Electroplex emission from a blend of poly(N-vinylcarbazole) and 2,9-dimethyl-4,7-diphenyl-1, 10-phenanthroline

The PL and EL spectra of poly(N-vinylcarbazole) (PVK) : 2,9-dimethyl-4,7-diphenyl-1, 10-phenanthroline (BCP) (1:1 w/w) film were found completely different. The PL spectrum is a single peak at 415 nm that originates from excitons emission from PVK, and the tail of the spectrum is suggested to be excimer emission from BCP molecules. However, a new emission at 595 nm was found in the EL spectra of devices ITO/PEDOT:PSS(50 nm)/PVK:BCP(1:1)(100 nm)/Al. After aggregate, exciplex and product of electrochemical reaction were ruled out, the new emission was proposed to be electroplex emission that occurred between PVK and BCP molecules. Under high voltage, only electroplex emission can be observed in the EL spectra.     

Electroluminescence of poly(N-vinylcarbazole) films: fluorescence, phosphorescence and electromers

We have studied the electroluminescence (EL) properties of pure poly(N-vinylcarbazole) (PVK) films. Three types of light emission in the EL spectrum were observed, attributed to fluorescence, phosphorescence and electromers, respectively. The observation of electrophosphorescence from PVK films at room temperature is very meaningful, indicating that PVK can produce a large number of triplet excitons under an electric field at room temperature. Our results demonstrate clearly the reason why PVK is an excellent host material for phosphorescent polymer light-emitting diodes (PLEDs).     

On the triplet state of poly(N-vinylcarbazole)

Triplet state properties including transient triplet absorption spectrum, intersystem crossing yields in solution at room temperature and phosphorescence spectra, quantum yields and lifetimes at low temperature as well as singlet oxygen yields were obtained for poly(N-vinylcarbazole) (PVK) in 2-methyl-tetrahydrofuran (2-MeTHF), cyclohexane or benzene. The results allow the determination of the energy value for the lowest lying triplet state and also show that triplet formation and deactivation is a minor route for relaxation of the lowest excited singlet state of PVK. In addition, they show the triplet state is at higher energy than reported heavy metal dopants used for electrophosphorescent devices, such that if this is used as a host it will not quench their luminescence.     

Study of the degradation of polyurethanes. III. Mechanism of the photodegradation of polyurethane

The effect of triplet sensitizers, benzophenone and anthraquinone and metal acetylacetonates [Co(II, III), Cu, Sn, and Ni] on the photodegradation of polyurethane was examined. Ultravioletvisible (UV-V) absorption spectra, gel formation, and luminescence emission of the polymer before and after irradiation were measured. Changes in UV-V absorption of the polymer and the formation of an insoluble fraction in the polymer were accelerated in the presence of the triplet sensitizers, and (Co(II, III)), Cu, and Sn acetylacetonates. Unirradiated polyurethane was excited by irradiation at 290 and 346 nm, and emitted light at 310 and 420 nm. After 1/2 hr irradiation emission of luminescence was observed at 430 nm, excitation at 290 and 346 nm; after 2hr irradiation at 530 nm, excitation at 420 nm was observed. The results suggest that photodegradation of the polyurethane proceeds via excited triplet states forming excimer between the polymers at the initial stage and exciplexes between the polymer and degradation products or intermediates after a certain irradiation.     

Observation of intramolecular singlet and triplet excimers of tethered naphthalene moieties under the geometric constraints imposed by the host framework of zeolites

