Abstract
Aminoazobenzene derivatives with four ortho substituents with respect to the N–N double bond are a relatively unexplored class of azo compounds that show promise for use as photoswitches in biology. Tetra-ortho-methoxy-substituted aminoazobenzene compounds in particular can form azonium ions under physiological conditions and exhibit red-light photoswitching. Here, we report the synthesis and characterization of two bis(4-amino-2-bromo-6-methoxy)azobenzene derivatives. These compounds form red-light-absorbing azonium ions, but only under very acidic conditions (pH < 1). While the low pKa makes the azonium form unsuitable, the neutral versions of these compounds undergo trans-to-cis photoisomerization with blue-green light and exhibit slow (τ1/2 ≈ 10 min) thermal reversion and so may find applications under physiological conditions.
Highlights
The application of photoswitches to control biological targets has been a driving force for the development of photoswitches that operate at wavelengths that are compatible with cells and tissues
Calculations were performed using density functional theory (DFT) methods (B3LYP/6-31+G**) to optimize geometry, and Time-dependent density functional theory (TD-DFT) with a Solvation Model based on Density (SMD) to calculate absorption wavelength maxima
The relative stability of different conformations of the molecule was calculated, i.e., with the methoxy substituents on the same or on the opposite side of the N–N double bond. The conformation where both methoxy groups were on the opposite side was found to be the most stable one, the conformation where both methoxy groups were on the same side was predicted to be significantly populated at 20 °C
Summary
The application of photoswitches to control biological targets has been a driving force for the development of photoswitches that operate at wavelengths that are compatible with cells and tissues. While many classes of photoswitches are known [1,2], few of these are adaptable to controlling targets, such as proteins [3], while simultaneously exhibiting robust photochemistry in the red or near-infrared (NIR) regions of the spectrum [4,5,6,7]. Azonium ions – protonated forms of azobenzenes – have recently been found to exhibit photoswitching properties suitable for in vivo use [8,9,10]. The elevated pKa of 1 has been attributed to resonance stabilization of the azonium cation together with intramolecular H-bonding between the azonium proton and methoxy groups in ortho-position to the azo double bond [10]. Since the azonium ion 1 forms under physiological conditions, i.e., at neutral pH value in an aqueous solution, it is useful as a Beilstein J.
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