电场诱导聚合物分子取向对单线态和三线态激子形成截面的影响

通过对聚乙烯咔唑(PVK) 掺杂三(2-苯基吡啶)铱(Ir(ppy)₃)和4-二氰亚甲基-2-叔丁基-6-(1,1,7,7-四甲基久咯呢定基-9-烯基)-4H-吡喃(DCJTB)，PVK 掺杂DCJTB和PVK掺杂Ir(ppy)₃聚合物在成膜时高压电场作用下分子取向变化对单线态和三线态激子形成截面的研究，发现，随着成膜时电场的增强，单线态激子的形成截面在增加，而三线态激子的形成截面却减小. 关键词： 分子取向 激子形成截面 三线态 单线态     

Color and Luminescence Centers in LuAG and Pr:LuAG Crystals Before and After <Superscript>60</Superscript>Co γ-Irradiation

Entire luminescence spectrum of a commercial photosensitizer Radachlorin in aqueous solution has been recorded under laser excitation at 660 nm and analyzed. The peak of singlet oxygen phosphorescence at 1274 nm has been observed. The results obtained were compared with those recorded with laser excitation at 405 nm and reported earlier. The comparison showed the similarity of relaxation and luminescence processes occurring in both cases. Effective absorption cross sections were determined at each excitation wavelength, it was also shown that the singlet oxygen quantum yield is independent of photosensitizer concentration. The lifetime of the first excited triplet state in Radachlorin was determined. The results obtained can be used for optimization of the conditions of singlet oxygen generation and detection in solutions and biological samples.     

铜配合物的光物理与电致发光性能

以中心原子为铜的磷光材料Cu4(C≡Cph)4L2[L=1，8-bis(diphenylphosphino)-3，6-dioxaoctane](简称Cu4)作为掺杂材料，选用空穴传输材料聚乙烯基咔唑(PVK)为母体材料，制作结构为ITO／Cu4PVK／TAZ／Mg：Ag／Ag的双层器件。其发光颜色随掺杂的变化而改变，在较高掺杂浓度的条件下，可观察到单纯Cu4的发光，即实现了单重态到三重态的能量转移。着重讨论了主客体材料间的能量转移过程，并研究了影响器件效率的外界因素如氧气的猝灭对Cu4发光强度的影响。     

Experimental quantum simulation of Avian Compass in a nuclear magnetic resonance system

Avian magnetoreception is the capacity for avians to sense the direction of the Earth’s magnetic field. Discovered more than forty years ago, it has attracted intensive studies over the years. One promising model for describing this capacity in avians is the widely used reference-and-probe model where radical pairs within the eyes of bird combines to form singlet and triplet quantum states. The yield depends on the angle between the Earth’s magnetic field and the molecules’ axis, hence the relative value of yield of the singlet state or triplet state enables avians to sense the direction. Here we report the experimental demonstration of avian magnetoreception in a nuclear magnetic resonance quantum information processor. It is shown clearly from the experiment that the yield of the singlet state attains maximum when it is normal to the Earth’s magnetic field, and the experimental results agree with theory very well.     

Pentacene Nanorods as Effective Photosensitizer for Photodynamic Therapy of Tumor via Singlet Fission

Singlet fission (SF), whereby a singlet exciton is converted into a pair of triplet excitons, can improve the efficiency of solar cells. Pentacene has been extensively studied as the most promising SF compound, owing to its 200% yield of triplet states. However, the easy degradation of pentacene in the presence of light and air owing to photooxidation cannot be explained by the classical ¹O₂ generation mechanism. To address this issue, in the present study, pentacene nanorods (Pc NRs) are prepared as a novel photosensitizer (PS); self-carried Pc NRs exhibited higher ¹O₂ generation capacity. Thus, a novel ¹O₂ generation mechanism is proposed based on the SF effect. The initial photon absorption occurs to access single-exciton states, S1–S3. Excited-state Pc pairs accelerate the SF effect in pentacene NRs, leading to a non-adiabatic transition to the dark D state. Dark D state is a singlet state by two triplets coupled overall, and it can transfer its energy to ³O₂ for generating ¹O₂. Using Pc NRs as PSs, photodynamic therapy (PDT) inhibits tumor growth in 4T1 tumor-bearing mice upon 405-nm-wavelength and 650-nm-wavelength laser irradiations. This study paves the way to discover novel PSs that are not considered with classical ¹O₂ generation mechanisms.     

