Photochemistry sensitization

The photochemistry of Cp(arene)Fe + complexes has been extensively examined since the discovery of photochemical arene loss by Nesmeyanov, and this class of compounds has received considerable attention as a cationic photocatalyst for the polymerization of epoxides, dicyanate esters, and styrene among others. As in the case of the bis(arene)metal complexes described above, photolysis is believed to initiate an r] - transformation whereupon solvent enters the open site and thermally displaces the arene ligand. Consistent with this the quantum yields indicate a significant associative component in the ligand displacement. Time-resolved studies indicate that the initial photochemical event occurs on the fs or ps scale. There has been substantial confusion concerning the role of singlet and triplet excited states in the observed photochemistry. Sensitization studies established a... 

N.M. Scully, W.F. Vincent, D.R.S. Lean, W.J. Cooper (1997). Implications of ozone depletion for surface-water photochemistry Sensitivity of clear lakes. Aquat. Sci., 59, 260-274. 

Lasers can be coupled efficiently to fiber optic devices to deHver intense monochromatic light precisely to the desired region of the body, including internal organs (see Fiber optics). As in other cases of laser-induced photochemistry, biphotonic effects may be important (87). Lasers also offer the advantage of being able to concentrate the incident energy in a spectral bandpass matched to the absorption band of the sensitizer. 

While di-i-butyl (34) and dicumyl hyponitrites (35) have proved convenient sources of Tbutoxy and cumyloxy radicals respectively in the laboratory,71 72 115"117 the utilization of hyponitrites as initiators of polymerization has been limited by difficulties in synthesis and commercial availability. Dialkyl hyponitrites (16) show only weak absorption at A>290 ntn and their photochemistry is largely a neglected area. The triplet sensitized decomposition of these materials has been investigated by Mendenhall et a .11 s... 

There is a multiplicity of pathways for thermal dediazoniations. An analogous situation is to be expected for photochemical dediazoniations. Based on the general experience that light-sensitive reactions often involve free radical intermediates, it was commonly assumed that all photolytic dediazoniations are free radical reactions. Horner and Stohr s results (1952), mentioned above, could lead to such a conclusion. More sophisticated methods of photochemistry also began to be applied to investigations on arenediazonium salts, e. g., the study of photolyses by irradiation at an absorption maximum of the diazonium ion using broad-band or monochromatic radiation. This technique was advocated by Sukigahara and Kikuchi (1967 a, 1967 b,... 

However, with the advent of the superacid solvents, a multitude of long-lived stable carbonium ions have been made available for extended study. Clearly, then, the door is open to quantum 5ueld determinations, kinetic treatment, studies on the effects of various solvents and sensitizers and quenching experiments (compare the very recent study by Bethell and Clare, 1972). In short, the photochemistry of carbonium ions is still in its infancy and there is a wealth of information yet to be gained. 

McEvoy AJ, Gratzel M (1994) Sensitization in photochemistry and photovoltaics. Sol Energy Mater Sol Cells 32 221-227... 

The photochemistry of the polynucleotides has been elucidated primarily by studies of the photochemical behavior of the individual pyrimidine and purine bases (the ribose and phosphate groups would not be expected to undergo photochemical reactions in this wavelength range). These studies have shown the pyrimidines (cytosine and thymine) to be roughly ten times more sensitive to UV than the purines (adenine and guanine.) Thus we would expect most of the photochemistry of the nucleic acids to result from the action of light on the pyrimidines. 

Merocyanine Dye Method for Acid Analysis. Resist photochemistry can often be monitored by the changes in ultraviolet absorption spectra associated with a bleaching of the sensitizer absorbance. In the case of resist systems with triphenylsulfonium salts, no change in the film absorption is observed on irradiation. In order to determine the amount of acid produced, a direct method for acid analysis was required. A highly sensitive method was desirable since the amount of acid produced is approximately 10 6 mmol for a 1 micrometer thick film on a 2 inch wafer. Furthermore a nonaqueous technique is preferred in order to avoid hydrolysis of the hexafiuoroantimonate salt. Hydrolysis gives hydrogen fluoride (14) which makes accurate acid determination more difficult. 

Milanesio ME, Alvarez MG, Rivarola V, Silber JJ, Durantini EN (2005) Porphyrin-fullerene C-60 dyads with high ability to form photoinduced charge-separated state as novel sensitizers for photodynamic therapy. Photochemistry and Photobiology 81 891-897. 

Rancan F, Helmreich M, Molich A, Jux N, Hirsch A, Roder B, Witt C, Bohm F (2005) Fullerene-pyropheophorbide a complexes as sensitizer for photodynamic therapy uptake and photo-induced cytotoxicity on Jurkat cells. Journal of Photochemistry and Photobiology B Biology 80 1-7. 

Several aromatic molecules undergo no efficient photochemistry of their own. Thus, these molecules are well suited for use as electron transfer sensitizers. According to the substrate and the conditions chosen, they may form either complexes (or tight radical ion pairs) or free, solvated radical ions (Figure 3.9). 

Photolysis of N-trimethylborazine with H2 yields a crystalline product, identified as l,2-di(3, 5 -dimethylborazinyl)ethane The following radical mechanism, involving Hg-sensitization is used to explain the photochemistry... 

Previous studies of the photochemistry of alkylchlorodiazirines have shown that the yield of trappable carbene is sensitive to the alkylcarbene structure. A laser flash photolysis study of phenanthridenes (91), precursors of alkylchlorocarbenes, in the presence of pyridine, has ruled out the intermediacy of a carbene-pyridine complex which partitions between pyridine-ylide formation and [1,2]-H shift. ... 

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