Azophenols thermal isomerization kinetics

Thermal Isomerization Kinetics of Azophenols in Isotropic Solvents... 

The starting point for this discussion will be 4-hydroxyazobenzene (1 in Figure 11.7). Although parent azophenol shows relatively fast thermal isomerization kinetics (t = 205 ms in ethanol), it is still far away from that expected for an efficient optical oscillator. In spite of this, the rate of the thermal back reaction for 1 can be significantly enhanced by introducing the well-known electron-withdrawing cyano and nitro functions (8 and 9 in Figure 11.7) in the position 4 of the azophenol platform. Indeed, this protocol results in both a considerable decrease of the relaxation time for cis-8 and cis-9 (27 and 4.6 ms, respectively) and a concomitant increase of their oscillation frequency up to 72.5 Hz. 

It is also highly noteworthy that the introduction within the very same azo platform of two (4 in Figure 11.3) or more (5 in Figure 11.3) -OH functions results in a significant increase of its cis-to-trans isomerization kinetics owing to a cooperative effect. In particular, cis-4 and cis-5 display thermal relaxation times as low as 6-12 and 33-53 ms in ethanol and toluene, respectively, under ambient conditions (Figure 11.3). In fact, these two azo compounds are the fastest thermally isomerizing non-push-pull azophenols reported heretofore. 

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