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1.
Photoinduced effects on the electron spin polarization (ESP) in the lowest excited triplet (T 1) states of porphyrins (PORs) and phthalocyanines (PCs) have been observed with a two-color time-resolved (TR) electron paramagnetic resonance (EPR) technique in a glassy matrix at low temperatures. On single-color excitation with the wavelengths of the ground state absorptions of PORs and PCs, polarized EPR spectra due to the corresponding T 1 state were observed. The polarization patterns match well with interpretation as anisotropic intersystem crossing (ISC) induced by the spinorbit coupling between the singlet excited (S 1) and the triplet states. In contrast, two-color excitation led to a change of the phase of the T 1 state polarization pattern to the opposite. The observed ESP in the T 1 state resulting from the excitation to the upper triplet state (T n ) was interpreted in terms of anisotropic ISC between the T n and S 1 states. From the analysis of the ESP, changes in the quantum yields of the reverse ISC processes were determined at different temperatures. The results could be best interpreted by the existence of thermal pathways with small activation energy in the relaxation processes. 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.
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
4.
A thin film of triphenylamine dimer, N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)-benzidine (TPD), doped with fac tris(2-phenylpyridine) iridium (Ir(ppy) 3) and platinum octaethyl porphine (PtOEP) is characterized by photoluminescence emission measurements at several excitation wavelengths and photoluminescence excitation measurements at relevant emission wavelengths in the temperature range from 10 K to room temperature. The investigated film is a phosphorescent OLED material with singlet absorbing host (TPD) and triplet emitting guests [Ir(ppy) 3 and PtOEP]. At short wavelength excitation simultaneous triple band emission from singlet TPD, triplet Ir(ppy) 3 and TPD, and from triplet PtOEP is observed. Förster-type singlet-singlet state energy transfer from TPD to Ir(ppy) 3 and PtOEP, intra-component intersystem crossing, and Dexter-type triplet-triplet energy transfer between the substituents are studied. 相似文献
5.
The dynamic nonlinear absorption of a chloroform solution of chlorophyll A was investigated using the Z-scan technique with
picosecond pulses at 532 nm. The nonlinear absorption exhibits a reverse saturation, indicating a strong intersystem crossing
(singlet–triplet) process. The time evolution of the optical nonlinearity, modeled by means of a five-level energy diagram,
allows the determination of excited-state cross sections and the lifetime of the intersystem crossing based on its absorption
characteristics and efficient formation of triplet states. Chlorophyll A was found to be a good candidate for a sensitizer
in photodynamic therapy.
Received: 14 February 2002 / Published online: 2 May 2002 相似文献
6.
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
7.
Quantum yields of pyrene fluorescence and bis[2-(2′-benzothienyl)-pyridinato-N,C 3′](acetylacetonate)iridium [Btp 2Ir(acac)] phosphorescence upon excitation via a matrix of poly-N-epoxypropylcarbazole (PEPC) or poly-N-epoxypropyl-3,6-dibromocarbazole (DBrPEPC), respectively, were found to be lower than those for the compounds directly excited in a polystyrene (PS) matrix. It was established that the energy in PEPC was transferred to an acceptor by both singlet excitons (by migration and long-range dipole–dipole interaction) and triplet excitons (through migration and short-range exchange electron interaction); however, only by triplet excitons in DBrPEPC, which did not show any fluorescence. The energy-transfer efficiency in PEPC by singlet excitons was higher than by triplet excitons. The observed effects were explained by the fact that energy transfer to the acceptor competed with such processes as localization of the excitons in the “tail” energy states, dissociation of singlet excitons into geminal electron–hole pairs (EHP), and capture of triplet excitons by polymer oxidation products. 相似文献
8.
The spin selectivity of electron transfer in a series of metalloporphyrin pyridyl-linked naphthalenediimides (MTPP-Pyr(CH 2)
n
NDI, where M = Zn, n = 2, 4, 7, and M = Al(OCOPh), n = 7) is studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy in the nematic liquid crystal 4-cyano-4′-pentylbiphenyl
(5CB). Following pulsed laser excitation, all of the complexes show a narrow antiphase doublet that is assigned to the triplet
state of the radical pair MTPP •+NDI •−. Initially, the antiphase doublet has an emission/absorption (E/A) polarization pattern characteristic of singlet electron
transfer. At later times the polarization inverts to an A/E pattern. The intensity of the late signal depends very strongly
on the nature of the metal in the porphyrin. A qualitative model that rationalizes this result is presented. It is proposed
that both singlet and triplet electron transfer occur in the dyads and that the differences in the intensity of the polarization
are the result of differences in the spin selectivity of intersystem crossing for the different metals. The consequences of
this model for magnetic field effects in such systems are briefly discussed. 相似文献
9.
