Singlet Species Research Articles

Photophysical Properties of Biphotochromic Dihydroindolizines. Ring-Opening into Extended Bis-Betaines

New photochromic spirodihydroindolizines (DHI) that are mono- (1a−e) or biphotochromic (3a−e) have been prepared. The monophotochromic DHIs 1d and 1e contain sterically highly demanding substituents. Photophysical studies, both static and time-resolved, were carried out with 1 and 3. Monophotochromic DHIs 1a−e show fluorescence and phosphorescence. Temperature decrease produces a hypsochromic shift in the fluorescence spectra. Time-resolved studies demonstrate that the photochromism is based on a one-photon electrocyclic photoreaction of 1 to the colored betaine. Only one transient of 0.6−6 μs lifetime could be detected for the transformation of 1a−e. For the bichromophoric DHIs 3a−e, two different transients were recorded with lifetimes of 1 and 45−680 μs. It was proved that the ring opening 1 → 4 (as well as 3 → 9) results from an excited singlet species, as demonstrated by the absence of an oxygen effect and by sensitization experiments. Viscosity effects underline the conformational transformations involved in the sequence 1 → T → 4 and 3 → T1 → T2 → 9 to occur in distinct steps, thus showing the detailed reaction paths of photochromic molecules 1 and 3.

Complexes of borane(1) [BH] and HBCN-, HBCO, HBCF+: a calculational and conceptual study

(BH,C,N)-, (BH,C,O) and (BH,C,F)+, designated by the general notation (BH,C,X)*, have been investigated ab initio with a double-zeta basis set augmented with polarization functions and diffuse functions on the nonhydrogen atoms (DZP(p)). Cyclic and acyclic systems with both B-C and B-X connectivities have been examined for both singlet and triplet states. Geometries were optimized at the MBPT(2) level and single-point energies were obtained at the CCSD, CCSD + T(CCSD) and CCSD(T) levels. Inclusion of electron correlation affects the equilibrium structures and the singlet-triplet gaps. A variety of new species and isomeric types have been identified from this work. For all three systems acyclic 3HBCX* is most stable. The acyclic triplet is linear for X = O and F and is bent for X = N. The analogous singlet species are stable only for X = N and F. At this point, for the acyclic HBXC* systems a well defined minimum has been found for only 1HBNC-. Cyclic singlet and triplet forms existfor HBCN and HBCO and a pseudo-cyclic form exists for 1HBCF-. As an aid in understanding these new species the computational results have been combined with variously derived intuition. Suggestions for possible routes to the gas-phase (BH,C,X)* complexes are made.

Intramolecular spin-interactions between two nitronyl nitroxide or imino nitroxide radical units linked to thieno[2,3- b]thiophene and thieno[2,3- b]thieno[3′,2′- d]thiophene chromophores

Diradical species 1N, 1I, 2N, and 2I, in which two nitronyl nitroxide ( N) or imino nitroxide ( I) radical centers are attached to thieno[2,3- b]thiophene ( 1) and thieno[2,3- b]thieno[3′,2′- d]thiophene ( 2) chromophores, respectively, were prepared and their intramolecular exchange interactions were investigated in frozen solutions by means of ESR spectroscopy and magnetic susceptibility measurements at cryogenic temperature. Temperature dependence of the ESR signal intensities due to the triplet state revealed that bis(nitronyl nitroxide) radicals, 1N and 2N, are ground-state singlet species with quite small singlet-triplet gaps. In bis(imino nitroxide) radical 1I the spin interaction was antiferromagnetic and the singlet-triplet gap was estimated to be 2J k B =−18 K . On the other hand, bis(imino nitroxide) 2I was revealed to be a singlet-triplet degenerate species. These results are discussed in terms of the intramolecular NO---S contact along with corroboration by the semi-empirical MO calculations.

Intramolecular Spin Interactions between Two Radical Units through Isomeric Chromophores, Thieno[3,2-b]thiophene and Thieno[2,3-b]thiophene

Abstract Diradical species which bear two nitronyl nitroxide groups or two imino nitroxide groups on thieno[3,2- b ]thiophene and thieno[2,3- b ]thiophene, respectively, were studied in a frozen solution by means of ESR spectroscopy to show those are the ground-state singlet species.

