Triplet Reaction Research Articles

The “Non‐Reaction” of Ground‐State Triplet Carbon Atoms with Water Revisited

We introduce a novel experimental setup for the generation of carbon atoms by means of pulsed laser ablation with a pulse rate optimized to avoid warming of the matrix. The combination of this technique with annealing of the matrix, recooling, and spectral recording (e.g. IR) allowed us to differentiate between the reactivity of triplet and singlet carbon atoms towards water under matrix-isolation conditions. Our experimental procedure assures the relaxation of all unreacted carbon atoms to their triplet ground state in the 10 K matrix before spectral recording. In agreement with CCSD(T)/cc-pVTZ+ZPVE computational data and earlier lower level results, we find that triplet carbon atoms indeed do not react with water, despite their high initial energy. Intersystem crossing of the triplet to singlet states of hydroxy carbene are less important, as the barriers for rearrangement of the initial complex of triplet carbon atoms and water to covalently bound species are too high, and dissociation is more likely. We found no evidence for a direct O--H bond-insertion path for triplet carbon atoms. Self-condensation reactions of triplet carbon atoms are clearly favored and yield carbon clusters that show reactivity of their own. The proposed formation of aldoses in extraterrestrial environments can therefore only derive from "hot" carbon atoms or through photoreactions.

Quantum Chemical Study of the Mechanism of Reaction between NH (X 3Σ-) and H2, H2O, and CO2 under Combustion Conditions

Reactions of ground-state NH (3sigma-) radicals with H2, H2O, and CO2 have been investigated quantum chemically, whereby the stationary points of the appropriate reaction potential energy surfaces, that is, reactants, products, intermediates, and transition states, have been identified at the G3//B3LYP level of theory. Reaction between NH and H2 takes place via a simple abstraction transition state, and the rate coefficient for this reaction as derived from the quantum chemical calculations, k(NH + H2) = (1.1 x 10(14)) exp(-20.9 kcal mol(-1)/RT) cm3 mol(-1) s(-1) between 1000 and 2000 K, is found to be in good agreement with experiment. For reaction between triplet NH and H2O, no stable intermediates were located on the triplet reaction surface although several stable species were found on the singlet surface. No intersystem crossing seam between triplet NH + H2O and singlet HNO + H2 (the products of lowest energy) was found; hence there is no evidence to support the existence of a low-energy pathway to these products. A rate coefficient of k(NH + H2O) = (6.1 x 10(13)) exp(-32.8 kcal mol(-1)/RT) cm3 mol(-1) s(-1) between 1000 and 2000 K for the reaction NH (3sigma-) + H2O --> NH2 (2B) + OH (2pi) was derived from the quantum chemical results. The reverse rate coefficient, calculated via the equilibrium constant, is in agreement with values used in modeling the thermal de-NO(x) process. For the reaction between triplet NH and CO2, several stable intermediates on both triplet and singlet reaction surfaces were located. Although a pathway from triplet NH + CO2 to singlet HNO + CO involving intersystem crossing in an HN-CO2 adduct was discovered, no pathway of sufficiently low activation energy was discovered to compare with that found in an earlier experiment [Rohrig, M.; Wagner, H. G. Proc. Combust. Inst. 1994, 25, 993.].

Regioselectivity in the [2 + 2] cyclo-addition reaction of triplet carbonyl compounds to substituted alkenes (Paterno-Büchi reaction): A spin-polarized conceptual DFT approach

Regioselectivity of the photochemical [2 + 2] cyclo-addition of triplet carbonyl compounds with a series of ground state electron-rich and electron-poor alkenes, the Paterno-Büchi reaction, is studied. Activation barriers for the first step of the triplet reaction are computed in the case of the O-attack. Next, the observed regioselectivity is explained using a series of DFT-based reactivity indices. In the first step, we use the local softness and the local HSAB principle within a softness matching approach, and explain the relative activation barriers of the addition step. In the final step, the regioselectivity is assessed within the framework of spin-polarized conceptual density functional theory, considering response functions of the system’s external potential v, number of electronsN and spin numberN s , being the difference between the number ofα andβ electrons in the spin-polarized system. Although the concept of local spin philicity, introduced recently within this theory, appears less suited to predict the regioselectivity in this reaction, the correct regioselectivity emerges from considering an interaction between the largest values of the generalized Fukui functionsf ss on both interacting molecules.

