Electronic transitions of the Soret band of reaction centers from Rhodobacter sphaeroides studied by femtosecond transient absorbance spectroscopy

The Soret band of reaction centers from Rhodobacter sphaeroides has been systematically studied using femtosecond transient absorption spectroscopy. When the excitation wavelength was scanned over the entire Soret band, the approximate absorption spectra of the bacteriochlorophyll dimer, the monomer bacteriochlorophylls, and the bacteriopheophytins within the Soret band were determined by analyzing the ground state bleaching with about 100 fs resolution. The main contribution of H is on the blue end of the spectrum, peaking near 350 nm, P absorbs mostly on the red side of the spectrum, but probably has multiple bands, and the main absorbance of B likely lies between H and P, overlapping with P on the red side (particularly near 390 nm). The energy transfer from B to P in the QY band takes about 300 fs when Soret-band excitation is used and the time constant of overall energy transfer from H to B to P in the QY band when H is specifically excited near 350 nm is about 500 fs. Internal conversion after Soret-band excitation is the rate-limiting step for the energy-transfer process. The time constant of internal conversion for B and P is less than 300 fs, and for H it is about 500 fs.     

四(4,4',4'',4'''-N,N-二氨基)四苯乙烯光致发光与分子的构象效应

利用还原偶联方法合成出新化合物四(4,4',4',4''-N,N-二氨基)四苯乙烯（ TDETE）。通过测定该化合物在溶液、掺杂聚合物中及晶体粉末的稳态-瞬态荧光光 谱、荧光量子产率和辐射衰变速率常数等。讨论了分子的构象效应等因素对TDETE 光致发光行为的影响。在一定浓度下TDETE溶液存在着三个发光带,分别为全扭曲 构象分子（位于345nm附近的发光I带）、半扭曲构象分子（位于430nm附近的发光 II带）和激基缔合物（530发光III带）的辐射衰变所致。在聚合物（PMMA）中,一 方面由于分子单键的自由旋转扭曲受到遏制,表现为II带的辐射衰变速率常数(kf 值)增大、同时非辐射衰变速率常数knf值减小;另一方面,TDETE分子之间相互作 用得到加强而有利一缔合物形成,结果,使发光II带和III带合二为一出现强而宽 的发射峰,荧光量子产率从溶液中的0.055提高到0.855。此外,在PMM介制裁中观 测到TDETE分子聚集体在626nm处的发光带（IV）,数粉末态中聚集体IV带的强度骤 增,峰值波长红移至650nm。     

Ground- and excited-state proton transfer reaction of 3-hydroxyflavone in dimyristoylphosphatidylcholine liposome membrane

3-Hydroxyflavone (3HF), a molecule that exhibits excited-state intramolecular proton transfer, has been studied for its fluorescence characteristics in dimyristoylphosphatidylcholine (DMPC) liposome membrane. 3HF partitions to the lipid bilayer membrane with a reasonably large partition coefficient. On excitation at 417 nm, a weak emission from the ground-state anion species was observed at 483 nm, whereas excitation at absorption maxima (345 nm) gives the usual intense fluorescence of the phototautomeric emission at 530 nm. In this article, we report the observation of a ground-state proton transfer reaction of 3HF in DMPC liposome membrane.     

Photophysical processes in aromatic polyimides. Studies with model compounds

Emission mechanism in an aromatic polyimide, PI(BPDA/PDA), derived from biphenyltetracarboxylic dianhydride and p-phenylene diamine were studied with ultraviolet visible absorption and fluorescence spectra of a series of the model compounds. The excitation spectrum of the intermolecular charge-transfer (CT) fluorescence peaking around 550 nm of PI(BPDA/PDA) thin film was completely consistent with the absorption spectrum, indicating that the intermolecular CT fluorescence emission of PI(BPDA/PDA) film is not caused by direct excitation of the CT absorption band, but by light absorption due to structural units in the polymer backbone. The UV-vis. absorption spectra of the model compounds corresponding to the structural units in PI(BPDA/PDA) showed that the longest wavelength absorption band is due to the biphenylbisimide moiety. The band was assigned as π, π* transition with the polarization spectrum of the model compound. The fluorescence spectra of the model compounds changed sensitively depending on the conformation around N-phenyl bond. The lifetime measurement for the model compounds suggested that intramolecular CT process occurs very rapidly. © 1993 John Wiley & Sons, Inc.     

