A platform for the development of highly red-shifted azobenzenes

Azobenzenes are versatile photoswitches that can be used to generate elaborate optical tools including photopharmaceuticals. However, the targeted application-guided design of new photoswitches with specific properties, such as the ability to use tissue-penetrating red light combined with defined thermal relaxation rates and stability towards biological reducing agents, remains challenging. We have developed synthetic protocols for derivatives of the dfdc (di-ortho-fluoro di-ortho-chloro) azobenzene scaffold with various chemical alterations in the para-/ortho-positions and performed an in-depth study into the effects of their structures on their photo-physical properties with an emphasis on the n → pi* absorption band using NMR, UV-vis and X-ray analysis. The data was used to establish and validate a computational approach that, among other parameters, allows to compute realistic UV-vis spectra by combining TD-DFT excited-state calculations from 6000 thermally accessible structures generated through ab-initio MD simulations while taking the high structural flexibility of ortho-substituted azobenzenes into account. With our platform, we added 15 new visible light-operated photoswitches to the toolbox for the development of optical devices that harbor relaxation rates across multiple orders of magnitude. We have identified several examples with stronger bathochromic shifts than the dfdc azobenzene lead structure and a high stability towards the biological reducing agent GSH, which is a prerequisite for in vivo applications. An intriguing example is dfdb (di-ortho-fluoro di-ortho-bromo) azobenzene which demonstrates accelerated photoswitching with deep red light at increased temperatures. Taken together, our combined experimental and computational study forms the foundation for the advanced in silico design and synthesis of new highly red-shifted azobenzene photoswitches.