C-S bond cleavage in the sensitized photooxygenation of tert-alkyl phenyl sulfides. The role of superoxide anion

The N-methylquinolinium tetrafluoroborate (NMQ⁺)-photosensitized oxidation of tert-alkyl phenyl sulfides 1a-c (1a, tert-alkyl=tert-butyl; 1b, tert-alkyl=2-phenyl-2-propyl; 1c, tert-alkyl=1,1-diphenylethyl) and benzyl phenyl sulfide (2) were investigated in CH₃CN by nanosecond laser flash photolysis (LFP) and steady-state irradiation either under nitrogen or in the presence of O₂. By laser irradiation, the formation of sulfide radical cations 1a⁺-c⁺ in the monomeric form (λ_max=520 nm) and of 2⁺ in both the monomeric (λ_max=520 nm) and dimeric form (λ_max=780 nm) were observed within the laser pulse. In both cases, the radical cations decayed by second-order kinetics without any apparent formation of transients attributable to C-S bond rupture. In line with these results, very small amounts of photoproducts were obtained under nitrogen thus suggesting that the sulfide radical cations mainly undergo a back electron transfer process with the reduced N-methylquinolinium (NMQ). A different situation was found in the presence of O₂ since steady-state photolysis produced substantial amounts of C-S bond cleavage products (alcohols, alkenes, and ketones from 1a-c and benzaldehyde from 2), in contrast with LFP experiments. Formation of products was, however, significantly reduced in the presence of benzoquinone, a trap for O₂⁻ generated by NMQ and O₂. For the tert-alkyl phenyl sulfides, 1a-c, these results have been interpreted by suggesting that C-S bond cleavage products in the presence of oxygen mostly derive from the decomposition of a thiadioxirane 6 formed by the reaction of the sulfide radical cation with O₂⁻. In this cleavage a sulfinate and a carbocation formed. The former is oxidized to sulfonate, whereas the carbocation can react with adventitious water to form the alcohol (and the alkene therefrom) and with O₂⁻ to produce the ketone. For 2 (a sulfide with α-CH bonds) probably a different mechanism holds, benzaldehyde coming from the α-phenylthio carbon radical formed from deprotonation by O₂⁻ of 2ⁿ⁺.     

The role of direct oxalate oxidation in electrogenerated chemiluminescence of poly(4-vinylpyridine)-bound Ru(bpy)2Cl/oxalate system on indium tin oxide electrodes

The electrochemical and electrogenerated chemiluminescence (ECL) properties of indium tin oxide (ITO) electrodes modified with poly(4-vinylpyridine) (PVP)-bound Ru(bpy)₂Cl⁺ (where bpy = 2,2′-bipyridine) have been studied. In a sodium oxalate solution, two irreversible oxidation waves as well as two ECL emission waves were observed during the potential scan in the range 0.4-1.4 V (versus Ag/AgCl/saturated KCl reference). The first ECL wave appeared at ca. 0.8 V, which was caused by the excited-state Ru^2+* generated through a bimolecular redox reaction between electrogenerated Ru³⁺ and the strong reducing agent, C⁻O₂. The latter was formed via a Ru³⁺-mediated oxidation of oxalate. Direct oxidation of oxalate was not involved in the first ECL process. The second ECL wave started at ca. 1.1 V, which was also from the excited-state Ru^2+* generated via the redox reaction between Ru³⁺ and C⁻O₂. However, both direct and Ru³⁺-mediated oxidation of oxalate contributed to the formation of C⁻O₂. The important role of the direct oxidation of oxalate in the ECL mechanism of PVP-bound Ru(bpy)₂Cl⁺/oxalate system was demonstrated. The relative contribution of direct oxidation of oxalate to the observed ECL depended upon the surface concentration of PVP-bound Ru²⁺, the concentration of oxalate and the electrode potential applied.     

Artificial photosynthetic reaction centers with carotenoid antennas

Two reaction center-antenna models based on a purpurin macrocycle linked to a C₆₀ and to a carotenoid polyene have been synthesized. In these systems the C₆₀ moiety is the primary electron acceptor, the purpurin is the primary electron donor and the carotenoid moiety acts both as an antenna and secondary electron donor. Formation of the initial charge separated state, C-Pur⁺-C₆₀⁻, following excitation with light absorbed by either the purpurin or C₆₀ takes place on the 10 ps time scale. The final charge separated state, C⁺-Pur-C₆₀⁻, is formed in one of the compounds with a quantum yield of 32% based upon light absorbed by the carotenoid. In order to function as an antenna, the carotenoid pigment must be electronically coupled to the purpurin. The purpurin C ring provides an excellent framework for locating a carotenoid polyene in partial conjugation with the macrocycle, leading to a relatively strong electronic communication between the chromophores; functionalization of a meso position of the purpurin provides a site for the covalent attachment of C₆₀.     

