Pyrene Radical Cation Research Articles

Formation and decay of pyrene radical cation and pyrene dimer radical cation in the absence and presence of cyclodextrins during resonant two-photon ionization of pyrene and sodium 1-pyrene sulfonate

Formation and decay of pyrene radical cation and pyrene dimer radical cation during resonant two-photon ionizations (TPI) of pyrene (Py) and sodium 1-pyrene sulfonate (NaPySA) in the absence and presence of cyclodextrins were studied with visible and near IR transient absorption measurement using 355-nm laser flash photolysis technique. Ionization of Py and 1-pyrene sulfonate anion (PySA−) occurred within the laser flash (5 ns) to give Py radical cation (Py˙+) and PySA− radical cation (Py˙+SA−) having Py˙+ chromophore. Dimerizations of Py˙+ and Py and of Py˙+SA− and PySA− were observed to give the dimer radical cations (Py2˙+ and Py2˙+(SA−)2) based on time-resolved measurements of the charge resonance (CR) band. The TPI of PySA− was also examined in the presence of β- and γ-cyclodextrins (βCD and γCD, respectively). Py2˙+(SA−)2 in the cavity of two γCDs having fully overlapped structure showed the CR band around 1700 nm, while Py2˙+(SA−)2 in the cavity of two βCD having partially and fully overlapped structures indicated the CR bands around 1500 or 1700 nm, respectively. On the basis of formation and decay rates of Py2˙+(SA−)2, bimolecular formation of Py2˙+(SA−)2 and the neutralization are inhibited in the cavities of CDs. Selective formation of the fully overlapped structure of Py2˙+(SA−)2 in the cavity of two γCDs within a laser flash is explained by direct TPI of two PySA−s with the parallel structure of two Py chromophores in two PySA−s in the cavity of two γCDs.

Kinetics of Multistep Hole Transfer in DNA by Monitoring the Transient Absorption of the Pyrene Radical Cation

To clarify the hole-transfer mechanism in DNA and the factor controlling the hole-transfer rates, the kinetics of hole transfer in DNA was studied by monitoring the transient absorption of the pyrene (Py) radical cation (Py•+) during the pulse radiolysis of oligodeoxynucleotides (ODNs) conjugated with Py. By analyzing the transient absorption of Py•+ formed by the hole transfer from DNA to Py, the rate constants of the hole transfer in various sequences of DNA were determined. The rate constants of the hole transfer from the nearest guanine (G) to Py were weakly dependent upon the distance between Py and the nearest G, indicating the occurrence of the multistep hole transfer in DNA. In contrast, in the hole transfer where the rate-determining step was the single-step hole transfer between Gs, the rate was strongly dependent upon the distance between the Gs. Comparing the intervening nucleobases between Gs, the rate constants of the multistep hole transfer increased in the order G•+AG > G•+AC > G•+TG. Thes...

Photoinduced electron transfer at solid surfaces: the TiO2-SiO2 system.

Small particles of TiO2 have been synthesized on porous silica. XRD and spectral measurements place the sizes of the particles at about 50 A. With these small particles the band gap of the semiconductor material is markedly blue shifted with respect to bulk TiO2. This enables the direct excitation of co-adsorbed arenes on the TiO2-SiO2 material. Direct excitation of TiO2 leads to Ti3+, by photoinduced extraction from the SiO2. Direct excitation of pyrene on the material leads to a shortened fluorescence lifetime and lower quantum yield. Meanwhile, the yield of pyrene cation radical increases compared to that on pure SiO2. The production of Ti3+ on the pyrene samples, by direct excitation of the TiO2, leads to a shortened pyrene fluorescence lifetime and decreased quantum yield, the quantum yield decreasing to zero at high enough Ti3+. Removal of Ti3+ by O2 causes the original properties of the pyrene fluorescence to return.

Hole Transfer in Pyrene‐Conjugated DNA

AbstractFor Abstract see ChemInform Abstract in Full Text.

