Azophenols optical oscillators

Slow thermally isomerizing azo derivatives are exceptional building blocks for information storage devices. By contrast, for other applications, like optical oscillators and fast-responding artificial muscles, which will be described in the next sections of this chapter, it is essential that the return to the thermodynamically stable tmns form in the dark occurs as fast as possible. Specifically, hydroxy-substituted azobenzenes, commonly known as azophenols, are particularly attracting chromophores for this purpose since they are endowed with fast thermal isomerization rates under ambient conditions. 

Importantly, not all azo dyes are useful for optical oscillation. In fact, for high values to be achieved, the light-sensitive azo compound should revert very quickly to its thermodynamically stable trans form in the dark, most desirably within the micro- or nanosecond temporal domain. Two different families of azo dyes have been described so far that could be suitable for this task 4-donor-4 -acceptor azobenzenes, also known as push-pull azo dyes, and azophenols. Indeed, both azo derivatives are well known to exhibit fast thermal isomerization rates under ambient conditions. Among all push-pull azo dyes, 4-A/ ,A/ -dimethylamino-4 -nitroazobenzene, has been considered, until a few years ago, as the fastest thermally isomerizing azo dye, with a relaxation time for its thermal back reaction ranging from 21 to 122 ms in alcoholic solutions. On the other hand, and as it has been described previously in this chapter, the... 

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. 

Sub-Millisecond Thermally Isomerizing Azophenols for Optically Triggered Oscillating Materials... 

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