Hemiindigo: Highly Bistable Photoswitching at the Biooptical Window

Abstract

Hemiindigo is a long known chromophore that absorbs in the blue part of the spectrum but has almost completely been ignored as potential photoswitch. Herein we show how the absorption of hemiindigo is shifted to the red part of the visible spectrum and how nearly perfect photoswitching can be achieved using blue or green and red light. Five derivatives were investigated giving very high isomeric yields in both switching directions, i.e. >90% E isomer after irradiation with 470 to 530 nm light and 99% Z isomer with 590 up to 680 nm light. At the same time the thermal bistability is extraordinarily high leading to half-lives of the pure isomeric states of up to 83 years at 25 °C. The herein developed photoswitches show photochromism in the visible enabling the two isomeric states to be distinguished by the naked eye. Substituted hemiindigos therefore constitute extremely promising new photoswitches with excellent properties for applications in biology, chemistry, or material sciences.

Reversible Photoswitching of Isolated Ionic Hemiindigos with Visible Light

“…The maxima of the Z → E PISA spectra of HIs 1, 2 and 3 occur at 450, 410, and 470 nm, respectively, whereas the corresponding E → Z PISA spectra have maxima at 505, 490, and 520 nm, respectively. The significant red shift observed for the peaks in the PISA spectra of HI 3 compared to HI 1 and 2 can be explained by the presence of the strong electron donating julolidine group, as found recently for the neutral analogue of this compound . Band maxima for the absorption spectra of HIs in acetonitrile solution are shifted by 30–70 nm to longer wavelength compared to the peaks in the PISA spectra (see Figures (g–i)).…”

“…Whereas DFT calculations predict a near planar configuration for E isomers of the three HIs, the optimized Z isomers show a pronounced twist around the single bond connecting the aniline (julolidine) and indigo planes (torsion angle θ in Figure (a)). The torsion angles θ for the lowest energy conformers of HIs 1 , 2 and 3 are 45°, 52° and 52°, respectively, which are somewhat larger than those calculated at the B3LYP‐GD3BJ/6‐311G(d,p) level of theory with polarizable continuum model for the neutral, solvated Z isomer analogues of 1 (34°) and 3 (37°) …”

“…Whereas HI 1 and 2 differ in the length of the alkyl chain carrying the charge‐tag, HI 3 incorporates a strong electron donating julolidine moiety. Recent work on the neutral analogues of HIs 1–3 revealed their high thermal bistability, almost perfect reversible photoswitching behaviour in the red region of the spectrum, and strong photo‐ and solvato‐chromism in organic solvents . The physical and photochemical properties of the neutral HIs were found to depend moderately on the nature of the surrounding solvent …”

Recent Implementations of Molecular Photoswitches into Smart Materials and Biological Systems

“…HI derivatives with electron‐rich aromatic substituents demonstrated mutual photoswitching with red and green light, with PSS>90 % for both isomers and thermal half‐lives of up to 83 years at 25 °C. Due to similar energies of both isomers E and Z , thermal equilibration does not favor either of them …”

Kontrolle über Kettenvernetzung und Einzelkettenverknüpfung durch zwei Farben des sichtbaren Lichts

“…[4][5][6][7] Während die intramolekulare Vernetzung synthetischer Polymere sich aller Werkzeuge der synthetischen Chemie bedienen kann, um Sekundär-und Te rtiärstrukturen nachzuahmen, [8][9][10][11][12] bestehen Proteine in der Natur aus mehreren Peptidsträngen, die sich zu einer Quartärstruktur zusammenlagern, die ihre Funktionsweise bestimmt. [14][15][16][17][18][19] Da photoreaktive Einheiten nur durch Photonen aktiviert werden kçnnen, welche die gleiche Energie wie ihr angeregter Zustand aufweisen, wurden in den letzten Jahren große Bemühungen angestrengt Systeme verschiedener Chromophore zu entwickeln, die nur bei bestimmten Wellenlängen absorbieren, um eine exklusive Adressierbarkeit mit exakt gewählten Wellenlängen zu ermçglichen. [13] Um einen synthetischen Zugang zu solch hierarchischen makromolekularen Architekturen zu erhalten, spielt die orthogonale Ligationschemie von intra-und intermolekularen Kettenreaktionen eine entscheidende Rolle.W ährend klassische, thermisch gesteuerte organische Chemie das Potential innehält solche Systeme zu verwirklichen, würde dies den Einsatz zusätzlicher Reagenzien und chemischer Modifikation der reaktiven Gruppen erfordern.…”