Time-Resolved Resonance Raman and Density Functional Theory Study of Hydrogen-Bonding Effects on the Triplet State of p-Methoxyacetophenone

Picosecond and nanosecond time-resolved resonance Raman spectroscopy combined with density functional theory calculations have been performed to characterize the structure, dynamics, and hydrogen-bonding effects on the triplet state of the phototrigger model compound p-methoxyacetophenone (MAP) in cyclohexane, MeCN, and 50% H2O/50% MeCN (v:v) mixed solvent. Analogous work has also been done to study the corresponding ground state properties. The ground and triplet states of MAP were both found to be associated strongly with the water solvent molecules in the 50% H2O/50% MeCN solvent system. A hydrogen-bond complex model involving one or two water molecules bonded with the oxygen atoms of the MAP carbonyl and methoxy moieties has been employed to explore the hydrogen-bond interactions and their influence on the geometric and electronic properties for the ground and triplet states of MAP. Among the various hydrogen-bond configurations examined, the carbonyl hydrogen-bond configuration involving one water molecule was calculated to lead to the most stable hydrogen-bond complex for both the ground and the triplet states with the strength of the hydrogen-bond interaction being stronger in the triplet state than the ground state. The increased carbonyl located hydrogen-bond strength in the triplet state results in substantial modification of both the electronic and the structural conformation so that the triplet of the hydrogen-bond complex can be considered as a distinct species from the free MAP triplet state. This provides a framework to interpret the differences observed in the TR3 spectral and triplet lifetime obtained in the neat MeCN solvent (attributed to the free MAP triplet state) and the 50% H2O/50% MeCN solvent (due to the triplet of the hydrogen-bond complex). Temporal evolution at early picosecond times indicates rapid ISC conversion, and subsequent relaxation of the excess energy of the initially formed energetic triplets occurs for both the free MAP and the hydrogen-bond complex. The triplet of the carbonyl hydrogen-bond complex appears to be generated directly from the corresponding ground state complex and it does not dissociate back to the free triplet state within the triplet state lifetime. We briefly discuss the influence of the carbonyl hydrogen-bond effect on the pi pi* triplet reactivity for MAP and closely related compounds.

Efficient photodecarboxylation of 3- and 4-acetylphenylacetic acids in aqueous solution

The photochemistry of 3- and 4-acetylphenylacetic acids (6 and 7) has been studied in aqueous solution. This work is a continuation of research efforts aimed at understanding the structural effects on the efficacy for benzyl carbanion photogeneration via photodecarboxylation. The nitro group (at the 3- and 4-positions) is known to be an exceptionally good activating group on the benzene ring — because of its enhanced electron-withdrawing effect in the excited triplet state — for photodecarboxylation and the related photo-retro-aldol type process. It is shown in this work that the acetyl group is an equally good activating group for the photodecarboxylation. Thus, the photochemistry of 6 and 7 parallels much of what was observed for the corresponding nitrophenylacetic acids 1 and 2. Both 6 and 7 photodecarboxylate efficiently (Φd = 0.60 and 0.22, respectively, at pH 7) via the carboxylate form, to give observable (by laser flash photolysis) benzyl carbanion or related intermediates. The meta isomer 6 displays an acid-catalyzed pathway for photodecarboxylation at pH < 3 and along with its enhanced overall reactivity, is consistent with a meta effect of the acetyl group. Triplet state reactivity is inferred from sensitization and laser flash photolysis experiments. Based on the results of this work, the acetyl group may now be viewed as an "enhanced" electron-withdrawing group (in the excited state) when attached to a benzene ring (i.e., in acetophenone derivatives) that is capable of inducing ionic reactions, which is not the traditional photochemistry that is expected from such compounds.Key words: photodecarboxylation, acid catalysis, acetophenone, photogenerated carbanion, enol, enolate.

Selectivity Control in Electron Spin Inversion Processes: Regio- and Stereochemistry of Paternò−Büchi Photocyclo- additions as a Powerful Tool for Mapping Intersystem Crossing Processes

Regio- and stereoselectivity in ene-carbonyl photocycloadditions depend on the spin multiplicity of the excited carbonyl state, although both singlet and triplet states produce the cycloadducts with comparable chemoselectivity. The correlation between selectivity and spin state was evaluated by concentration, temperature, and solvent viscosity studies. The higher selectivity observed for triplet reactions is rationalized by the optimal conformations of the intermediate 2-oxabutane-1,4-diyls for intersystem crossing (ISC) to the singlet manifold, controlled preferentially by spin-orbit coupling. This weak interaction connected with ISC can lead to substantial control of regio- and stereoselectivity. The role of hyperfine coupling is demonstrated by magnetic isotope effects.

