Room Light Conditions Research Articles

Photocatalytic paper using zinc oxide nanorods

Zinc oxide (ZnO) nanorods were grown on a paper support prepared from soft wood pulp. The photocatalytic activity of a sheet of paper with ZnO nanorods embedded in its porous matrix has been studied. ZnO nanorods were firmly attached to cellulose fibers and the photocatalytic paper samples were reused several times with nominal decrease in efficiency. Photodegradation of up to 93% was observed for methylene blue in the presence of paper filled with ZnO nanorods upon irradiation with visible light at 963 Wm–2 for 120 min. Under similar conditions, photodegradation of approximately 35% was observed for methyl orange. Antibacterial tests revealed that the photocatalytic paper inhibits the growth of Escherichia coli under room lighting conditions.

Bright light effects on ultradian rhythms in performance on hemisphere-specific tasks

Ultradian rhythms in indices of brain hemisphere activity and in cognitive performance have been found in numerous studies. Asymmetry of these rhythms with regard to phase and frequency have also been documented in some studies. There is some evidence that bright light can affect ultradian rhythms of arousal state and vigilance. A study on unilateral exposure to bright light has demonstrated more pronounced effects of bright light on the right hemisphere. The aim of this study was to examine whether daytime intermittent bright light could affect parameters of ultradian rhythms in performance speed on hemisphere-specific tasks, and whether the effect of bright light was symmetric for the rhythms in performance on hemisphere-specific tasks presented laterally. A counter-balanced, within-subject research design was applied. The performance of 15 participants on hemisphere-specific tasks exposed laterally was measured every 30 min starting at 08:00 h and ending at 20:30 h in intermittent bright light (IBL, pulses of 15 min of 4000 lux light regularly interspersed between 45 min of background light levels of 300 lux) and in ordinary room light (ORL) conditions (300 lux). Individual time series data were subjected to cosinor analysis. General linear model analyses (the factors were: level of processing, visual field, and the task) were performed on the rhythms’ parameters. There was a substantial lengthening of the rhythms’ periods in IBL conditions for performance speed on spatial tasks and an increase in amplitude of the rhythms of performance speed for spatial tasks in both visual fields and for verbal tasks in the left visual field in the IBL conditions when compared to the ORL conditions. The results showed that the schedule of light exposure affected ultradian rhythms of hemisphere-specific tasks differently and that the right hemisphere seems to be more “sensitive” to light than the left hemisphere.

Influence of light and time on bilirubin degradation in CSF spectrophotometry for subarachnoid haemorrhage

Aim. Investigation of the influence of light on bilirubin degradation over time in cerebrospinal fluid (CSF) specimens from patients with computed tomography (CT) positive subarachnoid haemorrhage (SAH).Methods. Twenty-nine CSF samples were analysed from 23 patients with CT-positive SAH. Samples were divided into two cohorts – one half being stored in the dark, the other in normal room light conditions. Samples were assayed by spectrophotometry 0, 1, 2, 6, 19, 24 and 48 h post-storage. Net bilirubin absorbance (NBA) was measured in absorbance units (AU) by determining the optical density at 476 nm.Results. In 27 of 28 (96%) samples stored in light conditions and 21 of 29 (72%) samples stored in dark, a decline in the amount of bilirubin detected in CSF over time was observed. The rate of bilirubin degradation over 24 h for CSF samples stored in the light ranged from 0.0001 to 0.0048 AU/h (mean 0.0020 AU/h), and was significantly greater (p < 0.01) than the rate of degradation in the dark, which ranged from 0 to 0.0024 AU/h (mean 0.0005 AU/h).Conclusion. CSF bilirubin samples are susceptible to the same photodegradation as shown in serum bilirubin samples. The rate of degradation is not predictable or consistent. This photodegradation may potentially produce false-negative results in the diagnosis of SAH. If spectrophotometry cannot be carried out immediately, all such CSF samples should at least be centrifuged immediately, the supernatant protected from light as soon as possible and the collection and analysis times provided in the report.

