Aromatic Ring-Fused Dipyrrins: Programmed [4 + 2]-Cycloaddition Pathway with Regio-selectivity upon Alkylamino-Substitution

Sulfolenodipyrrins are employed as building blocks to concisely and efficiently construct aromatic rings (e.g., naphthoquinone, anthraquinone, fullerenes, and phthalimide) from fused dipyrrins by programmed [4 + 2]-cycloaddition reactions. Notably, alkylamino-substitution at the α-position not only enhances the reactivity of sulfolenodipyrrins but also results in the regio-selectivity of the cycloaddition reactions. Theoretical calculations in terms of frontier orbitals of dienes, energy of dienes, steric hindrance, and aromaticity have been conducted to understand the reason in depth. Additionally, the fusion of aromatic groups enables bathochromic absorption with up to ∼130 nm for the monoadducts and to ∼200 nm for the bis-adducts. The phthalimide annulation dipyrrin displays red emission, while the other mono- or bis-adducts do not, owing to the presence of typical acceptors such as quinone analogs or fullerene.

Porphyrindiene-Based Tandem Diels-Alder Reaction for Preparing Low-Symmetry π-Extended Porphyrins with Push-Pull Skeletons

Tandem Diels-Alder reactions of masked porphyrindienes (i.e., sulfolenoporphyrins) with benzoquinones and stilbenes, followed by aromatization, have been developed to load porphyrin with mixed annulation units (i.e., terphenyl and naphthoquinone), furnishing the low-symmetry π-extended porphyrins (DxAy) with push-pull skeletons. All low-symmetrical chromophores display panchromatic absorption spectra, which look like a spectral combination of symmetrical congeners (D4/A4) in a certain ratio. Among them, tD2A2 with trans-arrangement of push/pull units possesses the largest maximum centered at 766 nm with the onset around 900 nm. The fusion of the electron-deficient naphthoquinone moiety on the porphyrin core results in the approximately quantitative regulation of the E_ox1 and HOMOs (i.e., 0.10-0.13 V increase for the E_ox1 and 0.14-0.16 eV decrease for the HOMOs per naphthoquinone unit). In brief, this work provides a new way to construct low-symmetry π-extended porphyrins with tunable properties resorting to the ratios and locations of the annulated push-pull units.

Loading Phlorins with Fullerenes by a [4 + 2]-Cycloaddition Reaction: Regulation of the Regioselectivity by Pyrrole Linkage Modes

Sulfolenopyrrole-based normal and N-confused phlorins have been constructed to address the seldom touched phlorin functionalization and simultaneously explore the effect of the pyrrole linkage modes (αα, αβ) on the [4 + 2] cycloaddition reaction. The common sulfolenophlorin 1 contains two sulfolenopyrroles with the same reactivity upon tautomerization and undergoes stepwise [4 + 2]-cycloaddition with fullerene to furnish monoadduct 1-C60 and bisadduct 1-2C60 with a total yield up to 76%. By contrast, the presence of the confused pyrrole in 2 fixes the π-system owing to the low tendency to tautomerize and enables the two sulfolenopyrroles to exhibit in different fashions (i.e., normal NH-type and imino-type). Notably, under milder conditions (120 °C), the monofullerenoadduct 2-C60 forms rapidly and has been isolated from the [4 + 2] cycloaddition reaction of 2 and fullerene as the predominant fraction, accompanied by a trace amount of bisadduct 2-2C60. Raising the temperature to 140 °C did not improve the yield of 2-2C60. The structural analysis of 2-C60 indicates the attachment of fullerene at the iminopyrrole part. The high regioselectivity in the [4 + 2] cycloaddition of the imino-type sulfolenopyrrole unit has been rationalized thermodynamically by the DFT calculation on the relative energy of the two diene intermediates.

Solvent-regulated biomorphs from the intense π,π-mediated assemblies of tetracenequinone fused porphyrin

Tetracenequninone fused porphyrin exhibits remarkable π,π-stacking, which can be regulated by solvents to afford various biomorphs or cubic-shaped architectures.

Introduction of an isoxazoline unit to the β-position of porphyrin via regioselective 1,3-dipolar cycloaddition reaction

Isoxazoline-linked porphyrins have been synthesized by a regioselective 1,3-dipolar cycloaddition reaction between vinylporphyrin 2 and nitrile oxides. The steric interaction directed the reaction trajectory, in which only the product with a link between the 5-position of the isoxazoline and the β-position of porphyrin was observed. The isoxazoline-porphyrins 3a,b have been characterized by absorption, emission, 1H NMR and mass spectra. Later, the crystal structure of 3a was obtained and confirmed the basic features of the NMR-derived structure. Furthermore, a pair of enantiomers of 3a presented in the crystal, which formed a dimeric complex through intermolecular coordination between the Zn2+ center and the carbonyl group of the second molecule.

