Expanded porphyrins. Functional photoacoustic imaging agents that operate in the NIR-II region

Photoacoustic imaging (PAI) relies on the use of contrast agents with strong molar absorptivity in the NIR-I/NIR-II region. Expanded porphyrins, synthetic analogues of natural tetrapyrrolic pigments (e.g. heme and chlorophyll),...

Triplet State Baird-Aromaticity in Macrocycles: Scope, Limitations and Complications

<p>The aromaticity of cyclic 4<i>n</i>p-electron molecules in their first pp* triplet state (T₁), labelled Baird-aromaticity, has gained growing attention in the last decade. Here we explore computationally the limitations of T₁ state Baird-aromaticity in macrocyclic compounds, <b>[<i>n</i>]CM</b>’s, which are cyclic oligomers of four different monocycles (M = <i>para</i>-phenylene (PP), 2,5-linked furan (FU), 1,4-linked cyclohexa-1,3-diene (CHD), and 1,4-linked cyclopentadiene (CPD)). We strive for conclusions that are general for various DFT functionals, although for macrocycles with up to 20 p-electrons in their main conjugation paths we find that for their T₁ states single-point energies at both canonical UCCSD(T) and approximative DLPNO-UCCSD(T) levels are lowest when based on UB3LYP over UM06-2X and UCAM-B3LYP geometries. This finding is in contrast to what has earlier been observed for the electronic ground state of expanded porphyrins. Yet, irrespective of functional, macrocycles with 2,5-linked furans (<b>[<i>n</i>]CFU</b>’s) retain Baird-aromaticity until larger <i>n</i> than those composed of the other three monocycles. Also, when based on geometric, electronic and energetic aspects of aromaticity, a <b>³[<i>n</i>]CFU</b> with a specific <i>n</i> is more strongly Baird-aromatic than the analogous <b>³[<i>n</i>]CPP</b> while the magnetic indices tell the opposite. To construct large T₁ state Baird-aromatic <b>[<i>n</i>]CM</b>’s the design should be such that the T₁ state Baird-aromaticity of the macrocyclic perimeter dominates over a situation with local closed-shell Hückel-aromaticity of one or a few monocycles and semi-localized triplet diradical character. Monomers with lower Hückel-aromaticity in S₀ than benzene (<i>e.g.</i>, furan) that do not impose steric congestion are preferred. Structural confinement imposed by, <i>e.g.</i>, methylene bridges is also an approach to larger Baird-aromatic macrocycles. Finally, by using the Zilberg-Haas description of T₁ state aromaticity we reveal the analogy to the Hückel-aromaticity of the corresponding closed-shell dications, yet, observe stronger Hückel-aromaticity in the macrocyclic dications than Baird-aromaticity in the T₁ states of the neutral macrocycles. </p>

Triplet State Baird-Aromaticity in Macrocycles: Scope, Limitations and Complications

<p>The aromaticity of cyclic 4<i>n</i>p-electron molecules in their first pp* triplet state (T₁), labelled Baird-aromaticity, has gained growing attention in the last decade. Here we explore computationally the limitations of T₁ state Baird-aromaticity in macrocyclic compounds, <b>[<i>n</i>]CM</b>’s, which are cyclic oligomers of four different monocycles (M = <i>para</i>-phenylene (PP), 2,5-linked furan (FU), 1,4-linked cyclohexa-1,3-diene (CHD), and 1,4-linked cyclopentadiene (CPD)). We strive for conclusions that are general for various DFT functionals, although for macrocycles with up to 20 p-electrons in their main conjugation paths we find that for their T₁ states single-point energies at both canonical UCCSD(T) and approximative DLPNO-UCCSD(T) levels are lowest when based on UB3LYP over UM06-2X and UCAM-B3LYP geometries. This finding is in contrast to what has earlier been observed for the electronic ground state of expanded porphyrins. Yet, irrespective of functional, macrocycles with 2,5-linked furans (<b>[<i>n</i>]CFU</b>’s) retain Baird-aromaticity until larger <i>n</i> than those composed of the other three monocycles. Also, when based on geometric, electronic and energetic aspects of aromaticity, a <b>³[<i>n</i>]CFU</b> with a specific <i>n</i> is more strongly Baird-aromatic than the analogous <b>³[<i>n</i>]CPP</b> while the magnetic indices tell the opposite. To construct large T₁ state Baird-aromatic <b>[<i>n</i>]CM</b>’s the design should be such that the T₁ state Baird-aromaticity of the macrocyclic perimeter dominates over a situation with local closed-shell Hückel-aromaticity of one or a few monocycles and semi-localized triplet diradical character. Monomers with lower Hückel-aromaticity in S₀ than benzene (<i>e.g.</i>, furan) that do not impose steric congestion are preferred. Structural confinement imposed by, <i>e.g.</i>, methylene bridges is also an approach to larger Baird-aromatic macrocycles. Finally, by using the Zilberg-Haas description of T₁ state aromaticity we reveal the analogy to the Hückel-aromaticity of the corresponding closed-shell dications, yet, observe stronger Hückel-aromaticity in the macrocyclic dications than Baird-aromaticity in the T₁ states of the neutral macrocycles. </p>

Benzene- and pyridine-incorporated octaphyrins with different coordination modes toward two PdII centers

Expanded porphyrins have received considerable attention due to their unique optical, electrochemical and coordination properties. Here, we report benzene- and pyridine-incorporated octaphyrins(1.1.0.0.1.1.0.0), which are synthesized through Suzuki-Miyaura coupling of α,α′-diboryltripyrrane with m-dibromobenzene and 2,6-dibromopyridine, respectively, and subsequent oxidation with 2,3-dicyano-5,6-dichlorobenzoquinone. Both octaphyrins are nonaromatic and take on dumbbell structures. Upon treatment with Pd(OOCCH3)2, the benzene-incorporated one gives a Ci symmetric NNNC coordinated bis-PdII complex but the pyridine incorporated one gives Ci and Cs symmetric NNNC coordinated bis-PdII complexes along with an NNNN coordinated bis-PdII complex bearing a transannular C–C bond between the pyrrole α-positions. In addition, these two pyridine-containing NNNC PdII complexes undergo trifluoroacetic acid-induced clean interconversion.