Hierarchical Self-Assembly of Curved Aromatics: From Donor-Acceptor Porphyrins to Triply Periodic Minimal Surfaces.

A saddle-shaped π-extended zinc porphyrin containing a peripheral pyridyl ligand undergoes quantitative self-assembly into a cyclic trimer. The trimer has a prismatic structure with negatively curved side walls, which promote the formation of supramolecular organic frameworks stabilized by dispersion interactions. The first framework type, UWr-1, has the npo topology, with a hexagonal structure analogous to the Schwartz H triply periodic minimal surface. Co-crystallization of the trimer with either C60 and C70 produces the isomorphous cubic UWr-2 and UWr-3 phases, characterized by the ctn network topology and a structural relationship to the Fischer-Koch minimal surface S. All three phases contain complex labyrinths of solvent-filled channels, corresponding to very large probe-accessible volumes (68 % to 76 %). The UWr-2 network could be partly desolvated while retaining its long range dimensional order, indicating remarkable strength of the dispersion interactions in the crystal. A theoretical analysis of noncovalent interactions shows the role of geometrical matching between the negatively curved porphyrin units and positively curved fullerenes.

Supramolecular Subphthalocyanine Cage as Catalytic Container for the Functionalization of Fullerenes in Water.

Herein we report the first example of a supramolecular cage that works as a catalytic molecular reactor to perform transformations over fullerenes in aqueous medium. Taking advantage of the ability of metallo-organic Pd(II)-subphthalocyanine (SubPc) capsules to form stable host:guest complexes with C60 , we have prepared a water-soluble cage that provides a hydrophobic environment for conducting cycloadditions over encapsulated C60 , namely, Diels-Alder reactions with anthracene. Indeed, the presence of catalytic amounts of SubPc cage dissolved in water promotes co-encapsulation of insoluble C60 and anthracene substrates, allowing the reaction to occur inside the cavity under mild conditions. The lower stability of the host:guest complex with the resulting C60 cycloadduct facilitates its displacement by pristine C60 , which grants catalytic turnover. Moreover, bis-addition compounds are regioselectively formed inside the cage when using excess anthracene.

An efficient metal-free and catalyst-free C-S/C-O bond-formation strategy: synthesis of pyrazole-conjugated thioamides and amides.

An operationally simple and metal-free approach is described for the synthesis of pyrazole-tethered thioamide and amide conjugates. The thioamides were generated by employing a three-component reaction of diverse pyrazole C-3/4/5 carbaldehydes, secondary amines, and elemental sulfur in a single synthetic operation. The advantages of this developed protocol refer to the broad substrate scope, metal-free and easy to perform reaction conditions. Moreover, the pyrazole C-3/5-linked amide conjugates were also synthesized via an oxidative amination of pyrazole carbaldehydes and 2-aminopyridines using hydrogen peroxide as an oxidant.

Synthesis of Unsymmetrical Heterobenzisapphyrins.

Two examples of highly unsymmetrical heterobenzisapphyrins containing one benzene, two pyrroles, one furan, and one thiophene ring connected via four meso-carbons and one direct bond were prepared over a sequence of five steps in 8-9% yields. The X-ray structure revealed that the furan ring was inverted from the macrocyclic framework and the macrocycle was nearly planar. Spectral and electrochemical studies indicated that the macrocycle was nonaromatic in nature due to lack of conjugation.

Dibenzidecaphyrins (1.0.0.1.1.1.0.0.1.1) and Their Bis-BF2 Complexes.

The largest dibenziporphyrinoids discovered until date, dibenzidecaphyrins (1.0.0.1.1.1.0.0.1.1), were synthesized in 8-10% yields by condensing m-phenylene-based pentapyrrane with pentafluorobenzaldehyde under mild reaction conditions. The crystal structure obtained for one of the decaphyrin macrocycles revealed that the macrocycle adopts a figure eight conformation and showed nonaromatic features. The decaphyrin macrocycles were further characterized by various spectral, electrochemical techniques, and computational studies. The dibenzidecaphyrin macrocycle showed half the number of proton NMR resonances, indicating the high symmetry of the macrocycle. The decaphyrins showed a broad absorption in the 600-900 nm region along with a well-defined band at 448 nm, and the macrocycles were stable under redox conditions. The decaphyrins were treated with BF3·OEt2/triethylamine in CHCl3 at reflux temperature, followed by column chromatography, to afford bis-BF2 complexes of decaphyrin in 34-40% yields. The spectral and DFT studies supported a figure eight conformation for the bis-BF2 complexes, and electrochemical studies indicated that the bis-BF2 complexes were electron deficient compared to the free-base decaphyrin macrocycles.

Stable Core-Modified Doubly N-Fused Expanded Dibenziporphyrinoids.

