Pyrene Radical Cation Research Articles

Self-Assembly and Photochemistry of a Pyrene-Methyl Viologen Supramolecular Fiber System.

This paper reports the self-assembly of a donor-acceptor system into nanoscopic structures and the photo processes taking place within these structures. The donor employed is pyrene linked to two β-cyclodextrin molecules (CD-PY-CD), and adamantane-linked methyl viologen attached to the three arms of mesitylene (Ms-(MV2+-AD)3) is the acceptor. CD-PY-CD and Ms-(MV2+-AD)3 when dissolved in water self-assembled into vesicles, which joined together to give long fibers. The self-assembly was studied using spectroscopic and microscopic techniques. Fluorescence of the pyrene chromophore was quenched within the self-assembled system due to efficient photoinduced electron transfer to methyl viologen. Photoinduced electron transfer within the assembly is confirmed through identification of product radical ions in flash photolysis experiments. Steady-state irradiation of the self-assembled system in an optical bench led to the formation of methyl viologen radical cation, which was stable for a few hours. Longevity of the radical cation was attributed to the fast reaction of pyrene radical cation with adjacent pyrene to give an unstable adduct, which slows down the back electron transfer process.

Pyrene radical cation and the diffuse interstellar band at 4430 Å: A theoretical survey

Multistate and multimode vibronic dynamics on the first six coupled (X˜2B1g - A˜2B2g - B˜2B3u - C˜2Au - D˜2B3u - E˜2B3g ) electronic states of pyrene radical cation are investigated with the aid of ab initio electronic structure calculations of potential energy surfaces (PESs) and first principle quantum dynamical methods. Several low-lying conical intersections among various electronic states are found and their impact on the nuclear dynamics is discussed. The simulation of nuclear motion on the underlying coupled electronic states of pyrene radical cation is examined by both time-independent and time-dependent quantum mechanical methods. The combination of time-independent and time-dependent quantum mechanical simulations render a satisfactory explaination of a large part of the complex vibronic spectrum of Py+ which is in very good agreement with the photoelectron spectroscopic data. Furthermore, this study also establishes that polycyclic aromatic hydrocarbons can be the molecular carrier of diffused interstellar bands and ultrafast non-radiative deactivation of their excited electronic states makes them photostable against strong UV irradiation in the interstellar medium.

Theoretical Study on Molecules of Interstellar Interest. II. Radical Cation of Compact Polycyclic Aromatic Hydrocarbons.

Radical cations of polycyclic aromatic hydrocarbons have been postulated to be molecular carriers of diffuse spectroscopic features observed in the interstellar medium. Several important observations made by stellar and laboratory spectroscopists motivated us to undertake a detailed theoretical study attempting to validate the recorded data. In continuation of our work on this subject, we here focus on a detailed theoretical study of the doublet ground (X̃) and low-lying excited (Ã, B̃, and C̃) electronic states of the radical cation of phenanthrene, pyrene, and acenaphthene molecule. A multistate and multimode theoretical model in a diabatic electronic basis is developed here through extensive ab initio quantum chemistry calculations. Employing this model, first-principles nuclear dynamics calculations are carried out to unravel the spectral assignment, time-dependent dynamics, and photostability of the mentioned electronic states of the radical cations. The theoretical results compare well with the observed experimental data.

A pyrene dihydrodioxin with pyridinium “arms”: A photochemically active DNA cleaving agent with unusual duplex stabilizing and electron trapping properties

Abstract Pyrene dihydrodioxins (PDHD) comprise effective DNA intercalation agents that are masked ortho-quinones, which can be released by near ultraviolet or visible irradiation. We have studied the binding and photoreactions of chiral dipyridinium PDHDs with herring sperm DNA and an 11-mer duplex DNA containing all 10 basepair steps. Binding affinities to herring sperm DNA were determined for purified enantiomers (Kb = 1.6 ± 0.15 × 105 and 2.3 ± 0.2 × 105 M−1). UV-melting experiments using the 11-mer DNA revealed significant stabilization of duplex DNA, (ΔTm = 11.5° and ΔTm = 15.3° C). Both enantiomers linearized (double-strand cleavage) supercoiled ΦX174 plasmid DNA with high efficiency. PDHDs have specificity to cleave and/or damage DNA duplexes at Gs and have preferable binding to GG DNA sites. A full range of transient absorption spectroscopy from the ultrafast femtosecond to the microsecond domains has been applied in the study of this system. These studies have revealed a novel mechanism for quinone release via the pyrene radical cation and the entrapment of the released electron by the coordinated action of the two pyridinium rings. These same studies have shown that the released pyrenequinone can photochemically initiate the further release of pyrenequinone. Thus, this reaction is autocatalytic and can be initiated with visible light.

