共查询到20条相似文献,搜索用时 15 毫秒
1.
By the delayed fluorescence activated by direct multiphoton excitation of triplet molecules by CO 2–laser radiation we have studied the prevailing deactivation pathways of triplet molecules with a high store of vibrational energy E
vib. The dependences of the kinetic characteristics of delayed fluorescence on the presence of vapors and foreign gases have been used to estimate the rates and efficiencies of intermolecular vibrational relaxation in the vibrational quasi–continuum of the triplet state T
1. By the changes in the intensities and decay rates over a wide range of vibrational energies we have established the E
vib dependences of reversible intercombination conversion between the states T
1 and S
1 and interconversion from T
1 to the ground electronic state S
0 for both the case of isolated excited molecules and at a steady vibrational temperature. It is shown that at high vibrational temperatures the radiationless transition from the T
1 state to S
0 has an activation character and is accomplished through the energy barrier. The conditions for going to an exponential dependence have been determined. It has been found that the obtained dependences are in good agreement with the known experimental results. The influence of molecular and environmental characteristics on the decay rate of triplet molecules is compared. 相似文献
2.
B-Type delayed fluorescence of 1,2,3,4-dibenzanthracene in PMMA was experimentally observed for the first time. Dibenzanthracene molecules were exerted in a two-step process. In the first step, an excited singlet S 1 is created, which undergoes intersystem crossing to T 1 then T-T absorption creates an excited triplet dibenzanthracene molecule, which returns to the first exerted singlet level by intersystem crossing. The recreated first excited singlet of dibenzanthracene decays back to the ground state by emitting this new type of delayed fluorescence. 相似文献
3.
The intermolecular photoinduced reactions between triplet ketone molecules and aliphatic amines and pyridine are studied by the quenching of delayed fluorescence of anthraquinone and benzophenone vapors by diethylamine, dibutylamine, cyclohexylamine, triethylamine, and pyridine. In the temperature range 423–573 K, the delayed fluorescence quenching rate constants k q are estimated from changes in the decay rate constant and the intensity of delayed fluorescence upon increasing pressure of bath gases. It is ascertained that, in the gas phase, the mixtures under study exhibit both a negative and a positive dependence of k q on temperature, which indicates that some photoinduced reactions do not have activation barriers. The rate constant k q is shown to increase with decreasing ionization potential of the electron donors. This points to the importance of interactions with charge transfer in the photoreaction of triplet ketone molecules with aliphatic amines and pyridine in the gas phase. The relationship between k q and the change in the free energy Δ G upon the photoinduced intermolecular electron transfer, which is the primary stage of the photochemical reaction, is studied. It is shown that the dependence k q (Δ G) for the donor-acceptor pairs under study is described well by the Marcus equation, in which the average vibrational energies of the donor and acceptor are taken into account for the estimate of Δ G. 相似文献
4.
B-type of delayed fluorescence was observed for the first time for rubreneperoxide. Rubreneperoxide molecules were excited in a two step process. In the first step an excited singlet S 1 is created, which undergoes intersystem crossing to T 1; then T-T absorption creates an excited triplet rubreneperoxide molecule, which returns to the first excited singlet level by intersystem crossing. The recreated first excited singlet of rubreneperoxide decays back to the ground state by emitting B-type of delayed fluorescence. 相似文献
5.
Intermolecular vibrational relaxation is studied in mixtures of polyatomic molecules (benzophenone and fluorene) with bath gases after multiphoton excitation of the triplet molecules by CO 2 laser radiation. The dependences of the decay rate and the intensity of laser-induced delayed fluorescence on the laser energy density E CO2 and pressure P fg of bath gases are analyzed. They are found to be different for the fast and slow components of delayed fluorescence, which decays nonexponentially. It is shown that a change in the decay rate of the fast fluorescence component with increasing pressure P fg is governed by the properties of vibration-translation relaxation. The efficiency β of this process is estimated in a broad range of vibrational energies. It is found that β weakly changes with increasing E vib upon excitation of molecules to high vibrational levels. The features of intermolecular vibrational relaxation at high densities of anharmonically coupled vibrational states are discussed. 相似文献
6.