Night‐time neuronal activation of Cluster N in a day‐ and night‐migrating songbird

Magnetic compass orientation in a night-migratory songbird requires that Cluster N, a cluster of forebrain regions, is functional. Cluster N, which receives input from the eyes via the thalamofugal pathway, shows high neuronal activity in night-migrants performing magnetic compass-guided behaviour at night, whereas no activation is observed during the day, and covering up the birds’ eyes strongly reduces neuronal activation. These findings suggest that Cluster N processes light-dependent magnetic compass information in night-migrating songbirds. The aim of this study was to test if Cluster N is active during daytime migration. We used behavioural molecular mapping based on ZENK activation to investigate if Cluster N is active in the meadow pipit (Anthus pratensis), a day- and night-migratory species. We found that Cluster N of meadow pipits shows high neuronal activity under dim-light at night, but not under full room-light conditions during the day. These data suggest that, in day- and night-migratory meadow pipits, the light-dependent magnetic compass, which requires an active Cluster N, may only be used during night-time, whereas another magnetosensory mechanism and/or other reference system(s), like the sun or polarized light, may be used as primary orientation cues during the day.

TH-B-204B-01: Advanced Optical Imaging and Tracking Techniques for IGRT

Optical guidance (OG) such as optical distance indicator (ODI) in‐room laser alignment and live‐video camera monitoring has been routinely used in conventional radiotherapy for patient setup and monitoring. Now emerging OG using optical tracking and topologic imaging (stereovision) techniques can provide remote operators with real‐time measurements of patient body (or support apparatus) position shape and movement. The temporal and spatial OG will be an essential component in any advanced image‐guidedradiotherapy(IGRT) when patient or organ motion are considered during the dose delivery. In assisting users to become familiar with the new OG techniques this education session will start with an introduction on basic photogrammetry general camera calibration triangulation principle for 3D optical imagingsurface extraction and registration and what OG systems can and cannot do. Then the session will move to the clinical use of infrared patient positioning and tracking systems (IPT) with particular attention to the combined use with other imagingsystems detection and correction with six degrees of freedom for rigid body displacement and issues in attachment of the infrared reflection markers. That will lead up to actual use ISG integrated stereovision‐guided patient reposition and monitoring systems focusing on quantification of the body position and displacement prior to and during beam or arc irradiation. ISG adaptive radiotherapy of the brain H&N lung and breast cancer will be demonstrated using examples from early clinical trials. The effects of target motion and deformation skin tones and room light conditions are also discussed. Learning Objectives: 1. Understand the spatial and temporal characteristics of advanced OG 2. Understand the basic operation principle of 3D optical systems 3. Know how to check and calibrate the optical camerasystems 4. Understand why multiple infrared reflection markers have to be fastened to the patient 5. Know how to quantify and correct the displacements with six‐degrees of freedom 6. Know how to surrogate optical surfaceimages with CT/MR volumetric images 7. Know how to acquire reliable surfaceimages prior to and during X‐ray irradiation 8. Understand the limits and benefits in application of OG in clinical settings

M1493: Steerable Ultrathin Cholangioscope Provides Wide-Field Trimodal Laser Imaging

Peroral cholangioscopy requires a long and torturous approach to reach the biliary ducts and is limited by poor video quality and lack of imaging modes. By using laser-light scanning, three different imaging modes can be provided in one endoscope: (1) high-resolution wide-field color imaging, (2) enhanced spectral imaging (ESI) of the same 90° field, and (2) fluorescence imaging of the same field. Low-power red, green, and blue (RGB) laser light is combined to a single optical fiber and scanned at the tip of a new cholangioscope to produce 500-line images at 30 Hz. ESI generates higher contrast of blood vessels and helps to discriminate tissue types by enhancing blue over red backscattered light. Fluorescence imaging is generated by exciting the tissue at laser wavelengths (B-442 nm, G-532 nm, and/or R-635 nm) while detecting fluorescence in a longer wavelength channel, respectively (G, R, or near infrared). Currently, ESI imaging is simultaneous with RGB-color imaging, while fluorescence video imaging is generated with a 2-second switch.A prototype ultrathin (1.7-mm diameter, 5 Fr) cholangioscope has been developed with thumbstick control of tip deflection. The forward-viewing cholangioscope extends 2 feet beyond a side-viewing duodenoscope (PENTAX ED-3470TK). At the tip of this cholangioscope, a rigid portion extends beyond the bend mechanism by less than 10 mm, which houses the microscanner and lenses. After a full 180° of bending in the cholangioscope shaft, the range of tip deflection is +/-30° in any direction. Once positioned, the cholangioscope can be used as a guidewire for inserting cannulas and biopsy tools.In vitro testing of the cholangioscope used a synthetic (silicone) casting of a biliary tree model (3 branches from 8 to 2 mm inner diameter) which extends a commercial ERCP trainer (Koken, Japan). Along the luminal wall of the synthetic bile duct were placed (a) rolled paper target with fluorescence and non-fluorescence painted markings, (b) rolled paper target with dried markings of methylene blue, and (c) fluorescence microspheres. Targets were imaged in air with RGB/ESI and fluorescence contrast under attenuated room lighting conditions. No averaging was required to unambiguously visualize the targets, although this method may be used to improve image contrast of weak autofluorescence signals. Future development will allow all three modes of operation concurrently at video rates with appropriate fluorescence biomarkers. Thus, the physician will have a vast arsenal of visual information for analyzing indeterminate biliary strictures and guiding biopsy and other interventions in the pancreatobiliary system.

