Towards low-energy-light-driven bistable photoswitches: ortho-fluoroaminoazobenzenes

Abstract

Thermally stable photoswitches that are driven with low-energy light are rare, yet crucial for extending the applicability of photoresponsive molecules and materials towards, e.g., living systems. Combined ortho-fluorination and -amination couples high visible light absorptivity of o-aminoazobenzenes with the extraordinary bistability of o-fluoroazobenzenes. Herein, we report a library of easily accessible o-aminofluoroazobenzenes and establish structure–property relationships regarding spectral qualities, visible light isomerization efficiency and thermal stability of the cis-isomer with respect to the degree of o-substitution and choice of amino substituent. We rationalize the experimental results with quantum chemical calculations, revealing the nature of low-lying excited states and providing insight into thermal isomerization. The synthesized azobenzenes absorb at up to 600 nm and their thermal cis-lifetimes range from milliseconds to months. The most unique example can be driven from trans to cis with any wavelength from UV up to 595 nm, while still exhibiting a thermal cis-lifetime of 81 days. Graphical abstract

Design and Synthesis of Visible-Light-Responsive Azobenzene Building Blocks for Chemical Biology

“…the symmetric bis-parasubstituted azobenzene had a significantly shorter half-life compared to its mono-substituted counterpart. 39,42 Particularly surprising was that compound 7, which contains a fluoride instead of a bromide in one para position, exhibited the longest half-life with over 100 d under the same conditions (Figure 4). An analogous azobenzene in the literature with two fluorines in the para positions was reported to have a somewhat shorter half-life (95 h in ACN at 60 °C), 28 which follows the observed phenomenon of shortened half-lives of symmetric fluorinated azobenzenes.…”

“…Usually, tetra- ortho -fluoro azobenzenes do not, or hardly, absorb in the red-light region of the spectrum (>600 nm). In an attempt to further red-shift the tetra- ortho -fluoro azobenzenes, Priimagi and co-authors report an extensive library of combined tetra- ortho -substituted fluorinated and aminated azobenzenes . The addition of tertiary amines in the ortho -positions of classical azobenzenes increases the molar absorptivity in the visible region and ensures resistance toward glutathione (GSH) reduction, yet results in diminishing the half-lives into the second range. , Toward applying wavelengths for photoswitching within the “therapeutic window” (600–900 nm) where light has the maximum tissue-penetration depth, Pianowski and colleagues reported that introducing sp 2 -hybridized substituents that extend the conjugated π-electron system resulted in further red-shifting of the n−π* absorption bands .…”

New tricks and emerging applications from contemporary azobenzene research

“…Thus, reaching good PSDs may take hours even with high-intensity irradiation [31,34]. To tackle this problem, the group of Priimagi combined ortho-amination with -fluorination; thus, increasing the n-π* absorptivity while maintaining good to moderate cis-stability [35,36]. As the most promising example, the azobenzene 5 carrying pyrrolidino and fluoro substituents in the ortho position could be isomerized fast (in seconds) between > 80% cis and trans with 405 and 595 nm irradiation, respectively, while having a cis-half-life of 50 h at 25 °C.…”

“…Multiple switching cycles have been demonstrated without noticeable fatigue for both ortho-methoxylated and -fluorinated compounds [26,28]. Some ortho-fluoroaminoazobenzenes were also found to be stable over repeated switching cycles, but others decomposed even upon storage [36]. Thinking of potential future applications, a study comparing the relative stability of the most promising azobenzenes in different environments would be highly beneficial.…”

Structure–Property Relationship for Visible Light Bidirectional Photoswitchable Azoheteroarenes and Thermal Stability of Z-Isomers

“…Various approaches such as diazocines (Herges), tetra- ortho substitutions with fluorination (Hecht), alkoxy and S -ethyl (Woolley), , cyclic amine connections (Woolley and Priimagi), push-pull type connections (Woolley and Velasco), azo-BF 2 bridging (Aprahamian), etc. have been successfully employed in bringing longer wavelength (visible and NIR) photoswitchable azoarenes.…”