Formation of a stable biradical triplet state cation versus a closed shell singlet state cation by oxidation of adducts of 3,6-dimethoxycarbazole and polychlorotriphenylmethyl radicals.

We report an experimental and theoretical study of two stable radical adducts of the triphenylmethyl series, 1 and 2, whose composition and molecular structure are distinguished by the content and position of chlorine atoms in phenyls. The electrochemical study through cyclic voltammetry of these open layer species shows the existence of two reversible processes, related to reduction and oxidation, to stable charged species. The chemical oxidation of both radical adducts gives rise to stable cations, whose fundamental state has a biradical triplet electronic structure or a closed shell singlet character, depending on the electronic conjugation between the donor and acceptor electron moieties. The presence of chlorines adjacent to the nitrogen in 1 breaks the conjugation between both halves, facilitating the formation of a triplet electronic state of the cation, while the absence of chlorines in these positions in 2 facilitates partial conjugation and stabilizes the closed shell singlet electronic state of the cation.

Picosecond Switchable Azo Dyes.

Azo dyes that combine electron-withdrawing thiazole/benzothiazole heterocycles and electron-donating amino groups within the very same covalent skeleton exhibit relaxation times for their thermal isomerization kinetics within milli- and microsecond timescales at room temperature. Notably, the thermal back reaction of the corresponding benzothiazolium and thiazolium salts occurred much faster, within the picosecond temporal domain. In fact, these new light-sensitive platforms are the first molecular azo derivatives capable of reversible switching between their trans and cis isomers in a subnanosecond 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.

Spatially close azo dyes with sub-nanosecond switching speeds and exceptional temporal resolution.

Photoswitchable bis-azo dyes with an outstanding temporal resolution of 10(15) times between the thermal relaxation rates of their two constituting photochromes are reported. Remarkably, the close spatial proximity of both azo photochromes in these molecular assemblies translates in an unprecedented 10(3) -fold acceleration of the thermal isomerization rate of their faster azo unit compared to the one displayed by the isolated counterpart. Indeed, the relaxation time of the fast-isomerizing platform of the herein reported bis-azobenzenes is as low as 200 ps under ambient conditions. In the wake of these results, the bis-azo dyes described herein are invaluable chromophores for the design of multifunctional light-addressable materials in which simultaneous switching in two very different timescales might be essential.

A photoswitchable bis-azo derivative with a high temporal resolution.

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.

Molecular photo-oscillators based on highly accelerated heterocyclic azo dyes in nematic liquid crystals.

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).