Showing papers on "Triplet state published in 1980"
TL;DR: In this article, the absorption spectra show that there is little interaction between the metal and the porphyrin π-system so that the excited states can be regarded as (ππ*) in nature.
Abstract: Metal-free and ZnTPP complexes exhibit both fluorescence and low temperature phosphorescence, MnIITPP shows only low temperature phosphorescence while NiIITPP is non-luminescent. The absorption spectra show that there is little interaction between the metal and the porphyrin π-system so that the excited states can be regarded as (ππ*) in nature. Where phosphorescence occurs, the Stokes shift is 3900 ± 100 cm–1 and decreases with increased metal–porphyrin interaction. The rate constants for intersystem crossing and for non-radiative decay from the lowest (ππ*) triplet state depend upon spin–orbital coupling properties. ZnII and NiI exhibit fairly weak spin–orbital coupling but the effectiveness of MnII is enhanced by the paramagnetic character and by favourable geometry. NiIITPP possesses a low energy (dd) excited singlet state which facilitates rapid internal conversion within the singlet manifold.
188 citations
TL;DR: In this paper, a laser flash photolysis study of the reaction is proposed to proceed via a short-lived biradical which leads to an enol, which is responsible for product formation.
Abstract: Photolysis of solutions of o-nitrobenzaldehyde leads to the formation of o-nitrosobenzoic acid with quantum yields in the neighborhood of 0.5. The reaction proceeds via the intermediacy of a triplet state with a lifetime of 0.6 ns. A laser flash photolysis study of the reaction is proposed to proceed via a short-lived biradical which leads to an enol, which is responsible for product formation.
106 citations
TL;DR: The high quantum yield of formation and the pronounced magnetic field effect for the carotenoid triplet generated by direct excitation at 530 nm can be explained by assuming that this triplet is not formed by intersystem crossing, but by fission of the singlet excitation into two triplet excitations and subsequent annihilation.
106 citations
TL;DR: In this article, the ESR nutation signals following laser pulse excitation are analyzed for the case of the photoexcited triplet state of acridine in a molecular crystal matrix.
95 citations
TL;DR: Gaseous CS2 was dissociated at 193 nm into CS and S, and the translational and internal energy distributions of the CS fragments were measured as discussed by the authors, showing that the upper electronic state S3 is predissociative.
Abstract: Gaseous CS2 was dissociated at 193 nm into CS and S. The translational and internal energy distributions of the CS fragments were measured. There is now overwhelming evidence that the upper electronic state S3 is predissociative. In fact there are three upper states of importance, the initially excited S3, a state which dissociates to CS(X 1Σ) and S(1D) and a triplet state which dissociates to CS(X 1Σ) and S(3P). The CS fragments were rotationally excited with an average rotational energy ∼3.5 kcal/mole. The vibrational populations were also strongly inverted for both the 1D and the 3P dissociations and their surprisal plots were linear. CS fragments were found with v?7. Of the dissociations resulting in CS fragments with v?6, 80±10% of the S atoms are produced in the 1D state and 20±10% in the 3P state.
94 citations
TL;DR: It is concluded that, for excited chlorophyll alpha, the relative rates of intersystem crossing to the triplet state and of fluorescence emission are the same in vivo as in organic solvent.
92 citations
TL;DR: In this paper, a very short-lived (≈ 30 μs) triplet state of pentacene in naphthalene was obtained by combining the electron spin-echo technique with pulsed-laser excitation.
89 citations
TL;DR: In this paper, it was shown that an important factor contributing to the previously reported excitation wavelength dependence of φF and φT1 (φisc) for nucleic-acid components is the equilibrium coexistence of H-bonded and non-H-Bonded forms each having different photophysical properties.
Abstract: — Low-temperature (and some room temperature) absorption and emission, fluorescence and phosphorescence, data including quantum yields and lifetimes have been obtained from the title pyrimidine bases as a function of the nature of the solvent environment. Modest vibrational resolution has been observed for the first time in the absorption spectra, particularly for thymine and uracil. The excitation spectra also show structure. The quantum yields of fluorescence (φF) and phosphorescence are independent of the excitation wavelength. Thymine, thymidine and uracil have profoundly different photophysical properties in polar-aprotic vs polar-protic solvents. The N, N-dimethyl substitution of thymine and uracil produces photophysical changes comparable to the solvent change for the unsubsti-tuted bases. The species involved in the emission processes is the keto (lactam) form. It is probable that 1,3(n,π*) state(s) has(have) changed order relative to a lowest 1(π,π*) state as a consequence of both the solvent change and N, N-dimethyl substitution. The lowest triplet state is assigned as 3(n π*). We propose that an important factor contributing to the previously reported excitation wavelength dependence of φF and φT1 (φisc) for nucleic-acid components is the equilibrium coexistence of H-bonded and non-H-bonded forms each having different photophysical properties. Consideration is given of the impact of the significantly different photophysical properties of nucleic-acid bases as a function of the nature of the solvent upon the photochemical properties.