This paper describes the emission spectroscopic investigation of singlet and triplet excimers of tethered bisnaphthalene species at room temperature. 1,1'-Dinaphthylethane (DNE) and 1,1'-dinaphthylpropane (DNP) were incorporated into a 3-D network of a 1.3 nm diameter pseudo-spherical supercage of zeolites Y (with an entry aperture of 0.74 nm), and a 1-D channel of zeolite L with a 0.71 nm aperture. In the straight channel of zeolite L, DNP adopts a parallel-sandwich conformation as suggested by the observation of entirely excimeric fluorescence, while DNE can only enter the channel in an open form. As a result, incorporation of DNP into zeolite L exchanged with Tl(+) ions, which induce enhanced intersystem-crossing, allowed the observation of a broad and featureless phosphorescence spectrum that is ascribable to the triplet excimer of DNP. In contrast, an entirely monomeric phosphorescence is observed for DNE. Furthermore, we observed a monomeric phosphorescence spectrum for DNP in Tl(+)-exchanged zeolite Y, in which DNP was shown to assume its open conformation. Thus the parallel-sandwich structure is responsible for the triplet excimer of the bisnaphthalene species as well as for the singlet counterpart. The parallel-sandwich conformation of the triplet excimer of DNP immobilized by the host framework is significantly different from the previously proposed L-shaped configuration in solution. However, it is very similar to that of the triplet excimer of triple-tethered carbazolophane, as revealed quite recently by Ohkita's group (J. Phys. Chem. B, 2007, 111, 10905). Thus the present study helps tackle the long-term issue of triplet excimer through the use of zeolites.     

STUDIES ON THE PHOTOTOXICITY OF COUMARIN DERIVATIVES — I. PHOTOCYCLODIMERIZATION OF 5,7-DIMETHOXYCOUMARIN

Abstract— 5,7-DimethoxJtcoumarin (DMC) dimerizes through the C₄-photo-cycloaddition of 3,4-double bonds to form a syn head-to-tail dimer on direct irradiation ( Λ≥ 300 nm) in acetonitrile or benzene solution. The quantum yield of the photocyclodimerization in acetonitrile is 0.068 which is greater than that of coumarin.
In the presence of triplet sensitizers such as benzophenone, 5,7-dimethoxycoumarin forms an anti dimer with the quantum yield greater than 0.08. The structure of the photodimers has been elucidated by IR, UV, NMR, and mass spectrometry. The results of luminescence studies, triplet quenching and sensitization revealed that the syn head-to-tail dimer was formed via an excited singlet precursor, while the anti dimer was formed via the excited triplet state.     

Normal and second excimers in macromolecules—I: Poly(1-methoxy-4-vinylnaphthalene) in fluid solution

The emission properties of poly(1-methoxy-4-vinylnaphthalene) (PMVN) in 2-methyltetrahydrofuran solution have been investigated over the range 77–350 K. It exhibits, in addition to the monomer fluorescence (348 and 360 nm), two structureless emissions derived from two different types of excimer, viz. the normal excimer (420 nm) and the second excimer (380 nm), the latter having a partially overlapping structure of aromatic rings. The intensity of second excimer emission of PMVN increases with decreasing temperature, while the normal excimer emission shows a maximum at 215 K. Kinetic analysis of transient decay curves for the fluorescence of PMVN gave results consistent with the previously reported kinetic scheme for the dimer model, 1,3-bis(4-methoxy-1-naphthyl)propane, showing that the second excimer is not formed from or converted to the normal excimer and that two types of excimer are formed independently from the excited monomer.     

Synthesis and characterization of light‐emitting oligo(p‐phenylene‐vinylene)s and polymeric derivatives containing three‐ and five‐conjugated phenylene rings. II. Electro‐optical properties and optimization of PLED performance

A series of multilayer polymeric light‐emitting diodes (PLEDs) containing an electron‐transporting layer (ETL), that is tris(8‐quinolinolato)‐aluminum(III) (Alq) and 2,2′,2″‐(1,3,5‐phenylene)‐tris[1‐phenyl‐1H‐benzimidazole] (TPBI), were fabricated by doping fluorescent oligo(p‐phenylene‐vinylene)s (BIII and BV) and polymer derivatives (PBV) into poly(N‐vinyl carbazole) (PVK). These PLEDs can be optimized by the design of multilayer device configurations (brightness increased 8–15 times by addition of ETL) and possess greenish electroluminescent (EL) spectra peaked about 500–540 nm. A remarkably high brightness of 56,935 cd/m² with a power efficiency of 3.25 lm/W was obtained in the device of PVK:BVOC₈‐OC₈ (100:20)/Alq (60 nm/60 nm). It suggests that the emission mechanism (including the conjugated and excimer emissions of BVOC₈‐OC₈ emitters) originates from both of BVOC₈‐OC₈ and ETL (Alq and TPBI) by varying the concentration of chromophores and adjusting the thickness of ETL. The concentration effect of the emitters in PVK (i.e. PVK:BVOC₈‐OC₈ = 100:5, 100:20, and 100:100 wt %) and the influence of the ETL (including its thickness) on the EL characteristics are also reported. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2922–2936, 2006     