Photobleaching kinetics of Photofrin in vivo and in multicell tumour spheroids indicate two simultaneous bleaching mechanisms

We present a detailed investigation of Photofrin® photobleaching and photoproduct accumulation. Fisher rats were sensitized with 10 mg kg−1 Photofrin and irradiated 24 h later with 514 nm light at 5 or 100 mW cm−2. Fluorescence spectra were collected from the skin throughout treatment, and sensitizer bleaching and fluorescent photoproduct formation were quantified using spectral analysis. Photofrin bleaching was slightly more rapid at the higher irradiance under these conditions. However, accumulation of photoproduct was significantly enhanced at lower irradiance. To interpret these unexpected findings, we developed a new mathematical model in which reactions between singlet oxygen (1O2) and the photosensitizer and reactions between the sensitizer triplet and biological targets are both allowed to contribute to bleaching. Predictions of this model were tested in experiments performed on EMT6 spheroids sensitized with concentrations of 2.5, 10 and 30 µg mL−1 Photofrin and subjected to PDT. Photofrin bleaching and photoproduct formation in these spheroids were measured using confocal fluorescence spectroscopy. In qualitative agreement with the mixed-mechanism model predictions, at the highest drug concentration Photofrin bleaching was more efficient via 1O2 reactions, while at the lowest concentration triplet reactions were more efficient. At all concentrations, photoproduct accumulation was greater under conditions of abundant oxygen.

Pitch pyrolysis kinetics from single TG curve

Global pitch pyrolysis kinetic parameters determined by different methods from single thermogravimetric curves are compared. Differential (Freeman–Carroll, Vachuška–Vobořil), integral (Coats–Redfern, Horowitz–Metzger) and the method of direct non-linear regression (NLR) were used. Direct non-linear regression is the most suitable when the whole kinetic triplet (activation energy, frequency factor and reaction order) is searched, but the results are not quite satisfactory. If the global first order kinetic of pitch pyrolysis is accepted, similar values of activation energy and frequency factor were obtained by different methods. Considering the quality of results and applicability Coats and Redfern method is recommended in this case.

Origin of Regioselectivity in Photocycloaddition Reactions of 2-Cyclohexenone with Cycloalkenecarboxylates

Abstract The regiochemical results in [2 + 2]photocycloaddition of 2-cyclohexenone (1) with 1-cycloalkenecarboxylates (2–4) significantly changed from head-to-head (hh) to head-to-tail (ht) adducts depending on increments of the ring-size, from four to six, of the carboxylates. The root of the drastic regioselective change was inferred from the transition state (TS) analysis of the triplet reactions, including the back-reaction at the PM5 and B3LYP levels. The TS energy of the first step (TS1) is the most important factor for this reaction. The change in TS energies at hh adducts was larger than that at ht adducts. For more detailed analysis, the TS energies of the hh and ht reactions (at the three reactions) were partitioned to the two reactant deformation (Edf) and the interaction (or repulsion, Eint) energies. The Edf and Eint energy changes were found to be dependent on the ring-size of the 1-cycloalkenecarboxylates 2–4. Thus, since the larger ring of 4 has relatively higher TS1 and deformation energies for giving the hh adduct, they may cause the preferential formation of the ht adduct, and result in the significant changes in the ht/hh ratio owing to the ring-size. The root of the significant hh diminution may be the dipole–dipole repulsion between the enone carbonyl and larger carboxylate (4) moieties. The improved PM5 and B3LYP/6-31+G(d) calculations are effective and economical for disclosing the origin of regioselectivity in [2 + 2]photocycloadditions.

New Results on the Triplet Reactivity of β,γ‐Unsaturated Carbonyl Compounds

AbstractFor Abstract see ChemInform Abstract in Full Text.

Photodimerization of coumarin revisited: Effects of solvent polarity on the triplet reactivity and product pattern

Download options Please wait... Article information DOI https://doi.org/10.1039/B312335A Article type Paper Submitted 03 Oct 2003 Accepted 19 Nov 2003 First published 12 Dec 2003 Download Citation Phys. Chem. Chem. Phys., 2004,6, 368-376 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Photodimerization of coumarin revisited: Effects of solvent polarity on the triplet reactivity and product pattern T. Wolff and H. Görner, Phys. Chem. Chem. Phys., 2004, 6, 368 DOI: 10.1039/B312335A To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Thomas Wolff Helmut Görner

Triplet photochemistry within zeolites through heavy atom effect, sensitization and light atom effect

Methods to generate triplets of organic molecules within zeolites have been established by employing the Zimmerman rearrangement of barrelenes, oxa-di-π-methane rearrangement of β,γ-unsaturated ketones and photodimerization of acenaphthylene as probe reactions. The two methods, heavy cation effect and triplet sensitization, are well established solution techniques and these work well within zeolites. The Zimmerman rearrangement of dibenzobarrelene is enhanced even within Li + and Na + exchanged zeolites and these are believed to be the result of slowing of the rearrangement to dibenzocyclooctatetraene from S 1 through cation-π interaction. The methods described here provide an opportunity to explore the control afforded by the zeolite environment on triplet reactions.

