Microwave-assisted rapid photocatalytic degradation of malachite green in TiO2 suspensions: mechanism and pathways

Microwave-assisted photocatalytic (MPC) degradation of malachite green (MG) in aqueous TiO2 suspensions was investigated. A 20 mg/L sample of MG was rapidly and completely decomposed in 3 min with the corresponding TOC removal efficiency of about 85%. To gain insight into the degradation mechanism, both GC-MS and LC-ESI-MS/MS techniques were employed to identify the major intermediates of MG degradation, including N-demethylation intermediates [(p-dimethylaminophenyl)(p-methylaminophenyl)phenylmethylium (DM-PM), (p-methylaminophenyl)(p-methylaminophenyl)phenylmethylium (MM-PM), (p-methylaminophenyl)(p-aminophenyl)phenylmethylium (M-PM)]; a decomposition compound of the conjugated structure (4-dimethylaminobenzophenone (DLBP)); products resulting from the adduct reaction of hydroxyl radical; products of benzene removal; and other open-ring intermediates such as phenol, terephthalic acid, adipic acid, benzoic acid, etc. The possible degradation mechanism of MG included five processes: the N-demethylation process, adduct products of the hydroxyl radical, the breakdown of chromophores such as destruction of the conjugated structure intermediate, removal of benzene, and an open-ring reaction. To the best of our knowledge, it is the first time the whole MG photodegradation processes have been reported.     

A theoretical study on the quenching mechanisms of triplet state riboflavin by tryptophan and tyrosine

The reactions of tryptophan (Trp) and tyrosine (Tyr) with endogenous photosensitizer riboflavin (RF) have gained much interest for their crucial roles in various photobiological processes. In this paper, the quenching mechanisms of triplet state RF by Trp and Tyr have been explored employing density functional theory calculations. It is revealed that the H-atom transfer reaction from Trp and Tyr to triplet state RF is more favorable on thermodynamic grounds compared with direct energy transfer or direct electron transfer pathways. During the photosensitization, RF can photogenerate various reactive oxygen species (ROS) as intermediates, while the present study provides some deeper insights into the photosensitizing behaviors of triplet state RF by reacting directly with Trp and Tyr.     

Photolysis of hexamethyldisilane at 206 nm

The photolysis of Me₆Si₂ at 206 nm results in two main decomposition processes: simple Si---Si bond breaking with a quantum yield of Φ = 0.21 ± 0.03, and Me₃SiH elimination with the concomitant formation of Me₂SiCH₂ with Φ = 0.18 ± 0.01. There is also a minor decomposition channel with a very small quantum yield, Φ = (5.6 ± 0.2) × 10⁻³, which results in the formation of Me₄Si and Me₂Si. The main fate of the excited Me₆Si₂ molecule produced during photolysis is stabilization by collisional deactivation. The end products observed indicate that the reaction pathways followed by the main intermediates, Me₃Si and Me₂SiCH₂, are the same as those found in the photolysis of Me₄Si (Ahmed et al., J. Photochem. Photobiol. A: Chem. 86 (1995) 33).     

Protective effect of Boldo and tea infusions on the visible light-mediated pro-oxidant effects of vitamin B2, riboflavin

The effect of Boldo and black tea infusions on the pro-oxidant effects of vitamin B2, riboflavin (RF), when exposed to the action of visible light was studied. The amounts of antioxidants present in Boldo and tea infusions were evaluated by a procedure based on the bleaching of preformed 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) radical cations and were expressed as 6-hydroxy-2,5,7,8-tetramethyl-chroman-2-carboxylic acid equivalent concentrations. The quenching rate constants of singlet oxygen (1O2; [kq]Boldo = 6.0 x 10(7) M(-1) s(-1) and [kq]Tea = 3.2 x 10(7) M(-1) s(-1)) and triplet RF (3RF; [3RFkq]Boldo = 10 x 10(8) M(-1) s(-1) and [3RFkq]TEA = 3.2 x 10(8) M(-1) s(-1)) with Boldo and tea were determined by flash photolysis. These data allow a quantitative interpretation of the results obtained. Our data suggest that most of the oxygen consumption observed in the photolysis of RF in the presence of tea and Boldo infusions is caused by 1O2 reactions. The oxygen consumption quantum yield is considerably smaller than the fraction of RF triplets trapped by the additives (AH) present in the infusion, indicating that their interaction with 3RF does not lead to chemical reactions or that the AH*+ radical ions initially formed participate in secondary processes that do not consume oxygen. Boldo and tea infusions have a significant protective effect when a system containing RF and tryptophan (Trp) is exposed to visible light, not only by quenching the 1O2 and interfering with the Type-I mechanism but also by repairing the damage to Trp molecules associated with the latter mechanism.     