Oligotriarylamines with a pyrene core: a multicenter strategy for enhancing radical cation and dication stability and tuning spin distribution.

Monoamine 1, diamines 2-4, triamine 5, and tetraamine 6 have been synthesized by substituting dianisylamino groups at the 1-, 3-, 6-, and/or 8-positions of pyrene. Diamines 2-4 differ in the positions of the amine substituents. No pyrene-pyrene interactions are evident in the single-crystal packing of 3, 4, and 6. With increasing numbers of amine substituents, the first oxidation potential decreases progressively from the mono- to the tetraamine. These compounds show intense charge-transfer (CT) emission in CH2 Cl2 at around 530 nm with quantum yields of 48-68 %. Upon stepwise oxidation by electrolysis or chemical oxidation, these compounds were transformed into radical cations 1(⋅+) -6(⋅+) and dications 2(2+) -6(2+) , which feature strong visible and near-infrared absorptions. Time-dependent density functional theory studies suggested the presence of localized transitions from the pyrene radical cation and aminium radical cation, intervalence CT, and CT between the pyrene and amine moieties. Spectroscopic studies indicated that these radical cations and dications have good stability. Triamine 5 and tetraamine 6 formed efficient CT complexes with tetracyanoquinodimethane in solution. The results of EPR spectroscopy and density functional theory calculations suggested that the dications 2(2+) -4(2+) have a triplet ground state, whereas 5(2+) and 6(2+) have a singlet ground state. The dication of 1,3-disubstituted diamine 4 exhibits a strong EPR signal.

Photodissociation of pyrene cations: structure and energetics from C16H10(+) to C14(+) and almost everything in between.

The unimolecular dissociation of the pyrene radical cation, C16H10(+•), has been explored using a combination of computational techniques and experimental approaches, such as multiple photon absorption in the cold ion trap Piège à Ions pour la Recherche et l'Etude de Nouvelles Espèces Astrochimiques (PIRENEA) and imaging photoelectron photoion coincidence spectrometry (iPEPICO). In total, 22 reactions, involving the fragmentation cascade (H, C2H2, and C4H2 loss) from the pyrene radical cation down to the C14(+•) fragment ion, have been studied using PIRENEA. Branching ratios have been measured for reactions from C16H10(+•), C16H8(+•), and C16H5(+). Density functional theory calculations of the fragmentation pathways observed experimentally and postulated theoretically lead to 17 unique structures. One important prediction is the opening of the pyrene ring system starting from the C16H4(+•) radical. In the iPEPICO experiments, only two reactions could be studied, namely, R1 C16H10(+•) → C16H9(+) + H (m/z = 201) and R2 C16H9(+) → C16H8(+•) + H (m/z = 200). The activation energies for these reactions were determined to be 5.4 ± 1.2 and 3.3 ± 1.1 eV, respectively.

Photodissociation of pyrene dimer radical cation during the pulse radiolysis–laser flash photolysis combined method

Photodissociation of pyrene (Py) dimer radical cation (Py2∙+) giving pyrene radical cation (Py∙+) and Py and subsequent regeneration of Py2∙+ by association of Py∙+ and Py were directly observed during the pulse radiolysis–laser flash photolysis combined method at room temperature. When Py2∙+ was excited at the local excitation band with the 532-nm laser flash, the rapid growth and decay of monomeric Py∙+ were observed at 460 nm. The dissociation of Py2∙+ proceeded via a one-photon process to give the ground-state Py∙+(D0) and Py in the quantum yield (Φdiss) of (2.9 ± 0.9) × 10−3. It was shown that Py∙+ decayed with a time constant of several tens of nanoseconds, indicating that the association of Py∙+ with Py regenerating Py2∙+ proceeds at a diffusion-controlled rate. The photodissociation proceeded from the lowest excited state of Py2∙+, even when Py2∙+ was excited to the higher excited state. The difference between the Φdiss value of Py2∙+ and that previously reported for naphthalene dimer radical cation (Np2∙+) is discussed.

