Synthesis of Extremely π‐Extended Porphyrin Tapes from Hybrid meso‐meso Linked Porphyrin Arrays: An Approach Towards the Conjugation Length

Directly meso‐meso, β‐β, β‐β triply linked porphyrin arrays are exceptional π‐conjugated molecules exhibiting remarkably red‐shifted absorption bands extending deeply in the IR region. In order to determine the effective conjugated length (ECL), we embarked on the synthesis of the porphyrin tapes far beyond the 12‐mer, which is the longest we have prepared so far. In this study, to find the compromise between the feasibility of the meso‐meso coupling reaction up to longer arrays and the sufficient solubility and chemical stability of the resultant porphyrin tapes, we prepared hybrid meso‐meso linked porphyrin arrays BOn up to 24‐mer, which have two different aryl groups, a 2,4,6‐tris(3,5‐di‐tert‐butylphenoxy) phenyl group (Ar¹) and a 3,5‐dioctyloxy phenyl group (Ar²). All these arrays were effectively converted into the corresponding triply linked porphyrin tapes TBOn by oxidation with DDQ‐Sc(OTf)₃. Importantly, the low energy Q‐band‐like absorption bands of TBOn are progressively red‐shifted with an increase in the number of porphyrins n until 16 but the red‐shift is saturated at n=16, indicating the ECL of the porphyrin tape to be around 14–16. The regularly introduced meso‐aryl bulky substituents impose facial encumbrance, hence leading to the effective suppression of π–π interactions as well as improvement of the chemical stabilities of TBOn .     

Exploration of electronically interactive cyclic porphyrin arrays

We have explored a variety of covalently and non-covalently assembled cyclic porphyrin arrays mainly as biomimetic models of light harvesting antenna in photosynthetic systems. The key reaction is Ag(I)-promoted coupling reaction of 5,15-diaryl zinc(II) porphyrin that provides a meso–meso linked diporphyrin. An advantage of this coupling reaction is its extremely easy extension to higher porphyrin arrays, since longer porphyrin arrays have practically the same reactivity as that of the monomer. On the basis of this strategy, we have prepared cyclic porphyrin arrays including directly meso–meso linked porphyrin rings CZ4–CZ8, large porphyrin wheels C12ZA and C24ZB, and three-dimensional porphyrin boxes D1–D3. Efficient excitation energy transfer along these cyclic porphyrin arrays has been revealed by the time-resolved transient absorption and fluorescence measurements.     

Directly Linked and Fused Oligoporphyrin Arrays from Oxidation of Metalloporphyrins

The Ag^I-promoted oxidative meso-meso coupling reaction of 5,15-diaryl Zn^II-porphyrins is advantageous in light of its high regioselectivity as well as its easy extension to large porphyrin arrays. Linear meso-meso linked porphyrin 128-mer and three-dimensionally arranged grid porphyrin 48-mer were isolated in a discrete form by repetitive oxidation reaction and subsequent gel-permeation chromatography (GPC)-HPLC. 5,15-Diaryl Cu^II-, Ni^II-, and Pd^II-porphyrins were converted to meso- doubly-linked diporphyrins by oxidation with(p-BrC₆H₄)₃NSbCl₆. End-aryl-capped meso-meso linked Cu^II-, Ni^II-, and Pd^II- diporphyrins were converted to completely fused meso-meso - -triply-linked diporphyrins through the oxidative ring closure (ODRC) reaction with (p-BrC₆H₄)₃NSbCl₆. Finally, we found that Sc^III-catalyzed oxidation with DDQ gave a very efficient ODRC reaction and hence allowed the synthesis of triply-linked oligoporphyrins up to 12-mer.     