Possible molecular hydrogen formation mediated by the radical cations of anthracene and pyrene.

Hydrogen molecules cannot be formed readily by the association of gaseous hydrogen atoms. Possible H(2) formation mediated by the radical cations of typical polycyclic aromatic hydrocarbons (PAHs), anthracene and pyrene, was studied at the B3LYP/6-31G** level of theory. We presumed that H(2) is formed by way of two elementary reactions: the addition of an H atom to a PAH molecular cation, and the H abstraction from the resulting monohydro-PAH cation (i.e., arenium ion) by a second H atom to yield H(2). The first reaction takes place without any activation energy. The second reaction is also predicted to proceed along almost barrierless pathways, although it is far from being a typical ion-molecule reaction. There is a possibility that these reactions might constitute one of the mechanisms for H(2) formation in extremely cold interstellar space. Deuterium enrichment in PAH cations is possibly accompanied by such H(2) formation because deuteration lowers the energies of polyatomic PAH cations appreciably.

Ionic and excited species in irradiated poly(ethylene terephthalate) doped with pyrene

Temperature effect on the fate of ionic and excited species in stationary irradiated poly(ethylene terephthalate), doped with pyrene, were examined using wavelength selected radiothermoluminescence and absorption spectroscopic methods. In general, all intermediates were found to disappear at temperatures higher than ∼180 K. Some small fraction of pyrene radical cations was still detected in temperatures close to room ones. The relaxation phenomena in this polymeric matrix certainly stimulated the behaviour of all intermediates trapped in the system. The detailed mechanism of reactions involved was proposed.

Pulse radiolysis of pyrene in aprotic polar organic solvents: simultaneous formation of pyrene radical cations and radical anions

Simultaneous formation of pyrene radical cations Py ·+ and radical anions Py ·− was observed by pulse radiolysis of pyrene in DMSO or sulfolane solutions containing 10 vol% toluene. Using 5×10 −5 mol dm −3 pyrene in the oxygen-free solvent mixture of 90 vol% DMSO and 10 vol% toluene, the radical cation Py ·+ (λ max =450 nm;ε 450=2.29×10 4 dm 3 mol −1 cm −1) was formed with the rate constant k=4.0×10 9 dm 3 mol −1 s −1 and decayed by a first-order reaction with k 1=1.2×10 4 s −1 , forming the dimer radical cation ( Py) 2 ·+ . Simultaneously, the radical anion Py ·− (λ max =495 nm; ε 495=2.17×10 4 dm 3 mol −1 cm −1) was produced with k=6.0×10 10 dm 3 mol −1 s −1 and disappeared by formation of the dimer radical anion ( Py) 2 ·− with k 1=4.6×10 4 s −1 . The first-order rate constants ( k 1) for the decay of the species Py ·+ and Py ·− with formation of the dimer radical ions ( Py) 2 ·+ and ( Py) 2 ·− increased with raising the substrate concentration. By scavenging all radiolytically generated electrons with oxygen (k≈8×10 8 dm 3 mol −1 s −1 in 90 vol% DMSO and 10 vol% toluene), the formation of the Py ·− transients was suppressed without interfering with the generation of the radical cations Py ·+ . In oxygen-free solutions of pyrene (5×10 −5 mol dm −3) in 90 vol% sulfolane and 10 vol% toluene the corresponding rate constants of the transient formation and the decay were for Py ·+ :k=1.5×10 10 dm 3 mol −1 s −1 and k 1=0.9×10 4 s −1 , and for Py ·− :k=2.3×10 10 dm 3 mol −1 s −1 and k 1=4,1×10 4 s −1 .

