Photochemical side effects

Bosca, F., Marin, M.L., and Miranda, M.A. (2001) Photoreactivity of the non-steroidal antiinflammatory 2-arylpropionic acids with photosensitizing side effects, Photochem. Photobiol., 74, 237-255. 

The interaction of intense laser pulses with molecular materials is, in general, quite complex. It is customary to delineate the induced processes in three types, namely thermal, photochemical, and photomechanical. Though this formalistic division provides a convenient basis for the discussion of the mechanisms and effects of UV ablation of polymers, the three phenomena are certainly closely interrelated. The inadequacies of this division will be most clearly illustrated in the examination of the chemical effects. The present review addresses all three types of side effects, but the major emphasis concentrates on the photochemical phenomena. 

Particular emphasis is placed on the side effects of the procedures. The thermal, mechanical, and in particular the chemical effects are discussed from a fimdamental/mechanistic standpoint and are exemplified in the case of the laser-based restoration of painted artworks. Certainly, the interaction of intense laser pulses with organic substrates is highly complex. In fact, in many respects, conventional photochemical concepts would suggest that the laser procedures would be particularly damaging for molecular/organic substrates. Yet these limitations or deleterious influences have been shown, at least empirically, to be avoidable to a large extent. This is best illustrated in the laser restoration of painted artworks. Despite their chemical and structural complexity, excimer lasers can be applied successfully to restoration, and irradiation conditions can be defined under which potentially damaging side effects to the substrate are minimal or at least inconsequential. 

The synthetic methods which have been used include modem verrions of established methods for metal colloid preparation such as the mild chemical reduction of solutions transition metal salts and complexes, newer methods such as radio-lytic and photochemical reduction, metal atom extrusion from labile organo-metallics, and metal vapor synthesis techniques. While some of these reactions have been in use for many years, others have resulted from research stimulated by the current resurgence in metal colloid chemistry. The list of preparative methods is being extended daily, and as examples of these methods are described below the reader will be made quickly aware that almost any organometallic reaction whose hitherto undesirable side effect is the facile deposition of metallic precipitates is a resource for the metal colloid chemist. The acquisition of new methods is limited only by the ingenuity of the synthetic chemist in turning a previously negative result into a synthetic possibility. Examples of each of these methods will be described below. 

Alternatively, during this relatively long lifetime, the molecule can undergo an internal structural change or form new reaction products by an activated collision (photochemistry). This deactivation route is promoted by high light intensities and is used in photochemical synthesis, but it can lead to undesired side effects (e.g.. in fluorescence spectroscopy) because photochemical processes... 

The decarboxylation of carboxylic acids is a photochemical process of significance in several areas of study thus, photodecarboxylation reactions are used in synthetic chemistry to generate radical intermediates or in agriculture to produce pesticides. In the case of pharmaceuticals, this process is responsible for a decreased stability of some compounds and seems to be directly or indirectly involved in adverse photosensitizing side effects. 

The i j -configuration of the 6,7-double bond in pre-vitamin D is critical to its subsequent thermal rearrangement to the active vitamin. A photochemical isomerization of pre-vitamin D to yield the inactive trans-isoTnen occurs under conditions of synthesis, and is especially detrimental if there is a significant short wavelength component, eg, 254 nm, to the radiation continuum used to effect the synthesis. This side reaction reduces overall yield of the process and limits conversion yields to ca 60% (71). Photochemical reconversion of the inactive side product, tachysterol, to pre-vitamin D allows recovery of the product which would otherwise be lost, and improves economics of the overall process (70). 

Phosphorus-containing pesticides la 254 Phosphorus insecticides lb 83 Phosphorus pesticides lb 32 Photochemical activation lb 13 Photochemical reactions lb 15,17 Photodiodes la 24,29 Photo effect, external la 24 -, internal la 24, 29 Photo element la 24,29 Photography, exposure times la 137 -, instmmentation la 137 Photomultiplier la 25ff -, disadvantages la 27 -, energy distribution la 26 -, head on la 27 -, maximum sensitivity la 28 -, side on la 27 -, spectral sensitivity la 28 -, window material la 28 Photocells la 25 Phloxime lb 116... 

A reaction of ozone provides an example of concentration effects. Ozone in the atmosphere near the Earth s surface is a serious pollutant that damages soft tissues such as the lungs. In major urban areas, smog alerts are issued whenever there are elevated concentrations of ozone in the lower atmosphere. Nitmgen oxide, another component of photochemical smog, is a colorless gas produced in a side reaction in automobile engines. One reaction that links these species is the reaction of NO and O3 to produce O2 and NO2 ... 

Photochemically Triggered Induced Circular Dichroism in Liposomes When an optically inactive chromophore is subject to the effect of optically active environment, optical activity may be induced at the absorption wavelength of the optically inactive chromophore. This phenomenon of induced circular dichroism(ICD) is often observed in polypeptides bearing various achiral chromophores on the side chain( ). The strong chiral environment caused by the peptide helix structure is responsible for this. Distance from, and orientation to, the chiral field decide the degree of ICD appearing on the achiral chromophore. 

Provided that an optically active molecular aggregate is photochemically perturbed to change the state of molecular alignment, the effect of a chiral environment on an achiral chromophore incorporated in the molecular aggregate will be also altered. It has been known that polypeptides bearing photochromic side groups change their optically active properties as a result of photochromic reaction(10-12). This phenomenon is likely to be related to non-linear photoresponsiveness. 

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