Journal of Photochemistry and Photobiology A: Chemistry

Published by Elsevier
Online ISSN: 1010-6030
Publications
Article
4-Chloro-1,2-phenylenediamine (4-Cl-o-PDA) is an aromatic diamine used as a precursor for manufacture of hair dyes and dyes of other purposes. 4-Cl-o-PDA has been found to be photomutagenic in bacteria when concurrently exposed to simulated sunlight irradiation. Irradiation of 4-Cl-o-PDA by either outdoor sunlight or indoor lamp, one main photoproduct appeared and it was found to be 2,3-diamino-7-chlorophenazine, a dimerized product through the excited state reaction of 4-Cl-o-PDA in the presence of oxygen. The isolated yield of 13% for 2,3-diamino-7-chlorophenazine is far better than the oxidation reaction of 4-Cl-o-PDA by H2O2 and may therefore be used as a synthetic method. The half live of transformation for 4-Cl-o-PDA in water (100μM) is 39min when exposed to outdoor sunlight. The photomutagenicity of 4-Cl-o-PDA and its photoproduct were tested in Salmonella typhimurium TA102. Under the same conditions, both compounds are photomutagenic. In addition, 2,3-diamino-7-chlorophenazine is both phototoxic and mutagenic.
 
Article
Two-photon-enhanced dendritic nanoprobes are being developed for two-photon (2P) laser scanning microscopy of oxygen [1]. In these molecular constructs, phosphorescence of metalloporphyrins is coupled to two-photon absorption (2PA) of electronically separate antenna dyes via intramolecular Förster-type resonance energy transfer (FRET). In the originally developed probes, competing electron transfer (ET) between the antennae and the long-lived triplet states of metalloporphyrins partially quenched the phosphorescence, reducing the probe's sensitivity and dynamic range. The rate of such ET can be reduced by tuning the redox potentials of the chromophores. In order to identify the optimal metalloporphyrin-2P antenna pairs, we performed screening of several phosphorescent Pt porphyrins (FRET acceptors) and 2P dyes (FRET donors) using dynamic quenching of phosphorescence. Phosphorescence lifetimes of Pt porphyrins were measured as a function of the dye concentration in organic solutions. The obtained Stern-Volmer quenching constants were correlated with the corresponding ET driving forces (DeltaG(ET)), calculated using the Rehm-Weller equation. FRET-pairs with minimal quenching rates were identified. The developed approach allows convenient screening of candidate-compounds for covalent assembly of 2P-enhanced triplet nanodevices. Systematic electrochemical measurements in a series of Pt porphyrins with varying peripheral substitution and conjugation pathways are presented.
 
Article
For a series of rhodamine dyes, two-photon absorption (TPA) and two-photon fluorescence (TPF) have been performed in different solvents. Solvent-dependent TPA spectra of these dyes were measured with open aperture z-scan method and compared to their respective single-photon spectra at equivalent energies. In the TPA spectra, relative peak intensities and positions are highly solvent dependent, which could be a result of vibronic couplings that depend on solvent environment. Measured TPA cross-sections of rhodamine dyes are consistently higher in nonpolar solvents. Certain complementary and similarity between TPA and TPF are also elucidated. Finally, a two-photon figure-of-merit is presented for these dyes in different solvents as a function of wavelength.
 
Article
The photodegradation of adsorbed perylene is strongly influenced by the environment in which it is found as demonstrated by the nature of the photoproducts characterized on the adsorbed state in comparison to its photochemical behavior in solution. The separation, characterization, and identification of the products were carried out using HPLC equipped with UV-Vis diode array and MS detection. Two of the products were identified as 1,12-perylenedione and 3,10-perylenedione. Three additional products were characterized as a perylenedione and two perylenediols based on their m/z ratio. Based on this information, two possible mechanisms of formation were proposed for the identified diones. The experimental data showed that both the radical cation and singlet oxygen participate in the route of photodegradation of perylene which occurs through a mixed Type I and Type II pathway. These results assist in the understanding of complex processes undergone by perylene and other PAHs in the environment.
 
Article
Lanthanide ions luminescence has long life time enabling highly sensitive detection in time-gated mode. The synthesis of reactive lanthanide probes for covalent labeling of the objects of interest is cumbersome task due to the large size of the probes, complex multi-step procedures and the presence of sensitive groups, which often prevents introduction of reactive cross-linking functions optimal for conjugation. We suggest simple synthetic protocol for luminescent europium chelates based on serendipitous reaction yielding acylating compounds, whose reactivity is comparable to that of commonly used N-hydroxysuccinimide (NHS) esters. The probes react with proteins at pH 7.0 within several minutes at ambient temperature displaying high coupling efficiency. The resulting conjugates survive electrophoretic separation under denaturing conditions, which makes the labels useful in proteomic studies that rely on high detection sensitivity.
 
Absorption (left) and emission (right) spectrum of APH in acetonitrile. Insert: APH structure.
Article
Phenothiazines derivatives are versatile compounds that are used in many fields, depending on the type and position of the substitution on the parent molecule. The photochemical, photophysical and electrochemical properties of several phenothiazine derivatives have been previously reported in detail. However, no reports have been presented for 2-aminophenothiazine (APH), a candidate that provides for the further chemical modification and the introduction of specific substituents. In this work, the photophysical and electrochemical properties of APH were measured in acetonitrile. The APH ground state absorption and fluorescence spectrum (phi(f) < 0.01) are similar to the corresponding that of PH parent molecule. A mono exponential decay fluorescence lifetime of 0.65 ns was determined for APH in acetonitrile. Characterization of the 355 nm nanosecond laser flash photolysis transient species reveals the presence of the triplet-triplet transient intermediate with a high intersystem crossing quantum yield (phi(T) = 0.72 +/- 0.07), indicating that the APH main excited state deactivation channel is intersystem crossing. The oxidation potential of APH is lower than phenothiazine parent molecule ((0.38 V vs 0.69 V vs Ag/AgCl(sat)). Altogether, these results show that APH has photochemical and photophysical properties similar to the phenothiazine parent molecule, but with the possibility of providing an amino functionality at 2-position for further chemical modification.
 
