Expansion and contraction: Shaping the porphyrin boundary via diradical reactivity

Carbene functionalization of porphyrinoids through tosylhydrazones

Here, we investigated methods for carbene functionalization of porphyrinoids through metal catalyst-free thermal decomposition of their tosylhydrazones. For the first time, tetrapyrrolyl substituted carbenes were obtained via thermolysis of tosylhydrazones of the corresponding tetrapyrrolyl aldehydes and ketones in the presence of a base. The carbenes formed reacted thermally with substrates without a metal catalyst or light irradiation. Carbenes at the β-pyrrolic position of porphyrinoids reacted with styrene leading to cyclopropane derivatives of tetrapyrroles. Carbenes also reacted with 1,4-dioxane with their insertion into the C-H bond yielding a tetrapyrrole 1,4-dioxane conjugate. Thermolysis of tosylhydrazones of meso-formyl-β-octaalkylporphyrinoids led exclusively to the corresponding cyclopentane fused porphyrinoids via intramolecular carbene C-H insertion. A plausible reaction mechanism was discussed based on DFT calculations of the intermediates. The tetrapyrrolyl carbenes were found to be considerably more stable than other carbenes. The products of the functionalization of porphyrinoids via hydrazone formation and subsequent carbene reactions exhibited modified optical spectra. The method for carbene functionalization of porphyrinoids through thermal decomposition of their tosylhydrazones created a new synthetic pathway for tailoring the perimeter of tetrapyrrolic macrocycles. Moreover, this method allows the obtainment of dyes with controllable spectral optical properties. In particular, new tetrapyrrole derivatives possessing phytoporphyrin carbon skeletons which have not been accessible were obtained using a convenient straightforward procedure.

Developing strong NIR absorption materials through linear planar π-conjugated cyclopalladated complex dimers

Strong near-infrared absorption complexes with linear planar π-conjugated structure were presented.

Anion Control of Lanthanoenediyne Cyclization

A suite of lanthanoenediyne complexes of the form Ln(macrocycle)X₃ (Ln = La³⁺, Ce³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Lu³⁺; X = NO₃^-, Cl^-, OTf^-) was prepared by utilizing an enediyne-containing [2 + 2] hexaaza-macrocycle (2). The solid-state Bergman cyclization temperatures, measured via DSC, decrease with the denticity of X (bidentate NO₃^-, T = 267-292 °C; monodentate Cl^-, T = 238-262 °C; noncoordinating OTf^-, T = 170-183 °C). ¹³C NMR characterization shows that the chemical shifts of the acetylenic carbon atoms also rely on the anion identity. The alkyne carbon closest to the metal binding site, C_A, exhibits a Δδ > 3 ppm downfield shift, while the more distal alkyne carbon, C_B, displays a concomitant Δδ ≤ 2.5 ppm upfield shift, reflecting a depolarization of the alkyne on metal inclusion. For all metals studied, the degree of perturbation follows the trend 2 < NO₃^- < Cl^- < OTf^-. This belies a greater degree of electronic rearrangement in the coordinated macrocycle as the denticity of X and its accompanying shielding of the metal's Lewis acidity decrease. Computationally modeled structures of LnX₃ show a systematic increase in the lanthanide-2 coordination number (CN_La-mc = 2 (NO₃^-), 4 (Cl^-), 5 (H₂O, model for OTf^-)) and a decrease in the mean Ln-N bond length (La-N_average = 2.91 Å (NO₃^-), 2.78 Å (Cl^-), 2.68 Å (H₂O)), further suggesting that a decrease in the anion coordination number correlates with an increase in the metal-macrocycle interaction. Taken together, these data illustrate a Bergman cyclization landscape that is influenced by the bonding of metal to an enediyne ligand but whose reaction barrier is ultimately dominated by the coordinating ability of the accompanying anion.

Diboron Porphyrins: The Raman Signature of the In-Plane Tetragonal Elongation of the Macrocycle

We describe an unusual in-plane type of porphyrin core distortion, tetragonal elongation (TE), observed experimentally in diboron porphyrins. The vibrational spectra of several of these complexes exhibit shifts that we have assigned to this TE distortion by comparing experimental spectra with DFT computational findings. The influence of TE in porphyrin systems was isolated using DFT analysis of the well-known model compounds Ni(II)porphine and Zn(II)porphine, with the macrocycle ring constrained to eliminate the influence of out-of-plane (OOP) distortions. A significant down-shift in frequencies was observed for porphyrin normal vibrational modes, particularly the in-plane A_1g/B_1g modes that are dominated by contributions from stretching and bending of C_α-C_m coordinates. In contrast, TE had little effect on the v(Pyr_halfring) and δ(Pyr_def) modes, though the lowered symmetry of the system resulted in significant splitting of the B_2u and B_3u modes. The impact of the TE distortion upon the diboron porphyrin vibrational spectrum was probed experimentally using Raman spectroscopy of B₂O₂(BCl₃)₂(TTP), B₂OF₂(TTP), and B₂OPhOH₂(TTP) (TTP = 5,10,15,20-(tetra- p-tolyl)porphyrin). Comparing the experimentally obtained spectral signatures to the computational findings allowed us to assign the large shifts observed for the v₂ and v₃ modes to the TE distortion in diboron porphyrins.

