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
Highlights
Scheme 1 Substitution patterns of studied azobenzene derivatives and the abbreviations used for different secondary amines. moiety, leading to excellent yields for either mono- or diaminated product [42]
The intrinsic reaction coordinate (IRC) calculations [66, 67] with subsequent geometry optimizations showed that the cis-isomers corresponding to these two types of inversion transition states generally differ in geometry from one another (Tables S10–S12)
Compared to the parent ortho-fluoroazobenzenes, the degree to which the properties of the amino-substituted products differ depends on the π donation strength of the amino substituent: a strong donor such as pyrrolidine promotes the visible light absorptivity more than other amines, whereas piperidine, a weaker donor, preserves the long cis-lifetime better
Summary
Photochromic molecular switches such as azobenzenes [1], diarylethenes [2, 3] and spiropyrans [4,5,6] pave the way towards next-generation pharmaceuticals [7,8,9,10,11], catalysts [12,13,14] and functional materials [15,16,17,18] that can be activated or controlled with a spatially and temporally precise external stimulus–light. While ortho-methoxylated [33], fluorinated [31, 34,35,36] and chlorinated [37,38,39] compounds enable efficient switching between two essentially bistable forms with visible light, high irradiation intensities or long illumination periods (hours for ortho-chlorinated compounds [38, 39]) are required for efficient switching because of the low molar absorptivity of the n–π* band This is a drawback when the photoswitch is illuminated through tissue or other light-scattering or -absorbing material. We show that commercial and inexpensive l-proline derivatives can be used to create azobenzenes with similar photoswitching properties as the pyrrolidine-substituted ones, while enhancing water solubility and providing easy synthetic access to ortho-linkage to, e.g., bioactive molecules or polymer networks
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