Subsequent Photobleaching Research Articles

Adaptive photoluminescence through a bioinspired antioxidative mechanism.

Transition metal complexes are archetypal luminescent probes that are widely used for various applications ranging from optoelectronics to biomedicine. However, they face significant challenges such as photobleaching and photooxidative stress, which limit their performance. Herein, we introduce a photosystem-inspired concept based on the use of a vitamin (ascorbic acid, Asc-Ac) to adaptively suppress photobleaching of molecular luminophores. As a proof-of-concept compound, we have selected a new bis-cyclometalated Pt(II) complex (Pt-tBu) and investigated its adaptive photoluminescence resulting from singlet dioxygen (1O2) photoproduction in the presence of Asc-Ac. Interestingly, the excited state quenching and subsequent photobleaching of Pt-tBu in aerated solutions is suppressed by addition of Asc-Ac, which scavenges the 1O2 photosensitized by Pt-tBu upon irradiation and results in an adaptive oxygen depletion with enhancement of luminescence. The adaptation is resilient for successive irradiation cycles with oxygen replenishment, until peroxidation overshooting leads to the degradation of Pt-tBu by formation of a dark Pt(iv) species. The complexity-related adaptation with initial overperformance (luminescence boost) relies on the external energy input and cascaded feedback loops, thus biomimicking inflammation, as the repeated exposure to a stressor leads to a final breakdown. Our antioxidative protection mechanism against photobleaching can be successfully extended to multiple coordination compounds (e.g., Ir(iii), Ru(ii) and Re(i) complexes), thus demonstrating its generality. Our findings broaden the scope of molecular adaptation and pave the way for enhancing the stability of molecular luminophores for multiple applications.

Photobleaching-induced changes in the optical and photochemical properties of algal organic matter.

Algal organic matter (AOM), a significant source of endogenous dissolved organic matter (DOM) is released in high concentrations during cyanobacterial blooms, along with cyanotoxins. Subsequent photobleaching of AOM is an important phenomenon to investigate. In this study, intracellular organic matter (IOM) and extracellular organic matter (EOM) were extracted from cultured cyanobacteria taken from Taihu Lake in China. The formation of photochemically produced reactive intermediates in different stages of IOM and EOM photobleaching was compared to Suwannee River DOM (SRDOM, reference standard DOM). Results revealed notable differences influenced by the pigment component among IOM, EOM, and SRDOM. The pigment in IOM contributed to a triplet state pool with strong energy-transfer but limited electron-transfer capabilities. Notably, IOM exhibited the highest triplets state quantum yield value in the visible region, suggesting its potential significance in pollutant degradation in deeper water layers. For EOM, one of the pools exhibits photolability and remarkable electron-transfer capability, indicating it as a high-energy triplet state component. Moreover, three cyanotoxins (MC-LR, ACA, and ATX-a) were detected in the extracted AOM, and their photodegradation was monitored during the AOM photobleaching process. This highlights the potential role of AOM as a photosensitizer in the natural self-cleaning mechanisms of water bodies, facilitating the degradation of organic pollutants through photochemical reactions. The findings of this study contribute to understanding the dynamic nature of AOM and its implications in environmental processes.

Ratiometric Two-Photon Near-Infrared Probe to Detect DPP IV in Human Plasma, Living Cells, Human Tissues, and Whole Organisms Using Zebrafish.

