Measurements of sub-nanosecond lifetimes by means of nanosecond laser pulses

It is shown that the lifetimes of excited states of molecules which give a reasonably strong absorption (molar decadic extinction coefficient ∈ > 10⁴M^?1cm^?1) at the frequency of the ruby laser can be determined from the saturation of the optical transitions. This is illustrated for DDI (1,1'-diethyl-2,2'-dicarbocyanine iodide) in glycerol, which appears to have a lowest excited state with a lifetime of 110 psec. This value agrees fairly well with that calculated from the formula for the rate of internal conversion derived from Siebrand's work. Under favourable conditions lifetimes as short as 10 psec can be measured.     

Hydrogen Bonding Effects on the Photophysical Properties of 2,3-dihydro-3-keto-1H-pyrido[3,2,1-kl]phenothiazine

Time-dependent density functional theory (TDDFT) and femtosecond transient absorption spectroscopy were used to investigate the photophysical properties of 2,3-dihydro-3-keto-1H-pyrido[3,2,1-kl]phenothiazine (PTZ4) and 3-keto-1H-pyrido[3,2,1-kl]phenothiazine (PTZ5). The calculated results obtained from TDDFT suggest that the red-shifts of the absorption spectra of these two fluorophores in methanol are due to the formation of hydrogen-bonded complexes at the ground state. Four conformers of PTZ4 were obtained by TDDFT. The two fluorescence peaks of PTZ4 in tetrahydrofuran (THF) came from the ICT states of the four conformers. The fluorescence of PTZ4 in THF showed a dependence on the excitation wavelength because of butterfly bending. The excited state dynamics of PTZ4 in THF and methanol were obtained by transient absorption spectroscopy. The lifetime of the excited PTZ4 in methanol was 53.8 ps, and its relaxation from the LE state to the ICT state was completed within several picoseconds. The short lifetime of excited PTZ4 in methanol was due to the formation of out-of-plane model hydrogen bonds between PTZ4 and methanol at the excited state.     

Determination of the rate constant of the solvent cage reorientation around a photoexcited molecule using phase modulation fluorometry

The fluorescence lifetime and quantum yield of ethylamino-9-methoxy-2-chloro-6-acridine in glycerol solution were measured at three temperatures using the phase modulation technique. The results were interpreted in terms of a relaxation process in the excited state that involved two emitting species: one (designated F) directly excited and one (designate R) resulting from the reorientation of the solvent cage around the F state. The rate constant of this process was evaluated; it was greatly increased on raising the temperature, which also enhanced the radiationless deactivation rate constants of both emitting species, although to a minor extent. At 60 °C, this relaxation process is so fast that the whole fluorescence is emitted from R species.     

AN UNUSUAL EXCITED STATE SPECIES OF ortho- HYDROXY-CINNAMIC ACID*

The fluorescence of ortho-hydroxycinnamic acid was studied in aqueous solution over the pH 0-14 range and in methanol. At pH values above 9.7, intense anion fluorescence is observed. In the pH 4-9 range emissions are from both the monoanion (albeit weakly) and from the dianion due to photodissociation. In the pH 0-4 range a blue and a green fluorescence was found, which is attributed to emissions from the unchanged molecule and from an unusual excited state tautomer respectively. The latter is assumed to be the phenolate anion of cinnamic acid and represents a tautomer with no equivalent in the electronic ground state. It is assumed to be formed by adiabatic photodissociation during the lifetime of the first excited singlet state.     

Charge recombination fluorescence in photosystem I reaction centers from Chlamydomonas reinhardtii

The fluorescence kinetics of photosystem I core particles from Chlamydomonas reinhardtii have been measured with picosecond resolution in order to test a previous hypothesis suggesting a charge recombination mechanism for the early electron-transfer steps and the fluorescence kinetics (Müller et al. Biophys. J. 2003, 85, 3899-3922). Performing global target analyses for various kinetic models on the original fluorescence data confirms the "charge recombination" model as the only acceptable one of the models tested while all of the other models can be excluded. The analysis allowed a precise determination of (i) the effective charge separation rate constant from the equilibrated reaction center excited state (438 ns(-1)) confirming our previous assignment based on transient absorption data (Müller et al. Biophys. J. 2003, 85, 3899-3922), (ii) the effective charge recombination rate constant back to the excited state (52 ns(-1)), and (iii) the intrinsic secondary electron-transfer rate constant (80 ns(-1)). The average energy equilibration lifetime core antenna/RC is about 1 ps in the "charge recombination" model, in agreement with previous transient absorption data, vs the 18-20 ps energy transfer lifetime from antenna to RC within "transfer-to-the-trap-limited" models. The apparent charge separation lifetime in the recombination model is about three times faster than in the "transfer-to-the-trap-limited" model. We conclude that the charge separation kinetics is trap-limited in PS I cores devoid of red antenna states such as in C. reinhardtii.     

