Photodegradation of poly(neopentyl isophthalate) part I: Laboratory and outdoor conditions

In this paper the mechanisms of photodegradation of poly(neopentyl isophthalate) (PNI) in laboratory (Suntest XXL+, λ > 300 nm) and outdoor conditions are compared. Changes in the chemical composition were studied with ATR-FTIR, SEC and MALDI-ToF MS. Furthermore, the results were compared with data presented in our previous paper on PNI coatings that were aged in the UVACUBE (λ > 254 nm). Two main aspects of photodegradation of PNI are addressed in the present paper: the influence of different wavelengths and the comparison of laboratory and outdoor exposure regarding the mechanism of degradation. Under short (λ > 254 nm) and long (λ > 300 nm) wavelength irradiation similar products of degradation are formed. However, the presence of short wavelength radiation dramatically accelerates the overall rate of photodegradation of PNI. UV light absorption calculations confirm this experimentally found acceleration. Exposure of PNI in laboratory and outdoor conditions, both with wavelengths λ > 300 nm resulted in similar degradation products in the initial stage of ageing.     

SPECTRAL AND PHOTOCHEMICAL PROPERTIES OF CURCUMIN

Curcumin, bis(4-hydroxy-3-methoxyphenyl)-l,6-heptadiene-3,5-dione, is a natural yellow-orange dye derived from the rhizome of Curcuma longa, an East Indian plant. In order to understand the photobiology of curcumin better we have studied the spectral and photochemical properties of both curcumin and 4-(4-hydroxy-3-methoxy-phenyl)-3-buten-2-one (hC, half curcumin) in different solvents. In toluene, the absorption spectrum of curcumin contains some structure, which disappears in more polar solvents, e.g. ethanol, acetonitrile. Curcumin fluorescence is a broad band in acetonitrile (λ_max= 524 nm), ethanol (λ_max= 549 nm) or micellar solution (λ_max= 557 nm) but has some structure in toluene (λ_max= 460, 488 nm). The fluorescence quantum yield of curcumin is low in sodium dodecyl sulfate (SDS) solution (φ= 0.011) but higher in acetonitrile (φ= 0.104). Curcumin produced singlet oxygen upon irradiation (φ > 400 nm) in toluene or acetonitrile (Φ= 0.11 for 50 μM curcumin); in acetonitrile curcumin also quenched ¹O₂ (k_q, = 7 × 10⁶ M^?1 s^?1). Singlet oxygen production was about 10 times lower in alcohols and was hardly detectable when curcumin was solubilized in a D₂O micellar solution of Triton X-100. In SDS micelles containing curcumin no singlet oxygen phosphorescence could be observed. Curcumin photogenerates superoxide in toluene and ethanol, which was detected using the electron paramagnetic resonance/spin-trapping technique with 5,5-dimethyl-pyrroline-.N-oxide as a trapping agent. Unidentified carbon-centered radicals were also detected. These findings indicate that the spectral and photochemical properties of curcumin are strongly influenced by solvent. In biological systems, singlet oxygen, superoxide and products of photodegradation may all participate in curcumin phototoxicity depending on the environment of the dye.     

Dual electrochemical microsensor for simultaneous measurements of nitric oxide and oxygen: Fabrication and characterization

A planar-type amperometric dual microsensor was developed for the simultaneous measurement of the nitric oxide (NO) and oxygen (O₂) concentrations. The sensor (overall diameter = 500 μm) consisted of a dual working electrode (WE) containing two platinized platinum microdisks (25 μm diameter, WE1, WE2, distance between two disks > 330 μm) and a Ag/AgCl wire reference electrode covered with an expanded poly(tetrafluoroethylene) gas-permeable membrane. The differentiation and concurrent measurements of NO and O₂ were obtained successfully using two sensing WEs with different applied potentials (+0.75 V for WE1 and −0.4 V for WE2). Cross-talk between WE1 and WE2 was eliminated with an optimized internal solution composition. Linear dynamic range, selectivity, sensitivity, detection limit (<5 nM for NO; <500 nM for O₂), and stability (>50 h) were evaluated.     