蓝色磷光材料FIrpic的发光特性

研究了掺杂浓度及热退火对磷光材料双(4,6-二氟苯基吡啶-N,C2?)吡啶甲酰合铱(FIrpic)发光性能的影响.不同掺杂浓度的薄膜及有机电致发光器件(OELDs)的发光颜色都随FIrpic浓度的增大由蓝色逐渐变化到黄绿色.纯FIrpic薄膜的吸收光谱和光致发光(PL)光谱在440-480nm范围内有明显的光谱重叠,476nm处的发光强度随FIrpic掺杂浓度增大而降低主要是由自吸收效应引起的.测量了不同激发密度下的光致发光光谱和不同掺杂浓度下的电致发光(EL)光谱,发现530nm处的发光强度随激发强度或掺杂浓度的增大而增强,证实了530nm处的发光是来源于FIrpic分子间的激基缔合物发光.通过比较热退火前后薄膜微观形貌及电致发光器件光谱的变化,进一步证实了热退火促进FIrpic分子聚集,增强了FIrpic分子间的辐射跃迁发光.通过调控FIrpic掺杂浓度和优化器件结构,并对器件进行热退火处理得到一系列发光颜色从蓝色逐渐变化到黄绿色的有机电致发光器件.     

Photochemistry and phototoxicity of aloe emodin

Photochemical pathways leading to the phototoxicity of the aloe vera constituent aloe emodin were studied. The results indicate a photochemical mechanism involving singlet oxygen to be the most likely pathway responsible for the observed phototoxicity. Aloe emodin was found to efficiently generate singlet oxygen when irradiated with UV light (phidelta = 0.56 in acetonitrile). The survival of human skin fibroblast cells in the presence of aloe emodin was found to decrease upon irradiation with UV light. A further decrease in cell survival was observed in D2O compared with H2O, suggesting the involvement of singlet oxygen as the primary pathway. Laser flash photolysis experiments were also carried out on aloe emodin alone and in the presence of various biological substrates. Aloe emodin proved to be relatively photostable (phi = 1 x 10(-4)) and a poor photo-oxidant (E*red = +1.02 V). Only absorption bands caused by the triplet state of aloe emodin (lambdamax = 480 nm) and the aloe emodin conjugate base (lambdamax = 520 nm) were observed in the transient spectra.     

Intramolecular singlet and triplet excimers of triply bridged [3.3.N](3,6,9)carbazolophanes

Intermoiety electronic interactions in the singlet and triplet excimer states of triply bridged [3.3.n](3,6,9)carbazolophanes ([3.3.n]Cz, n=3-6) were studied by emission and transient absorption measurements. In these [3.3.n]Cz molecules, the dihedral angle and the separation distance r between fully overlapped two carbazole rings change systematically from nearly parallel (n=3, r=3.35 A) to oblique (n=6, r=4.03 A). In rigid glass at 77 K, [3.3.n]Cz (n=3, 4) (r<4 A) exhibited red-shifted and structureless excimer fluorescence and phosphorescence while [3.3.n]Cz (n=5, 6) (r>4 A) exhibited monomer-like vibrational fluorescence and phosphorescence. In solution at 130 K, all [3.3.n]Cz molecules exhibited an excimeric fluorescence band while [3.3.5]Cz still exhibited monomer-like phosphorescence. Transient absorption spectra measured at 294 K exhibited local excitation and charge-transfer bands for all [3.3.n]Cz molecules in the excited singlet and triplet states, suggesting that not only singlet but also triplet excimers of carbazole are formed at room temperature. Furthermore, the singlet-triplet energy gap decreased with the decrease in n, suggesting that electrons are effectively delocalized over the two carbazole moieties. These findings showed that both singlet and triplet excimers of carbazole are formed with a separation distance shorter than about 4 A and are most stable in the parallel-sandwich structure and that the configurational mixing between exciton resonance and charge resonance states plays an essential role in the formation of singlet and triplet excimers of carbazole.     