Study on excited states of hexamethoxybenzene-NO+ charge-transfer complex: incorporation of excited radical cation

A charge-transfer (CT) complex of NOBF₄ and hexamethoxybenzene (HMB), which gives out HMB^?+ as a “fluorescent radical cation probe,” upon one-electron oxidation, has been designed to explore the excited state dynamics of contact radical ion pairs by laser-induced fluorescence and femtosecond transient absorption spectroscopic techniques. The acetonitrile solution of the CT complex showed weak fluorescence with a similar spectrum to that observed for free excited HMB radical cation (HMB^?+*), suggesting the formation of HMB^?+* upon the one-photonic excitation of the CT complex. The laser-power dependence of the fluorescence intensity supported the one-photonic excitation event. We have also observed a short-lived transient species but no long-lived species by femtosecond laser flash photolysis of the CT complex. The lifetime (6.5 ps) was in good accordance with its fluorescence quantum yield (2.5 × 10^?5) and was able to assign the transient species to the fluorescent state, an excited radical ion pair [HMB ^?+*/NO^?]. All the events were completed within the inner sphere and the short lifetime of the transient species could be attributed to rapid back-electron transfer. It is concluded that the excited radical cation character in the excited state of the CT complex originates from the radical ion character in the CT complex in the ground state and that a relatively long lifetime of HMB^?+* facilitates its observation even in the contact ion pair.     

Squaraine Rotaxane as a Reversible Optical Chloride Sensor

A mechanically interlocked squaraine rotaxane is comprised of a deep‐red fluorescent squaraine dye inside a tetralactam macrocycle. NMR studies show that Cl^? binding to the rotaxane induces macrocycle translocation away from the central squaraine station, a process that is completely reversed when the Cl^? is removed from the solution. Steady‐state fluorescence and excited‐state lifetime measurements show that this reversible machine‐like motion modulates several technically useful optical properties, including a three‐fold increase in deep‐red fluorescence emission that is observable to the naked eye. The excited states were characterized quantitatively by time‐correlated single photon counting, femtosecond transient absorption spectroscopy, and nanosecond laser flash photolysis. Cl^? binding to the rotaxane increases the squaraine excited singlet state lifetime from 1.5 to 3.1 ns, and decreases the excited triplet state lifetime from >200 to 44 μs. Apparently, the surrounding macrocycle quenches the excited singlet state of the encapsulated squaraine dye and stabilizes the excited triplet state. Prototype dipsticks were prepared by adsorbing the lipophilic rotaxane onto the ends of narrow, C18‐coated, reverse‐phase silica gel plates. The fluorescence intensity of a dipstick increased eighteen‐fold upon dipping in an aqueous solution of tetrabutylammonium chloride (300 mM ) and was subsequently reversed by washing with pure water. It is possible to develop the dipsticks for colorimetric determination of Cl^? levels by the naked eye. After dipping into aqueous tetrabutylammonium chloride, a dipstick’s color slowly fades at a rate that depends on the amount of Cl^? in the aqueous solution. The fading process is due primarily to hydrolytic bleaching of the squaraine chromophore within the rotaxane. That is, association of Cl^? to immobilized rotaxane induces macrocycle translocation and exposure of the electrophilic C₄O₂ core of the squaraine station, which is in turn attacked by the ambient moisture to produce a bleached product.     

Laser pulse trains for controlling excited state dynamics of adenine in water

We investigate theoretically the control of the ultrafast excited state dynamics of adenine in water by laser pulse trains, with the aim to extend the excited state lifetime and to suppress nonradiative relaxation processes. For this purpose, we introduce the combination of our field-induced surface hopping method (FISH) with the quantum mechanical-molecular mechanical (QM/MM) technique for simulating the laser-driven dynamics in the condensed phase under explicit inclusion of the solvent environment. Moreover, we employ parametric pulse shaping in the frequency domain in order to design simplified laser pulse trains allowing to establish a direct link between the pulse parameters and the controlled dynamics. We construct pulse trains which achieve a high excitation efficiency and at the same time keep a high excited state population for a significantly extended time period compared to the uncontrolled dynamics. The control mechanism involves a sequential cycling of the population between the lowest and higher excited states, thereby utilizing the properties of the corresponding potential energy surfaces to avoid conical intersections and thus to suppress the nonradiative decay to the ground state. Our findings provide a means to increase the fluorescence yield of molecules with an intrinsically very short excited state lifetime, which can lead to novel applications of shaped laser fields in the context of biosensing.     

