Showing papers on "Triplet state published in 1981"

Near Infra-Red Absorption Spectra of the Chlorophyll a Cations and Triplet State in vitro and in vivo

Abstract: Absorption spectra in the near infra-red have been obtained for transient species obtained from chlorophyll a, by flash absorption spectroscopy. The triplet state of Chl a presents an absorption band around 760 nm (ϵ = 7,500 M−1 cm−1) and a very broad band (ϵ = 1,700 M−1 cm−1) around 1100 nm. At low temperature, evidence was obtained for the formation of the triplet state of aggregated Chl a. Its t1/2 of decay is around 0.5 ms instead of 1 ms for the monomeric species. The radical-cation Chl a+ was formed by reaction of 3Chl a with a quinone; it has an absorption band at 840 nm (in cyclohexanol) with a shoulder around 740 nm and no absorption between 1000 and 1650 nm. The cation has also been formed in vivo, as the oxidized state of the photosynthetic primary electron donors, P680 and P700. The spectrum of P+680 resembles that of Chl a+; its absorption maximum is at 820 nm. The spectrum of P+680, maximum at 810 nm, is significantly broader. P+700 has no absorption between 1000 and 1650 nm. These spectra are in favor of recent suggestions on a monomeric nature of P680 and of a dimeric nature for P700.

Laser Flash Photolysis of Phthalocyanines in Solution and Microemulsion

Abstract: Triplet state formation (Φisc) and properties (eT-T, τT) of phthalocyanine (HPC) and zinc phthalocyanine (ZnPC) have been characterized in homogeneous solutions (1-chloronaphthalene, 1-propanol) and in microemulsion by investigating the variation of the transient optical density as a function of the intensity of the exciting laser. Experimental results follow the theoretically predicted dependence only for very low intensities of the exciting pulse. At higher intensities, a more complicated scheme of primary reactions has to be taken into account, implicating qualitative restrictions in the application of saturation experiments. The observed transients at high-intensity excitation are interpreted as being perturbed by aggregational phenomena.

Electron spin echoes of a photoexcited triplet: Pentacene in p‐terphenyl crystals

Abstract: Electron spin echo observations of the photoexcited triplet state of 0.1 mol % pentacene‐h14 and ‐d14 in p‐terphenyl crystals at room temperature are presented. Theory is presented for calculation of the echo envelope modulations for an S = 1, I = 1/2 spin system including zero field splittings in the high field limit. Echo envelope modulations due to proton and deuteron hyperfine interactions in the pentacene molecule have been observed. The echo decay data are used to calculate triplet state decay parameters.

Analytic configuration interaction (CI) gradient techniques for potential energy hypersurfaces. A method for open-shell molecular wave functions

Abstract: A formalism is presented for the determination of analytic energy first derivatives for the most common types of open‐shell correlated wave functions. These are the cases for which the electronic structure is qualitatively understood in terms of a set of molecular orbitals which are eigenfunctions of Roothaan’s restricted Hartree–Fock (RHF) operator. Using these RHF orbitals, configuration interaction (CI) wave functions of broad generality are included in the formalism. The method has been implemented in conjunction with the loop‐driven graphical unitary group approach. Application to the vibrational frequencies of methylene suggests that the triplet state has 0.5 kcal more zero‐point vibrational energy than does the lowest singlet state.

Average singlet–triplet coupling properties of biacetyl and methylglyoxal using quantum beat spectroscopy

Abstract: We have observed quantum beats in the reversible intersystem crossing of simple α‐dicarbonyls, and we have analyzed them to obtain information concerning the density of interacting states and the average intramolecular coupling energy. We have analyzed most of the biacetyl quantum beats using a method based on the properties of random matrices which is described elsewhere and we present the results here. We also present an analysis based on perturbation theory using the Fourier transforms of the quantum beats which is appropriate for understanding the methylglyoxal quantum beats. The density of vibrationally hot triplet states which interact with excited singlet states that are connected to the ground electronic state via optical excitation (∼22 000 cm−1) is found to increase with the amount of vibrational excitation in the accessible singlet state at roughly the same rate as the overall density of triplet vibrational stress increases. We find satisfactory agreement between the density obtained from the quantum beats and that calculated using well known analytical formulas and direct state counting. The density of interacting states increases with the rotational quantum number of the initially excited singlet rovibronic state. The average value of the magnitude of the spin–orbit interaction is 1–10 MHz independent of the amount of vibrational–rotational excitation present. Radiationless transitions in these highly excited molecules are evidently not subject to any overriding selection rules other than spatial symmetry, and conservation of total energy, total angular momentum, and nuclear spin. The biacetyl quantum beats are collisionally quenched with helium at the same rate as the overall fluorescence. The cross section for this process is roughly 300±200 A2.