Excitonic Coupling Strength and Coherence Length in the Singlet and Triplet Excited States of meso–meso Directly Linked Zn(II)porphyrin Arrays

Excitation‐energy transport (EET) phenomena in meso–meso directly linked Zn(II )porphyrin arrays in the singlet and triplet excited states were investigated with a view to electronic coupling strength and coherence length by steady‐state and time‐resolved spectroscopic measurements. To investigate energy transfer in the triplet states, we modified the Zn(II )porphyrin arrays with bromo substituents at both ends. The coupling strength of the Soret bands of the arrays was estimated to be about 2200 cm^?1, and that of the Q bands is about 570 cm^?1. The coherence length in the S₁ state of the Zn(II )porphyrin arrays was determined to be 4–5 porphyrin units, which is comparable to that of the well‐ordered two‐dimensional circular structure B850 in the peripheral light‐harvesting antenna (LH2) in photosynthetic purple bacteria. This indicates that the Zn(II )porphyrin arrays are well suited for mimicking natural light‐harvesting antenna complexes. On the other hand, the rate of energy transfer in the triplet state is estimated to be on the order of 100 μs^?1, and the very weak coupling between the triplet states (ca. 0.003 cm^?1), indicates that the triplet excitation energy is essentially localized on a single porphyrin moiety.     

Large Porphyrin Squares from the Self‐Assembly of meso‐Triazole‐Appended L‐Shaped meso–meso‐Linked ZnII–Triporphyrins: Synthesis and Efficient Energy Transfer

meso‐Triazolyl‐appended Zn^II–porphyrins were readily prepared by Cu^I‐catalyzed 1,3‐dipolar cycloaddition of benzyl azide to meso‐ethynylated Zn^II–porphyrin (click chemistry). In noncoordinating CHCl₃ solvent, spontaneous assembly occurred to form tetrameric array ( 3 )₂ from meso–meso‐linked diporphyrins 3 , and dodecameric porphyrin squares ( 4 )₄ and ( 5 )₄ from the L ‐shaped meso–meso‐linked triporphyrins 4 and 5 . The structures of these assemblies were examined by ¹H NMR spectra, absorption spectra, and their gel permeation chromatography (GPC) retention time. Furthermore, the structures of the dodecameric porphyrin squares ( 4 )₄ and ( 5 )₄ were probed by small‐ and wide‐angle X‐ray scattering (SAXS/WAXS) measurements in solution using a synchrotron source. Excitation‐energy migration processes in these assemblies were also investigated in detail by using both steady‐state and time‐resolved spectroscopic methods, which revealed efficient excited‐energy transfer (EET) between the meso–meso‐linked Zn^II–porphyrin units that occurred with time constants of 1.5 ps^?1 for ( 3 )₂ and 8.8 ps^?1 for ( 5 )₄.     

Photochemistry of covalently-linked multi-porphyrinic systems

Synthesis, structural characteristics, and optical and electrochemical properties of various covalently-linked porphyrin arrays are described. First, aromatic-spacer bridged diporphyrins were prepared in which the diporphyrin geometries were conformationally-restricted and thus suitable for detailed studies on the exciton coupling and the intramolecular energy and/or electron transfer reactions. Secondly, the Ag(I)-salt oxidation of 5,15-diaryl Zn(II) porphyrins provided meso–meso-linked Zn(II)-diporphyrins. This reaction is advantageous in light of its high regioselectivity and easy extension to longer porphyrin arrays. The doubling reaction was repeated up to the synthesis of a discrete 128-mer, which is, to the best of our knowledge, the longest man-made molecule. Finally, the oxidation of meso–meso-linked Zn(II) porphyrin arrays with a combination of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and Sc(III)(OTf)₃ produced fused porphyrin arrays with full π-conjugation, which displayed extremely small HOMO–LUMO gaps that reach into the infrared region.     