Photoinduced Charge Separation of Pyrene in Chromium Containing Silicoaluminophosphate (SAPO-5) Microporous Materials at Room Temperature

The photoinduced charge separation of pyrene in chromium containing microporous SAPO-5 materials with ultraviolet irradiation at room temperature was examined. Photoionization of pyrene in Cr containing SAPO-5 results in the formation of pyrene cation radicals that are characterized by electron spin resonance (ESR) and diffuse reflectance spectroscopy (DRS). ESR studies indicate that Cr(V) acts as an electron acceptor. The photoyield and stability of the photoproduced pyrene cation radical were found to depend on the pore size of the microporous material, the concentration, and the oxidation state of chromium ions. The photoyield and stability of the photoproduced pyrene cation are found to be higher than in mesoporous materials investigated in our laboratory earlier. The study thus demonstrates the importance of Cr containing microporous SAPO-5 materials as potential candidates for photochemical conversion and storage devices.

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To elucidate the hole transfer in DNA including multi-step processes, we used pulse radiolysis which provides a system for the hole transfer process free from charge recombination. Hole gen-erated in DNA was monitored by the transient absorption of radical cation of end-capped pyrene (pyrene.+) during pulse radiolysis of pyrene-conjugated oligodeoxynucleotides with different num-bers of intervening A-T base pairs between the G-region (5'-GTGTGTG-3') and pyrene. The results demonstrated a distance dependence of the hole transfer from G radical cation (G.+) to pyrene. This method revealed the hole transfer in DNA over the period of 100, us, which strongly suggests that G (-H.+) . : C (+H+) + can be present in DNA more than 100, us, and that this long life-time is at the very origin of the long-range hole transfer in DNA. We also determined a distance dependence of the single-step hole transfer process as β = 0.6 Å-1 which is consistent with that of the photoinduced single-step electron transfer in DNA.

Formation and Decay Behaviors of Laser-Induced Transient Species from Pyrene Derivatives 1. Spectral Discrimination and Decay Mechanisms in Aqueous Solution

The ultraviolet and visible absorption spectra of laser-induced transient species from the following pyrene derivatives were measured in aqueous or aqueous ethanol solution: sodium pyrenesulfonate (NaPS), tetrasodium pyrenetetrasulfonate (Na4PS4), pyrenecarboxylic acid (HPC), and pyrenebutyric acid (HPB). The major transient species contributing to the intense absorption peaks were revealed on the basis of the quenching experiments. The absorption spectra of triplets, as well as cation radicals, were separable from the other transients by selecting the experimental conditions (atmosphere, coexisting quencher, and delay time). The cation radicals produced from PS-, PB- and PC- anions showed an absorption maximum at ∼460 nm, close to that of pyrene cation radical. Their decay behaviors in the absence of any additional quenchers were dominated by bimolecular reaction kinetics with each parent molecule, of which the rate constants were very similar. This result is consistent with the previous proposal that these cation radicals exist as zwitterions such as P•+S-. The cation radical from Na4PS4 showed a strong absorption peak at 505 nm and exhibited different decay behaviors, suggesting that this cation radical appears not to be a simple zwitterion. Three specific quenchers, I-, OH-, and SO32-, were found to strongly accelerate the decay rates of these cation radicals. CH3SO3- anion, mimicking the headgroup of tentative anionic surfactants, and several inorganic anions such as ClO4- were poor quenchers for the cation radicals even at the highest concentration. We also discussed the origin of two additional peaks at 375 and 395 nm observed in the transient absorption spectra of NaPS and HPB. On the triplet−triplet absorption, the molar extinction coefficients of Na4PS4 and NaPS could be determined using the ground-state depletion method. These results are discussed in terms of applications to probe the micellar microenvironment.

