Triplet State Of Anthracene Research Articles

Visible-Light-Driven, Iridium-Catalyzed Hydrogen Atom Transfer: Mechanistic Studies, Identification of Intermediates, and Catalyst Improvements.

The harvesting of visible light is a powerful strategy for the synthesis of weak chemical bonds involving hydrogen that are below the thermodynamic threshold for spontaneous H2 evolution. Piano-stool iridium hydride complexes are effective for the blue-light-driven hydrogenation of organic substrates and contra-thermodynamic dearomative isomerization. In this work, a combination of spectroscopic measurements, isotopic labeling, structure–reactivity relationships, and computational studies has been used to explore the mechanism of these stoichiometric and catalytic reactions. Photophysical measurements on the iridium hydride catalysts demonstrated the generation of long-lived excited states with principally metal-to-ligand charge transfer (MLCT) character. Transient absorption spectroscopic studies with a representative substrate, anthracene revealed a diffusion-controlled dynamic quenching of the MLCT state. The triplet state of anthracene was detected immediately after the quenching events, suggesting that triplet–triplet energy transfer initiated the photocatalytic process. The key role of triplet anthracene on the post-energy transfer step was further demonstrated by employing photocatalytic hydrogenation with a triplet photosensitizer and a HAT agent, hydroquinone. DFT calculations support a concerted hydrogen atom transfer mechanism in lieu of stepwise electron/proton or proton/electron transfer pathways. Kinetic monitoring of the deactivation channel established an inverse kinetic isotope effect, supporting reversible C(sp2)–H reductive coupling followed by rate-limiting ligand dissociation. Mechanistic insights enabled design of a piano-stool iridium hydride catalyst with a rationally modified supporting ligand that exhibited improved photostability under blue light irradiation. The complex also provided improved catalytic performance toward photoinduced hydrogenation with H2 and contra-thermodynamic isomerization.

Energy and electron transfer reactions on silica gel and titania\u2013silica mixed oxide surfaces

The energy and electron transfer reactions of anthracene co-adsorbed with an electron donor on silica gel and titania–silica mixed oxides have been studied by a combination of steady-state reflectance, emission spectroscopy, and nanosecond diffuse reflectance laser flash photolysis. Bimolecular rate constants for energy and electron transfer between anthracene and azulene have been measured; kinetic analysis of the decay of the anthracene triplet state and radical cation show that the kinetic parameters depend on the titania content of the sample and the azulene loading. The rate of energy and electron transfer reactions increases as a function of azulene loading and decreases with increasing titania content in titania–silica mixed oxides. These findings indicate that the observed rate of reaction is determined by the rate of diffusion of anthracene on the titania–silica surfaces, whereas, in contrast, the observed rate of reaction on silica gel is predominantly governed by the rate of diffusion of azulene.Graphic abstract

Interplay between singlet and triplet excited states in a conformationally locked donor-acceptor dyad.

The synthesis and photophysical characterization of a palladium(II) porphyrin - anthracene dyad bridged via short and conformationally rigid bicyclo[2.2.2]octadiene spacer were achieved. A spectroscopic investigation of the prepared molecule in solution has been undertaken to study electronic energy transfer in excited singlet and triplet states between the anthracene and porphyrin units. By using steady-state and time-resolved photoluminescence spectroscopy it was shown that excitation of the singlet excited state of the anthracene leads to energy transfer to the lower-lying singlet state of porphyrin. Alternatively, excitation of the porphyrin followed by intersystem crossing to the triplet state leads to very fast energy transfer to the triplet state of anthracene. The rate of this energy transfer has been determined by transient absorption spectroscopy. Comparative studies of the dynamics of triplet excited states of the dyad and reference palladium octaethylporphyrin (PdOEP) have been performed.

