–triplet annihilation process. This work provides a useful way to develop efficient TTA‐UC systems with broad spectral response by using Pd–porphyrin oligomers as sensitizers.     

Molecular Topology Tuning of Bipolar Host Materials Composed of Fluorene‐Bridged Benzimidazole and Carbazole for Highly Efficient Electrophosphorescence

Two new molecules, CzFCBI and CzFNBI , have been tailor‐made to serve as bipolar host materials to realize high‐efficiency electrophosphorescent devices. The molecular design is configured with carbazole as the hole‐transporting block and N‐phenylbenzimidazole as the electron‐transporting block hybridized through the saturated bridge center (C9) and meta‐conjugation site (C3) of fluorene, respectively. With structural topology tuning of the connecting manner between N‐phenylbenzimidazole and the fluorene core, the resulting physical properties can be subtly modulated. Bipolar host CzFCBI with a C connectivity between phenylbenzimidazole and the fluorene bridge exhibited extended π conjugation; therefore, a low triplet energy of 2.52 eV was observed, which is insufficient to confine blue phosphorescence. However, the monochromatic devices indicate that the matched energy‐level alignment allows CzFCBI to outperform its N‐connected counterpart CzFNBI while employing other long‐wavelength‐emitting phosphorescent guests. In contrast, the high triplet energy (2.72 eV) of CzFNBI imparted by the N connectivity ensures its utilization as a universal bipolar host for blue‐to‐red phosphors. With a common device configuration, CzFNBI has been utilized to achieve highly efficient and low‐roll‐off devices with external quantum efficiency as high as 14 % blue, 17.8 % green, 16.6 % yellowish‐green, 19.5 % yellow, and 18.6 % red. In addition, by combining yellowish‐green with a sky‐blue emitter and a red emitter, a CzFNBI ‐hosted single‐emitting‐layer all‐phosphor three‐color‐based white electrophosphorescent device was successfully achieved with high efficiencies (18.4 %, 36.3 cd A^?1, 28.3 lm W^?1) and highly stable chromaticity (CIE x=0.43–0.46 and CIE y=0.43) at an applied voltage of 8 to 12 V, and a high color‐rendering index of 91.6.     

PHOTODYNAMIC DAMAGE TO EGG LECITHIN LIPOSOMES*

Abstract— Egg lecithin liposomes were lysed by singlet oxygen (¹O*₂) generated by red light irradiation of oxygenated methylene blue (MB⁺) and by superoxide (O^?₂) generated by blue-green irradiation of oxygenated riboflavin (RF) in the presence of ethylenediaminetetraacetic acid (EDTA) or reduced nicotinamide adenine dinucleotide (NADH). The MB⁺/O₂ sensitized process is more efficient in alkaline solutions, attributed to the stabilization of the triplet state di-cation. The small increase of the lysis efficiency from pH4 to pH9 for the RF/EDTA/O₂ system suggests that HO^?₂ may be more damaging than O^?₂. Estimates based on large-target reaction kinetics indicate that ¹O*₂ is about 200 times more effective than superoxide for inducing liposome lysis. Similar experiments with saturated 1 -α-phosphatidylcholine dipalmitoyl liposomes showed no photosensitized lysis for either system.     

Excitation‐Dependent Long‐Life Luminescent Polymeric Systems under Ambient Conditions

Organic room temperature luminescent materials present a unique phosphorescence emission with a long lifetime. However, many of these materials only emit single blue or green color in spite of external stimulation, and their color tunability is limited. Herein, we report a rational design to extend the emission color range from blue to red by controlling the doping of simple pyrene derivatives into a robust polymer matrix. The integration of these pyrene molecules into the polymer films enhances the intersystem crossing pathway, decreases the first triplet level of the system, and ensures the films show a sensitive response to excitation energy, finally yielding excitation‐dependent long‐life luminescent polymeric systems under ambient conditions. These materials were used to construct anti‐counterfeiting patterns with multicolor interconversion, presenting a promising application potential in the field of information security.     

Escherichia coli: Dominance of Red Light over Other Visible Light Sources in Establishing Viable but Nonculturable State

In this study, the effect of UV-A and different wavelengths of visible light irradiations combined with or without a photosensitizer (methylene blue, MB) on the establishment of viable but nonculturable (VBNC) state in Escherichia coli was investigated. Survival of the E. coli was investigated by measuring plate counts, respiring cell count (RCC), direct viable count (DVC) and total counts over a period of up to 72 h. The inhibition rates of various light sources in the presence or absence of MB on E. coli in seawater were ranked in the order UV-A>red light>white light>blue light>green light (from greatest to least activation). E. coli survived for 10.2, 19.0, 21.3 and 24.04 h under exposure to red, white, blue and green light and for 6.8 h under exposure to UV-A in the presence of MB according to t ₉₉ . Although the VC declined to undetectable levels in a relatively short time, the RCC showed that some cells were still capable of respiration and, therefore, are assumed to have entered the VBNC phase. This is the first time that red light has been shown to have a stronger effect on E. coli survival and VBNC than white, green and blue light in seawater environment.