Photoionization of Ferrocytochrome c by 248 nm Laser Light and the Observation of the Early Stages of Ferricytochrome c Unfolding in the Nanosecond-to-millisecond Timescale

Photolysis of ferrocytochrome c by 248 nm laser light in aqueous solution at pH 7 generates hydrated electrons (eaq-) by a monophotonic process with quantum yield phi = 0.034. Approximately three-quarters of the eaq- originate from the heme, which is converted from the ferrous to the ferric state in < 100 ns. The conformational changes associated with the change in the redox state of cytochrome c are either not detectable spectrophotometrically or complete in < 100 ns. Also, under conditions where ferrocytochrome c is stable but ferricytochrome c is unfolded (3 M guanidine, pH 7, 40 degrees C), photoionization of ferrocytochrome c generated ferricytochrome c with similar quantum yield. Under these conditions, the lifetime of native ferricytochrome c is 67 microseconds; it decays via two intermediates with lambda max > 410 nm, neither of which is the thermodynamically favored, unfolded form. These species are putatively identified as unfolding intermediates with nonnative iron ligands, similar to those found during folding of ferrocytochrome c. The results suggest that unfolding, like folding, proceeds by intrachain diffusion and ligand exchange.     

The rhodafluor family. An initial study of potential ratiometric fluorescent sensors for Zn2+

A new class of ratiometric Zn(2+) sensors that employ a hybrid fluorescein and rhodamine fluorophore has been designed, and two members of the rhodafluor family of sensors, RF1 and RF2, have been synthesized. The preparation of RF1 (9-(o-carboxyphenyl)-2-chloro-6-[bis(2-pyridylmethyl)amino]-3-xanthanone, Rhodafluor-1), uses conventional synthetic methods. Elaboration of the RF1 synthesis in an effort to enhance the Zn(2+) affinity was unsuccessful, so palladium-catalyzed aryl amination was applied to prepare RF2 (1-[9'-(o-carboxyphenyl)-6'-amino-2'-chloro-3'-xanthanone]-4,10-(diethyl)-7-(2-pyridylmethyl)-1,4,7,10-tetraazacyclododecane, Rhodafluor-2). The key step in the synthesis of RF2 is coupling of a triprotected tetraazamacrocycle (cyclen) to 3-bromoanisidine. RF2 binds Zn(2+) with a dissociation constant of 13.5 microM accompanied by an approximately 50% increase in quantum yield. Although only small shifts in absorption wavelength were observed, because protonation of the amino nitrogen atoms of the macrocycle prevents the uncomplexed sensor from adopting the desired mesomer, the intensity doubling makes the probe of value for immediate application in situations where our previous tight binding (<1 nM) sensors are inadequate.     

Selective oxygenation of 4,4'-dimethylbiphenyl with molecular oxygen, catalyzed by 9-phenyl-10-methylacridinium ion via photoinduced electron transfer

Photooxygenation of 4,4'-dimethybiphenyl with oxygen occurs efficiently in the presence of 9-phenyl-10-methylacridinium perchlorate (AcrPh(+)ClO(4)(-)) under visible light irradiation in O(2)-saturated chloroform (CHCl(3)) to yield 4-(4'-methylphenyl)benzaldehyde as a main oxygenated product. Prolonged photoirradiation afforded the further oxygenated product, 4,4'-diformylbiphenyl. The reactive radical intermediates involved in the photocatalytic cycle have successfully been detected by laser flash photolysis and electron spin resonance (ESR) measurements. The photocatalytic mechanism for the oxygenation of 4,4'-dimethybiphenyl via photoinduced electron transfer from 4,4'-dimethybiphenyl to the singlet excited state of AcrPh(+) is clarified based on the dependence of quantum yields on concentrations of substrates and the detected radical intermediates.     

