Triplet Multiplicity Research Articles

Mechanism of Photosensitized Generation of Singlet Oxygen during Oxygen Quenching of Triplet States and the General Dependence of the Rate Constants and Efficiencies of O2(1Σg+), O2(1Δg), and O2(3Σg-) Formation on Sensitizer Triplet State Energy and Oxidation Potential

Rate constants of photosensitized generation of O2(1Σg+), O2(1Δg), and O2(3Σg-) have been determined for a series of ππ* triplet sensitizers with strongly varying oxidation potential (Eox), triplet energy (ET), and molecular structure, in CCl4. We demonstrate that one common dependence on Eox and ET successfully describes these rate constants for the molecules studied here and also for all previously investigated ππ* sensitizers, independently of molecular structure or any other parameter. Photosensitized singlet oxygen generation during O2 quenching of ππ* triplet states can be generally described by a mechanism involving the successive formation of excited noncharge transfer (nCT) encounter complexes and partial charge transfer (pCT) exciplexes of singlet and triplet multiplicity 1,3(T13Σ), following interaction of O2(3Σg-) with the triplet excited sensitizer. Both 1,3(T13Σ) nCT and pCT complexes decay by internal conversion (ic) to yield O2(1Σg+), O2(1Δg), and O2(3Σg-) and the sensitizer ground state. ic is the rate-limiting step in the nCT channel, whereas exciplex formation is rate determining in the pCT channel. Rotation of the O2 molecule within the solvent cage of 1,3(T13Σ) nCT complexes is fast enough to allow for a completely established intersystem crossing (isc) equilibrium, whereas significant noncovalent binding interactions slow rotation and inhibit isc between 1(T13Σ) and 3(T13Σ) pCT complexes. Upon the basis of this mechanism, we propose a semiempirical relationship that can be generally used to estimate rate constants and efficiencies of photosensitized singlet oxygen generation during O2 quenching of ππ* triplet states in CCl4. The data set includes 127 rate constants for derivatives of naphthalene, biphenyl, fluorene, several ketones, fullerenes, porphyrins and metalloporphyrins, and other homocyclic and heterocyclic aromatics of variable molecular structure and size. It is suggested that the general relationship presented here can be used for the optimization of the singlet oxygen photosensitization ability of many molecules, including those used in biological and medical applications, such as the photodynamic therapy of cancer.

An ab initio study of water–oxygen complexes (O 2–W n, n=1–6) in the ground and lowest singlet excited states

Geometries of the O 2–W n ( n=1–4) complexes with triplet multiplicity were optimized at the UMP2/6-311++G** level of theory. The triplet ground and lowest singlet excited state geometries of O 2 and the O 2–W n ( n=1–6) complexes were optimized at the B3LYP/6-31+G* level of theory. Various methods were tested to study the lowest singlet excited states of O 2 and the O 2–W n ( n=1–6) complexes, and it was found that the B3LYP/6-31+G* method worked well and was economical enough to permit a detailed study of O 2–W n ( n=1–6) complexes. The CCD/AUG-cc-pVDZ and B3LYP/6-31+G* methods were found to yield similar results for the triplet ground and lowest singlet excited states of O 2 and the O 2–W 1 complex. Microsolvation of O 2 in water clusters ( n=1–5) in the ground state does not affect the structures of the latter. However, microsolvation of O 2 in the ground state modifies the structure of the cage form of water hexamer appreciably such that the latter acquires a non-planar cyclic structure. The structure of the O 2–W 1 complex is strongly modified in going from the triplet ground state to the singlet excited state. Thus W 1 in the singlet excited state gets dissociated into the H and OH fragments both of which are bound to the same oxygen atom of the O 2 molecule. This structure of the O 2–W 1 complex in the singlet excited state is retained in the O 2–W n ( n=2,3) complexes also but not in the O 2–W n ( n=4–6) complexes where none of the molecules is dissociated and are bound to one another as in the triplet ground state. The singlet O 2–W 2 complex is an isomer of a hydrogen bonded complex of 2H 2O 2 and the former represents an activated state of the latter by about 1.06 eV. An appreciable amount of electronic charge is transferred to O 2 in the O 2–W n ( n=1–3) complexes from the water molecules, particularly from the hydrogen atoms. Thus O 2 in the singlet excited state of each of these complexes would behave somewhat like O 2 −.