67 citations
66 citations
TL;DR: In this paper, the authors observed that the helium clusters of either isotope, or neon clusters of some 104 atoms, are observed to be excited by the impact of electrons of 30 to 100 eV energy.
Abstract: Metastable electronic states of helium clusters of 106 to 108 atoms of either isotope, or of neon clusters of some 104 atoms, are observed to be excited by the impact of electrons of 30 to 100 eV energy. Being detected through the release of electrons which results from the impact of the excited neutral or ionized clusters onto the first dynode of an electron multiplier, these metastable excitations allow the time‐of‐flight spectrometry of speeds and sizes of large clusters. The energy dependence of the excitation probability of the helium clusters indicates the initial excitation of the metastable atomic triplet state while the observed flight times of about 10−3 s point to the metastable triplet molecular state as the final helium cluster excitation.
65 citations
TL;DR: In this paper, photolysis of benzophenone in the presence of an amine transferred an electron to an excited triplet state, forming an ion pair that is stable relative to diffusional separation.
Abstract: Studies were initiated utilizing picosecond (ps) absorption spectroscopy, to directly monitor the dynamics of electron transfer from 1,4-diazabicyclo(2.2.2)octane (Dabco) to the excited states of benzophenone and fluorenone. These two systems were chosen because of their contrasting photochemistry. The quantum yield for photoreduction of benzophenone in polar solvents is generally greater than 0.1, while that of fluorenone is zero. In polar solvents, the proposed mechanism dictates that an electron is transferred to the excited singlet state fluorenone, which then back-transfers the electron, regenerating ground-state fluorenone and amine. Photolysis of benzophenone in the presence of an amine transfers an electron to an excited triplet state, forming an ion pair that is stable relative to diffusional separation. The results of this study verify this proposal.
TL;DR: The structure and energies of the lowest triplet states of four isomers of H2CN+ have been determined by self-consistent field and configuration interaction calculations as discussed by the authors, and it is suggested that the triplet H2NC+ isomer may play a role in determining the relative yields of HCN and HNC from the reaction of C+ and NH3.
Abstract: The structures and energies of the lowest triplet states of four isomers of H2CN+ have been determined by self‐consistent field and configuration interaction calculations. When both hydrogen atoms are attached to the nitrogen atom, H2NC+, the molecule has its lowest triplet state energy, which is 97.2 kcal mol−1 above the energy of the linear singlet ground state. The structure has C2v symmetry, with an HCH bond angle of 116.8°, and bond lengths of 1.009 A (H–N) and 1.268 A (N–C). Other isomers investigated include the H2CN+ isomer at 104.7, the cis‐HCNH+ isomer at 105.3, and the trans‐HCNH+ isomer at 113.6 kcal mol−1. The H2CN+ isomer has an unusual ’’carbonium nitrene’’ structure, with a C–N bond length of 1.398 A. It is suggested that the triplet H2NC+ isomer may play a role in determining the relative yields of HCN and HNC from the reaction of C+ and NH3. Specifically, a triplet path is postulated in which C+ and NH3 yield the triplet H2NC+ isomer, which then yields the singlet H2NC+ isomer by phospho...
TL;DR: In this article, the extinction coefficients for the triplet absorption of benzophenone and naphthalene in benzene solution, and of anthracene in ethanol and cyclohexane solutions were determined.
TL;DR: In this paper, a triplet state of carotenoids has been observed in reaction centers of Rhodopseudomonas sphaeroides, strain 2.4.