SINGLET OXYGEN YIELDS AND RADICAL CONTRIBUTIONS IN THE DYE-SENSITISED PHOTO-OXIDATION IN METHANOL OF ESTERS OF POLYUNSATURATED FATTY ACIDS (OLEIC, LINOLEIC, LINOLENIC AND ARACHIDONIC)

Abstract— The triplet state characteristics (spectrum, lifetime and quantum yield) for four dye sensi tisers [methylene blue (MB), erythrosin (ER), haematoporphyrin (HP) and riboflavin (RF)] were determined in methanol by laser flash photolysis and singlet oxygen yields (0.60 to 0.48) from time-resolved measurements of the 1270 nm near infrared emission. The reaction of singlet oxygen with four long chain unsaturated phenyl esters [oleate (18: 1), linoleate (18: 2), linolenate (18: 3) and arachidonate (20: 4)] was followed quantitatively using the singlet oxygen luminescence technique and also, after continuous420–700 nm irradiation, by HPLC and other analysis of the isomeric product monohydroperoxides. The overall quantum yield of photooxidation (∼10^-2) was shown to be consistent with the observed singlet oxygen quenching constants(2–12 times 10⁴ dm³ mol^-1 s^-1) for the four esters studied and the singlet oxygen lifetime in methanol (τ∼ 9 μs). The isomer product distribution was interpreted in terms of a dual singlet oxygen and radical mechanism, the radical contribution increasing with sensitiser in the order ER = MB < HP ≪ RF, but also showing some dependence on substrate unsaturation. Evidence is presented for singlet oxygen quenching by MB and RF ( k_O = 1.6 and 6.0 times 10⁷ dm³ mol^-1 s^-1) and for the accelerated photobleaching of the dye sensitisers in the presence of the unsaturated esters.     

A photoacoustic calorimetric characterization of the reaction enthalpy and volume for the preparation of a reactive intermediate from CpMn(CO)3

Photoacoustic calorimetry (PAC) allows measurement of the energetics of reactive intermediates. Here, we report the examination of the metal carbonyl η⁵-CpMn(CO)₃ (Cp, cyclopentadiene) via time-independent PAC, in a homologous series of solvents. The measured heat releases allow one to determine simultaneously the enthalpy and volume change resulting from the photodissociation of CpMn(CO)₃. While the photoacoustic signal results from both of these processes, it has often been assumed that the volume change contribution to the observed photoacoustic signal is negligible for small molecules undergoing photodissociation. The current study tests the assumption of a negligible reaction volume by using a more complete treatment. The reaction of an equimolar number of photons and CpMn(CO)₃ molecules, the subsequent photodissociation of the Mn–CO bond, and the ligation of a solvent molecule in an alkane solvent yields ΔH_obs = 32.7 ± 0.7 kcal/mol and ΔV_chem = 11.0 ± 1.3 mL/mol, both of which are independent of the quantum yield of photodissociation. A detailed analysis of the quantum yield is included (using both previously reported measurements, and new data from this work), from which we determine Φ_diss = 0.635. This quantum yield allows us to determine ΔH_rxn = 51.6 kcal/mol and ΔV_rxn = 17.3 mL/mol. These results demonstrate that if the contribution of the reaction volume change to the photoacoustic signal is ignored, the reaction enthalpy derived would underestimate the true value by 7%. We also estimate the BDE{Cp(CO)₂Mn–CO} to be 59.4 kcal/mol.     

