Chiral 1,2-Subnaphthalocyanines

Following the first suggestion of inherent
molecular chirality in asymmetrically substituted subphthalocyanines by Torres
and co-workers in 2000, elucidation of the relationship between structure and
chirality has become an important issue. However, separation of the enantiomers
has been prevented by the low solubility of the molecules synthesized to date,
and it has not been possible to link the CD signs and intensities to their
absolute structures. Recently, we observed that 1,2-subnaphthalocyanines possess
two diastereomers with respect to the arrangement of the naphthalene moieties
and that these novel chiral molecules exhibit moderate solubility in common
organic solvents. This has enabled us to separate all of the diastereomers and
enantiomers. The two diastereomers have been completely characterized by NMR
spectroscopy and X-ray diffraction analysis. The absorption and magnetic
circular dichroism spectra, together with theoretical calculation, reveal a
small variation in the frontier molecular orbitals of the
1,2-subnaphthalocyanines compared with conventional subphthalocyanines, except
for destabilization of the HOMO–3, which results in a characteristic absorption
in the Soret band region. The chirality of 1,2-subnaphthalcyanines, including
the CD signs and intensities, is discussed in detail for the first time with
enantiomerically pure molecules whose absolute structures have been
elucidated by single-crystal X-ray diffraction analysis.