Superexchange-Mediated Charge Separation and Charge Recombination in Covalently Linked Donor - Bridge - Acceptor Systems

Evidence is presented in support of the concept that electron transfer (ET) between a pair of chromophores may take place efficiently over large distances (>10 Å) by the mediation of an intervening saturated hydrocarbon medium. For example, ET is found to take place on a sub-nanosecond timescale through saturated norbornylogous bridges greater than 13 Å in length, by a superexchange (through-bond coupling) mechanism. The dependence of the ET dynamics on the bridge length and configuration are consistent with the operation of a superexchange mechanism. The distinction between molecular wire behaviour and superexchange-mediated ET is made. The distance dependence of ET dynamics through different types of bridges—saturated and unsaturated hydrocarbon bridges, proteins, and duplex DNA—is discussed and explained. Strategies for prolonging the lifetimes of charge-separated states are explored and discussed. In general, long-lived charge-separated species have been generated using giant multichromophoric systems in which the charges are separated by large distances, often exceeding 20 Å. In contrast, it is shown that very long-lived charge-separated states, possessing the triplet multiplicity, may be generated using short ‘dwarf’ dyads, in which the charges are less than 6 Å apart. Charge recombination in these species is slowed by the difference in electron spin multiplicity between the charge-separated state and the ground state.

Excited-State Properties of the Four Stereoisomers of 1-(9-Anthryl)-4-phenyl-1,3-butadiene: Evidence of Adiabatic and Diabatic Deactivation Pathways

The photophysics and photochemistry of the four geometrical isomers of 1-(9-anthryl)-4-phenyl-1,3-butadiene have been extensively studied by stationary and pulsed spectrometry in nonpolar solvent. Fluorimetric and laser flash techniques were used to measure the quantum yields and lifetimes of the lowest excited states of singlet and triplet multiplicity at different temperatures. This paper reports the spectral characterization of the four isomers and their derived parameters of the radiative, nonradiative, and reactive relaxation pathways. The isomerization proceeds through triplet diabatic and/or adiabatic mechanisms, generally with low quantum yield, owing to high torsional barriers in the S1 state and small T1 population quantum yields, due to the very fast S1 → S0 internal conversion. At high temperature, a singlet diabatic mechanism is also operative.

ChemInform Abstract: Recombination of Radical Ion Pairs of Triplet Multiplicity.

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The Reaction of CH2(X3B1) with O2(X3): A Theoretical CASSCF/CASPT2 Investigation

The reaction between CH 2 (X 3 B 1 ) + O 2 in the gas phase was investigated by carrying out CASSCF and CASPT2 calculations with the 6-31G(d,p) and 6-311+G(3df,2p) basis sets. The reaction proceeds by the addition of O 2 tomethylene and may follow a singlet and a triplet potential energy surface. In both cases, its fate is the carbonyl oxide (H 2 COO). The reaction in its singlet multiplicity is computed to be exothermic by 50.9 kcal/mol with an activation enthalpy of 1.9 kcal/mol at 298 K. The reaction in its triplet multiplicity is computed to be exothermic by 26.9 kcal/mol with an activation enthalpy of 5.4 kcal/mol at 298 K. According to classical transition state theory, the branching ratio for the reaction in its triplet state multiplicity changes from negligible at 298 K to about 26% at 1800 K, and this ratio equals the formation of atomic oxygen (O, 3 P). The following unimolecular decomposition of carbonyl oxides in its singlet and triplet multiplicity was also considered.

