Rafael Arce

Environmental photochemistry of nitro-PAHs: direct observation of ultrafast intersystem crossing in 1-nitropyrene.

Femtosecond broadband transient absorption experiments of 1-nitropyrene, a nitro-polycyclic aromatic hydrocarbon of environmental concern are presented in cyclohexane and hexane solutions. The transient absorption spectra show the presence of three species that are assigned to the Franck-Condon excited lowest singlet (S1) state, the structurally relaxed S1 state, and the lowest excited triplet state. The spectral changes at early times are interpreted in terms of conformational dynamics; primarily due to an ultrafast rotation of the nitro group in the S1 state. This excited state relaxation is followed by intersystem crossing with a time constant of 7 ps. CIS/6-31G(d,p) calculations predict planarization of the nitro-aromatic torsional angle as the major nuclear relaxation coordinate, from 32.8 degrees at the HF/6-31G(d,p) level of theory in the ground state (27.46 degrees at B3LYP/6-31++G(d,p)) to 0.07 degrees in the S1 state. Vertical excitation energies at the TDDFT/6-31++G(d,p) and TDDFT/IEFPCM/6-31++G(d,p) levels of theory predict a small energy gap (<0.12 eV) between the S1(pipi*) state and the third excited triplet state T3(npi*) in the gas phase and in cyclohexane, respectively. The small energy gap suggests a large spin-orbit coupling between the S1(pipi*) and T3(npi*) states, which explains the ultrafast intersystem crossing of 1-nitropyrene in nonpolar solvents.

Photophysics and Photochemistry of 1-Nitropyrene

1-Nitropyrene (1NPy) is the most abundant nitropolycyclic aromatic contaminant encountered in diesel exhausts. Understanding its photochemistry is important because of its carcinogenic and mutagenic properties, and potential phototransformations into biologically active products. We have studied the photophysics and photochemistry of 1NPy in solvents that could mimic the microenvironments in which it can be found in the atmospheric aerosol, using nanosecond laser flash photolysis, and conventional absorption and fluorescence techniques. Significant interactions between 1NPy and solvent molecules are demonstrated from the changes in the magnitude of the molar absorption coefficient, bandwidth at half-peak, oscillator strengths, absorption maxima, Stokes shifts, and fluorescence yield. The latter are very low (10 (-4)), increasing slightly with solvent polarity. Low temperature phosphorescence and room temperature transient absorption spectra demonstrate the presence of a low energy (3)(pi,pi*) triplet state, which decays with rate constants on the order of 10 (4)-10 (5) s (-1). This state is effectively quenched by known triplet quenchers at diffusion control rates. Intersystem crossing yields of 0.40-0.60 were determined. A long-lived absorption, which grows within the laser pulse, and simultaneously with the triplet state, presents a maximum absorption in the wavelength region of 420-440 nm. Its initial yield and lifetime depend on the solvent polarity. This species is assigned to the pyrenoxy radical that decays following a pseudo-first-order process by abstracting a hydrogen atom from the solvent to form one the major photoproducts, 1-hydroxypyrene. The (3)(pi,pi*) state reacts readily ( k approximately 10 (7)-10 (9) M (-1) s (-1)) with substances with hydrogen donor abilities encountered in the aerosol, forming a protonated radical that presents an absorption band with maximum at 420 nm.

Spectroscopy and photochemistry of fluorene at a silica gel/air interface

The emission spectroscopy of fluorene has been studied at a silica gel/air interface at surface loadings from 1.5% to 77.1% of a monolayer. Both monomer and excimer-like emissions are observed on the surface. The excimer-like emission, centered at 350 nm, arises from excitation of ground-state van der Waals pairs of fluorene molecules. Photolysis into the van der Waals dimers at room temperature does not lead to dissociation into constituent monomers; however, irradiation of fluorene monomers at a silica gel/air interface leads to production of 9-fluorenone as the only identifiable photoproduct in 72% yield. Fluorene triplet and cation radical are both observed at the silica gel/air interface by transient diffuse reflectance spectroscopy. Sensitization of singlet molecular oxygen by silica-sorbed methylene blue, in the presence of co-sorbed fluorene, does not lead to fluorene oxidation, thus suggesting that oxidation by direct photolysis does not involve an energy transfer mechanism. An electron transfer oxidation mechanism is proposed to account for the observed photochemical oxidation of fluorene at the silica gel/air interface.