Zum Mechanismus der Photochemie des Benzfurazans

Mechanistic studies on the photochemical reactions of benzfurazan . From other works it is known that irradiation of benzfurazan ( 1 ) in methanol gives the carbaminacid-ester 4 , whereas in benzene the azepinederivative 3 is obtained (Scheme 1). The compounds 5–8 (Scheme 2) have been proposed as intermediates. In our investigations we detected and characterized by means of UV.- and IR.-spectroscopy the two species 5 and 8 . Irradiation of 1 with 350 nm light at room temperature in a strongly polar solvent (e.g. H₂O) yields exclusively 5 (Fig. 1) with a quantum yield of 0.48. In non polar solvents (e.g. hexane) 5 isomerizes in a second photochemical step to 8 (quantum yield 0.43) (Fig. 3). Thermally, 5 can be converted back to 1 . The rate constant for this reaction at room temperature is 2 · 10^–5s^–1. The transformation 5 → 8 was also investigated at low temperature. There was no direct evidence for any intermediates of the type oxazirene ( 6 ) or nitrene ( 7 ). However, the formation of azepine 3 upon irradiation of 5 in benzene suggests as intermediate the nitrene 7 which could be converted into 8 in a fast thermal reaction (Scheme 3).     

Computational characterization of reaction intermediates in the photocycle of the sensory domain of the AppA blue light photoreceptor

The AppA protein with the BLUF (blue light using flavin adenine dinucleotide) domain is a blue light photoreceptor that cycle between dark-adapted and light-induced functional states. We characterized possible reaction intermediates in the photocycle of AppA BLUF. Molecular dynamics (MD), quantum chemical and quantum mechanical-molecular mechanical (QM/MM) calculations were carried out to describe several stable structures of a molecular system modeling the protein. The coordinates of heavy atoms from the crystal structure (PDB code 2IYG) of the protein in the dark state served as starting point for 10 ns MD simulations. Representative MD frames were used in QM(B3LYP/cc-pVDZ)/MM(AMBER) calculations to locate minimum energy configurations of the model system. Vertical electronic excitation energies were estimated for the molecular clusters comprising the quantum subsystems of the QM/MM optimized structures using the SOS-CIS(D) quantum chemistry method. Computational results support the occurrence of photoreaction intermediates that are characterized by spectral absorption bands between those of the dark and light states. They agree with crystal structures of reaction intermediates (PDB code 2IYI) observed in the AppA BLUF domain. Transformations of the Gln63 side chain stimulated by photo-excitation and performed with the assistance of the chromophore and the Met106 side chain are responsible for these intermediates.     

PHOTODISSOCIATION OF CARBON TETRACHLORIDE SENSITIZED BY NAPHTHALENE DERIVATIVES-I. RADICAL-CHAIN REACTION IN THE SYSTEM NAPHTHALENE-METHANOL-CARBON TETRACHLORIDE

Abstract— Irradiation of naphthalene in methanol in the presence of carbon tetrachloride with light of 313 nm wavelength forms hydrochloric acid, chloroform, and formaldehyde as the main products. The quantum yields of these products are larger than 1 (up to 50 , depending on the concentrations of reactants and sensitizer and on temperature). The quantum yield for the disappearance of the sensitizer naphthalene is smaller than 0.3. From these results it is concluded that the products are built up by a chain reaction with CC1₃ and CH₂OH radicals as intermediates.     

Anthraquinon-2-ylmethoxycarbonyl (Aqmoc): a new photochemically removable protecting group for alcohols

[see structure]. Synthesis and photochemistry of a new photochemically removable protecting group for alcohols is described. Four carbonates of galactose derivatives (1-4) were synthesized from the corresponding arylmethanols via 4-nitrophenyl carbonate intermediates. Among them, photolysis of anthraquinon-2-ylmethoxycarbonyl (Aqmoc) galactose (1) proceeded with overall photolysis efficiency of 150 (quantum yield 0.10, and molar absorptivity 1500 M(-1) x cm(-1)) and rate constant of approximately 10(6) s(-1). To demonstrate its application to a biologically related molecule, 5'-Aqmoc-adenosine (5) was synthesized and photolyzed to yield adenosine in 91% yield.     