Evaluation of chemiluminescence reagents for selective detection of reactive oxygen species

In order to evaluate the chemiluminescence (CL) reagents for selective detection of reactive oxygen species (ROS), we comprehensively measured the CL responses of 20 CL reagents (three luminol derivatives, two imidazopyrazinone derivatives, eight lophine derivatives, six acridinium ester derivatives and lucigenin) against six types of ROS (superoxide anion: O₂⁻, hydroxyl radical: OH, hydrogen peroxide: H₂O₂, hypochlorite anion: ClO⁻, singlet oxygen: ¹O₂, and nitric oxide: NO). As a result of the screening, it was found that nine CL reagents selectively detected O₂⁻ while one CL reagent selectively detected OH. However, no CL reagent had selectivity on the detection of H₂O₂, ClO⁻, ¹O₂ and NO. Our screening results could help to select the most suitable CL reagent for selective determination of different ROS.As an application study, 4-methoxyphenyl-10-methylacridinium-9-carboxylate (MMAC), one of the acridinium ester derivatives, showed high selectivity on the detection of O₂⁻, and thus was applied to the assay of superoxide dismutase (SOD) activity. The dynamic range and detection limit of the developed CL assay were 0.1-10 and 0.06 U mL⁻¹, respectively. Significant correlation (r = 0.997) was observed between the results by the CL assay using MMAC and the spectrophotometric assay using 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium monosodium salt.     

Chemiluminescence study of carbonate and peroxynitrous acid and its application to the direct determination of nitrite based on solid surface enhancement

Peroxynitrous acid (ONOOH) was produced by the on-line mixing of acidified hydrogen peroxide with nitrite in a flow system. A strong chemiluminescent (CL) emission was observed when ONOOH reacted with carbonate without any special CL reagents. When cotton was present in the CL cell, the CL emission was enhanced significantly. The method was developed to determine nitrite, which showed a key improvement that any CL reagents and sensitizers were not used, resulting in better selectivity. The applicability of the present CL system was demonstrated for the sensitive and selective determination of nitrite in natural water samples without any special pretreatment. Good agreements were obtained for the determination of nitrite in tap and well waters between the present approach and a standard spectrophotometric method. The average precision was 4.6% (n=7) and detection limit (S/N=3) was 1.0×10⁻⁷ M. Based on the CL spectrum, UV spectra, and dissolved oxygen measurement, a possible CL mechanism was proposed. ONOOH was an unstable compound in acidic solution and could be quenched into peroxynitrite (ONOO⁻) in basic solution. ONOO⁻ reacted with CO₂ to produce ONOOCO₂⁻, which can rapidly decompose into NO₂ and CO₃⁻ radicals. In the presence of H⁺, CO₃⁻ radicals can protonate to bicarbonate radical (HCO₃). The recombination of HCO₃ radicals and decomposition can lead to light emission.     

The role of oxygen acidity on the side-chain fragmentation of ring methoxylated benzocycloalkenol radical cations

The reactivity of 2,2-dimethyl-5-methoxyindan-1-ol (1) and 2,2-dimethyl-6-methoxytetral-1-ol (2) radical cations has been studied both in acidic and basic solution. At pH≤4 both 1⁺ and 2⁺ undergo C_αH deprotonation as the exclusive reaction with k=4.6×10⁴ and 3.2×10⁴ s⁻¹, respectively. In basic solution 1⁺ and 2⁺ behave as oxygen acids undergoing ⁻OH-induced αOH deprotonation in a diffusion controlled process (k_−OH≈10¹⁰ M⁻¹ s⁻¹). An intermediate alkoxyl radical is formed which undergoes a 1,2-hydrogen atom shift in competition with CC β-scission (with 1⁺) or as the exclusive pathway (with 2⁺). A behavior which is interpreted in terms of the greater ease of ring-opening of a five membered ring as compared to a six-membered one.     

NHC-Pd(II)-Im (NHC = N-heterocyclic carbene; Im = 1-methylimidazole) complexes as efficient catalysts for Suzuki-Miyaura coupling reactions of aryl chlorides

A new type of well-defined N-heterocyclic carbene (NHC)-palladium chloride-imidazole complexes derived from IPrHCl or IMesHCl, PdCl₂ and 1-methylimidazole exhibits high catalytic activity in the room-temperature Suzuki-Miyaura coupling reactions of aryl or heteroaryl chlorides. Moreover, the large-scale (20.0 mmol) couplings in the presence of 0.01 mol% catalyst loading can also give the corresponding coupling products in high yields.     

Twisted molecular geometry and localized electronic structure of the triplet excited gem-diphenyltrimethylenemethane biradical: substituent effects on thermoluminescence and related theoretical calculations

Substituent effects on the energies of electronic transitions (ETs) between the triplet excited and ground states of gem-diphenyltrimethylenemethane biradicals (³2a) were explored by using thermoluminescence (TL) spectroscopy and density functional theory (DFT) including time-dependent (TD) DFT. Linear free energy (Hammett) analyses of TL energies of a variety of para-substituted aryl derivatives of ³2* gave reasonable correlations with the substituent constant, σ. The slope of Hammett plots of the data are nearly identical to one obtained from a similar analysis of the photoluminescence (PL) energies of the structurally-related 1,1-diarylethyl radicals (3*). The results suggest that TL of ³2* and PL of 3* derive from a common diarylmethyl radical fluorophore. This interpretation is also supported by the DFT and TDDFT calculated electronic structures and ET energies of ³2 and 3. Thermodynamic and kinetic analyses of the charge recombination (CR) process between 2⁺ and 1⁻, which generates ³2*, revealed that substituents not only alter the TL energies but also the TL intensities of ³2*. The observations made in this effort demonstrate that ³2* as well as ³2 and 2⁺ have greatly twisted molecular geometries and highly localized electronic structures.     

