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PubMed2000

A computational study of the factors controlling triplet-state reactivity in 1,4-pentadiene

The triplet-state reactions of 1,4-pentadiene have been investigated using density functional theory (UB3LYP) and ab initio (CASSCF) calculations with a 6-31G basis set. Intramolecular [2 + 2] photocycloadditions and three different reaction pathways leading to vinylcyclopropane have been examined. The computed results are in good agreement with the experimental observations, predicting the dominant product to be vinylcyclopropane produced by a di-pi-methane rearrangement, and the favored [2 + 2] cycloaddition product to be bicyclo[2.1.0]pentane. Reaction pathways involving initial C-C or C-H bond cleavage were found to be too high in energy to be significant. Both the [2 + 2] cycloadditions and the di-pi-methane rearrangement proceed through cyclic biradical intermediates formed on the triplet surface. The relative rates of formation of these triplet biradicals are found to depend on three factors: biradical stability, the geometry of the transition structure, and orbital interactions through bonds.

Journal Article↗
PubMed2000

Generation, characterization, and kinetics of triplet Di[1,2,3,4,5,6, 7,8-octahydro-1,4:5,8-di(ethano)anthryl]carbene.

The title carbene has been generated by photolysis of the corresponding diazo precursors and studied by spectroscopic means, i. e., electron paramagnetic resonance (EPR) and UV/vis spectroscopy in matrixes at low temperature and laser-flash photolysis in solution at room temperature, with the product analysis. The results are compared with triplet di(2,3,5,6-tetramethylphenyl)carbene, an open-chain counterpart, which revealed that bicycloalkyl groups are acting as a fairly good kinetic protector for the triplet carbene as opposed to the open-chain counterpart. The formation of all-hydrocarbon triplet carbenes having a half-life over a second under normal conditions was realized for the first time. Effects of para-substituents on the structure and reactivities of the carbene are also investigated and discussed in terms of polar and spin electronic effects.

Journal Article↗
PubMed2002

Hydrogen atom tunneling in triplet o-methylbenzocycloalkanones: effects of structure on reaction geometry and excited state configuration.

The rates of phosphorescence decay of 4,7-dimethylindanone (2), 6,9-dimethylbenzosuberone (3), and several related compounds have been analyzed between 4 and 100 K to determine the contributions of intramolecular hydrogen atom tunneling from the o-methyl group to the excited state carbonyl oxygen. Changes in the benzocycloalkanone ring size from five to seven not only affect the geometry at the reaction center, but they also affect the electronic configuration of the triplet excited state in a significant manner. While the triplet state of 5,8-dimethyltetralone (1) in nonpolar glasses can be clearly described as having a predominant n,pi configuration, compounds 2 and 3 have a significantly larger contribution of the less reactive pi,pi state. 4,7-Dimethylindanone (2) is stable under cryogenic conditions and in solution at ambient temperature. In contrast, triplet lifetimes and product analysis indicate that 6,9-dimethylbenzosuberone (3) reacts by quantum mechanical tunneling at temperatures as low as 4 K. A surprisingly small isotope effect k(H)/k(D) approximately 1.1 between 4 and 50 K increases steadily up to k(H)/k(D) approximately 5.1 at 100 K. This unusual observation is interpreted in terms of a vibrationally activated quantum mechanical tunneling process with hydrogen atom transfer at the lowest temperatures being mediated by zero-point-energy reaction-promoting skeletal motions. Results presented here indicate that the combined effects of increasing pi,pi character and unfavorable reaction geometry contribute to the diminished reactivity of o-methyl ketones 2 and 3 as compared to those of tetralone 1.

Journal Article↗
PubMed2002

Triplet energy distribution in photoinitiators containing two dissociable groups.

Suitable probe molecules containing the benzophenone chromophore and one dissociable bond [perester 1 or C-X (4, 5)] or two such bonds (2, 3) have been synthesized and studied to examine intramolecular triplet energy dispersion. Triplet energies and phosphorescence quantum yields as well as quantum efficiencies of bond scissions have been studied, and the flow of triplet energy in such molecules is discussed. Upon irradiation at 350 nm in either benzene or methanol, the target peresters undergo dissociation of both cleavable groups, producing a pair of radicals. The presence of a benzylic chloride function has little influence on the efficiency of perester dissociation. However, the presence of a benzylic bromide function was found to decrease the quantum yield of decomposition of the perester function of 3. This can be explained by taking into account the effect of the heavy atom and the rate of cage geminate radical pair recombination. The nature of the heavy atom perturbation, however, was found to be different in 5 as compared with 3.