The photophysical properties of a julolidene-based molecular rotor

The photophysical properties of 9-dicyanovinyljulolidine are sensitive to solvent viscosity but are little affected by changes in polarity. In fluid solution, the lifetime of the first-excited singlet state is very short and triplet state formation cannot be detected by laser flash photolysis. Decay of the excited singlet state is strongly activated and weak phosphorescence can be observed in a glassy matrix at 77 K. Temperature dependent 1H NMR studies indicate that the molecule undergoes slow internal rotation in solution, for which the activation energy has a value of ca. 35 kJ mol(-1). This process is unlikely to account for the poor fluorescence quantum yield found in fluid solution. Instead, it is considered that the target compound undergoes rapid rotation around the dicyanovinyl double bond from the excited singlet state. The rate of rotation depends weakly on the viscosity of the solvent in a range of linear alcohols at room temperature. This might represent the fact that the rotor is relatively small and can pack into cavities in the solvent structure. In glycerol, the rate of rotation is more sensitive to viscosity effects but a quite complex temperature dependence is observed in ethanol. Here, the rate is almost activationless in a glassy matrix and in fluid solution at high temperature but strongly activated at intermediate temperatures.     

Emission spectroscopy and excited-state dynamics of chromyl-chloride vapor

A tunable dye laser has been used to excite single vibronic features in the low-pressure vapor of CrO₂Cl₂. The fluorescence spectrum, fluorescence excitation spectrum and time-resolved fluorescence decay are discussed. It is shown that the active ν′₄ and ν″₄ modes are the same frequency in the gas phase, thus collapsing the sequence congestion normally observed in gas-phase spectra. This degeneracy makes impossible the excitation of single vibronic levels. It is shown that the fluorescence lifetime of the excited state in all except the vibrationally cold level is severely shortened by unimolecular radiationless decay. This radiationless rate is strongly dependent upon the partitioning of energy into various excited-state modes. The radiative lifetime of the vibrationally cold excited state is (1.34 ± 0.08) μs and the apparent bimolecular quenching rate is (5.9 ± 0.2) × 10^?10 cm³/molecules. No evidence of emission from the lowest-energy excited electronic state recently reported by Spoliti [J. Mol. Spectrosc. 52 (1973) 146] is observed.     

Time-resolved Luminescence and Singlet Oxygen Formation After Illumination of the Hypericin–Low-density Lipoprotein Complex

Time-resolved fluorescence and phosphorescence study of hypericin (Hyp) in complex with low-density lipoproteins (LDL) as well as the evolution of singlet oxygen formation and annihilation after illumination of Hyp/LDL complexes at room temperature are presented in this work. The observed shortening of the fluorescence lifetime of Hyp at high Hyp/LDL molar ratios (>25:1) proves the self-quenching of the excited singlet state of monomeric Hyp at these concentration ratios. The very short lifetime (∼0.5 ns) of Hyp fluorescence at very high Hyp/LDL ratios (>150:1) suggests that at high local Hyp concentration inside LDL molecules fast and ultrafast nonradiative decay processes from excited singlet state of Hyp become more important. Contrary to the lifetime of the singlet excited state, the lifetime (its shorter component) of Hyp phosphorescence is not dependent on Hyp/LDL ratio in the studied concentration range. The amount of singlet oxygen produced as well as the integral intensity of Hyp phosphorescence after illumination of Hyp/LDL complexes resemble the dependence of the concentration of molecules of Hyp in monomeric state on Hyp/LDL until a concentration ratio of 60:1. This fact confirms that only monomeric Hyp is able to produce the excited triplet state of Hyp, which in aerobic conditions leads to singlet oxygen production. The value of singlet oxygen lifetime (∼8 μs) after its formation from the excited triplet state of Hyp in LDL proves that molecules of singlet oxygen remain for a certain period of time inside LDL particles and are not immediately released to the aqueous surrounding. That Hyp exists in the complex with LDL in the monodeprotonated state is also demonstrated.     

