Photoisomerization and thermal isomerization of shuttlecock- and bowl-equipped (phenylazo)pyridines.

The photoisomerization and thermal isomerization of sterically hindered (phenylazo)pyridine derivatives with a shuttlecock-shaped (TbetNNPy) or a bowl-shaped framework (BmtNNPy as well as the azoxy counterpart, BmtNN(O)Py) have been investigated. The crystal structures of these compounds revealed a planar conformation of the (phenylazo)pyridine moiety for TbetNNPy and severely distorted conformations for BmtNNPy and BmtNN(O)Py. The quantum yields of the trans-to-cis photoisomerization of TbetNNPy and BmtNNPy is lower than those of unsubstituted (phenylazo)pyridines. The low quantum yields may, to a large part, be attributed to electronic factors rather than steric factors. While the shuttlecock framework in TbetNNPy does not affect the thermal cis-to-trans isomerization, as the activation parameters for TbetNNPy are quite similar to those of azobenzene and azopyridine derivatives, the bowl framework in BmtNNPy renders the thermal isomerization process slower by lowering the frequency factor to an extent that more than compensates for the lowered activation energy. The process is characterized with a large negative activation entropy and a small activation enthalpy, implying that the isomerization proceeds through a limited range of intermediates stabilized by the presence of the bowl framework.