Azo dyes that combine electron-withdrawing thiazole/benzothiazole heterocyclesa nd electron-donating amino groups within the very same covalent skeleton exhibit relaxation times for their thermali somerization kinetics within milli-and microsecond timescales at room temperature. Notably,t he thermal back reactiono ft he corresponding benzothiazolium and thiazolium salts occurred much faster,w ithin the picosecond temporald omain. In fact, these new light-sensitive platforms are the first molecular azo derivativesc apable of reversibles witching between their trans and cis isomers in as ubnanosecond timescale under ambient conditions. In addition, theoretical calculations revealed very low activation energies for the isomerization process, in accordance with the fast subnanosecond kinetics that were observed experimentally.

Benzothiazole-pyrrole-based azo dyes greatly enhance their thermal isomerisation rate by up to 160 times when they are under the influence of the nematic mean field yielding the LC-based photochromic oscillators with the highest oscillation frequencies reported so far (2.6 kHz at 298 K).

The novel photoswitchable bis-azo derivative reported herein shows a high temporal resolution of 2 × 10(8) times between the thermal relaxation rates of its two constituting photochromes. Moreover, the slow and fast azo building blocks of this molecular construct can be triggered by using UV and visible light, respectively.