Formation of dimer radical cation of 1,1-dianisylethylene through two-step hole transfer processes during TiO2 photocatalytic reactions

The formation of the 1,1-dianisylethylene dimer radical cation ((DAE)₂⁺) during TiO₂ photocatalytic reactions was successfully investigated by the transient absorption measurement with time-resolved diffuse reflectance spectroscopy. This reaction was explained in terms of the cascade hole transfer processes from the photo-generated holes in the TiO₂ particles to free DAE (DAE_free) in the bulk solution mediated by the radical cation of p-phenylbenzoic acid adsorbed on the TiO₂ surface (PBA_ads⁺).     

Structures and EPR spectra of binary sulfur-nitrogen radicals from DFT calculations

The scattered electron paramagnetic resonance (EPR) spectroscopic data for binary sulfur-nitrogen (S,N) radicals have been compiled and critically assessed.Many of these are inorganic rings or cages.For each species, possible equilibrium structures in the gas phase and the EPR hyperfine coupling (hfc) constants have been calculated with DFT using the B3LYP functional and basis sets of triple-ζ (or better) quality.Good agreement is obtained between calculated and measured values for the well characterized [S₃N₂]⁺, a planar π-radical for which the s-component of the orbitals is likely to be reasonably independent of minor geometrical changes between gas-phase and condensed-phase states.The cage compounds [S₄N₄]⁻ and [S₄N₅]⁻², for which reliable experimental EPR spectra have been reported, show larger variation between calculated and measured hfc, as a consequence of the dependence of the s orbital content of the molecular orbitals on small structural changes.The very large disagreements between the DFT calculated and experimentally claimed hfc constants for [NS], [SNS] and [S₄N₄]⁻³ in condensed phases lead us to question their assignment.Among binary S,N radicals, ³³S hfc data has only been reported for [S₃N₂]⁺ (through isotopic enrichment).These values were essential for the correct identification of the EPR spectra of this important radical, which previously was misassigned to other species.Our results suggest that ³³S data will be equally important for the correct identification of the EPR spectra of other binary S,N species, many of which are cyclic systems, e.g.[S₃N₃], [S₄N₃] and[S₄N₅].     

OH-Induced shift from carbon to oxygen acidity in the side-chain deprotonation of 2-, 3- and 4-methoxybenzyl alcohol radical cations in aqueous solution: results from pulse radiolysis and DFT calculations

DFT calculations have been carried out for 2-, 3- and 4-methoxybenzyl alcohol radical cations (1⁺, 3⁺ and 4⁺, respectively) and the α-methyl derivatives 2⁺ and 5⁺ using the UB3LYP/6-31G(d) method. The theoretical results have been compared with the experimental rate constants for deprotonation of 1⁺-5⁺ under acidic and basic conditions. In acidic solution, the decay of 1⁺-5⁺ proceeds by cleavage of the C-H bond, while in the presence of ⁻OH all the radical cations undergo deprotonation from the α-OH group. This pH-dependent change in mechanism has been interpreted qualitatively in terms of simple frontier molecular orbital theory. The ⁻OH induced α-O-H deprotonation is consistent with a charge controlled reaction, whereas the C-H deprotonation, observed when the base is H₂O, appears to be affected by frontier orbital interactions.     

Optimization of hydroxyl radical formation using TiO2 as photocatalyst by response surface methodology

The determination of the optimum parameters for hydroxyl radicals (OH) formation by a TiO₂ solution has been investigated by measuring the emitted fluorescence after the reaction with terephthalic acid has occurred. After UV irradiation, the terephthalic acid was transformed into 2-hydroxyterephthalic acid whose fluorescence is directly proportional to the generated OH. Optimization of hydroxyl radicals’ formation using TiO₂ as catalyst was carried out by studying the effects of irradiation time, TiO₂ concentration and terephthalic acid concentration on the production of the fluorescent HTA with an experimental design. The aim of our research was to apply response surface methodology as a chemometric method for the optimization of the reaction conditions. The combination of irradiation time, TiO₂ concentration and terephthalic acid concentration was varied at designed points of a central composite rotatable design. The three factors were found to have a significant effect upon the reaction. The optimum conditions for the reaction achievement were estimated to be 10 min for the irradiation time, 25 μg mL⁻¹ TiO₂ concentration and 0.1 mmol L⁻¹ terephthalic acid concentration. Afterwards, using these parameters the method was applied for the determination of the ability of several plant extract samples to scavenge the formed OH.