Molecular reorientation in pyrene hexafluoroarsenate salts

The quasi-one dimensional pyrene (PY) organic conductor (PY)7(PY)4(AsF6)4 . 4CH2Cl2shows parallel as well as 60° rotated PY radical cations in its stacks, but crystallizes in two different modifications I and II. One of the seven intra-stack pyrene molecules is susceptible to a reorientation by 60°, that is stable already at room temperature for modification II, but occurs at a hysteretic first order transition between 170 K and 240 K for modification I. Crystal structure, microwave conductivity and static magnetic susceptibility are typical for a quasi-one dimensional organic conductor with Peierls transition at TP = 73 K (mod. I) or TP = 105 K (mod. II). The pyrene radical packing is analysed by continuous wave and pulsed electron spin resonance measurements, using 9.45 GHz as well as 425 MHz as measuring frequency. Anisotropy of the conduction electron diffusion constant exceeds 1000 in the metallic phase.

Photochemistry of chromophore-functionalized gold nanoparticles

Abstract It is generally believed that metal nanoparticles strongly quench the singlet-excited states of chromophores when attached to nanoparticle surfaces, through an energy-transfer mechanism, which limits their application in optoelectronic devices and photonic materials. Recent studies of fluorophore-linked metal nanoparticles reveal that there is a dramatic suppression in the quenching of the singlet-excited state of these molecules and they possess unusual photophysical properties. A summary of our work on the photophysical and excited-state properties of chromophore-functionalized gold nanoparticles is presented in this article. Pyrene-capped gold nanoparticles showed normal fluorescence in nonpolar solvents and an intermolecular excimer formation at higher loadings. The quenching of the emission, observed in pyrene-labeled gold nanoparticles in polar solvents, is attributed to the formation of pyrene radical cation through a photoinduced electron-transfer process. We have also functionalized gold nanoparticles using a thiol derivative of fullerene. The quenching of fluorescence and decreased yields of triplet-excited state, observed in these systems, are attributed to an energy-transfer process.

Photoinduced Charge Separation in a Fluorophore−Gold Nanoassembly

We report the first spectroscopic demonstration of direct electron transfer between a gold nanoparticle and a surface-bound fluorophore induced by pulsed laser irradiation. Binding of pyrene thiol directly to the gold nanoparticle results in quenching of its singlet excited state. The suppression of S1−T1 intersystem crossing process as well as the formation of pyrene radical cation confirm the excited-state interaction between the metal nanoparticle and the surface-bound fluorophore. The charge separation is sustained for several microseconds before undergoing recombination.

Observation of hole transfer through DNA by monitoring the transient absorption of pyrene radical cation.

Observation of hole transfer through DNA by monitoring the transient absorption of pyrene radical cation.

Photoinduced reaction of arene singlets with carbon tetrachloride in zeolite Y

Picosecond fluorescence methodologies combined with nanosecond, laser flash photolysis have been applied to examine the reaction of arene excited singlets with CCl 4 in dehydrated, zeolite NaY. Fluorescence quenching by CCl 4 was shown to originate mainly from a static process, with a much greater drop in fluorescence intensity than could be accounted for by a random distribution of CCl 4 among the anthracene-occupied supercages. A similar effect was observed in a previous study of arene fluorescence quenching by nitroalkanes in NaY, and was explained by a mechanism involving coadsorption of the arene and nitroalkane to conjugate acid–base sites. This places the reactants in close proximity – a requirement for the static quenching. The results of the present study support this quenching mechanism by showing that a different electrophilic species (i.e., CCl 4) also behaves like nitroalkanes, while reductive quenching by N, N-dimethylaniline and triethylamine does not. Quenching of pyrene fluorescence by CCl 4 produced pyrene radical cations through electron transfer to CCl 4, with the cation yield proportional to the level of fluorescence quenching. One-third of these radical cations reacted with the fragmentation products of CCl 4 anions to form stable products; the remaining fraction was stable, but reacted with added ferrocene. These results demonstrate that the addition of small quantities of CCl 4 to the zeolite leads to efficient charge separation by a single-photon mechanism.