Article
Photolysis of 9-methyl-10-nitroanthracene in chloroform or methanol produces mainly two products 9-methyl-9-nitrosoanthracen-10-one and 9,10-anthraquinone in about 4:1 ratio under ambient air. The formation of 9-methyl-9-nitrosoanthracen-10-one confirms the proposed excited state rearrangement reaction of the nitro group peri to two hydrogens and perpendicular to the aromatic rings. The nitro group rearranges to a nitrite, followed by breaking of the N–O bond producing NO radical. The NO radical further forms a bond with the carbon on the opposite site of the benzene ring through radical recombination. Photolysis of 12-methyl-7-nitrobenz[a]anthracene produced several nitroso ketone-like compounds which further convert to an aldehyde. Photolysis of the desmethyl nitro compounds, 9-nitroanthracene and 7-nitrobenz[a]anthracene, produced the respective quinones.
 
Article
The photochromic properties and thermal stability of a newly synthesized fluorinated N-ethoxycarbonylmethyl indolylfulgimide substituted on the imide nitrogen were examined in a protic and aprotic environment. The absorption spectra and extinction coefficients of the Z- and C-forms of the fluorinated indolylfulgimide (open and closed, respectively) were measured in a binary 70/30 ethanol/water system and in toluene. The results demonstrated a favorable bathochromic shift of the absorption maxima for both the open and closed forms of the fulgimide when the solvent was changed from aprotic toluene to protic aqueous ethanol. In addition, the photochemical stability of the new fulgimide was found to be high (0.056% and 0.020% degradation each time the fulgimide is cycled between the open and closed form in 70/30 ethanol/water and in toluene, respectively). The thermal stability of both forms of the fulgimide in 70/30 ethanol/water at 50 degrees C, toluene at 80 degrees C, and polymer film (PMMA) at 80 degrees C was measured using UV-Vis and/or (1)H NMR spectroscopy. Both forms of indolylfulgimide display high hydrolytic stability in 70/30 ethanol/water at 50 degrees C, with the Z- and C-forms degrading 1.3%/day and 1.2%/day respectively based on (1)H NMR data. At 80 degrees C in toluene the less stable Z-form lost about 20%/day.
 
Article
Photodegradation of a mixture of three polycyclic aromatic hydrocarbons fluorene (FLU), dibenzofuran (DBF), and dibenzothiophene (DBT) using UV and UV/H2O2 processes was studied. Treating a mixture of the PAHs stimulated a more realistic contamination composition present in polluted water. Effects of pH, PAH concentration, and water matrix composition on removal rates and efficiencies of these compounds are discussed. Batch experiments were conducted using both monochromatic low pressure (LP, 253.7 nm) and polychromatic medium pressure (MP, 200-400 nm) UV sources, in a quasi-collimated beam setup. A synergistic effect was observed during direct photolysis and LP-UV/H2O2 of the mixture as compared to photodegradation as a single component in an aqueous solution. Similar results were obtained for FLU using MP-UV/H2O2 whereas, degradation of DBF and DBT was inhibited in a mixture. Natural water enhanced the direct photolysis compared to laboratory buffered water, whereas, degradation of the PAHs in the natural water was inhibited using UV/H2O2 process. Toxicity testing using a luminescent inhibition bioassay was correlated to intermediates generated during UV-based oxidation reactions.
 
Article
The photophysical properties of four imidazolium-substituted metalloporphyrins have been assessed to gain insights into the relative efficacy of the compounds for photodynamic therapy (PDT). A set of zinc(II), palladium(II), and chloro-indium(III) porphyrins all bear a net positive charge owing to the diethylimidazolium unit; one zinc chelate bears a negative charge owing to a bis(sulfobutyl)imidazolium unit. The photophysical properties of the cationic and anionic zinc porphyrins are very similar to one another in organic solvents, phosphate-buffered saline, and in the presence of bovine serum albumin. The properties of the zinc and palladium porphyrins bearing charged peripheral groups are generally similar to those of neutral analogs in organic solvents. The palladium porphyrin shows an essentially quantitative yield (≥0.99) of the triplet excited state compared to the zinc porphyrins (∼0.9), and all are quantitatively quenched (at the diffusion limit) by molecular oxygen in air-saturated fluid solution. If the rate constant and yield of quenching of the triplet excited state by energy or electron transfer to molecular oxygen is the same in the cellular environment as in solution, then these processes combined with the triplet yield contribute only a factor of 1.3 to the higher PDT activity of analogous palladium versus zinc porphyrins, which is much smaller than what is observed. Therefore, other factors such as transient reduction of the excited porphyrin or delivery to the target site must predominantly underlie the difference in PDT efficacy of these sensitizers.
 
Article
The syntheses and photophysical properties of 1-(5-methylhexyl)-2,3,7,8-tetrahydro-1H-naphtho[2,1-e]indol-9(6H)-one (7a) and 1-(5-methylhexyl)-2,3,8,9-tetrahydro-1H-naphtho[2,1-e]indol-6(7H)-one (7b) are reported. They are prepared in eight steps from the corresponding bromonaphthylamines. These fluorescent compounds have PRODAN-like cores, and they are structurally similar to cholesterol. Compound 7a is the first reported PRODAN derivative where both the amino and carbonyl groups are constrained to be coplanar with the naphthalene core. Comparing the photophysical behavior of these compounds with related compounds indicates that locking the amino group in a five-membered ring enhances their desirable properties as solvent polarity sensors.
 
Article
Structurally diverse near-infrared (NIR) absorbing polymethine dyes were prepared and their fluorescence lifetimes (FLT) were evaluated in relation to their structural features. Comparative FLT analysis based on the modification of methine chain length and heterocyclic system showed that indolium or benz[e]indolium heptamethine dyes exhibited longer FLT than the benz[c,d]indolium trimethine dye. Modification of heterocyclic system alone with an intact chain length showed that indolium-based heptamethine dyes showed approximately 30% longer FLT than the benz[e]indolium-based dyes. In general, the FLT of polymethine dyes increased from polar to non-polar solvents. In addition, correlation study between the theoretical and the experimental FLT for indocyanine green (ICG) suggests that the lack of structural rigidity for these cyanine dyes is primarily responsible for the loss of the excited state energy via non-radiative pathway.
 