Synthesis and properties of hybrid porphyrin tapes

Hybrid porphyrin tapes 3 and 4 , consisting of a mixture of 3,5‐di‐tert‐butylphenyl‐substituted donor‐type Zn^II–porphyrins and pentafluorophenyl‐substituted acceptor‐type Zn^II–porphyrins, were prepared by a synthetic route involving cross‐condensation reaction of a Ni^II–porphyrinyldipyrromethane and pentafluorophenyldipyrromethane with pentafluorobenzaldehyde followed by appropriate demetalation, remetalation, and oxidative ring‐closure reaction. The Ni^II‐substituted porphyrin tapes 5 (Ni‐Zn‐Ni) and 6 (Ni‐H₂‐Ni) were also prepared through similar routes. The hybrid porphyrin tapes 3 and 4 are more soluble and more stable than normal porphyrin tapes 1 and 2 consisting of only donor‐type Zn^II–porphyrins. The solid‐state and crystal packing structures of 3 , 4 , and 5 were elucidated by single‐crystal X‐ray diffraction analysis. Singly meso–meso‐linked hybrid porphyrin arrays 12 and 14 exhibit redox potentials that roughly correspond to each constituent porphyrin segments, while the redox potentials of the hybrid porphyrin tapes 3 and 4 are positively shifted as a whole. The two‐photon absorption (TPA) values of 1–6 were measured by using a wavelength‐scanning open aperture Z‐scan method and found to be 1900, 21 000, 2200, 27 000, 24 000, and 26 000 GM, respectively. These results illustrate an important effect of elongation of π‐electron conjugation for the enhancement of TPA values. The hybrid porphyrin tapes show slightly larger TPA values than the parent ones.     

meso–meso‐Linked Diarylamine‐Fused Porphyrin Dimers

A meso–meso‐linked diphenylamine‐fused porphyrin dimer and its methoxy‐substituted analogue were synthesized from a meso–meso‐linked porphyrin dimer by a reaction sequence involving Ir‐catalyzed β‐selective borylation, iodination, meso‐chlorination, and S_NAr reactions with diarylamines followed by electron‐transfer‐mediated intramolecular double C?H/C?I coupling. While these dimers commonly display characteristic split Soret bands and small oxidation potentials, they produced different products upon oxidation with tris(4‐bromophenyl)aminium hexachloroantimonate. Namely, the diphenylamine‐fused porphyrin dimer was converted into a dicationic closed‐shell quinonoidal dimer, while the methoxy‐substituted dimer gave a meso–meso, β‐β doubly linked porphyrin dimer.     

Oligoporphyrin Arrays Conjugated to [60]Fullerene: Preparation,NMR Analysis,and Photophysical and Electrochemical Properties

We report the synthesis and physical properties of novel fullerene–oligoporphyrin dyads. In these systems, the C‐spheres are singly linked to the terminal tetrapyrrolic macrocycles of rod‐like meso,meso‐linked or triply‐linked oligoporphyrin arrays. Monofullerene–mono(Zn^II porphyrin) conjugate 3 was synthesized to establish a general protocol for the preparation of the target molecules (Scheme 1). The synthesis of the meso,meso‐linked oligopophyrin–bisfullerene conjugates 4 – 6 , extending in size up to 4.1 nm ( 6 ), was accomplished by functionalization (iodination followed by Suzuki cross‐coupling) of the two free meso‐positions in oligomers 21 – 23 (Schemes 2 and 3). The attractive interactions between a fullerene and a Zn^II porphyrin chromophore in these dyads was quantified as ΔG=−3.3 kcal mol⁻¹ by variable‐temperature (VT) ¹H‐NMR spectroscopy (Table 1). As a result of this interaction, the C‐spheres adopt a close tangential orientation relative to the plane of the adjacent porphyrin nucleus, as was unambiguously established by ¹H‐ and ¹³C‐NMR (Figs. 9 and 10), and UV/VIS spectroscopy (Figs. 13–15). The synthesis of triply‐linked diporphyrin–bis[60]fullerene conjugate 8 was accomplished by Bingel cyclopropanation of bis‐malonate 45 with two C₆₀ molecules (Scheme 5). Contrary to the meso,meso‐linked systems 4 – 6 , only a weak chromophoric interaction was observed for 8 by UV/VIS spectroscopy (Fig. 16 and Table 2), and the ¹H‐NMR spectra did not provide any evidence for distinct orientational preferences of the C‐spheres. Comprehensive steady‐state and time‐resolved UV/VIS absorption and emission studies demonstrated that the photophysical properties of 8 differ completely from those of 4 – 6 and the many other known porphyrin–fullerene dyads: photoexcitation of the methano[60]fullerene moieties results in quantitative sensitization of the lowest singlet level of the porphyrin tape, which is low‐lying and very short lived. The meso,meso‐linked oligoporphyrins exhibit ¹O₂ sensitization capability, whereas the triply‐fused systems are unable to sensitize the formation of ¹O₂ because of the low energy content of their lowest excited states (Fig. 18). Electrochemical investigations (Table 3, and Figs. 19 and 20) revealed that all oligoporphyrin arrays, with or without appended methano[60]fullerene moieties, have an exceptional multicharge storage capacity due to the large number of electrons that can be reversibly exchanged. Some of the Zn^II porphyrins prepared in this study form infinite, one‐dimensional supramolecular networks in the solid state, in which the macrocycles interact with each other either through H‐bonding or metal ion coordination (Figs. 6 and 7).     