DPP IV, otherwise known as CD26 lymphocyte T surface antigen, is a transmembrane glycoprotein also found in circulation in the blood. It plays an important role in several processes like glucose metabolism and T-cell stimulation. Moreover, it is overexpressed in renal, colon, prostate, and thyroid human carcinoma tissues. It can also serve as a diagnostic in patients with lysosomal storage diseases. The biological and clinical importance of having readouts for the activity of this enzyme, in physiological and disease conditions, has led us to design a near-infrared (NIR) fluorimetric probe that also has the characteristics of being ratiometric and excitable by two simultaneous NIR photons. The probe consists of assembling an enzyme recognition group (Gly-Pro) (Mentlein, 1999; Klemann et al., 2016) on the two-photon (TP) fluorophore (derivative of dicyanomethylene-4H-pyran, DCM-NH2) disturbing its NIR characteristic internal charge transfer (ICT) emission spectrum. When the dipeptide group is released by the DPP IV-specific enzymatic action, the donor-acceptor DCM-NH2 is restored, forming a system that shows high ratiometric fluorescence output. With this new probe, we have been able to detect, quickly and efficiently, the enzymatic activity of DPP IV in living cells, human tissues, and whole organisms, using zebrafish. In addition, due to the possibility of being excited by two photons, we can avoid the autofluorescence and subsequent photobleaching that the raw plasma has when it is excited by visible light, achieving detection of the activity of DPP IV in that medium without interference.

Accelerated weathering of Compreg with phenol-formaldehyde resin treated top veneers of European white oak (Quercus ssp., L.) and teak (Tectonia grandis, L.F.)

The weather resistance of Compreg surfaces depends on various factors, such as top veneer type, impregnating resin, and protective topcoats. Artificial weathering tests were performed for the first time on material types with oak and teak top veneer surfaces. Both showed strong surface brightening due to resin degradation and subsequent photobleaching of the exposed veneer surface. Directly pressed, phenolic resin-impregnated cellulose paper overlays caused significant surface stabilization.

Evolution of Aerosol Optical Properties from Wood Smoke in Real Atmosphere Influenced by Burning Phase and Solar Radiation.

Emissions of light-absorbing black carbon (BC) and organic aerosol (OA) from biomass burning are presented as complex mixtures, which introduce challenges in modeling their absorbing properties. In this study, we chose typical residential wood burning emission and used a novel designed chamber to investigate the early stage evolution of plumes from different burning phases under real ambient conditions. The detailed mixing state between BC and OA was evaluated, on the basis of which optical modeling was performed to achieve a closure of aerosol-absorbing properties. Intensive secondary OA (SOA) formation was observed under solar radiation. OA from flaming conditions showed a higher absorptivity than from smoldering conditions, as OA is mostly internally and externally mixed with BC, respectively. For flaming (smoldering), the imaginary refractive index of OA (kOA) was initially at 0.03 ± 0.01 (0.001) and 0.15 ± 0.02 (0.05 ± 0.02) at λ = 781 and 405 nm, respectively, with a half-decay time of 2-3 h in light but a <40% decrease under dark within 5 h. The production of less-absorbing SOA in the first 1-2 h and possible subsequent photobleaching of chromophores contributed to the decrease of kOA. The enhanced abundance but decreased absorptivity of coatings on BC resulted in a relatively maintainable absorptivity of BC-containing particles during evolution.

Regression Analysis of Protoporphyrin IX Measurements Obtained During Dermatological Photodynamic Therapy.

Photodynamic therapy (PDT) is a light activated drug therapy that can be used to treat a number of dermatological cancers and precancers. Improvement of efficacy is required to widen its application. Clinical protoporphyrin IX (PpIX) fluorescence data were obtained using a pre-validated, non-invasive imaging system during routine methyl aminolevulinate (MAL)-PDT treatment of 172 patients with licensed dermatological indications (37.2% actinic keratosis, 27.3% superficial basal cell carcinoma and 35.5% Bowen’s disease). Linear and logistic regressions were employed to model any relationships between variables that may have affected PpIX accumulation and/or PpIX photobleaching during irradiation and thus clinical outcome at three months. Patient age was found to be associated with lower PpIX accumulation/photobleaching, however only a reduction in PpIX photobleaching appeared to consistently adversely affect treatment efficacy. Clinical clearance was reduced in lesions located on the limbs, hands and feet with lower PpIX accumulation and subsequent photobleaching adversely affecting the outcome achieved. If air cooling pain relief was employed during light irradiation, PpIX photobleaching was lower and this resulted in an approximate three-fold reduction in the likelihood of achieving clinical clearance. PpIX photobleaching during the first treatment was concluded to be an excellent predictor of clinical outcome across all lesion types.

