Triplet state pseudo-stilbenes

As in the azobenzene type systems, the triplet pathway seems to be decoupled from the singlet route in pseudo-stilbenes. Little is known about the mechanism in the triplet state. The only information comes from calculations, and these show that the triplet surfaces are frequently similar in shape to the singlet surfaces. Thus, both mechanisms may be operative in the triplet state, too. 

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. 

E,Z-isomerization can be induced by a direct (singlet) or sensitized (triplet) route from the n,Jt or 7t,7t state. The thermal back-reaction from the Z-isomer is slow in azobenzene and fast in amino- or aminonitro-substituted azobenzenes (pseudo-stilbenes). The color change is more in intensity than in wavelength. In geometrically locked azobenzene (i.e., in 3,4, and 5), the Z-form might be stable and cannot isomerize (Scheme 2). ... 

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