Showing papers on "Flash photolysis published in 2015"

High Yield Ultrafast Intramolecular Singlet Exciton Fission in a Quinoidal Bithiophene

Abstract: We report the process of singlet exciton fission with high-yield upon photoexcitation of a quinoidal thiophene molecule. Efficient ultrafast triplet photogeneration and its yield are determined by photoinduced triplet–triplet absorption, flash photolysis triplet lifetime measurements, as well as by femtosecond time-resolved transient absorption and fluorescence methods. These experiments show that optically excited quinoidal bithiophene molecule undergoes ultrafast formation of the triplet-like state with the lifetime ∼57 μs. CASPT2 and RAS-SF calculations have been performed to support the experimental findings. To date, high singlet fission rates have been reported for crystalline and polycrystalline materials, whereas for covalently linked dimers and small oligomers it was found to be relatively small. In this contribution, we show an unprecedented quantum yield of intramolecular singlet exciton fission of ∼180% for a quinoidal bithiophene system.

A benzophenone-naphthalimide derivative as versatile photoinitiator of polymerization under near UV and visible lights

Abstract: A benzophenone-naphthalimide derivative (BPND) bearing tertiary amine groups has been developed as a high-performance photoinitiator in combination with 2,4,6-tris(trichloromethyl)-1,3,5-triazine or an iodonium salt for both the tree radical polymerization (FRP) of acrylates and the cationic polymerization (CP) of epoxides upon exposure to near UV and visible LEDs (385–470 nm). BPND can even produce radicals without any added hydrogen donor. The photochemical mechanisms are studied by molecular orbital calculations, steady state photolysis, electron spin resonance spin trapping, fluorescence, cyclic voltammetry and laser flash photolysis techniques. These novel BPND based photoinitiating systems exhibit an efficiency higher than that of the well-known camphorquinone- based systems (FRP and CP) or comparable to that of bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (FRP at 455 nm).

Specific cationic photoinitiators for near UV and visible LEDs: Iodonium versus ferrocenium structures

Abstract: Two iodonium salts based on a coumarin chromophore are investigated for polymerization upon light emitting diode irradiations (LEDs). They work as one-component photoinitiators. They initiate the cationic polymerization of epoxides (under air) and vinylethers (laminate) upon exposure to violet LEDs (385 and 405 nm). Excellent polymerization profiles are recorded. Their efficiency is quite similar to that of a ferrocenium salt. Interpenetrating polymer networks can also be obtained through a concomitant cationic/radical photopolymerization of an epoxy/acrylate blend monomer. The light absorption properties of these new salts as well as the involved photochemical mechanisms are investigated for the first time through electron spin resonance, laser flash photolysis, steady state photolysis experiments. Molecular orbital calculations are also used to shed some light on the initiation mechanisms

Photochromic Phenoxyl-Imidazolyl Radical Complexes with Decoloration Rates from Tens of Nanoseconds to Seconds

Abstract: We report a novel photochromic molecular system, phenoxyl-imidazolyl radical complex (PIC), in which both a phenoxyl radical site and an imidazolyl radical site are reversibly and simultaneously generated upon UV light irradiation. PIC consists of the three parts: an aromatic linker, a diarylimidazole moiety, and a 4H-cyclohexadienone ring. Upon UV light irradiation, the C–N bond between the 4H-cyclohexadienone ring and the imidazole ring in the colorless closed-ring isomer of PIC undergoes a homolytic cleavage, leading to the formation of the transient colored open-ring isomer. Based on the substituents on the imidazoyl/4H-cyclohexadienone rings and the nature of the aromatic linker, the half-life of the colored open-ring isomer can be varied between tens of nanoseconds and seconds. PIC derivatives containing a 1,2-phenylene linker exhibit high fatigue resistance toward repeated photochromic reactions. Analysis using laser flash photolysis reveals that the absorption spectra of the open-ring isomers are ...

