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

The half‐life (τ 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 (τ 1/2  < 1 min in methanol and acetonitrile and 6 min in cyclohexane), whereas the cis isomer is surprisingly stable in benzene (τ 1/2  = 125 min). However, addition of hydrogen chloride and triethylamine in benzene causes a remarkable decrease in τ 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 © 2005 John Wiley & Sons, Ltd.