A Dipyrrin Programmed for Covalent Loading with Fullerenes

We describe here a di-(β,β'-sulfoleno)pyrrin programmed for efficient and specific β,β'-functionalization via [4+2] cycloaddition reactions. At 120 °C and in the presence of an excess of C60 -fullerene the di-(β,β'-sulfoleno)pyrrin decomposed cleanly, furnishing a di-(β,β'-fullereno)pyrrin and the corresponding monofullereno-dipyrrin in an overall yield of 96 %. Hence, relatively mild thermolysis of the di-(β,β'-sulfoleno)pyrrin induced stepwise extrusion of two equivalents of SO2 , producing highly reactive dipyrrin-β,β'-diene intermediates readily, providing a very effective path to [4+2]-cycloadducts. As presented here by the example of the covalent attachment of C60 -fullerene units, a convenient general methodology for the efficient synthesis of covalent dipyrrin β,β'-cycloadducts is made available.

Quinone-fused porphyrins as contrast agents for photoacoustic imaging

Photoacoustic (PA) imaging is an emerging non-invasive diagnostic modality with many potential clinical applications in oncology, rheumatology and the cardiovascular field. For this purpose, there is a high demand for exogenous contrast agents with high absorption coefficients in the optical window for tissue imaging, i.e. the near infrared (NIR) range between 680 and 950 nm. We herein report the photoacoustic properties of quinone-fused porphyrins inserted with different transition metals as new highly promising candidates. These dyes exhibit intense NIR absorption, a lack of fluorescence emission, and PA sensitivity in concentrations below 3 nmol mL-1. In this context, the highest PA signal was obtained with a Zn(ii) inserted dye. Furthermore, this dye was stable in blood serum and free thiol solution and exhibited negligible cell toxicity. Additionally, the Zn(ii) probe could be detected with an up to 3.2 fold higher PA intensity compared to the clinically most commonly used PA agent, ICG. Thus, further exploration of the 'quinone-fusing' approach to other chromophores may be an efficient way to generate highly potent PA agents that do not fluoresce and shift their absorption into the NIR range.

Panchromatic π-Extended Porphyrins from Conjugation with Quinones

Typical porphyrins are coloured compounds due to their characteristic wavelength-selective visible light absorptions. An exceptional "blackened" Zn^II -porphyrin was recently obtained from fusing the β,β'-positions of the porphyrin core with four quinone units. Here, studies with metal-free and Ni^II -containing analogues are reported. These quinone-fused porphyrins were prepared from the corresponding β,β'-tetrasulfolenoporphyrins via thermally generated porphyrindienes, which were trapped by [4+2] cycloaddition as benzoquinone cycloadducts, which were subsequently oxidised. By using this strategy, metal-free and Ni^II -containing porphyrins with one, two, three or four conjugated naphthoquinone moieties were prepared efficiently. The presence of the π-conjugated naphthoquinone moieties changed the porphyrin chromophores profoundly, giving broadly absorbing "blackened" pigments. The influence of coordinated Ni^II ions, or of the absence of a metal ion in the modified porphyrin core, on their structural and spectroscopic properties was explored. Blackened quinone-conjugated porphyrins might be pigments suitable for solar energy conversion. With their unique peripheral functional groups they are also a set of porphyrins "programmed" for further covalent extension. Thus, they are building blocks for the preparation of supra-porphyrinoid assemblies that might be useful in optoelectronics and in the nanosciences.

A DFT and QTAIM study of the novel d-block metal complexes with tetraoxa[8]circulene-based ligands

The transition metal complexes with the tetraoxa[8]circulene-based ligand are designed. The complexation process strictly depends on matching of the d-metal cation size to the size of the tetraoxa[8]circulene formed cavity.

meso-Tetraphenylporphyrin with a pi-system extended by fusion with anthraquinone

Fusion of a porphyrin with anthraquinone through the synthesis of a suitably substituted pyrrole that can be cyclotetramerized and the optical properties of the resulting molecular system are described.

Alkali and alkaline-earth metal complexes with tetraoxa[8]circulene sheet: a computational study by DFT and QTAIM methods

The optimized structure of the sandwich-type complexes [M(TOC)₂]ⁿ⁺(M = K⁺, Ca²⁺).