SnIV Porphyrin Scaffolds for Axially Bonded Multiporphyrin Arrays: Synthesis and Structure Elucidation by NMR Studies

A simple, one‐step, supramolecular strategy was adopted to synthesize Sn^IV‐porphyrin‐based axially bonded triads and higher oligomers by using meso‐pyridyl Sn^IV porphyrin, meso‐hydroxyphenyl‐21,23‐dithiaporphyrin, and Ru^II porphyrin as building blocks and employing complementary and non‐interfering Sn^IV?O and Ru^II ??? N interactions. The multiporphyrin arrays are stable and robust and were purified by column chromatography. ¹H, ¹H–¹H COSY and NOESY NMR spectroscopic studies were used to unequivocally deduce the molecular structures of Sn^IV‐porphyrin‐based triads and higher oligomers. Absorption and electrochemical studies indicated weak interaction among the different porphyrin units in triads and higher oligomers, in support of the supramolecular nature of the arrays. Steady‐state fluorescence studies on triads indicated the possibility of energy transfer in the singlet state from the basal Sn^IV porphyrin to the axial 21,23‐dithiaporphyrin. However, the higher oligomers were weakly fluorescent due to the presence of heavy Ru^II porphyrin unit(s), which quench the fluorescence of the Sn^IV porphyrin and 21,23‐dithiaporphyrin units.     

Synthesis of Peptides by Silver‐Promoted Coupling of Carboxylates and Thioamides: Mechanistic Insight from Computational Studies

The mechanism of the recently described N→C direction peptide synthesis through silver‐promoted coupling of N‐protected amino acids with thioacetylated amino esters was explored by using density functional theory. Calculation of the potential energy surfaces for various pathways revealed that the reaction proceeds through silver‐assisted addition of the carboxylate to the thioamide, which is followed by deprotonation and silver‐mediated extrusion of sulfur as Ag₂S. The resulting isoimide is the key intermediate, which subsequently rearranges to an imide through a concerted pericyclic [1,3]‐acyl shift (O–sp²N 1,3‐acyl migration). The proposed mechanism clearly emphasises the requirement of two equivalents of Ag^I and basic reaction conditions, which is in full agreement with the experimental findings. Alternative rearrangement pathways involving only one equivalent of Ag^I or through O–sp³N 1,3‐acyl migration can be excluded. The computations further revealed that peptide couplings involving thioformamides require significant conformational changes in the intermediate isoformimide, which slow down the rearrangement process.     