The two-photon excitation cross section of 6MAP, a fluorescent adenine analogue

6MAP is a fluorescent analogue of adenine that undergoes Watson-Crick base pairing and base stacking in double-stranded DNA. The one-photon absorption spectrum of 6MAP is characterized by a maximum around 330 nm with moderate quantum yield fluorescence centered at about 420 nm. To take advantage of this probe for confocal and single-molecule microscopy, it would be advantageous to be able to excite the analogue via two photons. We report the first determination of the two-photon excitation cross section and spectrum for 6MAP from 614 to 700 nm. The power dependence of the fluorescence indicates that emission results from the absorption of two photons. The one-photon and two-photon emission line shapes are identical within experimental error. A study of the concentration dependence of the fluorescence yield for one-photon excitation shows no measurable quenching up to about 5 microM. The maximum in the two-photon excitation spectrum gives a two-photon cross section, delta(TPE), of 3.4 +/- 0.1 Goeppert-Mayer (G.M.) at 659 nm, which correlates well with the one-photon absorption maximum. This compares quite favorably with cross sections of various naturally fluorescent biological molecules such as flavins and nicotiamide. In addition, we have also obtained the two-photon-induced fluorescence emission spectrum of quinine sulfate. It is approximately the same as that for one-photon excitation, suggesting that two-photon excitation of quinine sulfate may be used for calibration purposes.     

High light-induced changes of 77 K fluorescence emission of pea thylakoid membranes with altered membrane fluidity

The effect of lipid phase order of isolated thylakoid membranes on fluorescent characteristics of both photosystems during illumination with high light intensity at 22 degrees C and 4 degrees C was investigated. For artificial modification of membrane fluidity two membrane perturbing agents were applied-cholesterol and benzyl alcohol. 77 K fluorescence emission and excitation spectra of control, cholesterol- and benzyl alcohol-treated thylakoid membranes were analysed in order to determine the high light-induced changes of emission bands attributed to different chlorophyll-protein complexes-F 735, emitted by photosystem I-light-harvesting complex I; and F 685 and F 695, emitted by photosystem II-light-harvesting complex II. Analysis of emission bands showed that high light treatment leads to a decrease of the area of band at 695 nm and a concomitant increase of intensity of the band at 735 nm. The involvement of different pigment pools (chlorophyll a and chlorophyll b) in the energy supply of both photosystems before and after photoinhibitory treatment was estimated on the basis of excitation fluorescence spectra. The dependence of the ratios F 735/F 685 and the band areas at 685 and 695 nm on the illumination time was studied at both temperatures. Data presented indicate that cholesterol incorporation stabilized the intersystem structure in respect to light-induced changes of fluorescence emission of PSI and PSII. It was shown that the effect of fluid properties of thylakoid membranes on the 77 K fluorescence characteristics of main pigment protein complexes of pea thyalkoid membranes depends on the temperature during high light treatment.     

GdVO_4∶Eu~(3 )的真空紫外激发光谱研究

报道了GdVO4∶Eu3 的光致发光光谱和真空紫外 紫外激发光谱。GdVO4∶Eu3 是高效率的真空紫外 紫外激发荧光材料。GdVO4∶Eu3 在6 0～ 35 0nm真空紫外 紫外波段的激发可能主要来源于基质的吸收 ,有明显的Eu3 及Gd3 的 4fn - 1 5d吸收。在GdVO4∶Eu3 中 ,存在如下能量传递过程 :VO3 - 4 →Eu3 ,Gd3 →Eu3 ,Gd3 →VO3 - 4 →Eu3 ;通过后两个过程 ,Gd3 Eu3 可实现量子剪裁。     

Highly luminescent water-dispersed silicon quantum dots for fluorometric determination of oxytetracycline in milk samples