Further Insight into the Photostability of the Pyrene Fluorophore in Halogenated Solvents

Pyrene fluorophores of pyrene-functionalized CdSe quantum dots (QD@Py), as well as alkylpyrene and pyrene itself (Py), undergo fast degradation in aerated chloroform under ultraviolet-A (UV-A, 316<λ<400 nm) illumination. Steady-state fluorescence studies of irradiated chloroform solutions of QD@Py show formation of new bands, red-shifted compared to that of the pyrene moiety. Similar behaviour is observed for pyrene and the alkylpyrene system. Column chromatography of the pyrene photolysate in chloroform allowed us to isolate photoproducts arising from pyrene degradation, and to obtain information on the structure of the photoproducts responsible for the emission bands. The most predominant photoproducts were those originating from the reaction of pyrene with dichloromethyl radicals. The phototransformation of QD@Py and the alkylpyrene involves mainly detachment of the alkyl chain from the aromatic ring, induced also by dichloromethyl radicals, and oxidation of the alkyl chain at the benzylic position was detected as well. By contrast, these pyrene systems show a high photostability in aerated dichloromethane. Transient absorption measurements showed formation of both pyrene triplet and pyrene radical cation for all pyrene systems in these halogenated solvents. The yield of pyrene radical cations for Py is higher than for QD@Py and the alkylpyrene. In addition, pyrene radical cations were longer-lived in dichloromethane than in chloroform. The reason for the pyrene photostability in dichloromethane is the different reactivity of chloromethyl and dichloromethyl radicals towards pyrene and oxygen. These studies show that the use of dichloromethane can be a suitable alternative to chloroform when the good solubility properties of these halogenated solvents are needed to dissolve pyrene when this chromophore is used as a fluorescent probe.

Dynamics and Efficiency of Electron Injection and Transport in DNA Using Pyrenecarboxamide as an Electron Donor and 5-Bromouracil as an Electron Acceptor

The photophysical and photochemical behavior of a series of hairpin-forming DNA conjugates possessing a 5'-tethered pyrenecarboxamide chromophore and one or two bromouracil bases has been investigated. Quenching of the pyrene fluorescence and transient absorption spectra characteristic of the pyrene cation radical are observed only when bromouracil is located at the first or second base pair position nearest to the point of pyrene attachment. These observations are consistent with an intercalated structure for these conjugates in which pyrene is adjacent to the second base pair. Selective quenching of singlet pyrene by bromouracil but not by thymine is consistent with the free energy for charge separation estimated using Weller's equation. Low quantum yields for loss of bromide when bromouracil is not adjacent to pyrene are attributed to inefficient charge separation via either a multistep electron transport or a single-step superexchange mechanism. Quantum yields are only weakly dependent upon the distance between pyrene and bromouracil, as expected for a multistep electron transport mechanism. Loss of bromide from conjugates possessing two bromouracils occurs sequentially. For adjacent bromouracils, competitive loss of bromide from both bromouracils is observed, whereas for nonadjacent bromouracils loss of bromide from the proximal bromouracil occurs prior to any loss from the distal bromouracil, consistent with a slower rate constant for electron transport vs loss of bromide.

