Quenching Of Excited States Research Articles

Periodic Trends in Intra-ionic Excited State Quenching by Halide.

The preassociation of reactants in a photoinitiated redox reaction through the use of noncovalent interactions can have a significant impact on excited state reactivity. As these noncovalent interactions render some stabilization to the associated species, they impact the kinetics and thermodynamics of photoinitiated electron transfer. Reported herein is a novel iridium(III) photocatalyst, equipped with an anion-sensitive, amide-substituted bipyridine ligand, and its reactivity with the halides (X = I-, Br-, Cl-) in acetonitrile and dichloromethane. A noteworthy periodic trend was observed, where the size and electron affinity dramatically altered the observed photoredox behavior. The binding affinity for the halides increased with decreasing ionic radius (Keq ∼103 to >106) in a polar medium but association was stoichiometric for each halide in a nonpolar medium. Evidence for the static quenching of iodide and bromide is presented while dynamic quenching was observed with all halides. These results highlight how the photophysics of halide adducts and the thermodynamics of intra-ionic photo-oxidation are impacted as a consequence of preassociation of a quencher through hydrogen bonding.

Reductive Dynamic and Static Excited State Quenching of a Homoleptic Ruthenium Complex Bearing Aldehyde Groups.

A new homoleptic Ru polypyridyl complex bearing two aldehyde groups on each bipyridine ligand, [Ru(dab)3](PF6)2, where dab is 4,4'-dicarbaldehyde-2,2'-bipyridine, was synthesized, characterized, and utilized for iodide photo-oxidation studies. In acetonitrile (CH3CN) solution, the complex displayed an intense metal-to-ligand charge transfer (MLCT) absorbance maximum at 475 nm (ε = 22,000 M-1 cm-1) and an infrared (IR) band at 1712 cm-1 assigned to the pendent aldehyde groups. Visible light excitation in air-saturated solution resulted in room temperature photoluminescence (PL) with a maximum at 675 nm, a quantum yield, ϕPL = 0.048, and an excited state lifetime, το = 440 ns, from which radiative and nonradiative relaxation rate constants were extracted, kr = 9.1 × 104 s-1 and knr = 1.8 × 106 s-1. Pulsed visible light excitation yielded transient UV-vis and IR absorption spectra consistent with an MLCT excited state; relaxation occurred with the maintenance of two isosbestic points in the visible region, and a lifetime that agreed with that measured by time-resolved PL. Cyclic voltammetry studies in a CH3CN solution with 0.1 M TBAPF6 electrolyte revealed a quasi-reversible oxidation, E°(RuIII/II) = +1.25 V vs. Fc+/0, and three sequential one-electron reductions at -1.10, -1.25, and -1.54 V vs. Fc+/0. An excited state reduction potential of E°(Ru*2+/+) = +0.89 V vs. Fc+/0 was estimated with the Rehm-Weller expression. Titration of tetrabutylammonium iodide, TBAI, into a CD3CN solution of [Ru(dab)3](PF6)2 resulted in significant shifts in the aldehyde H atom and 3,3'-biypridyl resonances that were analyzed with a 1:1 equilibrium model, from which Keq = 460 M-1 was extracted, increasing to 5800 M-1 when the solvent was changed to acetone-d6. Iodide titrations resulted in a significant quenching of the [Ru(dab)3]*2+ lifetime and quantum yield in both CH3CN and acetone solvents. In CH3CN, the quenching was mainly dynamic and well described by the Stern-Volmer model, from which a quenching rate constant, kq, of 4.5 × 1010 M-1 s-1 and an equilibrium constant, Keq, of 8.3 × 103 M-1 were obtained. In acetone, the static quenching pathway by iodide was greatly enhanced, with a Keq of 1.2 × 104 M-1 and a higher kq of 9.2 × 1010 M-1 s-1.

Intrinsic Photostability in Dithiolonaphthalimide Achieved by Disulfide Bond-Induced Excited-State Quenching.

