Bistable photochromic system

Signal Transduction by Combination of Energy Transfer with a Bistable Photochromic System... 

In a chiral photochromic system (Figure 1), A and B represent two different chiral forms of a bistable molecule, and a reversible change in chirality occurs upon light irradiation. The left-handed (S or M) and right-handed (R or P) forms of a chiral compound 91 represent two distinct states in a molecular binary logic element. 

The photochemistry of the dimer of the triphenylimidazolyl radical is an example of photochromism whose dynamic behavior resembles that of a bistable ABC system. It is characterized by the formation of colored, relatively persistent radicals (TPI, 5). 

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. 

Switching also implies molecular and supramolecular bistability since it resides in the reversible interconversion of a molecular species or supramolecular system between two thermally stable states by sweeping a given external stimulus or field. Bistability in isolated molecules or supermolecules is, for instance, found in optical systems such as photochromic [8.229] or thermochromic substances or devices, in electron transfer or magnetic processes [8.239], in the internal transfer of a bound substrate between the two binding sites of a ditopic receptor (see Section 4.1 see also Fig. 33) [6.77]. Bistability of polymolecular systems is of a supramolecular nature as in a phase transition or a spin transition, both of which involve an assembly of interacting species. 

Fulgides have been extensively investigated as erasable and rewritable optical memory systems (see Chapter 4) and these photochromic molecules are attractive candidates for chiroptical switch development.1471 Their bistability is based on the... 

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. 

Although the system in figure 5A and 5C is under identical constraints, two different stationary states can be observed, providing further evidence for bistability. The explanation of this phenomenon was derived from reaction mechanisms for photochromic and deactivation processes. 

---