Photosensitized Generation of Singlet Oxygen from Re(I) Complexes: A Photophysical Study Using LIOAS and Luminescence Techniques

Quantum yields and efficiencies of (1)O2 ((1)Δg) production along with photophysical properties for a number of Re(I) complexes in acetonitrile solutions are reported. Two different classes of Re(I) complexes, L(S)-CO2-Re(CO)3(bpy) (L(S) = 2-pyrazine, 2-naphthalene, 9-anthracene, 1-pyrene, 2-anthraquinone) and XRe(CO)3L (X = CF3SO3, py; L = bpy, phen), were probed as photosensitizers for (1)O2 ((1)Δg) production in air-saturated acetonitrile solutions. Depending on the nature of the Re(I) complex, the excited state responsible for the generation of (1)O2 ((1)Δg) is either a metal-to-ligand charge transfer ((3)MLCT) or a ligand centered ((3)LC) state. With L(S)-CO2-Re(CO)3(bpy) complexes, (1)O2 ((1)Δg) is produced by oxygen quenching of (3)LC states of anthracene and pyrene with high quantum yields (ΦΔ between 0.8 and 1.0), while the complexes bearing the ligands L(S) = 2-anthraquinone, 2-pyrazine, and 2-naphthalene did not yield (1)O2. XRe(CO)3L complexes generate (1)O2 ((1)Δg) mainly by oxygen quenching of their (3)MLCT luminescence with an efficiency of (1)O2 ((1)Δg) formation close to unity. Bimolecular rate constants for the quenching of the XRe(CO)3L complexes' emission by molecular oxygen range between 1 × 10(9) and 2 × 10(9) M(-1) s(-1), and they are all ≤ (1/9)kd, in good agreement with the predominance of the singlet channel in the mechanism of (1)O2 ((1)Δg) generation using these Re(I) complexes as photosensitizers. All the experimental singlet oxygen efficiencies are consistent with calorimetric and luminescence data for the studied complexes. With L(S)-CO2-Re(CO)3(bpy) complexes, calorimetric experiments were utilized in the calculation of the quantum yields of triplet formation; namely φT = 0.76 and 0.83 for the triplet states of anthracene and pyrene, respectively.

Triplet state kinetics of anthracene studied by pulsed electron paramagnetic resonance

Photoexcited triplet states of anthracene in a glassy toluene matrix were investigated using pulsed UV–laser photoexcitation and pulsed electron paramagnetic resonance spectroscopy. The triplet lifetime, the triplet relaxation rates between the high-field eigenstates and the populations of the triplet sublevels are quantitatively analysed. High-quality data allows for supporting the former conclusion that excitations into the individual zero-field components proceed via independent channels.

Intramolecular interactions and deuteration effect in nonradiative deactivation of lowest triplet state of anthracene and naphthalene

In the approximation of nonadiabatic interactions and considering all out-of-plane vibrational modes to be promoting ones, we calculate changes in the rate constants TsKdgs of the nonradiative degradation T1s ⇝ S0 of in-plane spin (s) triplet states as a result of the complete deuteration of anthracene and naphthalene molecules. We examine how the deuteration, the frequency factor, and the shape of promoting vibrational modes affect the squared matrix elements of both nonadiabatic coupling and adiabatic vibronically induced spin-orbit (VISO) coupling of electronic states. The compensation effect of spin-orbit interactions in structural elements of the carbon backbone of the anthracene molecule is ascertained.

Intramolecular Spin Alignment in Photomagnetic Molecular Devices: A Theoretical Study

Ground- and excited-state magnetic properties of recently characterized pi-conjugated photomagnetic organic molecules are analyzed by the means of density functional theory (DFT). The systems under investigation are made up of an anthracene (An) unit primarily acting as a photosensitizer (P), one or two iminonitroxyl (IN) or oxoverdazyl (OV) stable organic radical(s) as the dangling spin carrier(s) (SC), and intervening phenylene connector(s) (B). The magnetic behavior of these multicomponent systems, represented here by the Heisenberg-Dirac magnetic exchange coupling (J), as well as the EPR observables (g tensors and isotropic A values), are accurately modeled and rationalized by using our DFT approach. As the capability to quantitatively assess intramolecular exchange coupling J in the excited state makes it possible to undertake rational optimization of photomagnetic systems, DFT was subsequently used to model new compounds exhibiting different connection schemes for their functional components (P, B, SC). We show in the present work that it is worthwhile considering the triplet state of anthracene, that is, P when promoted in its lowest photoexcited state, as a full magnetic site in the same capacity as the remote SCs. This framework allows us to accurately account for the interplay between transient ((3)An) and persistent (IN, OV) spin carriers, which magnetically couple according to a sole polarization mechanism essentially supported by phenyl connector(s). From our theoretical investigations of photoinduced spin alignment, some general rules are proposed and validated. Relying on the analysis of spin-density maps, they allow us to predict the magnetic behavior of purely organic magnets in both the ground and the excited states. Finally, the notion of photomagnetic molecular devices (PMMDs) is derived and potential application towards molecular spintronics disclosed.

