The flourescence lifetime of the benzyl radical

Time resolved fluorescence of the benzyl, monomethylbenzyl and dimethylbenzyl radicals strapped in rigid solvents at low temperatures has been observed using the second harmonic of the ruby laser at the exciting source. The fluorescence lifetimes of these radicals are very long (10^?7–10^?6 sec), which are influenced considerably by the methyl substituents. The long fluorescence lifetimes of the benzyl radical and its methyl derivatives are interpreted in terms of the forbidden character of the first doublet-doublet electronic transition.     

Synthesis and Hybridization Properties of Oligodeoxynucleotides with Long‐Chain Linkers

New oligonucleotides with a long‐chain linker (6,9‐dioxa‐3,12‐diazatetradecane‐1,14‐diyl) in their backbone were synthesized, and their hybridization properties were studied by measurement of their T_m curves and fluorescence spectra. The T_m analyses revealed that these oligonucleotides could bind to their complementary strands despite the presence of the long‐chain linker. We also demonstrated interesting fluorescence properties of oligodeoxynucleotides with an anthracen‐9‐ylmethyl group on one of the two N‐atoms in the long‐chain linker. The fluorescence intensity of these oligonucleotides increased upon their hybridization to the complementary strands and was sensitive to the presence of the mismatch base pairs at a specific position.     

Unimolecular Reaction Properties for the Long‐Chain Alkenyl Radicals

Although alkenyl radicals are important intermediates involved in both alkanes and alkenes combustion, previous kinetic studies on them are very limited, especially for the long‐chain alkenyl radicals. To deeply investigate unimolecular reaction activities of long‐chain alkenyl radicals, a series of octenyl (C₈H₁₅) radicals were chosen to study the reaction kinetics of three typical types of reactions (i.e., intramolecular radical addition, internal H‐migration, and bond dissociation) in this work. The CBS‐QB3 method was used to build potential energy surfaces for these reactions, and the transition state theory was applied to obtain the high‐pressure limit rate constants. Some general rules are observed from our systematic calculations in respect of structure–activity relationships.     

Direct Observation of Aliphatic Peroxy Radical Autoxidation and Water Effects: An Experimental and Theoretical Study

The autoxidation of organic peroxy radicals (RO₂) into hydroperoxy‐alkyl radicals (QOOH), then hydroperoxy‐peroxy radicals (HOOQO₂) is now considered to be important in the Earth's atmosphere. To avoid mechanistic uncertainties these reactions are best studied by monitoring the radicals. But for the volatile and aliphatic RO₂ radicals playing key roles in the atmosphere this has long been an instrumental challenge. This work reports the first study of the autoxidation of aliphatic RO₂ radicals and is based on monitoring RO₂ and HOOQO₂ radicals. The rate coefficients, k_iso (s^?1), were determined both experimentally and theoretically using MC‐TST kinetic theory based on CCSD(T)//M06‐2X quantum chemical methodologies. The results were in excellent agreement and confirmed that the first H‐migration is strongly rate‐limiting in the oxidation of non‐oxygenated volatile organic compounds (VOCs). At higher relative humidity (2–30 %) water complexes were evidenced for HOOQO₂ radicals, which could be an important fate for HOO‐substituted RO₂ radicals in the atmosphere.     

Study on the long life radicals of hexanediol diacrylate in irradiated urea canal complexes in relation to post-polymerization by irradiation

The properties of radicals formed on irradiating long chain monomers in urea canal complexes were studied by ESR and related to the results of post-polymerzation. The initiating radicals (CH₃-?HCOOR) were trapped in canal complexes as a result of γ-irradiation at relatively high temperature such as 25° and the radicals converted to propagating radicals (～CH₂-?HOOR0 on heating to 70°. The temperature dependence of radical concentration was followed and the decay curves corresponded well with the results of post-polymerization.     