Synthesis and antiaromatic character of alkyl-substituted di-peri-dinaphthoporphyrin Ni(II) complex

Antiaromatic molecules have recently been proven to display unique properties such as small HOMO–LUMO gaps, ambipolar carrier transporting and [Formula: see text]-stack-induced aromatic stabilizations. These intriguing properties have been demonstrated by stable antiaromatic molecules that exhibit a distinct antiaromatic character. Di-peri-dinaphthoporphyrins are peripherally [Formula: see text]-extended porphyrinoids that exhibit a planar structure and a distinct antiaromatic character stemming from their 24[Formula: see text] electron circuit. Herein, we report an alkyl-substituted di-peri-dinaphthoporphyrin Ni(II) complex 5Ni bearing less peripheral bulkiness as compared with phenyl-substituted ones. 5Ni displayed upfield-shifted [Formula: see text]H NMR signals due to [Formula: see text]-protons, indicating its distinct antiaromaticity, and a slightly shorter intermolecular packing distance (ca. 3.402 Å) in the solid state. Cyclic voltammetry shows a larger potential difference between the first and second oxidation waves, implying that this molecule tends to stack favorably in its oxidized state.

Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications

Two-dimensionally extended, polycyclic heteroaromatic molecules (heterocyclic nanographenes) are a highly versatile class of organic materials, applicable as functional chromophores and organic semiconductors. In this Review, we discuss the rich chemistry of large heteroaromatics, focusing on their synthesis, electronic properties, and applications in materials science. This Review summarizes the historical development and current state of the art in this rapidly expanding field of research, which has become one of the key exploration areas of modern heterocyclic chemistry.

Synthesis of Di-peri-dinaphthoporphyrins by PtCl2 -Mediated Cyclization of Quinodimethane-type Porphyrins

Di-peri-dinaphthoporphyrins can be regarded as a key and common substructure of fused porphyrinoids. PtCl2 -mediated cycloisomerization reaction of quinodimethane-type porphyrins provided these doubly fused porphyrins, which exhibit characteristic paratropic ring currents that presumably arise from 24π antiaromatic circuit as a dominant resonance contributor. UV/Vis absorption spectra, cyclic voltammetry, and excited-state dynamics as well as theoretical calculation support this conclusion.

Porphyrin Macrocycle Modification: Pyrrole Ring-Contracted or -Expanded Porphyrinoids

In recent years, several synthetic strategies aiming at the peripheral functionalization of porphyrins were developed. Particularly interesting are those involving the modification of β-pyrrolic positions leading to pyrrole-modified porphyrins containing four-, five-, six- or seven-membered heterocycles. Azeteoporphyrins, porpholactones and morpholinoporphyrins are representative examples of such porphyrinoids. These porphyrin derivatives have recently gained an increasing interest due to their potential application in PDT, as multimodal imaging contrast agents, NIR-absorbing dyes, optical sensors for oxygen, cyanide, hypochlorite and pH, and in catalysis.

Isoxazolidine-fused meso-tetraarylchlorins as key tools for the synthesis of mono- and bis-annulated chlorins

An efficient synthetic approach to prepare novel annulated chlorin derivatives, a meso-tetraarylchlorin bearing an azepine ring and another bearing simultaneously an azepine and a 2H-pyran ring, is described.

LED lighting as a simple, inexpensive, and sustainable alternative for Wolff rearrangements

The Wolff rearrangement suffers from many drawbacks with respect to its practical execution in the laboratory. Herein, commercial LED lamps are employed as a sustainable alternative for the classic protocols typically used for Wolff rearrangements.

Porphyrins functionalized by quaternary pyridinium units

In this review, free-base and metalloporphyrins, functionalized on meso-positions by quaternary pyridinium units, also referred to as cationic porphyrins, are presented. The article consists of five parts. In the first part free-base porphyrins are described, especially taking account on generation of singlet oxygen; next parts concern metalloporphyrins. The second and third parts deal with zinc and manganese porphyrins, respectively; in the fourth part copper, palladium, and platinum porphyrins are presented. In the fifth part, describing porphyrins with various metal ions an attention is paid to porphyrin metal-organic frameworks (MOFs) and metal-organic materials (MOMs) in which metalloporphyrins are immobilized; syntheses and characterization of obtained products are shown.