Synthesis,characterization, and electroluminescence of novel copolyfluorenes and their applications in white light emission

New copolyfluorenes (PC8OF0–PC8OF50) comprised of 9,9‐dioctylfluorene and jacketed units 2,5‐bis[(5‐octyloxy‐phenyl)‐1,3,4‐ oxadiazole]‐1‐(3,5‐dibromophenyl)‐benzene (35C8) were synthesized by palladium‐catalyzed Suzuki coupling reaction. They were characterized by molecular weight determination, ¹H NMR, elemental analysis, DSC, TGA, absorption and emission spectroscopy, and cyclic voltammetry (CV). These copolymers were readily soluble in common organic solvents and exhibited high glass transition temperature and thermal stability.The copolymer films showed absorption peaks from 381 nm to 351 nm, and PL peaks from 432 nm to 421 nm with a blue shift originated from 35C8 units. Both the HOMO energy levels and LUMO levels changed little as the content of 35C8 units increased (?5.59 eV to ?5.48 eV and ?2.60 eV to ?2.49 eV). Electroluminescent devices: ITO/PEDOT:PSS[poly(ethylenedioxythiophene):polystyrenesulfonate]/polymer/Ca (25 nm)/Ag(80 nm) (a), ITO/PEDOT:PSS/polymer/TPBI [1,3,5‐ tris(N‐phenylbenzimidazol‐2‐yl)benzene](15 nm)/Mg:Ag(10:1, wt)/Ag (b), and ITO/ PEDOT:PSS/PVK[Poly(N‐vinylcarbazole)]/polymer/TPBI(15 nm)/Ca(25 nm)/Ag(80 nm) (c) were fabricated to investigate the influence of jacketed contents and device architectures on emission characteristics. The maximum brightness and current efficiency of the PC8OF25 device (5097.8 cd/m² and 0.484 cd/A) surpassed those of the PC8OF0 device (3122.8 cd/m² and 0.416 cd/A). The EL emissions of PC8OF0 – PC8OF50 were pure blue and low‐energy excimer emission bands were successfully suppressed, indicating that these copolymers could be good candidates for blue light‐emitting materials. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4555–4565, 2009     

On the photochemical conversion of an alpha, beta-unsaturated gamma, delta-epoxy ketone: 3-oxo-6-alpha, 7-alpha-oxide-17-beta-acetoxy-Delta-androstane

The α,β-unsaturated γ,δ-epoxyketone 7 is isomerized almost exclusively to the δ-diketone 9 both upon irradiation in the n → π* absorption band with light of wavelengths above 310 nm (in anhydrous dioxane or benzene solutions) and upon triplet sensitization using acetophenone in benzene. The reaction may be formulated by the cleavage of the C_γ? O oxide bond and the shift of the δ-hydrogen to the γ-position, and thus bears a formal “double bond homology” to the photochemical α,β-epoxyketone rearrangement. Excitation in the π → π* absorption band of 7 with light of wavelength 253,7 nm (in anhydrous dioxane solution) leads to the formation of product 10 as well as to the triplet rearrangement to 9 . With this result a novel partial synthesis of O-acetyl-B-nortestosterone has been accomplished, which has the advantages of fewer steps and higher product yield ( 7 → 10 : ～30% yield) than previously published syntheses. On the basis of the presently available experiments, the mechanism of the transformation 7 → 10 , which constitutes one of the still few examples of enone photoreactions induced selectively from the π,π* excited singlet, remains unknown.     