A fluorescent probe based on silicon quantum dots (SiQDs) was developed for the selective and sensitive detection of oxytetracycline (OTC) via the inner filter effect (IFE). The water-soluble fluorescent SiQD was synthesized based on the reaction of 3-Aminopropyltriethoxysilane (APTES) and sodium citrate as precursors by the one-pot hydrothermal process. The strong fluorescence emission of quantum dots (QDs) was obtained at 440 nm when excited at 350 nm and OTC had a broad absorption band between 200 and 400 nm. The excitation spectrum of SiQDs was completely overlapped with the absorption spectrum of OTC. The light at an excitation wavelength of QDs absorbed by OTC caused a decrease in fluorescence intensity with an increase in the concentration of OTC. Under optimal conditions, the linear concentration range was 0.92–9.2 µg mL1 with a detection limit (LOD; S/N = 3) of 0.19 µg mL ^-1 . The proposed method was applied to the determination of OTC in milk samples and satisfactory recoveries (98.8–100.5%) with low RSD % values (0.93–2.31%) were achieved. This simple, selective, sensitive, rapid, and cheap method can be used as a promising tool for OTC analysis in food safety.     

Changes in the structure of chlorophyll-protein complexes and excitation energy transfer during photoinhibitory treatment of isolated photosystem I submembrane particles

The activity of light-induced oxygen consumption, absorption spectra, low temperature (77 K) chlorophyll fluorescence emission and excitation spectra were studied in suspensions of photosystem (PS) I submembrane particles illuminated by 2000 microE m(-2) s(-1) strong white light (WL) at 4 degrees C. A significant stimulation of oxygen uptake was observed during the first 1-4 h of photoinhibitory treatment, which rapidly decreased during further light exposure. Chlorophyll (Chl) content gradually declined during the exposure of isolated PSI particles to strong light. In addition to the Chl photobleaching, pronounced changes were found in Chl absorption and fluorescence spectra. The position of the major peak in the red part of the absorption spectrum shifted from 680 nm towards shorter wavelengths in the course of strong light exposure. A 6-nm blue shift of that peak was observed after 5-h illumination. Even more pronounced changes were found in the characteristics of Chl fluorescence. The magnitude of the dominating long-wavelength emission band at 736 nm located in untreated particles was five times reduced after 2-h exposure, whereas the loss in absolute Chl contents did not exceed 10% of its initial value. The major peak in low-temperature Chl fluorescence emission spectra shifted from 736 to 721 nm after 6-h WL treatment. Individual Chl-protein complexes differed in the response of their absorption spectra to strong WL. Unlike light-harvesting complexes (LHC), LHCI-680 and LHC-730, which did not exhibit changes in the major peak position, its maximum was shifted from 678 to 671 nm in CPIa complex after PSI submembrane particles were irradiated with strong light for 6 h. The results demonstrated that excitation energy transfer represents the stage of photosynthetic utilization of absorbed quanta which is most sensitive to strong light in isolated PSI particles.     

Laser induced fluorescence and resonant two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone

We carried out laser induced fluorescence and resonance enhanced two-color two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone (1-HAQ). The 0-0 band transition to the lowest electronically excited state was found to be at 461.98 nm (21,646 cm(-1)). A well-resolved vibronic structure was observed up to 1100 cm(-1) above the 0-0 band, followed by a rather broad absorption band in the higher frequency region. Dispersed fluorescence spectra were also obtained. Single vibronic level emissions from the 0-0 band showed Stokes-shifted emission spectra. The peak at 2940 cm(-1) to the red of the origin in the emission spectra was assigned as the OH stretching vibration in the ground state, whose combination bands with the C=O bending and stretching vibrations were also seen in the emission spectra. In contrast to the excitation spectrum, no significant vibronic activity was found for low frequency fundamental vibrations of the ground state in the emission spectrum. The spectral features of the fluorescence excitation and emission spectra indicate that a significant change takes place in the intramolecular hydrogen bonding structure upon transition to the excited state, such as often seen in the excited state proton (or hydrogen) transfer. We suggest that the electronically excited state of interest has a double minimum potential of the 9,10-quinone and the 1,10-quinone forms, the latter of which, the proton-transferred form of 1-HAQ, is lower in energy. On the other hand, ab initio calculations at the B3LYP/6-31G(d,p) level predicted that the electronic ground state has a single minimum potential distorted along the reaction coordinate of tautomerization. The 9,10-quinone form of 1-HAQ is the lowest energy structure in the ground state, with the 1,10-quinone form lying approximately 5000 cm(-1) above it. The intramolecular hydrogen bond of the 9,10-quinone was found to be unusually strong, with an estimated bond energy of approximately 13 kcal/mol (approximately 4500 cm(-1)), probably due to the resonance-assisted nature of the hydrogen bonding involved.     