Long‐lived species generated via irradiation of poly(methyl methacrylate) containing pyrene

AbstractThe fate of long‐lived species [pyrene (Py) radical cations and Py–cyclohexadienyl‐type radicals] generated by electron‐beam irradiation at room temperature in poly(methyl methacrylate) (PMMA) doped with Py is described. The separation of reacting solute intermediates and the relaxation phenomena seem to be the main factors limiting the reactivity of long‐lived Py species in the PMMA matrix. The temperature‐stimulated recombination of ionic species in PMMA doped with Py results in Py excited‐state formation. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 4110–4118, 2001

A Fourier-transform ion-cyclotron-resonance study of the reactions of polycyclic aromatic hydrocarbon cations with molecules of interstellar interest

The reactivity of naphthalene and pyrene radical cations and their derivatives ( C 10 H n + , n=6,7,8,9) , C 16 H n (n=9,10,11) has been studied with molecules of interstellar interest in an ion cyclotron resonance apparatus. The radical cations C 10 H 8 + and C 16 H 10 + are unreactive with H 2,CO,H 2O and NH 3. Adduct formation is the only channel for almost all reactions of C 10 H 7 + with these molecules. The implications of these results for the stability of polycyclic aromatic hydrocarbon (PAH) cations in the interstellar medium are briefly discussed. Exploratory studies of the ion chemistry of a larger PAH, coronene, have also been done.

Peierls transition in pyrene radical cation salts

PY 12 (SbF 6 ) 7 and PY 7 PY 4 (AsF 6 ) 4 (CH 2 Cl 2 ) 4 belong to the group of quasi-one dimensional organic conductors. Measurements of the microwave electrical conductivity show that both substances undergo a Peierls transition (metal-insulator transition) at about T P =116 K and T P =74 K, respectively. The 3D-locking of the Peierls transition was investigated by following the temperature dependence of the conduction band energy gap, which was calculated from measurements of the magnetic susceptibility.

EPR Study on the Complex Formed by Charge‐Transfer Process between Ground‐state Acceptor 2, 3‐Dicyano‐5, 6‐dichloro‐1, 4‐benzoquinone and Some Donors and on Cation Radical of Perylene (or Pyrene)

AbstractEPR study showed that the semi‐quinone radical anion of 2,3‐dicyano‐5,6‐dichloro‐1,4‐benzoquinone (DDQ) was formed in a charge transfer process between ground‐state DDQ as acceptor and each one of following ground state donors, i.e., 4‐methyl‐4′‐tridecyl‐2, 2′‐bipyridyl; 4‐methyl‐4′‐nonyl‐2, 2′‐bipyridyl; bis (2,2′‐bipyridyl) (4‐methyl‐4′‐heptadecyl‐2, 2′‐bipyridyl)ruthenium(2+) perchlorate and perylene. EPR study also showed that there are perylene cation radical and pyrene cation radical in the following experimental conditions: (a) in 98% sulfuric add. (b) 10−3 mol/L perylene (or pyrene) was dissolved in trifluoroacetic acid‐nitrobenzene (1: 1 V/V).

Novel Charge-Transfer Materials via Cocrystallization of Planar Aromatic Donors and Spherical Polyoxometalate Acceptors

Spherical polyoxometalates (POMs) such as M6O192- and SiM12O404- (with M = Mo or W) and planar arene donors (anthracenes and pyrenes) can be cocrystallized (despite their structural incompatibility) by attaching a cationic “anchor” onto the arene which then clings to the POM anion by Coulombic forces. As a result, novel charge-transfer (CT) salts are prepared from arene donors and Lindqvist-type [M6O19]2- and Keggin-type [SiM12O40]4- acceptors with overall 2:1 and 4:1 stoichiometry, respectively. The CT character of the dark-colored (yellow to red) crystalline materials is confirmed by the linear Mulliken correlation between the CT transition energies and the reduction potentials of the POM acceptors, as well as by the transient (diffuse reflectance) absorption spectra (upon picosecond laser excitation) of anthracene or pyrene cation radicals (in monomeric and π-dimeric forms). X-ray crystallographic studies reveal a unique “dimeric” arrangement of the cofacially oriented arene couples which show contact ...