Interaction of substituted cobalt(III) cage complexes with DNA †

Polycyclic aromatic moieties covalently bound to the inert cationic cobalt(III)–sar cage complex (sar = sarcophagine = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane) intercalate with negatively supercoiled plasmid DNA, as shown by both spectrophotometric and gel electrophoresis studies. Although anthracene itself is not a good intercalating agent, the association constant of the complex between anthracene tied to the cobalt(III)–sar cage and DNA is ≈2 × 106 dm3 mol−1 and the average binding-site size on supercoiled plasmid DNA is ≈2.4 base pairs. The hydrophilic complex cation (a) solubilizes the anthracene moiety in water and (b) enhances DNA binding by its electrostatic interaction with the anionic phosphodiester backbone of DNA. While irradiation of the intercalated complexes at 254 nm led to single-strand cleavage of DNA, irradiation at higher wavelength, 302 and 365 nm, was much less effective. Neither molecular oxygen nor excited singlet or triplet states of anthracene appear to be involved in the cleavage process, and it is likely that the N-radical cation arising from ligand to metal charge or electron transfer in the phosphodiester–complex ion pair gives rise to oxidation of the deoxyribose moieties. Fission of the resulting deoxyribose diphosphate radical cation follows, leading to DNA single-strand cleavage. It also appears that the most easily reduced complex moieties are the most effective cleavage agents. Excited singlet and triplet states of anthracene arising from irradiation at 365 nm are efficiently quenched by DNA and the allowedness of the short-axis in-plane aromatic transitions is also sharply diminished. This in itself is unusual.

Anisotropic Intersystem Crossing from the Upper Excited Triplet States of Anthracenes: Two-laser Time-Resolved EPR Study

A two-laser, two-color time-resolved EPR (TREPR) technique has been applied to the investigation of the electron spin polarization in the excited triplet states of anthracene and its halogen anthracene in rigid glassy matrixes. The one-laser excitation of anthracenes with 355 nm gave the triplet TREPR spectra of the lowest excited triplet (T1) states with EEE/AAA polarization pattern. For 9-bromoanthracene and 9,10-dibromoanthracene, the reexcitation by a dye laser (442 nm) to the upper excited triplet (Tn: n > 3) states brought about the opposite phase polarization in the T1 states. The phenomena were interpreted in terms of the sublevel selective reverse intersystem crossing (RISC) from the Tn (n = 2 or 3) state to the S1 state. The time profiles of the polarization were analyzed taking into account the anisotropic RISC processes. On the other hand, for anthracene and 9,10-dichloroanthracene, little effect was observed on the electron spin polarization by two-step laser irradiation.

Quenching of electronically excited states by molecular oxygen in fluid solution

Abstract

Intramolecular deactivation of photoexcited anthracenes by aromatic ketones

The excited state properties of anthracenes non-conjugatively linked to various types of aromatic ketone were investigated in non-polar solution by determination of the quantum yields of fluorescence and intersystem crossing. In a series of substituted bichromophoric ω-(9-anthryl)propiophenones, the degree of intramolecular fluorescence quenching by the ketone was found to be associated with an increase in the quantum yield of anthracene triplet state formation. Excitation of 4′-piperazinoacetophenone tethered to anthracene resulted in deactivation within the acetophenone chromophore, rather than singlet energy transfer to anthracene. For photoexcited anthracene linked to 2-amido-substituted fluorenone, intramolecular exothermic transfer of the singlet excitation energy to the fluorenone chromophore was found to proceed with high efficiency.