Disulfide bridges common in proteins show excellent photostability achieved by ultrafast internal conversion and maintain the stability of the tertiary structure. When disulfide bonds exist in aromatic compounds, the rigid chemical structure may affect the cleavage and reforming dynamics of disulfide bonds. In this work, a model compound with a disulfide five-membered-ring structure, 4,5-dithiolo-N-(2,6-dimethylphenyl)-1,8-naphthalimide (DTDPNI), is selected to elaborate the effect of disulfide modification on the excited-state deactivation mechanism. Quantum chemical calculations show that the S-S stretching leads to a dramatic decrease in the energy gap between the S1 and S0 states, similar to the situation in 1,2-dithiane. Due to the efficient nonradiative process, the excited-state lifetime of DTDPNI resolved by ultrafast spectroscopy is determined to be ∼20 ps. It is found that the excellent photostability is achieved by ultrafast excited-state quenching induced by the S-S stretching, rather than the cleavage of the disulfide bond; even the disulfide bridge is in a very rigid aromatic molecular system.

J-Aggregated Dyes for Dye-Sensitized Solar Cells: Suppressing Aggregation-Induced Excited-State Quenching by Intramolecular Charge Separation

J-Aggregated Dyes for Dye-Sensitized Solar Cells: Suppressing Aggregation-Induced Excited-State Quenching by Intramolecular Charge Separation

Physicochemical properties of selected compounds used as ultraviolet filters in cosmetics

A ultraviolet filter is defined as a substance, mixture, or material that blocks or absorbs radiation in the UV range, and its key purpose is protect the skin from the harmful effects of the radiation. Consumer concern is mainly regarding organic filters and the frequent problems with their low photo-stability, which is highly undesirable in terms of safety. The aim of this work was to discuss the term “photo-stability” and to assess the safety of the three most popular compounds classified as ultraviolet filters. Cited studies and issued opinions by Scientific Committees on Customer Safety confirm the safety for human health of Avobenzone, Oxybenzone and Octinoxate in strictly defined maximum concentrations. Side effects of these filters, like accumulation in the bloodstream, may occur after prolonged use. In order to improve the level of photo-stability of Avobenzone and Octinoxate the following methods are used: the use of photo-stabilizers and excited state quenchers, encapsulation of the molecule in micelles or the addition of antioxidants such as vitamin C. Although these compounds do not pose a serious threat to human health, two of them (Oxybenzone and Octinoxate) have a negative impact on aquatic ecosystems.

Efficiency Enhancement of a Photocatalytic Decarbonylation of an Aminocyclopropenone by Benzothiophene Substitution.

To improve the efficiency of the photocatalytic decarbonylation of cyclopropenones, the effects of substituents on cyclopropenone were explored. A benzothiophene-substituted aminocyclopropenone exhibited significantly improved decarbonylation efficiency to produce the corresponding ynamine, which worked as a potent dehydration condensation agent. The benzothiophene derivative was applicable to the photocatalytic reaction in the presence of potential excited-state quenchers such as oxygen and anilines. The high catalyst sensitivity would be attributed to the involvement of triplet energy transfer reaction pathway, which was not observed in the reaction with previously reported aminocyclopropenones.

Unifying Mechanism for Thiol-Induced Photoswitching and Photostability of Cyanine Dyes.

Cyanines (Cy3, Cy5, Cy3B) are the most utilized dyes for single-molecule fluorescence and localization-based super-resolution imaging. These modalities exploit cyanines' versatile photochemical behavior with thiols. A mechanism reconciling seemingly divergent results and enabling control over cyanine photoreactivity is however missing. Utilizing single-molecule fluorescence on Cy5 and Cy5B, transient-absorption spectroscopy, and DFT modeling on a range of cyanine dyes, herein we show that photoinduced electron transfer (PeT) from a thiolate to Cy in their triplet excited state and then triplet-to-singlet intersystem crossing in the nascent geminate radical pair are crucial steps. Next, a bifurcation occurs, yielding either back electron transfer and regeneration of ground state Cy, required for photostabilization, or Cy-thiol adduct formation, necessary for super-resolution microscopy. Cy regeneration via photoinduced thiol elimination is favored by adduct absorption spectra broadening. Elimination is also shown to occur through an acid-catalyzed reaction. Overall, our work provides a roadmap for designing fluorophores, photoswitching agents, and triplet excited state quenchers for single-molecule and super-resolution imaging.

Enhanced Operational Durability of Thermally Activated Delayed Fluorescence-Based Organic Light-Emitting Diodes with a Triazine Electron Transporter.

