Photosensitizers absorption spectra

Figure 4.1 shows atypical absorption spectrum ([Ru(bpy)3]2+, bpy = 2,2 -bipyridine), which is the prototype inorganic photosensitizer and sensor. Note in particular the intense blue-ultraviolet (UV) absorptions and the wavelength independence of the luminescence efficiency. In the blue region, a 10 M solution would absorb over 90% of the light in a 1-cm path length. 

One of the first applications of this chopped-beam irradiation technitriplet spectra was reported by Labhart From a knowledge of the intensity of the irradiation light, he determined the quantum yield of triplet generation to be 0.55 0.11 for outgassed solutions of 1,2-benzanthrazene in hexane at room temperature. Hunziker 32) has applied this method to the study of the gas-phase absorption spectrum of triplet naphthalene. A gas mixture of 500 torr Na, 0.3 mtorr Hg, and about 10 mtorr naphthalene was irradiated by a modulated low-pressure mercury lamp. The mercury vapor in the cell efficiently absorbed the line spectrum of the lamp and acted as a photosensitizer. The triplet state of naphthalene was formed directly through collisional deactivation of the excited mercury atoms. 

Porphyrin [125-128] and phthalocyanine [129] derivatives are also employed as photosensitizers in DSSCs. A nanocrystalline Ti02 solar cell sensitized by Cu chlorophyllin produced 2.6% efficiency (./ . = 9.4 mA/cm2 and Voc = 0.52 V) under 100 mW/cm2 [127]. In order to develop new, efficient, metal dye photosensitizers, both an increase in the absorption coefficient of the metal complex and a greater red shift of the absorption spectrum is required. 

Figure 7 Photosensitized reduction of oxazine dye using humic acid as sensitizer. The transient absorption spectrum was recorded following 532-nm laser pulse excitation of humic acid containing the dye and 1102.

The usefulness of action spectra can readily be seen in Figure 4. In these plots, it is readily demonstrated that the absorption spectrum is not a good indicator of the photosensitive wavelengths of nifedipine. These plots also demonstrate the... 

The photosensitivity was unexpected as there is negligible overlap of the drug A absorption spectrum and either the ICH visible or UV lamp outputs (Fig. 3, bottom). Furthermore, the molecular components of the matrix (Table 1) are also nonlight absorbing in the 300 to 700 nm exposure regions. 

A 68-year-old man had a photosensitive drug eruption while taking flutamide (24). The minimal erjhhema dose with ultraviolet A light was reduced to 2 J/cm and recovered to over 16 J/cm after withdrawal, without changing reactivity to ultraviolet B. The absorption spectrum of flutamide was not altered after ultraviolet A irradiation. 

Singlet oxygen generation is used in photodynamic tumor therapy—for instance, with porphyrins as photosensitizers. (Cf. Dougherty, 1992.) The disadvantage of natural porphyrins such as hematoporphyrin is that they are of low chemical stability and have an absorption spectrum similar to that of... 

Degradable Plastics are not inherently biodegradable but, for the most part, are slightly modified conventional resins which are intentionally made more sensitive to the erythemal (sunburn) region of the sun s spectrum. One common approach to the required modification involves the addition of a photosensitive additive, hopefully with just the right UV absorption spectrum and subsequent excited state reaction mechanisms to produce early embrittlement. 

Figure 5. Observed photosensitized absorption in dye aggregate-viologen monolayer system compared with literature spectrum of the radical produced from one-electron reduction of dimethylviologen (36).

Before starting a photochemical reaction the absorption spectrum of the "photoactive" compound(s) should be recorded. The "photoactive compound is the material which should be electronically excited either to a singlet or a triplet state. This compound could further undergo (i) unimolecular transformations, (ii) bimolecular reactions with a ground-state substrate molecule, (iii) bimolecular reactions with another electronically excited molecule (a rare case), (iv) serve as a photosensitizer, i.e. activate another substrate... 

The action spectrum for any specific photosensitized reaction would normally overlay the absorption spectrum of the sensitizer (Grossweiner, 1989). The erythemal response (sunburn) spectrum in Figure 2.4 is an example. 

The absorption property is a first indication of a possible photochemical process leading to decomposition or other changes in a drug formulation. It is also an in vitro estimation of the drug s potential to photosensitize. However, in most cases, the absorption spectrum will not reveal the presence of other compounds in the preparation that can influence the photoreactivity of the drug. Such compounds can be impurities from the synthesis or (photo)degradation products formed during the... 

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