meso‐Free BIII Subporphyrins with Electron‐donating Groups

meso‐Free B^III 5,10‐bis(p‐dimethylaminophenyl)subporphyrins were synthesized. They display red‐shifted absorption and fluorescence spectra, bathochromic behaviors in polar solvents, a high fluorescence quantum yield (Φ_F=0.57), and a small HOMO–LUMO gap mainly due to destabilized HOMO as compared with meso‐free B^III 5,10‐diphenylsubporphyrin. This subporphyrin serves as a nice precursor of various meso‐substituted B^III subporphyrins such as B^III meso‐nitrosubporphyrin, B^III meso‐aminosubporphyrin, and meso‐meso’ linked B^III azosubporphyrin dimer. Reactions of meso‐free B^III subporphyrins with NBS or bis(2,4,6‐trimethylpyridine)bromonium hexafluorophosphate gave meso‐meso′ linked subporphyrin dimers, often as a major product along with meso‐bromosubporphyrins.     

Azobenzene‐Bridged Porphyrin Nanorings: Syntheses,Structures, and Photophysical Properties

Azobenzene‐bridged β‐to‐β and meso‐to‐meso porphyrin nanorings were successfully synthesized by a palladium‐catalyzed Suzuki–Miyaura coupling reaction in a logical synthesis. The dimeric structure was confirmed by XRD analysis. The azo linkages in di‐ and tetramers are in the all‐trans conformation, whereas in the trimers one azo linkage can be interconverted between cis and trans under external stimulation. When trimeric isomers are heated to 333 K or higher, the azo linkages will be in the all‐trans configurations: the pure all‐trans trimer can be kept in the dark for several months. Fluorescence anisotropy and pump‐power‐dependent decay results revealed excitation energy transfer for azobenzene‐bridged zinc–porphyrin nanorings. The distances between porphyrin units of these azobenzene‐bridged porphyrin arrays are almost the same, but the exciton energy hopping (EEH) times for each wheel are markedly different. The dimer and meso‐to‐meso tetramer possess relatively short excitation energy transfer (EET) times (1.28 and 2.48 ps, respectively) due to their good planarity and rigidity. In contrast, the EET time for the trimeric zinc(II)–porphyrin array (6.9 ps) is relatively long due to its nonradiative decay pathway (i.e., cis/trans isomerization of azobenzene). Both di‐ and tetramers exhibit relatively high fluorescence quantum yields, whereas the trimers show weak emission because of structural differences.     

Nonlinear Optical Properties of Porphyrin Derivatives with Electron‐donating or Electron‐withdrawing Substituents

To investigate photoelectric properties of meso‐extended porphyrin derivatives with electron‐donating or electron‐withdrawing substituents, a series of functionalized porphyrin materials have been designed and synthesized by Suzuki coupling reaction. The meso‐extended structures were fully characterized by ¹H NMR, IR spectroscopy and mass spectrometry. The photophysical properties of porphyrin derivatives were carefully examined by UV‐Visible and fluorescence spectra, and the solvatochromic effect was observed and discussed. In particular, Z‐scan technique was employed to characterize the third‐order nonlinear optical (NLO) properties of the products such as nonlinear absorption and refraction, the third‐order nonlinear refractive indexes (??⁽³⁾‐value) of these porphyrin derivatives achieved 3.9×10^?12 esu. In addition, the compounds could be self‐assembled into highly organized morphologies through phase‐exchange method. All the results indicated that the discotic materials have the potential for optoelectronic applications.     