Design and validation of an open-source modular Microplate Photoirradiation System for high-throughput photobiology experiments.

Research in photobiology is currently limited by a lack of devices capable of delivering precise and tunable irradiation to cells in a high-throughput format. This limits researchers to using expensive commercially available or custom-built light sources which make it difficult to replicate, standardize, optimize, and scale experiments. Here we present an open-source Microplate Photoirradiation System (MPS) developed to enable high-throughput light experiments in standard 96 and 24-well microplates for a variety of applications in photobiology research. This open-source system features 96 independently controlled LEDs (4 LEDs per well in 24-well), Wi-Fi connected control and programmable graphical user interface (GUI) for control and programming, automated calibration GUI, and modular control and LED boards for maximum flexibility. A web-based GUI generates light program files containing irradiation parameters for groups of LEDs. These parameters are then uploaded wirelessly, stored and used on the MPS to run photoirradiation experiments inside any incubator. A rapid and semi-quantitative porphyrin metabolism assay was also developed to validate the system in wild-type fibroblasts. Protoporphyrin IX (PpIX) fluorescence accumulation was induced by incubation with 5-aminolevulinic acid (ALA), a photosensitization method leveraged clinically to destroy malignant cell types in a process termed photodynamic therapy (PDT), and cells were irradiated with 405nm light with varying irradiance, duration and pulsation parameters. Immediately after light treatment with the MPS, subsequent photobleaching was measured in live, adherent cells in both 96-well and a 24-well microplates using a microplate reader. Results demonstrate the utility and reliability of the Microplate Photoirradiation System to irradiate cells with precise irradiance and timing parameters in order to measure PpIx photobleaching kinetics in live adherent cells and perform comparable experiments with both 24 and 96 well microplate formats. The high-throughput capability of the MPS enabled measurement of enough irradiance conditions in a single microplate to fit PpIX fluorescence to a bioexponential decay model of photobleaching, as well as reveal a dependency of photobleaching on duty-cycle—but not frequency—in a pulsed irradiance regimen.

Wide-Field Fluorescence Microscopy of Real-Time Bioconjugation Sensing.

We apply wide-field fluorescence microscopy to measure real-time attachment of photosynthetic proteins to plasmonically active silver nanowires. The observation of this effect is enabled, on the one hand, by sensitive detection of fluorescence and, on the other hand, by plasmonic enhancement of protein fluorescence. We examined two sample configurations with substrates being a bare glass coverslip and a coverslip functionalized with a monolayer of streptavidin. The different preparation of the substrate changes the observed behavior as far as attachment of the protein is concerned as well as its subsequent photobleaching. For the latter substrate the conjugation process is measurably slower. The described method can be universally applied in studying protein-nanostructure interactions for real-time fluorescence-based sensing.

Cover Picture: Molecular Composition of Sub-stoichiometrically Labeled α-Synuclein Oligomers Determined by Single-Molecule Photobleaching (Angew. Chem. Int. Ed. 35/2012)

In amyloid diseases, monomeric proteins self-assemble into supramolecular aggregates. In their Communication on page 8821 ff. V. Subramaniam et al. describe a new method that combines single-molecule photobleaching approaches with sub-stoichiometric labeling with fluorophores to determine the molecular composition of these large protein aggregates. The cover picture shows the aggregation of combinations of labeled and unlabeled monomers and the subsequent photobleaching analysis of single aggregates.