Transient signals in the fluorescence intensity induced by microwaves can be used to acquire quantitative information on the molecular rates of populating and depopulating of the lowest triplet state of organic molecules. This method seems especially promising when the molecule emits no detectable phosphorescence. Part of this paper concerns the introduction of an appropriate mathematical model for describing the fluorescence transients. The model is formulated in conjunction with a specific series of experiments from which one may evaluate the molecular quantities in a systematic way. The kinetics of the lowest triplet state of free base porphin (H 2P) are determined explicitly by this method. In the discussion the results for H 2P are compared with those for Zn porphin. A satisfactory qualitative explanation based on the theory of radiationless transitions can be given for the great difference in behaviour of these two porphyrin molecules. Short attention is given to effects due to the photochemical shift of the two H atoms in the centre of H 2P, which is proven to occur even at a temperature as low as 1·3 K. 相似文献
7.
Long-term luminescence of organic dyes (xanthene dyes, halogen substituted fluoroscein) was used for an in vitro study of the photodynamic effect of exogenic probes in malignant tumors and healthy tissues of mice. It is shown that the photodynamic activity of oxygen and the dynamics of its concentration in tissues can be estimated from the delayed fluorescence of exogenic probes caused by singlet–triplet annihilation of singlet oxygen and excited triplet states of the molecules of photosensitizer dyes. It is found that quenching of long-term luminescence of photosensitizers significantly differs in tumors and normal tissues. 相似文献
8.
The spectral and kinetic properties of photoluminescence of mixed Langmuir-Blodgett (LB) films of eosin decyl ether and palmitic acid on a solid substrate are studied. The electronic absorption and fluorescence spectra of the films are identical to the spectra of the dye in ethanol. An increase in the dye concentration in a monolayer results in the appearance of a dimer absorption band, quenching of fluorescence of monomers, and the red shift of the spectral bands. At 90 K, the distinct phosphorescence and delayed fluorescence bands of LB films were observed. The decay kinetics of phosphorescence and delayed fluorescence is nonexponential. It is shown that the decay curve of delayed fluorescence is determined by triplet-triplet annihilation (TTA) and T 1→S 1 triplet-singlet intersystem crossing (IS). The initial nonexponential phosphorescence decay is caused the dominant contribution of TTA to the decay of triplet molecules. The experimental data are interpreted based on the mechanisms of exchange-resonance and inductive-resonance annihilation. 相似文献
9.
We studied the annihilation kinetics for triplet excitations in isolated pairs of organic molecules and their delayed fluorescence
decay using a new mathematical model for the process. According to this model, the intensity of delayed annihilation fluorescence
(DAF) for isolated pairs is directly proportional to the number of pairs in which both molecules are found in the excited
triplet state. We show that the decrease in such pairs and the decay of their delayed annihilation fluorescence occur according
to an exponential law in the absence of random scatter in the deactivation rate constants for the triplet excitations. The
results of an experimental study of the DAF decay kinetics for 1,12-benzoperylene in n-hexane at 77 K, where triplet–triplet
annihilation occurs in isolated pairs, confirm the validity of the theoretical conclusions. 相似文献
10.
Abstract The fluorescence of anthracene vapors and its derivatives initiated by triplet-triplet excitation is observed. The quantum yield of inverce intersystem crossing of anthracene molecules is estimated (10 ?2). The fluorescence initiated by triplet excitation is quenched by foreign pentane gas, thus permitting an estimation of the lifetime of highly excited triplet molecules. 相似文献
11.
Experiments have been performed to determine the path of entry into and exit from the phosphorescent triplet state T 0 of quinoxaline in a durene host. First of all the decay of phosphorescence after flash excitation was followed at 4.2 and 1.34 °K. It was found that for both perdeutero- and perhydroquinoxaline the lifetime is shortened by a factor of about three when the temperature is lowered from 4.2 to 1.34 °K. At 1.34 °K relaxation between the spin components (i.e. re-orientation of the triplet spin angular momentum) is slow relative to the decay, and the observed reduction in lifetime indicates that entry into and exit from T 0, are through the same spin component. Similar decay experiments were then carried out at 1.34 °K in a 10 kG magnetic field or in a somewhat weaker field so chosen that the effect of microwave saturation of one of the E.S.R. transitions between the components could be observed. From the results it follows that on intersystem crossing the molecules enter the manifold T 0 through the top zero-field component and thus initially have their spins aligned. Decay departs almost exclusively from the same component, even in the case of perhydroquinoxaline, where at least 45 per cent of it must be radiationless. The decay route agrees with out-of-plane polarization of phosphorescence for the free molecule. Finally, spin alignment on intersystem crossing is discussed from the theoretical point of view. It appears that the phenomenon is clear-cut only in molecules such as those of the aza-aromatics, where strong spin-orbit coupling of the atomic type occurs betwen ππ? and nπ? states. 相似文献
12.