In a recent publication, Lobsiger et al. [Phys. Chem. Chem. Phys. 12, 5032 (2010)] presented infrared and electronic absorption spectra of supersonic jet-cooled 5-methyl-2-hydroxypyrimidine (5M2HP), the enol form of deoxythymine. In addition, they reported on the fast nonradiative decay of the S 1 population to a dark state. In the present paper, we have investigated the mechanism and rate constants of this nonradiative decay by means of quantum chemical multi-configuration methods. To this end, minima of the lowest excited singlet and triplet states as well as the minimum-energy crossing point of singlet and triplet potential energy hypersurfaces (PEHs) have been determined employing a numerical DFT/MRCI gradient where DFT/MRCI stands for a combination of density functional theory (DFT) and a semi-empirical multi-reference configuration interaction (MRCI) approach. Rate constants have been calculated in the Condon approximation using a time-dependent approach based on harmonic oscillator functions and electronic spin–orbit coupling matrix elements evaluated at the DFT/MRCI level. It is shown that the first excited triplet state possesses 3( n?→?π*) character in the gas phase. Fast intersystem crossing is mediated by the low-lying 3(π?→?π*) state whose PEH crosses both, the S 1 1( n?→?π*) and T 1 3( n?→?π*) PEHs. 相似文献
10.
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
11.
Single molecule spectroscopy requires molecules with low triplet yields and/or short triplet lifetimes. The intersystem crossing (ISC) rate may be dramatically enhanced by the host matrix. Comparing the fluorescence intensity of single terrylene molecules in para-terphenyl, naphthalene, and anthracene crystals, we found a reduction of the saturation intensity by three orders of magnitude in the latter case. The fluorescence autocorrelation function indicates that the bottleneck state is the terrylene triplet. We propose a ping-pong mechanism between host and guest. This intermolecular ISC mechanism, which can open whenever the host triplet lies lower than the guest singlet, was overlooked in previous single molecule investigations. 相似文献
12.
The effects of protonation on the excited states of trans-3-styrylpyridine (StP) and trans-4,4′-dipyridylethylene (DPE) have been studied through measurements of the time-resolved electron paramagnetic resonance (EPR), ultraviolet absorption, and fluorescence spectra in methanol-water mixtures at 77 K. The assignment of the transient EPR signals was carried out with the aid of the stretched poly(vinyl alcohol) films method. From the analysis of these spectra it is concluded that the single protonation appears to have little effect on the zero-field splitting parameters and the anisotropy in the sublevel populating rates of the lowest excited triplet (T 1) states of StP and DPE. However, the decay rate constants of the fluorescent states decrease and fluorescence quantum yields increase on single protonation. These experimental results suggest that the single protonation causes a decrease in the intersystem crossing (ISC) rates for the three T 1 sublevels. These results are explained in terms of the vibronic mixing between the 1nπ* and 1ππ* states in the lowest excited singlet state. The assignment of StP to the specified conformer was carried out through the analysis of the anisotropic ISC processes. 相似文献
13.
Time-resolved EPR spectra are reported for porphyrin-quinone-quinone and porphyrin-porphyrin-quinone triads obtained after photoexcitation in the nematic and soft glass phase of liquid crystals. Spin-polarized EPR spectra were observed for the triplet states of the porphyrin created by spin-selective intersystem crossing (ISC) from the excited singlet state and those of the charge-separated radical pair states (RP) generated by electron transfer (ET) processes. The EPR polarization patterns of the RP are discussed in terms of the favored decay channel of the photoexcited singlet state of the porphyrin donor. The decay pathway may either be singlet ET to the quinone(s) followed by singlet/triplet mixing to yield RPs with triplet character or triplet ET after ISC from the porphyrin singlet to the triplet state, or a superposition of both pathways. It is demonstrated that the nature of the linking bridge between donor and acceptor, i.e., aliphatic cyclohexylene or aromatic phenylene, significantly influences the ET mechanism and thus the polarization patterns of the RP spectra. Using liquid crystals, information about the orientation of the guest molecules in the liquid crystal matrix with respect to the long axes of the liquid crystal molecules can be obtained. In the porphyrin-porphyrin-quinone triads the energy and ET processes strongly depend on the type of metallation of the porphyrins, specifically, whether the distal, the vicinal or both porphyrins bear a zinc atom. 相似文献
14.