Intramolecular spin interactions between two radical units through isomeric chromophores, thieno[3,2-b]thiophene and thieno[2,3-b]thiophene

Diradical species which bear two nitronyl nitroxide groups or two imino nitroxide groups on thieno[3,2- b ]thiophene and thieno[2,3- b ]thiophene, respectively, were studied in a frozen solution by means of ESR spectroscopy to show those are the ground-state singlet species. The following diradicals are all ground-state singlet species.

Bridged versus classical structures in boron-substituted phosphinidenes. A theoretical study of singlet and triplet low energy species

Ab initio SCF-CI (6–31G**/MP2 and 6–31G**/MP4) computation of the low-lying singlet and triplet states of PBH 2 ( 1) and PBF 2 ( 2), has been carried out. In the triplet state, both compounds exhibit classical planar C 2v structures, 1t and 2t. In the singlet state, PBH 2 yields the bridged moiety 1s has only stable form, 1s being more stable than 1t. In contrast, PBF 2 yields two stable isomers. The most stable one, 2s has a planar C 2v structure and the second one, 2sa has a planar C s structure, where one F atom has migrated from B to P, via a sizeable activation barrier. In the latter case, 2t is more stable than 2s.

Some light on the concept of unreactivity arising from active center association in anionic polymerizations

AbstractA long held assumption in hydrocarbon based anionic polymerization involving organolithiums is that the aggregated species are unreactive and thus serve only as reservoirs from which the active singlet species momentarily emerge to participate in the initiation or propagation events. This work demonstrates that contrary to the notion of aggregate dormancy such species can participate in monomer addition. The approach involved freeze‐dried vitrified polystyryllithium which was found to be reactive toward butadiene and methanol vapors. The sub‐glass transition state of the polystyryllithium eliminates the establishment of the aggregate: singlet equilibrium as a result of the inability of chains to undergo diffusion. Furthermore, it is shown that the lower bound gradient for the dependence of propagation rate on active center concentration, within experimental error, is about 1/5 for the majority of those diene systems where the active center concentration covers at least two decades. A computer evaluation of the propagation rate: active center data was done with the aim of testing and determining the equilibrium constants of these aggregated systems.

Singlet oxygen in the environmental sciences

This review of the part played by the singlet states of molecular oxygen in the environment deals with atmospheric aspects. There are five bound excited states of molecular oxygen that correlate with two ground state, 3P, oxygen atoms. Of these, three are singlets, although the other two states (triplets) are closely associated with singlet oxygen processes, especially in the mesosphere. A weakly bound quintet state has been invoked, as well, in explaining some aspects of the physical chemistry of the singlet species. Of the three singlet states, the a1Δg is the most familiar. It has a low excitation energy, a long radiative lifetime, and is rather resistant to collisional deactivation in the gas phase. As a consequence, its chemistry has been susceptible to detailed study in the laboratory. These investigations, coupled with estimates of production rates, suggest that O2(a1Δg) is probably not important in initiating much chemical change in the lower atmosphere, at least in the gas phase; excited molecules dissolved in water droplets may promote chemical change under special circumstances. In the stratosphere and mesosphere, each of the bound excited states gives rise to characteristic emission features of the airglow, both by day and by night. The observational data, obtained from the ground, and from balloons, high-flying aircraft, rockets and satellites is surveyed as a background to examining the chemical and photochemical mechanisms by which the different states become excited. These mechanisms clearly differ by day and by night, and they also depend on the altitude from which the emission comes. The most intense feature of the oxygen dayglow, the Infrared Atmospheric Band, comes from O2(a1Δg) that is produced in the photolysis of ozone. Because dayglow measurements are sometimes used to derive ozone concentrations and altitude profiles in the atmosphere, the efficiency of production of the species in the photolysis of ozone is examined critically, and some unexpected laboratory findings are reported. The b1Σ+g state of oxygen is excited during the day largely by resonance scattering, although some is also populated by energy transfer from O(1D) to O2. At night, recombination of O(3P) atoms is the most likely source of excitation of all the states of oxygen. Laboratory experiments that bear on these processes are reviewed, and theoretical estimates of the partitioning of recombination events between the different states are presented. Direct recombination into the a1Δg and b1Σ+g states is unlikely to be efficient enough to produce the observed concentrations of these species, and some indirect process is thus implicated. Laser excitation experiments show that quenching of the three higher excited (ungerade) states of oxygen by O2 and, especially, N2, can generate O2(b1Σ+g) with high efficiency; similar experiments demonstrate explicitly that the quenching of O2(b1Σ+g) by the atmospheric gases yields O2(a1Δg). A consistent excitation scheme for the nightglow emissions is presented; this scheme also pays attention to the “auroral green” line produced by the 1S state of atomic oxygen, the intensities of which in the atmospheres of Earth and Venus provide some clues about the excitation of the molecular states. Finally, the laboratory studies are shown to indicate that the formation of excited molecular oxygen from vibrationally rich hydroxyl (OH) radicals is unlikely to be of major importance in the atmosphere.