The role of the triplet state in the photosensitization of cationic polymerizations by anthracene

AbstractThe photosensitization mechanism for cationic polymerizations initiated by diaryliodonium salts photosensitized by anthracene was investigated using fluorescence and phosphorescence spectroscopy. In situ photosensitizer fluorescence measurements confirmed that the photosensitization reaction proceeds by an electron transfer process. Transient phosphorescence studies demonstrated that electron transfer occurred from the triplet excited state of anthracene to the initiator, with an intrinsic kinetic rate constant of 2 × 108 L/mol s. Further evidence for the role of the triplet state was provided by an observed seven‐fold decrease in the polymerization rate upon addition of a triplet state quencher. Finally, numerical solution of the photophysical kinetic equations indicated that the triplet state concentration was approximately three orders of magnitude higher than that of the singlet state, and that 94‐96% of the active cationic centers are produced by reaction of the initiator with the triplet state. These results indicate that the electron transfer occurs primarily from the triplet state of anthracene, with the singlet state providing only a minor contribution to the photosensitization reaction. © 1995 John Wiley & Sons, Inc.

Time and temperature dependence of anthracene triplet yields in frozen aqueous micellar solutions: an electron paramagnetic resonance study

Abstract Anthracene in frozen aqueous micellar solutions of surfactants n -alkyltrimethyl-ammonium halide (C n TAX, n =10, 12, 14, 16 and 18; XF, Cl, Br, I) was exposed to laser pulse excitation at 308 nm in the temperature range 80–270 K. Relative triplet yields and lifetimes were measured by electron paramagnetic resonance (EPR) spectroscopy and found to depend on both temperature and environment. A temperature dependent and a temperature independent process is involved in the decay of the triplet state of anthracene. At least three processes compete with intersystem crossing generating triplets: fluorescence, internal conversion, and a photoreaction assigned to the formation of anthracene/halide exciplexes. The last process is strongly dependent on temperature and indicates significant mobility of reactants below the macroscopic melting point of the solvent.

Factors which determine the efficiency of sensitized singlet oxygen production

Nanosecond laser photolysis measurements of sensitized phosphorescence from oxygen have been used to obtain values for singlet oxygen formation efficiencies during oxygen quenching of excited singlet and triplet states of anthracene and naphthalene derivatives. Oxygen quenching of excited singlet states of anthracene and dicyanoanthracene in cyclohexane has been shown to lead to catalysed production of triplet states with unit efficiency in both cases, but concurrent production of singlet oxygen only occurs in the case of 9,10-dicyanoanthracene with efficiency close to unity whereas the efficiency for singlet oxygen production due to direct oxygen quenching of excited singlet anthracene is close to zero. In contrast to these results, oxygen quenching of the triplet states of anthracene and dicyanoanthracene in cyclohexane yields singlet oxygen with unit efficiency whereas the singlet oxygen formation efficiency during oxygen quenching of triplet 1-ethylnaphthalene is only 0.86 in cyclohexane and drops to 0.51 in acetonitrile. This solvent dependence demonstrates the role which charge transfer interactions play in determining singlet oxygen yields. Further information concerning the decay of excited oxygen-aromatic hydrocarbon charge-transfer complexes have been obtained from picosecond laser pump-probe studies where direct excitation is into the charge-transfer bands of oxygenated 1-ethylnaphthalene. Following the excitation of the charge-transfer complex, the triplet state of 1-ethylnaphthalene is rapidly produced with an efficiency which shows a marked solvent dependency, being 0.4 and 0.8 in acetonitrile and cyclohexane, respectively. The measured yields of singlet oxygen formation following excitation into 1-ethylnaphthalene-oxygen charge-transfer complexes are 0.36 and 0.78 in these two solvents which is greater than that expected on the basis of the measured triplet yields. Mechanisms of quenching of excited states by oxygen which explain these results are discussed.