Formation of Discrete Ladders and a Macroporous Xerogel Film by the Zipperlike Dimerization of Meso–Meso‐Linked Zinc(II) Porphyrin Arrays with Di(pyrid‐3‐yl)acetylene

Metal porphyrins assemble to form a supramolecular architecture with a characteristic structure and characteristic properties and functions upon complexation with appropriate ligands. However, there are few applications of these assembly processes to the construction of polymeric porphyrin arrays with useful functionalities. In this study, we found that meso–meso‐linked Zn^II porphyrin arrays underwent zipperlike dimerization upon complexation with di(pyrid‐3‐yl)acetylene (DPA) in chloroform to form discrete double‐stranded porphyrin ladders. Similarly, the assembly of poly(zinc(II) porphyrinylene) with DPA gave a thermoresponsive gel, whose three‐dimensional network structure was so strong that a macroporous xerogel film was obtained.     

The Marriage of Peripherally Metallated and Directly Linked Porphyrins: Bromidobis(phosphine)platinum(II) as a Cation‐Stabilizing Substituent on Directly Linked and Fused Triply Linked Diporphyrins

A meso‐bromidoplatiniobis(triphenylphosphine) η¹‐organometallic porphyrin monomer was prepared by the oxidative addition of meso‐bromoZnDPP (DPP=dianion of 5,15‐diphenylporphyrin) to a platinum(0) species. The meso–meso directly linked dimeric porphyrin ( 5 ) was prepared from this monomer by silver(I)‐promoted oxidative coupling and planarized to give a triply linked dizinc(II) porphyrin dimer ( 8 ). Acidic demetallation of 8 afforded the bis(free base) 9 . Dimer 5 was demetallated then remetallated with nickel(II) to give the dinickel(II) analogue 10 , the X‐ray crystal structure of which showed a twisted molecule with ruffled, orthogonal NiDPP rings, terminated by square‐planar trans‐[Pt(PPh₃)₂Br] units. New compounds were fully characterized spectroscopically, and the fused diporphyrin exhibited a broad, low‐energy, near‐IR electronic absorption band near 1100 nm. Electrochemical measurements of this series indicate that the organometallic fragment is a strong electron donor towards the porphyrin ring. The triply linked organometallic diporphyrin has a substantially lowered first one‐electron oxidation potential (?0.35 V versus the ferrocene/ferrocenium couple (Fc/Fc⁺)) and a narrow HOMO–LUMO gap of 0.96 V. Solutions prepared for NMR spectroscopy slowly decompose with degradation of the signals, which is attributed to partial oxidation to the cation radical. This paramagnetic species can be reduced in situ by hydrazine to restore the NMR spectrum to its former appearance. The combined influence of the two [Pt(PPh₃)₂Br] electron‐donating substituents is sufficient to make dimer 5 too aerobically unstable to allow further elaboration.     

Synthesis and Aggregation Behavior of meso‐Sulfinylporphyrins: Evaluation of S‐Chirality Effects on the Self‐Organization to S–Oxo‐Tethered Cofacial Porphyrin Dimers

The synthesis and aggregation behavior of meso‐sulfinylporphyrins are described. The copper‐catalyzed C–S cross‐coupling reaction of a meso‐iodoporphyrin with benzenethiol and n‐octanethiol has proved to be an efficient method for the synthesis of meso‐sulfanylporphyrins, which are oxygenated by m‐chloroperbenzoic acid to produce the corresponding meso‐sulfinylporphyrins. Optically active zinc meso‐sulfinylporphyrins were successfully isolated by means of optical resolution of the racemates on a chiral HPLC column. Zinc sulfinylporphyrins readily undergo self‐organization through S–oxo–zinc coordination to form cofacial porphyrin dimers in solution, in which the hetero‐ and homodimers are present as a diastereomeric mixture. The aggregation modes of the S–oxo‐tethered porphyrin dimers were fully characterized by ¹H NMR, IR, and UV/Vis spectroscopy as well as DFT calculations on their model compounds, thus revealing that the self‐aggregation behavior depends on the combination of S chirality. The absolute configurations at the sulfur center can be determined by the exciton‐coupled CD method. The observed self‐association constant for the S–oxo‐tethered dimerization of (S)‐phenylsulfinylporphyrin in toluene is larger than that in dichloromethane, which reflects the difference in dipole moments between the homodimer and the monomer. In cyclic and differential pulse voltammetry, the first oxidation process of the cofacial dimers is split into two reversible steps, which indicates that the initially produced π radical cations are delocalized efficiently between the two porphyrin rings. The present findings demonstrate the potential utility of meso‐sulfinyl groups as promising ligands for investigating the effects of peripheral chirality on the structures and optical and electrochemical properties of metal‐assisted porphyrin self‐assemblies.     

