Pseudo-Stilbene-Type Molecules

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-... 

The photoisomerization of the push-pull azobenzene derivative disperse red 1 (DRl) has also been shown recently to occur efficiently in poly-(methyl metacrylate) (PMMA) thin films at room temperature [23,24]. DRl is a pseudo-stilbene type azobenzene molecule, as classified by Rau. This means that the high-energy n-n transition is overlapping the low energy n-n transition, which leads to a large structureless band in the trans isomer and... 

Figure 1.11), and a pseudo-stilbene type with a (7t,7t ) state as the lowest-excited state (see also Figure 1.13). The assignment of an azo molecule or an azobenzene-containing macromolecule or system to one of these classes can be made by a simple inspection of the absorption spectrum. The spectroscopic properties of these types of azo compounds will be covered in Sections 1,3 to 1.5 of this chapter.

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 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. 

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.

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. 

---