Abstract: Triplet states of carotenoids have been detected by X-band electron paramagnetic resonance (EPR) and are reported here for the first time. The systems in which carotenoid triplets are observed include cells of photosynthetic bacteria, isolated bacteriochlorophyll-protein complexes, and detergent micelles which contain ..beta..-carotene. It is well known that if electron transfer is blocked following the initial acceptor in the bacterial photochemical reaction center, back reaction of the primary radical pair produces a bacteriochlorophyll dimer triplet. Previous optical studies have shown that in reaction centers containing carotenoids the bacteriochlorophyll dimer triplet sensitizes the carotenoid triplet. We have observed this carotenoid triplet state by EPR in reaction centers of Rhodopseudomonas sphaeroides, strain 2.4.1 (wild type), which contain the carotenoid spheroidene. The zero-field splitting parameters of the triplet spectrum are /D/ = 0.0290 +- 0.0005 cm/sup -1/ and /E/ = 0.0044 +-0.0006 cm/sup -1/, in contrast with the parameters of the bacteriochlorophyll dimer triplet, which are /D/ = 0.0189 +- 0.0004 cm/sup -1/ and /E/ = 0.0032 +- 0.004 cm/sup -1/. Bacteriochlorophyll in a light harvesting protein complex from Rps. sphaeroides, wild type, also sensitizes carotenoid triplet formation. In whole cells the EPR spectra vary with temperature between 100 and 10 K. Carotenoidmore » triplets also have been observed by EPR in whole cells of Rps. sphaeroides and cells of Rhodospirillum rubrum which contain the carotenoid spirilloxanthin. Attempts to observe the triplet state EPR spectrum of ..beta..-carotene in numerous organic solvents failed. However, in nonionic detergent micelles and in phospholipid bilayer vesicles ..beta..-carotene gives a triplet state spectrum with /D/ = 0.0333 +- 0.0010 cm/sup -1/ and /E/ = 0.0037 +- 0.0010 cm/sup -1/. 6 figures, 1 table.« less
TL;DR: In this paper, the exact ion yields and the kinetic parameters of ion formation and decay have been determined by transient conductimetry for two photochemical ionogenic reactions of zinc octaethyl porphyrin in a variety of inert organic solvents.
Abstract: Absolute ion yields and the kinetic parameters of ion formation and decay have been determined by transient conductimetry for two photochemical ionogenic reactions of zinc octaethyl porphyrin in a variety of inert organic solvents. One reaction (T–P) involves electron transfer in the encounter complex of the porphyrin triplet state (T) and ground state (P); it is relatively slow (kTP∼108M−1s−1, and solvent insensitive). The second (T–T reaction) involves reactive collision between two triplets; it occurs at the encounter limit (kTT∼1010M−1s−1). Neither rate constant depends on solvent dielectric constant. Reaction yields are very dielectric‐dependent, however, and provide unusually straightforward experimental access to the problem of geminate ion‐pair decorrelation. A two‐parameter model is presented in which the initial photochemically‐formed ion pair is created by electron tunneling in a specific spin state at nontrivial separation. The electron transfer radii for the two reactions are determined by co...
TL;DR: In this article, it was shown that the ability of catalytic fading via singlet oxygen mechanism is closely related to the quantum yield of triplet formation in commercial diaminoanthraquinonoid dyes.
Abstract: Taking various diaminoanthraquinones as model compounds for commercial anthraquinonoid dyes it was found that the ability of initiating catalytic fading via singlet oxygen mechanism is closely related to the quantum yield of triplet formation in these dyes.
The quantum yields of triplet formation show a marked difference between the various diaminoanthraquinones. Whereas 1, 4– and 1, 2–diaminoanthraquinones have very small quantum yields, the 1, 5– and 1, 8–derivatives pass into the triplet state with a fairly high efficiency. From this it was concluded that catalytic fading occuring via singlet oxygen mechanism must be expected in dye mixtures consisting of 1, 5– or 1, 8–diamino substituted anthraquinonoid dyes and easily oxidizable azo–compounds.
TL;DR: In this paper, the fate of excitation energy and electron transfer to quinones within Chl-a-containing phosphatidyl choline liposomes has been investigated.
Abstract: —
The fate of excitation energy and electron transfer to quinones within Chl-a-containing phosphatidyl choline liposomes has been investigated The bilayer membrane of the liposome stabilizes the Chl triplet state, as evidenced by a three-fold increase in the lifetime over that observed in ethanol solution The relative triplet yield follows the relative fluorescence yield, indicative of quenching at the singlet level Triplet state lifetimes are markedly shortened as the Chl concentration is increased, demonstrating that quenching occurs at the triplet level as well This process is shown to be due to a collisional de-excitation In the presence of quinones, the Chl triplet reduces the quinone resulting in production of long-lived electron transfer products The percent conversion of Chl triplet to cation radical when benzoquinone is employed as acceptor is approximately 60 ± 10%, which is slightly less than in ethanol solution (70 ± 10%) The lifetime of the radical, however, can be as much as 1900 times longer With respect to potentially useful photochemical energy conversion, the magnitude of this increased lifetime is far more significant than is the decreased radical yield
TL;DR: In this paper, the authors performed geometry optimizations of several low-lying diradical states of the ring-opened oxirane (·CH 2 CH 2 O·) by using the energy gradients of the UHF MINDO/3, STO-3G and 4-31G solutions.