Mechanistic Studies of Hydrogen Evolution in Aqueous Solution Catalyzed by a Tertpyridine–Amine Cobalt Complex

Abstract: The ability of cobalt-based transition metal complexes to catalyze electrochemical proton reduction to produce molecular hydrogen has resulted in a large number of mechanistic studies involving various cobalt complexes. While the basic mechanism of proton reduction promoted by cobalt species is well-understood, the reactivity of certain reaction intermediates, such as CoI and CoIII–H, is still relatively unknown owing to their transient nature, especially in aqueous media. In this work we investigate the properties of intermediates produced during catalytic proton reduction in aqueous solutions promoted by the [(DPA-Bpy)Co(OH2)]n+ (DPA-Bpy = N,N-bis(2-pyridinylmethyl)-2,20-bipyridine-6-methanamine) complex ([Co(L)(OH2)]n+ where L is the pentadentate DPA-Bpy ligand or [Co(OH2)]n+ as a shorthand). Experimental results based on transient pulse radiolysis and laser flash photolysis methods, together with electrochemical studies and supported by density functional theory (DFT) calculations indicate that, while...

Direct Observation of Guanine Radical Cation Deprotonation in G-Quadruplex DNA

Abstract: Although numerous studies have been devoted to the charge transfer through double-stranded DNA (dsDNA), one of the major problems that hinder their potential applications in molecular electronics is the fast deprotonation of guanine cation (G+•) to form a neutral radical that can cause the termination of hole transfer. It is thus of critical importance to explore other DNA structures, among which G-quadruplexes are an emerging topic. By nanosecond laser flash photolysis, we report here the direct observation and findings of the unusual deprotonation behavior (loss of amino proton N2–H instead of imino proton N1–H) and slower (1–2 orders of magnitude) deprotonation rate of G+• within G-quadruplexes, compared to the case in the free base dG or dsDNA. Four G-quadruplexes AG3(T2AG3)3, (G4T4G4)2, (TG4T)4, and G2T2G2TGTG2T2G2 (TBA) are measured systematically to examine the relationship of deprotonation with the hydrogen-bonding surroundings. Combined with in depth kinetic isotope experiments and pKa analysis, ...

Solvent-free one-step photochemical hydroxylation of benzene derivatives by the singlet excited state of 2,3-Dichloro-5,6-dicyano-p-benzoquinone acting as a super oxidant.

Abstract: Photoinduced hydroxylation of neat deaerated benzene to phenol occurred under visible-light irradiation of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), which acts as a super photooxidant in the presence of water. Photocatalytic solvent-free hydroxylation of benzene derivatives with electron-withdrawing substituents such as benzonitrile, nitrobenzene, and trifluoromethylbenzene used as neat solvents has been achieved for the first time by using DDQ as a super photooxidant to yield the corresponding phenol derivatives and 2,3-dichloro-5,6-dicyanohydroquinone (DDQH2 ) in the presence of water under deaerated conditions. In the presence of dioxygen and tert-butyl nitrite, the photocatalytic hydroxylation of neat benzene occurred with DDQ as a photocatalyst to produce phenol. The photocatalytic reactions are initiated by oxidation of benzene derivatives with the singlet and triplet excited states of DDQ to form the corresponding radical cations, which associate with benzene derivatives to produce the dimer radical cations, which were detected by the femto- and nanosecond laser flash photolysis measurements to clarify the photocatalytic reaction mechanisms. Radical cations of benzene derivatives react with water to yield the OH-adduct radicals. On the other hand, DDQ(.) (-) produced by the photoinduced electron transfer from benzene derivatives reacts with the OH-adduct radicals to yield the corresponding phenol derivatives and DDQH2 . DDQ is recovered by the reaction of DDQH2 with tert-butyl nitrite when DDQ acts as a photocatalyst for the hydroxylation of benzene derivatives by dioxygen.