The missing link: triplet fluorocarbonyl nitrene FC(O)N

The elusive triplet fluorocarbonyl nitrene, FC(O)N (X³A′′), has been generated in high yield from matrix‐isolated FC(O)N₃ by ArF excimer laser photolysis (λ=193 nm). As a side product FNCO was formed. The novel nitrene was characterized by IR, UV/Vis, EPR spectroscopy, and quantum‐chemical calculations. All six fundamental vibrations of FC(O)N at 1681.3, 1193.8, 879.8, 646.5, 588.7, and 434.8 cm^?1 (argon matrix, 16 K), their ^12/13C, ^16/18O, and ^14/15N isotopic shifts, and four electronic transitions at T₀=13 890, 25 428, 29 166, and 30 900 cm^?1 that exhibit vibrational fine structures have been detected. Under visible‐light irradiation at λ≥495 nm, FC(O)N reacted with molecular N₂ in the matrix cage at 6 K to give back FC(O)N₃, whereas near‐UV irradiation at λ≥335 nm yielded FNCO. The singlet–triplet energy gaps of different carbonyl nitrenes are discussed.     

PROBE FOR THE FORMATION RATIO BETWEEN EXCITED TRIPLETS AND SINGLETS AS GENERATED IN POLYMER LIGHT-EMITTING DIODES*

The electroluminescence (EL) produced by a highly luminescent phosphorescent dye Cu_4(C≡CPh)_4L_2 (L = 1.8-bis(di-phenylphosphino)-3, 6-dioxaoctane, Cu_4) doped polymer as emitting layer is reported. The effects o f the chargeinjection balance on the polymers, in particular, poly(N-vinylcarbazole) (PVK) have been studied by usingphotoluminescence and elecholuminescence spectroscopy. Changes in the emission spectra demonstrate the influence of thecharge injection balance on the formation ratio of triplet and singlet excitons. (This provides a new technical approach torealize the color patterning in polymer LEDs.     

Chemiluminescence upon Photooxidation of 4,4"-Diazidodiphenyl with Molecular Oxygen

It was found that irradiation of the solution of 4,4"-diazidodiphenyl in ethyl alcohol with UV-light at a wavelength longer than 280 nm in the presence of dissolved oxygen resulted in appearance of post-luminescence. The luminescence intensity increases with increasing the oxygen concentration; the luminescence virtually disappears after purging the solution with argon. The observed emission of light was attributed to chemiluminescence of the products of photooxidation of 4,4"-diazidodiphenyl. The rate dependence of the afterglow shows two maxima. The first is observed immediately after cessation of the UV irradiation of the solution and its transfer to a photometric cell. The intensity of the corresponding chemiluminescence decreases by an exponential law with _e= 5 s. The second maximum appears after a certain time interval after the first one. Addition of a triplet sensitizer (Michler's ketone) to the reaction mixture results in quenching of the chemiluminescence from the photooxidation products, whereas the addition of substances that stabilize electrophilic species in the singlet state leads to an increase in the chemiluminescence intensity. It was suggested that the chemiluminescence resulted from reactions involving the singlet nitrene adduct with oxygen.     

Synthesis of a [3.3]biphenylophane, and its photophysical and photochemical properties studied by emission and transient absorption measurements

[3.3](4,4′)Biphenylophane (BPP) is synthesized, and the photophysical and photochemical properties are studied by means of emission and transient absorption measurements. BPP emits excimer fluorescence at 295 and 77 K, and phosphorescence from the locally-excited (LE) triplet state at 77 K. Based on the transient absorption spectra of BPP, it is found that the excimeric triplet state of BPP is produced along with the LE triplet at 295 and 77 K. The triplet excimer of BPP is shown to be formed via intersystem crossing from the singlet excimer state, and concluded to be non-phosphorescent.     