Photochemistry of acenaphthene at a silica gel/air interface

Abstract The photolysis of acenapthene (ACE) has been studied at a silica gel/air interface. Direct photolysis leads to 1-acenaphthenol as the principal photoproduct. Secondary photochemical conversion of the alcohol into 1-acenaphthenone is also observed. Reactions leading to formation of both 1-acenaphthenol and 1-acenaphthenone are attributed to electron-transfer oxidation mechanisms and the cation radical of ACE is directly observed by transient diffuse reflectance spectroscopy. Preconditioning the silica gel by heating at 200°C in air partially removes physisorbed water from the silica surface and photochemical conversion of ACE is observed to occur faster.

CHARACTERIZATION OF THE TRANSIENT SPECIES IN THE 266-nm LASER PHOTOLYSIS OF ADENINE AND ITS DERIVATIVES

Abstract. Transient absorption spectra produced by 266‐nm ns laser flash photolysis of aqueous solutions of adenine, 2′‐deoxyadenosine, 2′‐deoxyadenosine‐5′‐phosphate, adenosine‐5′‐phosphate, 2′‐deoxyadenylyl‐(3′‐5′)‐2‐deoxyadenosine, 7‐methyladenine and 9‐methyladenine have been measured under different pH conditions. The transients observed after excitation consisted mainly of the hydrated electron, the radical cation, the radical anion and/or neutral radicals resulting from its protonation, and a minor contribution from the excited triplet state. The photoionization occurs mainly through an excited singlet state. For adenine, the cation and anion radical absorption spectra were determined using laser and pulse radiolysis techniques. The triplet state presence was established through sensitization experiments. The hydrated electron decays following a pseudo‐first‐order reaction with the neutral base.

Photodegradation Mechanisms of 1-Nitropyrene, an Environmental Pollutant: The Effect of Organic Solvents, Water, Oxygen, Phenols, and Polycyclic Aromatics on the Destruction and Product Yields

This work describes studies of the photodegradation mechanism of 1-nitropyrene (1-NO(2)Py) in a chemical model system consisting of an organic solvent and known constituents of an aerosol particle. Photoproducts such as 1-hydroxypyrene (1-OHPy), 1-hydroxy-x-nitropyrenes (1-OH-x-NO(2)Py), 1-nitrosopyrene, and 1,6- and 1,8-pyrenediones were identified by high-performance liquid chromatography (HPLC) and HPLC/mass spectrometry (HPLC/MS) techniques, and their quantum yields show a significant dependence on the type of solvent. The photodegradation quantum yield of 1-NO(2)Py, φ((-1-NO2Py)), was larger in toluene, benzene, and polar protic solvents (10(-3)) in comparison with nonpolar and polar aprotic solvents, where the yield is on the order of 10(-4). In solvents with an abstractable hydrogen atom, the products formed in higher yields were 1-OHPy and 1-OH-x-NO(2)Py. These represent 60-80% of the photodestruction yield and result from abstraction and recombination reactions of the pyrenoxy radical, an intermediate postulated to be formed as a result of a nitro-nitrite rearrangement in nitroaromatics. The small O(2) effect in the photodegradation yield and the quenching experiments with azulene demonstrate the small contribution of the (3)(π,π*) state in the 1-NO(2)Py photoreaction. The nitrosopyrene product was not observed under these conditions, demonstrating the participation of the (3)(π,π*) state in its formation. In the presence various phenol aerosol constituents, the photodegradation yield increased by 10-fold in all solvents. This effect is partly ascribed to the reaction of the (3)(π,π*) state with the phenol. The effect of water resulted in the reduction of the 1-NO(2)Py photodegradation yield and of its photoproducts. The phototodegradation of 1-NO(2)Py was also studied in a viscous solvent, hexadecane, and it was determined that this medium does not inhibit its photodecay.

A comparative photophysical and photochemical study of nitropyrene isomers occurring in the environment.

Ground state absorption, first excited-singlet state, and properties of reactive intermediates of mononitropyrene isomers encountered in the atmospheric aerosol have been studied under different conditions that could mimic the environment. The nitro group can present different orientations relative to the pyrene ring depending on its geometric location and could induce differences in the photochemistry of the isomers. The 2-NO(2)Py isomer has the largest red shift and lowest oscillator strength in the UV-visible band associated with the nitro group. The isomers show very low fluorescence yields (10(-3)-10(-4)). Only 1-NO(2)Py and 4-NO(2)Py have phosphorescence emission (Φ(p) ≈ 10(-4)), indicating that the lowest triplet state decays mainly through effective radiationless channels. Laser photolysis produces a low-lying triplet state (τ(T) = 10(-5)-10(-6) s), a long-lived pyrenoxy radical, and a PyNO(2)H radical in solvents in which the triplet can abstract a hydrogen atom. Similar triplet yields were calculated (0.1-0.6) for the isomers, while significant differences in the relative yield of the long-lived species were determined. Differences in the quenching rate constants of the triplet by water and phenols suggest a strong hydrogen-bond interaction with the nitro group in the C-2 position, which provides for radiationless deactivation routes.

