Picosecond Switchable Azo Dyes

Abstract

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.

The Photoisomerization Pathway(s) of Push-Pull Phenylazoheteroarenes

“…If a CoIn is not accessed therein, as in 2a, the system then evolves toward the rotation-like region, exhibiting slower photoisomerization kinetics. This perspective is in agreement with what has been characterized computationally for phenylazoheteroarenes 35 namely that the evolution along the nπ* surface implies an initial flattening of the CNN angle (i.e., inversion), followed by the CNNC torsion (i.e. rotation).…”

“…11 So far, their photoisomerization has been investigated in systems based on indole, 3 pyridine, 32, 33 pyrimidine 34 and thiazole. 35 These few cases suggest that the structural diversity of azoheteroarenes contributes to create a similar level of complexity as in the AB derivatives. A major limitation of these investigations is that the experiments do not provide direct information on the relaxation pathways, whereas computations are often limited to explore predefined regions of the ground-and low-lying excited states potential energy surfaces (PES).…”

Tuning the Thermal Stability and Photoisomerization of Azoheteroarenes through Macrocycle Strain

“…10,50 A possible explanation could be drawn from the thiazole-based heteroarenes reported by Velasco and coworkers. 66 These systems are able to undergo a rapid thermal isomerization under the ns timescale, and with a dependence on the solvent polarity, as expected for systems following the rotation mechanism. It is possible that these sulfur-containing heteroarenes increase the rate of (S0→T1→S0) intersystem crossing (associated with the rotational pathway) by virtue of the larger spin-orbit coupling of the heavy atom, making the non-adiabatic rotation pathway competitive to the inversion one.…”

“…The main CoIn are associated with the rotation (CoInA) and inversion (CoInB) motions. 66,[69][70][71] CoInA and CoInB are indeed the extremes of a crossing seam, 48 with the former (latter) at the low-(high-) energy end, as in AB. 34,72 Unsubstituted compounds 1 and 2 feature a similar distribution of CoInA-and CoInB-like structures within the crossing seam, which indicates that both rotation and inversion pathways are accessible.…”

The Photoisomerization Pathway(s) of Push–Pull Phenylazoheteroarenes**

“…[11] So far,t heir photoisomerization has been investigated in systemsb ased on indole, [3] pyridine, [34,35] pyrimidine, [36] and thiazole. [37] These few cases suggestt hat the structural diversity of azoheteroarenes contributes to create as imilarl evel of complexity as in the AB derivatives. Am ajor limitation of these investigations is that the experiments do not provide direct information on the relaxation pathways, whereas computations are often limited to exploring predefined regions of the ground-and low-lying excited states potential energy surfaces (PES).…”