pH-Dependent cis → trans Isomerization Rates for Azobenzene Dyes in Aqueous Solution

Abstract

Azobenzenes can function as molecular switches driven by their unusual cis <--> trans photoisomerization properties. The stability of an azobenzene-based switch depends on its rate of thermal relaxation, which is known to depend on the solvent environment, but few kinetic studies in aqueous media have been reported. We use nanosecond UV laser flash photolysis-transient absorption spectroscopy to measure thermal cis --> trans isomerization rates for mono- and disubstituted p-aminoazobenzenes and p-hydroxyazobenzenes in water at 23 degrees C over the pH range of 4 to 11. Observed absorption transients are fit to first-order relaxation rate constants between 10(5) and 10(1) s(-1), which is generally much faster than in nonpolar solvents, and the relaxation rates vary systematically and predictably with pH as the equilibrium shifts to ionized forms of the dyes that isomerize much more rapidly. Acid ionization constants for these dyes determined from our kinetic mechanism are compared with the pH dependence of their equilibrium UV-vis spectra. New kinetics results may enable pH control of azobenzene-based molecular switching times.

Ligands Modulate Reaction Pathway in the Hydrogenation of 4‐Nitrophenol Catalyzed by Gold Nanoclusters

“…However, the different reaction pathway identified in the present study involves the formation of other intermediates, most probably 4,4′‐dihydroxyazobenzene. It was reported that the cis isomer of 4,4′‐dihydroxyazobenzene showed a broad UV/Vis absorption band at approximately λ =460 nm in the range of λ =400 to 500 nm if the solution pH was increased from 7 to 12 . As the pH of the reaction mixture used in this study was 10, the additional absorption bands at λ =476 and 494 nm could be attributed to the cis isomer of the 4,4′‐dihydroxyazobenzene intermediate.…”

Synthesis and Photochromic Properties of Configurationally Varied Azobenzene Glycosides

“…[6, 19, 20] Moreover, azobenzene substitution can influence pH sensitivity of photoisomerization and solvent dependency of isomerization. [21] Our results strongly suggest, that substitution may also dramatically affect the tendency of the compounds to interact via π–π stacking or hydrogen-bond mechanisms, and thus may lead to strongly varying photochromic properties.…”

Azobenzene photoswitches for biomolecules

“…43,44 Thermal rates are enhanced at low pH where protonation at the azo moiety reduces the activation barrier. 16,[45][46][47] The dipolar character of the thermal transition state is proposed to be similar to that of the excited state. Thus in general, red-shifted azobenzenes are found to exhibit faster thermal relaxation processes.…”