AN EPR STUDY OF THE SPECIES PRODUCED DURING THE UV PHOTOLYSIS OF HETEROCYCLIC COMPOUNDS IN METHYLTETRAHYDROFURAN AT 77 K

Abstract—The ultraviolet irradiation (290 nm ≤Λ≤ 390 nm) of indole, purine, indazole, acridine and quinoline in 2‐methyltetrahydrofuran glass at 77 K produces trapped radicals. Two electron‐paramagnetic‐resonance (EPR) signals are found at 77 K during illumination, one at high magnetic field (3–25 times 10‐1 T) assigned to the matrix radical and the other at low field (1.3 times 10‐1 to 1–5 times 10‐1 T) attributed to the lowest triplet state of the heterocyclic molecule. Quantum yields for triplet production at 77 K are 0–34 for indole, 0.51 for purine, 0.55 for indazole, 0.15 for acridine, and 0.94 for quinoline. The rate of formation of matrix radicals varies as the nRth power of the incident light intensity, I0nR, where 1.6 ≤nR=≤ 2. Solvent radical yields, which depend on the light intensity, have been determined. Under the experimental conditions, no signals attributable to trapped electrons or cations have been observed. The dependence of the reciprocal value of the rise lifetime of the low field EPR signal as a function of the intensity of exposure is in accordance with a biphotonic mechanism.

Photoionization of DNA and RNA Bases, Nucleosides and Nucleotides Through a Combination of One- and Two-photon Pathways upon 266 nm Nanosecond Laser Excitation¶

Abstract The 266 nm nanosecond laser photolysis of various purine and pyrimidine derivatives results in their photoionization (PI) as one of the primary photochemical pathways. Electron photoejection occurs through a combination of one- and two-photon mechanisms. The PI values depend on the substituents attached to the chromophore of the base. The net PI of the purine bases at 266 nm are of the same order of magnitude (10−2) as those of the pyrimidine bases under similar experimental conditions. The monophotonic component is approximately one-third of the net PI yield of the bases. A nonrelaxed singlet excited state intermediate is tentatively proposed for this pathway. It is proposed that this state is significantly stabilized by water solvation, transforming it into a charge transfer to solvent state from which the hydrated electron evolves.

Photophysical Properties of Gilvocarcins V and M and Their Binding Constant to Calf Thymus DNA

Abstract— Absorption and emission techniques were used to characterize the ground (S0), singlet (S|) and triplet states (T1) of gilvocarcin V (GV) and gilvocarcin M (GM) in different solvents. Aggregation of GV with dimerization constant equal to 7800 M−1is observed in 10% dimethyl‐sulfoxide (DMSO)/water. The photophysical properties of the S, state of these molecules are more sensitive to changes in solvent characteristics than the corresponding ground states. The absorption of visible light by GV and GM results in a higher dipole moment of the excited state causing a red shift in the fluorescence spectra with increasing solvent polarity. The fluorescence quantum yield remains practically unchanged with changes in solvent properties unless water is present as a co‐solvent. Both φf and φf values corresponding to GV in DMSO are larger than those of GM, whereas in 10% DMSO/H2O the opposite is observed. Thus, GV is more susceptible to other deactivation pathways besides emission in the presence of water than GM. The relative phosphorescence quantum yield (φp= 0.03) and the triplet energy (ET= 52 kcal/mol) of GV and GM are similar. The S0‐S1 energy difference is 63 kcal/mol for GV, whereas for GM it is 67. Thus, the singlet‐triplet energy difference is 11 and 15 kcal/mol, respectively. The PM3/CI calculated electronic structures of these compounds are consistent with the observed photophysical properties. The dark binding constants of GV to calf thymus DNA ([1.1–0.08] × 106M−1) are about an order of magnitude larger than those of GM ([0.24–0.018] × 106M−1) at different ionic strengths (0–2.00 M NaCl). Also, the number of gilvocarcin molecules bound per base pair is smaller for GM than for GV. These differences in dark DNA binding parameters between GV and GM could have implications in the large photocytotoxic ability of GV as compared to GM.