In organic light-emitting diodes (OLEDs) based on materials that show thermally activated delayed fluorescence (TADF), the internal quantum efficiency of 100 % can be obtained without using phosphorescence-based organometallics that contain rare metals. Therefore, with TADF-based emitters, it is possible to fabricate high-performing OLEDs at a lower cost. However, compared with fluorescence- and phosphorescence-based OLEDs, an understanding of degradation mechanisms in TADF-based OLEDs is still insufficient for future commercialization. In particular, it is widely recognized that the development of electron transport materials is crucial for improving OLED characteristics, especially driving voltages and operational durability. In this study, it was demonstrated that the operational durability of TADF-based OLEDs was greatly improved by introducing a triazine-based material of 2,4,6-tris(1,1'-biphenyl-4-yl)-[1,3,5]triazine (pT2T) as a hole-blocking layer (HBL) compared with a conventional HBL material of 2,4,6-tris(biphenyl-3-yl)-[1,3,5]triazine (T2T). Several experiments were carried out to make the reasons of the improved durability clearer, and attributed the improved durability to the shift of a carrier recombination zone from the emitting layer/HBL interface and the suppressed formation of excited-state quenchers in the pT2T HBL, because of the higher electron mobility of pT2T and the better stability of its radical anion state.

Excited-state stability of quasi-two-dimensional metal halide perovskite films under optical and electrical excitations

Metal halide perovskites are promising as the emitter for efficient light-emitting diodes (LEDs). However, the operational stability of perovskite LEDs (Pe-LEDs) is still very low and, therefore, needs to be improved for future practical applications through the clarification of their basic degradation mechanisms. In this study, we investigated the stability of quasi-two-dimensional (q2D) perovskite films under three different conditions, i.e., carrier injection only, photoexcitation only, and photoexcitation in combination with carrier injection. As a result of the aforementioned comparison, we found that excited states (Wannier excitons) formed in q2D perovskite films by photoexcitation are relatively unstable. On the other hand, no degradation of q2D perovskite films was observed when electrons or holes were injected without the formation of excited states. The unstable excited states were associated with the change from the q2D phase to the three-dimensional phase and the formation of metallic lead working as an excited-state quencher in films. Furthermore, the excited-state stability became worse when the excited states and the charge carriers coexisted in films. This would be one of the reasons for the low stability of q2D Pe-LEDs under continuous electrical operation.

Photophysics and photochemistry of porphyrin core PAMAM dendrimers. Excited states interaction with quinones

Quenching of excited states of the meso-substituted tetraphenyl porphyrins with PAMAM branches, and one with PAMAM generation 1 peripherally-modified with thiazole groups by p-benzoquinones, were studied in DMF solution. Fluorescence lifetime measurements evidence a static component in the quenching process. The dynamic quenching rate constants were close to the diffusional limit except for the case of duroquinone. Apparent association constants of the porphyrin compounds with the quinones were determined by comparison of static and dynamic measurements of fluorescence quenching. The ground state interaction is very weak with the exception of tetrachlorobenzoquinone. The triplet state decay kinetics and the transient absorption spectra in the presence of the quinones were determined by laser flash photolysis. Triplet quenching rate constants were lower than those of singlet quenching. An electron transfer mechanism was proposed to explain the results. The long-time transient absorption spectra were assigned to the radical cation of the dyes. Relative radical yields were determined and a dependence on the side chains and the quencher was observed.

Heat and weather resistance of copolymer organic glasses

The parameters of light resistance, heat resistance and water absorption of organic glasses based on methyl methacrylate copolymers with various methacrylic monomers containing UV absorbers and UV stabilizers are investigated. It has been shown that UV stabilizers - quenchers of excited states: diphenyl and isomers of terphenyl have the best stabilizing eff ect. A decrease in water absorption of copolymer organic glasses with the introduction of cyclic fragments into the copolymer was established. It was shown that the introduction of methyl methacrylate - methacrylic acid into the system as a comonomer of cyclohexylmaleimide allows increasing the heat resistance of organic glasses.

Excited-State Quenching of Porphyrins by Hydrogen-Bonded Phenol-Pyridine Pair: Evidence of Proton-Coupled Electron Transfer

A series of porphyrins containing methoxy-substituted phenols were treated with different pyridine bases. Besides hydrogen bonding (H-bonding), the pyridine bases have imparted oxidation to the phe...

Tris -N-Nitrilotriacetic Acid Fluorophore as a Self-Healing Dye for Single-Molecule Fluorescence Imaging.