Paxillin focal adhesions, localization and implication: insight from Photo-Activated Localization Microscopy

Photo-Activated Localization Microscopy (P.A.L.M.) as described by E. Betzig (2006) optically resolves selected subsets of photo-activatable fluorescent probes within cells at mean separations of less than 25 nanometers through serial photo-activation and subsequent photobleaching of numerous sparse subsets of photo-activated fluorescent protein molecules.The position information from all subsets is then assembled into a super-resolution image, in which individual fluorescent molecules are isolated at high molecular densities. In this work COS-7 and ST14A tdEos-Paxillin transfected cells were used. We observed some features that limit the versatility of PALM, both in this setup and in its present version. It takes actually hours to go through the cycles of photo-activation and image acquisition, to collect all of data needed and to generate a single high-resolution image limiting the use to fixed specimens which precludes PALM.'s use for imaging of live cells. More important is the loss of data. Depending on the spatial concentration of the PA-FPs, most of the information about the position of molecules is lost during the photo-activation photobleaching phase, especially during the first cycles of data collection. From the biological point of view, we observe small paxillin clusters along the focal adhesions. Supported by U54 GM064346 CMC (MD, EG), NIH-P41-RRO3155 (EG, FC), P50-GM076516 (EG).Sponsored by Enrico Gratton

Heat stress causes inhibition of the de novo synthesis of antenna proteins and photobleaching in cultured Symbiodinium

Coral bleaching, caused by heat stress, is accompanied by the light-induced loss of photosynthetic pigments in in situ symbiotic dinoflagellate algae (Symbiodinium spp.). However, the molecular mechanisms responsible for pigment loss are poorly understood. Here, we show that moderate heat stress causes photobleaching through inhibition of the de novo synthesis of intrinsic light-harvesting antennae [chlorophyll a-chlorophyll c(2)-peridinin-protein complexes (acpPC)] in cultured Symbiodinium algae and that two Clade A Symbiodinium species showing different thermal sensitivities of photobleaching also show differential sensitivity of this key protein synthesis process. Photoinhibition of photosystem II (PSII) and subsequent photobleaching were observed at temperatures of >31 degrees C in cultured Symbiodinium CS-73 cells grown at 25-34 degrees C, but not in cultures of the more thermally tolerant control Symbiodinium species OTcH-1. We found that bleaching in CS-73 is associated with loss of acpPC, which is a major antennae protein in Symbiodinium. In addition, the thermally induced loss of this protein is light-dependent, but does not coincide directly with PSII photoinhibition and is not caused by stimulated degradation of acpPC. In cells treated at 34 degrees C over 24 h, the steady-state acpPC mRNA pool was modestly reduced, by approximately 30%, whereas the corresponding synthesis rate of acpPC was diminished by >80%. Our results suggest that photobleaching in Symbiodinium is consequentially linked to the relative susceptibility of PSII to photoinhibition during thermal stress and occurs, at least partially, because of the loss of acpPC via undefined mechanism(s) that hamper the de novo synthesis of acpPC primarily at the translational processing step.