TL;DR: In this paper, the triplet state quenching of a series of aromatic hydrocarbons by tetramethylpiperidine-Noxide in two solvents, acetonitrile and perfluoro n -hexane, is reported.
TL;DR: The authors showed conclusively that the primary photochemical act involves hydrogen abstraction from benzhydrol by the n,π* uncomplexed triplet state of benzophenone to give a triplet radical pair which does not couple to a measurable extent within the solvent cage.
TL;DR: In this article, the triplet-triplet absorption spectra of a series of carotenoid pigments in benzene solution have been determined by pulse radiolysis experiments and the natural lifetimes in deaerated solution have also been measured.
Abstract: — Triplet-triplet absorption spectra of a series of carotenoid pigments in benzene solution have been determined by pulse radiolysis experiments. The natural lifetimes in deaerated solution have also been measured. They fall in the range 2–10 µs as found for other carotenoids under similar conditions. Pulsed laser (337 nm) excitation of benzene solutions containing oxygen, carotenoid and a photosensitized molecule (anthracene) showed the generation of absorption spectra of the triplet states. These absorptions decayed by first order kinetics in such a way as to indicate that they were formed in reactions with singlet oxygen, itself generated by interaction with the anthracene triplet state. Bimolecular rate constants for energy transfer from O*2 (1△g), to carotenoid have been evaluated.
TL;DR: In this paper, a singlet pathway for the direct trans-cis photoisomerization of the cyanostilbenes was proposed, where the trans configuration of the lowest triplet state was identified as the planar trans configuration.
TL;DR: In this paper, the first ab initio calculation of the polarizability of H2 in the lowest triplet state was presented, providing the simplest quantum mechanical model for the effects of nonbonding interactions on molecular polarizabilities.
Abstract: Results of the first accurate ab initio calculation of the polarizability of H2 in the lowest triplet state are presented, providing the simplest quantum mechanical model for the effects of nonbonding interactions on molecular polarizabilities. When overlap is significant, the computed polarizability anisotropy is smaller than the dipole‐induced–dipole value, but somewhat larger than that obtained from the original Oxtoby–Gelbart continuum–dielectric model. Two models equivalent in first order, the simple electrostatic model introduced by Clarke, Madden, and Buckingham, and the continuum–dielectric model as modified by Oxtoby, give fairly good values for the pair polarizability if an additive exchange correction is included.
TL;DR: In this paper, a double zeta plus polarization basis set of contracted Gaussian functions was used in conjunction with self-consistent field and large scale configuration interaction wave functions to predict the equilibrium geometries and energies of acetylene in its excited singlet electronic states.
Abstract: Molecular electronic structure theory has been used to predict the equilibrium geometries and energies of acetylene in its excited singlet electronic states. A double zeta plus polarization basis set of contracted Gaussian functions was used in conjunction with self‐consistent field and large scale configuration interaction wave functions. The first excited singlet state of acetylene is the trans 1Au state, in agreement with the experimental studies of King, Ingold, and Innes. This result is particularly interesting because the lowest triplet state of C2H2 is not the 3Au state but rather the cis 3B2 state. The predicted geometry of the ? 1Au state is re(CC)=1.384 A, re(CH)=1.096 A, ϑe(HCC)=121.7 °, in good agreement with available spectroscopic data. The predicted relative energies of the excited singlet states are 5.06 eV (1Au), 5.54 eV (cis 1A2), 6.87 eV (1B2), and 7.29 eV (1Bu). Thus the energetic ordering of the singlet states is Au
TL;DR: In this article, a section of the potential energy surface of the Rydberg 3s 3 A″ 2 state of NH 3 leading to dissociation into NH 2 (2 B 1 ) and H( 2 S) was investigated at the RHF and CI levels.
TL;DR: In this paper, selective laser excitation was used to measure the radiative lifetimes of the ZnI triplet states 4sns3S1(n=5−7) and 4snd3D3,3D2 and3D1 (n=4−6).
Abstract: Selective laser excitation was used to measure the radiative lifetimes of the ZnI triplet states 4sns3S1(n=5–7) and 4snd3D3,3D2 and3D1(n=4–6). These states were excited from the metastable 4s4p3P states, which were collisionally populated in an atomic beam. The values are compared with the results of other experimental methods (beam-foil, pulsed electron excitation, Hanle effect) and with theoretical calculations. The corresponding oscillator strengths are discussed with respect to the astrophysical determination of the Zn photospheric abundance.