We describe one-pot synthesis of stable doubly N-fused expanded dibenziporphyrinoids using readily available precursors under acid-catalyzed conditions. The doubly N-fused expanded dibenziporphyrinoids have been synthesized by adopting an inversion followed by fusion strategy. The studies showed that the dibenziporphyrinoids undergo mono fusion initially, but due to the high stability of doubly fused dibenziporphyrinoids, the monofused macrocycles undergo further fusion to form doubly fused dibenziporphyrinoids. The mono fusion and double fusion in these dibenziporphyrinoids were established by X-ray crystallography.

Synthesis, Structural, Spectral, and Electrochemical Studies of Selenabenziporphyrin and Its Pd(II) Complex.

A new nonaromatic selenabenziporphyrin was synthesized by (3 + 1) condensation of m-benzitripyrrane and 2,5-bis[( p-tolyl)hydroxymethyl] selenophene under mild trifluoroacetic acid-catalyzed reaction conditions. The selenabenziporphyrin was characterized by high-resolution mass spectrometry, one- and two-dimensional NMR spectroscopy, and X-ray crystallography. The crystal structure revealed that the macrocycle was planar with moderately tilted m-phenylene ring and that the phenylene ring completely blocks the macrocyclic π-delocalization. The selenabenziporphyrin exhibits one broad absorption band at 645 nm along with one sharp band at 415 nm, and electrochemical studies revealed that the macrocycle was electron-deficient. The selenabenziporphyrin readily forms organometallic Pd(II) complex when treated with PdCl2 in CH3CN/CHCl3 at reflux followed by recrystallization. The X-ray structure revealed that the Pd(II) ion was coordinated with two pyrrole "N"s, selenophene "Se", and m-phenylene ring "C" in square-planar fashion, and the complex retained its nonaromatic nature. The Pd(II) complex exhibits ill-defined absorption bands, and it was more electron-deficient than free-base selenabenziporphyrin macrocycle. Time-dependent density functional theory studies supported the experimental observations.

Strongly Coupled Oxasmaragdyrin-BF2 Chelated Dipyrrin Dyads: Syntheses, X-ray Structure, Ground- and Excited-State Charge-Transfer Interactions.

Oxasmaragdyrin (osm), a 22-π electron aromatic macrocycle containing four pyrroles and a furan ring connected by three methine bridges and two direct bonds is shown to be an excellent electron donor-sensitizer exhibiting adequate spectral and redox properties. Further, this novel heterocyclic macromolecule has been utilized in building donor-acceptor systems involving BF2 -chelated dipyrromethenes (BODIPYs). The osm-BODIPY dyads were fully characterized by optical, electrochemical, computational, and X-ray diffraction studies. Due to close proximity and electronic structure of the donor and acceptor entities, strong intramolecular charge-transfer interactions in these dyads was witnessed and was supported by computational studies. Solvent-polarity-dependent rapid charge separation and charge recombination in these dyads is witnessed from photochemical studies involving time-resolved emission and femtosecond transient absorption studies. The present studies bring out the importance of oxasmaragdyrin as an electron donor in light-energy-harvesting artificial photosynthesis model systems.

Synthesis of Stable [28π] m-Benzihexaphyrins (1.0.0.1.1.1).

Four new expanded [28π] m-benziporphyrins (1.0.0.1.1.1) were synthesized by [3+3] condensation of 10,10'-bis[(p-toly)hydroxymethyl]-1,3-bis(2-thienyl)benzenediol with various 16-tripyrranes such as 16-thiatripyrrane, 16-oxatripyrrane, 16-azatripyrrane, and 16-selenatripyrrane under mild trifluoroacetic acid-catalyzed reaction conditions. The macrocycles are freely soluble in common organic solvents, and their identities were confirmed by HRMS and detailed 1D and 2D NMR spectroscopy. The macrocycles showed one sharp Soret-type band at ∼500 nm and broad ill-defined Q-type band(s) in the region 600-950 nm, which supports their nonaromatic nature. Upon protonation, the macrocycles exhibited bathochromically shifted absorption bands with a distinct change in the color of the solutions. The preliminary studies carried out with one of the macrocycles indicated that the macrocycles have a weak tendency to form coordination complexes.

Synthesis and Properties of Covalently Linked AzaBODIPY-BODIPY Dyads and AzaBODIPY-(BODIPY)2 Triads.

