meso-Aryl Phenanthroporphyrins: Synthesis and Spectroscopic Properties

The successful synthesis of tetraphenyltetraphenanthroporphyrin (TPTPhenP; 5a) in 2006 under modified Rothemund-Lindsey conditions yielded a tetraphenyl porphyrinoid with a B band redshifted to an unprecedented 576 nm. Radially symmetric fused-ring expansion of tetraphenylporphyrin with phenanthrene moieties results in very deep saddling due to steric crowding and very marked redshifts of the Q and B (or Soret) porphyrinoid absorption bands. The extent to which the TPTPhenP structure can be further modified is explored, and the optical properties of TPTPhenPs are analyzed based on a perimeter model approach that makes use of time-dependent DFT calculations and magnetic circular dichroism spectroscopy and also based on a detailed analysis of the fluorescence emission. Attempts to introduce substituents at the ortho and meta positions of the meso-phenyl groups and to insert a central metal proved unsuccessful. The synthesis of a series of TPTPhenPs with strong electron-with-drawing (-CN, -NO(2)) and -donating (-CH(3), -N(CH(3))(2)) substituents at the para positions of the meso-phenyl rings is reported. Marked redshifts of the main spectral bands were consistently observed. The most pronounced spectral changes were observed with -N(CH(3))(2) groups (5i) due to a marked destabilization of the HOMO, which has large MO coefficients on the meso-carbon atoms. Protonation of 5i at both the ligand core and at the -N(CH(3))(2) groups resulted in unprecedented Q(00) band absorption at wavelengths greater than 1200 nm.