A Benzodipyrrole-Derived Sapphyrin

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.

Theoretical analysis of expanded porphyrins: Aromaticity, stability, and optoelectronic properties

Expanded porphyrin systems are capable of binding a variety of substrates due to their increased cavity size and aromatic nature, holding important applications as magnetic resonance imaging contrast agents and as sensitizers for photodynamic therapy. It is there of fundamental interest to know the photoelectrical properties of expanded porphyrins using quantum chemistry calculations. In this work, we theoretically designed and screened a series of expanded porphyrins by incorporating terthiophene (TTH) and dithienothiophene (DTT) moieties. Our calculations showed that all the designed molecules exhibit excellent optoelectronic performance than the reference molecule. It is suggested that the porphyrin molecule with TTH moiety has better stability than the one with DTT moiety. Finally, we demonstrated that molecule 2 features with TTH moiety and the inverted selenophene ring outperform other molecules because it exhibits increased HOMO-LUMO gap, planar geometry, and strengthened aromaticity. We expect that this work can provide theoretical guidelines for the design of novel porphyrin materials.

Synthesis and Studies of Core-Modified Tellura Dithiasapphyrins

Four different aromatic meso-substituted tellurophene-containing dithiasapphyrins were synthesized by acid-catalyzed [3+2] condensation of 16-telluratripyrrane with bithiophene diol in 22-23% yields. The structural, spectral, and electrochemical properties of tellura dithiasapphyrins were studied and compared with those of previously reported aza (pyrrole)- and other heterocycle (furan, thiophene, and selenophene)-containing dithiasapphyrins. Nuclear magnetic resonance studies indicated that the tellurophene ring in tellura dithiasapphyrins is in the normal conformation, facing toward the inner core, but flips in diprotonated derivatives to an inverted conformation, facing away from the macrocyclic core, unlike aza- and other heterocycle-containing dithiasapphyrins in which pyrrole and the corresponding heterocycle ring always prefer to be in inverted conformation in their neutral and protonated forms. The crystal structure obtained for one of the tellura dithiasapphyrins showed that the macrocycle is highly planar and the tellurophene ring is in the normal conformation. The absorption spectra of tellura dithiasapphyrins exhibited slight hypsochromic shifts compared to those of pyrrole- and other heterocycle-containing dithiasapphyrins. The redox studies indicated that the tellura dithiasapphyrins are electron deficient and readily undergoes reductions. Density functional theory and nuclear independent chemical shift studies indicated that the macrocycles are aromatic, and the computational results closely matched the experimental observations.

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.

Doubly fused fluorene embedded heterosapphyrins

Synthesis and studies of first examples of nonaromatic doubly fused fluorene embedded heterosapphyrins and their intermediate macrocycles, monofused fluorene embedded hetero calixsapphyrins are reported.

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.

Palladium-Catalyzed Double Reductive Cyclization of 2,3-Dinitro-1,4-dialkenylbenzenes. Synthesis of 1H,8H-Pyrrolo[3,2-g]indoles

A flexible route to both symmetrical and unsymmetrical 1H,8H-pyrrolo[3,2-g]indole has been developed. The key and ultimate step is a double palladium-catalyzed, carbon monoxide mediated reductive cyclization of 1,4-dialkenyl-2,3-dinitrobenzenes. The cyclization precursors were prepared by a double Kosugi-Migita-Stille cross coupling of 1,4-dibromo-2,3-dinitrobenzene with an alkenyltin reagent to give symmetrical products. Unsymetrical cyclization precursors were prepared by two sequential cross couplings using 4-iodo-2,3-dinitrophenyl trifluoromethanesulfonate as the starting material.

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.

Conformationally Rigid Ethynylene-Cumulene Conjugated Aromatic [30] Heteroannulenes with NIR Absorption: Synthesis, Spectroscopic and Theoretical Characterization

Two hitherto unknown conformationally rigid Hückel aromatic ethynylene-cumulene conjugated [30] heteroannulenes have been synthesized and characterized. A thorough solution-state spectroscopic characterization, combined with in-depth theoretical calculations, has been performed to arrive at the proposed geometry of the macrocycles. The most stable optimized structures for the free base form of both the macrocycles showed absolute planar geometries without any ring inversion with mean plane deviation (MPD) values of 0.00 and 0.00 Å, respectively, in accordance with the NMR spectroscopic observations. The induced correspondence of rigid ethynylene-cumulene moieties leading to near-infrared (NIR) absorption in neutral and protonated forms of macrocycles is the important highlight of this article. This noteworthy finding has been supported by DFT-level theoretical calculations. There is an increasing pursuit in designing such NIR-absorbing/-emitting systems due to their immense applications in medicine and biology for recognizing and transportation of various substrates. The geometry of the novel 30π aromatic heteroannulenes shows promise for evolution of such novel systems in near future.