3D Nanoscopy: Bringing Biological Nanostructures into Sharp Focus

Understanding the nanometer-scale organization of living cells, tissues, and materials is an essential goal of present and future research efforts, and yet zooming into the small compartments of life remains one of the great challenges of science. Even though microscopy techniques such as electron microscopy can help us, these approaches are usually limited by the elaborate and destructive sample preparation and measurement, or by the inability to study details beyond the surface of the sample. Fluorescence microscopy, on the other hand, has proven to be one of the most convenient and widespread tools to study key issues in the life sciences, though the problems that it can address are still limited. As with any microscopy technique, there is a lower limit to the sizes of the details that can be discerned. This finite resolution is a direct consequence of the wavelike character of light, known as the diffraction limit, which is about 250 nm in optical microscopy. This means that it is not possible to discriminate between two objects that are separated by less than that distance, and thus it seems that the nanostructuring of cells and materials is not accessible by direct optical means. More specifically, (part of) the reason that the resolution is limited is because a fluorescent “point object” of negligible size (such as a single molecule) does not appear as a point after imaging through the microscope but rather appears as a three-dimensional shape of much larger dimensions (Figure 1). Because the distributions from adjacent molecules in close proximity overlap we lose the ability to distinguish between molecules that are close together, and hence to visualize very small details of the sample. Several schemes recently developed to address these limitations have succeeded in providing images with unprecedented detail. In particular, several papers appeared in the last half of 2006 that converged into a similar approach, which has become known as photoactivation–localization microscopy (PALM). PALM relies on the insight that individual, well-isolated emitters can be localized with nanometer precision simply by determining the centroid of the emission distribution (Figure 1). If we could determine the position of all the emitters in this way then we could construct an image of the sample with nanometer resolution by carefully processing the fluorescence image. Unfortunately, the obtained data is meaningful only if we are indeed looking at the emission of a single isolated emitter (Figure 1), yet if we are interested in imaging beyond the diffraction limit then our sample likely contains many fluorophores within nanometers of each other. As a result, the only way to apply this localization procedure to practical samples is to separate the emission of the fluorophores in time, so that only a single fluorophore emits within a diffraction-limited region at any instant. In PALM this is achieved by making use of photoactivation: the stochastic “on” switching of the fluorescence emission at the single-molecule level followed by subsequent photobleaching or other deactivation. Hence PALM measurements are based on repeating cycles of on/off switching events, where fluorescence can be registered only for those molecules that are in the “on” state during a particular cycle (Figure 2). Because the activation of the fluorescence is a stochastic process, different molecules will be activated in each cycle, and if these molecules are spaced sufficiently apart then we can reconstruct the high-resolution picture. The need to control the fluorescence emission of the sample, down to the single-molecule level, through photoirradiation has recently inspired many studies and developments of photoswitchable fluorescent proteins and other compounds, though the method was applied mainly to fluorescent proteins Figure 1. The effect of diffraction: the original pointlike fluorescence emitters appear as a blurred distribution after imaging through the microscope. The positions of the isolated emitters can be recovered by determining the center-of-brightness of the resulting spots (indicated by the markers), although molecules that are spaced closer together than the diffraction limit cannot be separated (the blue marker and blue circle).

A study of the radiation chemistry of poly(chlorotrifluoroethylene) by ESR spectroscopy

The ESR spectra of poly(chlorotrifluoroethylene) were recorded following γ-radiolysis under vacuum at room temperature and 77 K. The very broad spectrum at 77 K revealed little fine structure with which to identity the radicals formed upon irradiation, but subsequent photobleaching and annealing studies, together with radiolytic studies at higher temperatures, afforded scope for making radical assignments. Both main-chain radicals and a range of chain-end radicals have been identified. The G-values for radical formation were 1.55, 0.36 and 0.32 at 77 K, 273 K and room temperature, respectively.

Fluorescence correlation spectroscopy of flavins and flavoenzymes: photochemical and photophysical aspects

Fluorescence Correlation Spectroscopy (FCS) was used to investigate the excited-state properties of flavins and flavoproteins in solution at the single molecule level. Flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD) and lipoamide dehydrogenase served as model systems in which the flavin cofactor is either free in solution (FMN, FAD) or enclosed in a protein environment as prosthetic group (lipoamide dehydrogenase). Parameters such as excitation light intensity, detection time and chromophore concentration were varied in order to optimize the autocorrelation traces. Only in experiments with very low light intensity (<10 kW/cm 2), FMN and FAD displayed fluorescence properties equivalent to those found with conventional fluorescence detection methods. Due to the high triplet quantum yield of FMN, the system very soon starts to build up a population of non-fluorescent molecules, which is reflected in an apparent particle number far too low for the concentration used. Intramolecular photoreduction and subsequent photobleaching may well explain these observations. The effect of photoreduction was clearly shown by titration of FMN with ascorbic acid. While titration of FMN with the quenching agent potassium iodide at higher concentrations (> 50 mM of I −) resulted in quenched flavin fluorescence as expected, low concentrations of potassium iodide led to a net enhancement of the de-excitation rate from the triplet state, thereby improving the fluorescence signal. FCS experiments on FAD exhibited an improved photostability of FAD as compared to FMN: As a result of stacking of the adenine and flavin moieties, FAD has a considerably lower triplet quantum yield. Correlation curves of lipoamide dehydrogenase yielded correct values for the diffusion time and number of molecules at low excitation intensities. However, experiments at higher light intensities revealed a process which can be explained by photophysical relaxation or photochemical destruction of the enzyme. As the time constant of the process induced at higher light intensities resembles the diffusion time constant of free flavin, photodestruction with the concomitant release of the cofactor offers a reasonable explanation.

