Control of chromophore symmetry by positional isomerism of peripheral substituents

The first example of the control of porphyrinoid chromophore symmetry based on the positional isomerism of peripheral substituents has been achieved by preparing tetraazaporphyrins (TAPs) with C(4h), D(2h), C(2v), and C(s) symmetry due to the relative arrangement of peripheral tert-butylamino and cyano groups as push and pull substituents, respectively. The four structural isomers were successfully isolated and characterized by (1)H NMR spectroscopy and X-ray crystallography. The band morphology in the Q-band region varies depending on the molecular symmetry due to the significant perturbation introduced into the chromophore by the push and pull substituents. The C(4h) and C(2v) isomers exhibit a single Q band, whereas the Q bands of the D(2h) and C(s) isomers show a marked splitting. The magnetic circular dichroism spectra indicate that the push-pull TAPs retain the properties of the 16-membered 18π-electron perimeter generally observed for porphyrinoids. Theoretical calculations have demonstrated that the perturbation introduced by the substituents lowers the D(4h) symmetry of the parent TAP π-conjugated system, and this results in significant spectral changes. A novel approach to the fine-tuning of the spectral properties of porphyrinoids based on changes in the chromophore symmetry is described.