Intermolecular and Intramolecular Electron Transfer Processes from Excited Naphthalene Diimide Radical Anions

Abstract: Excited radical ions are interesting reactive intermediates owing to powerful redox reactivities, which are applicable to various reactions. Although their reactivities have been examined for many years, their dynamics are not well-defined. In this study, we examined intermolecular and intramolecular electron transfer (ET) processes from excited radical anions of naphthalene-1,4,5,8-tetracarboxydiimide (NDI(•-)*). Intermolecular ET processes between NDI(•-)* and various electron acceptors were confirmed by transient absorption measurements during laser flash photolysis of NDI(•-) generated by pulse radiolysis. Although three different imide compounds were employed as acceptors for NDI(•-)*, the bimolecular ET rate constants were similar in each acceptor, indicating that ET is not the rate-determining step. Intramolecular ET processes were examined by applying femtosecond laser flash photolysis to two series of dyad compounds, where NDI was selectively reduced chemically. The distance dependence of the ET rate constants was described by a β value of 0.3 A(-1), which is similar or slightly smaller than the reported values for donor-acceptor dyads with phenylene spacers. Furthermore, by applying the Marcus theory to the driving force dependence of the ET rate constants, the electronic coupling for the present ET processes was determined.

Iron complexes as photoinitiators for radical and cationic polymerization through photoredox catalysis processes

Abstract: Six iron complexes (FeCs) with various ligands have been designed and synthesized In combination with additives (eg, iodonium salt, N-vinylcarbazole, amine, or chloro triazine), the FeC-based systems are able to efficiently generate radicals, cations, and radical cations on a near UV or visible light-emitting diode (LED) exposure These systems are characterized by an unprecedented reactivity, that is, for very low content 002% FeC-based systems is still highly efficient in photopolymerization contrary to the most famous reference systems (Bisacylphosphine oxide) illustrating the performance of the proposed catalytic approach This work paves the way for polymerization in soft conditions (eg, on LED irradiation) These FeC-based systems exhibit photocatalytic properties, undergo the formation of radicals, radical cations, and cations and can operate through oxidation or/and reduction cycles The photochemical mechanisms for the formation of the initiat- ing species are studied using steady state photolysis, cyclic voltammetry, electron spin resonance spin trapping, and laser flash photolysis techniques V C 2014 Wiley Periodicals, Inc J Polym Sci, Part A: Polym Chem 2014, 00, 000-000

Naphthalimide-phthalimide derivative based photoinitiating systems for polymerization reactions under blue lights

Abstract: Naphthalimide-phthalimide derivatives (NDPDs) have been synthesized and combined with an iodonium salt, N-vinylcarbazole, amine or 2,4,6-tris(trichloromethyl)-1,3,5-triazine to produce reactive species (i.e., radicals and cations). These generated reactive species are capable of initiating the cationic polymerization of epoxides and/or the radical polymerization of acrylates upon exposure to very soft polychromatic visible lights or blue lights. Compared with the well-known camphorquinone based systems used as references, the novel NDPD based combinations employed here demonstrate clearly higher efficiencies for the cationic polymerization of epoxides under air as well as the radical polymerization of acrylates. Remarkably, one of the NDPDs (i.e., NDPD2) based systems is characterized by an outstanding reactivity. The structure/reactivity/efficiency relationships of the investigated NDPDs were studied by fluorescence, cyclic voltammetry, laser flash photolysis, electron spin resonance spin trapping, and steady state photolysis techniques. The key parameters for their reactivity are provided. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 665–674

Naphthalimide Derivatives: Substituent Effects on the Photoinitiating Ability in Polymerizations under Near UV, Purple, White and Blue LEDs (385, 395, 405, 455, or 470 nm)

