荧光素蒽醌甲酯分子内光诱导电子转移及电荷分离态的检测

合成了以荧光素为光敏剂的电子给体-受体二元化合物荧光素蒽醌甲酯(FL-AQ),用吸收光谱、荧光光谱、荧光寿命研究了该化合物在乙醇溶液中的光物理性质,并首次用纳秒级瞬态吸收光谱检测了此化合物分子内光诱导电子转移所形成的电荷分离态.在溶液中激发FL,电子可从FL有效地转移到AQ,其速率常数为3.95×10⁹s^-1,效率为95%.但由于电荷分离态寿命较短,瞬态吸收信号弱,若在此溶液中加入二氧化钛(TiO₂)纳米胶体,使FL-AQ吸附在胶体上,电荷分离态信号明显增强.480nm处FL^+·的寿命为11.1μs;560nm处AQ^-·的寿命为8.93μs.     

Photoinduced intramolecular electron-transfer processes in [60]fullerene and N,N-Bis(biphenyl)aniline molecular systems in solutions

Photoinduced intramolecular charge-separation and charge-recombination processes in covalently connected C(60)-(spacer)-bis(biphenyl)aniline (C(60)-sp-BBA) and C(60)-((spacer)-bis(biphenyl)aniline)(2) (C(60)-(sp-BBA)(2)) have been studied by time-resolved fluorescence and transient absorption methods. Since a flexible alkylthioacetoamide chain was employed as the spacer, the folded structures in which the BBA moiety approaches the C(60) moiety were obtained as optimized structures by molecular orbital calculations. The observed low fluorescence intensity and the short fluorescence lifetime of the C(60) moiety of these molecular systems indicated that charge separation takes place via the excited singlet state of the C(60) moiety in a quite fast rate and high efficiency even in the nonpolar solvent toluene, which was a quite new observation compared with reported dyads with different spacers. From the absorption bands at 880 and 1000 nm in the nanosecond transient absorption spectra, generations of C(60)(.-)-sp-BBA(.+) and C(60)(.-)-(sp-BBA(.+))(sp-BBA) were confirmed. The rates of charge separation and charge recombination for C(60)-(sp-BBA)(2) are faster than those for C(60)-sp-BBA, suggesting that one of the BBA moieties approaches the C(60) moiety by pushing another BBA moiety because of the flexible spacers.     

Photophysical properties of DASPMI as revealed by spectrally resolved fluorescence decays

Photophysical properties of 2-(4-(dimethylamino)styryl)-1-methylpyridinium iodide (DASPMI) in various solvents were investigated using time- and space-correlated single photon counting. DASPMI is known to selectively stain mitochondria in living cells.1,2 The uptake and fluorescence intensity of DASPMI in mitochondria is a dynamic measure of membrane potential. Hence, an endeavor has been made to elucidate the mechanism of DASPMI fluorescence by obtaining spectrally resolved fluorescence decays in different solvents. A biexponential decay model was sufficient to globally describe the wavelength-dependent fluorescence in ethanol and chloroform. While in glycerol, a three-exponential decay model was necessary for global analysis. In the polar low-viscous solvent water, a monoexponential decay model fitted the decay data. The sensitivity of DASPMI to solvent viscosity was analyzed using various proportions of glycerol-ethanol mixtures. The lifetimes were found to increase with increasing solvent viscosity. The negative amplitudes of the short lifetime component found in chloroform and glycerol at the longer wavelengths validated the formation of new excited-state species from the initially excited state. Time-resolved emission spectra in chloroform and glycerol showed a biphasic increase of spectral width and emission maxima. The spectral width had an initial fast increase within 150 ps and a near constant thereafter. A three-state model of generalized scheme, on the basis of successive formation of locally excited state (LE), intramolecular charge transfer state (ICT), and twisted intramolecular charge transfer (TICT) state, has been proposed to explain the excited-state kinetics. The presumed role of solvation dynamics of ICT and TICT states leading to the asymmetrical broadening and structureless fluorescence has been substantiated by the decomposition of time-resolved emission spectra in chloroform, glycerol, and ethanol/glycerol mixtures.     

