Porphyrin Arch-Tapes: Synthesis, Contorted Structures, and Full Conjugation

Porphyrin
tapes possessing meso–meso β–β β–β triple
direct linkages have been targets of extensive studies because of
their fully conjugated characteristic π-electronic networks.
In this paper, we report porphyrin arch-tapes that bear additional
carbonyl group(s) or methylene group(s) inserted between one of the
β–β linkage(s) of the porphyrin tapes. The carbonyl-inserted
porphyrin arch-tapes were efficiently synthesized by double fusion
reactions of β-to-β carbonyl-bridged porphyrin oligomers
with DDQ and Sc­(OTf)3, and were converted to the methylene-bridged
porphyrin arch-tapes via Luche reduction with NaBH4 and
CeCl3 followed by ionic hydrogenation with HBF4·OEt2 and BH3·NEt3. While
the conventional porphyrin tapes display rigid and planar structures
and low solubilities, these porphyrin arch-tapes show remarkably contorted
structures, flexible conformations, and improved solubilities because
of the presence of the incorporated seven-membered ring(s). Interestingly,
the methylene-inserted arch-tapes exhibited conjugative electronic
interactions that were comparable to those of porphyrin tapes probably
owing to through-space interaction in the contorted conformations.
The carbonyl-inserted arch-tapes displayed distinctly larger conjugative
interactions owing to an active involvement of the carbonyl group(s)
in the electronic conjugation. A similar trend was observed in the
nonlinear optical properties, as evidenced by their two-photon absorption
cross sections. Furthermore, as a benefit of the contorted structures,
these porphyrin arch-tapes can catch C60 fullerene effectively.
Naturally, the electron-rich methylene-bridged arch-tapes exhibited
larger association constants than the electron-deficient carbonyl-bridged
arch-tapes. Among these arch-tapes, a methylene-bridged syn-Ni­(II) porphyrin trimer recorded the largest association constant
of (1.5 ± 0.4) × 107 M–1 in
toluene at 25 °C.