ABSTRACT
The UV-visible absorption spectra of six new optical sensors based on acidochromic azobenzenes have been measured and assigned with the help of quantum chemical calculations. The investigated compounds are able to monitor the pH in the range from pH 3-10. Using the hybrid density functional PBE0 and including solvent effects with a polarized continuum model, the agreement between the experimental and theoretical UV/vis spectra of the dyes in their neutral and anionic forms is very good. The spectroscopic ππ* states, responsible for the optical properties of the sensors, are described within an accuracy of 0.1 eV. Similar accuracy is demonstrated in the nπ* states. The ππ* states can be assigned as a charge transfer from the aromatic π orbital localized in the azo-phenol moiety to the antibonding π* of the azo group. Under basic conditions, the spectrum is bathochromically shifted and more intense than in acid media. Upon substitution in the phenyl moiety, red- or blue-shifts of the UV-visible bands are observed depending on whether the substituent is electron-donor or -withdrawing, respectively. These effects are stronger at high pH values and can be rationalized in terms of the stabilization and/or destabilization of the involved frontier orbitals.
ABSTRACT
Eight new acidochromic dyes have been synthesised that can be used for optically monitoring pH in the range from 3 to 12. Their corresponding pK(a) values have been both measured and calculated theoretically by means of density functional theory. The synthesis of these new dyes is facile without the need for chromatographic purification. The dyes can be covalently linked to polymers containing hydroxyl functions such as cellulose, polyurethane hydrogel, and hydroxyalkyl methacrylate. The resulting sensor layers exhibit significant colour changes both in the UV and in the visible spectral range.
ABSTRACT
Three low molecular weight model compounds for poly(phenylene-ethynylene)s were inserted into the channels of zeolite L by using gas phase insertion. The absorption and emission spectra in solution and dye/zeolite L are reported. Two compounds show emission properties in zeolite L comparable to that in solution. In contrast, the bipyridine containing compound shows a red shift toward longer excitation wavelengths. Matrix rank analysis of the emission spectra gave three contributing species. Quantum chemical calculations provide different conformations depending on the Si/Al distribution of the framework and the extraframework cations and attached protons.