The use of crosslinking promoters in the ?-radiolysis of poly(tetrafluoroethylene-co-perfluoromethylvinyl ether).I

Incorporation of 1 wt % of triallyl isocyanurate (TAIC) significantly enhanced the radiation crosslinking of the perfluoroelastomer, poly(tetrafluoroethylene-co-perfluoromethylvinyl ether) (TFE/PMVE). The dose for gelation was lowered by 70% with the presence of TAIC. The additive also improved the tensile properties of TFE/ PMVE both before and after crosslinking by irradiation. Higher radical yields were obtained with the presence of TAIC at 77 K, indicating the crosslinking promoter was acting as a radical trap. ESR studies showed that radiolysis of TAIC and subsequent photobleaching cleaved an allyl branch from the ring structure. Upon thermal annealing, an allyl radical on the TAIC molecule was observed. (C) 1999 John Wiley & Sons, Inc.

The use of crosslinking promoters in the γ‐radiolysis of poly(tetrafluoroethylene‐co‐perfluoromethylvinyl ether).I

Incorporation of 1 wt % of triallyl isocyanurate (TAIC) significantly enhanced the radiation crosslinking of the perfluoroelastomer, poly(tetrafluoroethylene-co-perfluoromethylvinyl ether) (TFE/PMVE). The dose for gelation was lowered by 70% with the presence of TAIC. The additive also improved the tensile properties of TFE/PMVE both before and after crosslinking by irradiation. Higher radical yields were obtained with the presence of TAIC at 77 K, indicating the crosslinking promoter was acting as a radical trap. ESR studies showed that radiolysis of TAIC and subsequent photobleaching cleaved an allyl branch from the ring structure. Upon thermal annealing, an allyl radical on the TAIC molecule was observed. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 169–175, 1999

Protoporphyrin IX Fluorescence Photobleaching during ALA‐Mediated Photodynamic Therapy of UVB‐Induced Tumors in Hairless Mouse Skin

Fluorescence photobleaching of protoporphyrin IX (PpIX) during superficial photodynamic therapy (PDT), using 514 nm excitation, was studied in UVB-induced tumor tissue in the SKH-HR1 hairless mouse. The effects of different irradiance and light fractionation regimes upon the kinetics of photobleaching and the PDT-induced damage were examined. Results show that the rate of PpIX photobleaching (i.e., fluorescence intensity vs fluence) and the PDT damage both increase with decreasing irradiance. We have also detected the formation of fluorescent PpIX photoproducts in the tumor during PDT, although the quantity recorded is not significantly greater than generated in normal mouse skin, using the same light regime. The subsequent photobleaching of the photoproducts also occurs at a rate (vs fluence) that increases with decreasing irradiance. In the case of light fractionation, the rate of photobleaching increases upon renewed exposure after the dark period, and there is a corresponding increase in PDT damage although this increase is smaller than that observed with decreasing irradiance. The effect of fractionation is greater in UVB-induced tumor tissue than in normal tissue and the damage is enhanced when fractionation occurs at earlier time points. We observed a variation in the distribution of PDT damage over the irradiated area of the tumor: at high irradiance a ring of damage was observed around the periphery. The distribution of PDT damage became more homogeneous with both lower irradiance and the use of light fractionation. The therapeutic dose delivered during PDT, calculated from an analysis of the fluorescence photobleaching rate, shows a strong correlation with the damage induced in normal skin, with and without fractionation. The same correlation could be made with the data obtained from UVB-induced tumor tissue using a single light exposure. However, there was no such correlation when fractionation schemes were employed upon the tumor tissue.

