Synthesis of Phosphorescent Asymmetrically π‑Extended Porphyrins for Two-Photon Applications

Significant effort has been directed
in recent years toward porphyrins
with enhanced two-photon absorption (2PA). However, the properties
of their triplet states, which are central to many applications, have
rarely been examined in parallel. Here we report the synthesis of
asymmetrically π-extended platinum­(II) and palladium­(II) porphyrins,
whose 2PA into single-photon-absorbing states is enhanced as a result
of the broken center-of-inversion symmetry and whose triplet states
can be monitored by room-temperature phosphorescence. 5,15-Diaryl-syn-dibenzoporphyrins (DBPs) and syn-dinaphthoporphyrins
(DNPs) were synthesized by [2 + 2] condensation of the corresponding
dipyrromethanes and subsequent oxidative aromatization. Butoxycarbonyl
groups on the meso-aryl rings render these porphyrins
well-soluble in a range of organic solvents, while 5,15-meso-aryl substitution causes minimal nonplanar distortion of the macrocycle,
ensuring high triplet emissivity. A syn-DBP bearing
four alkoxycarbonyl groups in the benzo rings and possessing a large
static dipole moment was also synthesized. Photophysical properties
(2PA brightness and phosphorescence quantum yields and lifetimes)
of the new porphyrins were measured, and their ground-state structures
were determined by DFT calculations and/or X-ray analysis. The developed
synthetic methods should facilitate the construction of π-extended
porphyrins for applications requiring high two-photon triplet action
cross sections.