KINETICS OF STACKING INTERACTIONS IN FLAVIN ADENINE DINUCLEOTIDE FROM TIME-RESOLVED FLAVIN FLUORESCENCE

The time dependence of the fluorescence of flavin adenine dinucleotide (FAD) was measured with a subnanosecond-resolving fluorometer. In contrast to the fluorescence decay of FMN, the decay of FAD was proved to be nonexponential. The time-dependent fluorescence of FAD can be interpreted by assuming an equilibrium between closed and open conformers in the ground state. The rate constant for folding in the excited state and the fluorescence lifetime of the intramolecular complex could be evaluated from analysis of the observed fluorescence decay. The results on FAD were compared to those on NADH obtained earlier.     

Electronic spectroscopy of toluene-rare-gas clusters: the external heavy atom effect and vibrational predissociation

Toluene-X van der Waals clusters (where X = Ne, Ne2, Ar, Ar2, Kr, Xe) have been investigated by fluorescence excitation spectroscopy in the region of the S1-S0 transition. With the exception of Xe, for each rare-gas studied, we have assigned cluster transitions in the region of all the strong monomer vibrational bands up to 1000 cm(-1) above the origin band. We have further investigated the S1 relaxation dynamics for each vibrational level of each complex, via their fluorescence decay profiles. Clustering with neon has little appreciable effect on the vibrationless S1 lifetime. By contrast, the clusters with argon and krypton exhibit markedly shorter fluorescence lifetimes compared with the monomer. The effect is so severe in the case of toluene-Xe clusters that no fluorescence signals were observed. We interpret these results in terms of an external heavy atom effect in which the rate of intersystem crossing in toluene is influenced by the cluster partner. For clusters built upon excited S1 vibrational levels, the situation is potentially complicated by intramolecular vibrational redistribution and vibrational predissociation (VP). The majority of the fluorescence decay profiles were satisfactorily modeled using single exponential decays. The emission following pumping of the 37(1) level in the toluene-Kr cluster, however, is an exception. We have modeled the decay of this level with a simple kinetic scheme including VP and determined a predissociation rate of (1.04 +/- 0.54) x 10(7) s(-1).     

天线若丹明染料分子内能量与电荷传递的研究

染料三重态在染料激光的应用中起着重要作用，尤其是三重态一Z重态（T－T）吸收常常会造成谐振腔损耗＊．为了减少由基态吸收而造成的话振腔损耗，带有紫外吸收天线分子的三发色团染料已在研究问，2，5·二苯基螨喳（PP0）在紫外区（如308删）有很强的吸收，PPO－rhod．系列染料（见图1）在紫外区的吸收就很强，由PPO到若丹明母体的单线态一单线态（S功能量传递使这类三发色团染料具有较大的荧光量子效率，较小的基态重复吸收耗能卜，司．然而，在610N640nm区域中这些天线若丹明染料的激光输出效率远远小于若丹明Rh630＊，其原因正是…     

DNA-ligand binding mode discrimination by characterizing fluorescence resonance energy transfer through lifetime measurements with picosecond resolution

We describe a method for distinguishing between minor groove binders and base intercalators that is based on measurements of the fluorescence lifetime of a donor (D) in the presence of an acceptor (A). The D-A pair is separated by a short double helix DNA with which the ligands interact. By plotting the D fluorescence lifetime as a function of the ligand-to-base pair concentration ratio we find a clear signature that distinguishes between the two binding mechanisms: minor groove binding induces an asymptotic decrease of the D fluorescence lifetime, while intercalation gives a monotonically increasing lifetime and the appearance of an additional short lifetime. We assayed Quinacrine, Hoechst and 4'-6'diamidine-2-phenyl indole, which in control experiments performed on oligodeoxyribonucleotides (oligos) lacking the A are demonstrated not to interfere with the D fluorescence. The changes in fluorescence lifetimes measured in the case of dual-labeled oligos are thus caused by structural changes in the DNA that modify the D-A distance. The appearance of the short-lived transient in the fluorescence decay of Ds attached to dual-labeled oligos upon binding of an intercalator can be interpreted as denaturation.     