Photostabilization mechanisms of hindered amine light stabilizers: interaction of singlet and triplet anthracene with piperidine model compounds

The interaction of secondary and tertiary piperidines, hydroxylamines and piperidine- N- oxyl free radicals with the singlet and triplet state of anthracene in isopropylbenzene solution was studied. Piperidines and hydroxylamines react with the singlet excited anthracene but are quite poor quenchers of its triplet state. In contrast, piperidine- N-oxyl free radicals deactivate with good efficiency, but without any chemical reaction, both the singlet and the triplet state of anthracene. The relevance of these interactions to the overall photo- stabilization mechanism of polyolefins by hindered amine light stabilizers (HALS), the piperidine based photostabilizers most widely used, is briefly discussed.

Spin density distribution of the photoexcited triplet state of anthracene via electron spin echo envelope modulation spectroscopy

The values of spin density of the anthracene triplet have been determined from the analyses of electron spin echo envelope modulations and a program of non-linear least-squares fitting as follows: p 1 (α) = 0.116, p 2 (β) = 0.05, p 9(meso) = 0.285 and p 11 = −0.058. The values of anisotropic and isotropic proton hyperfine coupling constants per unit spin density on the adjacent carbon have also been determined. Comparisons with previously reported values are presented.

Quenching of emission and of photochemistry of pentacarbonyl(4-cyanopyridino)tungsten(0)

Abstract : The complex W(CO)5L,L = 4 cyanopyridine, is found to emit in room temperature methycyclohexane solution. The emission is centered around 630 nm and is 105 plus or minus 10 nsec in lifetime, gamma, at 25 C. Added 0.1 M ethanol does not change the charcteristics, and in this medium 1/gamma = 2.61x10 to the 8th power exp(-1960/RT). The mission is quenched by anthracene, as is also the photosubstitution reaction whereby L is replaced by ethanol ( phi = 0.028 plus or minus 10% at room tmperature), Stern-Volmer plots of gamma/gamma and of phi/phi vs. quencher concentration are linear, the common slope giving a bimolecular quenching rate constant of 3.7x10 to the 9th power M sec. The charge transfer (CT) emitting state is clearly implicated in the photochemistry; between the emission spectrum and the anthracene triplet state energy, the energy of the CT state is bracketed as between 14.7 and 20 kk. (Author)

KINETIC AND SPECTROSCOPIC FEATURES OF SOME CAROTENOID TRIPLET STATES: SENSITIZATION BY SINGLET OXYGEN

Abstract— Triplet‐triplet absorption spectra of a series of carotenoid pigments in benzene solution have been determined by pulse radiolysis experiments. The natural lifetimes in deaerated solution have also been measured. They fall in the range 2–10 µs as found for other carotenoids under similar conditions. Pulsed laser (337 nm) excitation of benzene solutions containing oxygen, carotenoid and a photosensitized molecule (anthracene) showed the generation of absorption spectra of the triplet states. These absorptions decayed by first order kinetics in such a way as to indicate that they were formed in reactions with singlet oxygen, itself generated by interaction with the anthracene triplet state. Bimolecular rate constants for energy transfer from O*2 (1△g), to carotenoid have been evaluated.

Intersystem crossing in anthracene-N,N-diethylaniline exciplex

The growing-in of the anthracene triplet state has been measured in the case of the anthracene-N,N-diethylaniline exciplex system in hexane. The anthracene triplet state is formed from the relaxed exciplex fluorescent state.

Photochemical reactions. X. External heavy-atom perturbed intersystem crossing from the excited singlet and triplet states of anthracene and 9,10-dibromoanthracene in fluid solution

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPhotochemical reactions. X. External heavy-atom perturbed intersystem crossing from the excited singlet and triplet states of anthracene and 9,10-dibromoanthracene in fluid solutionRobert P. DeToma and Dwaine O. CowanCite this: J. Am. Chem. Soc. 1975, 97, 12, 3283–3291Publication Date (Print):June 1, 1975Publication History Published online1 May 2002Published inissue 1 June 1975https://doi.org/10.1021/ja00845a001RIGHTS & PERMISSIONSArticle Views174Altmetric-Citations18LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (953 KB) Get e-Alerts Get e-Alerts