The photostability of fluorescent labels comprises one of the main limitations in single-molecule fluorescence (SMF) and super-resolution imaging. An attractive strategy to increase the photostability of organic fluorophores relies on their coupling to photostabilizers, e.g., triplet excited state quenchers, rendering self-healing dyes. Herein we report the self-healing properties of trisNTA-Alexa647 fluorophores (NTA, N-nitrilotriacetic acid). Primarily designed to specifically label biomolecules containing an oligohistidine tag, we hypothesized that the increased effective concentration of Ni(II) triplet state quenchers would lead to their improved photostability. We evaluated photon output, survival time, and photon count rate of different Alexa647-labeled trisNTA constructs differing in the length and rigidity of the fluorophore- trisNTA linker. Maximum photon output enhancements of 25-fold versus Alexa647-DNA were recorded for a short tetraproline linker, superseding the solution based photostabilization by Ni(II). Steady-state and time-resolved studies illustrate that trisNTA self-healing role is associated with a dynamic excited triplet state quenching by Ni(II). Here improved photophysical/photochemical properties require for a judicious choice of linker length and rigidity, and in turn a balance between rapid dynamic triplet excited state quenching versus dynamic/static singlet excited state quenching. TrisNTA fluorophores offer superior properties for SMF allowing specific labeling and increased photostability, making them ideal candidates for extended single-molecule imaging techniques.

Targeting NRF2 for Improved Skin Barrier Function and Photoprotection: Focus on the Achiote-Derived Apocarotenoid Bixin.

The transcription factor NRF2 (nuclear factor-E2-related factor 2) orchestrates major cellular defense mechanisms including phase-II detoxification, inflammatory signaling, DNA repair, and antioxidant response. Recent studies strongly suggest a protective role of NRF2-mediated gene expression in the suppression of cutaneous photodamage induced by solar UV (ultraviolet) radiation. The apocarotenoid bixin, a Food and Drug Administration (FDA)-approved natural food colorant (referred to as ‘annatto’) originates from the seeds of the achiote tree native to tropical America, consumed by humans since ancient times. Use of achiote preparations for skin protection against environmental insult and for enhanced wound healing has long been documented. We have recently reported that (i) bixin is a potent canonical activator of the NRF2-dependent cytoprotective response in human skin keratinocytes; that (ii) systemic administration of bixin activates NRF2 with protective effects against solar UV-induced skin damage; and that (iii) bixin-induced suppression of photodamage is observable in Nrf2+/+ but not in Nrf2−/− SKH-1 mice confirming the NRF2-dependence of bixin-induced antioxidant and anti-inflammatory effects. In addition, bixin displays molecular activities as sacrificial antioxidant, excited state quencher, PPAR (peroxisome proliferator-activated receptor) α/γ agonist, and TLR (Toll-like receptor) 4/NFκB (nuclear factor kappa-light-chain-enhancer of activated B cells) antagonist, all of which might be relevant to the enhancement of skin barrier function and environmental stress protection. Potential skin photoprotection and photochemoprevention benefits provided by topical application or dietary consumption of this ethno-pharmacologically validated phytochemical originating from the Americas deserves further preclinical and clinical examination.

Isotope Fractionation Associated with the Photochemical Dechlorination of Chloroanilines.

Isotope fractionation associated with the photochemical transformation of organic contaminants is not well understood and can arise not only from bond cleavage reactions but also from photophysical processes. In this work, we investigated the photolytic dechlorination of 2-Cl- and 3-Cl-aniline to aminophenols to obtain insights into the impact of the substituent position on the apparent (13)C and (15)N kinetic isotope effects (AKIEs). Laboratory experiments were performed in aerated aqueous solutions at an irradiation wavelength of 254 nm over the pH range 2.0 to 7.0 in the absence and presence of Cs(+) used as an excited singlet state quencher. Photolysis of 2-Cl-anilinium cations exhibits normal C and inverse N isotope fractionation, while neutral 2-Cl-aniline species shows inverse C and normal N isotope fractionation. In contrast, the photolysis of 3-Cl-aniline was almost insensitive to C isotope composition and the moderate N isotope fractionation points to rate-limiting photophysical processes. (13)C- and (15)N-AKIE-values of 2-Cl-aniline decreased in the presence of Cs(+), whereas those for 3-Cl-aniline were not systematically affected by Cs(+). Our current and previous work illustrates that photolytic dechlorinations of 2-Cl-, 3-Cl-, and 4-Cl-aniline isomers are each accompanied by distinctly different and highly variable C and N isotope fractionation due to spin selective isotope effects.