Room-temperature fluctuations in the fluorescence of a single polymer molecule

A theoretical model of light absorption and emission by a polymer molecule has been developed using recent experimental data on the room-temperature fluctuations in the fluorescence intensity of single molecules of a PPV-PPyV copolymer containing several tens of chromophores. An analysis of the experimental data based on the proposed model shows evidence of a change in the conformation of a polymer molecule in the triplet state. By applying the theory to the PPV-PPyV copolymer, it is possible to determine the rate constants of the conformation variation, the rates of the transition from the singlet to the triplet state, and the lifetime of the triplet state of the molecule. The theory also predicts some new effects which can be verified by experiment.     

EPR studies of photoinduced electron transfer in triad model compounds of photosynthesis

Time-resolved EPR spectra are reported for porphyrin-quinone-quinone and porphyrin-porphyrin-quinone triads obtained after photoexcitation in the nematic and soft glass phase of liquid crystals. Spin-polarized EPR spectra were observed for the triplet states of the porphyrin created by spin-selective intersystem crossing (ISC) from the excited singlet state and those of the charge-separated radical pair states (RP) generated by electron transfer (ET) processes. The EPR polarization patterns of the RP are discussed in terms of the favored decay channel of the photoexcited singlet state of the porphyrin donor. The decay pathway may either be singlet ET to the quinone(s) followed by singlet/triplet mixing to yield RPs with triplet character or triplet ET after ISC from the porphyrin singlet to the triplet state, or a superposition of both pathways. It is demonstrated that the nature of the linking bridge between donor and acceptor, i.e., aliphatic cyclohexylene or aromatic phenylene, significantly influences the ET mechanism and thus the polarization patterns of the RP spectra. Using liquid crystals, information about the orientation of the guest molecules in the liquid crystal matrix with respect to the long axes of the liquid crystal molecules can be obtained. In the porphyrin-porphyrin-quinone triads the energy and ET processes strongly depend on the type of metallation of the porphyrins, specifically, whether the distal, the vicinal or both porphyrins bear a zinc atom.     

Intramolecular energy transfer by singlet and triplet excitons in macromolecules

Absorption, luminescence and photo-oxidation of polyvinylcarbazole (PVCa) and polyvinylbenzocarbazole (PVBCa) in weak solutions have been studied. It is found that fluorescence and phosphorescence spectra, observed at 293, 77 and 4.2 K, depend on the molecular mass M of the polymer and on the experimental conditions. Due to efficient intramolecular energy transfer the fluorescence spectrum includes the emission of excimers, emission of end groups and that of internal structural defects of a macrochain. The relative role of each emission markedly depends on M. Phosphorescence is largely determined by the annihilation effects between triplet excitations, these effects being pronounced in long macrochains. The data show the excitonic nature of the singlet and triplet excitation energy transfer in the polymers studied. The migration of one-dimensional singlet and triplet excitons strongly influences the luminescent properties of PVCa and PVBCa macromolecules, as well as their photochemical reactions. The range of singlet excitons in PVCa and PVBCa macromolecules is determined both at room temperature and at low temperatures.     

