Neo-N-confused Phlorins and Phlorinone: Rational Synthesis and Tunable Properties

Pd(II) and Cu(III) Complexes of Multiply Fused Pentaphyrin Isomers with Tunable Structures and NIR Absorption

Fused porphyrinoids have received increasing interest in light of their extended conjugation and unique coordination behavior. On the basis of our previously reported multiply fused pentaphyrin isomers 1 and 2, a novel isomer 3 has been synthesized in this work. 3 possesses a hexacyclic fused moiety with a nearly coplanar CCNN cavity involving an inverted pyrrole, which is slightly different from the CNNN ones of 1 and 2 involving an N-confused pyrrole. 1-3 possess cavities with three depronatable protons and thus they all can generate Cu(III) complexes. However, only 3Cu is stable under ambient conditions. On the other hand, 3 remains intact upon treatment with Pd(II) ions, while 1 and 2 could undergo structural rearrangement to accommodate Pd(II), affording 1Pd and 2Pd accompanied by the formation of a lactone ring and the addition of a methoxy group, respectively. Compared with the free bases, the complexes show distinct aromaticity and more intense near-infrared (NIR) absorption up to ca. 1600, 1170, and 1500 nm, respectively. The results indicate that the subtle modification of the linking modes between the pyrrolic units in the fused pentaphyrinoids is effective in modulating the coordination behavior for synthesizing complexes with tunable aromaticity and NIR absorption.

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.

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.

Structural isolation of NIR absorbing ferrocenyl bridged N-confused fused expanded phlorin, N-confused porphodimethene and the π-extended corrorin isomer: synthesis and characterization

Concise syntheses and spectroscopic, solid state X-ray crystal structure and theoretical studies of three electronically appealing new generation hitherto unknown ferrocenyl bridged N-confused heterocyclic macrocycles with (without) fusion are reported. Intriguingly, the expanded N-confused fused phlorin (1.1.1.1.1) with the built-in tripentacyclic [5.5.5] moiety exhibits tailing of the NIR absorption band beyond 1000 nm while the nonconjugated porphodimethene and a new generation π-extended isomeric corrorin analogue exhibit UV-vis absorption.

Syntheses of thiophene appended N-confused phlorin isomers

A doubly confused thiapentapyrrane NSP-5 was synthesized by acid-catalysed condensation. Subsequent oxidation with DDQ did not afford the expected thiasapphyrin-like product. Instead, two tetrapyrrolic macrocycles, i.e. neo-N-confused phlorin (1) and N-confused phlorin-II (2) were obtained in the yields of 14% and 18%, respectively. The compounds were characterized by NMR, HRMS, and X-ray diffraction analyses. Single crystal structures clearly reveal that the thienyl units are not embedded into the macrocycles, but appended as meso-substituents, and the C[Formula: see text]-N and C[Formula: see text]-C[Formula: see text] cyclization modes can be clearly revealed by the crystal structures of 1 and 2, respectively. The observation that the thienyl unit is not involved in oxidative cyclization may be related to the relatively low reactivity of the thiophene moiety compared with the more electron-rich pyrrole unit. These results indicate that oxidative cyclization of linear thiaoligopyrranes containing terminal thiophene units may be developed as an effective approach for synthesizing nonconjugated macrocycles like phlorin analogues.

Doubly N-confused phlorin and phlorinone analogue

A doubly N-confused phlorin and phlorinone analogue were synthesized from a β,β'-linked dipyrromethane precursor and characterized by means of NMR and UV-Vis spectroscopies, X-ray crystallography, and electrochemistry. Solvents have a considerable impact on the optical absorption of the doubly N-confused phlorin so that it can differentiate simple alcohols such as methanol and ethanol.

Expanded N-Confused Phlorin: A Platform for a Multiply Fused Polycyclic Ring System via Oxidation within the Macrocycle