Article
The tetrazine chromophore holds promise as an effective photochemical trigger to achieve structural release, directed at the determination of peptide/protein motions that occur early in the folding processes. The photochemistry of 3,6-di-cysteine-S,S-tetrazines was examined by femtosecond IR transient absorption spectroscopy. Excitation of the tetrazine chromophore by visible and near UV light in the end yields chemically inert, structurally unobtrusive photoproducts that are not expected to interfere with the conformational dynamics of peptides and proteins. Dicysteine S,S-tetrazine is suggested to undergo photocleavage via a photochemical pathway different than the parent molecule s-tetrazine, based on kinetic measurements that reveal a stepwise reaction pathway of photofragmentation, whereby the initial ring cleavage event occurs prior to the formation of the SCN groups.
 
Article
Phenyl substituted arsenic compounds are widely used as feed additives in the poultry industry and have become a serious environmental concern. We have demonstrated that phenylarsonic acid (PA) is readily degraded by TiO(2) photocatalysis. Application of the Langmuir-Hinshelwood kinetic model for the initial stages of the TiO(2) photocatalysis of PA yields an apparent rate constant (k(r)) of 2.8 µmol/L·min and the pseudo-equilibrium constant (K) for PA is 34 L/mmol. The pH of the solution influences the adsorption and photocatalytic degradation of PA due to the surface charge of TiO(2) photocatalyst and speciation of PA. Phenol, catechol and hydroquinone are observed as the predominant products during the degradation. The roles of reactive oxygen species, •OH, (1)O(2), O(2) (-•) and h(VB) (+) were probed by adding appropriate scavengers to the reaction medium and the results suggest that •OH plays a major role in the degradation of PA. By-products studies indicate the surface of the catalyst plays a key role in the formation of the primary products and the subsequent oxidation pathways leading to the mineralization to inorganic arsenic. TiO(2) photocatalysis results in the rapid destruction of PA and may be attractive for the remediation of a variety of organoarsenic compounds.
 
Article
Relative rates of the photosensitized production of singlet oxygen ((1)O(2)) and of superoxide (O(2) (•-)) were determined using different couples of dyes and sacrificial electron donors (SEDs) of either high or low hydrophobicities. Such rates were also measured in the absence and presence of single unilamellar vesicles (SUVs) with 9DMPC:1DMPA mol ratio composition. The dyes aluminum phthalocyanine tetrasulfonate (AlPcS(4)) and pheophorbide-a (PHEO) were used as hydrophilic and hydrophobic photosensitizers, respectively. Xanthine (X) and glutathione (GSH) were used as hydrophobic and hydrophilic SEDs, respectively. The presence of SUVs in the aqueous sample produces the physical separation or encounter of SEDs and photosensitizers according to their membrane binding constants. When both the SED and the photosensitizer are localized within the same phase, a strong decrease in the rate of (1)O(2) formation, united to a strong increase in the rate of O(2) (•-) formation, is observed, relative to when both of these species are localized in different phases. The lipid phase is always present in the biological milieu. Thus, the use of a hydrophobic couple of both dye and SED (as in the case of X and PHEO), as well as a hydrophilic couple of both dye and SED (as in the case of GSH and AlPcS4), should strongly favor the Type I mechanism over the Type II. Since only a small number of hydroxyl radicals are needed to initiate a chain reaction of phospholipid peroxidation, the latter could be more toxic to the tumor tissue than peroxidation by a much higher concentration of singlet oxygen molecules.
 
Article
Luminescent lanthanide ion complexes are distinguished by unique light emitting properties that enable both highly sensitive detection of lanthanide labels attached to biomolecules and contrast imaging of various micro objects (cells, nanoparticles, etc.). Previously, we synthesized amine-reactive cs124-based luminescent lanthanide chelates with improved brightness and metal retention. Here we report the synthesis of new thiol-reactive derivatives of the same compounds including bromoacetamido-, and maleimido- forms of cs124 and cs124CF3 fluorophores. Maleimido-compounds displayed exceptional reactivity instantaneously coupling to thiols at physiological conditions at micromolar probes concentrations. Surprisingly, they displayed strong quenching by adjacent maleimido-group, which was completely eliminated after reaction with thiols, thereby enabling their simple detection by monitoring the light emission of the reaction mixture. This reaction can be used for hyper-sensitive determination of biologically important sulphydryl compounds (e.g. glutathione, co-enzyme A, etc.) in time-resolved mode.
 
Article
Thiopurines were examined for their ability to produce singlet oxygen ((1)O(2)) with UVA light. The target compounds were three thiopurine prodrugs, azathioprine (Aza), 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG), and their S-methylated derivatives of 6-methylmercaptopurine (me6-MP) and 6-methylthioguanine (me6-TG). Our results showed that these thiopurines were efficient (1)O(2) sensitizers under UVA irradiation but rapidly lost their photoactivities for (1)O(2) production over time by a self-sensitized photooxidation of sulfur atoms in the presence of oxygen and UVA light. The initial quantum yields of (1)O(2) production were determined to be in the range of 0.30-0.6 in aqueous solutions. Substitution of a hydrogen atom with a nitroimidazole or methyl group at S decreased the efficacy of photosensitized (1)O(2) production as found for Aza, me6-MP and me6-TG. (1)O(2)-induced formation of 8-oxo-7,8-dihydro-2'-dexyguanosine (8-oxodGuo) was assessed by incubation of 6-methylthiopurine/UVA-treated calf thymus DNA with human repair enzyme 8-oxodGuo DNA glycosylase (hOGG1), followed by apurinic (AP) site determination. Because more 8-oxodGuo was formed in Tris D(2)O than in Tris H(2)O, (1)O(2) is implicated as a key species in the reaction. These findings provided quantitative information on the photosensitization efficacy of thiopurines and to some extent revealed the correlations between photoactivity and phototoxicity.
 
