Aminoazobenzene-type molecules

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 photoisomerization of all types of azobenzenes is a very fast reaction on either the singlet or triplet excited-state surfaces according to the preparation of the excited state, with nearly no intersystem crossing. Bottleneck states have lifetimes on the order of 10 ps. The molecules either isomerize or return to their respective ground states with high efficiency. So photoisomerization is the predominant reactive channel, and the azobenKnes are photochemically stable. Only aminoazobenzene-type molecules and pseudo-stilbenes have small quantum yields of photodegradation. 

The lifetime of the Z-isomers of aminoazobenzene-type molecules is generally short. Approximate spectra are gained by extrapolation of a series of photoisomerization reaction spectra (Figure 1.12). 

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... 

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... 

The n—and n- n bands of -aminoazobenzene-type molecules are less clearly separated. Moreover, polar solvents lower the transition energy, in particular that of the n—band so that k—>k and n— bands may overlap and appear as one band. 

Thus, fluorescence may occur in aminoazobenzene-type compounds, but it is not prominent. Low-temperature, glassy solvents or other methods of external rigidification, such as adsorption to a surface at liquid nitrogen temperature, are necessary. For 4-dimethylaminoazobenzene, fluorescence in hydrocarbon solution at 77 K is on the red side of the n —> 7i band and assigned to n<—7t emission. With the high-power lasers available today, one would anticipate emission of many molecules of this class. 

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). 

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 azo produets obtained have an extended conjugate system having both the aromatie rings joined through the -N=N- bond. These compormds are often coloured and are used as dyes. Benzene diazonium chloride reacts with phenol in which the phenol molecule at its para position is coupled with the diazonium salt to formp-hydroxyazobenzene. This type of reaction is known as coupling reaction. Similarly the reaction of diazonium salt with anUtne yields p-aminoazobenzene. This is an example of electrophilic substitution reaction. 

Many substituted azobenzenes belong to the azobenzene type, as well. Substitution may shift the n —> it band from 440 nm (azobenzene), to 465 nm (hexamethylazobenzene), to 480 nm (2,2 -dimethyl-4,6,4 ,6 -tetra-tert.butylazobenzene), and even to 520 nm (hexaphenylazobenzene). Hydrocarbon, halogen, nitro, carboxy, acetyl, hydroxy, m-amino" and even 2,2 4,4 ,6,6 hexaphenyl substitution— influences the (7i,7t )-(n,7i ) state energy gap, but not so much as to shift the molecule into the aminoazobenzene group. In the context of this book, it is important that the long-chain and polymeric molecules containing azobenzene units coupled by means of hydroxy and carboxy substituents are of the azobenzene type. 

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