Triplet Reaction Research Articles

Modulation of iridium(iii) phosphorescence via photochromic ligands: a density functional theory study

The photochromic iridium(iii) complex (Py-BTE)(2)Ir(acac) synthesized by Tan et al. [W. Tan et al., Org. Lett. 2009, 11, 161-164] has shown distinct photo-reactivity and photo-controllable phosphorescence. We here present a density functional theory study on the (Py-BTE)(2)Ir(acac) complex to explore the mechanism at the molecular level and to help further design of photochromic iridium(iii) complexes with the desirable properties. The hybrid functional PBE0, with 25% Hartree-Fock exchange, is found to give an optimal structure compared with X-ray crystallographic data. The absorption bands are well reproduced by using time-dependent density functional theory calculations, lending the possibility to assign the metal-to-ligand and intra-ligand charge transfer transitions. The radiative and nonradiative deactivation rate constants, k(r) and k(nr), are rationalized for both the open-ring and closed-ring forms of the complex. The very large k(nr) and small k(r) make the closed-ring form of the complex non-emissive. The triplet reactivity of the Py-BTE ligand is also studied by performing density functional theory calculations on the potential energy surfaces of the ground state and the lowest triplet state.

Spin structure of $\bar{p}p\rightarrow\bar{\Lambda}\Lambda$

A complete determination of spin observables for \(\bar{p}p\rightarrow\bar{\Lambda}\Lambda\), at an antiproton lab momentum of 1.637 GeV/c, has been made by the PS185 Collaboration. Results of this experiment disagree strongly with the predictions of calculations which used either meson exchange or quark models for the reaction mechanism. We use a model-independent description of the spin structure of this reaction to show that, in the absence of initial and final state interactions, pure spin singlet or pure spin triplet reaction mechanisms require that depolarization D nn and spin transfer K nn are equal, in disagreement with experiment. We also show that a superposition of singlet and triplet reaction mechanisms allow D nn ≠ K nn , as required by experiment.

Theoretical Study on the Reaction Mechanism for the Formation of 2-Methylpyridine Cobalt(I) Complex from Cobaltacyclopentadiene and Acetonitrile

Using B3LYP level calculations, the reaction of acetonitrile with cobaltacyclopentadiene complex in singlet and triplet electronic states, to give pyridine cobalt complexes, was studied. While the most favorable path for the singlet reaction passes through a [4+2] cycloaddition transition state, the most favorable triplet reaction follows the reaction mechanism through azacobaltacycloheptatriene. Since the transition state for the singlet reaction path is lower in energy than those for the triplet reaction path, the singlet reaction seems to be more favorable. However, the reactant in the triplet state is more stable than that in the singlet state, suggesting that the two-state reactivity (TSR) mechanism with spin changes is followed. Determination of energy minimum crossing points between singlet and triplet energy surfaces led to the conclusion that the TSR mechanism is more favorable than both the singlet- and triplet-state (single-state reactivity) (SSR) mechanisms. Comparison of a reaction profile be...

RRKM and Ab Initio Investigation of the NH (X) Oxidation by Dioxygen

We performed a detailed study of the NH + O(2) potential energy surface by means of a number of multireference (CASSCF, MC-QDPT2, MR-AQCC, MR-CISD(18;13)+Q with 6-311+G(d,p), and aug-cc-pVTZ basis sets) and composite (G3B3, G3MP2B3, CBS-QB3, W1U) methods. Parent nitroso oxide, HNOO, was found to be the key intermediate of this process. In its ground state, (1)A', HNOO exists in two conformations, where the cis form is 8.1-10.9 kJ x mol(-1) more stable than the trans-nitroso oxide. The mechanism of nitrene oxidation by dioxygen may be represented as a set of various transformations of vibrationally excited HNOO, namely, decomposition into NO and OH radical pair, O-O dissociation reaction, and a number of thermal deactivation processes. We localized all stationary points of these transformations on both the singlet and the triplet reaction PES. The energies of reactants, products, and transition states were calculated at the RI-MR-CISD(18;13)+Q/aug-cc-pVTZ level of theory; the vibrational analysis of these species was done by means of CASSCF(18;13)/6-311+G(d,p). Apparent rate constants of the NH + O(2) reaction were calculated using RRKM theory. The total rate constant k(total) corresponds well to available experimental data. The temperature dependence of k(total) is rather nontrivial and consists of three quasi-linear intervals. At low temperatures (up to room temperature) the slope of log(k(total)) vs 1/T is negative due to prevailing stabilization of HNOO. The rate-determining channel of the "NH + O(2)" reaction in the medium-temperature interval (up to approximately 1000 K) was found to be formation of the NO + OH radical pair via H transfer to the terminal oxygen atom. This reaction is accelerated by a factor of 4.2 (214 K) and 1.2 (2500 K) due to tunnel effect. The distinctive feature of the NH + O(2) high-temperature chemistry is the increase of the effective activation energy due to prevailing dissociation of the HNOO peroxide bond.