A gain in detection sensitivity of more than three orders of magnitude is achieved in high-resolution solid-state 2H nuclear magnetic resonance of monocrystalline fluorene-d 10 by applying optical nuclear polarization via excited triplet states of acridine-h 9 guest molecules. The sensitivity gain is utilized to measure the angular dependence (rotation pattern) of the 2H nuclear magnetic resonance lines. In this way the principal values and orientations of all 2H quadrupolar tensors are determined. Except for the methylene deuterons, all tensors belonging to the same molecule have one principal axis in common, namely the axis perpendicular to the molecular plane, showing that in the crystal lattice the fluorene molecule is in a planar configuration. 相似文献
13.
Excited singlet ( S
1) and triplet ( T
1) state quenching by O 2 and by (O 2 + H 2O) gas-vapor mixtures was studied in the gas phase for polycyclic aromatic hydrocarbons (PAHs, anthracene, 2-aminoanthracene,
pyrene). Addition of water vapor is shown not to influence quenching of both fluorescence and delayed fluorescence of PAHs
by oxygen. The role of complexes stabilized by charge transfer and hydrogen bonds in quenching the excited states of PAHs
by atmospheric gases was analyzed.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 3, pp. 342–348, May–June, 2009. 相似文献
14.
Amino-acyl-quinoxalinone yellow dyes are cyclised analogues of the yellow azomethine dyes developed for, and still used in, silver halide colour photography. Unlike image azomethine dyes, which are rapidly deactivated in their excited states by torsion about the azomethine bond, amino-acyl-quinoxalinone dyes have an interesting photophysics because torsion is not possible due to their cyclised structure. We report results from studies on singlet and triplet state properties, and singlet oxygen yields, of the yellow dye, 7-diethylamino-3-(2,2-dimethyl-propionyl)-5-methyl-1-phenyl-1H-quinoxalin-2-one, in polar and nonpolar solvents. The dye photophysics is characterised by a weak fluorescence, with a solvent dependent emission yield (ΦF?≈?0.002–0.004), and short singlet state lifetime (τexpt?≈?20–50 ps), both increasing by a factor of ≈2 in going from polar acetonitrile to non-polar dioxane as solvent. DFT ZINDO calculations show a transition involving significant electron transfer from the diethyl-amino group into the carbonyl region of the molecule. In solution, in the presence of oxygen, the triplet state decays almost exclusively by oxygen quenching, and singlet oxygen is produced in high yield (Φ??≈?0.5–0.55). The triplet state absorbs across the 450–750 nm region with maxima around 480 and 650 nm, and moderate molar absorption coefficients (ca. 6000–8000 M?1 cm?1). In a glass at 77 K, triplet decay gives a red phosphorescence, with λmax?≈?640–650 nm, and a ?≈?0.25 s lifetime. If singlet oxygen yields are a good indication of triplet yields, then internal conversion and intersystem crossing occur with roughly equal efficiency. 相似文献
15.
Based on measurements of the time-resolved delayed fluorescence, the influence of universal interactions on the collisional vibrational energy transfer is studied in mixtures of vibrationally excited polyatomic molecules (acetophenone, benzophenone, and anthraquinone) with inert bath gases (Ar, C 2H 4, SF 6, CCl 4, and C 5H 12). From the dependences of the decay rates of delayed fluorescence on bath gas pressure, the efficiencies of the establishment of vibrational ( V-V relaxation) and thermal ( V-T relaxation) equilibrium after excitation of molecules into the vibrational quasi-continuum of a triplet state are estimated. The main emphasis is on studying the dependences of the efficiency of collisional vibrational energy transfer on temperature and the molecular characteristics of collision partners. It is found that the efficiency increases with the complication of the structure of bath gases for the V-V process and decreases upon the increasing of their mass for the V-T process. For the temperature range 273–553 K, the negative temperature dependence of the V-V transfer probability and the positive (Landau-Teller) dependence of the V-T transfer probability were obtained. It is concluded that the above regularities reflect the dominant role of long-range attractive forces in initiating the quasi-resonant V-V transfer and of short-range repulsive forces in the V-T transfer of vibrational energy. 相似文献
16.
