Unsymmetric Pentacene- and Pentacenequinone-Fused Porphyrins: Understanding the Effect of Cross- and Linear-Conjugation

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.

不对称的五苯并芘醌桥联（交叉共轭）和五苯并芘桥联（线性共轭）的卟啉被设计并合成。交叉共轭的（AM1–AM3）和线性共轭的（AM5–AM7）卟啉展现出截然不同的光学和电子性质，这些性质均异常且非典型，不符合卟啉的常规特性。分子轨道对称性守恒（MCD）、密度泛函理论（DFT）和时间依赖密度泛函理论（TDDFT）的计算表明，这两种π共轭系统中存在多个电荷转移状态，这导致了这些分子系统复杂的吸收和分子轨道对称性守恒光谱。用于解释卟啉光谱的Gouterman四轨道模型在理论数据和实验数据上存在矛盾，因此不适用于这些分子系统。通过推断，得出了“2 + 4”和“3 + 3”的有效活性空间，分别有效地解释了五苯并芘醌桥联（交叉共轭）和五苯并芘桥联（线性共轭）卟啉的吸收和分子轨道对称性守恒光谱。AM5–AM7的光谱电化学揭示了在1500–2500 nm范围内存在宽而强烈的红外吸收，这展示了这些五苯并芘桥联系统对容纳正电荷的卓越能力。对于五苯并芘桥联卟啉，观察到明显的金属效应。尽管五苯并芘桥联镍(II)卟啉（AM6）显示出异常稳定五苯并芘的能力，其半衰期超过28.3天，而游离碱和锌(II)同类物的半衰期则正常，与五苯并芘类似物相似。本研究为引导新材料设计提供了重要且实用的信息。