Azobenzene-Type Molecules

Even after the advent of femtosecond spectroscopy, a potential energy 

FIGURE 1.14 The rotation and inversion mechanisms of isomerization of azobenzene. (Adapted from reference 7, by permission.) 

For pseudo-stilbene-type molecules, the question of the mechanism of thermal isomerization was taken up again in the early eighties by Whitten et al. and later by Kobayashi et al., who, on the basis of their isomerization experiments with donor/acceptor-substituted azobenzenes in polar solvents, postulate rotation. Asano and coworkers infer from the isomeriza- 

Isomerization needs some extra sweep volume. The volumes for the two mechanisms of azobenzene should be quite different—ca. 0.25 nm for rotation and ca. 0.12 nm for in version. This bears out in restricted spaces. In some zeolites azobenzene can isomerize whereas stilbene does not. Kuriyama and Oishi found that there are two separate AH versus AS lines for azobenzenes isomerizing by inversion (azobenzene type) and rotation (pseudo-stilbene type).  

In their 1971 review, Ross and Blanc expressed doubts as to the operation of the inversion mechanism in the excited states. This opened another round of heated discussion. The rotation/inversion controversy invoked much theoretical and experimental work. 

---

The absorption bands of aminoazobenzene-type molecules have some charge transfer characteristics. So the vibrational structure is weakly expressed and the spectra are sensitive to the polarity of the solvent (Figure 1.11). Note that the intensity of the n —> ti band is increased relative to the n K compared to the azobenzene type molecules. This may indicate increased state mixing. 

Although the activation energies of aminoazobenzene-type compounds (Ea between 75 and 88 kj moH) are not very different from those of azobenzene-type molecules, therm Z -> E isomerization of aminoazobenzene-type molecules is in general much faster than that of the azobenzene-type compounds. Conventional flash experiments are necessary to monitor the changes. The half-life of the Z-form of dimethyl-aminoazobenzene in toluene at 298 K is 220 s. A Linear Free Energy Rehitionship and Hammett relation is established, which includes azobenzene- and aminoazobenzene type compounds.A linear In vs, n, the Taft parameter of solvent polarity, is also observed. The dependence of the isomerization rate on pressure is weak In most solvents, it increases less than 35% at 2100 bar, AV -1.65 ml moLL Methanol is exceptional, with AV = -17 ml... 

The prototype molecule for donor/acceptor substitution is 4-dimethyl-amino-4 -nitroazobenzene. Here, the n- n band is shifted far to the red due to the charge transfer character of the transition. The band has few vibrational features, and its energy is influenced by the polarity of the solvent. The weak n Tt band cannot be seen under the intense it —> it band (Figure 1.13). Most commercial azo dyes are pseudo-stilbenes rather than azobenzene-type molecules. 

Pseudo-stilbenes may emit fluorescence that is, contrary to true stilbenes, generally weak at room temperature and often weak even at low temperatures. Protonated azobenzene-type molecules and many protonated azo dye molecules emit strong fluorescence in sulfuric acid at 77 K with quantum yields of about 0.1. Inclusion of azobenzene in the channels of AIPO4-5 crystals provides complexation of the n-electrons and space confinement. This leads to emission by protonated azobenzene at room temperature. For their cyclopalladated azobenzenes, Ghedini et al. " report quantum yields of ca. 1T0 and lifetimes of ca. 1 ns. In contrast, donor/acceptor pseudo-stilbenes, if emitting at low temperatures or when adsorbed to surfaces, are weak emitters. In textile chemistry, it has long been known that azo dyes adsorbed to fibers may show fluorescence. ... 

The dependence of the photoisomerization process on the polarity of the environment varies greatly for differently substituted compounds. King et al. found that p-nitro- or cyanosubstituted azobenzenes had photostation-ary states vv ith similar E/Z ratios, but that an additional p -amino function stopped photoisomerization in acetonitrile. This was not true, however, in methylcyclohexane, where lifetimes of Z-isomers were determined to be in the order of seconds at room temperature. These findings are retained at -35°C, which is taken as proof that it is not the fast thermal Z —> E isomerization that fakes the lack of photoisomerization. This conclusion may be questionable, however, considering the weak temperature dependence for azobenzene-type molecules (Figure 1.10). 

