Bistable photochemical system

Between these two extreme values of incident photon flux, the behavior of the system will be more complicated. If A is concentrated, it absorbs the largest fraction of Iq, and B is little exposed to irradiation. The photochemical reaction B — C, Bc does not take place, and the system stabilizes at A. Thus, if A is concentrated at the start, it will remain concentrated in the steady state. On the other hand, if A is low, B is well exposed to irradiation, and the photochemical reaction B - C (4>BC) occurs, C then accumulates instead of A. Thus, if A is low at the start, A will remain low in the steady state. This will be reflected by the existence of two different photostationary states (A low or A concentrated) for the same value of incident photon flux Iq and the same initial global concentration. This represents a bistable photochemical system (Figure 12). 

Photochromic behaviour of salicylidene anilines incorporated in a Langmuir-Blodgettmultilayer shows that thermal decoloration is suppressed by the highly ordered densely packed environment Bistability has been observed with the triphenylimidazolyl radical dimer when irradiated at 350nm . A transition between two states is induced by changing either the flow rate or incoming light flux. This is believed to be the first example of chemical instability induced in an isothermal photochemical system. 

Some ABC systems can give rise to unusual and complex dynamic behavior such as photochemical bistability, corresponding to the possible presence of two different photostationary states for the same irradiation and initial concentration conditions. The system ABC, 2cj)s, 2kc (see Table 3) is bistable under conditions in which ba/ ca 1 and AbsJ, > 5. We assume that the molar extinction coefficients of A and B are equivalent (eA eB) and that C does not absorb (e = 0). 

A dithienylperfluorocyclopentene derivative was reported by Lemieux et al. for the photochemical switching of polarization in SmC and this system exhibited the advantage of thermally stabOity at irradiated state over the azobenzene-doped systems [136, 137]. FLC mixtures MDW950-PhP doped with 1.0 and 3.0 mol % of 30 was irradiated with UV light (Fig. 5.23), the photoisomerization of 30 caused a destabihzation in SmC and the Ps versus temperature plots shifted to lower temperatures. The phase transition temperature from ferroelectric SmC to non-ferroelectric SmA was decreased by 2 °C upon irradiation of the 3 mol % FLC mixture. The photoswitching was bistable and fatigue resistance (Fig. 5.24). 

Noise-induced transitions have been studied theoretically in quite a few physical and chemical systems, namely the optical bistability [12,13,5], the Freedricksz transition in nematics [14,15,16,5], the superfluid turbulence in helium II [17], the dye laser [18,19], in photochemical reactions [20], the van der Pol-Duffing oscillator [21] and other nonlinear oscillators [22]. Here I will present a very simple model which exhibits a noise-induced critical point. The so-called genetic model was first discussed in [4]. I will not describe its application to population genetics in this paper, see [5] for this aspect, but use a chemical model reaction scheme ... 

We report here an investigation of an entirely new bistable system based on an organic photochemical reaction. This system thus expands the experimental scope of -chemical dynamics since ... 

We present here the first experimental demonstration of photochemical bistability in an open reactor. This bistable reaction results from the non-linear properties of a photochromic system the dimer of the triphenylimidazyl radical in chloroform. Hysteresis is observed on the plots of the stationary states of the system over a wide range of flow rates. Within this region, the system is bistable and can be made to flip from one state to the other by an external manipulation. One of the stable states is characterized by a high concentration of violet radicals 2 while in the other the violet radicals are replaced by highly fluorescent compounds. Mechanistic studies showed that this bistability was due to a positive feedback loop. This was thought to arise from the screening effect of the violet radicals 2 with respect to the irradiation of the triphenyl imidazole 3 in combination with an inhibition of the violet radicals 2 by the products of photolysis of triphenylimidazole 3. 

MULLER - Do you think that this photochemically bistable system is a candidate for spatial pattern formation (without convective effects) under externally homogeneous illumination with the appropriate wavelength The chemical "feed-back loop" combined with diffusive transport might constitute a source of spatial symmetry-breaking, e.g. in an extended solution layer. 

MICHEAU - It is now very well known that the overhelming majority of photochemical spatial patterns are in fact due to a coupling of a linear photochemical reaction with a convection process coming from an evaporative cooling phenomenon. The patterns do not appear if convection is not allowed concerning now the new photochemical bistable system this system must be used in a CSTR under UV irradiation (360 nm) so, I don t see any special condition where spatial patterns could be experimentally observed. 

---