Efficient emission from charge recombination during the pulse radiolysis of electrochemical luminescent donor-acceptor molecules with an ethynyl linkage

Efficient monomer and excimer emission from various donor-acceptor substituted phenylethynes (PE), which are known as efficient electrogenerated chemiluminescent molecules, was observed with time-resolved fluorescence measurement during the pulse radiolysis in benzene. On the basis of the transient absorption and emission measurements, and steady-state measurements, the formation of PE in the singlet excited state (1PE*) and the excimer (1PE2*) can be interpreted by the charge recombination between the PE radical cation (PE.+) and the PE radical anion (PE.-) which are generated initially from the radiolytic reaction in benzene. It is suggested that the positive and negative charges are localized on the donor and acceptor moieties in the radical cation and anion, respectively. This mechanism is reasonably explained by the relationship between the annihilation enthalpy changes (-DeltaH' degrees ) and singlet excitation energies of donor-substituted phenyl(9-acridinyl)ethynes (1(a-e)). In addition to the monomer emission, the compounds bearing weak donors (1(a-d)) show the excimer emission due to a very small twist angle between the donor and acceptor moieties. For the phenyl(9-cyano-10-anthracenyl)ethynes (2(c) and 2(f)), although they also show the monomer and excimer emissions, it cannot be explained by the relationship between -DeltaH' degrees values and their singlet excitation energies, suggesting the formation of the ICT state and H-type excimer in which two 9-cyano-10-anthracenyl moieties are stacked face-to-face with donor bearing a benzene ring projecting perpendicularly away from each other through the charge recombination between 2.+) and 2.-) and/or triplet-triplet annihilation.     

NARROW BANDWIDTH BRIGHT SCARLET ORGANIC ELECTROLUMINESCENT DEVICE BASED ON N,N'-BIS[4-(N,N-DIMETHYLAMONO)-BENZYLIDENE]-DIAMINOMALEONITRILE DYE

A novel red dye, N, N'-bis[4-(N,N-dimethylamino)-benzylidene]diaminomaleonitrile (BAM), was prepared by reacting diaminomaleonitrile with 4-(N,N-dimethylamino)-benzaldehyde and were characterized by ¹H NMR, UV absorption and photoluminescence. The BAM dye showed an absorption peak wavelength of 530 nm and bright photoluminescence with a peak wavelength at 675 nm. It was used as the doped emitter for fabricating a bright scarlet organic electroluminescent (EL) device. The structure of the double-layer EL device consisted of a hole-transport layer and a luminescent layer between ITO glass and magnesium electrodes. The hole-transport layer was a poly(N-vinylcarbazole) (PVK) film. The luminescent layer consisted of a host material, 8-hydroxyquinoline aluminum (Alq₃), and BAM dye as the dopant. A bright light with the peak of 620 nm and narrow bandwidth of 50 nm was obtained in the device with a maximum luminance of 6230 cd/m². The emission spectra almost unchanged as the luminance increased with increasing injection current and the bias voltage. A tentative explanation from both the electronic distribution viewpoint and the molecular geometric analysis for the narrow bandwidth of this red dye was offered.     

Mechano- and Photoresponsive Behavior of a Bis(cyanostyryl)benzene Fluorophore

The mechanoresponsive behavior and photochemical response of a new bis(cyanostyryl)benzene fluorophore (CSB-5) were investigated. Green fluorescence with λ_em,max of 507 nm was found for CSB-5 in chloroform solution, mirroring the behavior of a previously reported similar dye (CSB-6). Alternatively, crystalline samples of CSB-5 exhibited orange fluorescence with λ_em,max of 620 nm, attributable to excimer emission. Although the emission color change was not clearly noticeable by naked eye, CSB-5 exhibited mechanochromic luminescence, due to transformation into the amorphous state upon grinding the crystalline powder. Interestingly, rubbed films of CSB-5 prepared on glass substrates exhibited a pronounced emission color change from orange to green when exposed to UV light. This response is the result of a photochemical reaction that occurs in the amorphous state and which causes a decrease of the excimer emission sites so that the emission color changes from excimer to monomer. The crystalline material did not display such a photoinduced emission color change and the difference in photochemical reactivity between crystalline and amorphous states was exploited to pattern the emission color of rubbed films.