THE USE OF EXOGENOUS FLUORESCENT PROBES FOR TEMPERATURE MEASUREMENTS IN SINGLE LIVING CELLS

The fluorescent membrane probes 7-nitrobenz-2-oxa-1,3-diazo1-4-y1 (NBD) and 6-dodeca-noy1-2-dimethylamino-naphthalene (laurdan) have been studied for use as optical thermometers in living cells. The thermal sensitivity of NBD is primarily a consequence of rapid, heat-induced electronic changes, which increase the observed fluorescence decay rate. As a result, fluorescence intensity and lifetime variations of membrane-bound NBD-conjugated phospholipids and fatty acids can be directly correlated with cellular temperature. In contrast, laurdan fluorescence undergoes a dramatic temperature-dependent Stokes shift as the membrane undergoes a gel-to-liquid-crystalline phase transition. This facilitates the use of fluorescence spectra to record the indirect effect of microenvironmental changes, which occur during bilayer heating. Microscope and suspension measurements of cells and phospholipid vesicles are compared for both probes using steady-state and fluorescence lifetime (suspension only) data. Our results show that NBD fluorescence lifetime recordings can provide reasonable temperature resolution (approximately 2°C) over a broad temperature range. Laurdan's microenvironmental sensitivity permits better temperature resolution (0.1-1°C) at the expense of a more limited dynamic range that is determined solely by bilayer properties. The temperature sensitivity of NBD is based on rapid intramolecular rotations and vibrations, while laurdan relies on a slower, multistep mechanism involving bilayer rearrangement, water penetration and intermolecular processes. Because of these differences in time scale, NBD appears to be more suitable for monitoring ultrafast phenomena, such as the impact of short-pulse microirradiation on single cells.     

Enhanced emission efficiency and excited state lifetime due to restricted intramolecular motion in silole aggregates

The aggregation-induced emission (AIE) properties of 1,1,2,3,4,5-hexaphenylsilole (HPS) and poly{11-[(1,2,3,4,5-pentaphenylsilolyl)oxy]-1-phenyl-1-undecyne} (PS9PA) were studied by time-resolved fluorescence technique. The enhanced fluorescence and long fluorescent lifetime were obtained for the sample in an aggregate state as compared to the sample in solution. The time-decay of fluorescence of HPS and PS9PA in high viscosity solvents and low-temperature glasses has also been measured in detail to further investigate the possible mechanism for AIE. Enhanced light emission and long fluorescence lifetime were detected for both HPS and PS9PA in the solution-thickening and -cooling experiments. These results provided direct evidence that the enhanced photoluminescence (PL) efficiency is due to restricted intramolecular motion, which ascribes AIE to the deactivation of nonradiative decay caused by restricted torsional motions of the molecules in the solid state or aggregate form.     

Oligosilane chain-length dependence of electron transfer of zinc porphyrin-oligosilane-fullerene molecules

A new series of zinc porphyrin-fullerenes bridged by flexible oligosilane chains ZnP-[Sin]-C60 (n = 1-5) was synthesized, and the photophysical properties of these molecules were investigated using steady-state and time-resolved spectroscopic methods. The spectral observations can be well explained by assuming the coexistence of extended conformers and folded conformers, that is, the observed emissions are from the extended conformers while the folded conformers form very short lifetime nonfluorescent excited-state charge-transfer (CT) complexes. Time-resolved transient absorption spectra suggest the generation of intramolecular radical-ion pairs that have sub-microsecond lifetimes. With the number of silicon atoms of the bridged oligosilane, the lifetimes of the radical-ion pairs do not vary regularly, indicating that intramolecular collision of the radical-cation moiety with the radical-anion moiety controls the charge-recombination rate. The attenuation factor of the electron transfer of the silicon chain was evaluated by the bridge-length dependence of charge-separation rate to be 0.16 A-1 on the basis of the oligosilane chain-length dependence of fluorescence lifetimes. This is the first evaluation of the attenuation factor for the one-dimensional Si-Si chain to the best of our knowledge.     