The dependence of DNA luminescence on excitation wavelength in the UV/VUV region

Luminescence emission and excitation spectra have been obtained for DNA films at 77 K under vacuum ultraviolet excitation (150–280 nm). The emission spectra, which cover the wavelength range 310 to 490 nm, consists of two components, a short-lived component around 350 nm which is attributed to fluorescence and a longer-lived component around 410 nm believed to be phosphorescence. The excitation spectra, as functions of emission wavelength, are similar in profile with a fairly broad peak around 9240–260 nm) with a shoulder around 200 nm followed by a gradual but constant decrease into the vacuum ultraviolet region of the spectrum. No evidence of autoionization was seen in the region investigated.     

Energy transfer in unsymmetrical phenylene ethynylene dendrimers

We have applied the fluorescence upconversion technique to explore the electronic excitation energy transfer in unsymmetrical phenylene ethynylene dendrimers. Steady-state emission spectra show that the energy transfer from the dendrons to the core is highly efficient. Ultrafast time-resolved fluorescence measurements are performed at various excitation wavelengths to explore the possibility of assigning absorption band structures to exciton localizations. We propose a kinetic model to describe the time-resolved data. Independent of the excitation wavelength, a typical rise-time value of 500 fs is measured for the fluorescence in the dendrimer without an energy trap, indicating initial delocalized excitation. While absorption is into delocalized exciton states, emission occurs from localized states. When an energy trap such as perylene is introduced on the dendrimer, varying the excitation wavelength yields different energy-transfer rates, and the excitation energy migrates to the trap through two channels. The interaction energy between the dendrimer backbone and the trap is estimated to be 75 cm(-1). This value is small compared to the vibronic bandwidth of the dendrimer, indicating that the monodendrons and the energy trap are weakly coupled.     

Evidence for excited state intramolecular charge transfer in benzazole-based pseudo-stilbenes

Two azo compounds were obtained through the diazotization reaction of aminobenzazole derivatives and N,N-dimethylaniline using clay montmorillonite KSF as catalyst. The synthesized dyes were characterized using elemental analysis, Fourier transform infrared spectroscopy, and (13)C and (1)H NMR spectroscopy in solution. Their photophysical behavior was studied using UV-vis and steady-state fluorescence in solution. These dyes present intense absorption in the blue region. The spectral features of the azo compounds can be related to the pseudo-stilbene type as well as the E isomer of the dyes. Excitation at the absorption maxima does not produce emissive species in the excited state. However, excitation around 350 nm allowed dual emission of fluorescence, from both a locally excited (LE, short wavelength) and an intramolecular charge transfer (ICT, long wavelength) state, which was corroborated by a linear relation of the fluorescence maximum (ν(max)) versus the solvent polarity function (Δf) from the Lippert-Mataga correlation. Evidence of TICT in these dyes was discussed from the viscosity dependence of the fluorescence intensity in the ICT emission band. Theoretical calculations were also performed in order to study the geometry and charge distribution of the dyes in their ground and excited electronic states. Using DFT methods at the theoretical levels BLYP/Aug-cc-pVDZ, for geometry optimizations and frequency calculations, and B3LYP/6-311+G(2d), for single-point energy evaluations, the calculations revealed that the least energetic and most intense photon absorption leads to a very polar excited state that relaxes non-radioactively, which can be associated with photochemical isomerization.     

