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

Abstract

Unsymmetric pentacenequinone-fused (cross-conjugated) and pentacene-fused (linear-conjugated) porphyrins were designed and synthesized. The cross-conjugated (AM 1 –AM 3 ) and linear-conjugated (AM 5 –AM 7 ) 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 AM 5 –AM 7 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.

Excited-State Charge Transfer in Push–Pull Platinum(II) π-Extended Porphyrins Fused with Pentacenequinone

“…By the application of appropriate potentials in a thin-layer optical cell (usually 100 mV past the redox potential), the spectrum of the oxidized and reduced species could easily be obtained and the diagnostic peaks could be utilized to identify the charge transfer products. This becomes especially important when the donor and acceptor entities are structurally different and/or fused, similar to the systems investigated here . Alternatively, chemical oxidizing and reducing agents could also be used in a controlled fashion to generate the spectra of the oxidized and reduced compounds.…”

“…Figure panels a and b show the spectral changes during the first oxidation and first reduction of SW-Pt1 in DCB, respectively. As expected for π-extended systems, considerable spectral changes were witnessed. During oxidation, diminished peak intensities of the neutral porphyrin were accompanied by new peaks at 455, 473, 574, and 613 nm.…”

“…Our previous work demonstrated that free-base, zinc, and nickel pentacenequinone-fused porphyrins possess unusually broad absorption bands covering a large range of electromagnetic spectrum from 350 to 800 nm. − It would be interesting to investigate the impact of pentacenequinone on the physicochemical properties of the platinum analogues of these π-extended porphyrins. In particular, conventional platinum porphyrins reported in the literature are based on porphyrin monomers.…”

“…This becomes especially important when the donor and acceptor entities are structurally different and/or fused, similar to the systems investigated here. 52 Alternatively, chemical oxidizing and reducing agents could also be used in a controlled fashion to generate the spectra of the oxidized and reduced compounds.…”

π-Extended Porphyrin–Phthalocyanine Heterojunction Devices Exhibiting High Ammonia Sensitivity with a Remarkable Light Effect

“…In this work, we report the use of two original highly π-conjugated porphyrin complexes in double-layer heterojunction devices in combination with LuPc 2 as the top layer and their response to ammonia. π-Extended porphyrins have received particular attention due to their unique photophysical and optoelectronic properties. , Recently, one of us (Wang) reported the synthesis of unsymmetrical polycyclic aromatic hydrocarbon (PAH)-fused porphyrins, i.e., pentacenequinone-fused (cross-conjugated) and pentacene-fused porphyrins (linear-conjugated), which can potentially impart original electrical properties. Despite they exhibit the same symmetry (D2h), they display very different optical and electronic properties due to the type of conjugation involved.…”