Excited state behaviour of pentahelicene dinitriles

The excited singlet and triplet states 2,13-dicyano[5]helicene (1) and two para-dicyno[5]helicenes containing one and two methyl groups (2 and 3, respectively) were studied in solvents of different polarity as a function of temperature. Fluorescence quenching by electron donors such as triethyl amine indicated photoinduced electron transfer. In the absence of additives triplet states were observed by flash photolysis. The triplet lifetime at room temperature was rather short (<1 μs) and the decay limited by intramolecular processes, e.g. charge transfer in the cases of 2 or 3. Luminescence of singlet molecular oxygen, O₂(¹Δ_g), was observed with moderate and low quantum yield for 1 and 3, respectively. For 1–3, the triplet lifetime increases by six orders of magnitude on going to −196°C. Two subsequently formed triplet states were observed for 3 at lower temperatures. The effects of temperature and solvent polarity on the quantum yields of fluorescence and phosphorescence and the spectroscopic and kinetic triplet absorption properties were examined. The influence of substituents on the deactivation pathways of excited pentahelicenes are discussed.     

Intriguing H-aggregate and H-dimer formation of coumarin-481 dye in aqueous solution as evidenced from photophysical studies

Photophysical properties of coumarin-481 (C481) dye in aqueous solution show intriguing presence of multiple emitting species. Concentration and wavelength dependent fluorescence decays and time-resolved emission spectra and area-normalized emission spectra suggest the coexistence of dye monomers, dimers, and higher aggregates (mostly trimers) in the solution. Because of the efficient intramolecular charge transfer (ICT) state to twisted intramolecular charge transfer (TICT) state conversion, the dye monomers show very short fluorescence lifetime of ~0.2 ns. Fluorescence lifetimes of dimers (~4.1 ns) and higher aggregates (~1.4 ns) are relatively longer due to steric constrain toward ICT to TICT conversion. Observed results indicate that the emission spectra of the aggregates are substantially blue-shifted compared to monomers, suggesting H-aggregation of the dye in the solution. Temperature-dependent fluorescence decays in water and time-resolved fluorescence results in water-acetonitrile solvent mixtures are also in support of the dye aggregation in the solution. Though dynamic light scattering studies could not recognize the dye aggregates in the solution due to their small size and low concentration, fluorescence up-conversion measurements show a relatively higher decay tail in water than in water-acetonitrile solvent mixture, in agreement with higher dye aggregation in aqueous solution. Time-resolved fluorescence results with structurally related non-TICT dyes, especially those of coumarin-153 dye, are also in accordance with the aggregation behavior of these dyes in aqueous solution. To the best of our knowledge, this is the first report on the aggregation of coumarin dyes in aqueous solution. Present results are important because coumarin dyes are widely used as fluorescent probes in various microheterogeneous systems where water is always a solvent component, and the dye aggregation in these systems, if overlooked, can easily lead to a misinterpretation of the observed results.     

Donor–σ–Acceptor Motifs: Thermally Activated Delayed Fluorescence Emitters with Dual Upconversion

A family of organic emitters with a donor–σ–acceptor (D‐σ‐A) motif is presented. Owing to the weakly coupled D‐σ‐A intramolecular charge‐transfer state, a transition from the localized excited triplet state (³LE) and charge‐transfer triplet state (³CT) to the charge‐transfer singlet state (¹CT) occurred with a small activation energy and high photoluminescence quantum efficiency. Two thermally activated delayed fluorescence (TADF) components were identified, one of which has a very short lifetime of 200–400 ns and the other a longer TADF lifetime of the order of microseconds. In particular, the two D‐σ‐A materials presented strong blue emission with TADF properties in toluene. These results will shed light on the molecular design of new TADF emitters with short delayed lifetimes.     