Article
On photolysis of 4-methyl-2,4,6-triphenyl-4H-thiopyran-1,1-dioxide (1d) in chloroform solution with a low-pressure mercury lamp at under an argon atmosphere, a high regioselective rearrangement takes place via a gem-methylphenyl migration to yield two bicyclic stereoisomers anti-6-methyl-1,3,6-triphenyl-2-thiabicyclo[3.1.0]hex-3-ene-2,2-dioxide (anti-2d) and syn-6-methyl-1,3,6-triphenyl-2-thiabicyclo[3.1.0]hex-3-ene-2,2-dioxide (syn-2d). The results observed are discussed in the light of a thia-di-π-methane rearrangement through a vinyl–vinyl bonding interaction.
 
Article
The far-UV (193 nm) laser flash photolysis of nitrogen-saturated isooctane solutions of 1,1-dimethylsiletane allows the direct detection of 1,1-dimethylsilene as a transient species, which (at low laser intensities) decays with pseudo-first-order kinetics (τ ∼ 10 μs) and exhibits a UV absorption spectrum with λmax ∼ 255 nm. Characteristic rapid quenching is observed for the silene with methanol (kMcOH = (4.9 ± 0.2) × 109 M−1 s−1), tert-butanol (kBuOH = (1.8 ± 0.1) × 109 M−1 s−1) and oxygen (kO2 = (2.0 ± 0.5) × 108 M−1 s−1). The Arrhenius activation parameters for the reaction with methanol have been determined to be Ea = −2.6 ± 0.6 kcal mol−1 and log A = 7.7 ± 0.3.
 
Article
The photophysical properties of [1,1′-binaphthalene]-2,2′-diol (BINOL) have been examined with particular emphasis to the effects of chirality and chiral recognition. Few studies have specifically examined the photophysical properties of BINOL. This study was prompted by preliminary observations in our laboratory that revealed anomalously high fluorescence anisotropy values for BINOL. The analyte (BINOL) was examined in various solvents and in aqueous solutions of various pH values in order to perturb the effects of proton transfer, which was observed as a red-shift in the fluorescence emission. The anomalously high anisotropy value could not, however, be attributed to proton transfer phenomena. The anisotropy was found to be concentration dependent and was attributed to the formation of dimers or other higher order aggregates. The chiral recognition of BINOL was also examined by fluorescence anisotropy measurements and compared to that of other binaphthyl analogs.
 
Article
The photochemical behaviour of 4,4-dimethyl-, 4-tert-butyl-4-methyl- and 4-benzyl-4-methyl-2,6-diphenyl-4H-thiopyran-1,1-dioxides has been investigated and compared with those of 4-methyl-2,4,6-triphenyl-4H-thiopyran-1,1-dioxide and 2,4,4,6-tetraphenyl-4H-thiopyran-1,1-dioxide as model compounds under identical experimental conditions followed by 1H NMR spectroscopy. The high yields of the stereoisomeric bicyclic photoproducts of dialkyl analogues in the absence of SO2 extruded byproducts are discussed on the basis of a vinyl–vinyl di-π-methane (DPM) rearrangement.
 
Article
The chemical behaviour of 32 selected 1,1-dicyano-1-alkenes (DCNA) that are devoid of additional unsaturation and of additional hetero-atoms, upon direct excitation by continuous irradiation with light of 253.7 nm wavelength into the long-wavelength flank of their longest wavelength UV absorption band has been studied in solvents ranging from cyclohexane to methanol. The predominant reaction products in the majority of cases were 1,1-dicyano-cyclopropanes formed via 1,2-migration of either hydrogen or methyl/alkyl from C-3 to C-2 (olefin to cyclopropane photorearrangement, OCPR). Photoreactions competing with OCPR were hydrogen atom abstraction from solvent by the C-2 of the DCNA and, in characteristically favourable cases only, 3,4-CC bond cleavage. In cases of low OCPR quantum yields, hydrogen abstraction from solvent was dominant in cyclohexane or methanol but it could be suppressed by the choice of a solvent (methylene chloride, acetonitrile, tert-butanol) that more strongly resisted hydrogen abstraction. Further minor by-products were isomeric DCNA and 1,1-dicyano-3-alkenes. No carbene-derived products were observed. Supplementary experiments included quenching experiments and an investigation of the DCNA triplet state. The DCNA triplet state was formed at only ca. 1% on direct irradiation but it could be efficiently produced by sensitisation with benzophenone; in the absence of olefins as inter- or intramolecular substrates, it was fairly unreactive. All observed reactions occur from the lowest excited DCNA singlet state. According to the quenching experiments, this state is short-lived as compared to diffusional movements. Other than OCPR which appears to be due to cationic reactivity at C-2 exhibited by the perpendicular geometry of the excited double bond, hydrogen abstraction and 3,4-CC bond cleavage appear to be due to radical reactivity at C-2 exhibited by geometries of the excited double bond that are intermediate between planar and perpendicular and are due to vibration about the perpendicular conformation.
 
Article
Photolysis of 1,1-diphenyl-2-haloethene in methanol solution in the presence of copper(II) acetate, copper(II) triflate or cadmium(II) acetate results in the formation of ionic and radical products. The ratio of ionic to ionic and radical products depends on the halogen atom and the salt used. The effect of a solid support (silica gel, silica gel impregnated with copper(II) or cadmium(II) salts) was studied and it was found that the product distribution and the ratio of ionic to ionic and radical products are different from the results obtained during irradiation in solution.
 
Article
The photophysical properties of 1,1′-binaphthyl-2,2′-diyl hydrogenphosphate (BNP) have been examined with particular emphasis to the effects of the chiral recognition of BNP by α-, β-, and γ-cyclodextrins (CDs). Fluorescence spectra and anisotropy values of the BNP enantiomers were measured in the presence and absence of the CDs under various conditions and the chiral recognition behavior are discussed based on the acquired fluorescence data. Small spectral shifts in the fluorescence emission allowed evaluation of the binding constant and enantioselectivity of the binding in β-, and γ-cyclodextrin, but significant shifts were not observed in the case of α-cyclodextrin. Fluorescence anisotropy proved to be an effective technique to study the enantioselective interactions, even in the case of α-cyclodextrin where a smaller binding constant and enantiodiscrimination were confirmed. The differential thermodynamics of binding were examined by fluorescence anisotropy and are discussed with reference to the general structural characteristic of the cyclodextrins examined. Interestingly, BNP was found to exhibit opposite thermodynamic trends for β-cyclodextrin at low (1 mM) and high (11 mM) cyclodextrin concentrations, indicating the possibility of complex of even different modes of enantiodiscrimination in this system.
 