The role of spin-forbidden processes in N+(3P) + NH3 reactions in the gas phase

The singlet and triplet potential energy surfaces involved in the gas-phase reactions between N+ and ammonia have been explored through the use of high-level ab initio techniques. Our calculations at the CCSD(T)/6-311 + G(3df,2p) level of theory provide mechanisms for the formation of both NH3+ and NH2+ products in these reactions. Quite importantly, spin-forbidden processes associated with the cross-over between both potential energy surfaces play an important role in the formation of NH2+. This cross-over is favored by the fact that, although the entrance channel has an overall triplet multiplicity, some of the [H3, N2]+ singlet state molecular ions are more stable than the corresponding triplet analogs, because they exhibit N–N bonds significantly stronger. On the other hand, the triplet–singlet transition involves a typical one-center spin–orbit coupling, through the corresponding one-center orbital rotation, which renders this transition particularly effective.

Excited-State Behavior of Someall-trans-α,ω-Dithienylpolyenes

The photophysics and photochemistry of all-trans-α,ω-di(2-thienyl)polyenes (with chain length n = 2, 3, 4) and α,ω-di(3-thienyl)polyenes (with n = 3, 4) have been extensively studied by stationary and pulsed spectrometry. Fluorometric and laser flash techniques were used to measure the quantum yields and lifetimes of the lowest excited states of singlet and triplet multiplicity in nonpolar and polar solvents at different temperatures. The trans−cis photoisomerization was also investigated, at least for the shorter oligomers. The kinetic parameters of the radiative, nonradiative, and reactive relaxation pathways were evaluated and discussed by comparison with the parent compound having n = 1 (trans-1,2-dithienylethene) and the better known α,ω-diphenylpolyenes.

Effect of pyridyl and thienyl groups on the excited state properties of stilbene-like molecules

The relaxation properties of the lowest excited states of singlet and triplet multiplicity of sometrans-stilbene-like molecules with two-ring or three-ring polycyclic groups containing nitrogen and sulphur heteroatoms have been studied by stationary and pulsed fluorimetric and photolytic techniques. The kinetic parameters of the competitive relaxation processes, derived from the measured quantum yields and lifetimes, have been compared with the corresponding values of the hydrocarbon analogues to have information on the heteroatom effect on the photoisomerization mechanism. Then, π* states introduced by the nitrogen atom can play a deactivating role, mainly through vibronic coupling and consequent internal conversion to the ground state. The heavy atom introduced with the thiophene ring favours the spin-orbit coupling and then the rate of intersystem crossing to the triplet manifold. In any case, the position of the heteroatom is an important factor affecting the nature of the lowest excited states and then the radiative and reactive behaviour of these compounds. The presence of polycondensed rings leads to a decrease in the quantum yield of photoisomerization to thecis isomers, as reported for the corresponding hydrocarbons, owing to an increase in the torsional energy barrier. The latter is drastically reduced in the polar solvents.

The Mechanisms of Photochemical Reactions of o-Sulfobenzoic Imides

Abstract The photochemical processes of N-propyl-o-sulfobenzoic imide to the amides were investigated in ethanol and monocyclic aromatics, such as benzene, anisole, toluene, polymethylbenzenes, and benzonitrile. It was clarified that although both the S1 and T1 states took part in those photochemical reactions, the mechanisms were different in ethanol and those aromatics. The mechanism in ethanol is thought to involve hydrogen-atom abstraction from ethanol by a diradical formed after the extrusion of sulfur dioxide. The mechanism in aromatics involved an energy transfer from the singlet excited aromatic molecules to the sulfobenzoic imide and the addition of the resulting diradicals to the partner aromatics. Although the sulfobenzoic imide sensitized to a triplet state by acetophenone in benzene resulted as well in the formation of the arylated benzamide, this process was minor in comparison with the process via the S1 state. This was attributed to the triplet multiplicity of the diradical. The reactions of the sulfobenzoic imide with p-xylene and mesitylene were less predominant than those with toluene and anisole, which was attributable to a steric effect of plural methyl groups on the ring. The fact that no formation of the arylated benzamides took place in the case of benzonitrile could be explained by a strong inductitive effect of a cyano group of benzonitrile.

