Mechanism of UV photodegradation of fluoroquinolone antibiotic ciprofloxacin in aqueous solutions.

Mechanism of direct UV photolysis of the zwitterionic and anionic forms of the quinolone antibiotic ciprofloxacin (CIP) was revealed by combination of nanosecond laser flash photolysis, steady-state photolysis coupled with high resolution LC-MS and DFT quantum-chemical calculations. For both forms, the main intermediate is a dissociative triplet state, which loses a fluorine ion to form a triplet carbocation; subsequent solvent attack of the latter leads to the formation of products of hydroxylation both the aromatic ring and the piperazyl substituent. Correspondingly, the quantum yield of photolysis of both CIP forms does not depend on the excitation wavelength, but depends on the concentration of dissolved oxygen. Secondary photolysis leads to a number of products of oxidation of the aromatic system, as well as oxidation, opening and full destruction of the piperazinyl substituent. The results obtained may be important for understanding the fate of quinolone antibiotics in UVC disinfection processes and in natural waters under the action of sunlight.

Laser flash photolysis and quantum chemical studies of UV degradation of pharmaceutical drug chloramphenicol: Short-lived intermediates, quantum yields and mechanism of photolysis.

Using methods of time-resolved and stationary photolysis, HPLC-MS and quantum-chemical calculations by the DFT method, the mechanism of direct UV photolysis of the antibiotic chloramphenicol (CAP) was established. For the first time, short-lived intermediates formed during photolysis were detected. The primary photoprocess is the cleavage of the ß-C-C bond relative to the aromatic system with the formation of 4-nitrobenzylalcohol radical and residual aliphatic radical. The first radical in deoxygenated solutions predominantly transforms into para-nitrobenzaldehyde and its secondary photolysis products. In the presence of oxygen, the aromatic radical and para-nitrobenzaldehyde are transformed into para-nitrosobenzoic and para-nitrobenzoic acids as a result of reaction with reactive oxygen species (ROS). Formation of ROS is provoked by reactions of aliphatic radical with dissolved oxygen, so this radical is very important for CAP degradation. The quantum yield of direct photolysis of CAP is ∼3% and does not depend on the presence of dissolved oxygen and on the change of the excitation wavelength in the range of 254-308 nm. Obtained data are important for further understanding of the transformation pathways of CAP and similar PPCP in natural and wastewaters under the action of sunlight and artificial UV radiation.

Photophysics of 1,8-naphthalimide/Ln(III) dyads (Ln = Eu, Gd): naphthalimide → Eu(III) energy-transfer from both singlet and triplet states.

Transient absorption and time resolved luminescence spectroscopy were used to study photophysical processes in the macrocycle-appended 1,8-naphthalimide compound H3L, and its Eu(III) and Gd(III) complexes Eu·L and Gd·L, in particular the naphthalimide-Eu(III) energy-transfer process. In all cases aggregation of the naphthalimide chromophores results in a low-energy emission feature in the 470-500 nm region in addition to the naphthalimide fluorescence; this lower-energy emission has a lifetime longer by an order of magnitude than the monomer naphthalimide fluorescence. Transient absorption spectroscopy was used to measure the decay of the naphthalimide triplet excited state, which occurs in the range 30-50 µs. In Eu·L, partial energy-transfer from the naphthalimide chromophore results in sensitized Eu(III)-based emission in addition to the naphthalimide-based fluorescence features. Time-resolved measurements on the sensitized Eu(III)-based emission reveal both fast (~10(9) s(-1)) and slow (~10(4) s(-1)) energy-transfer processes from the naphthalimide energy-donor, which we ascribe to energy-transfer occurring from the singlet and triplet excited state of naphthalimide respectively. This is an unusual case of observation of sensitization of Eu(III)-based emission from the singlet state of an aromatic chromophore.

A new view on the mechanism of UV photodegradation of the tricyclic antidepressant carbamazepine in aqueous solutions.