Abstract: Naphthalimide derivatives (NDAs) with amino- or alkylthio-substituents have been synthesized and combined in photoinitiating systems (PIS) with an iodonium salt, N-vinylcarbazole, an amine, or 2,4,6-tris(trichloromethyl)-1,3,5-triazine to produce radicals, cation radicals, or cations. The photochemical mechanisms are investigated by fluorescence, cyclic voltammetry, laser flash photolysis, electron spin resonance spin trapping, and steady state photolysis techniques. The generated reactive species are capable of initiating the cationic polymerization (CP) of epoxides under air or the free radical polymerization (FRP) of acrylates in laminate upon exposure to visible lights delivered by a halogen lamp, a cold white light-emitting diode (LED), or LEDs at 385, 395, 405, 455, or 470 nm. Compared to the well-known camphorquinone based systems, the novel NDA containing PISs exhibit higher efficiencies both for CP and FRP. Remarkably, the photo­bleaching of some NDAs is significant, and colorless final polymer coatings are obtained. The structure/photochemical property relationships of NDAs are discussed in detail (redox potentials, fluorescence, rate constants of reactions, etc.) to shed some light on their respective photoinitiating ability.

Enhanced solar photodegradation of toxic pollutants by long-lived electrons in Ag–Ag2O nanocomposites

Abstract: Ag–Ag2O was prepared by precipitation and thermal decomposition and characterized by X-ray diffraction and high-resolution transmission electron microscopy. A well-matched interfacial contact was formed between the Ag and Ag2O nanoparticles (NPs) by drying Ag2O at 70–100 °C. The nanocomposites exhibited a higher photoactivity and stability for the degradation and mineralization of toxic persistent organic pollutants compared to Ag2O, which was demonstrated using 2-chlorophenol, 2,4-dichlorophenol and trichlorophenol under visible or near-infrared light irradiation and simulated solar irradiation. Based on electron storage and transient photocurrent, laser flash photolysis, and electron spin resonance analyses under a variety of experimental conditions, two sequential electron transfer processes were verified from photoexcited Ag2O to Ag NPs to a thionine molecule or surface adsorbed oxygen as well as from water to the Ag2O, resulting in O2 − and OH. The maximum electron storage, the longest lifetime (757 μs) of photogenerated electrons and the strongest steady state photocurrent were observed for Ag–Ag2O-70 °C, which resulted in the highest solar photoactivity. However, electron-hole recombination was dominant in the Ag–Ag2O dried in other temperature ranges, leading to a much lower photoactivity. The results indicated that the interfacial contact with the metal/semiconductor played a key role in charge separation and migration, which improved their photocatalytic efficiency. These results will allow for the application of narrow bandgap semiconductors that can harvest the full spectrum of sunlight to be employed in photocatalysis and photovoltaic fuel cells.

Panchromatic photoinitiators for radical, cationic and thiol-ene polymerization reactions: A search in the diketopyrrolopyrrole or indigo dye series

Abstract: A series of diketopyrrolopyrrole derivatives (DPPs) and indigo derivatives (IDGs) are synthesized and applied as photoinitiators (PIs) for the cationic polymerization (CP) of epoxides, the free radical polymerization (FRP) of acrylates or the thiol-ene polymerization under different lights (purple, blue, green, yellow, red LEDs and green laser diode). The photochemical mechanisms involved in the presence of these PIs and additives (iodonium salt (Iod), N -vinylcarbazole (NVK), amine (MDEA) or 2,4,6- tris (trichloromethyl)-1,3,5-triazine (R-Cl)) are investigated by steady state photolysis, electron spin resonance spin trapping, fluorescence, cyclic voltammetry, and laser flash photolysis techniques. Real-time infrared spectroscopy studies reveal that DPP4/Iod/NVK is very efficient for both CP and FRP and, compared to two references (Eosin-Y and camphorquinone), displays noticeably higher polymerization efficiencies. Panchromatic curable formulations exhibiting an almost constant photosensitivity from the blue to the red are proposed.