The electronic states of but-2-yne studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio configuration interaction methods

The photoabsorption spectrum of but-2-yne in the range 5.5–11 eV (225–110 nm) has been recorded using a synchrotron radiation source. The spectrum is dominated by three d-type Rydberg series, converging to the first ionisation energy (IE) (π⁻¹, 9.562 eV). Origins of the π3d members are 7.841, 7.977 and 8.018 eV, respectively. Transitions of low intensity, arising from excitation of the π3s state (origin, 6.35 eV) and two π3p Rydberg states (7.38 and 7.51 eV, respectively) have also been identified in the spectrum. Near-threshold electron energy-loss spectra reveal valence excited triplet states at about 5.2 and 5.8 eV, respectively.Electronic excitation energies for valence and Rydberg-type states have been computed using ab initio multi-reference multi-root CI methods. These studies used a triple zeta + polarisation basis set, augmented by diffuse (Rydberg) orbitals, to generate the theoretical singlet and triplet energy manifolds. The correlation of theory and experiment shows the nature of the more intense Rydberg state types, and identification of the main valence and Rydberg bands. Calculated energies for Rydberg states are close to those expected, and there is generally a good correlation between the theoretical and experimental envelopes. It was possible to generate singlet Rydberg states which relate to the 5-lowest IEs of but-2-yne; furthermore, the separation of these sequences shows that the IE order (under D_3h symmetry) is: , also supported by direct calculation of the IEs by CI.The lowest valence singlet states are ππ^*, optically forbidden, and calculated to lie near 7.3 and 7.6 eV. The states which contribute strongly to the observed spectrum are πσ^* near 7.9 eV having excitation, followed by several ππ^* and πσ^* states between 10.0 and 10.5 eV; an ¹E′ antisymmetric combination(2e′2e″ − 2e′2e″) is by far the strongest in intensity. A further group of symmetry-allowed valence states are calculated to lie near 12.3 and 12.9 eV. The two lowest triplet states, both of E′ symmetry (ππ^*), have vertical excitation energies of 5.7 and 6.2 eV, but are strongly bent with a trans-CCCC unit (C_S and C_2h). The theoretical work confirms that, on intensity grounds, valence excited states do not contribute significantly to the spectrum. CI calculations of the ionic states give the ionisation energy sequence (D_3h): . Adiabatic structures for the first cation, two triplets, and a singlet (C_2h) were obtained; these show shortening of C–C, and lengthening of CC, in a trans-CCCC, as is found with ethyne.     

Photophysical characterization of imidazolium-substituted Pd(II), In(III), and Zn(II) porphyrins as photosensitizers for photodynamic therapy

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.     

Preparation and characterization of mesoporous N-doped and sulfuric acid treated anatase TiO2 catalysts and their photocatalytic activity under UV and Vis illumination

Nitrogen-doped TiO₂ catalysts were prepared by a precipitation method. The samples were calcined at 400 °C for 4 h in air. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), low temperature N₂-adsorption was used for structural characterization and UV-diffuse reflectance (UV-DR) was applied to investigate the optical properties of the as-prepared samples. It was found that microporous N-doped catalysts have solely anatase crystalline structure. Acidic treatment of the calcined samples was performed using sulfuric acid agitation. The crystalline structure remained unchanged due to surface treatment, while the porosity and the surface areas were decreased dramatically. Optical characterization of the doped catalysts showed that they could be excited by visible light photons in the 400–500 nm wavelength range (λ_g,1=390 nm, λ_g,2=510 nm). It was also established that surface treatment enhances the Vis-light absorption of the N-TiO₂ powders. Finally the catalysts were tested in the photocatalytic degradation of phenol in aqueous suspensions. Two different light sources were used; one of them was a UV-rich high pressure Hg-lamp, while the other was a tubular visible light source. We found that using visible light illumination N-doped, acid treated TiO₂ samples were more catalytically active than non-doped TiO₂ catalysts.     

Reduced N-methyl-pyridylethynyl porphyrins exhibit strong near-IR absorptions

Porphines bearing two N-methyl-4-pyridylethynyl substituent reversibly undergo two one-electron reductions at room temperature. The anion radicals and di-anions show diminished visible bands (450 nm and 600–700 nm) and intense absorptions in the 800-nm and 1100-nm region, respectively. Some of the near-IR bands have extinction coefficients greater than 1.5 × 10⁵ M⁻¹ cm⁻¹.     

Two-photon induced luminescence, singlet oxygen generation, cellular uptake and photocytotoxic properties of amphiphilic Ru(II) polypyridyl-porphyrin conjugates as potential bifunctional photodynamic therapeutic agents

