Weak acid triggers the ring opening of an otherwise long-lived triangle terthiazole closed isomer

Abstract

The stability of the colored cyclized form of a compound from the new class of photochromic triangle terthiazoles was found to be strongly decreased by the presence of trifluoroacetic acid (TFA) in acetonitrile at room temperature. The apparent first-order process rate constant of the observed bleaching is of the same order of magnitude as if the acid-free sample was heated to about 100 1C. Association constants with TFA were determined from the kinetics analysis and by NMR titration and were found to be moderate for the colored form and very weak for the colorless form. These results were interpreted considering the basicity of the molecules and the solvent effect on the acid strength. The structural consequences of the protonation on the ring-closed compound and on the entire ring-opening process were examined by DFT calculations. Protonated closed and open forms were found to be geometrically ''closer'' than their non-protonated parents and the altered bond-length alternation may explain the diminution of the activation barrier. The puzzling instability of the protonated terthiazole was compared to a related thermally stable imidazolium homologue. From structural DFT considerations, it was concluded that this different behavior may be explained in term of charge stabilization, the sulfur being less efficient than the nitrogen.

Proton‐Gated Ring‐Closure of a Negative Photochromic Azulene‐Based Diarylethene

“…The closed isomer c ‐ATE‐H + undergoes thermal ring‐opening reaction with a half‐life of around 7 min at room temperature (Figure c), similar to reported proton‐responsive DAE‐type photoswitches . Notably, the reaction did not lead to the exact same absorption spectrum of o ‐ATE‐H + , suggesting the thermal back reaction was accompanied with some decomposition of ATE (Figure c and S2).…”

“…In principle, proton‐response can be implemented into a photochromic system by incorporation of an acidic/basic group in conjugation to or as part of the photoactive reaction center. Commonly used moieties are heteroatom‐based, for example, pyridine or phenol .…”

Proton Catalysis in the Redox Responsivity of a Mini‐Sized Photochromic Diarylethene

“…To tackle this challenging question, a quantitative approach based on numerical modeling and kinetic curve-fitting has been chosen, based on an improved kinetic analysis method. [19] A detailed kinetic model has been created by using readily accessible chemical knowledge. It is constructed from a series of pseudo-elementary reaction processes involving the corresponding transient and stable species.…”

Protonenvermittelter Ringschluss eines negativ photochromen, Azulen‐basierten Diarylethens

“…Generell kann die Ansprechbarkeit auf Protonen eines photochromen Systems durch den Einbau einer Säure/Base‐Gruppe in Konjugation zu oder als Teil des photoaktiven Reaktionszentrums realisiert werden. Häufig verwendet werden heteroatombasierte Gruppen, z.…”

“…Analog zu bekannten protonensensitiven DAE‐Photoschaltern durchläuft das geschlossene Isomer c ‐ATE‐H + eine thermische Ringöffnung mit einer Halbwertszeit von etwa 7 Minuten bei Raumtemperatur (Abbildung c) . Es anzumerken, dass die Reaktion nicht zu dem exakt gleichen Absorptionsspektrum von o ‐ATE‐H + führte, was darauf hindeutet, dass die thermische Rückreaktion mit einer gewissen Zersetzung von ATE einherging (Abbildung c und S2).…”