The reactions of N-methylformamide and N,N-dimethylformamide with OH and their photo-oxidation under atmospheric conditions: experimental and theoretical studies

Abstract: The reactions of OH radicals with CH3NHCHO (N-methylformamide, MF) and (CH3)2NCHO (N,N-dimethylformamide, DMF) have been studied by experimental and computational methods. Rate coefficients were determined as a function of temperature (T = 260–295 K) and pressure (P = 30–600 mbar) by the flash photolysis/laser-induced fluorescence technique. OH radicals were produced by laser flash photolysis of 2,4-pentanedione or tert-butyl hydroperoxide under pseudo-first order conditions in an excess of the corresponding amide. The rate coefficients obtained show negative temperature dependences that can be parameterized as follows: kOH+MF = (1.3 ± 0.4) × 10−12 exp(3.7 kJ mol−1/(RT)) cm3 s−1 and kOH+DMF = (5.5 ± 1.7) × 10−13 exp(6.6 kJ mol−1/(RT)) cm3 s−1. The rate coefficient kOH+MF shows very weak positive pressure dependence whereas kOH+DMF was found to be independent of pressure. The Arrhenius equations given, within their uncertainty, are valid for the entire pressure range of our experiments. Furthermore, MF and DMF smog-chamber photo-oxidation experiments were monitored by proton-transfer-reaction time-of-flight mass spectrometry. Atmospheric MF photo-oxidation results in 65% CH3NCO (methylisocyanate), 16% (CHO)2NH, and NOx-dependent amounts of CH2NH and CH3NHNO2 as primary products, while DMF photo-oxidation results in around 35% CH3N(CHO)2 as primary product and 65% meta-stable (CH3)2NC(O)OONO2 degrading to NOx-dependent amounts of CH3NCH2 (N-methylmethanimine), (CH3)2NNO (N-nitroso dimethylamine) and (CH3)2NNO2 (N-nitro dimethylamine). The potential for nitramine formation in MF photo-oxidation is comparable to that of methylamine whereas the potential to form nitrosamine and nitramine in DMF photo-oxidation is larger than for dimethylamine. Quantum chemistry supported atmospheric degradation mechanisms for MF and DMF are presented. Rate coefficients and initial branching ratios calculated with statistical rate theory based on molecular data from quantum chemical calculations at the CCSD(T*)-F12a/aug-cc-pVTZ//MP2/aug-cc-pVTZ level of theory show satisfactory agreement with the experimental results. It turned out that adjustment of calculated threshold energies by 0.2 to 8.8 kJ mol−1 lead to agreement between experimental and predicted results.

Primary reactions in the photochemistry of hexahalide complexes of platinum group metals: A minireview

Abstract: A B S T R A C T We review the results obtained for Pt IV Cl6 2� , Pt IV Br6 2� , Ir IV Cl6 2� , Ir IV Br6 2� , and Os IV Br6 2� complexes in aqueous and alcoholic solutions using ultrafast pump–probe spectroscopy, laser flash photolysis, ESR, and photoelectron spectroscopy. We discuss the correlations between the photophysics and the photochemistry of these complexes. The key reaction for Pt IV Cl6 2� is the inner-sphere electron transfer, which results in an Adamson radical pair that lives for several picoseconds, and the subsequent photoaquation in aqueous solutions and photoreduction in alcohols. The chlorine atom formed as the primary product escapes the solvent cage in aqueous solutions or oxidizes a solvent alcohol molecule via secondary electron transfer, producing secondary intermediates that react on the microsecond time scale. The photoexcitation of Pt IV Br6 2� results in the formation of pentacoordinated Pt IV intermediates, i.e. 3 Pt IV Br5 � and 1 Pt IV Br5 � , with characteristic lifetimes of approximately 1 and 10 ps, respectively. Subsequent reactions of these intermediates result in the complexation of a solvent molecule. Photoreduction is also possible in alcohols. Similar reactions occur with rather low quantum yields for Ir IV Cl6 2� , therefore, only the ground-state recovery could be monitored in ultrafast experiments, which occur on the 10-ps time scale. The photochemical behaviours of the Ir IV Br6 2� and Os IV Br6 2� complexes are similar to those of Ir IV Cl6 2� and Pt IV Br6 2� , respectively.