Janus-faced charge recombinations in photosystem II photoinhibition

Light-induced damage of the photosynthetic apparatus in plants is an important phenomenon that primarily affects the photosystem II complex. Here, we propose a new model of photoinhibition in which charge recombination processes have a double-faced role: first, photodamage is induced by singlet oxygen, which is produced via interaction with the triplet reaction center chlorophyll ((3)P(680)) arising from the recombination of the charge-separated state between P(680) and the pheophytin electron acceptor ((3)[P(680)(+)Phe(-)]). Second, photoprotection is provided by competition between (3)[P(680)(+)Phe(-)] formation and direct recombination of the (1)[P(680)(+)Phe(-)] and P(680)(+)Q(A)(-) states. The efficiency of these two pathways is under control of the redox potential of the Phe and Q(A) electron acceptors, which is utilized during adaptation to high light conditions.

Photosensitizing properties of formulation adjuvants

Protein hydrolysed fertilizers (PHFs) are currently used to stimulate plant metabolism, mainly through foliar application, and are proposed for use as pesticide adjuvants. However, the effects of PHFs on pesticides upon irradiation are unknown. Here, several herbicides and model compounds were irradiated with simulated solar light in the presence of four PHFs extracted from animal proteins and from vegetal. PHFs enhanced the photodegradation of fenuron, mesotrione, 2-mercaptobenzothiazole and 2,4,6-trimethylphenol. Using the scavenging technique, we could evidence the direct reaction of triplet excited states of PHFs towards substrates and we evaluated the contribution of triplets, singlet oxygen and hydroxyl radicals. Hydroxyl radicals were not detected; they were either not formed or were efficiently scavenged by the PHFs. The four PHFs showed distinct photosensitizing abilities. Production processes, rather than origin of the PHFs, seem to govern the photochemical reactivity of the PHFs.

Photochemistry of N-Acetyl-, N-Trifluoroacetyl-, N- Mesyl-, and N-Tosyldibenzothiophene Sulfilimines

Time-resolved infrared (TRIR) spectroscopy, product studies, and computational methods were applied to the photolysis of sulfilimines derived from dibenzothiophene that were expected to release acetylnitrene, trifluoroacetylnitrene, mesylnitrene, and tosylnitrene. All three methods provided results for acetylnitrene consistent with literature precedent and analogous experiments with the benzoylnitrene precursor, i.e., that the ground-state multiplicity is singlet. In contrast, product studies clearly indicate triplet reactivity for trifluoroacetylnitrene, though TRIR experiments were more ambiguous. Product studies suggest that these sulfilimines are superior sources for sulfonylnitrenes, which have triplet grounds states, to the corresponding azides, and computational studies shed light on the electronic structure of the nitrenes.

New horizons of adenosinetriphosphate energetics arising from interaction with magnesium cofactor

MD DFT:B3LYP (6-31G** basis set, T = 310 K) method is used to study interactions [singlet (S) and triplet (T) reaction paths] between adenosinetriphosphate, ATP(4-), and [Mg(H(2)O)(6)](2+) in water environment, modeled with 78 water molecules. Computations reveal the appearance of low and high-energy states (stable, quasi-stable, and unstable), assigned to different spin symmetries. At the initial stage of interaction, ATP donates a part of its negative charge to the Mg complex making the Mg slightly charged. As a result, the original octahedral Mg complex loses two (S state) or four (T state) water molecules. Moving along S or T potential energy surfaces (PESs), Mg(H(2)O)(4 )or Mg(H(2)O)(2) display different ways of complexation with ATP. S path favors the formation of a stable chelate with the O1-O2 fragment of ATP triphosphate tail, whereas T path favors producing a single-bonded complex with the O2. The latter, being unstable, undergoes a further conversion into a spin-separated complex, also unstable, and two metastable S complexes, which finally arise in two stable, low-energy and high-energy, chelates. The spin-separated complex experiences rapid decomposition resulting in the production of a highly reactive adenosinemonophosphate ion-radical *AMP, early observed in the CIDNP experiment (Tulub 2006). Biological consequences of the findings are discussed.