Automated Landmark Extraction for Orthodontic Measurement of Faces Using the 3-Camera Photogrammetry Methodology

To set up a three-dimensional photogrammetric scanning system for precise landmark measurements, without any physical contact, using a low-cost and noninvasive digital photogrammetric solution, for supporting several necessity in clinical orthodontics and/or surgery diagnosis. Thirty coded targets were directly applied onto the subject's face on the soft tissue landmarks, and then, 3 simultaneous photos were acquired using photogrammetry, at room light conditions. For comparison, a dummy head was digitized both with a photogrammetric technique and with the laser scanner Minolta Vivid 910i (Konica Minolta, Tokyo, Japan). The precise measurement of the landmarks is ranged between 0.017 and 0.029 mm. The system automatically measures spatial position of face landmarks, from which distances and angles can be obtained. The facial measurements were compared with those done using laser scanning and manual caliper. The adopted method gives higher precision than the others (0.022-mm mean value on points and 0.038-mm mean value on linear distances on a dummy head), is simple, and can be used easily as a standard routine. The study demonstrated the validity of photogrammetry for accurate digitization of human face landmarks. This research points out the potential of this low-cost photogrammetry approach for medical digitization.

Limited Roles of Rdh8, Rdh12, and Abca4 in all-trans-Retinal Clearance in Mouse Retina

Although the retinoid cycle is essential for vision, all-trans-retinal and the side products of this cycle are toxic. Delayed clearance of all-trans-retinal causes accumulation of its condensation products, A2E, and all-trans-retinal dimer (RALdi), both associated with human macular degeneration. The protective roles were examined of the all-trans-RDHs, Rdh8 and Rdh12, and the ATP-binding cassette transporter Abca4, retinoid cycle enzymes involved in all-trans-retinal clearance. Mice genetically engineered to lack Rdh8, Rdh12, and Abca4, either singly or in various combinations, were investigated because all-trans-retinal clearance is achieved by all-trans-RDHs and Abca4. Knockout mice were evaluated by spectral-domain optical coherence tomography (SD-OCT), electroretinography, retinal morphology, and visual retinoid profiling with HPLC and MS. ARPE19 cells were examined to evaluate A2E and RALdi oxidation and toxicity induced by exposure to UV and blue light. Rdh8(-/-)Abca4(-/-) and Rdh8(-/-)Rdh12(-/-)Abca4(-/-) mice displayed slowly progressive, severe retinal degeneration under room light conditions. Intense light-induced acute retinal degeneration was detected by SD-OCT in Rdh8(-/-)Rdh12(-/-)Abca4(-/-) mice. Amounts of A2E in the RPE correlated with diminished all-trans-retinal clearance, and the highest A2E amounts were found in Rdh8(-/-)Rdh12(-/-)Abca4(-/-) mice. However oxidized A2E was not found in any of these mice, and A2E oxidation was not induced by blue light and UV illumination of A2E-loaded ARPE19 cells. Of interest, addition of all-trans-retinal did activate retinoic acid receptors in cultured cells. Rdh8, Rdh12, and Abca4 all protect the retina and reduce A2E production by facilitating all-trans-retinal clearance. Delayed all-trans-retinal clearance contributes more than A2E oxidation to light-induced cellular toxicity.

The formation of light emitting cerium silicates in cerium-doped silicon oxides

Cerium-doped silicon oxides with cerium concentrations of up to 0.9 at. % were deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition. Bright cerium related photoluminescence, easily seen even under room lighting conditions, was observed from the films and found to be sensitive to film composition and annealing temperature. The film containing 0.9 at. % Ce subjected to anneal in N2 at 1200 °C for 3 h showed the most intense cerium-related emission, easily visible under bright room lighting conditions. This is attributed to the formation of cerium silicate [Ce2Si2O7 or Ce4.667 (SiO4)3O], the presence of which was confirmed by high resolution transmission electron microscopy.