PICOSECOND FLUORESCENCE KINETICS IN SPINACH CHLOROPLASTS AT LOW TEMPERATURE

Abstract— At 77 K the fluorescence from spinach chloroplasts excited using picosecond mode-locked laser pulses at 620 nm is made up of 5 separate kinetic components. Three of these are predominant at short wavelengths. between 650 and 690 nm, and they appear to correspond to the 3 decay phases seen at room temperature. The 2 new components. a 100 ps rise and a 3000 ps decay, characterize the longer (730–770 nm) wavelength fluorescence. The temperature dependence of the kinetic components of the long wavelength fluorescence shows that the 3000 ps decay accounts for essentially all of the large increase in fluorescence yield observed at low temperature. Furthermore, it appears that this increase does not result entirely from an increase in the fluorescence lifetime, as has been proposed. The dependences of these 2 new components (the 100 ps rise and 3000 ps decay) on emission wavelength and temperature are similar enough to suggest they have a common origin, presumably the chlorophyll pigment component C705. The amplitudes (yield/lifetime) of these 2 phases are approximately equal, and they are opposite in sign. Thus. we see evidence of time-resolved excitation transfer from those pigment molecules that absorb the 620 nm radiation to those that give rise to the long wavelength fluorescence at low temperature.     

Generalized two-dimensional heterocorrelation analysis of spectrally resolved and temporally resolved fluorescence of the 8-anilino-1-naphthalenesulfonate-apomyoglobin complex with pH perturbation

We demonstrate two-dimensional heterocorrelation analysis between spectrally resolved and temporally resolved fluorescence to investigate the decay dynamics of the 8-anilino-1-naphthalenesulfonate- (ANS-) apomyoglobin complex. The dynamic changes of the lifetime components are disclosed across the emission spectrum with an external pH-perturbation. Two different fluorescence lifetime schemes of the ANS-apomyoglobin complex are revealed. From pH 8.5 to 4.5, the transition of protein conformation from the native state to the folding intermediate, a short lifetime component is found to correlate with a short-wavelength emission whose population diminishes with decreasing pH. The lifetime components reflect the excited-state populations of the nascent and the charge-transfer species. From pH 4.2 to 1.0, the transition from the folding intermediate to the acid-unfolded state, the short lifetime is responsible for a long-wavelength emission and the fraction of this component increases when the solution becomes more acidic. In this pH range, the decay components reflect the ground-state populations of microenvironments. The relative decay dynamics across the emission spectrum are revealed without collecting decays at each wavelength. More importantly, these conclusions are reached without the necessity of statistical fitting of the decay data with an a priori decay model.     

Picosecond laser studies of the effects of reactants on intramolecular energy relaxation of diphenylcarbene: Reaction of diphenylcarbene with alcohols

Picosecond laser induced fluorescence measurements provide for the first time the direct measurement of the intramolecular and intermolecular energy decay dynamics of singlet diphenylcarbene (¹ DPC) in the presence of reactive molecules. As exemplified by the reaction of ¹DPC with alcohols it is found that reactive molecules provide ¹DPC with not only a chemical decay channel but also an intramolecular decay channel which is due to a solvent polarity effect. These chemical and physical effects can act in opposite directions leading to novel results such as a significant increase in the singlet state lifetime upon addition of reacting molecules. The absolute reaction rate constants of ¹DPC with alcohols, in different solvents, obtained by direct measurements are also reported.     

Polar‐Solvent Effect on the Photocycloisomerization of Symmetrical Bis[anthracenes]: A Transient Ultrafast Kinetic Study

Bis[anthracenes] are the few among the fluorescing nonconjugated bichromophores that possess photoreactive properties. The 9,9′‐[methylenebis(oxy)]bis[anthracenes] 1 (AOCH₂OA) exhibit the highest known intramolecular photocycloaddition quantum yield from the S₁ state and, moreover, display a higher yield in polar solvents, an unexpected result for symmetrical systems. No excimer fluorescence was detected in solution at room temperature. The 10,10′‐dimethoxy derivative 1b was studied by picosecond (ps) laser spectroscopy. In nonpolar solvents (methylcyclohexane), S_n←S₁ was the only transient absorption detected, whereas, in polar solvents (MeCN), the growth and the decay of a second transient were recorded, and the second transient was attributed to a zwitterion A^+.−A^−.. The kinetics data were derived, and it was concluded that electron transfer and ion recombination should be at the origin of the observed rate enhancement of cycloadduct formation in polar solvents.     