Novel panchromatic photopolymerizable matrices: N,N'‐dibutylquinacridone as an efficient and versatile photoinitiator

Abstract: N,N'-dibutylquinacridone (DBQA) is utilized here for the first time as a high-performance panchromatic photoinitiator for the cationic polymerization (CP) of epoxides, the free radical polymerization (FRP) of acrylates, the thiol-ene polymerization and the synthesis of interpenetrated polymer networks (epoxide/acrylate) under violet, blue, green and yellow lights (emitted from LED at 405 nm, 470 nm, 520 nm, or 594 nm, or laser diode at 532 nm). It confers a panchromatic character to the photopolymerizable matrices. Remarkably, the proposed DBQA based photoinitiating systems exhibit quite excellent efficiency (the final monomer conversion for multifunctional monomers at room temperature can reach 62% and 50% in CP and FRP, respectively) and appear as much more powerful than the camphorquinone or Eosin-Y containing reference systems for visible light. For green light, DBQA is much more reactive than the literature reference (Eosin-Y) and for blue light, a good reactivity is found compared with camphorquinone. The photochemical mechanisms are studied by molecular orbital calculations, steady state photolysis, fluorescence, cyclic voltammetry, laser flash photolysis, and electron spin resonance spin trapping techniques. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1719–1727

Photodeamination Reaction Mechanism in Aminomethyl p-Cresol Derivatives: Different Reactivity of Amines and Ammonium Salts

Abstract: Derivatives of p-cresol 1–4 were synthesized, and their photochemical reactivity, acid–base, and photophysical properties were investigated. The photoreactivity of amines 1 and 3 is different from that for the corresponding ammonium salts 2 and 4. All compounds have low fluorescence quantum yields because the excited states undergo deamination reactions, and for all cresols the formation of quinone methides (QMs) was observed by laser flash photolysis. The reactivity observed is a consequence of the higher acidity of the S1 states of these p-cresols and the ability for excited-state intramolecular proton transfer (ESIPT) to occur in the case of 1 and 3, but not for salts 2 and 4. In aqueous solvent, deamination depends largely on the prototropic form of the molecule. The most efficient deamination takes place when monoamine is in the zwitterionic form (pH 9–11) or diamine is in the monocationic form (pH 7–9). QM1, QM3, and QM4 react with nucleophiles, and QM1 exhibits a shorter lifetime when formed from 1...

photochemistry of Fe(III) complexes with salicylic acid derivatives in aqueous solutions

Abstract: The photochemistry of a series of 1:1 Fe(III) complexes with salicylic acid derivatives (SADs) in aqueous solution has been investigated, using the following model compounds: salicylic acid (SA), 5-sulfoSA (5SSA), 4-hydroxySA (2HSA), 6-hydroxySA (6HSA), 4-nitroSA (4NSA) and 5-nitroSA (5NSA). The results of optical spectroscopy, steady-state and nanosecond laser flash photolysis experiments show that all Fe(III)–SAD complexes exhibit good photochemical stability upon 355 nm excitation (quantum yields of photolysis do not exceed 0.01). No evidence of hydroxyl radical formation was observed in laser flash experiments. Therefore, the primary photoprocess was assumed to be an electron transfer from coordinated SAD to the Fe(III) ion in the excited complex, leading to the formation of Fe(II) and the corresponding SAD phenoxy radical. SADs containing electron-withdrawing groups (4NSA, 5NSA, 5SSA) exhibit the smallest quantum yields of photolysis of the series, whereas the highest values were observed for SA and SADs containing electron-donating groups (4HSA, 6HSA). Therefore, in contrast to Fe(III)-complexes with aliphatic carboxylates, sunlight photolysis of Fe(III)–SAD complexes cannot be an effective way for the degradation of SADs in natural water systems.

Synthesis and characterization of TiO₂ and TiO₂/Ag for use in photodegradation of methylviologen, with kinetic study by laser flash photolysis.