Theoretical studies on the reaction of atomic oxygen (O(3P)) with acetylene. II

Abstract: Ab initio electronic structure calculations have been used to characterize the equilibrium and saddle points for the addition of O(/sup 3/P) to HCCH on the two lowest triplet surfaces. Structures, frequencies, and energetics are presented and, when possible, directly compared to those from experiment. The saddle point for abstraction of H from HCCH by O(/sup 3/P) has also been characterized for the two lowest triplet surfaces by analogy with recent ab initio calculations on similar abstraction reactions. Systematic errors in the calculated energetics are corrected by semiempirical arguments. Comparisons of the energetics are also made to semiempirical BAC-MP4 calculations. The final characterizations of the stationary points on the addition and abstraction surfaces are then used in RRKM calculations of the rate constants and product branching ratios. These calculations are compared to experiment. The comparisons show (1) abstraction is not an important process, (2) theory and experiment are in good agreement for the overall rate of reaction at temperatures below 1000 K but predict substantially lower rates than observed in high-temperature shock tube experiments, (3) the calculated branching ratio, production of H + HCCO over the total rate of reaction, is not small (certainly over 25% and most likely over 50%)more » with a relatively weak temperature dependence and no pressure dependence, and (4) the ground-state triplet surface (/sup 3/A') dominates both the overall rate constant and the branching ratio with the excited triplet surface (/sup 3/A') introducing only minor perturbations.« less

Theory of two coupled triplet states

Abstract: We present a theory of the magnetic dipolar interaction of two electrostatically coupled carbene triplet states. Using a model spin hamiltonian of a bicarbene system the spin functions and energy eigenvalues of the singlet (S = 0), triplet (S = 1), quintet (S = 2) and mixed singlet-quintet spin pair states are calculated. The resulting zero field and high field E.S.R. spectra are sensitively dependent on the energy separation ΔeSQ resulting from different electronic configurations of the same symmetry between the individual total singlet and quintet spin pair states. The results of this theory are compared with the E.S.R. data of the quintet states observed during low temperature photopolymerization of diacetylene single crystals.

High resolution optical studies on C-phycocyanin via photochemical hole burning

Abstract: We have shown that both the native C-phycocyanin and its corresponding free biline chromophore undergo reversible, low-temperature photochemistry. We attribute this photochemistry to reversible proton-transfer processes and utilize the observed photoreaction for photochemical hole burning (PHB). Using narrow-band PHB experiments, we have been able to perform high-resolution optical studies and show that the protein-chromophore assembly forms a very rigid structure. The results lead to the conclusion that the light-induced proton transfer occurs most probably in the triplet state.

Transient Photochemistry of Safranin-O.

Abstract: — We have characterized the spectra, acidity constants and decay kinetics of the triplet and semireduced radical species of Safranin-O. Between pH 3.0 and 10.6, there are three triplet species denoted 3DH2 +2, 3DH+ and 3D, the pKas being 7.5 and 9.2. All three triplet species exhibit first order decay, the rate constant for 3DH+ being ca. 5-fold lower than the rate constants of 3DH+ and 3D. Ascorbic acid and ethylenediaminetetraacetic acid (EDTA) quench the triplet state under appropriate pH conditions and the pH dependencies of the yield of semireduced dye indicate that 3DH+ is more reactive than 3DH+ or 3D. With EDTA as the reducing agent, there is the additional requirement that at least one of the amino nitrogens be deprotonated to obtain a significant yield of semireduced dye. In these reactions, ascorbic acid is oxidized reversibly, but EDTA is oxidized irreversibly, so that with the latter reducing agent photolysis causes buildup of the leucodye, which on subsequent photolysis can reduce triplet state dye. With ascorbic acid as the reducing agent, the regeneration of the ground state dye is reversible, the decay of the semireduced radical being second order. In general, the transient photochemistry of Safranin-O resembles that of Thionine, the major difference being that the lifetimes of 3DH2 +2 and 3DH+ are much longer for Safranin-O than for Thionine.

pH-Dependent fluorescence spectroscopy. Part 12. Flavone, 7-hydroxyflavone, and 7-methoxyflavone

Abstract: Flavone is found to be non-fluorescent in neutral aqueous solution, but strongly fluorescent in sulphuric acid. The pKa(S1) value of its conjugated acid was calculated from ground-state data and is found to be in poor agreement with the pKa value obtained from a plot of fluorescence intensity versus Hammett acidity. This is interpreted in terms of rapid intersystem crossing to the triplet state, before a prototropic process can take place. Conversely, 7-hydroxyflavone is found to be fluorescent in its neutral, conjugate acid, and conjugate base form. Excited state pKa values have been obtained from Forster–Weller calculations and from fluorescence titration curves, but are in poor agreement. 7-Hydroxyflavone undergoes adiabatic photodissociation in its S1 state and forms an exciplex (or a phototautomer) in pH 3 to Ho–3 solution, and similarly in acidified methanol. Fluorescence quantum yields are given for alkaline, neutral, and acidic solutions. Compared with 7-hydroxycoumarins, flavones are only weakly fluorescent in alkaline solution. This behaviour is explained by the small energy difference between the S1 and T1 states giving rise to more efficient intersystem crossing. To demonstrate the dynamic nature of excited state proton transfer processes and the involvement of water molecules in quenching processes, deuteriation experiments have been performed. Drastic changes are found in both fluorescence intensity and shape of the curves.