Coupled electron and proton transfer processes in 4-dimethylamino-2-hydroxy-benzaldehyde

TDDFT calculations, picosecond transient absorption, and time-resolved fluorescence studies of 4-dimethylamino-2-hydroxy-benzaldehyde (DMAHBA) have been carried out to study the electron and proton transfer processes in polar (acetonitrile) and nonpolar (n-hexane) solvents. In n-hexane, the transient absorption (TA) as well as the fluorescence originate from the ππ* state of the keto form (with the carbonyl group in the benzaldehyde ring), which is produced by an intramolecular proton transfer from the initially excited ππ* state of the enol form (OH group in the ring). The decay rate of TA and fluorescence are essentially identical in n-hexane. In acetonitrile, on the other hand, the TA exhibits features that can be assigned to the highly polar twisted intramolecular charge transfer (TICT) states of enol forms, as evidenced by the similarity of the absorption to the TICT-state absorption spectra of the closely related 4-dimethylaminobenzaldehyde (DMABA). As expected, the decay rate of the TICT-state of DMAHBA is different from the fluorescence lifetime of the ππ* state of the keto form. The occurrence of the proton and electron transfers in acetonitrile is in good agreement with the predictions of the TDDFT calculations. The very short-lived (～1 ps) fluorescence from the ππ* state of the enol form has been observed at about 380 nm in n-hexane and at about 400 nm in acetonitrile.     

Twisted Intramolecular Charge Transfer Emission of 2-(4'-N,N-dimethylaminophenyl)benzimidazole in Micelles

Spectral characteristics of 2-(4'N,N-dimethylaminophenyl)benzimidazole have been studied as a function of surfactant concentration and as a function of acid concentration in three surfactants. Dual fluorescence is observed in all the micelles. Fluorescence intensities of the local emission (B band) and twisted intramolecular charge transfer (A band, TICT) increase by an 17-30% and 38 to 64% respectively. When dissolved in micelles lifetimes of both the states also increase in the presence of micelles. The increase in the fluorescence intensities is attributed to the decrease in the non-radiative decay constant. cmc of the surfactants can be determined from the variation in the fluorescence intensity and the lifetime data. The equilibrium constants are determined for the prototropic reactions of the fluorophore in all the micelles in S0 and S1 states and the values obtained are discussed. Copyright 1999 Academic Press.     

Intense fluorescence of metal-to-ligand charge transfer in [Pt(0)(binap)(2)] [binap = 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl

[Pt(0)(binap)(2)] (binap = 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) is found to exhibit a luminescence from metal-to-ligand charge transfer state (MLCT) with a quantum yield of 0.12 and a lifetime of 1.2 micros in toluene at an ambient temperature. Prompt fluorescence with a quantum yield of 1.6 x 10(-)(4) is observed by means of a picosecond time-correlated single photon counting technique. The spectrum of the steady-state luminescence is almost identical to that of the prompt fluorescence, indicating that the intense luminescence is mainly delayed fluorescence from thermally activated (1)MLCT. The analysis of the temperature-dependent emission indicates that the energy difference between the (1)MLCT and (3)MLCT is 1.15 x 10(3) cm(-)(1). The lifetime of the prompt fluorescence is determined to be 3.2 ps from the decay of stimulated emission overlapped on subpicosecond transient absorption spectra. The lifetime of the (1)MLCT is much longer than expected from the large spin-orbit coupling constant of 5d (Pt) electrons (4000 cm(-)(1)). Theoretical analysis based on density functional theory reveals that structural distortion in the MLCT states causes large energy splitting between HOMO and HOMO - 1, which prevents a very fast ISC induced by strong spin-orbit interactions between these orbitals. The relatively slow ISC is therefore induced by weak spin-orbit interactions (ca. 50 cm(-)(1)) between ligand-centered molecular orbitals. Theoretical calculations indicate that the phosphorescence observed at lower temperatures is due to intensity borrowing from 4(1)B(2) --> GS transition. However, the large energy difference between HOMO and HOMO - 2 reduces the extent of mixing between the lowest (3)MLCT and 4(1)B(2) due to spin-orbit interaction, thereby decreasing the radiative rate of the phosphorescence.