Abstract: This paper reports the synthesis, characterization, and application of TiO2 and TiO2/Ag nanoparticles for use in photocatalysis, employing the herbicide methylviologen (MV) as a substrate for photocatalytic activity testing. At suitable metal to oxide ratios, increases in silver surface coating on TiO2 enhanced the efficiency of heterogeneous photocatalysis by increasing the electron transfer constant. The sol-gel method was used for TiO2 synthesis. P25 TiO2 was the control material. Both oxides were subjected to the same silver incorporation process. The materials were characterized by conventional spectroscopy, SEM micrography, X-ray diffraction, calculation of surface area per mass of catalyst, and thermogravimetry. Also, electron transfers between TiO2 or TiO2/Ag and MV in the absence and presence of sodium formate were investigated using laser flash photolysis. Oxides synthesized with 2.0 % silver exhibited superior photocatalytic activity for MV degradation.

Ultrafast Photoinduced Charge Separation Leading to High-Energy Radical Ion-Pairs in Directly Linked Corrole-C60 and Triphenylamine-Corrole-C60 Donor-Acceptor Conjugates.

Abstract: Closely positioned donor-acceptor pairs facilitate electron- and energy-transfer events, relevant to light energy conversion. Here, a triad system TPACor-C60 , possessing a free-base corrole as central unit that linked the energy donor triphenylamine (TPA) at the meso position and an electron acceptor fullerene (C60) at the β-pyrrole position was newly synthesized, as were the component dyads TPA-Cor and Cor-C60. Spectroscopic, electrochemical, and DFT studies confirmed the molecular integrity and existence of a moderate level of intramolecular interactions between the components. Steady-state fluorescence studies showed efficient energy transfer from (1) TPA* to the corrole and subsequent electron transfer from (1) corrole* to fullerene. Further studies involving femtosecond and nanosecond laser flash photolysis confirmed electron transfer to be the quenching mechanism of corrole emission, in which the electron-transfer products, the corrole radical cation (Cor(⋅+) in Cor-C60 and TPA-Cor(⋅+) in TPACor-C60) and fullerene radical anion (C60(⋅-)), could be spectrally characterized. Owing to the close proximity of the donor and acceptor entities in the dyad and triad, the rate of charge separation, kCS , was found to be about 10(11) s(-1), suggesting the occurrence of an ultrafast charge-separation process. Interestingly, although an order of magnitude slower than kCS , the rate of charge recombination, kCR , was also found to be rapid (kCR ≈10(10) s(-1)), and both processes followed the solvent polarity trend DMF>benzonitrile>THF>toluene. The charge-separated species relaxed directly to the ground state in polar solvents while in toluene, formation of (3) corrole* was observed, thus implying that the energy of the charge-separated state in a nonpolar solvent is higher than the energy of (3) corrole* being about 1.52 eV. That is, ultrafast formation of a high-energy charge-separated state in toluene has been achieved in these closely spaced corrole-fullerene donor-acceptor conjugates.

Direct detection of a triplet vinylnitrene, 1,4-naphthoquinone-2-ylnitrene, in solution and cryogenic matrices.

Abstract: The photolysis of 2-azido-1,4-naphthoquinone (1) in argon matrices at 8 K results in the corresponding triplet vinylnitrene 32, which was detected directly by IR spectroscopy. Vinylnitrene 32 is stable in argon matrices but forms 2-cyanoindane-1,3-dione (3) upon further irradiation. Similarly, the irradiation of azide 1 in 2-methyltetrahydrofuran (MTHF) matrices at 5 K resulted in the ESR spectrum of vinylnitrene 32, which is stable up to at least 100 K. The zero-field splitting parameters for nitrene 32, D/hc = 0.7292 cm–1 and E/hc = 0.0048 cm–1, verify that it has significant 1,3-biradical character. Vinylnitrene 32 (λmax ∼ 460 nm, τ = 22 μs) is also observed directly in solution at ambient temperature with laser flash photolysis of 1. Density functional theory (DFT) calculations support the characterization of vinylnitrene 32 and the proposed mechanism for its formation. Because vinylnitrene 32 is relatively stable, it has potential use as a building-block for high-spin assemblies.