The effects of high‐intensity uv‐radiation on nucleic acids and their components—i. thymine

Abstract: — The set of final products of thymine conversion induced by high-intensity UV irradiation (λ= 266nm, intensity 1024-5 × 1029 photons·s−1·m−2, pulse duration 10ns) of the dilute aqueous solution to the first approximation is similar to that formed with ionizing irradiation (γ-irradiation of aqueous solution or autoradiolysis of a solid 2-[14C]-thymine). The data obtained suggest that high-intensity UV-induced photochemical conversion of thymine involves photoionization and/or photodissociation. These processes pass through the higher excited state(s) populated as a result of the second photon absorption by excited (most probably in the T1 triplet state) thymine molecules.

Matrix‐isolation triplet (T1) state IR spectroscopy of acridine and phenazine

Abstract: The matrix‐isolated infrared (IR) spectra of the lowest triplet (T1) states of acridine and phenazine have been obtained by direct measurement. Population of T1, sufficient for IR detection, is achieved by pulsed indirect pumping with an Ar+ laser source. The ground and triplet state IR spectra were 180° out of phase, providing unambiguous indentification of the excited metastable state absorptions. Approximate T1 vibrational assignments are made on the basis of analogous experimental and theoretical results for ground state acridine, phenazine, and anthracene.

Molecular Spectroscopy of the Triplet State through Optical Detection of Zero-Field Magnetic Resonance

Abstract: Research on the phosphorescent states of organic molecules has experienced a very active and rapid growth since World War II. The growth was undoubtedly initiated in 1944 by the identification of phosphorescence in organic molecules with the triplet state by Lewis and Kasha [1]. Since then, a great number of experimental and theoretical studies on the triplet state have been accumulated [2], probably because of the recognition of the key role of the triplet state in photochemistry, photophysics, and photobiology. The paramagnetism due to two parallel spins characteristic of the triplet state has also attracted many researchers in the field of magnetic resonance.

Study of OCS by electron energy loss spectroscopy

Abstract: A high resolution study of the OCS molecule is presented using low‐energy electron energy loss spectroscopy. Measurements have been carried out at various incident electron energies and scattering angles, covering the whole range from 5 to about 17 eV. The high energy resolution (∼30 meV) has allowed the detailed analysis of the vibrational progressions and Rydberg series, and some identifications are supported by the plots of their differential cross sections. For the first time, the two overlapping progressions of the (10σ) 1Π state, around 7.4 eV, are resolved by electron impact. A new triplet state is observed at 7.59 eV, assigned to the (11σ) 3Π known state, whose 1Π term would correspond to a known progression lying under the strong 1Σ+ band. Above the first ionization limit, the spectrum presents a striking similarity with the photoionization efficiency curves, with the strong autoionizing features which form Rydberg series converging to the ? 2Σ+ state of OCS+. Even the weak series VIII is present, appearing as a small dip. Further measurements carried out at low residual electron energy (?5 eV) reveal that such features then become peaks of strength comparable with the other features. Typical triplet behavior are pointed out for some of them. Two new series (X and XI) are thus proposed, while another feature is assigned to the n = 3 term of series VII converging to the ? 2Σ+ state. For all of the series observed in this region, the quantum defects are determined and orbital assignments are proposed.

Ab initio CI study of the nitric oxide dimer (N2O2)

Abstract: Configuration interaction (CI) studies of the ground and electronically excited states are reported for nitric oxide dimer (N2O2) in itscis equilibrium geometry. The lowest triplet state (3 B 2) is found to lie only 0.43 eV above the ground state (1 A 1). The1 A 1 →1 B 1 transition is shown to be responsible for the rising absorption in the near infrared region observed experimentally. The transition of1 A 1→1 A 2 calculated in the visible spectrum range of 701 nm (1.77 eV) is symmetry forbidden.

Photochemistry of anthraquinone‐2,6‐disodium sulphonate in aqueous solution

Abstract: — Irradiation of anthraquinone-2,6-disodium sulphonate in aqueous solution leads to formation of hydroxyquinone and semiquinone, which undergoes disproportionation to form the fully reduced hydroquinone. The quantum yield for formation of the semiquinone depends upon pH, the results being attributed to the relative efficiency with which the various acid/base forms of the hydroquinone react with quinone. The photoreaction does not involve production of hydroxyl radicals and luminescence studies suggest that the quantum yield for formation of the triplet state is unity. Indeed, flash photolysis studies indicate the presence of a short-lived triplet (τ < 1 μs) which reacts with water to form a photosolvolate (τ= 5 μs) which, in turn, decays to form the observed products. These findings are in agreement with a mechanism previously proposed by Stonehill.