Higher singlet states can play an important role in various intramolecular processes. Recent investigations of the time-resolved
(with a picosecond resolution) spectra of the dual fluorescence of 3-hydroxyflavone molecules excited in the region of the
S
1 and S
2 absorption bands by pulses with a duration of ∼44 ps have directly shown the occurrence of the proton transfer from the carboxyl
to the carbonyl group of the molecule upon excitation into the second singlet absorption band. The reaction times estimated
from the emission characteristics are comparable with the electronic level lifetime (several picoseconds), as a result of
which the direct measurements are rather difficult. The proton transfer through the S
2 state is also recorded in the steady-state fluorescence excitation spectra. In this study, it is shown how the reaction rate
can be estimated from these data. 相似文献
15.
The photochemistry of suprofen (SPF) was investigated by femtosecond transient absorption (fs‐TA), resonance Raman (RR) and nanosecond time‐resolved resonance Raman (ns‐TR 3) spectroscopic methods to gain additional information so as to better elucidate the possible photochemical reaction mechanism of suprofen in several different solvents. In neat acetonitrile (MeCN), the fs‐TA and ns‐TR 3 experimental data indicated that the lowest lying excited singlet state S 1 (nπ*) underwent an efficient intersystem crossing process (ISC) to the excited triplet state T 3 (ππ*), followed by an internal conversion (IC) process to T 1 (ππ*). In the aqueous solution, a triplet biradical species ( 3ETK‐1) was obtained as the product of a decarboxylation process from triplet suprofen anion ( 3SPF −) and the reaction rate of the decarboxylation process was determined by the concentration of H 2O. A protonation process for 3ETK‐1 leads to formation of a neutral species ( 3ETK‐3) that was directly observed by ns‐TR 3 spectra, then this 3ETK‐3 species decayed via ISC process to generate final product. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
16.
合成了掺杂三价铽离子的邻苯二甲酸锶发光材料。测定了材料的热稳定性、晶体结构及紫外吸收光谱、激发光谱和发射光谱。研究了晶体中邻苯二甲酸根单线态Sл、л1*、Sn、л1*跃迁吸收和由三线态Tл、л1*、Tn、л1*到铽(Ⅲ)离子5D4能级的能量传递机理。 相似文献
17.
The photocurrent-action spectrum of an anthracene crystal doped with tetracene (in a concentration of 10 ?5) exhibits the tetracene singlet exciton absorption spectrum. The two Davydov components of the first excited state are found at 2.39 and 2.47 eV. The charge carrier generation process involves intersystem crossing from guest to host resulting in anthracene triplet excitons and hole detrapping in the host lattice. A lower limit of the intersystem crossing rate kisc = 3 × 10 4sec?1 is determined from the ratio of photocurrents in the guest and host absorption regions. 相似文献
18.
A series of results from different photophysical experiments on indium and gallium phthalocyanine (Pc) compounds is reported. Gallium Pc's have much longer singlet and triplet excited state lifetimes in comparison with indium Pc's. The strong reverse saturable absorption observed at 532 nm excitation is a consequence of the increase in absorbance of Pc's in the triplet excited state in the optical window comprised between Q- and B-bands, as verified upon laser irradiation with ns pulses at 355 nm. Using C 60 as a reference, the intersystem cross quantum yields of tBu 4PcInCl and tBu 4PcGaCl are 0.70 and 0.36, respectively. 相似文献
19.
0.4 Se 0.6 microcrystals was investigated by using time-resolved differential transmittance spectroscopy. The electron trapping at microcrystal–glass
interfaces was found to occur within less than 1 ps after photoexcitation. At low excitation energy density, the excited electrons
are trapped at point defects distributed over the nanocrystal interfaces. Such electrons give rise to long-lived photoinduced
absorption with a lifetime of 3.2 ns. On the other hand, at high excitation energy density, transient absorption with a fast
(60 ps) and simultaneously a slow decay component (3.2 ns) was observed. This short-lived photoinduced absorption is attributed
to the electrons trapped at the shallow trap states of the semiconductor–glass interfaces.
Received: 11 September 1996/Accepted: 9 December 1996 相似文献
20.
A review of data on the photophysics of carotenoids is presented. Results of investigations of spectroscopic, temporal, and
energy parameters of excited S 1 and S 2 singlet states of β-carotene and related compounds are critically examined. These states give rise to extremely high probabilities
(10 11–10 13 sec −1) of radiationless deactivation of the electronic excitation energy in carotenoids. Results of investigations of photophysical
properties of triplet states of carotenoids are considered mainly from the standpoint of quenching of singlet oxygen and triplet
states of organic molecules by carotenoids.
Institute of Molecular and Atomic Physics, Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated
from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 5–19, January–February, 1997. 相似文献
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