The absorption spectrum of cis-azobenzene

Abstract

Azobenzene is a prototypical photochromic molecule existing in two isomeric forms, which has numerous photochemical applications that rely on a precise knowledge of the molar absorption coefficients (ε). Careful analysis revealed that the previously reported absorption spectra of the "pure" isomers were in fact mutually contaminated by small amounts of the other isomer. Therefore, the absorption spectra of both trans- and cis-azobenzene in methanol were re-determined at temperatures of 5-45 °C. The thermodynamically more stable trans-azobenzene was prepared by warming the solution in the dark. To obtain the spectrum of cis-azobenzene three methods were used, which gave consistent results within the limits of error. The method based on the subtraction of derivative spectra coupled with a global analysis of the spectra recorded during thermal cis-trans isomerization is shown to give slightly more reliable results than the method using isomeric ratios determined by H-NMR. The described methods are readily generalizable to other azobenzene derivatives and to other photochromic systems. The practical implication of the re-determined ε values is demonstrated by a very high precision of spectrophotometric species analysis in azobenzene isomeric mixtures. The new ε values imply that the previously reported quantum yields must be revised.

A versatile method for the determination of photochemical quantum yields via online UV-Vis spectroscopy

“…Very recently, the molar absorption coefficients and photo-isomerization quantum yields in methanol have been re-determined and reviewed. 53,54 Here we examine the conversion of a pre-irradiated solution containing predominantly Z-7 with a 400 nm LED in a kinetic approach. In the case of azobenzene, the change in absorbance dA/dt induced by the continuous irradiation I 0 is given by: 50…”

A Natural Glycyrrhizic Acid‐Tailored Light‐Responsive Gelator

“…After standing for 20 min, the bottle-inversion method [15] was used to determine whether as upramolecular gel had formed or not. Additionally,d ifferent mixed solvents were also tested.A sc an be seen, trans-GAG formed at ransparent gel and ap artial gel in DMSO/H 2 Oa nd MeOH/H 2 O, respectively (Table S1,e ntries 14-15), buto nly clear solutionsi no ther systems (TableS1, entries [16][17][18][19][20][21]. Additionally,d ifferent mixed solvents were also tested.A sc an be seen, trans-GAG formed at ransparent gel and ap artial gel in DMSO/H 2 Oa nd MeOH/H 2 O, respectively (Table S1,e ntries 14-15), buto nly clear solutionsi no ther systems (TableS1, entries [16][17][18][19][20][21].…”

Principles of coaxial illumination for photochemical reactions: Part 2. Model validation

“…The reactor is illuminated radially by a series of 10 LEDs for cross-current illumination, mounted to a heat sink (3), and illuminated axially by an LED mounted to a bigger heat sink (4) the isomerization via absorption of UV-light and can be omitted as the samples are measured immediately after production. [7] The actinometric method used in this work is based on the model of Achi et al [7] and the measuring technique is based on the technique employed by Gauglitz and Hubig, [8] quantum yield values are reported by Gauglitz and Hubig [8] and absorbance values for cisazobenzene are reported by Vetrakova et al [20] This actinometer is used for cross-and co-current illumination validation. The method utilizes azobenzene (98%, VWR, Geel, Belgium) dissolved in methanol (Chromasolv 99.9%, Sigma Aldrich, Diegem, Belgium) at various concentrations that are pumped through the reactor and illuminated via either cross-or co-current illumination.…”

“…The concentration of trans-azobenzene is determined by UV-VIS at 358 nm. This wavelength is chosen as it was reported as optimal by Gauglitz and Hubig [8] and later shown by Vetráková et al [20] that ε trans ε cis ) 22 at 358 nm, indicating the absorbance of cis-azobenzene to be minimal, although it will not be neglected in this work. The concentration is determined by the following formula:…”

“…The calibration line is shown in Section S3.4, the specific absorbance value of ε cis = 73 ± 13 (M −1 cm −1 ) is used. [20] Once the concentration of trans-azobenzene is known, the received light flux is calculated based upon the obtained conversion. Two different models will be utilized, for cross-and co-current, respectively.…”