Dioxabenzosapphyrin: A New Benzodifuran-Derived Sapphyrin Analogue

Synthesis and Studies of Fused Benzo-Benzisapphyrins

A series of rare examples of fused benzo-benzisapphyrins were synthesized readily by (3 + 2) condensation of benzodipyrrole-derived diol and para-benzitripyrrane in the presence of 0.5 equiv of TFA in CH₂Cl₂ under inert atmosphere conditions accompanied by DDQ oxidation in open air. The crude compounds were separated by basic alumina column chromatography and afforded pure fused benzo-benzisapphyrins in 20-22% yields. The fused sapphyrins were characterized in detail by high-resolution mass spectrometry (HRMS) and one-dimensional (1D) and two-dimensional (2D) NMR spectroscopy. The ¹H NMR spectra recorded at both 298 and at 233 K clearly exhibited the presence of a strong diatropic ring current in benzo-benzisapphyrins, and the macrocycles are of aromatic nature. The DFT-optimized structure of benzo-benzisapphyrin revealed that the macrocycle was planar to a great extent due to the rigid structure of the dibenzopyrrole moiety, and the NICS(0) value of -11.2 ppm supports the aromatic nature of macrocycles. The absorption spectra of benzo-benzisapphyrins showed three weak Q bands approximately in the region of 650-900 nm and a strong Soret band at 480 nm, along with a shoulder band at ∼510 nm. The diprotonated derivative generated by the addition of excess TFA to the benzo-benzisapphyrin macrocycle exhibited bathochromically shifted absorption bands compared to the free base macrocycle.

Tailor-made aromatic porphyrinoids with NIR absorption

The highlight of this article is the recent progress in the state-of-the-art synthetic design and isolation of artificial porphyrinoids by swapping pyrrole component(s) with diverse functionalized pyrrolic(heterocyclic)/carbacycle building block(s) to compare the impact on the electronic absorption spectra and aromaticity of the incorporated isomeric/expanded porphyrinoids. Attention has been directed towards five distinct criteria of utilizing functionalized pyrrolic(heterocyclic)/aromatic hydrocarbons as synthons for NIR absorbing aromatic isomeric (N-confusion)/expanded porphyrinoids (with five/six heterocycles): (i) fused or annelated pyrrole (heterocycle), (ii) functionalized bi-pyrrole/bi-thiophene/bi-furan building blocks, (iii) azulene based carbacycle building block, (iv) vinylogous aromatic carbacycle/heterocycle(s) building block and (v) N-confused pyrrole ring(s), and N-confused fused pyrrole ring(s) leading to π-extension. These hybrid porphyrinoids are ideal candidates for basic research into macrocyclic aromaticity and for many potential applications owing to NIR absorption.

Stability, Aromaticity, and Photophysical Behaviors of Macrocyclic Molecules: A Theoretical Analysis

The macrocyclic molecules with terthiophene (TTH) isomers unit exhibit intriguing properties in terms of aromaticity, stability, and absorption. In this work, we theoretically designed a series of macrocyclic molecules featured with TTH and dithienothiophene (DTT) π-conjugated building units, which are used to permute pyrrole unit in porphyrin skeleton. Density functional theory and time-dependent DFT methods are used to evaluate the performance of the designed molecules. Our simulations show that molecules 1–3 exhibit excellent optoelectronic performance. Specifically, the molecule with the DTT unit is more stable than the one with TTH unit in terms of aromaticity and aromatic stabilization energy. This is because DTT unit enhances the coplanarity of the molecular material, facilitating electronic communication. Calculation of vertical electronic excitations suggests the absorption feature of these molecules is mainly contributed by the electronic excitations of higher occupied molecular orbital (HOMO) → lowest unoccupied molecular orbital (LUMO)+1 and HOMO-1 → LUMO. Judging from the key parameters determining the overall performance, 3 stands out because of its good planarity, large HOMO–LUMO gap, and strong aromaticity among all molecules. Interestingly, molecule 1 has the current density flow distributes around the outer section of TTH unit; in contrast, molecule 3 with DTT unit has the current density flow located at the inner section of DTT, which is beneficial for stability and aromaticity. Second-order perturbation energies are calculated to rationalize this observation. We expect that these research results can provide valuable insights into the rational design of novel molecular materials for a variety of applications.

Dibenzoylbenzodipyrroles: Key Precursors for the Synthesis of Fused meso-Aryl Sapphyrins

A simple strategy has been developed for the synthesis of α,α'-dibenzoylbenzodipyrroles, which are key synthons for the synthesis of fused porphyrinoids. α,α'-Dibenzoylbenzodipyrroles were characterized by high-resolution mass spectrometry (HRMS), NMR, and X-ray crystallography. To show the use of α,α'-dibenzoylbenzodipyrrole, we synthesized five different fused meso-aryl sapphyrins under acid-catalyzed reaction conditions. α,α'-Dibenzoylbenzodipyrrole was reduced to diol and condensed with five different tripyrranes such as aza, oxa, thia, selena, and telluratripyrranes under mild acid-catalyzed conditions to afford fused meso-aryl sapphyrins in 15-18% yields. One-dimensional (1D) and two-dimensional (2D) NMR studies revealed that in fused sapphyrins, the furan ring in oxabenzosapphyrin and the pyrrole ring in benzosapphyrin, which are present opposite to the benzodipyrrole moiety, attained ring inversion (inverted sapphyrins), whereas the selenophene ring in selenabenzosapphyrin and the tellurophene ring in tellurabenzosapphyrin did not show ring inversion (normal sapphyrins). However, thiabenzosapphyrin exhibits both normal and inverted conformations in different ratios. All fused sapphyrins showed typical aromatic absorption features; however, the absorption features of normal fused sapphyrins are different from the inverted fused sapphyrins. Redox studies indicate that normal fused sapphyrins are difficult to oxidize but easier to reduce compared to inverted fused sapphyrins. Density functional theory (DFT) studies support the experimental observations.