Mechanism of direct UV photolysis of the tricyclic antidepressant carbamazepine (CBZ) at neutral pH was revealed by a combination of nanosecond laser flash photolysis, steady-state photolysis combined with high resolution LC-MS and DFT quantum-chemical calculations. The detection of short-lived intermediates and the detailed identification of final products were performed for the first time. The quantum yield of CBZ photodegradation (282 nm) is about 0.1% and 0.18% in air-equilibrated and argon-saturated solutions. The primary stage is photoionization with the formation of CBZ cation radical followed by a rapid nucleophilic attack by a solvent molecule. The primary photoproducts are 10-oxo-9-hydro-carbamazepine, 9-formylacridine-10(9H)-carboxamide (a result of ring contraction) and various isomers of hydroxylated CBZ. Prolonged irradiation results to accumulation of acridine derivatives, which should lead to an increase of the toxicity of photolyzed CBZ solutions. The obtained results may be important for understanding the fate of tricyclic antidepressants in processes of UVC disinfection and in natural waters under action of sunlight.

Direct UV photodegradation of herbicide triclopyr in aqueous solutions: A mechanistic study.

Mechanism of direct UV photolysis of pyridine herbicide triclopyr (TRI) was revealed by the combination of nanosecond laser flash photolysis, steady-state photolysis coupled with high resolution LC-MS and DFT quantum-chemical calculations. Both the detection of short-lived intermediates and the detailed identification of final products were done for the first time. The quantum yield of TRI photodegradation is about 4% at both UVC (254 nm) and UVB (308 nm) excitation. The primary stage is the heterolytic cleavage of C-Cl bond in dissociative triplet state of TRI with the formation of phenyl cation followed by a fast nucleophilic attack by a solvent molecule. The minor channel is the photohydrolysis leading to the formation of 3,5,6-trichloropyridin-2-ol. Primary photoproducts undergo secondary photolysis by the mechanism similar to initial TRI with the formation of products of acetic group elimination, sequential substitution of chlorine atoms to hydroxyl groups and, finally, oxidation and opening of the pyridine ring. Obtained results can be important for understanding the fate of pyridine herbicides in the processes of UVC disinfection and in natural waters under action of the sunlight.

Photolysis of Fe(III) complex with ethylenediamine-N,N'-disuccinic acid and its efficiency in generation of •OH radical.

The mechanism of photolysis of the Fe(III) complex with ethylenediamine-N,N'-disuccinic acid ([FeEDDS]-) was revealed using a combination of time resolved and stationary photochemical methods. Using laser flash photolysis (λex = 355 nm), the formation of the primary intermediate, the radical complex of Fe(II) with quantum yield (φ0 = 0.21) was detected for the first time. The lifetime (1.8 ms) and the spectral characteristics (λmax = 520 nm, ε520 nm = 160 M-1cm-1) of this intermediate were also determined. The dependence of the quantum yield of photolysis of the [FeEDDS]- complex (φFeEDDS) and the hydroxyl radical quantum yield (φOH) on the excitation wavelength, pH, and concentrations of the ligand and iron ions were obtained for the first time. It has been established that under optimal conditions at neutral pH, the value of φFeEDDS is about 0.8, and φOH is about 0.15. It was found that φFeEDDS does not depend on the initial concentrations of Fe(III), EDDS, but depends on pH, the excitation wavelength and the presence of oxygen. φOH does not depend on the initial concentrations of Fe(III), EDDS, but depends on pH and the excitation wavelength. The high φOH values make the [FeEDDS]- complex a suitable system for the generation of •OH radical at neutral pH under UV radiation.

Photochemistry of dithiocarbamate Cu(II) complex in CCl4.

Laser pulse photolysis was used to study the nature and reactions of intermediates in the photochemistry of the flat dithiocarbamate complex Cu(Et(2)dtc)(2) in CCl(4). A nanosecond laser pulse (355 nm) is shown to induce intermediate absorption bands of bivalent copper complex whose coordination sphere contains a dithiocarbamate radical Et(2)dtc(•) and a chloride ion at the axial position ([(Et(2)dtc)Cu(Et(2)dtc(•))Cl(a)]). At room temperature during some microseconds after the laser pulse, this intermediate interacts with the initial complex to form presumably a dimer [Cu(2)(Et(2)dtc)(3)(Et(2)dtc(•))Cl]. The latter vanishes in the second-order reaction. Analysis of kinetic and spectral features gives the arguments for the formation of a cluster [Cu(2)(Et(2)dtc)(3)Cl-tds-Cu(2)(Et(2)dtc)(3)Cl], which produces a new absorption band at 345 nm. The cluster decomposes in ∼5 ms into final products, a binuclear complex [Cu(2)(Et(2)dtc)(3)Cl] and tetraethylthiuramdisulfide (Et(4)tds).