Stability ladder of various HC2N conformers and their excitation energies

The molecular structure of various stationary points of HC2N has been studied using the quadratic configuration interaction including single and double substitutions with triples contributions (QCISD(T)). A Huzinaga–Dunning double-zeta plus polarization (D95**) basis set was used. A stability ladder of these species is calculated using the configuration interaction including single and double substitutions (SDCI) with the Davidson correction (SDCI+Q). The general contraction scheme of the [5s3p2d1f(C and N)/3s2p1d(H)] atomic natural orbital basis set was chosen. The natural orbitals obtained in the preceding complete active space self-consistent field (CASSCF) calculations were applied to the open-shell SDCI. The triplet cyanomethylene is bent, and the barrier to linearity is 1.4 kcal/mol at the single-reference SDCI+Q level of theory in accord with the Schaefer and Roos’ values (0.8–1.0 kcal/mol). The nine-reference SDCI+Q calculation increases the energy separation by 0.7 kcal/mol. The thermal energy change between the two (1 and 2) including the calculated zero-point vibrational energies is 1.4 kcal/mol. The enthalpy barrier (ΔH≠) to linearize 1 to transition state 2 in a (hypothetical) reaction is predicted to be 0.0 kcal/mol using the single-reference SDCI+Q bent-linear energy separation. As a result, experiments might find no barrier to inversion of bent triplet 1. The most stable singlet species 6 (ring form of HC2N) lies 7.7 kcal/mol above the triplet bent cyanomethylene. Vertical excitation energies from the ground-state (3A″) of triplet bent cyanomethylene to 1A′ and 1A″ are studied using the CASSCF and SDCI methods; the SDCI+Q energies are 0.93 and 1.11 eV, respectively. These excitation energies are compared with those in the methylene and oxygen molecules having a qualitatively similar electronic structure to the present HCCN species, where two electrons occupy near-degenerate π-like (or degenerate π) highest occupied molecular orbitals (HOMO) forming a triplet ground-state. The excitation energies of 0.93 and 1.11 eV are close to the relative energies of the 3, 8, and 9 to the most stable triplet bent cyanomethylene.

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...

Singlet ⇌ quintet transition in a six-coordinate iron(II) complex of a tridentate bis(pyrazolyl)pyridine ligand

The bis(ligand)iron(II) complex [Fe(L 1) 2](ClO 4) 2·H 2O ( 1) [L 1 = 2,6-bis-(pyrazol-1-ylmethyl)pyridine] displays in the solid-state anomalous magnetic properties which are associated with a temperature-induced singlet ( 1 A 1) ⇌ quintet ( 5 T 2) transition. Mössbauer spectral data confirm this and reveal separate contributions from the singlet and quintet species with a strong temperature dependence of their relative intensities. Variable-temperature magnetic and Mössbauer studies establish that [Fe(L 2) 2](ClO 4) 2·2H 2O ( 2) [L 2 = 2-(3,5-dimethylpyrazol-1-ylmethyl)-6-(pyrazol-1-ylmethyl)pyridine] and [Fe(L 3) 2](ClO 4) 2·2H 2O ( 3) [L 3 = 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine] are purely high-spin species. For 1 three d-d transitions originating from the quintet and singlet state have been identified.