Methoxy-substituted stilbenes, styrenes, and 1-arylpropenes: photophysical properties and photoadditions of alcohols

The photochemistry of trans-stilbene and four methoxy-substituted stilbene derivatives has been investigated in a variety of solvents. The fluorescence of all five trans isomers was quenched by 2,2,2-trifluoroethanol (TFE). Upon irradiation of the five substrates in TFE, the products derived from photoaddition of the solvent were detected. Nuclear magnetic resonance spectroscopy of the products formed by irradiation in TFE-OD indicated that the proton and nucleophile are attached to two adjacent atoms of the original alkene double bond. Irradiation of the corresponding methoxy-substituted styrenes and trans-1-arylpropenes in TFE produced the analogous solvent adducts. The photoaddition of TFE proceeded with the general order of reactivity: styrenes > trans-1-arylpropenes > trans-stilbenes. Transient carbocation intermediates were observed following laser flash photolysis of the stilbenes in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). The results are consistent with a mechanism that involves photoprotonation of the substrates by TFE or HFIP, followed by nucleophilic trapping of short-lived carbocation intermediates. Compared to the other stilbene derivatives, trans-3,5-dimethoxystilbene displayed a large quantum yield of fluorescence and a low quantum yield of trans-cis isomerization in polar organic solvents. The unique photophysical properties of trans-3,5-dimethoxystilbene are attributed to formation of a highly polarized charge-transfer excited state (mu(e) = 13.2 D).     

Spectral kinetic characteristics of the photoisomerization products of naphthylmethylideneiminospironaphthopyran induced by photolysis at different wavelengths

The spectral kinetic characteristics of intermediates generated by photolysis with light at the wavelengths 337 and 430 or 470 nm of the photobifunctional compound (PBC), 1,3-dihydro-5-(2-hydroxy-1-naphthylmethylideneimino)-1,3,3-trimethylspiro[2H-indole-2,3-[3H]-naphtho[2,1-b]pyran], whose molecule combines the spironaphthopyran and hydroxyazomethine fragments, and also parameters of model compounds, viz., naphthylmethylideneimine and spironaphthopyran, were studied in methanol and toluene. The relative quantum yields of formation of different intermediates of PBC were measured relatively to model compounds, namely, trans-keto isomer formed due to cis-trans-isomerization and prototropic equiliration in the azomethine fragment, and in the merocyanine form generated by spiro bond opening. It was found that the photolysis of the PBC with light at the wavelengths ?? = 430 or 470 nm nearly no produces the merocyanine form, whereas the relative yield of the trans-keto tautomer is ??0.6. For PBC photolysis at ?? = 337 nm, the yield of the merocyanine form is ??0.2 and the yield of the trans-keto isomer decreases substantially (??0.2). The solvent nature affects the kinetic behavior of the system. The consistency of the isomerization and proton transfer processes is discussed.     

Photophysical and photochemical processes of 9,10-dihydro-9-silaphenanthrene derivatives: photochemical formation and electronic structure of 9-silaphenanthrenes

Photophysical and photochemical processes of 9-methyl- and 9-phenyl-9,10-dihydro-9-silaphenanthrene derivatives have been studied at room temperature and 77 K in comparison with the carbon analogue, 9,10-dihydrophenanthrene. These 9,10-dihydro-9-silaphenanthrene derivatives show smaller fluorescence quantum yield and remarkably larger Stokes shifts than those of the carbon analogue. In contrast, their phosphorescence quantum yields are two times larger than those of the carbon analogue, although the absolute value is not so large (approximately 0.1). Reaction products and intermediates produced by the 266 nm light photolysis have been studied, and it has been confirmed that 9-methyl- and 9-phenyl-9-silaphenanthrenes have been photochemically formed in methylcyclohexane at 77 K, in addition to the formation of radical cations of 9,10-dihydro-9-silaphenanthrene derivatives and the carbon-centered radical: 9-hydro-9-silaphenanthrenyl radical.     