C?H bond activation of ethylene by bare neutral palladium and platinum atoms: a theoretical investigation

The reactions of bare neutral palladium (Pd) and platinum (Pt) atoms with ethylene on both singlet and triplet surfaces were investigated at B3LYP and CCSD (T) levels of theory. The calculated potential energy profiles clearly show that Pt has higher reactivity than Pd toward ethylene. For both Pd and Pt, the reactions on singlet surfaces are energetically more favorable than those on triplet surfaces. However, notable barriers lie on the singlet and triplet surfaces for Pd +ethylene. This result rationalizes the experimental observation that Pd mainly forms π‐complex with ethylene. But under high‐energy condition, the reaction can proceed to yield dehydrogenation products, Pd‐CCH. and Pd (HCCH). For Pt, triplet‐singlet surface crossing was suggested to occur in the region where Pt forms π‐complex with ethylene to lead the reactions to the energetically more favorable singlet surfaces. For both the two metals, π‐complex and C? H bond insertion species are the reaction intermediates and the H.‐elimination products are the final products. Copyright © 2010 John Wiley & Sons, Ltd.     

Energy transfer by singlet and triplet excitons in carbazole-containing polymers

Quantum yields of pyrene fluorescence and bis[2-(2′-benzothienyl)-pyridinato-N,C^3′](acetylacetonate)iridium [Btp₂Ir(acac)] phosphorescence upon excitation via a matrix of poly-N-epoxypropylcarbazole (PEPC) or poly-N-epoxypropyl-3,6-dibromocarbazole (DBrPEPC), respectively, were found to be lower than those for the compounds directly excited in a polystyrene (PS) matrix. It was established that the energy in PEPC was transferred to an acceptor by both singlet excitons (by migration and long-range dipole–dipole interaction) and triplet excitons (through migration and short-range exchange electron interaction); however, only by triplet excitons in DBrPEPC, which did not show any fluorescence. The energy-transfer efficiency in PEPC by singlet excitons was higher than by triplet excitons. The observed effects were explained by the fact that energy transfer to the acceptor competed with such processes as localization of the excitons in the “tail” energy states, dissociation of singlet excitons into geminal electron–hole pairs (EHP), and capture of triplet excitons by polymer oxidation products.     

Enhanced triplet formation in polycrystalline tetracene films by femtosecond optical-pulse shaping

Polycrystalline tetracene films have been explored using weak ～ 30 fs visible laser pulses that excite the lowest singlet exciton as well as coherent vibrational motion. Transient difference spectra show a triplet absorption which arises following singlet fission (SF) and persists for 1.6 ns without decay. Adaptive pulse shaping identifies multipulse optimal fields which maximize this absorption feature by ～ 20%. These are comprised of subpulses separated by time delays well correlated with the period of lattice vibrations suggesting such modes control the yield of SF photochemistry.     

Singlet and Triplet State Properties and Singlet Oxygen Yield of an Amino-Acyl-Quinoxalinone Yellow Dye

Amino-acyl-quinoxalinone yellow dyes are cyclised analogues of the yellow azomethine dyes developed for, and still used in, silver halide colour photography. Unlike image azomethine dyes, which are rapidly deactivated in their excited states by torsion about the azomethine bond, amino-acyl-quinoxalinone dyes have an interesting photophysics because torsion is not possible due to their cyclised structure. We report results from studies on singlet and triplet state properties, and singlet oxygen yields, of the yellow dye, 7-diethylamino-3-(2,2-dimethyl-propionyl)-5-methyl-1-phenyl-1H-quinoxalin-2-one, in polar and nonpolar solvents. The dye photophysics is characterised by a weak fluorescence, with a solvent dependent emission yield (Φ_F?≈?0.002–0.004), and short singlet state lifetime (τ_expt?≈?20–50 ps), both increasing by a factor of ≈2 in going from polar acetonitrile to non-polar dioxane as solvent. DFT ZINDO calculations show a transition involving significant electron transfer from the diethyl-amino group into the carbonyl region of the molecule. In solution, in the presence of oxygen, the triplet state decays almost exclusively by oxygen quenching, and singlet oxygen is produced in high yield (Φ_??≈?0.5–0.55). The triplet state absorbs across the 450–750 nm region with maxima around 480 and 650 nm, and moderate molar absorption coefficients (ca. 6000–8000 M^?1 cm^?1). In a glass at 77 K, triplet decay gives a red phosphorescence, with λ_max?≈?640–650 nm, and a ?≈?0.25 s lifetime. If singlet oxygen yields are a good indication of triplet yields, then internal conversion and intersystem crossing occur with roughly equal efficiency.