THE PHOTOSENSITIZERS BENZOPHENOXAZINE and THIAZINES: COMPREHENSIVE INVESTIGATION OF PHOTOPHYSICAL and PHOTOCHEMICAL PROPERTIES

Eight differently substituted title dye compounds have been investigated regarding intersystem crossing, triplet state, fluorescence and singlet excited state pKa properties. In general, non-halogenated oxazines and thiazines as well as a mono bromooxazine show very low triplet quantum yields, phi tau (less than 0.03) and relatively long triplet lifetimes (approximately 40 microseconds) in acidic methanol. The phi tau data correlate well with known singlet oxygen yields. In basic methanol no triplet transient is observed but a significant yield of a ground state transient protonated (base dye) form is produced with a short lifetime, approximately 400 ns. Fluorescence can be seen simultaneously from both the excited base and the protonated base dye forms in basic methanol. For iodinated oxazine or thiazines, the triplet yield increases and can be as high as 0.5 (diiodo case) in acidic methanol. The triplet lifetimes are further shortened to approximately 10 microseconds compared to the non-iodinated derivatives above. The triplet yields of the iodo compounds are higher or equal to known singlet oxygen yields. In basic methanol triplet yields up to 0.2 can be seen, the triplet lifetime are shortened still further to 1 microsecond but no observable protonated form is produced (in distinction to the non-iodinated cases). Consideration is given to the correlation of triplet and singlet oxygen yields, ground and excited pKa properties, spin-orbit coupling and internal conversion properties, solvent effects, and phototherapeutic activity of these dyes.     

RADIATIVE LIFETIME AND THERMODYNAMIC POTENTIAL OF EXCITED STATES

Abstract— To clarify their similarities and common assumptions, a unified derivation is presented of the methods of evaluating the radiative lifetime of an electronically-excited state by integration of the absorption spectrum or of the fluorescence spectrum. Previous examinations of the accuracy of these methods are briefly reviewed. The radiative lifetime of eleven aromatic molecules is computed by the fluorescence spectrum method, and compared with published lifetimes determined experimentally and by integration of the absorption spectrum; from this it is concluded that the fluorescence spectrum method is accurate to about 25 per cent, and that the common assumption of equal partition functions for the ground and excited states is often not true. Extension of the derivation of formulas for the radiative lifetime shows that the standard chemical potential of the excited state may be determined from the experimental radiative lifetime and appropriately scaled integration of the fluorescence spectrum.     

Ultrafast excited-state dynamics of tetraphenylethylene studied by semiclassical simulation

Detailed simulation study is reported for the excited-state dynamics of photoisomerization of cis-tetraphenylethylene (TPE) following excitation by a femtosecond laser pulse. The technique for this investigation is semiclassical dynamics simulation, which is described briefly in the paper. Upon photoexcitation by a femtosecond laser pulse, the stretching motion of the ethylenic bond of TPE is initially excited, leading to a significant lengthening of ethylenic bond in 300 fs. Twisting motion about the ethylenic bond is activated by the energy released from the relaxation of the stretching mode. The 90 degrees twisting about the ethylenic bond from an approximately planar geometry to nearly a perpendicular conformation in the electronically excited state is completed in 600 fs. The torsional dynamics of phenyl rings which is temporally lagging behind occurs at about 5 ps. Finally, the twisted TPE reverts to the initial conformation along the twisting coordinate through nonadiabatic transitions. The simulation results provide a basis for understanding several spectroscopic observations at molecular levels, including ultrafast dynamic Stokes shift, multicomponent fluorescence, viscosity dependence of the fluorescence lifetime, and radiationless decay from electronically excited state to the ground state along the isomerization coordinate.     