Article
Di-derivatives of 1,1′-methylene-bis(2-naphthol) (L1) possessing ester (L2), carboxylic acid (L3) or picolyl (L4) as end groups of the pendants have been synthesized and characterized. Titration of these derivatives for their recognition towards M2+ (where M = Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn) were performed by following the fluorescence emission intensities in methanol solution. The picolyl derivative (L4) has been found to be well suited for Cu2+ recognition (≤630 ppb) via the formation of 1:1 complex that was further proven based on absorption as well as ESI MS studies. On the other hand, L1 and L2 were mainly insensitive towards M2+, where as the carboxylic derivative (L3) shows fluorescence changes with almost all the metal ions and hence none of the L1, L2 and L3 are suited for any M2+ recognition. The studies clearly suggested that the chemical nature of the functional groups and the coordination preferences of the metal ion seem to play important role in the selective recognition of the metal ion.
 
Article
Coumarins are well known for reversible dimer formation with wavelengths greater than 300 nm and dimer cleavage below 300 nm. In a photochemical [2+2]-cycloaddition a cyclobutane ring is formed. Formation as well as cleavage of the cyclobutane ring may be accomplished by a single-photon-absorption as well as by a two-photon-absorption triggered reaction. The coumarin system is of interest for various kinds of applications, ranging from drug delivery for ophthalmic implants to optical data storage. However, the two-photon-absorption coefficient of coumarin dimers is rather low falling in the range of 1 GM in the visible range. We present here a substitute for the coumarin dimer system which not only has an about one order of magnitude higher two-photon-absorption coefficient, but also overcomes several other problems of the coumarin dimer system. Coumarines and in particular coumarine dimers have a very limited solubility in common solvents and are susceptible to hydrolysis of the lactone ring, which leads to an undesired complexity in the photochemical cleavage reaction. The 1,1-dimethylnaphtalenone dimers introduced here show excellent stability, lead only to a single cleavage product, and have a two-photon-absorption coefficient of about 10 GM at 532 nm. These properties make the 1,1-dimethylnaphtalenone dimers a superior substitute over the well-known coumarin dimers in particular in applications where two-photon-absorption induced photocleavage is required.
 
Article
Time-resolved electron paramagnetic resonance and luminescence spectra have been observed for the lowest excited states of 1,10-phenanthroline (phen), 2-methyl-1,10-phenanthroline (2-Mephen), 2,9-dimethyl-1,10-phenanthroline (2,9-Me2phen), 4,7-Me2phen, 5,6-Me2phen and their Zn2+ complexes in rigid solutions at 77 K. The sublevel preferentially populated by intersystem crossing (ISC) is the out-of-plane sublevel in Zn-2-Mephen-Cl2 and Zn-2,9-Me2phen-Cl2. The observed anisotropy in the ISC of these Zn2+ complexes is anomalous for the ligand-localized 3ππ* state. The luminescence spectra and fluorescence lifetimes indicate that the increment of the ISC rate for the out-of-plane sublevel is responsible for the anomalous anisotropy in the ISC of these Zn2+ complexes.
 
Article
[M(phen)(XX)] complexes (where M is PdII or PtII), phen is 1,10-phenanthroline and XX is a dianion of catechol (CAT), 4tert-butylcatechol (BCAT), 3,4-dimercaptotoluene (DMT) or thiosalicylic acid (TSA) photosensitize the photo-oxidation of 2,2,6,6,-tetramethyl-4-piperidinol in a molecular-oxygen-saturated solution of dimethylformamide (DMF) when irradiated with light of wavelent 300 – 800 nm. This photo-oxidation reaction involves singlet molecular oxygen (1O2) as an intermediate. The presence of 1O2 is demonstrated by quenching with bis(di-n-butyldithiocarbomato)-nickel(II) or 10%waterDMF. The ralative efficiency of the various complexes is as follows: [Pd(phen)(BCAT)] > [Pt(phen)(BCAT)] > [Pd-(phen)(CAT)] > [Pt(phen)(CAT] > [Pd(phen)(TSA)] > [Pd(phen)(DMT)]. This order depends on the net ligand field of the mixed ligands attached to palladium(II) and platinum(II) in these complexes.
 
Article
Mixed-ligand complexes of nickel(II) with 2,2′-bipyridine (bipy) and 3,4-dimercaptotoluene (DMT) and with 1,10-phenanthroline (phen) and 3,4-dimercaptotoluene were prepared and characterized. These complexes give sharp 1H nuclear magnetic resonance (NMR) spectra in dimethyl-d6-sulphoxide which suggests that they are diamagnetic and square planar. They show five absorption bands in the region 250 – 600 nm. One of these bands shows a bathochromic shift of about 50 nm when the solvent is changed from CH3OH to CHCl3. They also act as physical quenchers of the photo-oxidation of 2,2,6,6-tetramethyl-4-piperidinol sensitized by haemotoporphyrin IX with quenching rate constants of 6.8 × 109 and 1.1 × 1010 M−1 s−1 for [Ni(bipy)(DMT)] and [Ni(phen)(DMT)] respectively. These values approach the quenching rate constant (1.1 × 1010 M−1 s−1 of a diamagnetic square planar complex of bis(di-n-butyldithiocarbamato)nickel-(II) which is a singlet molecular oxygen quencher.
 