Structure Determination of an AgI‐Mediated Cytosine–Cytosine Base Pair within DNA Duplex in Solution with 1H/15N/109Ag NMR Spectroscopy

The structure of an Ag^I‐mediated cytosine–cytosine base pair, C–Ag^I–C, was determined with NMR spectroscopy in solution. The observation of 1‐bond ¹⁵N‐¹⁰⁹Ag J‐coupling (¹J(¹⁵N,¹⁰⁹Ag): 83 and 84 Hz) recorded within the C–Ag^I–C base pair evidenced the N3–Ag^I–N3 linkage in C–Ag^I–C. The triplet resonances of the N4 atoms in C–Ag^I–C demonstrated that each exocyclic N4 atom exists as an amino group (?NH₂), and any isomerization and/or N4–Ag^I bonding can be excluded. The 3D structure of Ag^I–DNA complex determined with NOEs was classified as a B‐form conformation with a notable propeller twist of C–Ag^I–C (?18.3±3.0°). The ¹⁰⁹Ag NMR chemical shift of C‐Ag^I‐C was recorded for cytidine/Ag^I complex (δ(¹⁰⁹Ag): 442 ppm) to completed full NMR characterization of the metal linkage. The structural interpretation of NMR data with quantum mechanical calculations corroborated the structure of the C–Ag^I–C base pair.     

RhodiumIII/SilverI Relay Catalyzed C—H Aminomethylation with Imine Equivalents and Lewis Acid Catalyzed [4+2] Cycloaddition of Indoles with Triarylhexahydrotriazine†

A Rh^III/Ag^I relay‐catalyzed C(sp²)—H coupling of indoles with triarylhexahydrotriazine (THT) is reported in this context. Upon merging Rh^III‐catalyzed C(sp²)—H bond activation and silver promoted THT dissociation, an efficient indole's C3 aminomethylation protocol is uncovered, providing C3 aminomethyl indoles in good yields and exhibiting potential applications for the synthesis of complicated bioactive compounds. We revealed the C3‐selectivity of this reaction through a detailed mechanistic investigation. Meanwhile, during the examination of the reaction conditions, we discovered another [4+2] cycloaddition pathway to afford tetrahydro‐indolo[3,2‐c]quinoline scaffold products via silver or Lewis acid catalysis.     

Synthesis of three-dimensionally arranged porphyrin arrays via intramolecular meso-meso coupling

The synthesis and photophysical properties of three-dimensionally arranged porphyrin arrays with through-space electronic communication are reported. 1,3,5-Trioxamethylphenylene bridged Zn(II) porphyrin trimer 3 was coupled by Ag(I)-promoted oxidative coupling reaction to give porphyrin cage 5 comprising three meso-meso linked diporphyrins, which was then transformed by oxidation with DDQ and Sc(OTf)₃ into porphyrin cage 7 comprising three fused diporphyrins. Intramolecular meso-meso coupling reaction was applied to porphyrin pentamer 11 to provide porphyrin array 12 consisting of a porphyrin core flanked by two meso-meso linked diporphyrins. Further oxidation of 12 with DDQ and Sc(OTf)₃ afforded triply stacked porphyrin array 13 that is comprised of a porphyrin core flanked by two porphyrin tapes. UV-vis-NIR absorption and fluorescence spectra of 5, 7, 12, and 13 showed their distorted conformations and electronic interaction within the stacked porphyrin arrays.