Irradiation cycles, repeating

The entire photolysis sequence can be reversed readily by H2, as summarized in Scheme 1 (27). A sealed, degassed benzene solution of [IrClH2(PPh3)3], from which the photoreleased H3 is not allowed to escape, can be cycled repeatedly (>50 cycles) through the photoinduced H2 elimination-thermal H2 addition reactions without any observable loss of complex. Elimination of H2 can be induced by irradiation with X < 400 nm. The quantum yield of elimination, measured at 254 nm by monitoring the growth of the 449-nm band of [IrCl(PPh3)3], is 0.56 0.03. 

Figure 17d shows the switching behavior of the LB film of APT(8-6), which is very different from the two cases above. The conductivity increases with the trans-to-cis isomerization of azobenzene as in the case of APT(8-12), and the increase in conductivity is about 6% in the photostationary state. The conductivity stays, however, unchanged with the cis-to-trans isomerization. Further irradiation with UV light causes the conductivity to show another increase. This stepwise behavior is observed about 30 times. The increment per cycle decreases with an increasing number of cycles and the conductivity tends to become constant in a cycle repeated more than 30 times. 

The photoinduced expansion of thin films of polymers (4) containing azobenzene chromophores was explored in real time by single wavelength ellipsometry (Fig. 3.6 Tanchak and Barret, 2005). The initial expansion of the azobenzene polymer films of thickness ranging from 25 to 140 nm was irreversible and amounted to 1.5% %. Subsequently, reversible expansion was observed with repeated irradiation cycles the relative expansion was 0.6%-1.6%. 

Fig. 8 also illustrates another feature of difference spectroscopy. The out-of-phase signal at the centre of the control spectrum comes from the residual CHCI3, folded in from outside the spectral window. The difference artifact from this signal is worst in the spectra at the centre of Fig. 8. The experiments were performed in the usual way, by doing a series of on-resonance irradiations (in this case 8) followed by one off-resonance control irradiation, and repeating the whole cycle several times for signal averaging. It is apparent that the further away in time the spectrum is from the control, the worse the difference artifact.

If the reaction is incomplete after the first irradiation, a residual heat of reaction will be detected on the second irradiation. A good practice is to repeat irradiation cycles until two cycles are obtained that are indistinguishable from each other. This same methodology can be used to ascertain the completeness of cure of samples cured in laboratory and production processes, although additional measurement of Tg as an indicator of conversion is recommended. 

The Irradiated component Is then deposited Into a bucket, the component transferred out of the reactor vessel and a new component transferred Into the vessel. The new component Is then accepted by the charge machine and the cycle repeated. 

Chain reaction (Section 5.3) A reaction that., once initiated, sustains itself in an endlessly repeating cycle of propagation steps. The radical chlorination of alkanes is an example of a chain reaction that is initiated by irradiation with light and then continues in a series of propagation steps. 

Based on encouraging results from an early pilot study completed by Rush-Presbyte-rian-St. Luke s Medical Center, ECOG evaluated previously irradiated patients who presented with locally recurrent and/or metastatic disease (81). Patients received seven cycles of cisplatin (60 mg/m2, d 1), continuous infusion 5-FU (800 mg/m2, d 1-5), and standard daily radiotherapy (d 1-5) to be repeated every other week. All patients developed grade 2/3 mucositis. Clinically, the ORR was 74% (48% CR, 24% PR). Median TTP was 5 mo. Of interest were three patients that remained disease-free at 44 mo (T2N0), 86 mo (T0N2), and 88 mo (T2N0). 

There are a number of 1,2-dihydroquinolines that are known to ring open when irradiated at low temperature. For example, irradiation of (74) at -196 °C in an EPA matrix results in the development of a pink color attributed to (75). Warming to room temperature causes reversion to (74). This cycle can be repeated several times with high conversions (72JCS(P2)17). The appropriate choice of substituents can result in transformations that have potential synthetic utility (equations 24 and 25) (69JA6513,73CC922). 

Irradiation Studies in Solution. The degradation studies on both PIPTBK and PMIPK in solution were carried out in an automatic UV irradiation-viscometer apparatus that has been described elsewhere (10). Each solution was irradiated at 313 nm for a fixed time period and then automatically transferred to a viscometer, where nine repetitive measurements of the efflux time were performed. The solution was then returned to the UV cell for further irradiation, and the cycle was repeated seven times. The intrinsic viscosity of... 

The reactions were carried out in an Erlenmeyer flask, employing a conventional domestic microwave oven. To limit the presence of solvent vapours inside the cavity, the reaction mixtures were irradiated for 1 min and the deprotection reaction was subsequently monitored by thin layer chromatography (TLC). If starting material was present, additional cycles of 1 min were repeated until no traces of starting material could be detected by TLC. 

The maximum degree of photoconversion to the cis isomer (85%) is achieved by irradiating at 350-370 nm, whereas the maximum yield of the back-reaction from the cis to the trans isomer (80%) is achieved by irradiating at 450 nm. Using a 200 W lamp, irradiation for 1 or 2 minutes is enough to achieve the photostationary state. The thermal decay in the dark is much slower. At room temperature, it takes more than 200 hours to restore the fully trans isomeric composition. The photochromic cycles are completely reversible and can be repeated at will, without any apparent fatigue 21,22 ... 

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