Article
A series of mononuclear and binuclear ruthenium(II) heteroleptic complexes as potential sensitizers for DNA photocleavage has been synthesized and characterized. This series based on 1,10-phenanthroline (phen) ligands is of the type [Rux(phen)2x(L)]2x+ (x=1 or 2). L is dmbpy (dmbpy=4,4′-dimethyl-2,2′-bipyridine) or a bridging ligand of bis-bpy type linked by a polymethylene or polyamine chain (see Scheme 1). The spectroscopic and photophysical properties determined in Tris buffer and ethanol as well as the redox potentials are not affected by the complex nuclearity. Therefore, the two metallic sites of the binuclear complexes are not electronically coupled neither in the ground state nor in the excited state. The results also show that the luminescence properties of the complexes in Tris buffer solution are sensitive to the presence of amine functions in the linker chain. In particular, the excited state is better stabilized in the case of the binuclear complex including three amine groups. Improvement of binding with the possibility of a double-interaction of these systems with DNA is discussed.
 
Article
[Pt(NN)X2] complexes (where NN is 1,10-phenanthroline or 2,2′-bipyridine, and X is N3 CN, Cl, SCN, Br or I) have been found to be photosensitizers in the photo-oxidation of 2,2,6,6-tetramethyl-4-piperidinol to the nitroxide radical. This photo-oxidation reaction involves singlet molecular oxygen as an intermediate and its presence has been confirmed in quenching studies using bis-(di-n-butyldithiocarbamato)nickel(II) as an efficient 1O2 quencher with a quenching rate of 1.9 × 1010 M−1 s−1. The ability of [Pt(NN)(X)2] complexes to photosensitize the above photo-oxidation reaction follows the order [Pt(NN)(N3)2]> [Pt(NN)(CN)2]> [Pt(NN)Cl2]>[Pt(NN)(SCN)2]> [Pt(NN)Br2]>[PT(NN)I2]. With the exception of the cyanide ion, this order parallels the positions of the ligand-field splitting of X in the spectrochemical series.
 
Article
The photocatalyzed degradation of 1,2-diethyl phthalate (DEP) in aqueous suspensions of titanium dioxide has been investigated under a variety of conditions employing a pH-stat technique. The degradation was studied by monitoring the change in substrate concentration employing HPLC analysis and by the decrease in the total organic carbon (TOC) content as a function of irradiation time. The depletion of DEP and TOC was studied under different conditions such as pH, catalyst concentration, substrate concentration, different types of TiO2 as the photocatalyst and in the presence of several electron acceptors (H2O2, KBrO3 and (NH4)2S2O8) in addition to molecular oxygen. Highest degradation rates were observed with Degussa P25 being the photocatalyst at pH 6, while with Sachtleben Hombikat UV 100 smaller decomposition rates were obtained. The degradation of DEP was found to be only slightly enhanced by the addition of electron acceptors, whereas the mineralization is markedly improved in the presence of bromate ions. The degradation products were analyzed by GC–MS technique and probable pathways for the formation of products are proposed.
 
Article
E → Z photoisomerization of quaternary salts of E-1,2-bispyrazinyl-ethylene proceeds through the excited triplet state. The quantum yield of the iodide salt is lower than that of the chloride salt, suggesting that the electron transfer in the excited singlet state competes efficiently with intersystem crossing to the excited triplet state when the anion of the salt is an efficient electron donor, while in the salt with a weak electron-donating anion the intersystem crossing plays an important role in the excited singlet state and thereby photoisomerization becomes efficient.
 
Article
The radiative and reactive excited state relaxations of the ZE and ZZ geometrical isomers of 1,2-distyrylbenzene [1,2-(PhE)2B] and two hetero-analogues, where the side phenyl groups were replaced by 4′-pyridyl or 2′-thienyl groups, were studied and compared with the photobehaviour of the EE isomers previously investigated.The ZE and ZZ isomers of the hydrocarbon photoisomerize by a predominant adiabatic mechanism (a “one photon – two bonds” process is also operative in the case of ZZ) whereas the contribution of the adiabatic pathway is reduced for the thienyl-analogues, which isomerize by a mixed (diabatic/adiabatic) mechanism, whereas a prevalent diabatic mechanism characterizes the behaviour of the pyridyl analogue.
 
Article
The photochromic and thermochromic properties of trans-N,N′-bis(salicylidene)-1,2-cyclohexanediamine (1), trans-N,N′-bis(3,5-dichloro-salicylidene)-1,2-cyclohexanediamine (2), trans-N,N′-bis(3,5-di-t-butyl-salicylidene)-1,2-cyclohexanediamine (3) and trans-N,N′-bis(2-hydroxy-naphylidene)-1,2-cyclohexanediamine (4) were investigated by UV and fluorescence spectroscopies in the crystalline state at various temperatures and the molecular structures of 2 and 4 were determined by single-crystal X-ray diffraction. The existence of the two Schiff base groups on a single molecule does not seem to differentiate the chromobehavior of the present compounds (except possibly for 3) with respect to the usual Schiff bases of salicylaldehyde. It is suggested that for this class of compounds also, what determines the thermochromic behavior is the enhanced basicity of the nitrogen atom, due the absence of π,π- and n,π-conjugation with an aryl ring bound to it. The role of the crystal structure in this case is important only in so far as it affects the electron density on the nitrogen atom. Photochromism, however, is structure dependent and requires space for the generation of the photoproduct that involves cis to trans isomerization in the excited state.
 
Article
An unexpected (1,2) Wittig rearrangements occurs upon photodecarboxylation of fenofibric acid in neutral aqueous medium, by contrast with the usual experimental conditions employed in this type of rearrangement (alkyllithiums in dry THF or ethyl ether). Solvent effects suggest a mechanism involving cyclization of the decarboxylated carbanion to a resonance-stabilized oxacyclopropyl intermediate, rather than the generally operating radical cleavage-recombination pathway.
 