O( 3P) versus O( 1D) reaction dynamics with n-pentane: a crossed-beam imaging study

We present crossed-beam scattering results for reaction of ground state O( 3P) and electronically excited O( 1D) with n-pentane at a collision energy of 13 kcal/mol. The results were obtained using velocity map imaging with 157 nm single-photon ionization of the products. Using this probe, the only reaction channel observed was that producing the hydrocarbon radical and OH. The results show dramatically contrasting dynamics, with backward-sideways scattering dominating the ground state triplet reaction and isotropic/forward scattering for the singlet reaction.

Photoreaction between 2-benzoylthiophene and phenol or indole.

Laser flash photolysis, density functional theory (DFT) calculations, and product studies have been performed to understand the mechanism of photoreduction of 2-benzoylthiophene (BT) in the presence of phenol or indole. Time-resolved experiments showed that BT ketyl (BTH) and phenoxy (PhO) or indolyl (In) radicals are generated with high rate constants and quantum yields close to 1. However, low conversions (specially in the case of indole) of the starting reagents are obtained upon prolonged lamp irradiation, indicating that recombination within the radical pairs must occur to a large extent, regenerating the starting materials. The solvent-dependence of the quenching rate constants, together with DFT theoretical studies, indicate fundamental differences between the mechanisms of the reaction of BT triplet with phenol and indole. Thus, data for phenol agree with the involvement of a hydrogen-bonded exciplex BT(.)HOPh, where concerted electron and proton transfer leads to the BTH(.)OPh radical pair. However, in the case of indole, electron transfer at the BT(.)HIn stage precedes proton transfer. Finally, C-C cross-coupling products have been isolated and characterized in the preparative irradiation of BT in the presence of phenol and indole. The structures of the products have been confirmed by alternative synthesis.

2-Mercaptothioxanthone as a Novel Photoinitiator for Free Radical Polymerization

2-Mercaptothioxanthone (TX-SH), a hydrogen abstraction type photoinitiator for free radical polymerization, is synthesized and characterized. Its capability to act as an initiator for the polymerization of methyl methacrylate (MMA), styrene (St), and multifunctional monomers is examined. The relative efficiencies of TX-SH, the parent thioxanthone (TX), and their combination with an amine synergist such as N-methyldiethanolamine (MDEA) are compared in the photopolymerization. The postulated mechanism is based on the intermolecular reaction of triplet, 3TX-SH*, with the thiol moiety of ground-state TX-SH. Thus, when TX-SH is irradiated in the presence of a monomer, it can serve as both a triplet photosensitizer and a hydrogen donor. The resulting thiyl radicals initiate the polymerization. Incorporation of TX-S groups into polymers was demonstrated by spectroscopic methods.

Laser flash photolysis study of the photochemical formation of colloidal Ag nanoparticles in the presence of benzophenone

Sensitized photoreduction of Ag+ ions and subsequent formation of colloidal Ag nanoparticles take place when an aqueous solution of a mixture of AgClO4, sodium dodecylsulfate (SDS) and benzophenone (BP) is irradiated with near-UV light. In this study, the mechanism and kinetics of the reaction have been studied by means of nanosecond laser flash photolysis, time-resolved emission spectroscopy and also steady-state photolysis. Benzophenone ketyl radial (BPK) is formed by the hydrogen abstraction reaction of the benzophenone triplet (BPT) from SDS, which reduces Ag+ ions. It is found that the yield of colloidal Ag nanoparticle formation decreases remarkably with increasing concentration of Ag+ ions, indicating the efficient quenching process of BPT by Ag+ ions. Rate constants for the oxidation of BPK as well as the quenching of BPT by Ag+ ions in SDS micellar solution are considerably larger than those expected in the bulk solution, reflecting that Ag+ ions and BP molecules are concentrated in the SDS micelles. The rate of colloidal Ag nanoparticle formation, VAg, and BP photobleach, VBP, under steady-state irradiation with near-UV light (λ = 365 nm) were evaluated and the dependence of VAg and VBP on the concentration of Ag+ ions is interpreted quantitatively in terms of rate constants estimated by laser flash photolysis.