     

A novel photo-dielectric effect

We have measured dielectric displacement currents induced by irradiation in the absorption band of organic molecules, that are dissolved and oriented in a polymer matrix. The novel photo-dielectric effect is the result of marked changes in the dipole moment of the molecules upon excitation. The time constants of the observed effect indicate that both singlet and triplet excited states are involved.     

Ultrafast solvent-induced spin-flip and nonadiabatic coupling: ClF in argon solids

Femtosecond pump-probe spectra show direct evidence for ultrafast solvent-induced spin flip in photodissociation-recombination events of ClF, a light diatomic molecule, for which the spin-orbit coupling is weak. The bound triplet states ((3)Pi) of ClF are probed and the dynamics for excitation to the singlet state ((1)Pi(1)) is compared with excitation to the triplet state B((3)Pi(0)). The population initially excited to the singlet state (1)Pi(1) is transferred to the bound triplet states (3)Pi within tau(f)=0.5 ps. Oscillations in the spectra indicate wave packet dynamics with the triplet state period of 300 to 400 fs in both cases. According to simulations of F(2)/Ar, most of the initially excited singlet state population is converted to repulsive and weakly bound triplet states within approximately 60 fs. In the first ps, 40% of the triplet population accumulates in the weakly bound (3)Pi states, in good accord with the experiment.     

Searching for doubly charged Higgs bosons in Möller scattering by resonance effects at linear <Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> collider

We discuss the parity-violating left–right asymmetries (LRAs) in Möller scattering at the International Linear Collider (ILC) induced by doubly charged Higgs bosons in models with SU(2) _L triplet and singlet scalar bosons, which couple to the left- and right-handed charged leptons, respectively. These bosons are important in scenarios for the generation of the neutrino mass. We demonstrate that the contributions to the LRAs from the triplet and singlet bosons are opposite to each other. In particular, we show that the doubly charged Higgs boson from the singlet scalar can be tested at the ILC by using the resonance effect.     

Singlet-triplet splitting of geminate electron-hole pairs in conjugated polymers

The singlet-triplet splitting of geminate polaron pairs in a ladder-type conjugated polymer has been studied by the thermally stimulated luminescence technique. The energy gap separating the singlet and triplet states of the geminate pairs is measured to be in the range of 3-6 meV, depending on the polymer morphology. The results of correlated quantum-chemical calculations on a long ladder-type oligomer are fully consistent with the observed values of the geminate polaron pair singlet-triplet gap. Such low splitting values have important implications for the spin-dependent exciton formation in conjugated polymers.     

A theory of the collision-induced singlet to triplet transition of methylene

This paper begins with a survey and critique of past experimental and theoretical studies of the inert gas catalysed singlet to triplet transition of methylene. Next we develop a transition-state theory in which the process of activation consists of a collisional distortion of methylene to the extent that degeneracy occurs between adiabatic singlet and triplet vibronic states. Establishment of this collision induced degeneracy is followed by a spin-orbit coupled singlet to triplet transition which is then subsequently stabilized by the splitting of the degeneracy which accompanies the continued relative motion of catalyst and radical. This theory appears to be able to account for what is presently known about the bimolecular singlet to triplet reaction. It is capable of generating rate constants with the correct order of magnitude and of accounting for the experimentally observed isotope and catalytic dependence of these constants.     

Singlet and triplet exciton formation rates in conjugated polymer light-emitting diodes

By applying a molecular orbital perturbation approach, we calculate the formation rates for singlet and triplet molecular excitons associated with intermolecular charge-transfer processes. It is found that the interchain bond-charge correlation has a strong influence on the relative probabilities for generating singlet and triplet excitons. Most importantly, application of our approach to a model system for poly-(paraphenylenevinylene) shows that the ratio between the electroluminescence and photoluminescence quantum yields generally exceeds the 25% spin-degeneracy statistical limit.