Development of Flexible and Transparent Upconversion Display by Using Polymer-Ceramics Composite

Fabrication and characterization of a flexible and transparent upconversion display made of a composites of YF3: Er3+/Yb3+ nanoparticles and polymer matrices. To decrease the light scattering loss, the refractive indices of them were adjusted between the particles and polymers in visible wavelength range. The composites were prepared by an UV polymerization of a monomer mixture dispersed with the particles. The obtained materials are flexible and highly transparent as a transmittance of 80 % for a sample with 1 mm thickness at 589 nm. The fabricated flexible and transparent composite sheets showed green upconversion luminescence under a 980-nm excitation. A luminescent graphic on the bent surface of the display drawn by scanning an IR pump laser was clear and bright enough to be visible in ambient room lighting conditions.

On the Portability of Computer-Generated Presentations: The Effect of Text-Background Color Combinations on Text Legibility

The aim of our study was to investigate which text-background (TB) color combinations improve legibility and pleasantness of texts either presented on a computer screen or projected on a wider surface. Our work stems from the observation that multimedia presentations, even those by professionals in visual disciplines, are often spoiled by illegibility, which affects readability and the pleasantness of attending to such presentations. We investigated this problem through three experiments. Experiment 1 assessed the best TB color combinations in slides presented on a laptop. Experiment 2 verified the correlation between legibility and pleasantness for TB presentations. Experiment 3 investigated the legibility of a slide projected on a wide screen in different room lighting conditions. In all experiments participants expressed a rating score for stimulus legibility or pleasantness. Experiments 1 and 3 showed that it is best to have a dark text on a light background for both displayed and projected texts. Experiment 2 showed that pleasantness is tightly correlated with legibility, though legibility depends on contrast between text and background. Our findings are not in complete agreement with the literature concerning legibility of text displayed on a computer screen, and they are in contrast with the common belief that for projection purposes it is best to have a light text on a dark background. Some practical rules on combining TB colors are given to enhance the legibility of presentations, especially important for the legibility of projected texts.

Efficacy of brimonidine tartrate 0.2% ophthalmic solution in reducing halos after laser in situ keratomileusis

To quantitatively evaluate the effect of brimonidine tartrate 0.2% (Alphagan) on halo and pupil size in patients who had symptomatic night-vision difficulties after laser in situ keratomileusis (LASIK). Nune Eye Hospital, Seoul, Korea. This study comprised 28 eyes of 14 patients with symptomatic night-vision difficulties after LASIK. Pupil diameter was measured with a Colvard pupillometer (Oasis Medical, Inc.). Quantitative analysis of halos was performed by measuring the area using a new computerized method. Pupil size and halo size were evaluated under scotopic and normal room light conditions. Alphagan was administered, and the effect was measured after 30 minutes and 1, 6, 12, and 24 hours. There was a statistically significant correlation between pupil size and halo size (r = 0.527; P<.0001; slope = 691.6 pixel/mm). Pupil size and halo size decreased significantly 30 minutes after Alphagan instillation under both luminance conditions (all P< .0001). Under normal room light, the pupil and halo remained decreased until the last measurement at 24 hours. Under scotopic conditions, the pupil returned to its preinstillation size at 24 hours while the halo remained decreased. The maximum effect on halos was observed after 6 hours, when the mean reduction over preinstillation size was 28.2% and 29.1% under normal room light conditions and scotopic conditions, respectively. Alphagan effectively reduced halo size and pupil size in postoperative LASIK patients with night-vision symptoms.