时间分辨激光荧光光谱分析技术研究——Ⅱ.微秒级时间分辨激光荧光光谱测量

本文主要以进行稀土元素分析为目的,在已建立的激光荧光寿命测量装置的基础上,建立了时间分辨激光荧光光谱测量装置。结合在此装置上取得的初步实验结果,对装置的原理、特点、使用范围、工作条件进行了讨论。为进行稀土元素的时间分辩激光荧光光谱分析建立了必要的手段,也为某些发光材料的发光动力学研究提供了一定的方法。     

Photochemistry and Photophysics of the 3‐Styrylidenebenz[e]indanes

The photochemical and photophysical properties of the extended conformers of trans ‐ and cis ‐1‐(2‐naphthyl)‐2‐phenylethenes (t ‐ and c‐ NPE_B ) are strikingly different than those of their rigid analogues, trans ‐ and cis‐ 3‐styrylidenebenz[e]indane (t ‐ and c‐ BPE ). The fluorescence quantum yield and lifetime at 25°C in methylcyclohexane drop from 0.76 and 22.2 ns in t ‐NPE_B to 0.0051 and 0.25 ns in t‐ BPE , and there are complementary changes in the photoisomerization quantum yields. In both cases, photoisomerization occurs in S₂, a stilbenic excited state. The differences in behavior are traced to the diminished S₁/S₂ energy gap on the alkyl substitution afforded by the five‐membered ring in BPE . The effect of viscosity on the torsional relaxation of t ‐BPE , evaluated in glycerol/methanol mixtures at 25°C and in 95/5 and 99.9/0.1 glycerol/methanol (% v/v) as a function of T , is well accounted for by the medium‐enhanced barrier model. Absorption and fluorescence spectra of t ‐BPE in pure glycerol reveal aggregate formation. Quenching of t ‐BPE fluorescence in methylcyclohexane by tri‐n ‐butylamine (TBA ) leads to exciplex fluorescence. Comparison with analogous results for t ‐NPE_B suggests that charge transfer involves primarily the naphthalenic S₁ state in t ‐NPE_B and the stilbenic S₂ state in t ‐BPE .     

Fluorescence of phthalocyanines: Emission from an upper excited state

The weak (φ_f < 10³) fluorescence at around 430 nm of the S² upper excited singlet state of metal-free phthalocyanine and metallophthalocyanines is presented. Polarization measurements indicate that the emission is short-lived (< 800 ps) contrary to the intense (φ_f > 0.3) normal emission at around 700 nm originating from S¹ and having a lifetime in the 4.1 to 10.6 ns range, depending on the solvent. The short wavelength emitting S² excited state has been populated by a two photon absorption process using the excitation light at 695 nm of a pulsed ruby laser. This process is shown not to involve the triplet state but the following stepwise two photon absorption process: .     

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.     

THE FLUORESCENCE PROPERTIES OF ORTHO AMINOBENZOATE ANESTHETICS IN DEFINED SOLVENTS AND PHOSPHOLIPID VESICLES

Abstract— The fluorescence properties of three ortho aminobenzoate local anesthetics have been determined in a variety of solvents. Results from these studies have been used to deduce how these drugs interact with phosphatidylcholine bilayers. The emission energy, fluorescence quantum yield and lifetime exhibited a biphasic dependence on solvent polarity. In aprotic solvents, alcohols and in ethanol-water mixtures containing less than 40% water, quantum yields and lifetimes were high (approximately 0.55 and 8.5 ns respectively). In ethanol-water mixtures containing >40% water, the strong fluorescence quenching was primarily due to an increase in the rate of non-radiative deactivation of the excited state. Both the radiative ( k_r ) and non-radiative ( k_nr ) rate constants show a biphasic dependence on solvent polarity. These studies suggest the presence of two singlet excited states for these molecules, a polar singlet excited state, S_1-p and a charge transfer excited state, S_1-ct with the latter predominating in ethanol-water mixture containing >40% water. In egg phosphatidylcholine bilayers, the fluorescence, lifetime and quantum yield are consistent with the view that these drugs are localized within the lipid head group region where the charge-transfer excited state can be stabilized by intermolecular hydrogen bonding.     

Electric field effects on photoinduced electron transfer processes of methylene-linked compounds of pyrene and N,N-dimethylaniline in a polymer film

Time-resolved measurements of the electric-field-induced change in fluorescence intensity have been made for methylene-linked compounds of pyrene and N,N-dimethylaniline (DMA) doped in a polymer film. The lifetime of the fluorescence emitted from the locally excited state of pyrene chromophore becomes shorter in the presence of electric field (F), when the dopant concentration is high. The lifetime of the excipelx fluorescence resulting from the photoinduced electron transfer (PIET) from DMA to the excited state of pyrene chromophore between different molecules also becomes shorter in the presence of F. Based on the simulation of the electric field effect on fluorescence decay, the mechanism of intermolecular PIET between DMA and pyrene chromophore in a polymer film is discussed.