End-to-end diffusion coefficients and distance distributions from fluorescence energy transfer measurements: Enhanced resolution by using multiple donors with different lifetimes

We describe a method to improve the resolution of donor-to-acceptor distance distributions in molecules which are flexing on the timescale of the fluorescence lifetime. We measured the timedependent donor decays of two donor (D)-acceptor (A) pairs, where the donor lifetimes were substantially different. The donors were an indole residue (5.7 ns) and a naphthalene residue (24.4 ns). The same dansyl acceptor was used for both D-A pairs. The donor decays are complex due to both a distribution of D-A distances and D-A diffusion. Using the donor decay data for each D-A pair alone, it is difficult to resolve both the distance distribution and the D-to-A diffusion coefficient. However, these values are unambiguously recovered from global analysis of the data from both D-A pairs. The increased resolution from the global analysis is apparently the result of the complementary information content of the data for each D-A pair. The shorter-lived indole donor provides more information on the time-zero distance distribution because there is less time for D-A diffusion, and the longer-lived naphthyl donor is quenched to a greater extent than indole due to the longer time for diffusion-enhanced energy transfer. Simulations were also used to demonstrate the increased resolution of global analysis with different lifetime donors to obtain distance distribution parameters in the presence of D-A diffusion.     

电场对ZnCdSe-ZnSe应变超晶格光致发光的影响

n=1重空穴激子和施主-受主(D-A)对的光致发光在常压MOCVD生长的Zn0.85Cd0.15Se-ZnSe应变超晶格中被观测到了.激子和施主-受主对发光峰值位置随着增加正向偏置电压都相继产生蓝移和红移.这是由量子限制斯塔克效应引起的,究竟是蓝移还是红移则取决于由肖特基势垒引起的内部自建电场与外加正向电压引起的外电场之间的竞争.     

静态模型中激活剂(受主)发光的衰减

本文通过敏化剂(施主)和激活剂(受主)间能量传递速率的分布函数,得到了CA<1时静态模型中激活剂发光衰减曲线的表示式:Xe-βt+e-γt d/dt f(t)激活剂发光衰减具有的形式。     

Exchange broadened,optically detected ESR spectra for luminescent donor-acceptor pairs in Li doped ZnO

Application of optically detected ESR to the yellow photoluminescence of Li doped ZnO gives ESR spectra for shallow donor - lithium acceptor pairs, showing that at least a fraction of the yellow emission is donor-acceptor (D-A) luminescence. The distribution of separations r_DA gives a spectrum of D-A exchange interactions J which broaden the ESR lines, particularly at high microwave power, where pairs with long excited state lifetimes, that is large r_DA, are power saturated.     

氧族元素对D-A和D-π-A共聚物光吸收谱红移的第一性原理研究

D-A型共聚物作为有机聚合物太阳能电池的电子给体材料近年来引起广泛关注. 本文以苯并二噻吩(BDT)为电子给体单元, 苯并噻二唑(BT)为电子受体单元来模拟D-A共聚体; 并用噻吩环作为π桥, 构造出D-π-A(PBDT-DTBX, X = O, S, Se, Te)结构. 采用第一性原理的密度泛函理论, 系统地计算相应的电子结构和光吸收谱. 比较不同氧族元素和噻吩π-键桥对聚合物光吸收谱的影响. 研究结果表明: D-A共聚体中当X位元素以O, S, Se, T_e 替换时, 其体系的最高占有分子轨道(HOMO)能级变化不大, 最低未占有分子轨道(LUMO)能级逐渐靠近费米能级, 带隙逐渐减小. 在可见光区有两个较强的吸收峰, 随着X位元素原子序数增大, 位于4.0 eV左右的光吸收峰位基本不变, 另一光吸收峰强度明显增大并发生红移. 与D-A结构相比, D-π-A结构的带隙均有所减小, 其中X为Te时带隙最小; 光吸收峰强度随着氧族元素原子序数的增大也明显增大并发生红移. 通过比较光吸收系数和相应态密度, 结果表明, 4.0 eV 左右的光吸收峰主要是BDT单元的贡献, 氧族元素的改变主要影响519.4-703.9 nm范围的光吸收.     