Generation of electronically excited triplet species at the cellular level: A potential source of genotoxicity

Selected enzymatic systems can efficiently produce a product in the electronically excited triplet state. Earlier, only the formation of electronically excited singlet species was known. The formation of triplet species has been demonstrated with both normal substrates/metabolites and with xenobiotics, even at the cellular level. Triplet excited species have intrinsically much longer lifetimes than excited singlets, whereby they can be potentially important agents for normal and/or deleterious processes, including mutagenesis. Enzymically generated triplet species can damage DNA, even when protein coated, as in the case of the λ-phage of Escherichia coli. Some evidence of damage by triplet species has also been reported for intact cells. Triplet excited species may produce their effects through type I and/or type II dark photosensitization, that is, the events may be started by H • abstraction and/or singlet oxygen/superoxide ion production. The induction of lipid peroxidation, with concomitant clastogenic effects, appears to be of special importance.

A theoretical study of Te6 4+ and Te4S4 2+

Geometry optimizations and molecular orbital calculations were performed on Te64+ and Te4S42+ ions based on approximate density functional theory within the local-density approximation. The Te64+ ion was found to have an elongated trigonal-prismatic geometry (D3h) as its ground-state structure, in agreement with experimental results. A molecular orbital analysis of this non-classical structure in comparison to an alternative C2v boat structure for the Te64+ ion was performed and an explanation is offered for their relative stabilities. Experimentally, the Te4S42+ ion is reported to adopt a geometry isostructural to As4S4. The present calculations on Te4S42+ with the experimental geometry indicate that the ion has a half-filled doubly degenerate highest occupied molecular orbital which is strongly antibonding between the tellurium atoms. The geometries of Te4S42+ with triplet and singlet electronic structures have been optimized. These geometries differed from the experimental geometry, with a considerable lengthening of the Te–Te bonds for the triplet species, and cleavage of a Te–Te bond for the singlet species to produce a C2v structure similar to that of the isoelectronic Se82+ ion.

The reactions of silicon monoxide and aluminium monofluoride with 2,2′-bipyridyl

The high temperature species, SiO and AlF, have each been treated with 2,2′-bipyridyl in tetrahydrofuran solution at − 105 °C and in an adamatane matrix at − 196 °C to give products which have spectroscopic properties closely resembling the known compound [SiCl 2(bipy) 2]. When initially formed the products are highly coloured and paramagnetic in solution or in the adamantane matrix and their EPR spectra show the presence of singlet and triplet species. In the triplet species, two electrons are transferred from the silicon or aluminium centre to two bipyridyl ligands which must lie orthogonal to each other across the metal centres but it is uncertain whether the species are monomeric or oligomeric. Evaporation of the solutions causes irreversible precipitation of pyrophoric solids of composition SiO(bipy) or AlF(bipy).

A kinetic study of monomer and excimer fluorescence of pyrene lecithin in Langmuir-Blodgett films

The fluorescence of Langmuir-Blodgett films of 1-palmitoyl-2-pyrene-decanoylphosphatidylcholine in matrices composed of dipalmitoyl-, 1-palmitoyl-2-oleyl- and dioleylphosphatidylcholine was studied quantitatively at different concentrations and temperatures. Stationary quantum yields, time-resolved life-times and spectral data were used to determine the kinetics of pyrene fluorescence. Possible schemes for monomer and excimer decays are presented and rate parameters for the individual steps are evaluated. The main paths for the monomer decay involve two kinds of non-interacting singlet species. The first is quenched by interaction with another pyrene moiety or an aggregate in the ground state and does not form excimers. The other pyrene moiety is aggregated in such a way that only a small rotational motion is required for it to attain a dimeric configuration which might be a precursor of an excimer. The formation of pyrene excimer is mainly determined by a reorganization, diffusional rotation in or out of the plane of the pyrene moieties in the lattice. Evidence for energy migration in the lattice in the excimer formation process is presented. The excimer is kinetically distinguishable from the monitored monomeric species. Relative intensities of excimer emission are considerably reduced when the temperature decreases, indicating diffusional excimer formation at higher temperatures. Activation energies for excimer formation are evaluated at different temperatures and concentrations. Evidence for the reorganization of the molecular lattice by light and temperature is presented.