Evidence of Neutral Radical Induced Analyte Ion Transformations in APPI and Near-VUV APLI

We report on the reactions of neutral radical species [OH, Cl, O(3P), H], generated in a typical atmospheric pressure ionization (API) source upon irradiation of the sample gases with either 193 nm laser radiation or 124 nm VUV light, the latter commonly used in atmospheric pressure photoionization (APPI). The present investigations focus on the polycyclic aromatic hydrocarbon pyrene as representative of the aromatic compound class. Experimental results are supported by computational methods: simple kinetic models are used to estimate the temporal evolution of the concentrations of reactants, intermediates, and final products, whereas density functional theory (DFT) energy calculations are carried out to further elucidate the proposed reaction pathways. The neutral radicals are generated upon photolysis of background water and oxygen always present in appreciable mixing ratios in typical API sources. Substantial amounts of oxygenated analyte product ions are observed using both techniques. In contrast, upon atmospheric pressure laser ionization (APLI) with 248 nm radiation, oxygenated products are virtually absent. In addition, kinetic data evaluation yielded a bimolecular rate constant of k = (1.9 +/- 0.9) x 10(-9) cm3 molecule(-1) s(-1) for the reaction of the pyrene radical cation with OH radicals.

Metal Ion Enhanced Charge Transfer in a Terpyridine-bis-Pyrene System

The synthesis, electrochemical and photophysical properties of a branched molecule 3,5-bis(pyrene-1-yl)-4′-phenyl-2,2′:6′,2″-terpyridine are reported. Spectroscopy in different solvents reveals that an optical electron transfer from the pyrene donor to the terpyridyl electron acceptor can occur in polar media, as the system displays both charge transfer (CT) absorption and CT emission. Furthermore, the study of the zinc complex as well as the bis-protonated form shows an enhancement of the electron transfer character of the system, by an increase of the acceptor strength. This is accompanied by a large increase of the non-radiative processes. With sub-nanosecond transient absorption spectroscopy, the CT state, consisting of the pyrene radical cation and the terpyridine radical anion, has been detected. At room temperature, the study of the nanosecond transient absorption spectra reveals the formation of a low-lying triplet excited state that we attribute to the pyrene moiety through which the CT state decays. At 77K, the absence of the terpyridine triplet emission also suggests the population of a low-lying triplet state of the pyrene unit.

Excited-State Dynamics in the Covalently Linked Systems: Pyrene−(CH2)n−Aryl Azide

The primary physical and chemical processes which follow photoexcitation of the covalently linked systems, 1-[(4-azido-2,3,5,6-tetrafluorobenzoyloxy)methyl]pyrene (2) and 1-[3-(4-azido-2,3,5,6-tetrafluorobenzoyloxy)propyl]pyrene (3), have been studied using femto- and nanosecond transient absorption spectroscopy and computational chemistry. Excitation of 2 and 3 at 336 nm results in the population of the second excited singlet state of the pyrene moiety (S2). Internal conversion to the lowest excited singlet state of the pyrene moiety (S1) occurs with a time constant of ∼140 fs, a value which is similar to that of unsubstituted pyrene. The S1 local pyrene state, initially formed with excess vibrational energy, undergoes vibrational cooling with a time constant ∼2 ps. The decay of the local pyrene S1 state measured in the pump−probe experiments was described by a two exponential function with time constants which agree well with the values obtained previously from the fluorescence decay kinetics (Barabanov, I. I.; Pritchina, E. A.; Takaya, T.; Gritsan, N. P. Mendeleev Commun. 2008, 18, 273). In both systems decay of the local pyrene S1 state on the time scale of tens of picoseconds is accompanied by formation of a product with a narrow band at ∼460 nm. This transient absorption was assigned to the radical cation of the pyrene moiety. Calculations predict very fast dissociation of the counterpart aryl azide radical anion with formation of the corresponding arylnitrene radical anion. Decay of the local pyrene S1 state on the longer time scale (hundreds of ps) is not accompanied by formation of a noticeable amount of pyrene radical cation. Most likely, both electron transfer and energy transfer from pyrene to aryl azide moiety are responsible for the photosensitization of the perfluorinated aryl azide decomposition.