Hydrophobic effects on photophysical and photochemical processes: 1. Aggregation and excimer formation of alkyl β-naphthoates with long chain in poor solvents

The fluorescence spectra of alkyl β-naphthoates with various chain lengths (A_n) in DMSO-H₂O and ethylene glycol-water (EG-H₂O) mixtures were studied. The β-naphthoates with short chain show monomer fluorescence only in both solvent mixtures, while fluorescence spectra of long chain alkyl β-naphthoates are dominated by excimer emission. Addition of long chain hydrocarbon or amylose resulted in the reduction of excimer emission and enhancement of monomer fluorescence. All these experimental results supported intermolecular aggregation of long chain alkyl β-naphthoates in poor solvents. The kinetic parameters of the formation and dissociation of excimer as well as fluorescence polarization in aggregates were measured. These data provided an insight into the characteristics of aggregates.     

Determination of the antioxidant capacity of different food natural products with a new developed flow injection spectrofluorimetry detecting hydroxyl radicals

This paper presents an automatic spectrofluorimetric method (flow injection analysis spectrofluorimetry) for detecting hydroxyl radicals. Based on H₂O₂ catalyzed by Co²⁺ yielding HO, the method utilized sodium terephthalate to trap hydroxyl radicals and obtained sodium 2-hydroxyterephthalate by aromatic hydroxylation, which resulted in an increase in the fluorescence intensity. The relative fluorescence intensity was proportional to the concentration of hydroxyl radicals. Hydroxyl radicals could be determined indirectly, based on the increase of fluorescence. It was a simple, rapid, precise, sensitive and automatic technique for the determination of HO. The relative standard deviation of 11 determinations was 0.57%. The method can be used to sieve antioxidant medicines and will have theoretical and practical guidance on the mechanism of hydroxyl radicals-damaging biology.     

π‐Conjugated Carbon Radicals at Graphene Oxide to Initiate Ultrastrong Chemiluminescence

Graphene oxide has widely been employed in various fields, but its structure and composition has still not been fully understood. Here we report that freshly prepared graphene oxide exhibits a large number of π‐conjugated carbon radicals at its π‐network plane, which result from the addition reaction of hydroxyl radicals from H₂O₂ onto the conjugated double bonds of graphene oxide. The π‐conjugated carbon radicals can directly initiate the long‐lasting visible chemiluminescence of luminol, which is even stronger than that obtained when horseradish peroxidase and H₂O₂ are used. Previously, graphene oxide was mainly reported to be a quencher of chemiluminescence instead. Remarkably, the reacted radicals can be regenerated, thereby enabling the repetitive initiation of chemiluminescence by re‐treatment of graphene oxide. The results reported here provide a new understanding of the structure, properties, and applications of graphene oxide.     

Integrated In Situ Characterization of a Molten Salt Catalyst Surface: Evidence of Sodium Peroxide and Hydroxyl Radical Formation

Sodium‐based catalysts (such as Na₂WO₄) were proposed to selectively catalyze OH radical formation from H₂O and O₂ at high temperatures. This reaction may proceed on molten salt state surfaces owing to the lower melting point of the used Na salts compared to the reaction temperature. This study provides direct evidence of the molten salt state of Na₂WO₄, which can form OH radicals, using in situ techniques including X‐ray diffraction (XRD), scanning transmission electron microscopy (STEM), laser induced fluorescence (LIF) spectrometry, and ambient‐pressure X‐ray photoelectron spectroscopy (AP‐XPS). As a result, Na₂O₂ species, which were hypothesized to be responsible for the formation of OH radicals, have been identified on the outer surfaces at temperatures of ≥800 °C, and these species are useful for various gas‐phase hydrocarbon reactions, including the selective transformation of methane to ethane.     

ESR study of free radicals in UHMW-PE fiber irradiated by gamma rays

ESR spectra of the trapped radicals in an ultra-high molecular weight polyethylene (UHMW-PE) fiber irradiated by gamma rays showed well-resolved hyperfine splitting at room temperature since the c-axis of the crystallites is aligned with the fiber direction and the radicals are trapped in crystallites. The alkyl radical (?CH₂?^?CH?CH₂?) was the major product after irradiation in vacuum and in air at room temperature. Some of the alkyl radicals converted to allyl radicals (?CH₂?^?CH?CH=CH?) and polyenyl radicals (?CH₂?^?CH?(CH=CH)_n?CH₂?) during storage in vacuum. Upon storage in air atmosphere, the alkyl radicals decayed by reaction with oxygen. Of particular interest is the very slow decay rate of the alkyl radical trapped in UHMW-PE fiber, the half-life is 26 days in vacuum, and 13 days in air at room temperature, which is about 1/30 and 1/100 of that reported for high density polyethylene (HDPE), respectively. The extremely long lifetime of the alkyl radical is supposed to be caused by the large size of crystallites in UHMW-PE fiber. The rate of radical decay was accelerated by annealing at elevated temperature.     