The azaBODIPYs containing one and two formyl functional groups on the 1,7-aryl groups present at the azaBODIPY core were synthesized over sequence of steps and characterized by mass, NMR, absorption, and electrochemical techniques. The monoformylated and diformylated azaBODIPYs are very useful synthons to prepare a wide variety of new fluorescent compounds. The mono- and diformylated azaBODIPYs were treated with pyrrole under mild acidic conditions followed by column chromatographic purification to afford azaBODIPYs appended with one and two dipyrromethanyl groups. The dipyrramethanyl groups of azaBODIPYs were oxidized with DDQ and complexed with BF3·Et2O to obtain covalently linked azaBODIPY-BODIPY dyads and azaBODIPY-(BODIPY)2 triads. The dyads and triads were characterized in detail by HR-MS, 1D and 2D NMR, absorption, fluorescence, and electrochemical techniques and the structure of one of the triads was deduced by X-ray crystallography. The crystal structure of azaBODIPY-(BODIPY)2 triad revealed that the two BODIPY units were in perpendicular orientation with azaBODIPY unit. The absorption and electrochemical studies indicated a weak interaction among the BODIPY and azaBODIPY moieties and the moieties retain their independent characteristic features in dyads and triads. The preliminary fluorescence studies supported an efficient energy transfer from BODIPY unit(s) to azaBODIPY unit in dyads and triads.

Directly Connected AzaBODIPY-BODIPY Dyad: Synthesis, Crystal Structure, and Ground- and Excited-State Interactions.

Directly connected, strongly interacting sensitizer donor-acceptor dyads mimic light-induced photochemical events of photosynthesis. Here, we devised a dyad composed of BF2-chelated dipyrromethene (BODIPY) directly linked to BF2-chelated tetraarylazadipyrromethene (azaBODIPY) through the ß-pyrrole position of azaBODIPY. Structural integrity of the dyad was arrived from two-dimensional NMR spectral studies, while single-crystal X-ray structure of the dyad provided the relative orientation of the two macrocycles to be ∼62°. Because of direct linking of the two entities, ultrafast energy transfer from the (1)BODIPY* to azaBODIPY was witnessed. A good agreement between the theoretically estimated Förster energy transfer rate and experimentally determined rate was observed, and this rate was found to be higher than that reported for BODIPY-azaBODIPY analogues connected with spacer units. In agreement with the free-energy calculations, the product of energy transfer, (1)azaBODIPY* revealed additional photochemical events such as electron transfer leading to the creation of BODIPY(•+)-azaBODIPY(•-) radical ion pair, more so in polar benzonitrile than in nonpolar toluene, as evidenced by femtosecond transient spectroscopic studies. Additionally, the spectral, electrochemical, and photochemical studies of the precursor compound azaBODIPY-dipyrromethane also revealed occurrence of excited-state events. In this case, electron transfer from the (1)azaBODIPY* to dipyrromethane (DPM) yielded DPM(•+)-azaBODIPY(•-) charge-separated state. The study described here stresses the role of close association of the donor and acceptor entities to promote ultrafast photochemical events, applicable of building fast-response optoelectronic and energy-harvesting devices.

Synthesis of a donor-acceptor heterodimer via trifunctional completive self-sorting.

Selective self-assembly of heterodimers consisting of two non-identical subunits plays important roles in Nature but is rarely encountered in synthetic supramolecular systems. Here we show that photocleavage of a donor-acceptor porphyrin complex produces an heterodimeric structure with surprising selectivity. The system forms via a multi-step sequence that starts with an oxidative ring opening, which produces an equimolar mixture of two isomeric degradation products (zinc(II) bilatrien-abc-ones, BTOs). These two isomers are susceptible to water addition, yielding the corresponding zinc(II) 15-hydroxybiladien-ab-ones (HBDOs). However, in the photocleavage experiment only one HBDO isomer is formed, and it quantitatively combines with the remaining BTO isomer. The resulting heterodimer is stabilized by a Zn-O coordination bond and extended dispersion interactions between the overlapping π-surfaces of the monomers. The observed selectivity can be seen as a case of completive self-sorting, simultaneously controlled by three types of complementary interactions.

Porphyrin-Ryleneimide Hybrids: Tuning of Visible and Near-Infrared Absorption by Chromophore Desymmetrization.

Unsymmetrically fused porphyrins containing one or two naphthalimide subunits were prepared in modular syntheses relying on electron-rich and electron-poor pyrrole building blocks. These new chromophores show progressive changes in their electron-deficient character, while retaining comparably small optical and electrochemical band gaps. The intrinsic curvature and extended optical absorption of these systems make them of interest as mono- and difunctional components of multichromophoric assemblies.

Synthesis of Tellurabenziporphyrin and Its Pd(II) Complex.

An unprecedented tellurabenziporphyrin containing C, N, and Te donor atoms was synthesized by condensing benzitripyrrane and tellurophene diol under acid catalyzed conditions. The tellurabenziporphyrin readily forms a Pd(II) complex when treated with PdCl2 in CHCl3/CH3CN. The crystal structures of tellurabenziporphyrin and its Pd(II) complex revealed that the benzene ring hinders the π-electron delocalization. An unusual five-membered ring formed inside the macrocycle due to the strong interaction between "Te" and "N" in the Pd(II) complex.