Theoretical Study of the Reaction CH(X2Π) + NO(X2Π). I. Determination of Some Reaction Paths in the Lowest Triplet Potential Energy Surface

In this paper, which is the first of a series devoted to some aspects of the title reaction, we present the theoretical results concerning the topology of the lowest potential energy surface of triplet multiplicity. Results at the ab initio CASPT2 level with a complete active space self-consistent field (CASSCF) reference wave function built on a D95 Dunning basis set and involving 10 electrons in 10 orbitals are reported. This triplet surface is expected to play an important role in this reaction since experimental results show HCN(X1Σ+) as the main product, and this product can only be obtained via an HCNO(3A) hypersurface. We find that the channel leading to the formation of HCN(X1Σ+) + O(3P) is the most likely product channel, in agreement with available experimental findings. To a lesser extent, we can also observe the formation of the product channels HCO(X2A‘) + N(4S), NCO(X2Π) + H(2S), and CO(X1Σ+) + NH(X3Σ-). Conversely, the formation of CN(X2Σ+) + OH(X2Π) is highly unlikely because of the existence of high potential energy barriers along this reaction path.

4,5‐Diazafluorenylidene: Preparation, Reactivity, Multiplicity, and Selectivity Studies

AbstractThe chemistry of 4,5‐diazafluorenylidene (3), generated from the diazo compound 3a by photolysis, has been studied with regard to reactivity, multiplicity and selectivity. Reaction of 3 in 2,3‐dimethylbutene gave exclusively ketazine 24. The [1 + 2] cycloaddition with styrenes 5–11 afforded cyclopropanes 16–23 in 25–54% yields and 24 as a side product. In these reactions no diazirine intermediate was involved. Irradiation of 3a in methanol afforded the ether 26 (25% yield). The stereoselectivity of 3 was studied by its addition to (E)‐ and (Z)‐β‐methylstyrene. A mixture of the (E)‐ and (Z)‐cyclopropanes 22 and 23, respectively, was detected. Thus for 3, triplet multiplicity was concluded. The results of competition experiments with singlet/triplet scavengers (methanol/dimethylbutadiene) demonstrated that an equilibrium between singlet and triplet manifold of 3 exists at room temperature. Selectivity studies with carbene 3 were carried out by evaluating the competition experiments of the cycloaddition of 3 with differently substituted styrenes. A plot of the krel values from the competition studies versus Hammett σ constants gave ρ = −0.65 indicating that carbene 3 reacts as an electrophile.

Pressure effect on the quenching of singlet oxygen by amines in different solvents

AbstractRate constants kq of the quenching of singlet oxygen in its a1Δg‐state by five amines (piperazine (P), N,N′‐dimethylpiperazine (DMP), N‐methylpiperidine (MPP), quinuclidine (Q), N,N,N′,N′‐tetramethylphenylene‐1,4‐diamine (TMPD)) have been determined in solvents with different polarity in dependence on pressure up to 1200 or 1600 bar using time‐resolved phosphorescence techniques. In the case of the efficient O2(a1Δg) quencher TMPD in o‐dichlorobenzene and in benzonitrile at about 700 bar the transition is observed from the reaction‐controlled rate to the diffusion‐controlled rate limit. For all amines the highest negative value of the experimental activation volume was found in n‐hexane (−33.0 ≥ ΔV ≥ −42.0 cm3 · mol−1) and the lowest negative value in benzonitrile (−9.0 ≥ ΔV ≥ −19.0 cm3 mol−1). These results are consistent with the assumption that quenching of O2(a1Δg) by amines occurs via polar exciplexes with singlet and triplet multiplicity, respectively. The data were analysed taking into account contact complex formation. The reaction volume ΔVc of the formation of the contact complex was evaluated using the statistical model developed by Yoshimura and Nakahara. On the basis of Kirkwoods electrostriction model the dipole moments of the singlet exciplexes were calculated in the range between 5.7 (P) and 11.5 Debye (TMPD), indicating that only a partial charge transfer takes place in the quenching process.

Optical and ODMR Studies of Luminescent Halo(dimethylphenylphosphine)gold(I) Crystals.