Picosecond UV photolysis and laser-induced fluorescence probing of gas-phase nitromethane

Ground-state NO₂ radicals are formed in <5 ps with a quantum yield of ≈ 70% in 264 nm photolysis of low pressure nitromethane, as revealed by laser-induced fluorescence probing. Raman shifted photolyzing pulses indicate the NO₂ yield varies little with wavelength, although a small yield of excited state NO^*₂ produced at 264 nm increases significantly at 238 nm.     

Synthesis, fluorescence study and biological evaluation of three Zn(II) complexes with Paeonol Schiff base

The synthesis of three Paeonol Schiff base ligand and their Zn(II) complexes are reported. The complexes were fully characterized by IR, ¹H NMR, elemental analysis and molar conductivity. The experiment results show the three Zn(II) complexes can emit bright fluorescence at room temperature in DMF solution and solid state. The fluorescence quantum yields (Φ) of three Schiff base ligands and their Zn(II) complexes were calculated using quinine sulfate as the reference with a known Φ_R of 0.546 in 1.0N sulfuric acid. Furthermore, in order to develop these Zn(II) complexes’ biological value, the antioxidant activities against hydroxyl radicals (OH) were evaluated. The results show the three complexes possess excellent ability to scavenge hydroxyl radicals.     

Kinetic Study of the CCl2 Radical Recombination Reaction by Laser‐Induced Fluorescence Technique

An experimental setup that coupled IR multiple‐photon dissociation (IRMPD) and laser‐induced fluorescence (LIF) techniques was implemented to study the kinetics of the recombination reaction of dichlorocarbene radicals, CCl₂, in an Ar bath. The CCl₂ radicals were generated by IRMPD of CDCl₃. The time dependence of the CCl₂ radicals’ concentration in the presence of Ar was determined by LIF. The experimental conditions achieved allowed us to associate the decrease in the concentration of radicals to the self‐recombination reaction to form C₂Cl₄. The rate constant for this reaction was determined in both the falloff and the high‐pressure regimes at room temperature. The values obtained were k₀ = (2.23 ± 0.89) × 10^?29 cm⁶ molecules^?2 s^?1 and k_∞ = (6.73 ± 0.23) × 10^?13 cm³ molecules^?1 s^?1, respectively.     

Radical heterogeneous polymerization behavior of N‐methyl‐α‐fluoroacrylamide in benzene

The polymerization of N‐methyl‐α‐fluoroacrylamide (NMFAm) initiated with dimethyl 2,2′‐azobisisobutyrate (MAIB) in benzene was studied kinetically and with electron spin resonance. The polymerization proceeded heterogeneously with the highly efficient formation of long‐lived poly(NMFAm) radicals. The overall activation energy of the polymerization was 111 kJ/mol. The polymerization rate (R_p) at 50 °C is given by R_p = k[MAIB]^0.75±0.05 [NMFAm]^0.44±0.05. The concentration of the long‐lived polymer radical increased linearly with time. The formation rate (R_p?) of the long‐lived polymer radical at 50 °C is expressed by R_p? = k[MAIB]^1.0±0.1 [NMFAm]^0±0.1. The overall activation energy of the long‐lived radical formation was 128 kJ/mol, which agreed with the energy of initiation (129 kJ/mol), which was separately estimated. A comparison of R_p? with the initiation rate led to the conclusion that 1‐methoxycarbonyl‐1‐methylethyl radicals (primary radicals from MAIB), escaping from the solvent cage, were quantitatively converted into the long‐lived poly(NMFAm) radicals. Thus, this polymerization involves completely unimolecular termination due to polymer radical occlusion. ¹H NMR‐determined tacticities of resulting poly(NMFAm) were estimated to be rr = 0.34, mr = 0.48, and mm = 0.18. The copolymerization of NMFAm(M₁) and St(M₂) with MAIB at 50 °C in benzene gave monomer reactivity ratios of r₁ = 0.61 and r₂ = 1.79. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2196–2205, 2001     

