Photochromic and Reductive Electrochemical Switching of a Dithiazolylethene with Large Redox Modulation

Abstract

A new dipyridylthiazolylethene (1a) and its dicationic analogue (2a), with two N-methylated pyridyl rings, have been synthesized and structurally characterized. Due to the N-methylation of the pyridyl rings, 2 a displays not only very different photochromic properties, but also undergoes a reductive ring-closing reaction to generate its closed-ring isomer 2b. Careful electrochemical studies coupled with EPR spectroscopy show that this reductive ring-closing reaction takes place when 2a is two-electron reduced. DFT calculations suggest that such a ground-state electrocyclization is driven by a very large stabilization of the reduced closed-ring isomer 2b relative to the reduced open-ring isomer 2a. In addition, 2b exhibits two successive and reversible one-electron reductions at half-wave potentials of 0.04 and -0.14 V versus SCE and a redox modulation as large as 1 V is achieved when passing from 2a to 2b.

Oxidative and reductive cyclization in stiff dithienylethenes

“…Except for rare examples of methylpyridinium substituted DTEs [ 27 – 28 ] and dithiazolylethenes [ 29 ], ring closure under reductive conditions has not been reported for DAEs. For most structures the reduction potential of the open isomer is too negative to be reached within the redox window determined by the electrolyte.…”

“…Either cyclization [ 5 – 13 ] or cycloreversion [ 14 – 21 ] can be observed, while correlation of both reaction modes to the molecular structure is still under discussion [ 22 – 26 ]. There are only few reports about reductive isomerization, each involving the ionic methylpyridinium group as a substituent on the photochromic unit [ 27 – 29 ]. However, by a combination of suitable substituents, a bidirectional system able to operate in both switching directions either electrochemically or photochemically has been reported [ 28 ].…”

A Photo‐ and Redox‐Driven Two‐Directional Terthiazole‐Based Switch: A Combined Experimental and Computational Investigation

“…[3][4][5][6][7][8][9][10][11] In comparison, examples of reductive electrocyclization are rare and generally require the presence of strong electron withdrawing groups such as N-methylpyridinium connected to the photochromic dithienylethene or dithiazolylethene core. [12][13][14] However, recent works showed that dithienylethenes with appropriate cyano-substitution or an extended π system could also lead to reductive ring-closure, albeit with much more negative potentials. [11] It is worth noting that in most cases the redox input triggers only one-way isomerization and cannot be used to bring the diarylethene back to its initial state.…”

“…[23] With regard to redox-responsive behavior, different terarylenes (aryl = thiazolyl, thienyl group or a combination of the two) have been reported to undergo oxidation induced ring-opening of their respective closed forms and, moreover, this ring-opening reaction was found to proceed via an auto-catalytic chain reaction mechanism with an efficiency well above 100 %, [24,25] but no reductive cyclization is known for terarylenes, to our knowledge. To investigate the possibility of combining both the oxidative ringopening and the reductive ring-closing reaction within terar-ylenes, we set up to design appropriately N-methylpyridiniumsubstituted terarylenes, since this functionalization is the most effective for triggering reductive cyclization of diarylethenes at accessible potentials, [12,14] and also to study how the properties of such a bi-directional photo-and redox-switch, if confirmed, could possibly be modulated by tuning the structure of the central aryl group. To this end, two dicationic terthiazole-based molecules are synthesized and fully characterized (Scheme 1).…”

Synthesis and Properties of Photochromic Diarylethenes Bearing Thiazole Moiety

“…But, most of them are nonfluorescent or show rather small photochromic fluorescence changes that restrain their applications in fluorescent modulation switches. To solve the problem, novel diarylethene systems with other heteroaryl moieties have been designed and synthesized, such as pyrrole , indole , furan , pyridine , and thiazole . Among them, thiazole is an attractive aryl unit because of its low aromatic stabilization energy and its structure is similar to those of thiophene.…”