Crystals of {(Me(2)PhP)AuX}(n) (Me = methyl; Ph = phenyl; X = Cl, Br, I; n = 2, 3) show emission from two excited states. Both states are assigned a triplet multiplicity, on the basis of their lifetimes and zero-field splittings. The structured, higher energy emission originates at approximately 360 nm and has the greater relative intensity at low temperatures. It is assigned as intraligand phosphorescence from a phenyl-localized (3)pipi state. The unstructured, lower energy emission has a peak wavelength that varies in the range 630-730 nm. It is assigned as phosphorescence from the triplet state due to the gold-based sigma(p) <-- sigma(s,d) excitation. The corresponding singlet state is observed at 290-310 nm. The results of SCF-Xalpha-SW calculations on the model complexes H(3)PAuX and (H(3)PAuX)(2) are also presented.

Role of oxygen in phagocyte microbicidal action.

Immune information in the form of inflammatory mediators directs phagocyte locomotion and increases expression of opsonin receptors such that contact with an opsonized microbe results in receptor ligation and activation of microbicidal metabolism. Carbohydrate dehydrogenation and O2 consumption feed reactions that effectively lower the spin quantum number (S) of O2 from 1 to 1/2 and finally to 0. Oxidase-catalyzed univalent reduction of O2 (S = 1; triplet multiplicity) yields hydrodioxylic acid (HO2) and its conjugate base superoxide, O2- (S = 1/2; doublet multiplicity). Acid or enzymatic disproportionation of superoxide yields H2O2 (S = 0; singlet multiplicity). Haloperoxidase catalyzes H2O2-dependent oxidation of Cl- yielding HOCl (S = 0), and reaction of HOCl with H2O2 yields singlet molecular oxygen, 1O2 (S = 0; singlet multiplicity). The Wigner spin conservation rule restricts direct reaction of S = 1 O2 with S = 0 organic molecules. Lowering the S of O2 overcomes this spin restriction and allows microbicidal combustion. High exergonicity dioxygenation reactions yield electronically excited carbonyl products that relax by photon emission, i.e., phagocyte luminescence. Addition of high quantum yield substrates susceptible to spin allowed dioxygenation, i.e., chemiluminigenic substrates, greatly increases detection sensitivity and defines the nature of the oxygenating agent. Measurement of luminescence allows high sensitivity, real-time, and substrate-specific differential analysis of phagocyte dioxygenating activities. Under assay conditions where immune mediator and opsonin exposure are controlled, luminescence analysis of the initial phase of opsonin-stimulated oxygenation activity allows functional assessment of the opsonin receptor expression per circulating phagocyte and can be used to gauge the in vivo state of immune activation.

Magnetic field effect in the geminate recombination of radical ion pairs: Possible pitfalls in microsecond measurements of triplet reaction products

In the geminate recombination reaction of radical ion pairs with overall triplet multiplicity, locally excited triplet molecules are generated. The magnetic field effect (of the hyperfine type) on the triplet yield is usually determined by measuring the triplet absorbance both at zero and at higher applied magnetic fields. It is shown that the magnitude of this magnetic field effect decreases with time if the decay of the triplet concentration measured in the microsecond range is not of first order. This has been demonstrated experimentally using the linked acceptor–donor compound pyrene-(CH2)16-DMA as the test molecule in acetonitrile.

Theoretical study of the electronic spectrum of the CoH molecule

Calculated potential energy curves and spectroscopic parameters of the ground and various low-lying excited states of the cobalt hydride molecule are presented. Over 30 electronic states of singlet, triplet, and quintet multiplicity have been obtained with adiabatic excitation energies below 4 eV. In addition, the electronic structure of several negative ion states has been determined. CoH− possesses a 4Φ electronic ground state and at least three other electronic states that are stable with respect to electron autodetachment. The calculations include relativistic effects variationally by employing a one-component no-pair operator with external-field projectors. Electron correlation is accounted for by using multireference single and double excitation configuration interaction methods. Several experimentally observed bands in the optical spectrum of CoH are ascribed to transitions between the X 3Φ ground state and excited states of 3Φ, 3Δ, and 1Γ symmetry.