Stereochemical memory in the temperature-dependent photodenitrogenation of bridgehead-substituted DBH-type azoalkanes: inhibition of inverted-housane formation in the diazenyl diradical through the mass effect (inertia) and steric hindrance.

The photochemical denitrogenation of the cyclopentene-annelated DBH-type azoalkanes 1 has been examined in solution as a function of bridgehead substitution and temperature. For all derivatives, namely, the unsubstituted 1a(H/H), monomethyl 1b(Me/H) dimethyl 1c(Me/Me), monophenyl 1d(Ph/H), and diphenyl 1e(Ph/Ph), the temperature-dependent ratio of syn and anti housanes 2 provides experimental support for a competition between the singlet (high temperature) and triplet (low temperature) reaction channels in the direct photolysis. The syn/anti ratio of the housanes 2 depends on the extent and type of bridgehead substitution; the amount of the anti diastereomer (retention) follows the order Ph > Me > H, and double substitution is more effective than single. This stereochemical memory is interpreted in terms of the mass effect (inertia) of the substituents and steric interaction (size) between the substituents at the bridgehead and the methylene bridge during the deazetation step of the transient diazenyl diradical conformations (1)DZ (exo-ax) and (1)DZ (exo-eq). These conformers are impulsively generated upon decay of the (1)(n,pi)-excited azoalkane, a trajectory assessed through computational work. The new mechanistic feature disclosed by the unprecedented anti stereoselectivity (retention) is the intervention of a puckered 1,3-cyclopentanediyl singlet diradical (1)DR as product bifurcation step, whose conformational relaxation to the planar species (loss of stereochemical memory) is encumbered by bridgehead substitution.

Intramolecular [2+2] cycloaddition of some 1, n-diene-1,1-dicarbonitriles : Difference between singlet and triplet reactivity

Intramolecular [2+2] cycloadditions on direct irradiation have been observed with four 1-alkene-1,1-dicarbonitriles bearing an additional CC bond positioned in Δ 5, Δ 6, or Δ 7within a flexible chain. The cycloadducts formed are the “parallel” regioisomers. These cycloadditions occur by excitation of an intramolecular charge transfer complex formed by the two double bonds. Triplet sensitisation has also been investigated in one case and found also to lead to intramolecular [2+2] cycloaddition, however furnishing the “crossed” regioisomer. It appears that the triplet cycloaddition path, other than the singlet one, does not involve an exciplex but rather proceeds in two steps via the most stable triplet 1,4-diradical.

Quantum-Chemical Calculations of the Structure of the Triplet Reaction Complexes in Anionic Polymerization of Butadiene

A new mechanism of anionic polymerization of butadiene, radically changing the existing concepts on the structure and properties of the reaction complex, is suggested. It is stated that an elementary chemical process involves excitation into the low-lying triplet state of the “living” polymer–monomer complex, characterized as a transfer of a charge (electron) and cation (Li+ or Na+) from the terminal unit to the monomer molecule. In the framework of this concept, the probability of chemical bonding is determined by the spin density on the radical centers of reagent molecules. The semiempirical and ab initio 6-31G** calculations have revealed a strong interaction in the ground electronic state of the complex (5-10 kcal/mole) and low energies of the excited triplet levels (< 10 kcal/mole). A mechanism of cis-trans-isomerization in the terminal unit of the “living” polymer is suggested. The mechanism involves concerted rotation around the Cβ - Cβ bond and migration of Li between the Cα and Cγ atoms. The new approach is shown to be fruitful for microstructure analysis of polybutadiene in relation to the type of cation and the state of the terminal unit of the “living” polymer.

Kinetics of triplet–triplet annihilation in disordered organic solids on short time scale

In the present paper, the results of the investigation of the decay kinetics of delayed luminescence of disordered chrysene on short times are presented. A strong deviation of the decay of both phosphorescence and annihilation delayed fluorescence from the exponential law is observed. This effect is shown to be due to the relaxation process of electronic excitation energy in the system with large energetic disorder. At the same time, the reaction of triplet–triplet annihilation is well described by the second-order classical equation with a time-independent rate coefficient. This permits concluding that delayed fluorescence results from the annihilation of triplet excitations cascading down in the system of energetically disordered centres, and this process is in no way spatially limited. The mechanisms leading to the power law of annihilation delayed fluorescence decay in the region of low temperatures and their universality are also discussed.