Distance Dependence of Electron Spin Polarization during Photophysical Quenching of Excited Naphthalene by TEMPO Radical.

Quenching of excited states by a free radical is generally studied in systems where these two are separate entities freely moving in a liquid solution. Random diffusive encounters bring them together to cause the quenching and leave the spins of the radical polarized. In the dynamics of the radical-triplet pair mechanism of the generation of electron spin polarization (ESP), the distance-dependent exchange interaction plays a crucial role. To investigate how the distance between the excited molecule and the radical influences the ESP, we have covalently linked a naphthalene moiety to a TEMPO free radical through a spacer group of three different lengths. We compared the ESP process of these linked compounds with that of the usual "unlinked system" of naphthalene and TEMPO through time-resolved EPR experiments at low temperature in n-hexane solution. The time evolution of both the linked and the "unlinked system" was treated on a similar footing. The time-dependent EPR signal was analyzed by combining photophysical kinetics and time-dependent Bloch equations incorporating spin dynamics. Sequential quenching of the singlet state and the triplet state of naphthalene was seen in all the systems, as revealed through the spin-polarized TREPR spectra of opposite phase. The magnitudes of the ESP in the linked molecules were higher than those of the "unlinked system," showing that when the two moieties are held together greater mixing of quartet-doublet states takes place. The magnitudes of ESP steadily decrease with increasing the length of the spacer group. The polarization magnitudes due to triplet quenching and singlet quenching are very similar, differing by a factor of only ∼2. These characteristics show that for all the linked molecules the quenching takes place in the "weak exchange" regime and at almost the same distance of separation between the two moieties. Our results also showed that observation of small absorptive TREPR signals does not necessarily imply that its magnitude of polarization is small.

Amine Functionalization via Oxidative Photoredox Catalysis: Methodology Development and Complex Molecule Synthesis.

ConspectusWhile the use of visible light to drive chemical reactivity is of high importance to the development of environmentally benign chemical transformations, the concomitant use of a stoichiometric electron donor or acceptor is often required to steer the desired redox behavior of these systems. The low-cost and ubiquity of tertiary amine bases has led to their widespread use as reductive additives in photoredox catalysis. Early use of trialkylamines in this context was focused on their role as reductive excited state quenchers of the photocatalyst, which in turn provides a more highly reducing catalytic intermediate.In this Account, we discuss some of the observations and thought processes that have led from our use of amines as reductive additives to their use as complex substrates and intermediates for natural product synthesis. Early attempts by our group to construct key carbon–carbon bonds via free-radical intermediates led to the observation that some trialkylamines readily behave as efficient hydrogen atom donors under redox-active photochemical conditions. In the wake of in-depth mechanistic studies published in the 1970s, 1980s and 1990s, this understanding has in turn allowed for a systematic approach to the design of a number of photochemical methodologies through rational tuning of the amine component. Minimization of the C–H donicity of the amine additive was found to promote desired C–C bond formation in a number of contexts, and subsequent elucidation of the amine’s redox fate has sparked a reevaluation of the amine’s role from that of reagent to that of substrate.The reactivity of tertiary amines in these photochemical systems is complex, and allows for a number of mechanistic possibilities that are not necessarily mutually exclusive. A variety of combinations of single-electron oxidation, C–H abstraction, deprotonation, and β-scission result in the formation of reactive intermediates such as α-amino radicals and iminium ions. These processes have been explored in depth in the photochemical literature and have resulted in a firm mechanistic grasp of the behavior of amine radical cations in fundamental systems. Harnessing the synthetic potential of these transient species represents an ongoing challenge for the controlled functionalization of amine substrates, because these mechanistic possibilities may result in undesired byproduct formation or substrate decomposition. The presence of tertiary amines in numerous alkaloids, pharmaceuticals, and agrochemicals lends credence to the potential utility of this chemistry in natural product synthesis, and herein we will discuss how these transformations might be controlled for synthetic purposes.

