Synthesis and characterization of bis(4-amino-2-bromo-6-methoxy)azobenzene derivatives

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 pK _a makes the azonium form unsuitable, the neutral versions of these compounds undergo trans-to-cis photoisomerization with blue-green light and exhibit slow (τ_1/2 ≈ 10 min) thermal reversion and so may find applications under physiological conditions.

Keywords: azobenzene; azonium; molecular switches; ortho substitution; photoisomerization; photoswitch; visible light.

Figure 1

Structures of azonium ions studied.

Figure 2

a) Structures of model compounds used for computations (see Experimental section; in calculations, water was set as the solvent). b) Calculated spectra of neutral forms i and ii (black spectra) and azonium forms iii and iv (red spectra), carrying either para-piperidino (i and iii, dashed lines) or para-pyrrolidino (ii and iv, dash-dotted lines) substituents. Solid lines show experimental spectra of 4 in DCM (c ≈ 15 µM) without (black) or with TFA added (red).

Scheme 1

Synthesis of bis(4-amino-2-bromo-6-methoxy)azobenzene compounds.

Figure 3

a) UV–vis spectra of 4 in DCM (ca. 15 µM) at the PSS and 440 nm irradiation (thick dotted line; ca. 60% Z-isomer), and during thermal reversion to the dark-adapted state (solid black line). b) Time course of thermal reversion at 22 °C. Data were fitted to a single exponential decay equation (solid line).

Figure 4

a) UV–vis spectra of 5 in aqueous solution (c ≈ 15 µM, 5% methanol, pH 7) at the PSS and 440 nm irradiation (thick dotted line; ca. 60% Z-isomer) and during thermal reversion to the dark-adapted state (solid black line). b) Time course of thermal reversion at 22 °C. Data were fitted to a single exponential decay equation (solid line). c) Spectra of an aqueous solution of 5 upon addition of hydrochloric acid, as indicated by pH values. The wavelength of the absorption maximum of the azonium ion was 575 nm.