Novel interrupted π-conjugated macrocycles derived from expanded porphyrinoids were synthesized, and their unique reactivity was investigated in this work. The specific porphyrin analogs, so-called phlorins and isoporphyrins, possess a meso-sp³ methylene moiety, showing inner 3NH and 1NH pyrrolic cores, respectively, and extended near-infrared (NIR) absorption. Expanded N-confused pentapyrrolic phlorin analog 1 bears an interrupted cyclic π-conjugated system that is featured by a distinct higher HOMO and a lower LUMO. Oxidation of 1 allowed structural transformations through the expanded isoporphyrin-like species 2. One of the representative products is a spiro-carbon-bridged multiply N-fused product 3 comprising a fused [5.6.5.7.6.5]-hexacyclic ring obtained by oxidation with 2,3-dichloro-5,6-dicyano-p-benzoquinone. When magic blue was used as the oxidant, an aromatic N-confused pentaphyrin 4 was obtained via migration of one of the meso-phenyl groups to the β-position of the neighboring pyrrolic ring. By employing the flexible cavity of 1 for metal coordination, Pd(II) complexation occurred with a specific meso oxygenation to give a bimetallic complex 5. In contrast to the rich oxidation reactions, reduction of 1 with NaBH₄ resulted in the regioselective nucleophilic hydrogen substitution reaction at the para position of one of the meso-C₆F₅ groups. These results provide a practical approach for synthesizing novel interrupted or aromatic π-conjugated frameworks showing NIR absorptions.

Reversible π-system switching of thiophene-fused thiahexaphyrins by solvent and oxidation/reduction

The concept of chemical topology has generated considerable interest among chemists and one of the state-of-the-art topics is Möbius topology in cyclic π-conjugated molecules. In this regard, expanded porphyrins have been extensively studied because of their facile topological interconversions and attractive optoelectronic properties. A typical example involves [28]hexaphyrins: they show topological conversion between planar Hückel and twisted Möbius topologies owing to their flexible structure. With this in mind, we designed a [28]hexaphyrin where one dimethine pyrrole unit was replaced with dithieno[3,4-b:3',4'-d]thiophene (β-DTT), aiming at a reversible switching between macrocyclic and cross-conjugated π-systems by a change in molecular topologies. Considering that the β-DTT unit can offer both macrocyclic and cross-conjugated π-circuits, we envisioned that a combination of the topological interconversion of [28]hexaphyrin with the two π-circuits of the β-DTT unit would enable a reversible switching between macrocyclic and cross-conjugated π-circuits on Möbius and Hückel topologies, respectively, by a simple conformational change. Unexpectedly, the hexaphyrin revealed a unique, unprecedented π-system switching between a Möbius cross-conjugated π-system and a Hückel antiaromatic π-system, which was fully supported by both experimental and theoretical investigations. Meanwhile, the [28]hexaphyrin was also found to be redox interconvertible with the corresponding [26]hexaphyrin with a Hückel cross-conjugated π-system. These results demonstrate that the β-DTT unit is a new effective motif to realize π-system switching by changing molecular and π-system topologies. Importantly, external stimuli, i.e., solvent, as well as oxidation/reduction can be used to trigger the topological changes in expanded porphyrins with the help of the β-DTT unit.

Synthesis of a doubly SO2-fused phlorin: Tuning the structure and properties by the SO2 groups

A doubly SO2-fused phlorin 4 has been synthesized by the [2 + 2] condensation of dipyrromethanecarbinol 2 and SO2-fused dipyrromenthane 3 in the presence of TFA, followed by DDQ oxidation. The SO2-fused phlorin 4 has been characterized by absorption, fluorescence, mass and NMR spectra, as well as X-ray analysis. Compared to the [Formula: see text]-unsubstituted phlorin 5, the SO2-fused phlorin 4 exhibits a red-shifted absorption spectrum (around 12 nm), a more distorted molecular conformation, as well as nice photostability even with an electron-donating meso-3,5-di-tert-butylphenyl group. The titration of 4 and 5 with TBAF has been monitored by absorption spectroscopy. The deprotonated phlorin 4 shows a peak at 870 nm which is red shifted by 26 nm compared to that of deprotonated 5.

Calix[5]phyrin for Fluoride Ion Sensing with Visible and Near Infrared Optical Responses

Fluoride (F- ) ion sensing is an important topic due to its roles in health, medical, and environmental sciences. In this regard, colorimetric sensors with a near infrared (NIR) optical response are useful in biological systems because they can avoid interference from endogenous chromophores. Although calix[n]phyrins are highly attractive as sensors with the NIR optical response, studies on calix[n]phyrins are still limited owing to their intrinsic instability against ambient light and air. In this study, we report the synthesis and characterization of a new calix[5]phyrin bearing one sp3 -hybridized carbon atom as a π-expanded calix[n]phyrin. Upon addition of tetrabutylammonium fluoride, the calix[5]phyrin exhibited distinct NIR absorptions at 908 and 1064 nm as well as a visible color change. Importantly, it revealed an excellent selectivity for F- ion. These results demonstrate that calix[5]phyrins are promising colorimetric and NIR sensors of F- ion.