Photostability via Sloped Conical Intersections: A Computational Study of the Pyrene Radical Cation

The photophysics of the pyrene radical cation, a polycyclic aromatic hydrocarbon (PAH) and a possible source of diffuse interstellar bands (DIBs), is investigated by means of hybrid molecular mechanics-valence bond (MMVB) force field and multiconfigurational CASSCF and CASPT2 ab initio methods. Potential energy surfaces of the first three electronic states D 0, D 1, and D 2 are calculated. MMVB geometry optimizations are carried out for the first time on a cationic system; the results show good agreement with CASSCF optimized structures, for minima and conical intersections, and errors in the energy gaps are no larger than those found in our previous studies of neutral systems. The presence of two easily accessible sloped D 1/D 2 and D 0/D 1 conical intersections suggests the pyrene radical cation is highly photostable, with ultrafast nonradiative decay back to the initial ground state geometry predicted via a mechanism similar to the one found in the naphthalene radical cation.

Pyrene—p-tert-butylcalixarenes inclusion complexes formation: a surface photochemistry study

Diffuse reflectance and luminescence techniques were used to study the photophysics and photochemistry of pyrene within p-tert-butylcalix[n]arenes with n = 4, 6, and 8, and to study their ability to form inclusion complexes in heterogeneous media. Evidences for inclusion complex formation were found for the three hosts under study. Ground state diffuse reflectance results have shown the formation of ground state dimers of pyrene inside the cavity of calix[6]arene and calix[8]arene, with this feature much more evident for calix[6]arene. For calix[4]arene, only a monomer fits inside the cavity and the presence of pyrene microcrystals outside the cavity was detected. A luminescence lifetime distribution analysis was performed, revealing the presence of prompt emissions from the pyrene microcrystals outside the cavity in the case of calix[4]arene and from the constrained dimers inside the cavities of calix[6]arene and calix[8]arene. Transient absorption results have shown the presence of pyrene radical cation and also of trapped electrons for the three hosts under study. The formation of the phenoxyl radical of the calixarene following the laser pulsed excitation of pyrene at 355 nm is increased for calix[6]arene and calix[8]arene. This feature is particularly relevant for calix[6]arene, suggesting a very favourable situation for the hydrogen atom abstraction to occur. The analysis of the degradation products revealed the presence of hydroxypyrene as a major photodegradation product for the three hosts. Dihydro-hydroxypyrene was also formed in the case of calix[6]arene and calix[8]arene. The formation of the calixarene's phenoxyl radical and subsequent hydrogen abstraction is consistent with the formation of dihydro-dihydroxypyrene.

Reduction of tetranitromethane by electronically excited aromatics in acetonitrile: Spectra and molar absorption coefficients of radical cations of anthracene, phenanthrene and pyrene

Room temperature absorption spectra of the long-lived transient species produced by photolysing solutions containing tetranitromethane (TNM) or a TNM/arene mixture have been recorded with a multi-channel spectrophotometer, capable of μs time resolution and covering a wide spectral range (200–1010 nm). The arene was anthracene ( A), naphthalene ( N), phenanthrene ( Φ), or pyrene ( P). In methanol, A + , N + and Φ + disappear within the available time resolution, but all four cations live for tens or hundreds of μs in acetonitrile; the molar absorption coefficients of A + , Φ + and P + in the latter solvent match those of the respective anions in tetrahydrofuran.

Real-time observation of hydrogen bond-assisted electron transfer to a DNA base

Electron transfer (ET) in a pyrene-labeled DNA base ( PydU) has been investigated with fs-broadband pump–probe spectroscopy. By comparing the dynamics of PydU in a nonprotic solvent (acetonitrile) and a protic solvent (methanol) with similar dielectric properties, one can observe the formation of a planar intramolecular charge transfer state and the build-up of the pyrene radical cation population in real time. The temporal evolution of the excited state absorption spectrum provides direct evidence for a greatly enhanced ET yield in methanol. The results emphasize the critical role of hydrogen bonding for ET involving DNA bases. No spectroscopic evidence for (de)protonation of the ET product could be manifested.

