Bleaching kinetics

A thorough theoretical analysis of the bleaching kinetics for DMA and DPA in several polymers, using transmittance at 365 nm to avoid any complications due to possible absorbance in the medium or photoproducts, is being published elsewhere (J.R. Sheats, J. Phys. Chem., 1989). The reactions known from solution phase studies (20) lead (13) to the following rate equation for anthracene (A) (same for oxygen) ... 

Figure 19 Parent bleach kinetics (open circles) of Re2(CO)io in (a) hexanes, (b) CC14, (c) CHCI3, and (d) CH2CI2. Fits to a diffusion model to account for geminate recombination are shown as solid lines. Except for the macroscopic viscosity, identical molecular parameters (see Fig. 20) are used for all the solvents studied.

The effect of pulse excitation energy was studied on the 480-nm bleaching kinetics of 1PA2N dissolved in pure methylcyclohexane and a 5 2 mixture of methylcyclohexane-cyclohexanol. For the methylcyclohexane solution, we observed that, when the excitation energy was increased by a factor of 2 from 70 to 140 mj/cm, the decay time constant of the short component was found to decrease from 14 3 to 9 1 ps. This decrease is caused by a self-damping process similarly observed for DODCI. The short time constant was also observed to decrease from 28 5 to 22 8 ps for the 5 2 mixed solvent when the excitation energy was increased from 70 to 140 mj/cm. In addition, we found that when the concentration of the sample was increased by a factor of... 

Table 26. Spirwxazinc Bleaching Kinetics on a Polyurethane Matrix Measured after 30 minutes of Xenon Continuous Irradiation ( uv — cm 2 , — 14 klux r— 25 1 ) or during Hash-Photolysis Excitation ( , = 130J 50jlsl 7"= 331 C f...

L.l.Ji-lnmeLhylinJnHrcspirti-S-rrH.iItOt.V-fi iiLlrtihenAip rai (11) in anhydrous toluene. An increasi in temperature mulls in filler bleaching kinetics and higher fatigue ncsisianec. (Reprinted from Ref. 41 u-iih permission of the American Institute of Physics. I... 

Mesoporous tungsten oxide films were fabricated via different block copolymer templates and an ethanolic WCla solution (Cheng, 2001 Ozkan, 2002, 2003) showing improved kinetics performance over its sol-gel counterpart in protonic or lithium conducting electrolyte. The block copolymers were removed by calcination (300°C or 4(X)°C, Cheng, 2001 Ozkan, 2002) or by room temperature ultraviolet (UV) illumination/ ozone treatment method (Ozkan, 2002, 2003). llie mesoporous tungsten oxide layers show a faster coloration and bleaching kinetics, due to the faster diffusion of Ii+ or H+ ions, but the... 

Figure 40.12 Thermal bleaching kinetics of the photochromic molecules (a) in phenyl-modified matrices (SiO-Ph) and (b) in samples SiO-R7o%, where R is iBu, Ph, and Me.

Pottier, E., Du Best, R., GugHehnetti, R.J., Taridieu, P., Kellmann, A., Tfibel, E, Lenoir, P., and Aubard, J., Substituent, heteroatom and solvent effects on the thermal-bleaching kinetics and absorption spectra of photomerocyanines issued from spiro(indoline-oxazines), Helv. Chim. Acta, 73, 303, 1990 Hori, T., Tagaya, H., Nagaoka, T, Kadokawa, J., and Chia, K., Photochromism of sulfonated spiropyran in a silica matrix, Appl. Surf. ScL, 121/122, 530,1997. 

Fig.l shows the decrease of total chl and total car during illumination with high light intensities in thylakoids, D-10, and D-144. In all cases car are bleached faster than chl, as has been reported previously (Ridley, 1977). However, there are important differences with respect to the bleaching kinetics within the different fractions. In thylakoids,chl and car show a distinct lag phase of about 150 min before rapid bleaching commences whereas in D-10 only chl is retained for some 30 min, and car decreases with the onset of illumination. In D-144, chl and car are immediately degraded with the start of illumination. Fig.2 shows the corresponding PS-II activities of thylakoids and D-10. 

Table 1 indicated that the three membrane preparations possess different pigment compositions although the concentration of total chi and total car per ml sample are similar. Therefore, the different bleaching kinetics of chi and car cannot be explained with a lack of carotenoids per se but point to a specific function of each car species. 

A closer examination of the breakdown of each carotenoid in thylakoids, D-10, and D-144 (Fig.3) allows a more specific statement to be made. In thylakoids, neoxanthin violaxanthin, and 6-carotene show essentially the same bleaching kinetic with a significant lag phase whereas lutein degradation starts later and continues slower. Since both PS-II activity (Fig.2) and PS-I activity (data now shown) are inhibited before massive breakdown of pigments occurs, either the bleaching of car is not responsible for the rapid loss in activity, or only very few car and chi molecules are affected during the early phase of the experiment. From the data of Fig. 3, these specific pigments cannot be identified. 

FIGURE 3, Bleaching kinetics of carotenoids (as percent of initial) in a) thylakoids, b) D-10, and c) D-144 Pigment identification o Viola-xanthin, Neoxanthin, A Lutein, A B-Carotene. 

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