Thermal isomerization rates

Pressure dependence was thoroughly investigated by Asano and his group. It turns out that the partial volumes of the Z-forms of 4-dimethylamino-4 nitorazobenzene and related molecules are ca. 250 cm moP in all solvents. Those of the E-forms are smaller and solvent-dependent. Thermal isomerization rates are weakly dependent on pressure in nonpolar solvents, but contrary to azobenzene- and aminoazobenzene-type compounds, they are strongly dependent in polar solvents in hexane 10%, in acetone 475% for 2100 bar (AV = -0.7 and -25.3 em mol, respectively). This has implications for the discussion of the mechanism of isomerization (Section 1.6). 

FIGURE 3.8 Dependence of the absorbance of linearly polarized probe light at 360 nm (squares) and 450 nm (circles) on the angle. between the probe and the UV light polarization.TMs behavior is fitted by a cos with an amplitude that decays with the cis trans thermal isomerization rate. After reference 22, redrawn by permission of ACS. 

A comment must be made about the cis— trans thermal isomerization rate at pressure. At room temperature, the thermal back reaction of DRl-PMMA follows a complex, nonexponential recovery, most of which is completed after a few seconds with a rate of 0.25 s" and deviates from a single exponential decay after the first 10 seconds.Larger relaxation times at Tg -98°C include slow polymer motion coupled with the chromophores rotational diffusion. We confirmed that this behavior is true in the polymer... 

Studied in this chapter. In particular, we adjusted an exponential decay to the first 10 seconds of the recovery at all 6 pressures, and we found a thermal isomerization rate for PMMA-DRl in the range 0.17 - 0.23 s, with no particular dependence on pressure. This result rules out pressure-induced static effects and reinforces the friction effects discussed it also shows that if trans has enough sweep volume to isomerize to cis, cis will also have enough sweep volume to isomerize back, a feature supported by the more compact and globular, i.e., twisted, conformation of the cis- versus the trans-DRl. 

C. Barret, A. Natansohn, and P. Rochon, Cis-trans thermal isomerization rates of bound and doped azobenzenes in a series of polymers, Chem. Mater. 7, 899-903 (1995). 

Slow thermally isomerizing azo derivatives are exceptional building blocks for information storage devices. By contrast, for other applications, like optical oscillators and fast-responding artificial muscles, which will be described in the next sections of this chapter, it is essential that the return to the thermodynamically stable tmns form in the dark occurs as fast as possible. Specifically, hydroxy-substituted azobenzenes, commonly known as azophenols, are particularly attracting chromophores for this purpose since they are endowed with fast thermal isomerization rates under ambient conditions. 

As it has been already mentioned, the potential applicability of azobenzene-based materials is mainly determined by the thermal isomerization rate of the photoactive azo dye used, which, in turn, is related to its molecular architecture. In LC-based materials, however, the azo dye must operate within an ordered host matrix. In this medium, both the chemical nature and the intrinsic order of the LC matrix might influence the rate of the isomerization step and, thereby, the properties of the final material. Remarkably, this point is especially critical for azophenolic dyes since their thermal isomerization rate depends dramatically on the local environment where they are embedded. However, after a careful design, azophenol-based liquid-crystalline and glassy materials can exhibit thermal isomerization rates as fast as those of the monomers in solution, even without the presence of any solvent [41]. Thus, it will be the main aim of the present subsection to provide the reader the main clues for this purpose to be achieved. 

To summarize, achieving control over intermolecular interactions occurring either between the azophenol moieties and their environment or between themselves enables to transfer the fast thermal isomerization rate exhibited by hydroxy-substituted azo dyes in ethanol and acetonitrile solutions to both nematic and glassy thin films. Such a feature permits the generation of fast responding solvent-free photoactive systems with great potential applicability within materials science and actual technology. 

Importantly, not all azo dyes are useful for optical oscillation. In fact, for high values to be achieved, the light-sensitive azo compound should revert very quickly to its thermodynamically stable trans form in the dark, most desirably within the micro- or nanosecond temporal domain. Two different families of azo dyes have been described so far that could be suitable for this task 4-donor-4 -acceptor azobenzenes, also known as push-pull azo dyes, and azophenols. Indeed, both azo derivatives are well known to exhibit fast thermal isomerization rates under ambient conditions. Among all push-pull azo dyes, 4-A/ ,A/ -dimethylamino-4 -nitroazobenzene, has been considered, until a few years ago, as the fastest thermally isomerizing azo dye, with a relaxation time for its thermal back reaction ranging from 21 to 122 ms in alcoholic solutions. On the other hand, and as it has been described previously in this chapter, the... 

A possible protocol to accelerate the thermal back reaction of push-pull azophenolic dyes beyond the millisecond timescale involves the replacement of one of the benzene rings of 1 by a pyridine ring in the shape of 10 (Figure 11.8), where the pyridinic nitrogen atom bears a permanent positive charge [62]. This structural modification introduces an extremely powerful electron-withdrawing unit in the azophenol platform inducing thereby a dramatic enhancement of its thermal isomerization rate and oscillation frequency. Indeed, azopyridinium... 

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