1,4-双(4′-N,N-二甲基氨基苯乙烯基)萘的合成及荧光性质

利用Wittig反应合成了一个以萘为π-Center的对称型“D-π-D”有机绿色发光化合物1,4 双(4′-N,N-二甲基氨基苯乙烯基)萘(BDASN),并测试了其在不同环境中的光谱性质.在378nm激发波长的激发下,BDASN显示出很强的荧光发射峰,峰位在521nm(CH₂Cl₂).随着溶剂极性增大,最大发射波长红移且荧光强度降低,与“D-π-A”分子具有相似的分子内电荷转移(TICT)行为.在β-环糊精(β-CD)中BDASN的绿色发光带被猝灭,同时在450nm附近蓝发光带的荧光强度骤增.     

3,5-二氯水杨醛缩邻苯二胺铜配合物的合成、晶体结构及光谱学性质

合成了3,5-二氯水杨醛缩邻苯二胺铜配合物[Cu(C₂₀H₁₀Cl₄O₂N₂)]·DMF。 通过元素分析、红外光谱、热重测试技术对其进行了表征,同时用X射线单晶衍射确定了其晶体结构;利用紫外-可见光吸收光谱、荧光激发和发射光谱研究了该配合物的光物理性能。 结果表明,该晶体属于单斜晶系,空间群为P2(1)/n,a=0.81316(8) nm,b=1.53101(18) nm,c=1.87819(19) nm,β=92.4530(10)°,Z=4,最终偏差因子R₁=0.0584,ωR₂=0.1482,配合物的中心铜离子与席夫碱的2个O和2个N配位,形成1个五元环和2个六元环,从而构成了1个四配位的平面构型;配合物的热分解温度为384 ℃,具有很好的热稳定性;在DMF溶液体系中,配合物的荧光激发带位于360～480 nm,荧光发射峰在507 nm处,为蓝绿色荧光,最佳激发波长为440 nm,禁带宽度2.59 eV。     

Dual fluorescence of graphene oxide: a time-resolved study

The fluorescence properties of graphene oxide (GO) was studied by recording the fluorescence lifetime, fluorescence emission, and excitation spectra, as well as UV-visible and near-IR absorption spectra. For the first time, we showed that a blue band (ca. 440 nm) and a long wavelength (LW) band (ca. 700 nm) are coexistent, which can be recorded simultaneously by controlling concentration, excitation wavelength, and pH values. Two bands are closely related by the protonation or deprotonation of GO. The blue band is favored by low GO concentration, short excitation wavelength, and high pH value, while the LW band is favored by low pH and long excitation wavelength. To reveal the nature of the dual emission of GO, the fluorescence lifetimes under various conditions were also measured. The blue band contains three emitting components; one of them has a lifetime as long as 10 ns, and its emitting intensity is fairly sensitive to pH, showing the potential for applications in sensing H(+) and fluorescence lifetime imaging. Combining the results under various conditions, we conclude that the electronic transition for this component is very likely due to n-π* transition. The LW band contains two main emitting components (0.2 and 2.1 ns) that also appear in the blue band as minor contributors; the related emission is assigned to π-π* transition. In summary, GO emission is of broadband (300-1250 nm), long-lived, pH sensitive, and excitation wavelength dependent. This makes it easily tailored for versatile applications.     

Bio serves nano: biological light-harvesting complex as energy donor for semiconductor quantum dots

Light-harvesting complex (LHCII) of the photosynthetic apparatus in plants is attached to type-II core-shell CdTe/CdSe/ZnS nanocrystals (quantum dots, QD) exhibiting an absorption band at 710 nm and carrying a dihydrolipoic acid coating for water solubility. LHCII stays functional upon binding to the QD surface and enhances the light utilization of the QDs significantly, similar to its light-harvesting function in photosynthesis. Electronic excitation energy transfer of about 50% efficiency is shown by donor (LHCII) fluorescence quenching as well as sensitized acceptor (QD) emission and corroborated by time-resolved fluorescence measurements. The energy transfer efficiency is commensurable with the expected efficiency calculated according to F?rster theory on the basis of the estimated donor-acceptor separation. Light harvesting is particularly efficient in the red spectral domain where QD absorption is relatively low. Excitation over the entire visible spectrum is further improved by complementing the biological pigments in LHCII with a dye attached to the apoprotein; the dye has been chosen to absorb in the "green gap" of the LHCII absorption spectrum and transfers its excitation energy ultimately to QD. This is the first report of a biological light-harvesting complex serving an inorganic semiconductor nanocrystal. Due to the charge separation between the core and the shell in type-II QDs the presented LHCII-QD hybrid complexes are potentially interesting for sensitized charge-transfer and photovoltaic applications.     