Nanosecond laser photolysis of 4,4′-biphenylbisazide in solution

The primary photochemical processes of 4,4′-biphenylbisazide (N 3RN 3) in cyclohexane at room temperature were investigated by nanosecond laser photolysis. In the photodissociation process, only one nitrogen molecule is eliminated from the two equivalent azide groups attached to the biphenyl ring. The eventual photoproduct (azo compound) is mainly formed through the dimerization of two azide—nitrene molecules (N 3RN) in the triplet ground state, as in arylmonoazides (RN 3). A precursor of the azide—nitrene molecule is produced immediately after excitation with a 5 ns laser pulse at 266 nm. The precursor with a lifetime of 19 ± 2 ns is a singlet species such as azide—nitrene, dehydroazepine and its valence isomers.

Ascorbate- and hemoglobin-dependent brain chemiluminescence

It has been indicated recently that ascorbic acid is responsible for the hemoglobin-mediated oxidative damage to the central nervous system (Sadrzadeh & Eaton, J. Clin. Invest. 82: 1510–1515, 1988). In this paper we describe the changes in chemiluminescence accompanying hemoglobin- and ascorbate-dependent oxidative injury to brain tissue. Addition of either hemoglobin (15 μM) or ascorbate (1 or 2 mM) to rat brain homogenates stimulated spontaneous chemiluminescence in a synergistic manner. This increase in chemiluminescence was inhibited by desferrioxamine indicating that free iron was involved in the reactions leading to lipid peroxidation. Preincubation with ascorbate oxidase inhibited both spontaneous and hemoglobin-dependent chemiluminescence, suggesting that ascorbate was required for the reactions leading to lipid peroxidation. Supplementation with aminotriazole (an irreversible inhibitor of the catalase-H 2O 2 complex) increased chemiluminescence in a time- dependent manner, as catalase reacted with accumulated H 2O 2, suggesting that ascorbic acid has a dual action being involved in the production of H 2O 2 and also maintaining Fe in the reduced state to catalyze a Fenton-like reaction. The excited species responsible for the chemiluminescence were partly characterized by adding specific fluorescent energy acceptors: dibromoanthracene (DBA) and diphenylanthracene (DPA). Both DBA and DPA stimulated chemiluminescence several-fold indicating that triplet and singlet species are responsible for the observed chemiluminescence. Excited singlet carbonyls (identified with DPA) may be produced during the collision of two ROO·. Singlet oxygen may also be generated during the same reaction. It decays to the triplet state (emitting chemiluminescence at 634 nm) and reacts with double bonds producing dioxetanes, which may breakdown generating triplet carbonyls (identified with DBA). These results support the hypothesis that during stroke, the release of hemoglobin into the central nervous system (with high concentration of ascorbic acid) leads to the formation of oxidative species responsible for lipid peroxidation.

ChemInform Abstract: Photochemistry of Matrix‐Isolated Di‐tert.‐butyldiazidosilane. Observation of Di‐tert.‐butylsilylene and N,N′‐Di‐tert.‐butylsilanediimine.

AbstractThe major product from 254 nm irradiation of the matrix‐isolated title compound (I) is di‐tert.‐butylsilylene (III), a highly ground‐state singlet species with λmax 480 nm, which undergoes a subsequent photochemical C‐H insertion to give the stable silacyclopropane (IV).

Metal-Complexes of 1,10-Phenanthroline Derivatives. XV. Complexes of 2-(3,5-Dimethylpyrazol-1-yl)-1,10-phenanthroline

Iron(II) and nickel(II) bis ( ligand ) complexes of 2-(3,5-dimethylpyrazol-1-yl)-1,10-phenanthroline (L) have been prepared. The field strength of L as determined from spectral data for the nickel complex is in the range encompassing the critical value at the singlet (1A1) ↔ quintet (5T2) crossover for iron(II). The magnetic properties of the iron(II) complex both in solution and in the solid state are anomalous, and indicative of the occurrence of a thermally induced spin transition. Mossbauer spectral data confirm this, and reveal separate contributions from the singlet and quintet species with a strong temperature-dependence of their relative intensities. Magnetic data for the complex in solution are consistent with a simple high spin ↔ low spin equilibrium, and lead to values of ΔH = 23�0.5 kJ mol-1 and ΔS = 66�5 J K-1 mol-1 for the low spin → high spin transformation.

Theoretical study of the U.V. spectrum of acetylene

Electronic ground state and lowest lying excited singlet species of predominantly valence character of acetylene are studied by means of the ab initio method. The potential curves are calculated fo...