Photonics azobenzenes

A similar type of azobenzene dendrimers was reported by Jiang and Aida [37]. Interestingly, the aryl groups in such a dendrimer were found to be able to simultaneously absorb five infrared photons and then transfer the energy to the azobenzene core moiety to cause trans/cis isomerizations. 

To realize the above system, it is required to design a polymer which reversibly changes the molecular properties, such as hydrophilicity, by the external stimulation. Many molecules are known to be reversibly transformed to other isomers by external stimulation, such as photons, electrons or chemicals. Table 1 lists a few examples. Azobenzene shows the property change by photoirradiation. It isomerizes from the trans to the cis form by ultraviolet irradiation, and the dipole moment increases from 0.5 to 3.1 deb ye. The polar cis form returns to the less polar tram form by visible irradiation. Electrochemical oxidation of ferrocene changes the hydrophilicity. When it is oxidized from Fe(II) to Fe(III), the hydrophilicity increases. The Feflll) state returns to the Fe(II) state by either electrochemical or chemical reduction. Host molecules also change the properties in the presence of suitable guest ions. Benzo[18]crown-6, for example, captures potassium ions in the cavity, and increases the hydrophilicity. 

Here, it is worthwhile to note the difference in the response times of the dissolution and phase separation processes. This is important from the view point of energy conversion efficiency. At 19.5 °C, which is very close to Tc of the polymer solution with all trans azobenzene chromophores, the isomerization of a small number of chromophores, in other words, a small number of photons, was enough to raise Tc above 19.5 °C. Therefore, the transmittance increase took place immediately by irradiation for a very short time. The polymer chain was efficiently expanded by a small number of photons. 

In the phase separation process, however, it needed some induction period for the polymer to start the phase separation. Almost complete isomerization of the azobenzene pendant groups from the cis to the trans form is required to decrease the phase separation temperature below 19.5 °C. The phase separation process exhibited a non-linear response to the irradiation time or the number of photons. When the number of absorbed photons reached a critical value, the system underwent the phase separation and the polymer chain was shrunk. The photo-stimulated phase separation/dissolution cycle was not observed below 19.4 and above 26.0 °C. 

Photoresponsive systems are seen ubiquitously in nature, and light is intimately associated with the subsequent life processes. In these systems, a photoantenna to capture a photon is neatly combined with a functional group to mediate some subsequent events. Important is the fact that these events are frequently linked with photoinduced structural changes in the photoantennae. This suggests that chemical substances that exhibit photoinduced structural changes may serve as potential candidates for the photoantennae. To date, such photochemical reactions as E/Z isomerism of azobenzenes, dimerization of anthracenes, spiropyran-merocyanine interconversion, and others have been exploited in practical photoantennae. It may be expected that if one of these photoantennae were adroitly combined with a crown ether, it would then be possible to control many crown ether family physical and chemical functions by means of an ON/OFF photoswitch. This is the basic concept underlying the designing of photoresponsive crown ethers. We believe that this is one of the earliest examples of molecular machines . 

These azobenzene LCs display the liquid crystalline phase only when the azobenzene moiety is in the trans form, and no liquid crystalline phase at any temperature when the azobenzene moiety is in the cis form. In these azobenzene LC system, it was predicted that phase transition should be induced on essentially the same time-scale as the photochemical reaction of the photoresponsive moiety in each mesogen, if the photochemical reactions of a large number of mesogens were induced simultaneously by the use of a short laser pulse (Figure 7).1391 On the basis of such a new concept, the photoresponse of azobenzene LCs with the laser pulse was examined, and it was found that the N to I phase transition was induced in 200 xsJ39 40 This fast response, on the microsecond timescale, had been demonstrated for the first time in NLCs. From the viewpoint of application of LCs to photonic devices, such a fast response is quite encouraging. 

Despite great interest in azobenzene photophysics, the basic photoisomerization mechanism remains disputed [173] in contrast to the expectations of Kasha s rule, the isomerization quantum yield decreases rather than increases with increasing photon energy. In Fig. 22, the two possible isomerization channels, proceeding via either a planar pathway (inversion) or a nonplanar, twisted pathway (torsion) are shown. Previous studies determined that isomerization in the first excited state S1 state proceeds along the inversion coordinate [171]. The second excited state is generally thought to be... 