Laser flash photolysis study of the triplet reactivity of β-lapachones

The photochemical reactivity of beta-lapachone (1), nor-beta-lapachone (2) and beta-lapachone 3-sulfonic acid (3) has been examined by laser flash photolysis. Excitation (lambda = 266 nm) of degassed solutions of , in acetonitrile or dichloromethane, resulted in the formation of detectable transients with absorption maxima at 300, 380 and 650 nm. These transients, with lifetimes of 5.0 micros, were quenched by beta-carotene at a diffusion-controlled rate constant and assigned to the triplet excited states of 1-3. Addition of hydrogen donors, such as 2-propanol, 1,4-cyclohexadiene, 4-methoxyphenol or indole led to the formation of new transients, which were assigned to the corresponding ketyl radicals obtained from the hydrogen abstraction reaction by the triplets 1-3 . In the presence of triethylamine it was observed the formation of the long-lived anion radical derived from , which shows absorption maxima at 300 and 380 nm. The low values observed for the hydrogen abstraction rate constants for the beta-lapachones 1-3 using 2-propanol and 1,4-cyclohexadiene as quenchers led us to conclude that their triplet excited states show pi pi* character.

Time‐resolved resonance Raman and density functional theory study of the photochemistry of 4‐benzoylpyridine in acetonitrile and 2‐propanol

AbstractA nanosecond time‐resolved resonance Raman (ns‐TR3) spectroscopic investigation of the intermolecular hydrogen‐abstraction reaction of the triplet state of 4‐benzoylpyridine (4‐BPy) in 2‐propanol solvent is reported. The TR3 results reveal a rapid hydrogen abstraction (<10 ns) by the 4‐BPy triplet state (nπ*) with the 2‐propanol solvent, leading to formation of a 4‐BPy ketyl radical and an associated dimethyl ketyl radical partner from the solvent. The recombination of these two radical species occurs with a time constant about 200 ns to produce a para‐N‐LAT (light absorbing transient). The structure, major spectral features, and identification of the ketyl radical and the para‐N‐LAT coupling complex have been determined and confirmed by comparison of the TR3 results with results from density functional theory (DFT) calculations. A reaction pathway for the photolysis of 4‐BPy in 2‐propanol deduced from the TR3 results is also presented. The electron‐withdrawing effect of the heterocyclic nitrogen for 4‐BPy on the triplet state makes it have a significantly higher chemical reactivity for the hydrogen abstraction with 2‐propanol compared to the previously reported corresponding benzophenone triplet reaction under similar reaction conditions. In addition, the 4‐BPy ketyl radical reacts with the dimethyl ketyl radical to attach at the para‐N atom position of the pyridine ring to form a cross‐coupling product such as 2‐[4‐(hydroxy‐phenyl‐methylene)‐4h‐pyridin‐1‐yl]‐propan‐2‐ol instead of attacking at the para‐C atom position as was observed for the corresponding benzophenone reaction reported in an earlier study. Copyright © 2008 John Wiley & Sons, Ltd.

Oxidation of catechol to 2-hydroxymuconic semialdehyde

Singlet and triplet reaction pathways for the oxidation reaction of catechol to 2-hydroxymuconic semialdehyde were examined at the B3LYP/6-311G ∗∗ level of theory for the uncatalyzed process. In three step mechanisms, the reactions proceed through a peroxy-like transition state, with initial oxygen attack at different carbon atom sites. The final product of the (open shell) singlet reaction is the semialdehyde that has been observed experimentally. The reaction barriers along the singlet surface are lower than those on the triplet pathway.

Reaction Mechanism of the Vitamin K-Dependent Glutamate Carboxylase: A Computational Study

In the reaction cycle of glutamate carboxylase, vitamin K epoxidation by O2 has been proposed to generate a very strong base able to remove a proton from the gamma carbon of a Glu residue, thus yielding a Glu-based carbanion that readily reacts with CO2. We have used hybrid density functional theory to study this appealing mechanism. Our calculations show a very exergonic four-step mechanism with the reaction of (triplet) O2 with the singlet vitamin K anion as the rate-limiting step, with a rate similar to the experimental value. Our study also establishes the need to apply continuum models when performing the optimization of minimum-energy crossing points between potential energy surfaces of different multiplicities for enzyme model systems.

