Azobenzenes, reorientation

Positional changes of atoms in a molecule or supermolecule correspond on the molecular scale to mechanical processes at the macroscopic level. One may therefore imagine the engineering of molecular machines that would be thermally, photochem-ically or electrochemically activated [1.7,1.9,8.3,8.109,8.278]. Mechanical switching processes consist of the reversible conversion of a bistable (or multistable) entity between two (or more) structurally or conformationally different states. Hindered internal rotation, configurational changes (for instance, cis-trans isomerization in azobenzene derivatives), intercomponent reorientations in supramolecular species (see Section 4.5) embody mechanical aspects of molecular behaviour. 

Figure 10 Reorientation of an azobenzene dye by photoisomerization with polarized light. Molecular representations of the lruns and cis forms of an azobenzene are represented by straight and bent shapes to illustrate how a trans-cis-trans cycle can effect a reorientation of the traru-azobenzene. If the inducing hght is polarized in the horizontal direction, the vertically oriented molecule will be unable to absorb the hght, and this orientation will therefore be photostable.

Pedersen T G, Johansen P M. (1997) Mean-field theory of photoinduced molecular reorientation in azobenzene liquid crystalline side-chain polymers. Phys Rev Lett 79 2470-2473... 

Equation was derived without approximations. It is noteworthy that these solutions do not couple tensorial components of different orders and that they confirm that rotational diffusion and cis—>trans thermal isomerization are isotropic processes that do not favor any spatial direction. In Section 3.4, I discuss, through the example of azobenzene, how Equation 3.11 can be used to study reorientation processes during cis—>trans thermal isomerization after the end of irradiation. The next subsection gives analytical expressions at the early-time evolution and steady-state of photo-orientation, for the full quantification of coupled photo-orientation and photoisomerization in A<- B photoisomerizable systems where B is unknown. 

Equation 3.22. This type of experiment will be discussed eventually for spiropyran and diarylethene chromophores in films of PMMA. Next, I compare reorientation observations after cis—>trans thermal isomerization of azobenzene to the theoretical developments in Section 3.2.3.2. 

The process of reorientation during cis—>trans thermal isomerization can be seen at the value of in Equation 3.11, which shows that the cis anisotropy does not contribute to the trans anisotropy if the trans isomer loses total memory of the orientation in the cis isomer Q2 = 0). It is informative to note that in the realistic physical case—i.e., the case of the azobenzene molecule chemically attached to a polymer, where the cis and trans diffusion rates are negligible in comparison to the cis— trans isomerization rate—the relaxation of the cis and trans anisotropy, AA and can be written respectively in the form ... 

A similar behavior (not shown) was found for an azobenzene self assembled monolayer. The value of Q2 can be estimated by comparing reorientation measurements to Equation 3.28. In the next section, I discuss the photoorientation of push-pull azo dyes. 

For all Azo-PURs, the quantum yields of the forth, i.e., trans—>cis, are small compared to those of the back, i.e., cis—>trans, isomerization—a feature that shows that the azo-chromophore is often in the trans form during trans<->cis cycling. For PUR-1, trans isomerizes to cis about 4 times for every 1000 photons absorbed, and once in the cis, it isomerizes back to the trans for about 2 absorbed photons. In addition, the rate of cis—>trans thermal isomerization is quite high 0.45 s Q 1 shows that upon isomerization, the azo-chromophore rotates in a manner that maximizes molecular nonpolar orientation during isomerization in other words, it maximizes the second-order Legendre polynomial, i.e., the second moment, of the distribution of the isomeric reorientation. Q 1 also shows that the chromophore retains full memory of its orientation before isomerization and does not shake indiscriminately before it relaxes otherwise, it would be Q 0. The fact that the azo-chromophore moves, i.e., rotates, and retains full orientational memory after isomerization dictates that it reorients only by a well-defined, discrete angle upon isomerization. Next, I discuss photo-orientation processes in chromophores that isomerize by cyclization, a process that differs from the isomeric shape change of azobenzene derivatives. 

As mentioned in Section 5.1, the reorientation of azobenzene photo-chromic moieties under illumination with polarized light and the birefringence that is induced from it are well known and covered in many publications. 

Wu, Y., Mamiya, J.-L, Kanazawa, A., Shiono, T., Ikeda, T., Nagase, Y., and Zhang, Q. Photoinduced alignment of polymer liquid crystals containing azobenzene moieties in the side chain. 6. Biaxiality and three-dimensional reorientation. Macromolecules 32, 8829... 

Fischer, X., Menzel, H., and Stumpe, J, Photo-reorientation of azobenzene side groups of thermotropic hairy rod polyglutamate in LB multilayers. Supramol. Sci. 4, 543 (1997). 

In polyurethane 33, the azobenzene moieties are separated from each other along the polymer backbone by isophorone units and have the free volume necessary for isomerization and molecular reorientation. This polymer s azobenzene moieties can be photoisomerized readily in mixed LBK films, as demonstrated by measuring the optically induced birefringence that originates from the photoreorientation of the chromophore upon polarized irradiation see Section 6.5.2). 

Azobenzene has been incorporated in the shell of hydrophilic microparticles, too. Here, the azobenzene moieties have sufficient free volume for isomerization, as shown by the fact that they can be efficiently reoriented by polarized irradiation (see Section 6.5.2). 

Photoreorientation of azobenzene chromophores by irradiation with polarized light is a very important photoinduced structural change. For azobenzene moieties, there is a widely accepted mechanism for the photo-reorientation The azobenzene moieties that are parallel with their long axis (and therefore with their transition dipole) to the electric field vector... 

Hill, R. A., Dreher, S., Knoesen, A., and Yankelevich, D. R. Reversible optical storattf utilizing pulsed, photoinduced, electric-field-assisted reorientation of azobenzenes. Apfihetl Physics Letters, vol. 66, (no. 17), 24 April 1995, p. 2156-2158. 

The photoselection in azobenzene polymers (Figure 11) is rather a general phenomenon.77-84 Systematic studies have been conducted by Natansohn and coworkers on the photoinduced molecular reorientation of azobenzene units to induce birefringence and dichroism of thin films of acrylate amorphous polymers with Dispose Red (DR) pendant residues, which have been attracting extensive... 

---