Optical Absorption and Magnetic Field Effect Based Imaging of Transient Radicals.

Abstract: Short-lived radicals generated in the photoexcitation of flavin adenine dinucleotide (FAD) in aqueous solution at low pH are detected with high sensitivity and spatial resolution using a newly developed transient optical absorption detection (TOAD) imaging microscope. Radicals can be studied under both flash photolysis and continuous irradiation conditions, providing a means of directly probing potential biological magnetoreception within sub-cellular structures. Direct spatial imaging of magnetic field effects (MFEs) by magnetic intensity modulation (MIM) imaging is demonstrated along with transfer and inversion of the magnetic field sensitivity of the flavin semiquinone radical concentration to that of the ground state of the flavin under strongly pumped reaction cycling conditions. A low field effect (LFE) on the flavin semiquinone-adenine radical pair is resolved for the first time, with important implications for biological magnetoreception through the radical pair mechanism.

Experimental and Computational Studies of the Kinetics of the Reaction of Atomic Hydrogen with Methanethiol

Abstract: The overall rate constant for H + CH3SH has been studied over 296–1007 K in an Ar bath gas using the laser flash photolysis method at 193 nm H atoms were generated from CH3SH and in some cases NH3 They were detected via time-resolved resonance fluorescence The results are summarized as k = (345 ± 019) × 10–11 cm3 molecule–1 s–1 exp(−692 ± 016 kJ mol–1/RT) where the errors in the Arrhenius parameters are the statistical uncertainties at the 2σ level Overall error limits of ±9% for k are proposed In the overlapping temperature range there is very good agreement with the resonance fluorescence measurements of Wine et al Ab initio data and transition state theory yield moderate accord with the total rate constant, but not with prior mass spectrometry measurements of the main product channels leading to CH3S + H2 and CH3 + H2S by Amano et al

Direct Spectroscopic Detection and EPR Investigation of a Ground State Triplet Phenyl Oxenium Ion

Abstract: Oxenium ions are important reactive intermediates in synthetic chemistry and enzymology, but little is known of the reactivity, lifetimes, spectroscopic signatures, and electronic configurations of these unstable species. Recent advances have allowed these short-lived ions to be directly detected in solution from laser flash photolysis of suitable photochemical precursors, but all of the studies to date have focused on aryloxenium ions having closed-shell singlet ground state configurations. To study alternative spin configurations, we synthesized a photoprecursor to the m-dimethylamino phenyloxenium ion, which is predicted by both density functional theory and MRMP2 computations to have a triplet ground state electronic configuration. A combination of femtosecond and nanosecond transient absorption spectroscopy, nanosecond time-resolved Resonance Raman spectroscopy (ns-TR3), cryogenic matrix EPR spectroscopy, computational analysis, and photoproduct studies allowed us to trace essentially the complete ar...

Aryl tosylates as non-ionic photoacid generators (PAGs): photochemistry and applications in cationic photopolymerizations†

Abstract: The irradiation of various substituted aryl tosylates was investigated in a solution and homolysis of the ArO–SO2C6H4CH3 bond was the path exclusively observed. The corresponding phenols and photo-Fries adducts were obtained and p-toluenesulfinic and p-toluenesulfonic acids were liberated. The nature and amount of the acid photoreleased were tuned by changing the reaction conditions and the nature and position of the aromatic substituents. In deaerated solutions p-toluenesulfinic acid was formed exclusively, whereas under oxygenated conditions the stronger p-toluenesulfonic acid was released, as shown by HPLC ion chromatography analyses. ArO–S bond photocleavage takes place from the singlet state, as confirmed by laser flash photolysis experiments, and competitive intersystem crossing can make the aryl tosylate unreactive when a nitro group is present. The application of these aryl tosylates as non-ionic photoacid generators (PAGs) in hybrid organic/inorganic sol–gel photoresists has been explored.