Experimental and Theoretical Studies of the Spectroscopic Properties of Chalcone Derivatives

Newly synthesized, differently substituted chalcones (1,3-diaryl-2-propen-1-ones) have been studied using steady-state and time-resolved techniques combined with quantum-chemical modelling. To explore spectroscopic structure - property relationships the substituent (acceptor moiety) was chosen according to systematic variation in the Hammett parameter. It was shown that photophysical properties of the studied donor-acceptor (D-A) molecules can be predicted in terms of a simple model from the properties of individual chromophores (composite-model of decoupled moieties: donor (D) and acceptor (A)). The results of spectroscopic measurements also indicate that for investigated D-A fluorophores in medium-polar solvent, the initially populated, locally excited (LE) state (where the fundamental role plays donor moiety (D*-A)) reacts further to produce intramolecular charge transfer (ICT) state. Additionally, the experimental absorption (M _ge ) and fluorescence (M _eg ) transition dipole moments were calculated on the basis of spectroscopic data and compared with results of our quantum-chemical calculations. The absorption transition dipole moment was found to vary linearly with the Hammett substituent coefficient (σ).     

Distance distributions recovered from steady-state fluorescence measurements on thirteen donor-acceptor pairs with different Förster distances

The end-to-end distance distribution of a flexible molecule was recovered from steady-state fluorescence energy transfer measurements using the method suggested by Cantor and Pechukas (Proc. Natl. Acad. Sci. USA 68, 2099–2101, 1971). In this method, the Förster distance (R ₀) is varied by attaching different donor-acceptor (D-A) pairs to the flexible linker of interest. Distance distributions are then recovered from energy transfer efficiency measurements on the set of D-A pairs with differentR ₀ values. Thirteen D-A pair compounds were synthesized withR ₀ values ranging from 6 to 32 Å. Each compound contained a tryptamine donor linked by an alkyl chain (10 carbons) to 1 of 13 acceptors. Using these compounds, we have experimentally confirmed the Cantor and Pechukas method for recovering distance distributions. The measured transfer efficiencies, as a function ofR ₀, were fit to the transfer efficiencies predicted for both Gaussian and skewed Gaussian distance distributions. The data support the existence of a skewed Gaussian distribution, and we believe that this is the first experimental observation of an asymmetric distribution for a flexible molecule using fluorescence resonance energy transfer measurements. Finally, the experimentally recovered distance distribution was found to be in good agreement with the distribution predicted from the rotational isomeric state model of Flory (Statistical Mechanics of Chain Molecules, John Wiley & Sons, New York, 1969, Chaps. 1, 3, and 5) but not with the predicted distribution for a freely rotating or freely jointed chain.     

Luminescence of ZnO films implanted with Au+ ions and annealed in oxygen radicals

Visible and ultraviolet luminescence spectra of ZnO:Au⁺ samples with various dose of implanted gold were studied at a temperature of 77 K. Luminescence peaks of bound and free excitons, D-A pairs, and electron recombination to the gold acceptor level were detected. The optical depth (0.117 eV) of the Au_Zn impurity acceptor level was determined. Diffusive spreading of the implanted impurity profile upon annealing of the zinc oxide film was observed by SIMS.     

Laserlike vibrational instability in rectifying molecular conductors

We study the damping of molecular vibrations due to electron-hole pair excitations in donor-acceptor (D-A) type molecular rectifiers. At finite voltage additional nonequilibrium electron-hole pair excitations involving both electrodes become possible, and contribute to the stimulated emission and absorption of phonons. We point out a generic mechanism for D-A molecules, where the stimulated emission can dominate beyond a certain voltage due to the inverted position of the D and A quantum resonances. This leads to current-driven amplification (negative damping) of the phonons similar to laser action. We investigate the effect in realistic molecular rectifier structures using first-principles calculations.     

基于α-肉桂酰环乙烯酮二硫缩酮的一类新型D-A有机发光材料的理论

用密度泛函理论B3LYP方法对四种新型D-π-A分子(PKD,NKD,TKD和CKD)进行基态几何构型全优化,计算分子的电离势IP和电子亲和势EA等相关能量。用含时密度泛函(TDDFT)方法计算吸收光谱,用单组态相互作用方法(CIS)优化四种化合物分子的S₁激发态结构,分析其能量与发射光谱的关系。根据化合物组成的不同恰当地选择泛函计算分子的发射光谱,并与实验结果对照表明,计算结果比较可靠。     

Effect of chain length and donor–acceptor substitution on the electrical responsive properties of conjugated biphenyls: a DFT-based computational study