Facile fabrication of aggregation-induced emission based red fluorescent organic nanoparticles for cell imaging

A cyano-substituted diarylethlene derivative aggregation-induced emission （ALE） dye with two amino end-groups and 4,4＇-（hexafluoroisopropylidene）diphthalic anhydride were facilely incorporated into red fluorescent organic nanoparticles （FONs） via room temperature anhydride ring-opening polymerization under an air atmosphere. These obtained RO-HFDA FONs were characterized by a series of techniques including gel permeation chromatography, Fourier transform infrared spectroscopy, size distribution and zeta potential measurements, UV-Vis absorption spectrum, fluorescent spectroscopy and transmission electron microscopy. Biocompatibility evaluation and cell uptake behavior of RO-HFDA FONs were further investigated to explore their potential biomedical application. We demonstrated that such FONs showed high water dispersibility, stable uniform spherical morphology （150-200 nm）, broad excitation band （350-605 nm）, intense red fluorescence （627 nm） and excellent biocompatibility, making them promising for cell imaging applications.     

Two-photon excitation of substituted enediynes

Electronic spectroscopy of nine benzannelated enediynes and a related fulvene was studied under one-photon and two-photon excitation conditions. We utilize measured absorbance and emission spectra and time-resolved fluorescence decays of these molecules to calculate their radiative lifetimes and fluorescence quantum yields. The fluorescence quantum yields for the other compounds were referenced to the fluorescence quantum yield of compound 3 and used to determine relative two-photon absorption cross-sections. Further insight into experimental studies has been achieved using time-dependent density functional (TD-DFT) computations. The probability of two-photon absorption (TPA) increases noticeably for excitation to the higher excited states. The photophysical properties of benzannelated enediynes are sensitive to substitutions at both the core and the periphery of the enediyne chromophore. Considerably enhanced two-photon absorption is observed in an enediyne with donor substitution in the middle and acceptor substitution at the termini. Excited states with B symmetry are not active in TPA spectra. From a practical point of view, this study extends the range of wavelengths applicable for activation of the enediyne moiety from 350 to 600 nm and provides a rational basis for future studies in this field. Our theoretical computations confirmed that lowest energy TPA in benzannelated enediynes involves different orbitals than lowest energy one-photon absorbance and provided further support to the notion that introduction of donor and acceptor substituents at different ends of a molecule increases TPA.     

Novel red-emitting Ba2Tb(BO3)2Cl:Eu phosphor with efficient energy transfer for potential application in white light-emitting diodes

A novel red-emitting Ba(2)Tb(BO(3))(2)Cl:Eu phosphor possessing a broad excitation band in the near-ultraviolet (n-UV) region was synthesized by the solid-state reaction. Versatile Ba(2)Tb(BO(3))(2)Cl compound has a rigid open framework, which can offer two types of sites for various valence's cations to occupy, and the coexistence of Eu(2+)/Eu(3+) and the red-emitting luminescence from Eu(3+) with the aid of efficient energy transfer of Eu(2+)-Eu(3+)(Tb(3+)) and Tb(3+)-Eu(3+) have been investigated. Ba(2)Tb(BO(3))(2)Cl emits green emission with the main peak around 543 nm, which originates from (5)D(4) → (7)F(5) transition of Tb(3+). Ba(2)Tb(BO(3))(2)Cl:Eu shows bright red emission from Eu(3+) with peaks around 594, 612, and 624 nm under n-UV excitation (350-420 nm). The existence of Eu(2+) can be testified by the broad-band excitation spectrum, UV-vis reflectance spectrum, X-ray photoelectron spectrum, and Eu L(3)-edge X-ray absorption spectrum. Decay time and time-resolved luminescence measurements indicated that the interesting luminescence behavior should be ascribed to efficient energy transfer of Eu(2+)-Eu(3+)(Tb(3+)) and Tb(3+)-Eu(3+) in Ba(2)Tb(BO(3))(2)Cl:Eu phosphors.