Solid-state photocycloaddition of 6,6′-dimethyl-4,4′-[bis(methylenoxy)phenylene]-di-2-pyrones with benzophenone

Photocycloaddition reactions of 6,6′-dimethyl-4,4′-[bis(methylenoxy)phenylene]-di-2-pyrones ( 4a– c) with benzophenone ( 2a) by mixing in the solid state (solid solution) afforded the corresponding oxetane derivatives ( 5a– c; 1:2 adducts) with high site- and regioselectivity across the C5–C6 and C5′–C6′ double bonds in 4 via the triplet excited state of benzophenone. The oxetane formation proceeded more effectively in the solid state than in solution. The reaction mechanism was inferred by MO methods to be initiated by electrostatic interaction between the C6 position of 4a– c and the carbonyl oxygen of 2a in their ground states. The solid-state interaction may be enhanced by the electron density at the carbonyl oxygen of the triplet 2a. The transition state (TS) analysis of the [2+2] cycloaddition reactions also suggested some triplet complexes and high regioselectivity. The hydrogen-bonding interaction between 2a and 4a– c and the triplet reaction mechanism were also explained by the IR analyses and the quenching experiments, respectively.

Photodissociation dynamics of the reaction H2CO→H+HCO via the singlet (S) and triplet (T1) surfaces

We have explored the photodissociation dynamics of the reaction H(2)CO+hnu-->H+HCO in the range of 810-2600 cm(-1) above the reaction threshold. Supersonically cooled formaldehyde was excited into selected J(Ka,Kc) rotational states of six vibrational levels (1(1)4(1), 5(1), 2(2)6(1), 2(2)4(3), 2(3)4(1), and 2(4)4(1)) in the A((1)A2) state. The laser induced fluorescence spectra of the nascent HCO fragment provided detailed product state distributions. When formaldehyde was excited into the low-lying levels 1(1)4(1), 5(1), and 2(2)6(1), at E(avail)<1120 cm(-1), the product state distribution can be modeled qualitatively by phase space theory. These dynamics are interpreted as arising from a reaction path on the barrierless S0 surface. When the initial states 2(2)4(3) and 2(3)4(1) were excited (E(avail)=1120-1500 cm(-1)), a second type of product state distribution appeared. This second distribution peaked sharply at low N, Ka and was severely truncated in comparison with those obtained from the lower lying states. At the even higher energy of 2(4)4(1) (E(avail) approximately 2600 cm(-1)) the sharply peaked distribution appears to be dominant. We attribute this change in dynamics to the opening up of the triplet channel to produce HCO. The theoretical height of the barrier on the T1 surface lies between 1700 and 2100 cm(-1) and so we consider the triplet reaction to proceed via tunneling at the intermediate energies and proceed over the barrier at the higher energies. Considerable population was observed in the excited (0,0,1) state for all initial H(2)CO states that lie above the appearance energy. Rotational populations in the (0,0,1) state dropped more rapidly with (N,Ka) than did the equivalent populations in (0,0,0). This indicates that, although individual rotational states are highly populated in (0,0,1), the total v3=1 population might not be so large. Specific population was also measured in the almost isoenergetic Kc and J states. No consistent population preference was found for either asymmetry or spin-rotation component.

Organic Photochemical Rearrangements of Triplets and Zwitterions; Mechanistic and Exploratory Organic Photochemistry

AbstractThere has been some controversy about the mechanisms of reactions of enones and dienones. The question has been whether a zwitterion or diradical is involved as the intermediate. In the case of ground state species it has not been recognized that zwitterions may have diradical character. In triplet reactions there is the question of whether the rearrangements take place at the initial T1 stage or subsequently as S0 ground state zwitterions or diradicals. We now have some new rearrangements bearing on these questions. In some cases it is the triplet which rearranges, while in others it is the S0 zwitterion. The zwitterion vs. diradical nature of the S0 species has been assessed by both experimental as well as theoretical means. Similarly, both experiment and computational theory have been used to determine at what stage the triplet rearrangements occur and to determine the characteristics of the rearranging species. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2007)

Femtosecond Transient Absorption and Nanosecond Time‐Resolved Resonance Raman Study of the Solvent‐Dependent Photo‐Deprotection Reaction of Benzoin Diethyl Phosphate