Dithienothiophene fused 30π heptaphyrin and 34π octaphyrins: Syntheses, characterization and spectral properties

Syntheses of DTT fused 30[Formula: see text] heptaphyrin and 34[Formula: see text] octaphyrins are reported. The inclusion of fused group in the precursor itself allowed easy syntheses of these macrocycles. Fusion makes the macrocycles planar thus allowing strong [Formula: see text] electronic conjugation. Both 30[Formula: see text] heptaphyrin and 34[Formula: see text] octaphyrins are aromatic in freebase and protonated forms as revealed by large [Formula: see text] values in 1H NMR spectrum and sharp-intense Soret like bands in UV-vis spectrum. Interestingly one of the DTT ring undergoes fliping upon protonation in 34[Formula: see text] doubly fused octaphyrin. Such a large ring fliping is unprecedented in the literature. Single crystal X-ray analysis confirms planar nature of 34[Formula: see text] octaphyrins. Furthermore, the structure reveals two type of hydrogen bonding interactions: (a) CH[Formula: see text] (thiophene) and (b) CH[Formula: see text] (mesityl) leading to self assambled dimer separated by a distance of ∼3.0Å.

Thiophene-fused dithiaoctaphyrins: π-system switching between cross-conjugated and macrocyclic π-networks

We synthesized for the first time octaphyrins with a dithieno[3,4-b:3′,4′-d]thiophene core as thiophene-fused dithiaoctaphyrins with a cross-conjugated structure.

Flexible Porphyrinoids

This review underscores the conformational flexibility of porphyrinoids, a unique class of functional molecules, starting from the smallest triphyrins(1.1.1) via [18]porphyrins(1.1.1.1) and concluding with a variety of expanded porphyrinoids and heteroporphyrinoids, including the enormous [96]tetracosaphyrin(1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0). The specific flexibility of porphyrinoids has been documented as instrumental in the designing or redesigning of macrocyclic frames, particularly in the search for adjustable platforms for coordination or organometallic chemistry, anion binding, or mechanistic switches in molecular devices. A structural prearrangement to coordinate one or more metal ions has been outlined. The coverage of the topic focuses on representative examples of geometry or conformational rearrangements for each selected class of the numerous porphyrinoids accordingly categorized by the number of built-in carbo- or heterocycles.

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.

A facile synthetic route to meso-tetraaryl substituted N-5 sapphyrins and first single crystal X-ray analysis confirming the pyrrole inverted structure

Three meso-tetraaryl substituted N -5 sapphyrins were synthesized by a facile synthetic route from the corresponding open-chain pentapyrroles in high yield using acetic acid as the solvent and catalyst. Each compound was characterized by spectroscopic and electrochemical methods and the inverted pyrrole structure was confirmed in the solid state for the first time by single crystal X-ray analysis.

β-Decamethoxysapphyrin and Its N-Benzyl Analogue

The synthesis of a highly electron-rich decamethoxysapphyrin and its 27-N-benzyl analogue is reported for the first time. The effects of β-methoxy and 27-N-benzyl substitution on structure, anion binding, absorption, and electrochemical properties were explored in detail. Upon 27-N-benzyl substitution, counteranion-induced structural deformation arises in the diprotonated state, which could be clearly noticed both in solution (1)H NMR study and solid-state structural analysis. This type of anion-induced structural deformation is noted for the first time in β-substituted sapphyrins. Further, the free base sapphyrins generate singlet oxygen with moderate efficiency (∼42%); hence, they may act as good photosensitizers.

Deprotonation reactions and electrochemistry of substituted open-chain pentapyrroles and sapphyrins in basic nonaqueous media

Electrochemical and spectroelectrochemical properties of three open-chain pentapyrroles and the corresponding sapphyrins were examined in pyridine containing 0.1 M tetra-n-butylammonium perchlorate and dichloromethane (CH2Cl2) or benzonitrile (PhCN) containing tetra-n-butylammonium hydroxide (TBAOH). The investigated compounds are represented as (Ar)4PPyH3 and (Ar)4SH3, where Ar is a F(-) or Cl(-) substituted phenyl group, PPy is a trianion of the open-chain pentapyrrole, and S is a trianion of the sapphyrin. The pentapyrroles, (Ar)4PPyH3, undergo two reversible one-electron reductions in pyridine, while the structurally related sapphyrins exhibit four reductions in this solvent, the first two of which are irreversible due to coupled chemical reactions following the electron transfers. Both series of neutral compounds could be deprotonated in CH2Cl2 or PhCN by addition of TBAOH to solution, and the progress of these reactions was monitored as a function of the base concentration by cyclic voltammetry and UV-vis spectroscopy. The neutral pentapyrroles were spectroscopically shown to undergo a loss of two protons in a single step to generate the [(Ar)4PPyH](2-) dianion while the sapphyrins could only be monodeprotonated, leading to formation of the [(Ar)4SH2](-) monoanion under the same solution conditions. The deprotonation constants were measured for each series of compounds in benzonitrile, and oxidation-reduction mechanisms are examined as a function of the solution 'basicity'.