The metallophilic bond is a weak interaction between closed‐shell ions and has been widely used a probe for various sensing of toxic chemicals for environmental safety concerns. Here, the interaction between Au nanoclusters (NCs) and metallic ions (mercury (Hg 2+) and copper (Cu 2+) ions) is explored using steady‐state and time‐resolved luminescence and transient absorption measurements. For Hg 2+ ions, the delayed fluorescence (DF) of bovine serum albumin (BSA) protected Au 25 (Au 25@BSA) NCs is quenched via an effective triplet state electron transfer through the metallophilic bond. However, the Cu 2+ ions do not alter the DF in Au 25@BSA NCs because of the absence of the metallophilic interaction. Furthermore, for Au 8@BSA and Au 10@histidine, in which there are no Au + ions on the surface, the fluorescence is not quenched by Hg 2+ ions. Such a novel triplet electron transfer process through metallophilic bonds are observed and reported for the first time. The reduction of the reverse intersystem crossing is the crucial for Hg 2+ ion sensing in the fluorescent Au 25@BSA NCs. 相似文献
17.
Lifetimes of TPPS 4 (meso-tetraphenylporphine tetrasulfonate) triplet states were measured for liquid solutions of different acidity and viscosity and as a function of acceptor concentration for different acceptors (bovine serum albumin, tryptophan and furfuryl alcohol). Triplet lifetimes were estimated by monitoring the decay of TPPS 4 delayed fluorescence of E-type. The lifetime of delayed fluorencence depends on the concentration of O 2, since the latter is an effective quencher of the triplet state. The lifetime is shown to be influenced mainly by degree of aggregation state of TPPS 4 and, therefore, by the pH of the solution, decreasing with pH and for each pH remaining constant over a wide range of acceptor concentrations. The monomeric species is found to have the longest triplet lifetime in aqueous phosphate-buffered saline solution at neutral pH, especially when bound to albumin, despite of the low viscosity and protonated nature of the medium. 相似文献
18.
A sketch is presented of the path that has led from Zavoisky’s pioneering experiments to modern investigations by electron paramagnetic resonance (EPR) of the phosphorescent ( S = 1) triplet state of polyatomic molecules or ions. The group-theoretical method first introduced by Wigner in his analysis of the multiplets of atomic spectroscopy, likewise provides a key for understanding the zero-field splitting and selection rules for radiative decay of the phosphorescent triplet state. Examples to illustrate the progress made through EPR experiments are selected from three fields. (i) Conformational instability on excitation. Both the zero-field splitting and the electron spin density distribution provide unique fingerprints of a triplet state’s geometry — structural information of a kind that is nonexistent for singlet states! Illustrations are provided by benzene C 6H 6 and fullerene C 60. (ii) The optical pumping cycle. The spin selectivity of singlet-to-triplet intersystem crossing and radiative decay of the individual spin components of the triplet state is discussed. In practice this selectivity is put to advantage by performing EPR on triplet states in zero-field by means of optical detection. In turn, such experiments have led to a detailed insight into the spin-orbit coupling mechanisms responsible for the spin selectivity of the above processes. The high sensitivity attainable with optical detection has recently culminated in EPR experiments on single molecules. (iii) Quantum interference. In a triplet state of low symmetry two of the spin sublevels may decay to the ground state by the emission of photons of a common polarization (i.e., out of plane for an aromatic hydrocarbon). In such a situation quantum interference between the two decay channels can be induced by an appropriate preparation of the excited state. An example is shown where flash-excitation in the singlet manifold followed by rapid intersystem crossing causes the S = 1 spin angular momentum to be created in a spin state which is not an eigenstate of the zero-field splitting tensor. This nonstationary character of the initial triplet state, which reflects the spin-orbit coupling pathway, is observed through the detection of a spontaneous microwave signal following the 25 ps laser flash. 相似文献
19.
The advantages and disadvantages of electrogenerated chemiluminescence (ECL) as a method for estimating triplet energies ET of organic compounds are demonstrated in several examples involving strong, weak or non-luminescent compounds. It is shown that in many cases ET can be determined within an error of ±0.1 eV from the thermodynamic relationships between electrochemical and spectroscopic data, from ECL quenching or from sensitized ECL. The method can also be successfully applied to substances in which phosphorescence and delayed fluorescence investigations have failed. It was found that formation of exciplexes and irreversible reactions of the ion radicals may lead to misinterpretation of the results. In such cases, additional measurements were carried out to confirm the interpretation of the triplet mechanism and to rule out chemical complications. 相似文献
20.
We propose a procedure for determining the contribution from the concentrations of triplet molecules and their interaction
constants to the intensity of delayed annihilation fluorescence of isolated molecular pairs. We have used the proposed procedure
to determine the contribution of these quantities to the intensity of the components in the doublet in the quasi-line delayed
fluorescence spectrum of coronene in n-octane at 77 K. 相似文献
|