Journal Article↗
PubMed2003

Spin delocalization by triple-bonded functionalities in propargyl and heteropropargyl radicals, assessed from the EPR-spectral D parameter of 1,3-cyclopentanediyl triplet diradicals.

The cyclopentane-1,3-diyl triplet diradicals T and T' with the triplet-bonded acetylene, cyano, and isocyano functionalities at one of the radical sites are readily prepared from the corresponding azoalkanes by photodenitrogenation in a 2-methyltetrahydrofuran (2-MTHF) matrix at 77 K. The EPR-spectral D values of these triplet diradicals show that the spin delocalizing ability of the triple-bonded pi substituent follows the order -Ctbd1;CH > -NC approximately -CN. Good correlations of the D values have been obtained with the hyperfine coupling constants (a(H)) and with the calculated spin densities (PM3/AUHF-CI method) of the corresponding monoradicals M. The propargyl-type mesomeric structure is favored over the allenyl-type contributor for all three triple-bonded functionalities; spin delocalization is less pronounced in the heteropropargyl derivatives due to the electronegativity effect of the nitrogen atom.

Journal Article↗
PubMed2004

Stereodifferentiation in the decay of triplets and biradicals involved in intramolecular hydrogen transfer from phenols or indoles to pi,pi aromatic ketones.

Laser flash photolysis studies on (R,S) and (S,S) diastereoisomers of the bichromophoric compounds 1-6 have been used to investigate the possible chiral discrimination in the quenching of triplet excited ketones, resulting in formal hydrogen abstraction. Deuterium isotopic effects show that triplet deactivation in these bichromophores is dominated by hydrogen atom transfer. A remarkable stereodifferentiation is found in the intramolecular quenching of the ketone triplets of 1-3 and 5 by the phenolic or indolic moieties, either in methanol or acetonitrile as solvents. This indicates the existence of specific structural requirements for hydrogen transfer. On the other hand, the lifetimes of the generated biradicals show large solvent dependence; solvation appears to slow their reversion to the starting ketone. The considerable stereodifferentiation observed for the biradical lifetimes suggests that the kinetics of biradical decay is faster when the approach of the two radical termini becomes easier.

Journal Article↗
PubMed2004

Photonucleophilic aromatic substitution of 6-fluoroquinolones in basic media: triplet quenching by hydroxide anion.

Photoreaction of 1-ethyl-6-fluoro-7-(1-piperazinyl)-1,4-dihydro-4-oxoquinoline-3-carboxylic acid (norfloxacin, NFX) and other 6-fluoroquinolones in aqueous solution gives rise to the corresponding 6-hydroxy derivatives. Although two mechanisms have been proposed for this photonucleophilic aromatic substitution, direct evidence for any of them is still missing. Obtaining such evidence requires work in basic media, where intramolecular electron transfer from the piperazine ring to the quinolone system is the almost exclusive singlet deactivation pathway. To overcome this problem, the 4'-N-acetyl derivative of norfloxacin (ANFX) has been employed in the present paper due to the lower availability of the N lone pair. The photochemical and photophysical properties of ANFX have been studied in aqueous solutions at pH between 7.4 and 13. As expected, fluorescence of ANFX is not significantly quenched in basic media. Furthermore, the excited triplet state (lambda(max) = 620 nm) reacts with hydroxide anions with a rate constant of (0.3 +/- 0.1) x 10(6) M(-)(1) s(-)(1). This supports a direct attack by hydroxide anions to the excited triplet state with subsequent release of fluoride as the operating mechanism. The fact that the reaction is inhibited by the presence of naproxen (a water-soluble naphthalene derivative) as triplet quencher clearly confirms the mechanistic assignment.

Journal Article↗
PubMed2006

Photochemistry of 1,2-dihydronaphthalene oxide: concurrent triplet and singlet processes via singlet excitation.

The photochemistry of 1,2-dihydronaphthalene oxide (254 nm) was reexamined and indan was found to be a primary photoproduct, as well as the traditionally assumed secondary photoproduct. Quenching studies demonstrated that indan, as a primary photoproduct, is derived from a triplet pathway, competing with a singlet route, back to the ground state surface. CASSCF calculations strongly suggest that the triplet pathway consists of a dissociation of the oxirane moiety to give a triplet carbene and aldehyde, which via hydrogen abstraction-decarbonylation-ISC recloses to give indan. Conical intersections corresponding to the presumed 1,2-hydrogen shift and 1,2-alkyl shift to give 2-tetralone and 1-indancarbaldehyde, respectively, were located computationally.