Light Emission from Rare-Earth Doped Silicon Nanostructures

Rare earth (Tb or Ce)-doped silicon oxides were deposited by electron cyclotron resonance plasma-enhanced chemical vapour deposition (ECR-PECVD). Silicon nanocrystals (Si-ncs) were formed in the silicon-rich films during certain annealing processes. Photoluminescence (PL) properties of the films were found to be highly dependent on the deposition parameters and annealing conditions. We propose that the presence of a novel sensitizer in the Tb-doped oxygen-rich films is responsible for the indirect excitation of the Tb emission, while in the Tb-doped silicon-rich films the Tb emission is excited by the Si-ncs through an exciton-mediated energy transfer. In the Ce-doped oxygen-rich films, an abrupt increase of the Ce emission intensity was observed after annealing at 1200∘C. This effect is tentatively attributed to the formation of Ce silicate. In the Ce-doped silicon-rich films, the Ce emission was absent at annealing temperatures lower than 1100∘C due to the strong absorption of Si-ncs. Optimal film compositions and annealing conditions for maximizing the PL intensities of the rare earths in the films have been determined. The light emissions from these films were very bright and can be easily observed even under room lighting conditions.

Daytime Intermittent Bright Light Effects on Processing of Laterally Exposed Stimuli, Mood, and Light Perception

Little is known about intermittent bright light (IBL) exposure during the daytime on oscillations in human performance and mood. The aims of this study were to determine whether the applied regime of IBL can differentially influence the daily oscillations of processing of laterally exposed stimuli, as well as the daily course of alertness and affective state, and the participants' perception of light conditions. A counter‐balanced, within‐subject study design was applied. Performance and mood of 20 student volunteers were measured every 30 min starting at 08:00 h and ending at 20:30 h in IBL and ordinary room light (ORL) conditions. Near to statistical significance, effects of the IBL regime on performance (i.e., main effect on speed of verbal logical task, and interactive effect of light conditions and measurement time on daily oscillations in speed of processing of spatial tasks) and mood (i.e., interactive effect of bright light and measurement time on global vigor assessment) were found. An assessment of IBL conditions as significantly less comfortable and natural and more glaring when compared to ORL conditions draws attention to the importance of participants' perception of the light, which may affect the application of bright light in real life situations.

Natural preference in luminosity for frame composition

We investigated whether or not frame composition spontaneously selected by individuals exhibit a natural left shift in preference of luminosity, as previously suggested, by examining biases in luminosity imprinted in photographs taken under three different conditions: (i) daytime pictures taken outside without determination of frame composition (N=4153); (ii) daytime pictures taken outside with determination of frame composition (N=4035); and (iii) pictures taken inside with determination of frame composition (N=4045). As anticipated, there was a strong vertical gradient of luminance grayscale intensity, reflecting natural day light distribution, for pictures of categories (i) and (ii). Similarly, a minor vertical gradient was detected for pictures of category (iii), indicating common room lighting conditions. To our great interest, there was a consistent leftward tilt of ca. 2 and 9 degrees found in the vertical gradients for pictures of categories (ii) and (iii), respectively, but not for pictures of category (i). The result provides clear-cut evidence that there is indeed a natural preference for a leftward bias in luminosity for frame composition.

Investigations of combinations of mutations in the jellyfish green fluorescent protein (GFP) that afford brighter fluorescence, and use of a version (VisGreen) in plant, bacterial, and animal cells

Among the GFPs used for imaging green fluorescence, the Emerald version has been considered the best GFP to use but there is no formal report on its construction or the relevance of the amino acid (aa) substitutions in it relative to the commonly used GFPs. Here, we have shown that a version of Emerald makes Escherichia coli host cells visibly green even under dim room light conditions. Exploiting this feature, we have determined for the first time whether the changes in the structure of Emerald protein brought about by the aa substitutions are all indeed essential for brightness. F64L and S72A accompanying the classical S65T substitution on the chromophore-bearing helix are essential. Two amino acid changes, one on the surface (N149K) of the β barrel that encases the helix and the other (I167T) near the chromophore enhance the visible green colour individually and additively when present together. The other two substitutions, M153T (on the surface) and H231L (on the surface), do not contribute to the visible green phenotype, even though in earlier studies M153T has been reported to enhance GFP fluorescence. The GFP version with F64L–S65T–S72A–N149K–I167T is referred to as VisGreen. We found VisGreen and Emerald to be indistinguishable in their quantum yield, molar extinction coefficient, folding efficiency, or photosensitivity. VisGreen rendered bacterial, plant, and animal cells highly fluorescent. Interestingly, N149K in the above combination was not essential to render bacterial cells highly fluorescent.

