Effect of solvent on cis-to-trans isomerization of 4-hydroxyazobenzene aggregated through intermolecular hydrogen bonds

The half-life (tau(1/2)) for thermal isomerization of cis-4-hydroxyazobenzene (c-HOAB) to its trans isomer is highly dependent on the solvent used. For non-aromatic solvents, the cis isomer is extremely unstable at 293 K (tau(1/2) < 1 min in methanol and acetonitrile and 6 min in cyclohexane), whereas the cis isomer is surprisingly stable in benzene (tau(1/2) = 125 min). However, addition of hydrogen chloride and triethylamine in benzene causes a remarkable decrease in tau(1/2). On the basis of ab initio molecular orbital calculations of the binding energies and the optimum structures of c-HOAB-solvent complexes, a mechanism for the thermal cis-to-trans isomerization reaction involving hydrogen-bonded dimers and complexes is suggested. Copyright (c) 2005 John Wiley & Sons, Ltd.