Journal Article↗
PubMed1997

Additivity of the Electronic Meta-Substituent Effect in 3,5-Disubstituted Cumyl Radicals Assessed by the EPR D Parameter of 1,3-Arylcyclopentane-1,3-diyl Triplet Diradicals.

The D parameter (EPR zero-field splitting) of the 3,3',5,5'-tetrasubstituted triplet diradicals 6 (X = X' = H, NO(2), CH(3), OAc, OCH(3), NH(2), and OH) were determined in a MTHF matrix at 77 K and serves as a spectroscopic tool for the determination of electronic substituent effects in multiply-substituted benzyl-type monoradicals through its spin density dependence. The linear correlation (m = 2.00 +/- 0.01, r(2) = 0.974) of the experimental D values of these meta-disubstituted triplet diradicals 6 versus the 3,3'-disubstituted triplet diradicals 5 demonstrates the additivity of the electronic meta-substituent effect in the corresponding 3,5-disubstituted cumyl monoradicals 4 and is corroborated by theoretical (PM3-AUHF) spin density calculations for the latter. Thus, the combined use of the experimental D parameter and semiempirically calculated alpha spin density has provided for the first time the unambiguous demonstration of the additivity of electronic effects exerted by meta-substituents in the cumyl monoradicals 4.

Journal Article↗
PubMed2000

Paterno-Buchi photocyclization of 2-siloxyfurans and carbonyl compounds. Notable substituent and carbonyl (Aldehyde vs ketone and singlet- vs triplet-excited state) effects on the regioselectivity (Double-bond selection) in the formation of bicyclic exo-oxetanes

Paterno-Buchi coupling, photochemical [2 + 2] cycloaddition, of carbonyl compounds 2a-f with 2-siloxyfurans 1a-d has been investigated in detail. The stereoselective formations of exo-oxetanes 3 and 4 were observed in high yields. The regioselectivity (double-bond selection, 3 vs 4) was found to be largely dependent upon the carbonyls, the substituents at the furan ring, and the excited state of the carbonyls (singlet vs triplet). The photoreaction of aldehydes 2a-c gave bicyclic exo-oxetanes 3 and 4 at regio-random, independent upon their excited states and the substituents at furan ring. However, the photoreaction of the triplet state of ketones 2d-f was found to give regioselectively exo-oxetanes 4, except for the 4-methyl-2-siloxyfurane 1d case. The singlet-excited state of acetone 2f gave both oxetanes 3 and 4 at regio-random. For the singlet-state photochemistry, the approach direction of the electrophilic oxygen of the excited carbonyls to the furan ring is proposed to be an important factor for the exo-stereoselection. The Griesbeck model can rationalize the regio- and exo-selective formation of oxetanes in the triplet-state photoreaction.

Journal Article↗
PubMed2005

Exploring two-state reactivity pathways in the cycloaddition reactions of triplet methylene.

Spin forbidden 1,2-cycloadditions of triplet methylene to alkenes have been theoretically studied as an example of the two-state reactivity paradigm in organic chemistry. The cycloadditions of triplet methylene to ethylene and the (E)- and (Z)-2-butene isomers show spin inversion after the transition state and therefore with no effect on the reaction rate. A local analysis shows that while triplet methylene addition to alkenes leading to the formation of a biradical intermediate is driven by spin polarization, the ring closure step to yield cyclopropane is a pericyclic process. We have found that at the regions in the potential energy surface where the spin crossover is likely to occur, the spin potential in the direction of increasing spin multiplicity, mu(+)(s), tends to equalize the one in the direction of decreasing spin multiplicity, mu(-)(s). This equalization facilitates the spin transfer process driven by changes in the spin density of the system.

Journal Article↗
PubMed2005

Photoinduced omega-bond dissociation in the higher excited singlet (S2) and lowest triplet (T1) states of a benzophenone derivative in solution.

