ortho‐Fluoroazobenzenes: Visible Light Switches with Very Long‐Lived Z Isomers

Abstract

Improving the photochemical properties of molecular photoswitches is crucial for the development of light-responsive systems in materials and life sciences. ortho-Fluoroazobenzenes are a new class of rationally designed photochromic azo compounds with optimized properties, such as the ability to isomerize with visible light only, high photoconversions, and unprecedented robust bistable character. Introducing σ-electron-withdrawing F atoms ortho to the NN unit leads to both an effective separation of the n→π* bands of the E and Z isomers, thus offering the possibility of using these two transitions for selectively inducing E/Z isomerizations, and greatly enhanced thermal stability of the Z isomers. Additional para-electron-withdrawing groups (EWGs) work in concert with ortho-F atoms, giving rise to enhanced separation of the n→π* transitions. A comprehensive study of the effect of substitution on the key photochemical properties of ortho-fluoroazobenzenes is reported herein. In particular, the position, number, and nature of the EWGs have been varied, and the visible light photoconversions, quantum yields of isomerization, and thermal stabilities have been measured and rationalized by DFT calculations.

Determining the Photoisomerization Quantum Yield of Photoswitchable Molecules in Solution and in the Solid State

“…Azobenzene is one of the oldest photoswitches known, but new derivatives thereof are still being developed27282930. The low energy state of azobenzene is the trans-isomer and the high energy is the cis-isomer, which spontaneously isomerizes back to the trans isomer at ambient temperatures.…”

Reversible and Tunable Photoswitching of Protein Function through Genetic Encoding of Azobenzene Amino Acids in Mammalian Cells

“…This can be a useful property when biological processes on a similar timescale are being optically controlled; however, it may impact control of protein function at longer time scales and may complicate experimental designs and interpretation of experimental outcomes. In contrast, the half-life of cis - F 4 AzoF (t ½ at 25 °C is 2 years) [4] is dramatically increased, reducing thermal relaxation and showing complete stability for at least 24 h in proteins in live human cells. Due to the enhanced thermal stability of the cis - F 4 AzoF , it could be used to investigate long term biological events as well as for kinetics experiments in live animals.…”

“…[3] Another possibility is to enhance the thermal stability of the trans -isomer through lowering the energy of the n orbital and thereby increasing the energy of the n → π* transition. [4] …”

Introduction: Organic Photochromic Molecules

“…Within the literature on T-type molecule families, such as spiropyrans and azobenzene, thermal back-reaction rate is probably among the most discussed properties in feature chapters and reviews [29,34,52,60]. For the latter, substituents such as amino groups or further conjugated push-pull structures speed up the thermal back-reaction, and recent work reports on the capability of fluorine substituents in slowing down the back-reaction rate drastically [100]. As mentioned previously, HABI undergoes a photochromic reaction between a TPID dimeric form and a pair of radicals TPIR (see Section 1.2.3,Figure 1.19).…”