Iron (III) hydroxocomplex-methyl viologen dication system as a prospective tool for determination of hydroxyl radical reaction rate constants with environmental pollutants.

Reactivity of oxidative species with target pollutants is one of the crucial parameters for application of any system based on advanced oxidation processes (AOPs). This work presents new useful approach how to determine the hydroxyl radical reaction rate constants (kOH) using UVA laser flash photolysis technique. Fe (III) hydroxocomplex at pH 3 was applied as a standard source of hydroxyl radicals and methyl viologen dication (MV2+) was used as selective probe for •OH radical. Application of MV2+ allows to determine kOH values even for compounds which do not generate themselves optically detectable transient species in reaction with hydroxyl radicals. Validity of this approach was tested on a wide range of different persistent pesticides and its main advantages and drawbacks in comparison with existing steady-state and time-resolved techniques were discussed.

Photodegradation of para-arsanilic acid mediated by photolysis of iron(III) oxalate complexes.

Organic arsenicals are important environment pollutants due to wide use in livestock and toxicity of degradation products. In this work we report about the efficient photodegradation of the p-arsanilic acid (p-ASA) and its decomposition products in the Fe(III)-oxalate assisted approach under nature-relevant conditions. At neutral pH under near-visible UV irradiation the Fe(III) oxalate complexes generate the primary oxidizing intermediate, OH radical (the quantum yield of ϕOH âˆ¼ 0.06), which rapidly reacts with p-ASA with high rate constant, (8.6 ± 0.5) × 109 M-1s-1. Subsequent radical reactions result in the complete photooxidation of both p-ASA and basic aromatic photoproducts with the predominant formation of inorganic arsenic species, mainly As(V), under optimal conditions. Comparing with the direct UV photolysis, the presented Fe(III)-oxalate mediated degradation of p-ASA has several advantages: higher efficiency at low p-ASA concentration and complete degradation of organic arsenic by-products without use of short-wavelength UV radiation. The obtained results illustrate that the Fe(III)-oxalate complexes are promising natural photosensitizers for the removal of arsenic pollutants from contaminated waters.

Mechanistic investigation of humic substances assisted photodegradation of imipramine under simulated sunlight.

Imipramine (IMI) is a frequently prescribed tricyclic antidepressant and widely detected in the natural waters, while the environmental fate of IMI is yet poorly understood. Here, we investigated the photodegradation of IMI under simulated sunlight in the presence of humic substances (HS), typically including humic acid (HA) and fulvic acid (FA). The direct and indirect IMI photodegradation was found to increase both with increasing pH and with deoxygenation of the reaction solutions. The excited triplet state of HS (3HS⁎) was mainly responsible for the photosensitized degradation of IMI according to the steady-state quenching and direct time-resolved experiments. The electron transfer interaction between 3HS⁎ and IMI was observed by laser flash photolysis (LFP) with bimolecular reaction rate constants of (4.9 ± 0.4) × 109 M-1 s-1. Evidence of electron transfer from IMI to 3HS⁎ was further demonstrated by the photoproduct analysis. The indirect photodegradation was triggered off in the side chain of IMI with the nonbonding nitrogen electron transferring to 3HS⁎, followed by hydroxylation, demethylation and cleavage of the side chain. Very important that HS photosystem does not lose its efficiency with decreasing of IMI concentration, meaning that the studied photosystem still be used at environmentally relevant concentrations of IMI. These results suggest that photodegradation could be an important attenuation pathway for IMI in HS-rich and anaerobic natural waters.

Excited state dynamics of a PtII diimine complex bearing a naphthalene-diimide electron acceptor.