Photochemical properties of photoinduced omega-bond dissociation in p-benzoylbenzyl phenyl sulfide (BBPS) in solution were investigated by time-resolved EPR and laser flash photolysis techniques. BBPS was shown to undergo photoinduced omega-bond cleavage to yield the p-benzoylbenzyl radical (BBR) and phenyl thiyl radical (PTR) at room temperature. The quantum yield (phi(rad)) for the radical formation was found to depend on the excitation wavelength, i.e., on the excitation to the excited singlet states, S2 and S1 of BBPS; phi(rad)(S2) = 0.65 and phi(rad)(S1) = 1.0. Based on the CIDEP data, these radicals were found to be produced via the triplet state independent of excitation wavelength. By using triplet sensitization of xanthone, the efficiency (alpha(rad)) of the C-S bond fission in the lowest triplet state (T1) of BBPS was determined to be unity. The agreement between phi(rad)(S1) and alpha(rad) values indicates that the C-S bond dissociation occurs in the T1 state via the S1 state due to a fast intersystem crossing from the S1 to the T1 state. In contrast, the wavelength dependence of the radical yields was interpreted in terms of the C-S bond cleavage in the S2 state competing with internal conversion from the S2 to the S1 state. The smaller value of phi(rad)(S2) than that of phi(rad)(S1) was proposed to originate from the geminate recombination of singlet radical pairs produced by the bond dissociation via the S2 state. Considering the electronic character of the excited and dissociative states in BBPS showed a schematic energy diagram for the omega-bond dissociation of BBPS.

Benzophenones↗
PubMed2005

Nanosecond time-resolved infrared studies of visnagin and khellin triplets and radical ions.

Time-resolved infrared spectroscopy (TRIR) and density functional theory (DFT) calculations were used to directly observe and assign the vibrational spectra of the triplet states of visnagin and khellin, and to investigate their electron-transfer chemistry. The TRIR spectra of triplet visnagin and triplet khellin, and of their radical cations and anions, were obtained upon 266 nm laser flash photolysis in acetonitrile and in deuterated acetonitrile. The radical cations were observed in the presence of chloranil, and the radical anions were formed in the presence of NaI and KSCN. The TRIR spectra are in good agreement with the calculated vibrational spectra. We did not observe the related neutral radicals by TRIR spectroscopy upon laser flash photolysis (LFP) of khellin in the presence of hydroquinone, but we found evidence for the formation of semiquinone and neutral visnagin radicals upon LFP of visnagin and hydroquinone.

Ions↗
PubMed2006

Spin-lattice relaxation and rotational motion of aromatic triplet-state molecules in supercooled alkane solvents (part 1).

The phosphorescence of phenazine (PZ) and quinoxaline (QX) was investigated after pulsed laser excitation in the glass-transition range of several alkane solvents. Three relaxation processes of PZ and QX in the metastable triplet state, T1, were studied as a function of temperature: (1) the decay of the selective population of the strongly phosphorescent triplet substate T1x due to spin-lattice relaxation (SLR), (2) the time-dependent red shift of the phosphorescence spectrum due to the solvation of triplet-state molecules, and (3) the decay of the phosphorescence polarization due to orientational relaxation (OR). Various aspects and connections of the mechanisms governing the three relaxation phenomena are discussed. The relaxation dynamics were characterized at temperatures above the glass-transition temperature of the respective solvent, where the fundamental processes involved are strongly dependent upon the solvent viscosities. For the systems treated here, OR and solvation were satisfactorily described by a Vogel-Fulcher-Tammann temperature behavior. SLR also depends on properties of the alkane solvent above the glass transition. Upon cooling, SLR becomes independent of the specific solvent properties and is based on mechanisms that are typical for amorphous glasses or solids. (This particular aspect will be the subject of a subsequent publication, part 2).

Alkanes↗
PubMed2006

Quenching mechanism of Rose Bengal triplet state involved in photosensitization of oxygen in ethylene glycol.

The quenching rate constants of the excited triplet state of Rose Bengal (RB) by oxygen (k(obs)) were measured in ethylene glycol (EG) at different temperatures using nanosecond laser flash photolysis. Although a plot of the quenching rate constant k(obs) for RB triplet state vs oxygen concentration is linear at 20 degrees C, the oxygen dependence of k(obs) does not exhibit linearity but upward curvature at high temperatures from 130 to 140 degrees C. The upward curvature at high temperatures is not well-described by a kinetic scheme first postulated by Gijzeman et al., which is characterized by exciplex formation and a unimolecular dissociation of the exciplex to products, but instead by a more comprehensive mechanism involving a bimolecular dissociation in addition to a unimolecular one. The measurements of the oxygen dependence of k(obs) for RB triplet state at different temperatures yielded a reaction enthalpy for the exciplex formation of 150 kJ mol(-1). Due to the large exothermic reaction enthalpy, equilibrium was obtained for the exciplex at 20 degrees C even at low oxygen concentration and the bimolecular quenching by oxygen became the major dissociation process. The equilibrium attainment and bimolecular dissociation provide a linear oxygen dependence of k(obs) to all outward appearances. Therefore, linearity does not always mean that exciplex dissociation proceeds solely through a unimolecular mechanism.