Article
A photochromic dithienylethene, 1,2-bis(2-methyl-5-phenylthien-3-yl)perfluorocyclopentene (1), was synthesized and incorporated into the lipid bilayer of liposomes as a strategy for demonstrating the potential for complete photocontrol over the permeability of a lipid membrane. Absorption spectroscopy and differential scanning calorimetry (DSC) were used to characterize the photochromism and inclusion of 1 in dipalmitoylphosphatidylcholine (DPPC) large unilamellar liposomes (LUVs), respectively. Absorption studies confirm that the photoisomerization of 1 in DPPC LUVs is completely reversible, although hindered due to the more restrictive and rigid environment posed by the lipid bilayer. DSC studies strongly suggest that 1 is partially buried in the hydrocarbon core of the bilayer, interacting primarily with the C2–C8 methylene region of the acyl chains of DPPC. Photoinduced changes in the membrane permeability of DPPC LUVs were assessed by monitoring changes in the fluorescence of an encapsulated fluorophore, 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS), upon photocycling 1 between the two photoisomers using ultraviolet and visible light. The fluorescence studies show that release of encapsulated HPTS is not observed following the irradiations. Thus, the photoisomerization of 1 does not lead to a large disruption in the local lipid order within the bilayer.
 
Article
Five new pyrrolo[1,2-b]pyridazine derivatives: 5,6-dicarbomethoxy-2,7-dimethylpyrrolo[1,2-b]pyridazine (I), 5,6-dicarbomethoxy-7-methyl-2-phenylpyrrolo[1,2-b]pyridazine (II), 5,6-dicarbomethoxy-7-ethyl-2-phenylpyrrolo[1,2-b]pyridazine (III), 5-carboethoxy-7-methyl-2-phenylpyrrolo[1,2-b]pyridazine (IV) and 7-ethyl-2-phenylpyrrolo[1,2-b]pyridazine (V), with aryl or methyl substituents in pyridazinic ring and with alkyl, aryl and/or carbometoxy in the pyrrolic ring have been investigated by spectroscopic methods and electronic structure calculations. UV–vis absorption together with steady-state and time-resolved fluorescence measurements have been conducted in cyclohexane, n-hexane (as inert solvents), and in solid state to evidence comparatively the effect of the structure of the compounds on the absorption and fluorescence properties. All five compounds have an intense fluorescence with high quantum yields. The intense fluorescence is evidenced also in solid state. The electronic structure calculations have been performed in the framework of density functional theory (DFT) and time dependent DFT (TD-DFT) in order to elucidate the differences observed in absorption spectra as an effect of the substituents.
 
Article
Irradiation of cis, trans-l,2-dibenzoyl-3-methylcyclopropane (cis-1a) produced trans, trans-1,2-dibenzoyl-3-methylcyclopropane (trans-1a). This cis–trans isomerization was shown to be reversible through cleavage of the bond beta to both carbonyl (1,2-bond fission) while irradiation of cis, trans-l,2-dibenzoyl-3-phenylcyclopropane (cis-1b) gave trans, trans-1,2-dibenzoyl-3-phenylcyclopropane (trans-1b) irreversibly through cleavage of the bond beta to both carbonyl groups (1,2-bond fission) and the bond beta to phenyl group (1,3-bond fission). 1,2-bond fission of optically active trans-1b showed racemization only under irradiation. This racemization of trans-1b was considered to be responsible for irreversible cis–trans isomerization of cis, trans-1,2-dibenzoyl-3-phenylcyclopropane (cis-1b).
 
Article
The fluorescence quantum yield of the 5-(4-methoxyphenyl)-7,7-dimethyl-7H-indolo[1,2-a]quinolinium perchlorate and its analogues was determined in 12 selected solvents of the hydroxyl and CH-acid type and in the dichloromethane–acetonitrile binary system at constant and variable temperature. The quenching process, which depends on the dye and solvent structure, in hydroxylic ones was caused by dissociation of the solvent–dye complex. A rare phenomenon of the dye fluorescence quantum yield raising along with the increase in temperature is observed. It has been proved that methoxy group and its derivatives is a negatively charged complexing centre and the complex stability raises along with the increasing electron donor character of alkyl fragment. This conclusion is supported by theoretical calculations of bonding energy. The important role of a local dielectric constant and the method of its calculation are discussed.
 
Article
Photolysis of 1,2,3-indanetrione (1) in dichloromethane, in the presence of 2,3-diphenyl-1,4-dioxene, leads to the formation of the corresponding oxetane 6, through a Paterno-Büchi reaction. The irradiation of 1 in the presence of 2-methyl-2-butene gives, in addition to the oxetane 7, products arising from an allylic hydrogen abstraction process, i.e. E-8 and 9. On the other hand, in the presence of 2,4,4-trimethyl-1-pentene, the only product formed, i.e. 12, results from an allylic hydrogen abstraction reaction from the primary center. The regioselectivity found in some of these reactions was associated to strong steric effects in the approach between triketone triplet and the olefin.
 
Article
The photolysis of 1,2,3-triazine in hexane and methanol solvents and in the vapour phase has been studied at a number of wavelengths in the main absorption band. For solutions of the triazine in hexane, two forms have been spectroscopically identified. They correspond to hydrogen-bonded, owing to the presence of trace quantities of water, and non-bonded species, In methanol solutions, such spectroscopic resolution was not achieved. Photodissociation quantum yields were found to decrease with increasing wavelength. For photolysis of hexane solutions conducted at the shorter wavelengths, the quantum yields were larger for the hydrogen-bonded species. The quantum yields were independent of solvent, the presence of air and the incident light intensity. The identification of acetylene and the observation of an unidentified photoproduct(s) suggest that the concerted photofragmentation mechanism cannot be the one and only photodecomposition pathway in hexane and methanol solutions. However, for the vapour the identification of acetylene and HCN suggest that it may be the mechanism.
 
Article
The solvent and pH dependence of the absorption and fluorescence spectra, fluorescence quantum yields, fluorescence lifetimes, low temperature luminescence spectra and quantum yields and photochemical reactivity of a series of 4-(1,2,4-triazol-1-yl)-pyrimidin-2(1H)-ones have been examined and the data obtained were compared with the results reported previously for uracil and thymine nucleosides. Stacking interaction occurs between triazolyl-pyrimidinones and common nucleic acid bases and was detected by UV absorption and fluorescence spectroscopy. The compounds studied undergo [2+2] photocycloaddition. This was determined on the basis of photochemical studies of synthesized dinucleotide analogues.
 