Some reports on fluorescence occurring in, for instance, porous materials such as Nafion or aluminophosphates, " do not refer to azobenzene but to protonated azobenzene, which is classified as a pseudostilbene see Section 1.5). Emission from nonprotonated, isolated azobenzene-type molecules is still very rare. Aggregated systems, however, seem more prone to sho%v fluorescence emission. Shinomura and Kunitake have detected fluorescence bands with a maximum of near 600 nm in bilayer systems built from the monomers of 15. They have shown that the ability to emit is tied to the type of aggregation Head-to-tail aggregates emit relatively strongly, with quantum yields of up to < ) = 10" and lifetimes below 2 ns. Their prototype of card-packed dimers does not emit at all. This is expected because of the low transition probability at the lower band edge, which favors radiationless deactivation, probably via the Si state (see Figure 1.7). 

Pseudo-stilbenes may emit fluorescence that is, contrary to true stilbenes, generally weak at room temperature and often weak even at low temperatures. Protonated azobenzene-type molecules and many protonated azo dye molecules emit strong fluorescence in sulfuric acid at 77 with quantum... 

Azobenzenes can be separated into three spectroscopic classes, well described by Rau (1990) azobenzene-type molecules, aminoazobenzene-type molecules, and pseudo-stilbenes (refer to Fig. 1.1 for examples). The particulars of their absorption spectra (shown in Fig. 1.2) give rise to their prominent colors yellow, orange, and red, respectively. Many azos exhibit absorption characteristics similar to the unsubstituted azobenzene archetype. These molecules exhibit... 

Figure 1.2. Schematic of typical absorbance spectra for trans-azobenzenes. The azobenzene-type molecules solid line) have a strong absorption in the UV, and a low intensity band in the visible (barely visible in the graph). The aminoazo-benzenes dotted line) and pseudo-stilbenes dashed line) typically have strong overlapped absorptions in the visible region.

According to the spectral features and isomerization behavior, the aromatic azo compounds have been classified as azobenzene type, aminoazobenzene type, and pseudostilbene type (Rau, 1990). For the azobenzene-type molecules, the 71-71 transition band (320 nm) appears at shorter wavelength than n-n transition band (430 nm) (Kumar and Neckers, 1989). The cis-to-trans isomerization is relatively slow at room temperature, and the existence of cis isomers can be easily identified by the spectroscopic method. For aminoazobenzene-type and pseudos-tilbene-type molecules, the ti-ti and n-n transition bands are overlapped. The cis state of the molecules is unstable, which relaxes back to the trans state quickly. According to the definition, the azo polymers given in Figs. 5.1 and 5.2 can be assigned to contain pseudostilbene-type and azobenzene-type chromophores, respectively. 

The water content in the medium plays a key role to control the colloid formation. The influence of the water content can be studied by using dynamic light scattering (DLS) and UV-vis spectroscopy. DLS indicates that when the water content increases above CWC, the hydrodynamic radius (i b) gradually increases as the water content increases. When the water content is above a certain value, i h starts to decrease as the water content further increases and then stabilizes at the final value. The structure evolution in the process can be better understood from the UV-vis spectroscopic investigation. It is well known that the photoisomerization rate and isomerization degree at the photostationary state are related to the free volume surrounding the azo chromophores (Kumar and Neckers, 1989). For azobenzene-type molecules, the isomerization behavior can be monitored by UV-vis spectroscopy and used as a molecular probe to detect the environmental variation in the systems. 

Two phenyl rings separated by an azo (-N=N-) bond known as Azobenzene, functions as the base molecule for a wide class of aromatic azo compounds. Azobenzenes almost cover the full rainbow, and up to and almost 70 % of the world s commercial dyes are still azobenzene-based. These chromophores are adaptable molecules and have received much attention, in both fundamental and applied research. Azobenzenes can be categorized into three spectroscopic classes azobenzene-type molecules, aminoazobenzene-type molecules, and pseudo-stilbenes. 

Azobenzene-Type Molecules Aminoazobenzene-Type Molecules Pseudostilbene-Type Molecules Photoisomerization of Azobenzenes Conditions... 

---