Synthesis of Nonaromatic and Aromatic Dithia Benzisapphyrins

A series of [24π] dithia meta-benzisapphyrins and [22π] dithia para-benzisapphyrins were synthesized by 3 + 2 condensation of appropriate benzitripyrrane with bithiophene diol under mild acid catalyzed conditions. The dithia m-benzisapphyrins and dithia p-benzisapphyrins were thoroughly characterized by HR-MS, 1D and 2D NMR, absorption, and electrochemical techniques. Our studies showed that dithia m-benzisapphyrins are nonaromatic, whereas the dithia p-benzisapphyrins are aromatic in nature. Thus, we demonstrated here that the dithiabenzisapphyrins can be made aromatic by replacing the m-phenylene moiety of the benzisapphyrin macrocycle with a p-phenylene unit. Furthermore, the studies also indicated that the aromaticity of the dithia p-benzisapphyrins was relatively more compared to the reported heterosapphyrins. The structural and spectral characteristics including aromaticity of the m-benzisapphyrins and p-benzisapphyrins were also discussed with the help of DFT, NICS, and TD-DFT studies.

Magnetic Circular Dichroism of Transition-Metal Complexes of Perfluorophenyl-N-Confused Porphyrins: Inverting Electronic Structure through a Proton

Neutral and deprotonated anionic Ni(II), Pd(II), Cu(II), and Cu(III) complexes of tetrakis(perfluorophenyl)-N-confused porphyrin (PF-NCP) were prepared and investigated by UV-visible and magnetic circular dichroism (MCD) spectroscopies. As in the previously reported Ni(II) adduct of tetraphenyl N-confused porphyrin, we observe sign reverse (positive to negative intensities with increasing energy) features in the MCD spectra of the neutral Ni(II), Pd(II), and Cu(II) complexes of PF-NCP, which is indicative of rare ΔHOMO < ΔLUMO relationships. Upon deprotonation of Ni(II), Pd(II), and Cu(II) complexes, these features revert to those of more typical porphyrin MCD spectra consistent with a ΔHOMO > ΔLUMO condition. The Cu(III) PF-NCP complex shows features similar to those of the deprotonated divalent metal systems. Spectroscopic features in all target complexes as well as previously published metal-free and Ni(II) NCP systems were correlated with the density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. Calculation data are consistent with the tautomeric rearrangement of the electronic structures of NCP cores playing dominant roles, with smaller contribution from the central metal ions in the observed optical and magneto-optical properties. This is true for all described NCP systems to date, as they affect the stabilization/destabilization of the N-confused porphyrin-centered Gouterman orbitals.

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.

2,2'-Bipyrrole-Based Porphyrinoids

A large number of porphyrinoids containing 2,2'-bipyrrole subunits have appeared since they were originally found as a component of sapphyrin and corrole, and it was found that the bipyrrole subunit endowed macrocycles with specific geometric features and electronic properties. Synthetic methods for bipyrrole-containing precursors for porphyrinoid are summarized in this review; these include coupling reactions of pyrrole rings, pyrrole ring-forming reactions leading directly to bipyrrole units, and synthetic reactions for oligopyrrolic compounds. Some hybrid oligopyrroles having nonpyrrole (hetero)aromatic ring(s) are also included. This review also describes porphyrinoids composed of bipyrrole subunits. Interesting electronic properties derived from strong cyclo-π-conjugation are highlighted in the bipyrrole-based porphyrinoids with or without meso-like carbons. Anion-binding chemistry is one of the main topics for bipyrrole-based macrocycles with less efficient or deficient cyclo-π-conjugation, such as those linked with electronically localized aromatic ring(s), with sp³ carbon(s), and with amido or imine connection(s). The principal concern in this review is porphyrinoids of relatively large ring size, composed of more than five units of pyrroles and (hetero)aromatic substitutes in total, and so bipyrrole-based porphyrinoids up to five pyrrolic units, such as corroles, porphycenes, sapphyrins, and smaragdyrin, will not be covered here except for some special cases.

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.

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'.

Dinaphthoporphycenes

Naphthobipyrrole is a potentially useful building block for porphyrin and porphyrin analogue synthesis. Reported here is a simple, generalizable synthetic route to α-formylated, β-substituted naphthobipyrroles and their use in the preparation of binaphthoporphycenes. The resulting binaphthoporphycenes possess a planar geometry as determined via a single-crystal X-ray diffraction analysis; they also display absorption maxima that are bathochromically shifted compared to simple porphycenes.