Listening to Lanthanide Complexes: Determination of the Intrinsic Luminescence Quantum Yield by Nonradiative Relaxation

The photophysical properties of lanthanide complexes have been studied extensively; however, fundamental parameters such as the intrinsic quantum yield as well as radiative and nonradiative decay rates are difficult or even impossible to measure experimentally. Herein, a photoacoustic (PA) method is proposed to determine the intrinsic quantum yield of lanthanide complexes with lifetimes in the order of milliseconds. This method is used to determine the intrinsic quantum yields for europium(III)‐containing metallomesogens as well as terbium(III) complexes. The results show that the PA signal is sensitive to both the lifetime and the ratio of the fast‐to‐slow heat component of the samples. It is found that there is an efficient ligand sensitization and a moderate intrinsic quantum yield for the complexes. The intrinsic quantum yield of Eu³⁺ in the metallomesogens exhibits an obvious increase upon the isotropic liquid to smectic A transition. The proposed PA method is quite simple, and can contribute to a clearer understanding of the photophysical processes in luminescent lanthanide complexes.     

Scandium ion-enhanced oxidative dimerization and N-demethylation of N,N-dimethylanilines by a non-heme iron(IV)-oxo complex

Oxidative dimerization of N,N-dimethylaniline (DMA) occurs with a nonheme iron(IV)-oxo complex, [Fe(IV)(O)(N4Py)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), to yield the corresponding dimer, tetramethylbenzidine (TMB), in acetonitrile. The rate of the oxidative dimerization of DMA by [Fe(IV)(O)(N4Py)](2+) is markedly enhanced by the presence of scandium triflate, Sc(OTf)(3) (OTf = CF(3)SO(3)(-)), when TMB is further oxidized to the radical cation (TMB(?+)). In contrast, we have observed the oxidative N-demethylation with para-substituted DMA substrates, since the position of the C-C bond formation to yield the dimer is blocked. The rate of the oxidative N-demethylation of para-substituted DMA by [Fe(IV)(O)(N4Py)](2+) is also markedly enhanced by the presence of Sc(OTf)(3). In the case of para-substituted DMA derivatives with electron-donating substituents, radical cations of DMA derivatives are initially formed by Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+), giving demethylated products. Binding of Sc(3+) to [Fe(IV)(O)(N4Py)](2+) enhances the Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+), whereas binding of Sc(3+) to DMA derivatives retards the electron-transfer reaction. The complicated kinetics of the Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+) are analyzed by competition between binding of Sc(3+) to DMA derivatives and to [Fe(IV)(O)(N4Py)](2+). The binding constants of Sc(3+) to DMA derivatives increase with the increase of the electron-donating ability of the para-substituent. The rate constants of Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+), which are estimated from the binding constants of Sc(3+) to DMA derivatives, agree well with those predicted from the driving force dependence of the rate constants of Sc(3+) ion-coupled electron transfer from one-electron reductants to [Fe(IV)(O)(N4Py)](2+). Thus, oxidative dimerization of DMA and N-demethylation of para-substituted DMA derivatives proceed via Sc(3+) ion-coupled electron transfer from DMA derivatives to [Fe(IV)(O)(N4Py)](2+).     

The Femtochemistry of a Ferracyclobutadiene

The eminent role of metallacyclobutadienes as catalytic intermediates in organic synthesis and polymer chemistry is widely acknowledged. In contrast, their photochemistry is as yet entirely unexplored. Herein, the photo‐induced primary processes of a ferracyclobutadiene tricarbonyl complex in solution are revealed by femtosecond mid‐infrared spectroscopy. The time‐resolved vibrational spectra expose an ultrafast substitution of a basal CO ligand by a solvent molecule in a consecutive dissociation–association mechanism. Following optical excitation, the system relaxes non‐radiatively to the triplet ground state from which a CO is expelled. Since the triplet state is bound with respect to Fe−CO cleavage, the dissociation can only occur from vibrationally excited states. The excitation energy, vibrational relaxation, and intersystem crossing to the singlet ground state control the primary quantum yield for formation of the ferracyclic dicarbonyl–solvent product complex.