A large number of azobenzene-based amorphous and liquid crystalline polymers, particularly polyacrylates and polymethacrylates with chiral azobenzene pendants, have been prepared for the development of data storage and photonic devices [1-3,11-14]. For instance, the introduction of optically active mesogenic azobenzene residues into the side groups of the polymers produces chiral nematic and cholesteric phases, which are regulated by photoisomerization of the azobenzene units [10,14]. In most cases, however, the optical activity and chiroptical... 

The short pulse duration combined with the high photon density of ps-and fs-lasers have provided the means to study the properties of the excited states by emission and transient absorption measurements. Fluorescence of the lowest and higher excited states of azobenzene can be detected, but most work is being directed toward the dynamics of isomerization. Because questions about the isomerization mechanism are prominent in this field, this work will be discussed in Section 1.6 The Isomerization Mechanism. 

Not every photon absorbed by azo compounds induces isomerization. Table 1.1 shows a series of quantum yields of azobenzene collected from different authors. The spread of the values reflects the experimental problems of a seemingly simple system. 

The lower part of Figure 3.1 shows a simplified model of the excited states. Only two excited states are represented, but each represents a set of actual levels. The lifetimes of all these levels are assumed to be very short in comparison of those of the two excited states, and form the cross section for absorption of one photon by the trans and the cis isomers, respectively. The cross sections are proportional to the isomers extinction coefficients, y is the thermal relaxation rate it is equal to the reciprocal of the lifetime of the cis isomer (x ). tc and ct are the quantum yields (QYs) of photoisomerization they represent the efficiency of the trans->cis and cis—>trans photochemical conversion per absorbed photon, respectively. They can be calculated for isotropic media by Rau s method, which was adapted from Fisher see Appendix A) for anisotropic media, they can be calculated by a method described in this chapter. Two mechanisms may occur during the photoisomerization of azobenzene derivatives—one from the high-energy 7C-7t transition, which leads to rotation around the azo group, i.e., - M=N-double bond, and the other from the low-energy transition, which... 

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. 

To summarize, there are three types of photoinduced motions at the molecular level, at the nanometer, or domain, level, and at the micrometer (macroscopic) level. All are the result of photoinduced isomerization of the azobenzene groups. One interesting direction of our research was to try to exploit these phenomena for their photonic applications. We have demonstrated, at least as proof of principle, a few possible photonic functions for the new materials we synthesized. Some of these are summarized in publications they will also be reviewed here. 

Natansohn, A., Rochon, P. (1997). Azobenzene-containing polymers Diptal and holc aphic storage. In ACS Symposium Series Photonic and Optoelectronic Polymers (K. J. Wynne and S. A. Jenekhe, Eds.), Vol. 672,236-249, American Chemical Society Washington, DC. Natansohn, A., Rochon, P. (1999). Photoinduced motions in azobenzene-bashd mnorphous polymers. Adv. Mater. 11,1387-1391. 

It seems possible to amplify the photostimulated conformational changes in solution at the molecular level into shape changes of polymer gels or solids at the visible macro level. The first proposal to use the structural changes at the molecular level for direct conversion of photon energy into mechanical work has been made by Merian (13.) in 1966. Since then, many materials, most of which contained azobenzene chromophores, have been reported to show photostimulated deformation(JM). Till now, however, the reported deformations were limited to less than 10%. In addition, Matejka et. al. have pointed out that in many cases photo-heating effect instead of photochemical reaction plays a dominant role in the deformation(15,16). 

The light-induced isomerization of the azobenzene moiety is a classical example of controlled molecular motion and has provided the basis for the construction of some of the first archetypes of molecular machines [17]. In system 5, the pendant-arm/ring interaction concurs to improve the efficiency of the azobenzene-based engine, which converts photonic energy into a mechanical work, at the molecular level. 

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