Ultraviolet regions photosensitization

When a reverse bias of 2-5 V is applied to the devices with polymer-C o blends, the external photosensitivity is 0.2-0.3 AAV and the external quantum yield is 50-80% el/ph. Because polymer photodiodes exhibit a relatively flat response over a broad spectral range, they are especially suitable as detectors for spectroscopic applications. They are particularly advantageous compared to silicon for detecting light in the blue and ultraviolet regions of the spectrum. At 370 nm the photosensitivity is approximately 0.3 AAV for a polymer photodiode and 0.05 AAV for a UV-enhanced silicon photodiode [69]. 

Photosensitizers are agents that sensitize cells to radiation in the visible and near ultraviolet region of the radiation spectrum. Therapeutic use has been made of naturally occurring psoralens to treat vitiligo and psoriasis, in so-called photodynamic therapy. With newly developed UV irradiation systems that emit high-intensity UVA radiation, the principle has been extended to the treatment of severe psoriasis, mycosis fungoides and many other skin diseases. Agents used in psoralen photochemistry (PUVA) medicine include methoxypsoralen (8-MOP), trioxysalen and other synthetic psoralens. 

Photosensitization. The chief absorption bands for onium salts I-III lie below 230 nm in the ultraviolet region. Photolysis at longer wavelengths is, therefore, ineffective. However, the photolysis of these compounds can be readily photosensitized to respond to long wavelength uv and to visible light. The mechanism of photosensitization involves an electron transfer process... 

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. 

Crivello et al., described the synflieses of 5-arylthianthrenium salts that form efficient triarylsulfonium salt photoinitiators for cationic polymerization. Through the use of electron-transfer photosensitizers, the response of these photoinitiators can be spectrally broadened to the long wavelength ultraviolet-visible light regions of the spectrum. The results obtained suggest that. 5-arylthianthrenium salts are potential replacements for now available triarysulfonium salt photoinitiators in many applications. 

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