Low-Field Optically Detected EPR Spectroscopy of Transient Photoinduced Radical Pairs

The effects of simultaneously applied weak static and weak radio frequency magnetic fields on the recombination of transient (<100 ns) radical pairs have been investigated using a low-field optically detected electron paramagnetic resonance technique. Measurements on the photoinduced electron-transfer reaction of perdeuterated pyrene with 1,3-dicyanobenzene using a approximately 0.3 mT radio frequency field at three separate frequencies (5, 20, and 65 MHz) in the presence of 0-4 mT static fields yield spectra that are strikingly sensitive to the frequency of the time-dependent field, to the strength of the static field, and to the relative orientation of the two fields. The spectra are simulated using a modified form of the gamma-COMPUTE algorithm originally devised for calculating magic angle spinning NMR spectra of polycrystalline samples. The essential features of the spectra are consistent with the radical pair mechanism and were satisfactorily simulated using parameters whose values are either known independently or for which estimates are readily available. The calculations included hyperfine couplings to four deuterons in the pyrene cation radical and three protons in the 1,3-dicyanobenzene anion radical. Spin-selective recombination was modeled using an exponential distribution of radical encounter times. The results are discussed in the context of the proposal that radical pair chemistry forms the basis of the magnetoreceptor that allows birds to sense the Earth's magnetic field as a source of compass information during migration.

Kinetic analysis of the reactions of 1-substituted pyrene cation radicals with water in acetonitrile

Although the spectroscopic detection of electrochemically generated pyrene cation radical (PY +) is generally difficult at room temperature in acetonitrile (AN), we could successfully observe the time changes in the absorption spectra of short-lived PY + using an electron transfer stopped-flow (ETSF) method by adopting the tris(2,4-dibromophenyl)amine cation radical (TDBPA +) as a reaction initiator. The decrease of PY + was observed to be complete within 30 ms after generating PY + via the mixing of the AN solutions of PY and TDBPA +. This decay reaction of PY + can be attributed to the reaction with a trace amount of water in AN. Thus, the reactions of 1-substituted pyrene cation radicals (XPY +) with water were systematically analyzed using the ETSF method by adding a constant amount of water. Consequently, the reactions were confirmed to be second-order in the cation radicals in all the cases. Among the derivatives, the higher stability of 1-aminopyrene + was confirmed on the basis of the slow reaction rates, and the order of the reactivity toward water was clarified to be pyrene +>1-bromopyrene +>1-methylpyrene +>1-aminopyrene +. The present results of XPY + were compared with our previous results on the reactions of 9-substituted anthracene derivative cation radicals.

Influence of Oxygen on the UV−Photocatalyzed Generation of Pyrene Cation Radicals in the Zeolites X and Y

The photoinduced generation of pyrene cation radicals (Py+·) has been examined as a function of oxygen pressure in the internal cavities of the zeolites NaX and NaY. Excitation of pyrene was carrie...

Surface photochemistry: Photodegradation study of pyrene adsorbed onto microcrystalline cellulose and silica

Ground‐state diffuse reflectance, time resolved laser‐induced luminescence, diffuse reflectance laser flash‐photolysis transient absorption and chromatographic techniques were used to elucidate the photodegradation processes of pyrene adsorbed onto microcrystalline cellulose and silica. Ground‐state diffuse reflectance showed that on both substrates low concentrations display absorption of pyrene monomers. At high concentrations spectral changes attributed to aggregate formation were observed. Laser induced fluorescence showed that pyrene onto microcrystalline cellulose mainly presents fluorescence from monomers, while for silica, excimer‐like emission was observed from low surface loadings (≥0.5 μmol g−1). Transient absorption and photodegradation studies were performed at concentrations where mainly monomers exist. On silica, pyrene presents transient absorption from its radical cation. On microcrystalline cellulose both radical cation, radical anion and pyrene triplet‐triplet absorption were detected. Irradiation followed by chromatographic analysis showed that pyrene decomposes on both substrates. For pyrene on microcrystalline cellulose 1‐hydroxypyrene was the main identified photoproduct since in the absence of oxygen further oxidation of 1‐hydroxypyrene was very slow. For pyrene on silica photodegradation was very efficient. Almost no 1‐hydroxypyrene was detected since in the presence of oxygen it is quickly oxidized to other photooxidation products. On both substrates, pyrene radical cation is the intermediate leading to photoproducts and oxygen it is not involved in its formation.