Pseudostilbene-type molecules

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

Rau classified azobenzenes as azobenzene-type, aminobenzene-type, and pseudostilbene-type molecules according to their UV/VIS spectra, based on different energies for the transitions of n- and 7t-electrons into the 7t -obitals. > ... 

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). 

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. 

---