A combination of picosecond time-resolved infrared spectroscopy, picosecond transient absorption spectroscopy, and nanosecond flash photolysis was used to elucidate the nature and dynamics of a manifold of the lowest excited states in Pt(phen-NDI)Cl 2 ( 1), where NDI = strongly electron accepting 1,4,5,8-naphthalene-diimide group. 1 is the first example of a Pt (II)-diimine-diimide dyad. UV/vis/IR spectroelectrochemistry and EPR studies of electrochemically generated anions confirmed that the lowest unoccupied molecular orbital (LUMO) in this system is localized on the NDI acceptor group. The lowest allowed electronic transition in Pt(phen-NDI)Cl 2 is charge-transfer-to-diimine of a largely Pt-->phen metal-to-ligand charge-transfer (MLCT) character. Excitation of 1 in the 355-395 nm range initiates a series of processes which involve excited states with the lifetimes of 0.9 ps ( (1)NDI*), 3 ps ( (3)MLCT), 19 ps (vibrational cooling of "hot" (3)NDI and of "hot" NDI ground state), and 520 mus ( (3)NDI). Excitation of 1 with 395 nm femtosecond laser pulses populates independently the (1)MLCT and the (1)NDI* excited states. A thermodynamically possible decay of the initially populated (1)MLCT to the charge-transfer-to-NDI excited state, [Pt (III)(phen-NDI (-*))Cl 2], is not observed. This finding could be explained by an ultrafast ISC of the (1)MLCT to the (3)MLCT state which lies about 0.4 eV lower in energy than [Pt (III)(phen-NDI (-*))Cl 2]. The predominant decay pathway of the (3)MLCT is a back electron transfer process with approximately 3 ps lifetime, which also causes partial population of the vibrationally hot ground state of the NDI fragment. The decay of the (1)NDI* state in 1 populates vibrationally hot ground state of the NDI, as well as vibrationally hot (3)NDI. The latter relaxes to form (3)NDI state, that is, [Pt(phen- (3)NDI)Cl 2]*, which possesses a remarkably long lifetime for a Pt (II) complex in fluid solution of 520 mus. The IR signature of this excited state includes the nu(CO) bands at 1607 and 1647 cm (-1), which are shifted considerably to lower energies if compared to their ground-state counterparts. The assignment of the vibrational bands is supported by the density-functional theory calculations in CH 2Cl 2. Pt(phen-NDI)Cl 2 acts as a modest photosensitizer of singlet oxygen.

Mechanistic study of fulvic acid assisted propranolol photodegradation in aqueous solution.

Laser flash (355 nm) and stationary (365 nm) photolysis were used to study the mechanisms of propranolol photolysis in the presence of fulvic acid in aqueous solutions. The FA-assisted photodegradation of propranolol was observed using UV-A irradiation (where propranolol is stable). Direct evidence indicated that the photodegradation resulted from the static quenching of the FA triplet state by propranolol via the electron transfer mechanism. The triplet state yield (ϕT≈0.6%) and the T-T absorption coefficient (ɛT(620 nm)≈5×10(4) M(-1) cm(-1)) were estimated for the first time by modeling the yields of the FA triplet state in the presence of propranolol. Thus, fulvic acid is a promising agent for accelerating propranolol photodegradation in aqueous solutions under UV-A light irradiation.

Fast processes and intermediates in photochemistry of 7-dimethyl-germanorbornadiene.

Laser pulse photolysis experiments have shown that the triplet excited-state of 1,2,3,4-tetraphenylnaphthalene (TPN) is one of the primary intermediates of the photochemical transformation of 7,7'-dimethylgerma-1,4,5,6-tetraphenyl-2,3-benzo-norbornadiene (GNB) in hexane solution. The molar absorption of T-T absorption and the quantum yield of the intersystem crossing of TPN were determined from the triplet-triplet energy transfer. The scheme of GNB photocleavage has been suggested where the triplet excited TPN originated from the triplet state of biradical through cleavage of the second C-Ge bond, the latter being generated from the excited singlet state of the initial GNB after the cleavage of the first C-Ge bond and the intersystem crossing. Other channels of GNB's chemical transformation have been discussed.