Dynamic fluorescence changes during photodynamic therapy in vivo and in vitro of hydrophilic A1 (III) phthalocyanine tetrasulphonate and lipophilic ZN(II) phthalocyanine administered in liposomes

The fluorescence emission of hydrophilic tetrasulphonated aluminium phthalocyanine (AlPcS 4) and hydrophobic zinc phthalocyanine (ZnPc), bound to the membrane of liposomes, was investigated in vivo in an appropriate tumour model of the rat bladder and in RR 1022 epithelial cells of the rat. The sensitizers were administered systemically to the rats and photodynamic therapy (PDT) was performed 24 h later. During PDT treatment, the fluorescence was measured every 30 s. The fluorescence was excited with 633 nm light from an HeNe laser and the fluorescence spectra were detected with an optical multichannel analyser system. PDT was performed for both sensitizers using 672 nm light from an Ar + dye laser. The fluorescence changes during PDT were significantly different for the two phthalocyanines. For AlPcS 4, an initial fluorescence intensity increase, followed by subsequent photobleaching, was observed. In contrast, ZnPc fluorescence showed an exponential decrease and no increase at the start of treatment. Tumour necrosis 24 h after PDT was significant only for ZnPc. RR 1022 cells incubated for 24 h with AlPcS 4 revealed a granular fluorescence pattern, whereas ZnPc was localized diffusely in the cytoplasm of the cells. In agreement with the in vivo measurements, subcellular relocalization and a fluorescence intensity increase were detected exclusively in the case of AlPcS 4. Morphological changes at this time were significant only for ZnPc. The subcellular localization and fluorescence kinetics were obtained using a confocal laser scanning microscope.

Photobleaching of light-induced paramagnetic defects in amorphous silicon nitride films

We report for the first time the photobleaching of light-induced paramagnetic defects in nitrogen-rich hydrogenated amorphous-silicon nitride thin films. We have determined the photon energy dependence and the stretched exponential time dependence for photoproduction and the subsequent photobleaching of the light-induced electron spin resonance signal. Photobleaching is a reversible process that occurs at low temperature (100 K) and at room temperature. These findings are in support of an optically induced rearrangement of charges in existing defects as an important light-induced effect in a-SiN1.6:H.

Influence of amine structure on the post-cured photo-yellowing of novel amine diacrylate terminated ultraviolet and electron beam cured coatings

The post ultraviolet (UV) and electron beam (EB) cured photo-yellowing of nine novel amine terminated diacrylate monomers has been compared with that of standard commercial diethylamine diacrylate monomer using second order derivative UV absorption spectroscopy. Whilst all the UV cured monomers exhibited an initial rapid growth in UV absorption followed by a rapid photo-bleaching, the EB cured monomers exhibited a very slow growth in absorption followed by a plateau and subsequent slow photo-bleaching. In the former case the residual benzophenone photo-initiator is sensitising the photo-yellowing reaction and its subsequent photo-bleaching. Differences in the rates may be determined by the nature of the exciplex between the terminal amine groups and the benzophenone initiator. With regard to the nature of the amine structure all the simple alkylamines exhibit the greatest degree of photo-yellowing whilst hydroxyl containing amines are generally lower. In the former case methylene hydrogen atoms alpha to the nitrogen atom are important for abstraction. Dicyclohexylamine provides the most stable monomer toward photo-yellowing due to the stability of the alpha methylene hydrogen atoms and steric hindrance by the two bulky cyclohexane rings towards the formation of conjugated chromophores. For the EB cured monomers the degree of photo-yellowing increases with increasing alkyl chain length of the amine group due to the increased possibility of the formation of conjugated chromophores.