Ethylene Glycol↗
PubMed2006

Spectroscopic characterization of a series of platinum acetylide complexes having a localized triplet exciton.

In this work, we describe the spectroscopic properties of a series of platinum complexes containing one acetylide ligand per platinum, having the chemical formula trans-Pt(PBu(3))(2)((C[triple bond]CC(6)H(4))(n)()-H)Cl, n = 1-3 (designated as half-PEn-Pt) and compare their spectroscopic behavior with the well-characterized series trans-Pt(PBu(3))(2)((C[triple bond]CC(6)H(4))(n)-H)(2), n = 1-3 (designated as PEn-Pt). This comparison aims to determine if the triplet state of PEn-Pt is confined to one ligand or delocalized across the central platinum atom. We measured ground-state absorption spectra, fluorescence spectra, phosphorescence spectra, and triplet-state absorption spectra. The ground-state absorption spectra and fluorescence spectra both showed a blue shift when comparing half-PEn-Pt with PEn-Pt, showing the S(1) state is delocalized across the platinum. In contrast, the phosphorescence spectra of the two types of compounds had the same 0-0 band energy, showing the T(1) state was confined to one ligand in PEn-Pt. The triplet state absorption spectra blue shifted when comparing half-PEn-Pt with PEn-Pt, showing the T(n) state was delocalized across the central platinum. This comparison supports recently published work that suggested this confinement effect (Rogers, J. E et al. J. Chem. Phys. 2005, 122, 214701).

Journal Article↗
PubMed2006

Products of the triplet excited state produced in the radiolysis of liquid benzene.

The radiation chemical yields of the products derived from the triplet excited state produced in the radiolysis of liquid benzene with gamma-rays, 10 MeV 4He ions, and 10 MeV 12C ions have been determined. Iodine scavenging techniques have been used to examine the formation and role of radicals, especially the H atom and phenyl radical. For all irradiation types examined here, the increase in hydrogen iodide yields with increasing iodine concentration matches the increase in iodobenzene yields. This agreement suggests that the benzene triplet excited state is the common precursor for the H atom and the phenyl radical. Pulse radiolysis studies in liquid benzene have determined the rate coefficients for the reactions of phenyl radicals with iodine and with the solvent benzene to be 9.3 x 10(9) M(-1) s(-1) and 3.1 x 10(5) M(-1) s(-1), respectively. Direct measurements of polymer formation, which refers to trimers (C18) and higher order compounds (>C18), in liquid benzene radiolysis using gamma-rays, 4He ions, and 12C ions at relatively high doses have been performed using gel permeation chromatography. The yields of trimers increase from gamma-rays to 12C ions due to the increased importance of intratrack radical-radical reactions that can be scavenged by the radical scavenging reactions of iodine. On the other hand, the >C18 product yields decrease from gamma-rays to 12C ions. The structure of the polymer consists of a partly saturated ring as determined by infrared and gas chromatography/mass spectrometry studies. A schematic representation for the radiolytic decomposition of the benzene triplet excited state is presented.

Alpha Particles↗
PubMed2006

Triplet topology of self-assembled zinc porphyrin-pyridylfullerene complex.

This work extends a recent EPR study on light-driven electron and energy transfer in a self-assembled zinc porphyrin-pyridylfullerene (ZnP-PyrF) complex. We report on a triplet line shape analysis of the photoexcited PyrF monomer and the ZnP-PyrF complex dissolved in isotropic and anisotropic matrixes of different polarity, namely, toluene, tetrahydrofuran (THF), and the nematic liquid crystals (LCs), E-7 and ZLI-4389. The line shape of the unbound *(3)PyrF obtained in both isotropic matrixes exhibits triplet parameters similar to those obtained for other monoadducts of C(60) under similar experimental conditions. On the other hand, 8(3)PyrF oriented in the LCs shows a complicated line shape, which is attributed to two conformers: (a) an axial dominant (85%) configuration characterized by triplet parameters, similar to those obtained in the isotropic matrixes and (b) a bent configuration associated with spin density localized about the poles accompanied by sign reversal of the ZFS parameter D of the *(3)C(60) moiety. Further, since in both LCs the ZnP-PyrF complex mainly exhibits a conformation with axial symmetry, the differences between the electron and the energy transfer routes in each LC are attributed to their different polarity. This study reflects the strength of LC matrixes to serve as a topological tool, enabling us to determine the conformers' distribution and to differentiate between electron and energy transfer routes.

Journal Article↗