Article
The irradiation of 1-(veratryl)-2-(3′-methoxyphenoxy)glycerol in alkaline methanol solution was performed in the presence of hydrogen peroxide (UV and visible light) and sensitized by methylene blue (MB) in the presence of O2 (visible light). The photoproducts were analysed by gas chromatography-mass spectrometry (GC-MS). The β-O-4 ether bond cleavage, which usually occurs in reactions involving singlet oxygen, was not operative, since the corresponding phenol 3 was detected in trace amounts only and the carbonyl derivative 4 was not detected at all. Irradiation in the presence of MB/O2 induced the oxidation of the benzyl group into a carbonyl function and produced the hydroxymethylation of one benzene ring of compound 1. In contrast, such reactivity was not observed when 1 was irradiated in the presence of H2O2: nevertheless, hydroxylation of one benzene ring was detected. For both types of irradiation, four stereoisomers were formed. The mass spectrum showed the addition of hydrogen peroxide to the B henzene ring of compound 1. This is the most important reaction, and was ascribed to the radical addition of a superoxide anion assisted by the hydroxyl groups in the β or γ positions of the glycerol moiety.
 
Article
The latest results concerning the photochemistry and sensitized photoreduction of transition metal 1,3-diketonates are examined. The physical and chemical aspects of the interaction mechanisms between excited singlet and triplet states of organic compounds and 1,3-diketonates are discussed. Some examples of the uses of transition metal 1,3-diketonates in photocatalysis are presented.
 
Article
Synthesis, thermo-optical and glass forming properties of a series of photoresponsive luminescent mesogenic materials containing a butadiene moiety linked to a cholesterol group (CBIN, CBIN8 and CBIN12) as well as colour imaging devices based on them are described. These molecules exhibited relatively high fluorescence quantum efficiency in the solid and liquid crystalline states. The cholesteric phase was observed to be the major phase in these molecules and the pitch of cholesteric helix was sensitive to external stimuli such as temperature and light, making it possible to tune their iridescent colour in the visible region. The colour reflected by the cholesteric films could be stabilized by converting them to the glassy state by sudden cooling from their cholesteric phase to ∼0 °C. Selective exposure of the material in its liquid crystalline phase to light resulted in trans–cis photoisomerization of the butadiene chromophore. Consequently the pitch of the irradiated portion changed depending upon the intensity of the illuminating light. The coloured images thus formed could be stored for long periods by converting the films to glasses by sudden cooling. The images stored in the glassy state were stable over long periods of time (>one year).
 
Article
Two isomeric water-soluble 1,3-diphenylisobenzofuran derivatives, containing an m- or p-trimethylammonium iodide group in one of the phenyl substituents ( respectively), were prepared and characterized, and their ability to quench was evaluated in both homogeneous aqueous media and dioctadecyldimethylammonium chloride (DODAC) vesicles. The solubility in water of each isomer was higher than 2×10−4M and the respective total quenching rate constants (kq) were (2.3 ± 0.2) × 109and (1.1 ± 0.1) × 109M−1s−1. The reactive pathway fraction for isomer m-4 in D2O was 0.72 ± 0.10, a value obtained by comparison with unsubstituted 1,3-diphenylisobenzofuran in the system cetyltrimethylammonium chloride-D2O. Both quenchers can be used with generators, e.g. thionine and methylene blue, but not with anionic dyes, such as rose bengal, because of the formation of ground state, ion pair complexes. When methylene blue and isomer p-4 were separated by a DODAC bilayer, the quencher was consumed with zero-order kinetics, but the consumption rate was much smaller than in the absence of the bilayer. A rough estimate indicates that the quencher p-4 reacts with approximately 25% of the generated escaping from the vesicle.
 
Article
The absorption and emission spectroscopic investigations of a β-diketone, 1,3-diphenyl-propane-1,3-dione (1k) in various polar, non-polar as well as protic and aprotic solvents are reported. The solvatochromic shifts of the spectral bands were studied in a multitude of solvents followed by multilinear regression in which several solvent parameters were simultaneously analysed. This π-conjugated β-diketone exhibits solvatochromism. Variation in the electronic absorption and emission spectral characteristics of 1k were studied in solution in presence of magnesium perchlorate. Absorption spectral studies indicate complex formation between magnesium ion and 1k in the ground state. The values of equilibrium constant, enthalpy and entropy of complex formation together with the molar absorbance of the absorbing species have been determined for the diketone–cation 1:1 interaction. Fluorescence spectral changes with the addition of Mg(ClO4)2 revealed the existence of hydrogen bonded excited state of 1k. The values of equilibrium constant for 1k/Mg2+ interaction have been measured in acetone and methanol media.
 
Article
Spiropyran compounds exhibit photochromism in solution or as polymer matrix films. Pure bulk films of spiropyran in the solid amorphous state also exhibit photochromism. However, spiropyrans are easily decomposed by cyclic light irradiation in air. The decomposition process of bulk spiropyran (1,3,3-trimethyl-6′-hydoxy spiro[2H-1-benzopyran-2,2′-indoline]; HBPS) films was investigated using UV-visible, IR and proton nuclear magnetic resonance (1H-NMR) spectroscopy and elemental analyses. Spiro carbon and oxygen bonds are cleaved on exposoure to UV light in air, and new OCH3 and N+H bonds are generated.It is clarified that the HBPS decomposition process comprises photo-oxidation and a change in methyl groups.
 
Article
Two novel 1,3,5-triphenyl-2-pyrazoline moiety containing derivatives of 3-hydroxychromone were synthesized and studied by 1H NMR, absorption and fluorescence spectroscopy. The prospects of practical application of these compounds exhibiting high solvatofluorochromism into analytical chemistry and biophysics as effective ratiometric polarity probes were discussed proceeding from the data on their fluorescent properties.
 
Top-cited authors
Nick Serpone
  • University of Pavia
J. Kiwi
  • École Polytechnique Fédérale de Lausanne
Kazuhito Hashimoto
  • National Institute for Materials Science
Hironori Arakawa
  • Tokyo University of Science
César Pulgarin
  • École Polytechnique Fédérale de Lausanne