Two Ru(II) polypyridyl-porphyrin and Zn(II) porphyrin conjugates (Ru-L and Ru-Zn-L) have been synthesized and their photophysical properties studied. The two conjugates, which contained a hydrophobic tetraphenylporphyrin L conjugated via an acetylide linker at its β-position with a hydrophilic Ru(II) polypyridyl complex, showed high singlet oxygen quantum yields (>70%) and substantial two-photon absorption cross-sections (~500 GM). Ru-L gave strong emissions at ~660 and ~733 nm through linear or two-photon excitation. Solvatochromism was observed in the fluorescence spectra of Ru-L and Ru-Zn-L, where in less polar solvents (i.e., toluene and dichloromethane) their fluorescence emissions became slightly blue-shifted with a 3-fold reduction in intensity relative to those observed in polar solvents (i.e., acetonitrile and methanol). Cell-based studies of these complex conjugates were conducted using human nasopharyngeal carcinoma HK-1 and cervical carcinoma HeLa cells on which Ru-L showed rapid cellular uptake, low dark-cytotoxicity, and high photo-cytotoxicity. Furthermore, Ru-L can be excited and emits in the "biological window"in vitro, making it a potential potent new generation photodynamic therapeutic agent capable of singlet oxygen generation and in vitro near-infrared emission.     

Photophysical Characterization of the Naturally Occurring Dioxobacteriochlorin Tolyporphin A and Synthetic Oxobacteriochlorin Analogues

Tolyporphins are tetrapyrrole macrocycles produced by a cyanobacterium‐containing culture known as HT‐58‐2. Tolyporphins A–J are free base dioxobacteriochlorins, whereas tolyporphin K is an oxochlorin. Here, the photophysical characterization is reported of tolyporphin A and two synthetic analogues, an oxobacteriochlorin and a dioxobacteriochlorin. The characterization (in toluene, diethyl ether, ethyl acetate, dichloromethane, 1‐pentanol, 2‐butanone, ethanol, methanol, N,N‐dimethylformamide and dimethylsulfoxide) includes static absorption and fluorescence spectra, fluorescence quantum yields and time‐resolved data. The data afford the lifetime of the lowest singlet excited state and the yields of the nonradiative decay pathways (intersystem crossing and internal conversion). The three macrocycles exhibit only modest variation in spectroscopic and excited‐state photophysical parameters across the solvents. The long‐wavelength (Q_y) absorption band of tolyporphin A appears at ~680 nm and is remarkably narrow (full‐width‐at‐half‐maximum ~7 nm). The position of the long‐wavelength (Q_y) absorption band of tolyporphin A (~680 nm) more closely resembles that of chlorophyll a (662 nm) than bacteriochlorophyll a (772 nm). The absorption spectra of tolyporphins B–I, K (which were available in minute quantities) are also reported in methanol; the spectra of B–I closely resemble that of tolyporphin A. Taken together, tolyporphin A generally exhibits spectral and photophysical features resembling those of chlorophyll a.     

Effect of high rates of mass transport on oxygen reduction at copper electrodes: Implications for aluminium corrosion

The reduction of oxygen at copper microelectrodes (25 μm diameter) in aqueous solution (pH 7) has been studied under conditions of high mass transport, similar to those experienced by μm sized copper-containing inclusions in aluminium alloys during corrosion. Contrary to previous studies at copper electrodes operating at lower mass transport rates, oxygen reduction limiting currents show an apparent number of electrons which decrease towards 2 as the mass transport rate increases (mass transfer coefficients up to ca. 0.55 cm s⁻¹), rather than a 4-electron process. These new data suggest that the treatment of oxygen reduction as a 4-electron transfer process at micron and smaller-sized copper intermetallics in aluminium alloys may require revision.     

Charge transport modulation of silicon nanowire by O2 plasma

The modification of the electrical characteristics of field-effect transistors (FETs) with channels composed of n- or p-type silicon nanowires (SiNWs) by oxygen plasma treatment is investigated in this study. The SiNWs obtained from silicon bulk wafers are <111> surface-oriented and their doping concentrations are 10²¹ and 10¹⁷ cm⁻³ for the n- and p-type SiNWs, respectively. After the back-gate SiNWFETs were subjected to oxygen plasma treatment, the magnitude of the drain current of the n-type SiNWs was decreased, whereas that of the p-type SiNWs was increased, while the gate-dependent characteristics of both of types of SiNWs were improved. The changes in the electrical characteristics are due to the adsorption of oxygen ions on the surface of the SiNWs. To verify the effect of the oxygen ions, the SiNWFETs were kept in a vacuum for 24 h whereupon their electrical characteristics tended to revert to their inherent state.     

Role of electrolytes in the preparation of nanoparticles via the emulsion polymerization of vinyl pivalate

By controlling both the kind of ion and the ionic strength of electrolytes in an emulsion polymerization system of vinyl pivalate containing about 1% sodium lauryl sulfate as a surfactant, nanoparticles of polyvinylpivalate having a diameter of about 25 nm were successfully prepared. The use of high concentrations of lithium chloride and lithium sulfate (1.0 mol L⁻¹) prevented the nanoparticles from aggregating and produced nanoparticles sizes of 25–50 nm. Ammonium acetate and sodium acetate, on the other hand, accelerated the aggregate of the nanoparticles. These phenomena were examined in detail and found to be similar to the Hofmeister phenomena and the combination rule proposed by Craig et al.     

