Indicator photostability

It has been shown that photoexcitation of the guanine-cytosine (G-C) base pair leads to proton transfer [231], Watson-Crick (WC) base pairs have excited state lifetimes much shorter than other non-WC base pairs indicating once again that the natural occurring WC base pairs are more photostable than other alternative configurations [115, 118, 232-235], Much work has been done in the gas phase where many different base pair isomers exist. The ultrafast relaxation of the WC base pair has also been confirmed in solution using fluorescence up-conversion measurements [117]. 

The excitation and emission maxima of Alexa Fluor 647, 680, and 750 (the numbers indicate approximate absorption maxima in nanometers), and the molar absorptivities are in the same range as those for Cy5, Cy5.5, and Cy7, but Alexa Fluor dyes are reported to have major advantages in terms of their photostability,... 

The formation of silyl radicals in the exhaustive photolysis of the silane polymers was indicated by the isolation of disilanes of general structure (HSiRR SiRR H) as shown in Table 1. These materials accumulate in the photolysate and are photostable as they absorb only weakly at the irradiation wavelength (254 nm). Longer chain silanes are presumably continuously degraded under the conditions of the exhaustive irradiation. 

The results obtained in this work indicate that both the structure of the stabilizer and the nature of the surrounding environment are important factors in determining the efficiency of the energy dissipation processes in these derivatives. Molecular structures in which the planar form is favoured and where the intramolecular hydrogen bond is protected from interactions with the medium by the incorpoation of bulky substituent groups, should exhibit highest photostability and impart improved photoprotection to polymer substrates. 

Photodimerization behavior of 4-formyl-, 3,4-dichloro-, and several other cinnamic acid derivatives is greatly influenced by other molecules outside of the crystal (9,10). For example, 4-formylcinnamic acid 1 crystallizes in two modifications, photoreactive and photostable forms. The photoreactive crystals of 1 (mp 249 °C), on photoirradiation at room temperature in the presence of even a trace of moisture, dimerize to crystalline dimer 2 containing one molecule of water. The continuous change of the x-ray diffraction pattern during the photodimerization indicates a typical crystal-to-crystal transformation process. On the other hand, the same crystal 1 photodimerizes into amorphous dimer 2 in the absence of water. The same cyclobutane derivative is produced in very high yield in both reactions. However, highly crystalline dimer 2 is obtained only by the photodimerization of 1 in the presence of water and is not regenerated by any attempted recrystallization procedures from various aqueous solutions of 2. 

Hie dye-doped ORMOSIL materials exhibit much higher photostability than the reported polymer-doped materials. For example, optical gain was reported for several coumarin dyes in PMMA (13), It is significant to note that the best dyes reported could only be used to several hundred pulses and that most of the coumarin dyes could not even oscillate for 100 pulses. The ORMOSIL gels thus represent an improvement of at least 1-2 orders of magnitude in photostability. Optical gain was also reported for R6G doped into modified poly(methyl methacrylate) (14L These materials exhibited loss of 20% output after 180 pump pulses (1 J/cm ) at 532 nm. The reported plots indicate that these materials would have dropped by 90% after 275-300 pulses due to rapid steady state photodegradation once the pulse count exceeded the critical pulse number. In this case the R6G doped ORMOSIL gel laser offers a useful lifetime improvement by a factor of more than 15 over the reported polymer material. 

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