A combined femtosecond transient absorption (fs-TA) and nanosecond time-resolved resonance Raman (ns-TR3) study was performed to directly detect the dynamics and elucidate the mechanism of the excited state deactivation and solvent-dependent photo-deprotection pathways for benzoin diethyl phosphate (BDP) in neat acetonitrile (MeCN) and 75 % H2O/25 % MeCN. Comparison of the TA spectral evolution observed in the two solvents provides explicit evidence that the photophysical deactivation of the BDP singlet excited state has little solvent dependence. The TA spectra also indicate the related internal conversion (IC) and intersystem crossing (ISC) processes occur rapidly on hundreds of femtoseconds and approximately 2-3 ps time scales, respectively. From this and in conjunction with a photochemistry study and ground state resonance Raman (RR) measurements, the TA results reveal that the phenacyl localized BDP triplet state (that is mainly npi* nature) is the common and immediate precursor to the photo-deprotection reaction in both solvents. However, the triplet deprotection follows different pathways in neat MeCN versus the largely water containing solvent. The deprotection reaction in MeCN was determined to occur with a approximately 11 ns time constant and the reaction was found to be an unimolecular process leading to elimination of the diethyl phosphoric acid apparently concurrent with cyclization to yield the benzofuran product. In the water mixed solvent, the triplet reaction was observed to proceed with a approximately 15 ns time constant and the reaction leads to not only the deprotection-cyclization but also a heterolytic dissociation to release the diethyl phosphate anion through a branching and competing mechanism. The ns-TR3 spectra combined with relevant DFT calculations have been used to characterize the dynamics, structure and vibrational frequencies to help identify the important intermediates as well as to explore the reaction pathway leading to formation of the solvolysis product in the largely water solvent. A consecutive mechanism has been revealed for the heterolysis-solvolysis reaction in the water mixed solvent. The present work provides direct and irrevocable evidence for the dynamics and mechanistic description of the overall photophysics and deprotection related photochemistry for BDP.

Woodward-Hoffmann rules in density functional theory: Initial hardness response

The Woodward-Hoffmann rules for pericyclic reactions, a fundamental set of reactivity rules in organic chemistry, are formulated in the language of conceptual density functional theory (DFT). DFT provides an elegant framework to introduce chemical concepts and principles in a quantitative manner, partly because it is formulated without explicit reference to a wave function, on whose symmetry properties the Woodward-Hoffmann [J. Am. Chem. Soc. 87, 395 (1965)] rules are based. We have studied the initial chemical hardness response using a model reaction profile for two prototypical pericyclic reactions, the Diels-Alder cycloaddition of 1,3-butadiene to ethylene and the addition of ethylene to ethylene, both in the singlet ground state and in the first triplet excited state. For the reaction that is thermally allowed but photochemically forbidden, the initial hardness response is positive along the singlet reaction profile. (By contrast, for the triplet reaction profile, a negative hardness response is observed.) For the photochemically allowed, thermally forbidden reaction, the behavior of the chemical hardness along the initial stages of the singlet and triplet reaction profiles is reversed. This constitutes a first step in showing that chemical concepts from DFT can be invoked to explain results that would otherwise require invoking the phase of the wave function.

Quantum chemical study on the population of the lowest triplet state of psoralen

The efficient population of the low-lying triplet ππ * state of psoralen is studied with the quantum chemical CASPT2 method. Minima, singlet–triplet crossings, conical intersections, and reaction paths on the low-lying singlet and triplet states hypersurfaces of the system have been computed together with electronic energy gaps and spin–orbit coupling terms. A mechanism is proposed, favorable in the gas phase, for efficient deactivation of the initially populated singlet excited ππ * state, starting with an intersystem crossing with an nπ * triplet state and evolving via a conical intersection toward the final lowest-lying ππ * triplet state, protagonist of the reactivity of psoralen.

Effect of stereoisomerism on the radiative and reactive relaxation channels of two thio-analogues of distyrylbenzene

The photophysical and photochemical properties of the ZE and ZZ stereoisomers of two thio-analogues of distyrylbenzene, where thiophene rings replaced the central or the side benzene rings, have been investigated. The kinetic competition between the radiative and reactive relaxation channels of the lowest excited singlet state has been compared with that previously found for the corresponding EE stereoisomers and the parent hydrocarbon. The photobehaviour markedly depends on the position of the heteroatom (central or side rings). Its main effect is an increase in the S 1 → T 1 intersystem crossing, which may open the way to isomerization in the triplet manifold. The triplet reactivity, sensitized by biacetyl, was also investigated. Adiabatic formation of the excited singlet and triplet EE ∗ isomer by irradiation of the other isomers (in the case of ZZ, a “one photon – two bonds” mechanism is operative) was evidenced by fluorescence and flash photolysis measurements.

Phase transition of triplet reaction-diffusion models

The phase transitions classes of reaction-diffusion systems with multiparticle reactions are an open challenging problem. Large scale simulations are applied for the 3A --> 4A, 3A --> 2A and the 3A --> 4A, 3A --> [formula : see text] triplet reaction models with site occupation restriction in one dimension. Static and dynamic mean-field scaling are observed with signs of logarithmic corrections suggesting d(c) = 1 upper critical dimension for this family of models.