posted on 2022-07-29, 15:34authored byAusten Moss, Dustin E. Nevonen, Yi Hu, Vladimir N. Nesterov, Victor N. Nemykin, Hong Wang
Unsymmetric pentacenequinone-fused (cross-conjugated)
and pentacene-fused
(linear-conjugated) porphyrins were designed and synthesized. The
cross-conjugated (AM1–AM3) and linear-conjugated (AM5–AM7) porphyrins displayed strikingly different sets of
optical and electronic properties, both of which are unusual and nontypical
of porphyrins. MCD, DFT, and TDDFT calculations suggest that multiple
charge transfer states exist in both π-conjugated systems, which
contributes to the complex absorption and MCD spectra of these molecular
systems. The general Gouterman’s four-orbital model used to
explain porphyrin spectroscopy led to contradicting theoretical and
experimental data, and is thus not applicable for these molecular
systems. The “2 + 4” and “3 + 3” active
spaces have been deduced and have proven effective to interpret the
absorption and MCD spectra of the pentacenequinone-fused (cross-conjugated)
and pentacene-fused (linear-conjugated) porphyrins, respectively.
Spectroelectrochemistry of AM5–AM7 revealed broad and
intense IR absorptions in the range of 1500–2500 nm, illustrating
the exceptional ability of these pentacene-fused systems to accommodate
positive charges. A pronounced metal effect was observed for pentacene-fused
porphyrins. While pentacene-fused Ni(II) porphyrin (AM6) demonstrated an abnormal ability to stabilize pentacene
with a half-life of >28.3 days, the half-life of the free base
and
Zn(II) counterparts were normal, similar to those of pentacene analogues.
This work provides important and useful information on guiding new
material designs.