Novel photochromic 2,2′-bithiophene azo dyes

Abstract

The photochromic behaviour of two series of 2,2'-bithiophene azo dyes in THF solutions was studied. The photochromic properties and colour constancy were strongly dependent on the substitution pattern of the dyes. Under visible irradiation (> 420 nm) while some dyes exhibited a significant change in the colour intensity others exhibited an almost stable absorption. The photokinetic parameters of these systems are described.

Enhancement of the photochromic switching speed of bithiophene azo dyes

“…[29][30] In the last years we have synthesized a large variety of heterocyclic azo dyes substituted by electron-donating and electron-withdrawing groups ('push/pull' substitution pattern) that possess solvatochromic, photochromic and nonlinear optical (NLO) properties which are influenced by the electronic nature of the -conjugated bridge (thienylpyrrole or bithiophene) and by the substitution pattern on the aryl and heteroaryl-diazene moieties (Figure 1). [31][32][33][34][35][36][37][38] Figure 1 Earlier, we observed that bithiophene azo dyes exhibit photochromic properties at room temperature, although the speed of the photo and thermal isomerization is quite slow: visible light irradiation of phenyl substituted 2,2'-bithiophene azo dyes, in solution, leads in less than 30 s to a significant decrease of the absorbance at  max (35-50%) due to the conversion of the E-isomer to the Z-isomer. When the irradiation ceases the system returns to the initial state following a mono-exponential kinetics with an half-life time between 18 to 69 s, depending on the aryl substituents.…”

“…As a result a noticeable colour intensity change is easily perceivable by a naked eye, due to the switching between the two isomers. 38 In order to increase the speed of this process we decided to substitute the phenyl ring by thiazole, benzothiazole or thiadiazole heterocycles which are known to have a marked influence on the thermal stability of the Z-isomer. 33,35 The thiazolyl-, benzothiazolyl-and dithiazolyl-2,2'-bithiophene azo dyes presented in this study can be divided in four groups according to the nature of the aromatic ring linked to the azo group and the position of that linkage ( Figure 2).…”

“…42 As observed before, the change on the azo linkage position from the 5 to the 4 position of the bithiophene moiety decreased the photochromic properties and for compounds 2a,b a near constancy of the absorbance was observed with or without visible light irradiation. 38 The annelation of a benzene ring to the thiazole group (compound 3) had a strong effect on the absorption variation while keeping the thermal Z-E isomerization kinetics at the same level. This compound shows a very fast and pronounced response to the light stimulus ( Figure 5): visible irradiation leads in 3 seconds to 19% decrease of the absorption and, after removal of the light source, the system returns to the initial state in another 3 seconds.…”

“…For these two dyes the switching between the two photoisomers can be performed in less than 5 seconds with a notable absorption variation in the visible region. Compared to previously described bithiophene aryldiazenes (see compound 5 in table 1) 38 this set of thiazole, benzothiazole or thiadiazole bithiophene azo dyes shows much faster switching speeds between the two photoisomers (up to 180 times) and for benzothiazole azo dye 3 and thiadiazole azo dyes 4b-d the visible light irradiation leads to a noteworthy change in the visible spectrum which is indicative of a significant conversion (18 to 32%) of the trans-isomer into the cis-isomer.…”

Synthesis and Photochromism Properties of Anionic Waterborne Polyurethane Containing Azobenzene Chromophores

“…Azo dyes constitute a dominant class of synthetic dyes. These azo dyes exist stably in the trans-isomers form but can be photochemically converted into the cis-isomers form without any bond breakage, leading to a realignment of the molecular structure and an observable change in the UV absorption spectra (1)(2)(3). Because of their characteristic photochromism, azo dyes are widely used in optical memory, optical switches, information converters and ophthalmic sun-screening applications and are also potentially useful in security printing, solar energy, storage, nonlinear optics and biological systems (4)(5)(6).…”