The effect of donor–acceptor (D-A) substituent and chain length on the electrical polarisabilities and first hyper polarisability of cis and trans biphenyl oligomeric compounds have been investigated using density functional theory-based hybrid functional CAM-B3LYP with 6-311G (2d,2p) basis set. Our extensive computational study reveals that both average polarisability and first hyper polarisability of the studied compounds increase with the increasing ethylene spacer chain length. Again the substitution of donor (NMe₂) and acceptor (C≡N) at the para position of the phenyl rings to each oligomer shows order of magnitude increase of both α_av and β_av value compared to the unsubstituted one. This increased α_av and β_av values have been explained due to increasing charge transfer contribution resulting from decreasing optical energy gap (ΔE?=?S₁???S₀) upon D-A substitution. It is also observed that the charge transfer contribution to first hyperpolarisability (β_CT) is more than the polarisability (α_CT) for the studied molecules. The electronic spatial extent (<R²>) which is a measure of electron density volume around the molecule is found to play a major role along with the intramolecular charge transfer character to explain the non-linear variation of first hyperpolarisability (β_av) as a function of ethylene spacer chain length (n) and D-A substitution.     

采用非绝热动力学模拟探究溶剂效应对桨-轮形状的给体-受体复合物的激发态弛豫动力学的影响

充分理解给体-受体复合物的激发态动力学在实验和理论上都具有非常重要的意义. 本文首次结合电子结构计算和非绝热动力学模拟,探索了最近合成的一个桨-轮形状的给体-受体复合物的光致动力学过程,该复合物由三个氟硼二吡咯基团和一个六氧杂并三苯构成. 根据计算结果,得出结论,桨-轮形状的给体-受体复合物共轭物在激发时将被提升到氟硼二吡咯片段的局部激发状态,然后发生超快局部激发电子态到电荷转移电子态的激子转移. 与先前实验工作中提出的光致电子转移机理不同,这种激子转移过程伴随着从氟硼二吡咯基团到六氧杂并三苯基团上的空穴. 此外,还发现溶剂效应在该体系的光致动力学中起着重要作用. 具体而言,强极性的乙腈溶剂加速了空穴转移动力学,这可以归因于溶剂对电荷转移状态的显著影响,即局部激发和电荷转移激子之间的能隙大大减小,而与此同时非绝热耦合增加,这两个因素都可以促进空穴转移过程. 目前的工作不仅为桨-轮形状的给体-受体复合物的潜在光诱导机制提供了有价值的见解,而且有助于未来设计具有更好光电性能的新型给体-受体复合物.     

Donor-acceptor conjugated copolymer with high thermoelectric performance: A case study of the oxidation process within chemical doping

The doping process and thermoelectric properties of donor-acceptor(D-A)type copolymers are investigated with the representative poly([2,6-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene]3-fluoro-2-[(2-ethylhexyl)-carbonyl]thieno[3,4-b]thiophenediyl))(PTB7-Th).The PTB7-Th is doped by Fe Cl;and only polarons are induced in its doped films.The results reveal that the electron-rich donor units within PTB7-Th lose electrons preferentially at the initial stage of the oxidation and then the acceptor units begin to be oxidized at a high doping concentration.The energy levels of polarons and the Fermi level of the doped PTB7-Th remain almost unchange with different doping levels.However,the morphology of the PTB7-Th films could be deteriorated as the doping levels are improved,which is one of the main reasons for the decrease of electrical conductivity at the later stage of doping.The best electrical conductivity and power factor are obtained to be 42.3 S·cm^-1;and 33.9μW·mK^-1,respectively,in the doped PTB7-Th film at room temperature.The power factor is further improved to 38.3μW·mK^-1;at 75℃.This work may provide meaningful experience for development of D-A type thermoelectric copolymers and may further improve the doping efficiency.     

ODMR investigations of recombination processes in ZnSe:Cu

New ODMR signals have been observed in ZnSe:Cu crystals by monitoring the Cu-green, Cu-red and the i.r. emissions. The results are interpreted in terms of direct D-A recombination involving Cu_Zn and Cu-X centres and indirect recombination via excited states of these centres.     