On the initial stage of the free-radical polymerizations of styrene and methyl methacrylate in the presence of fullerene C<Subscript>60</Subscript>

It has been demonstrated experimentally and theoretically that the essentially different inhibiting effects of fullerene C₆₀ on the initial stage of the polymerizations of styrene and methyl methacrylate (including complete hampering of styrene polymerization throughout a long induction period) are of common kinetic nature. The difference arises from the competition between C₆₀ and the monomer not for initiating radicals but for radicals originating from the monomer; that is, the difference stems from the competition between the chain propagation reactions and the termination reactions on fullerene molecules. As a consequence, the further development of the process is determined by the relative reactivities of the radicals toward C₆₀ and towards their parent monomers.     

Multiphoton ultraviolet photochemistry

Multiphoton photodissociation has been observed in C₂N₂, C₂H₂, C₂H₄, CH₃OH, and C₂H₅OH. In all of these molecules fluorescence from small free radicals such as CN, CH, and OH has been observed. None of the observed fluorescence can result from the absorption of a single 193 nm photon. Arguments are presented which suggest that the observed results are best explained by invoking a sequential absorption scheme where the excited molecule absorbs a second laser photon rather than predissociating. Emissions have also been observed when H₂O is photolyzed with the ArFlaser. In H₂O the evidence suggest that this emission arises from collisional dissociation of two excited H₂O molecules.     

Spektroskopische Untersuchungen über die Rolle des Käfig-Effektes bei der Prädissoziation aromatischer Nitroverbindungen

Photolysis of the bond R_ar–NO₂ contributes to quenching of the fluorescence of aromatic nitro compounds. Since no nitro compound is known which fluoresces above 20,000 cm^?1 photolysis must occur via a predissociation process. Either a fluorine-substituted nitro compound or fluorobenzene as the solvent was used for the irradiation experiments so that ¹⁹F-NMR. spectroscopy could be used to analyse the reaction products. Cage effects play an important role. With a large distance between the radicals R _ar and NO ₂, the phenyl radical forms a diphenyl compound with a benzenetype solvent molecule, and with small distance recombination will occur. For medium to long distances geminal recombination will also occur, not to the initial nitro compound, but to the corresponding nitrite, which in the presence of oxygen forms o-nitrophenol. Mass spectrometry showed that the added oxygen atom is located in the nitro group.     

A Macrocyclic Fluorophore Dimer with Flexible Linkers: Bright Excimer Emission with a Long Fluorescence Lifetime

Bright fluorescent molecules with long fluorescence lifetimes are important for the development of lifetime‐based fluorescence imaging techniques. Herein, a molecular design is described for simultaneously attaining long fluorescence lifetime (τ) and high brightness (Φ_F×?) in a system that features macrocyclic dimerization of fluorescent π‐conjugated skeletons with flexible linkers. An alkylene‐linked macrocyclic dimer of bis(thienylethynyl)anthracene was found to show excimer emission with a long fluorescence lifetime (τ≈19 ns) in solution, while maintaining high brightness. A comparison with various relevant derivatives revealed that the macrocyclic structure and the length of the alkylene chains play crucial roles in attaining these properties. In vitro time‐gated imaging experiments were conducted as a proof‐of‐principle for the superiority of this macrocyclic fluorophore relative to the commercial fluorescent dye Alexa Fluor 488.     

酪胺修饰石墨烯量子点检测血清中游离胆固醇

以柠檬酸为原料合成具有模拟过氧化氢酶活性的石墨烯量子点(GQDs),经酪胺修饰的GQDs(TYR-GQDs)可以将过氧化氢还原成羟基自由基,通过酚羟基之间的交联聚集,GQDs的荧光被猝灭。由于胆固醇氧化酶可以催化胆固醇氧化生成H₂O₂,因此,基于TYR-GQDs建立了一种检测胆固醇的荧光传感器。在pH=7.4的条件下,TYR-GQDs的荧光猝灭率与胆固醇浓度(2.67×10^-8～2.67×10^-3mol/L)的对数呈良好的线性关系,检出限为9.32×10^-9mol/L。干扰实验表明,该荧光传感器对胆固醇具有高度的选择性,可以用于人体血清中游离胆固醇的检测,加标回收率为96.55%～100.14%。