Quenching of diphenylmethyl radical fluorescence by cyanoaromatics and phenols

We have used double-laser nanosecond laser flash photolysis to study the kinetics of bimolecular quenching of the lowest doublet excited state of the transient diphenylmethyl radical (2Ph2CH·) in acetonitrile at 25 °C. We chose a series of cyanoaromatic compounds as acceptor-type quenchers and a series of 4-substituted phenols as donor-type quenchers. The observed bimolecular quenching rate constants (k Q) are in the range 2 × 106–2 × 1010 M −1 s−1 for cyanoaromatics and 1 × 105–3 × 109 M −1 s−1 for phenols. The Hammett plots of the k Q values for these two series have positive and negative slopes, respectively, and establish the enhanced behavior of the photo-excited diphenylmethyl radical as both a donor and an acceptor. For cyanoaromatics, the trend in the dependence of 2Ph2CH·* quenching rate constants on the free energy change of electron transfer (ΔG el) coincides well with that of Rehm–Weller data on acceptor-type and donor-type quenchers of singlet excited states. An approximate fit of the k Q data for cyanoaromatics to the Agmon–Levine model of electron transfer gave a value of 4.5 kcal mol−1 for the free energy of activation at ΔG el = 0. For phenols, the trend in the dependence of k Q on the free energy change of electron transfer deviates significantly from that of cyanoaromatics data. This deviation was traced to the use of irreversible oxidation potentials of phenols to calculate ΔG el. No isotope effect is observed from substitution of phenolic hydrogen by deuterium, i.e., k Q,H/k Q,D ≈ 1. Phenolate ions quench 2Ph2CH·* with large rate constants, 1–2 × 1010 M −1 s−1, i.e., well in the limit of diffusion control.

Cy3 photoprotection mediated by Ni2+ for extended single-molecule imaging: old tricks for new techniques.

The photostability of reporter fluorophores in single-molecule fluorescence imaging is of paramount importance, as it dictates the amount of relevant information that may be acquired before photobleaching occurs. Quenchers of triplet excited states are thus required to minimize blinking and sensitization of singlet oxygen. Through a combination of single-molecule studies and ensemble mechanistic studies including laser flash photolysis and time-resolved fluorescence, we demonstrate herein that Ni(2+) provides a much desired physical route (chemically inert) to quench the triplet excited state of Cy3, the most ubiquitous green emissive dye utilized in single-molecule studies.

Photodegradation and photostabilization of polymers, especially polystyrene: review.

Exposure to ultraviolet (UV) radiation may cause the significant degradation of many materials. UV radiation causes photooxidative degradation which results in breaking of the polymer chains, produces free radical and reduces the molecular weight, causing deterioration of mechanical properties and leading to useless materials, after an unpredictable time. Polystyrene (PS), one of the most important material in the modern plastic industry, has been used all over the world, due to its excellent physical properties and low-cost. When polystyrene is subjected to UV irradiation in the presence of air, it undergoes a rapid yellowing and a gradual embrittlement. The mechanism of PS photolysis in the solid state (film) depends on the mobility of free radicals in the polymer matrix and their bimolecular recombination. Free hydrogen radicals diffuse very easily through the polymer matrix and combine in pairs or abstract hydrogen atoms from polymer molecule. Phenyl radical has limited mobility. They may abstract hydrogen from the near surrounding or combine with a polymer radical or with hydrogen radicals. Almost all synthetic polymers require stabilization against adverse environmental effects. It is necessary to find a means to reduce or prevent damage induced by environmental components such as heat, light or oxygen. The photostabilization of polymers may be achieved in many ways. The following stabilizing systems have been developed, which depend on the action of stabilizer: (1) light screeners, (2) UV absorbers, (3) excited-state quenchers, (4) peroxide decomposers, and (5) free radical scavengers; of these, it is generally believed that excited-state quenchers, peroxide decomposers, and free radical scavengers are the most effective. Research into degradation and ageing of polymers is extremely intensive and new materials are being synthesized with a pre-programmed lifetime. New stabilizers are becoming commercially available although their modes of action are sometimes not thoroughly elucidated. They target the many possible ways of polymer degradation: thermolysis, thermooxidation, photolysis, photooxidation, radiolysis etc. With the goal to increase lifetime of a particular polymeric material, two aspects of degradation are of particular importance: Storage conditions, and Addition of appropriate stabilizers. A profound knowledge of degradation mechanisms is needed to achieve the goal.Electronic supplementary materialThe online version of this article (doi:10.1186/2193-1801-2-398) contains supplementary material, which is available to authorized users.