Photoisomerization of azobenzenecarboxylic acids and their potassium salts: Evidence of structural volume changes associated with hydrogen bond formation

The contractions determined by laser-induced optoacoustic spectroscopy (LIO AS) for the E --> Z photoisomerization of the three isomers (para, meta, an d ortho) of azobenzenecarboxylates in 0.01 M potassium phosphate buffer (pH 8) are rationalized in terms of the difference in the chromophore-water hy drogen bonds strength between the photoisomer and the parent compound. The relatively high concentration of K+ is needed to solubilize the compounds b y means of interrupting the intermolecular interactions. At this salt conce ntration a large portion of the anions is paired with potassium ions. Due t o the reduced conjugation, a stronger interaction of the nitrogens' lone pa irs and water is expected in the Z isomers, leading to the contraction upon isomerization. A higher K+ concentration reduces the structural volume cha nge as a consequence of its perturbing the hydrogen bond network. No struct ural volume change was observed for the E --> Z photoisomerization of meta- azobenzenecarboxylic acid in a series of cyclic alkanes, consistent with th e assignment of the changes to variations in the hydrogen bonds strengths, in view of the lack of those bonds in the cycloalkanes. Despite the relativ ely large error of the Z-E energy differences derived from LIOAS, they are near those reported for nonsubstituted azobenzene and larger than those cal culated for both free acid and lithium salts. With the LIOAS-derived struct ural volume changes Delta V-R and the quantitative relationship Delta S-R = (c(p)rho/beta)(ik) Delta V-R/T [(c(rho)p/beta)(ik) = 14 kJ cm(-3), the rat io of thermoelastic parameters at the isokinetic temperature in water, ca. 300 K], a large entropic term, in turn determined by the specific solute-wa ter interactions, is calculated for the Z --> E thermal isomerization.