Intra- and interobserver agreement and performance score of breast phantom image interpretation: influence of ambient room lighting levels

The influence of ambient room lighting conditions on soft-copy breast phantom image interpretation was evaluated by comparing cathode ray tube (CRT) monitors with liquid crystal displays (LCDs). Nine observers were asked to use a three-point scale to rate the visibility of various phantom objects (masses, specks, and fibers) displayed on a 21-inch CRT (2,560 x 2,048) and a 21-inch LCD (2,560 x 2,048) under three different levels of ambient lighting (20, 100 and 420 lux at the display center). Each phantom image was interpreted twice, and the reproducibility of judgment and inter-observer agreement was evaluated using kappa statistics. Except for the "mass" score, the LCD score showed a significantly higher value (p<0.05) compared with that of CRT. Nevertheless, no significant differences were found among the three lighting levels. Furthermore, intra- and inter-observer agreement in judgments showed no effects of room illumination. Although the breast phantom objects were better visualized on LCDs than on CRT monitors, room illumination did not affect the performance score of soft-copy reading.

Low light and low ammonium are key factors for guayule leaf tissue shoot organogenesis and transformation

A new method has been developed for guayule tissue culture and transformation. Guayule leaf explants have a poor survival rate when placed on normal MS medium and under normal culture room light conditions. Low light and low ammonium treatment greatly improved shoot organogenesis and transformation from leaf tissues. Using this method, a 35S promoter driven BAR gene and an ubiquitin-3 promoter driven GUS gene (with intron) have been successfully introduced into guayule. These transgenic guayule plants were resistant to the herbicide ammonium-glufosinate and were positive to GUS staining. Molecular analysis showed the expected band and signal in all GUS positive transformants. The transformation efficiency with glufosinate selection ranged from 3 to 6%. Transformation with a pBIN19-based plasmid containing a NPTII gene and then selection with kanamycin also works well using this method. The ratio of kanamycin-resistant calli to total starting explants reached 50% in some experiments.

Etoposide-loaded nanoparticles made from glyceride lipids: Formulation, characterization, in vitro drug release, and stability evaluation

The aim of the study was to prepare etoposide-loaded nanoparticles with glyceride lipids and then characterize and evaluate the in vitro steric stability and drug release characteristics and stability. The nanoparticles were prepared by melt emulsification and homogenization followed by spray drying of nanodispersion. Spray drying created powder nanoparticles with excellent redispersibility and a minimal increase in particle size (20-40 nm). Experimental variables, such as homogenization pressure, number of homogenization cycles, and surfactant concentration, showed a profound influence on the particle size and distribution. Spray drying of Poloxamer 407-stabilized nanodispersion lead to the formation of matrix-like structures surrounding the nanoparticles, resulting in particle growth. The in vitro steric stability test revealed that the lipid nanoparticles stabilized by sodium tauroglycocholate exhibit excellent steric stability compared with Poloxamer 407. All 3 glyceride nanoparticle formulations exhibited sustained release characteristics, and the release pattern followed the Higuchi equation. The spray-dried lipid nanoparticles stored in black polypropylene containers exhibited excellent long-term stability at 25 degrees C and room light conditions. Such stable lipid nanoparticles with in vitro steric stability can be a beneficial delivery system for intravenous administration as long circulating carriers for controlled and targeted drug delivery.

Substituent Effects on the Electrochemistry and Photostability of Model Compounds of Calcium Channel Antagonist Drugs

We have synthesized three nitrocompounds in order to mimic the model structure of some relevant drugs such as nifedipine, nitrendipine, nisoldipine, nimodipine, and related ones, in order to investigate the effect of structural change on its electrochemical and photodegradation properties. We have found that ortho substitution with a nitro group produces a distortion of the coplanar arrangement, thus decreasing the resonance interaction between the nitro group and the aromatic system. A consequence of this fact is the observed cathodic peak potential shift toward negative values in the ortho-substituted isomer. It is observed that the position of the nitro substitution also produced differences in the photostability of the studied compounds. Stability studies of the compounds in buffered solutions (pH 7) stored in room light conditions show that only the ortho derivative was unstable in the time scale of the experiment (2 h). The instability of the ortho derivative may be explained if we consider that in this isomer, the photoexcited state of the nitro group is not stabilized by resonance as a consequence of the loss of coplanarity between the nitro group and the aromatic ring. Using molecular modeling, we have found that the configuration of minimal energy shows torsion angles of 70, 7.5, and 2.2° for ortho- meta- and para-derivatives, respectively, confirming the lost of coplanarity between the nitro group and the aromatic ring in the ortho derivative. © 2001 The Electrochemical Society. All rights reserved.