Photoisomerization of Azobenzenecarboxylic Acids and Their Potassium Salts: Evidence of Structural Volume Changes Associated with Hydrogen Bond Formation

Resumen

The contractions determined by laser-induced optoacoustic spectroscopy (LIOAS) for the E → Z photoisomerization of the three isomers (para, meta, and ortho) of azobenzenecarboxylates in 0.01 M potassium phosphate buffer (pH 8) are rationalized in terms of the difference in the chromophore-water hydrogen bonds strength between the photoisomer and the parent compound. The relatively high concentration of K+ is needed to solubilize the compounds by means of interrupting the intermolecular interactions. At this salt concentration a large portion of the anions is paired with potassium ions. Due to the reduced conjugation, a stronger interaction of the nitrogens' lone pairs and water is expected in the Z isomers, leading to the contraction upon isomerization. A higher K+ concentration reduces the structural volume change as a consequence of its perturbing the hydrogen bond network. No structural volume change was observed for the E → Z photoisomerization of meta-azobenzenecarboxylic acid in a series of cyclic alkanes, consistent with the assignment of the changes to variations in the hydrogen bonds strengths, in view of the lack of those bonds in the cycloalkanes. Despite the relatively large error of the Z-E energy differences derived from LIOAS, they are near those reported for nonsubstituted azobenzene and larger than those calculated for both free acid and lithium salts. With the LIOAS-derived structural volume changes ΔVR and the quantitative relationship ΔSR = (cpρ/β)ik ΔVR/T [(Cpρ/β)ik = 14 kJ cm-3, the ratio of thermoelastic parameters at the isokinetic temperature in water, ca. 300 K], a large entropic term, in turn determined by the specific solute-water interactions, is calculated for the Z → E thermal isomerization.