Ab initio studies of the lowest singlet and triplet potential energy surfaces of CO2S

Portions of the lowest singlet and triplet potential energy surfaces of CO2S were investigated by ab initio self-consistent field methods with the inclusion of electron correlation by Moller-Plesset perturbation theory. Earlier experiments indicated that the reaction of oxygen atoms with carbonyl sulphide occurred by a direct stripping mechanism to form the products CO(1Σ+) and SO(3Σ-). This reaction may proceed on the singlet potential energy surface through an OCSO intermediate. This singlet surface lies approximately 60 kcal mol-1 below the triplet and the barrier heights leading to the OCSO singlet minima are less than 30 kcal mol-1. Multiple intersystem crossings are required to connect ground state reactants to products as both have triplet multiplicity in contrast to the singlet intermediates. Two sets of bent cis and trans triplet structures were located, the SCOO and OCOS isomers, both of which are considerably higher in energy than the singlet species. Another series of low-lying singlet structu...

A Tr Esr study of the quenching of photoexcited dioxouranium (VI) salts by stable nitroxyl free radicals

TR ESR spectroscopy was applied to the study of the quenching of excited dioxouranium (VI) (uranyl) nitrate and sulfate by stable nitroxyl radicals of the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) family. Photoexcitation of uranyl in solutions of alcohols of moderate viscosity (η = 3-10 cP) in the presence of TEMPO leads to CIDEP signals of TEMPO due to a radical triplet pair mechanism (RTPM). Polarized nitroxyls were also observed in solutions of polyelectrolyte sodium poly(styrenesulfonate), NaPSS, in the presence of the nitroxyl with a positively charged trimethylammonium group. Photolysis of uranyl salts in solutions of alcohols leads to the generation of free radicals of alcohols. No CIDEP of these radicals was observed, distinguishing U2 2+* from its organic analog, the triplet benzophenone. The probable reason for the lack of polarization in uranyl photoreduction reactions is the difficult access of free radicals to the U atom of the solvated radical UO2+ (V); this atom bears the unpaired electron. The role of polyelectrolytes in the enhancement of the quenching of excited states is discussed. Results are in agreement with the statement that photoexcited uranyl has a triplet multiplicity.

Formation of electronically excited states during the interaction of p-benzoquinone with hydrogen peroxide.

The reaction between p-benzoquinone and H2O2 in slightly alkaline solutions yields three major quinoid products that accumulate in the reaction mixture: (a) 2,3-epoxy-p-benzoquinone, (b) 2-hydroxy-p-benzoquinone and (c) p-benzohydroquinone. The reaction is accompanied by photoemission, probably originating from excited triplet 2-hydroxy-p-benzoquinone. These products originate from hydrogen peroxide and hydroxide nucleophilic addition to the C2 = C3 double bond, as well as secondary redox interactions. The hydroxy substituent and the epoxide ring exert a substantial influence on the electronic distribution in the p-benzoquinone molecule leading to a decrease in the half-wave potential, as compared to the parent p-benzoquinone. The generation of electronically excited states is the result of reactions secondary to the nucleophilic additions involving 2-hydroxy-p-benzosemiquinone, H2O2 and hydroxyl radical. The process involves the primary oxidation of 2-hydroxy-p-benzosemiquinone by hydrogen peroxide, followed by oxidation of the semiquinone by hydroxyl radical leading to the formation of the electronically excited quinone. The decay of the excited triplet to the ground state is accompanied by photoemission with maximal intensity at 485-530 nm. Thermodynamic calculations along with an observed increase of photoemission intensity in anaerobiosis point to the triplet (n, pi*) multiplicity of the excited state. The efficiency of chemiluminescence could be calculated as 10(-8) photons/2-hydroxy-p-benzoquinone molecule formed. Photoemission arising from the p-benzoquinone/H2O2 reaction was inhibited efficiently by addition of GSH to the reaction mixture. This may be due to deactivation of the triplet quinone by a 2-glutathionyl-p-benzohydroquinone adduct, involving thioether alpha-hydrogen atom-transfer to the triplet ketone.