Bonding the superalkali M3O (M = Li and K): An effective strategy to improve the electronic and nonlinear optical properties of the inorganic B40 nanocage

Inspired by the fascinating finding of all-boron fullerene B₄₀ (Nat Chem, 2014, 6, 727), we propose a new and effective strategy to construct a series of typical Donor-Acceptor (D-A) frameworks via linking the superalkali M₃O (M = Li and K) unit with the low ionization potential to the B₄₀ nanocage with large electron affinity. By means of the density functional theory computations, we have systematically investigated the structures, electronic properties, the first and second hyperpolarizabilities of these modified B₄₀ nanocage systems. Owing to the formation of a B–O chemical bond, these composite systems (M₃O)_n-B₄₀ (M = Li and K, n = 1 and 2) can possess the considerably large binding energy ranging from 57.0 to 99.8 kcal/mol, indicating their high structure stabilities. Compared with the pristine B₄₀ nanocage, linking the superalkali M₃O can effectively narrow the wide energy gap from the original 2.86 eV to 0.61–1.11 eV, and significantly increase the first and second hyperpolarizabilities to as large as 5.00 × 10⁴–2.46 × 10⁵ au and 1.48 × 10⁷–4.85 × 10⁸ au, respectively, owing to the occurrence of evident electron transfer process in this kind of typical D-A framework. These fascinating findings will be advantageous for promoting the potential applications of the inorganic boron-based nanosystems in the new type of electronic nanodevices and high-performance nonlinear optical materials.     

真空热处理对VPE ZnSxSe1-x外延膜发光的影响

本文测量了经真空热处理的VPE ZnSxSe1-x(X=0.055,0.22)外延膜液氮温度下的光致发光(PL)光谱,观察到一个新的蓝色发光带,并把这一谱带归结为导带中自由电子与束缚在深受主中心上空穴的复合.实验表明,深受主中心是在真空热处理时形成,并与真空热处理时在外延层中产生的Zn空位有关.     

Photoluminescence in fluorescent 4H-SiC single crystal adjusted by B,Al,and N ternary dopants

This paper reports the sensitive effect of photoluminescence peak intensity and transmittance affected by B, Al, and N dopants in fluorescent 4H-SiC single crystals. The crystalline type, doping concentration, photoluminescence spectra,and transmission spectra were characterized at room temperature. It is found that the doped 4H-SiC single crystal emits a warm white light covering a wide range from 460 nm to 720 nm, and the transmittance increases from ~10% to ~60%with the fluctuation of B, Al, and N ternary dopants. With a parameter of C_(D-A), defined by B, Al, and N concentration, the photoluminescence and transmittance properties can be adjusted by optimal doping regulation.     

双噻吩基四硫富瓦烯富勒烯C60几何构型与电子结构的理论研究

利用半经验AM1法研究双噻吩基四硫富瓦烯富勒烯 C60 (BTTTF C60 )和四硫富瓦烯 C60 (TTF C60 )的几何构型、电子结构和前线轨道 .计算结果显示 ,两化合物的TTF面发生弯曲 ,形成独特的空间构型 ,电子结构的分析表明其原因是由C60与TTF或BTTTF的相互作用引起的 .C60的LUMO能与BTTTF的HOMO能接近 ,易发生D A反应 ,形成BTTTF C60 .BTTTF C60和TTF C60的LUMO能仍较低 .LUMO分布集中在C60部分 ,表明BTTTF C60的C60母体仍可接受电子 .另外对两分子的电荷分布、HOMO及LUMO的分析比较 ,表明所设计的BTTTF C60分子可能产生与TTF C60分子类似的电荷分离态     

Two-photon absorption in two-dimensional conjugated quadrupolar chromophores

We present ultrafast z-scan measurements of the two-photon absorption (TPA) spectra of a pair of two-dimensionally conjugated quadrupolar donor/acceptor (D/A) chromophores. The all-donor substituted species displays a peak TPA cross section [sigma(2)=520+/-30 GM] that is more than twice that of the D-A species [sigma(2)=240+/-20 GM]. Unlike previous structure-property studies that have evaluated TPA behavior for D/A molecules through the comparison of dipolar and quadrupolar compounds, both molecules investigated herein are quadrupolar, ultimately providing a more consistent evaluation of the effects of donor and/or acceptor substitution on the TPA of conjugated chromophores.     

Observation of a donor-acceptor pair recombination in the edge emission of ZnSe crystal by electro-luminescence

An injection type electro-luminescence in ZnSe crystal has been studied by using ZnSe-SnO₂ hetero-junction at 20°K. In the emission peak observed around 2·70 eV, a clear energy shift toward the higher energy side with increasing injection current density has been found at low temperatures, which could be associated with the D-A pair (donor-acceptor pair) recombination process. The energy sum of the donor and acceptor activation is estimated to be larger than 137 meV. In the higher temperature region, this emission line turns out due to the ‘free-to-bound’ recombination, and the related acceptor ionization energy is considered to be ～120 meV. By taking into consideration the energies of bound exciton emission, the exciton localization energies and the related donor and acceptor ionization energies are evaluated.