Rate constants of the thermal cis-trans isomerization of azobenzene dyes in solvents, acetone/water mixtures, and in microheterogeneous surfactant solutions

Abstract

Rate constants k iso of the thermal cis-trans isomerization of four 4,4'-nitro-aminoazobenzenes with different amino groups have been determined in homogeneous aprotic solvents and polyglykol oligomers, primarily by means of conventional flash photolysis. The rate constants have been correlated with polarity (according to max from UV/Vis absorption spectra of the trans isomers) and bulk viscosity of the solvents. Qualitative conclusions about the influence of varying concentrations of water with respect to polarity and hydrogen bonding on k iso -and max -values in acetone/water mixtures were derived. Based on these results the data from microheterogeneous solutions have been interpreted.In microheterogeneous water/surfactant solutions k iso -values of selected azo dyes were strongly dependent on the concentrations of SDS, Triton ᭨ X-100, C 12 EO 8 in water, and varied with the composition of bicontinuous microemulsions of Igepal ᭨ CA-520/ heptane/water. The large spread of isomerization rate constants is in part due to varying microviscosity.Replacement of H 2 O by D 2 O in aqueous surfactant solutions produced surprisingly large kinetic solvent isotope effects.

Kinetics of the Z-E isomerization of monosubstituted azobenzenes in polar organic and aqueous micellar solvents

“…The surprisingly high polarities 15,16,17 for these media were probably the result of a "polar gradient" experienced by the probe in the interfacial region of the aggregate. In such a situation the amino end of the molecule sits in a relatively non polar environment while the nitro end projects into the highly polar region at the interface with the aqueous solution.…”

“…The reactivity of this derivative increases on going from methanol, to ethanol to water probably due to a higher sensitivity of the rotation mechanism towards additional solvophilic interactions of the n-butyl group in ethanol solution and/or towards solvophobic interactions in aqueous solution. It is interesting to note that similarly to our n-butyl derivative, also push-pull disubstituted derivatives exhibit higher rate constants in protic/polar than in apolar solvents due to the formation of hydrogen bonds 17,18 or to the competition of the rotational over the inversion mechanism. 31 In micellar solutions 4-X-azobenzenes, due to their low solubility in water, prefer to partition into the surfactant aggregates.…”

“…17,18 In this study we have investigated the Z-E isomerization of azobenzene and seven 4-X substituted azobenzenes (where X = CH3, n-Butyl, CF3, OCH3, Ot-Butyl, Br, F), bearing either an electron withdrawing or an electron donating group. We deliberately avoided the push-pull disubstituted azobenzenes, for which the competition of the two independent mechanisms had been previously attributed 17,18 to specific hydrogen bonding interactions with the solvent. We propose to use the thermal isomerization rate constants of the selected monosusbtituted azo dyes to estimate the polarity of micro-heterogeneous media.…”

Slower processes of the ultrafast photo-isomerization of an azobenzene observed by IR spectroscopy

“…A difference spectrum of NA in DMSO-d 6 could not be recorded. The cis form of NA in polar solvents is so short lived [3] that not enough cis isomer can be accumulated in a cw-experiment. In toluene the lifetime is prolonged ($70 s [11]) and a steady state difference spectrum is readily recorded.…”

“…This results in charge transfer (CT) absorptions and concomitantly a positive solvatochromism (see Ref. [3] for NA and Ref. [4] for para-nitroanilines).…”