Studies on curcumin and curcuminoids. XXXIV. Photophysical properties of a symmetrical, non-substituted curcumin analogue

Curcumin is the main constituent of curry. In its ground state it shows chemo-preventive, chemo-therapeutic, anti-inflammatory and immune stimulating effects, and it is considered as a drug or drug model in the treatment of AIDS and cystic fibrosis. Further biological activity is induced in curcumin by light exposure: cytotoxicity is enhanced and photosensitized antibacterial effects are achieved. For the curcumin cis enol conformer, the fastest deactivation mechanism of the first excited singlet state is an excited-state intra-molecular proton transfer, which brings curcumin back to the ground state. This mechanism, as well as reketonization, interaction with the solvent and photodegradation, compete with the phototherapeutic action. The native compound curcumin carries phenolic hydroxyl and methoxy groups that influence the molecular charge distribution and hence the excited-state intra-molecular proton transfer rate in an unpredictable way. In this work we study static and time-resolved spectroscopic properties of a non-substituted curcuminoid that lacks both the phenolic hydroxyl and the phenolic methoxy groups. The photophysical properties of this compound are compared to those of native curcumin, in order to provide a rationale to the design of curcuminoids with molecular structures optimized for a photosensitizer.     

A computational study and fragment analysis of the back-bonding in Titanocenyl Complexes containing a five-member L,L′-cyclic ligand, L,L′ = O,O′; S,S′ or Se,Se′

A DFT computational study is performed on different Cp₂Ti^IV(L,L′-BID) complexes with L,L′-BID = dioxolene, dithiolene or diselenolene. A fragment analysis of the titanocene-ligand bonding in the DFT optimized geometries showed that out of plane folding for maximum Ti ← L π donation increases Cp₂Ti^IV(O,O′-BID) (35°) < Cp₂Ti^IV(S,S′-BID) (43–49°) < Cp₂Ti^IV(Se,Se′-BID) (48–53°).     

Isoconversional kinetics of degradation of polyvinylpyrrolidone used as a matrix for ammonium nitrate stabilization

Thermogravimetric analyzer (TGA) has been applied to measure the kinetics of the thermal degradation of virgin polyvinylpyrrolidone (PVP) and a phase stabilized PVP–ammonium nitrate (AN) material. The PVP–AN samples have been prepared by using 20 wt.% of AN and PVP of three different molecular weights. Virgin PVP undergoes a major mass loss in the region 380–550 °C leaving a small amount of nonvolatile residue. The application of an advanced isoconversional method to the respective degradation process demonstrates that its effective activation energy increases from 70 kJ mol⁻¹ to a plateau value at 250–300 kJ mol⁻¹, which is independent of the molecular weight. The PVP–AN materials lose spontaneously 20% of their mass on heating above the glass transition temperature of the PVP matrix (160–180 °C). After the escape of AN, the remaining PVP matrix degrades in the same temperature region as virgin PVP, however, the effective activation energy of this degradation is 150–200 kJ mol⁻¹.     

Singlet oxygen generation in PUVA therapy studied using electronic structure calculations

The ability of furocoumarins to participate in the PUVA (Psoralen + UV-A) therapy against skin disorders and some types of cancer, is analyzed on quantum chemical grounds. The efficiency of the process relies on its capability to populate its lowest triplet excited state, and then either form adducts with thymine which interfere DNA replication or transfer its energy, generating singlet molecular oxygen damaging the cell membrane in photoactivated tissues. By determining the spin–orbit couplings, shown to be the key property, in the intersystem crossing yielding the triplet state of the furocoumarin, the electronic couplings in the triplet–triplet energy transfer process producing the singlet oxygen, and the reaction rates and lifetimes, the efficiency in the phototherapeutic action of the furocoumarin family is predicted as: khellin < 5-methoxypsoralen (5-MOP) < 8-methoxypsoralen (8-MOP) < psoralen < 4,5′,8-trimethylpsoralen (TMP) < 3-carbethoxypsoralen (3-CPS), the latter being the most efficient photosensitizer and singlet oxygen generator.     

Excited state intramolecular proton transfer in a new o-hydroxy Schiff base in non polar solvents at room temperature and 77 K

A new orthohydroxy Schiff base, 7-ethylsalicylidenebenzylamine (ESBA) has been synthesised. The excited state intramolecular proton transfer (ESIPT) processes have been investigated by means of absorption, emission and nanosecond spectroscopy at room temperature and at 77 K in non polar solvents. The ESIPT is evidenced by a large Stokes shifted emission (11 000 cm⁻¹) only at 77 K. From fluorescence and excitation spectra it is suggested that at least three different species are present in the excited state at room temperature. Our theoretical calculation at AM1 level confirm the cis-isomer to be the only viable form in the ground state.     

Role of hydrogen bonding in the photophysical properties of isomeric tetrapyridylporphyrins in aprotic solvent

Extensive photophysical properties of isomeric tetra-2-pyridylporphyrin (TpyP(2)), tetra-3-pyridylporphyrin (TpyP(3)), and tetra-4-pyridylporphyrin (TpyP(4)) have been studied in the presence of a series of phenols of increasing hydrogen bonding power in dichloromethane solution by employing UV/vis spectroscopy; steady-state, time-resolved fluorescence spectroscopy; and transient absorption spectroscopic techniques. The change of absorption spectra of all three porphyrins as a function of different phenol concentrations established the preference of hydrogen bonded complex formation to the peripheral pyridyl nitrogen rather than the pyrrole nitrogen of the porphyrin macrocycle. The fluorescence behaviors of the porphyrins which were observed upon addition of different phenols point to a marked dependence on the nature of the added phenols. Phenols with an electron withdrawing group do not quench the fluorescence of porphyrins, whereas phenols with an electron donating group quench the singlet porphyrin both in static and dynamic pathways. A remarkable difference in quenching behaviors of singlet excited porphyrin by 4-methylphenol (4-MePhOH) and 4-MeOPhOH/4-EtOPhOH (4-EtOPhOH = 4-ethoxyphenol) are observed. The quenching of singlet excited porphyrins by 4-MePhOH is attributed to be purely static in nature, and the H-bond provides a strong nonradiative channel to singlet excited porphyrins. However, the quenching of singlet excited porphyrins by 4-MeOPhOH/4-EtOPhOH is mostly dynamic, and it is ascribed to be the reductive quenching of single excited porphyrins. Picosecond transient absorption study with TpyP(2) and 4-MeOPhOH provides the evidence of porphyrin radical anion and phenol radical cation of equal lifetime, which indicates the fact that electron transfer occurs from phenol to singlet excited porphyrin. The temperature effect on dynamic quenching by 4-MeOPhOH/4-EtOPhOH and kinetic deuterium isotope effect established the reaction to be a photoinduced concerted proton coupled electron transfer.     

Femtosecond dynamics of a non-steroidal anti-inflammatory drug (piroxicam) in solution: The involvement of twisting motion

In this contribution, we report on fast and ultrafast dynamics of a non-steroidal anti-inflammatory drug, piroxicam (PX), in methyl acetate (MAC) and triacetin (TAC), two solvents of different viscosities. The enol form of PX undergoes a femtosecond (shorter than 100 fs) electronically excited state intramolecular proton-transfer reaction to produce keto tautomers. These structures exhibit an internal twisting motion to generate keto rotamers in 2–5 ps, a time being longer in TAC. The transient absorption/emission spectrum is very broad indicating that the potential-energy surface at the electronically excited state is very flat, and reflecting the involvement of several coordinates along which the wavepacket of the fs-produced structures evolve.     

Ultrafast excited-state dynamics of RNA and DNA C tracts

The excited-state dynamics of the RNA homopolymer of cytosine and of the 18-mer (dC)₁₈ were studied by steady-state and time-resolved absorption and emission spectroscopy. At pH 6.8, excitation of poly(rC) by a femtosecond UV pump pulse produces excited states that decay up to one order of magnitude more slowly than the excited states formed in the mononucleotide cytidine 5′-monophosphate under the same conditions. Even slower relaxation is observed for the hemiprotonated, self-associated form of poly(rC), which is stable at acidic pH. Transient absorption and time-resolved fluorescence signals for (dC)₁₈ at pH 6.8 are similar to ones observed for poly(rC) near pH 4, indicating that hemiprotonated structures are found in DNA C tracts at neutral pH. In both systems, there is evidence for two kinds of emitting states with lifetimes of 100 ps and slightly more than 1 ns. The former states are responsible for the bulk of emission from the hemiprotonated structures. Evidence suggests that slow electronic relaxation in these self-complexes is the result of vertical base stacking. The similar signals from RNA and DNA C tracts suggest a common base-stacked structure, which may be identical with that of i-motif DNA.