Strong Electronic Communication Research Articles

Synthesis of inter-[60]fullerene conjugates with inherent chirality

Coalescence of [60]fullerenes potentially produces hypothetical nanocarbon assemblies with non-naturally occurring topologies. Since the discovery of [60]fullerene in 1985, coalesced [60]fullerene oligomers have only been observed as transient species by transmission electron microscopy during an oligomerization process under a high electron acceleration voltage. Herein, we showcase the rational synthesis of covalent assemblies consisting of inherently chiral open-[60]fullerenes. The crystallographic analyses unveiled double-caged structures of non-conjugated and conjugated inter-[60]fullerene hybrids, in which the two [60]fullerene cages are bounds to each other through a covalent linkage. The former one further assembles via a heterochiral recognition so that four carbon cages are arranged in a tetrahedral manner both in solution and solid state. Reflecting radially-conjugated double π-surface nature, the inter-[60]fullerene conjugate exhibits strong electronic communication in its reduced states, intense absorption behavior, and chiroptical activity with a dissymmetry factor of 0.21 (at 674 nm) which breaks the record for known chiral organic molecules.

Spin-Frustrated Trisradical Trication of PrismCage.

Organic trisradicals featuring threefold symmetry have attracted significant interest because of their unique magnetic properties associated with spin frustration. Herein, we describe the synthesis and characterization of a triangular prism-shaped organic cage for which we have coined the name PrismCage6+ and its trisradical trication─TR3(•+). PrismCage6+ is composed of three 4,4'-bipyridinium dications and two 1,3,5-phenylene units bridged by six methylene groups. In the solid state, PrismCage6+ adopts a highly twisted conformation with close to C3 symmetry as a result of encapsulating one PF6- anion as a guest. PrismCage6+ undergoes stepwise reduction to its mono-, di-, and trisradical cations in MeCN on account of strong electronic communication between its 4,4'-bipyridinium units. TR3(•+), which is obtained by the reduction of PrismCage6+ employing CoCp2, adopts a triangular prism-shaped conformation with close to C2v symmetry in the solid state. Temperature-dependent continuous-wave and nutation-frequency-selective electron paramagnetic resonance spectra of TR3(•+) in frozen N,N-dimethylformamide indicate its doublet ground state. The doublet-quartet energy gap of TR3(•+) is estimated to be -0.08 kcal mol-1, and the critical temperature of spin-state conversion is found to be ca. 50 K, suggesting that it displays pronounced spin frustration at the molecular level. To the best of our knowledge, this example is the first organic radical cage to exhibit spin frustration. The trisradical trication of PrismCage6+ opens up new possibilities for fundamental investigations and potential applications in the fields of both organic cages and spin chemistry.

Radical‐Bridged Heterometallic Single‐Molecule Magnets Incorporating Four Lanthanoceniums

AbstractThe syntheses and magnetic properties of organometallic heterometallic compounds [K(THF)6]{CoI[(μ3‐HAN)RE2Cp*4]2} (1‐RE) and [K(Crypt)]2{CoI[(μ3‐HAN)RE2Cp*4]2} (2‐RE) containing hexaazatrinaphthylene radicals (HAN⋅3−) and four rare earth (RE) ions are reported. 1‐RE shows isolable species with ligand‐based mixed valency as revealed by cyclic voltammetry (CV) thus leading to the isolation of 2‐RE via one‐electron chemical reduction. Strong electronic communication in mixed‐valency supports stronger overall ferromagnetic behaviors in 2‐RE than 1‐RE containing Gd and Dy ions. Ac magnetic susceptibility data reveal 1‐Dy and 2‐Dy both exhibit slow magnetic relaxation. Importantly, larger coercive field was observed in the hysteresis of 2‐Dy at 2.0 K, indicating the enhanced SMM behavior compared with 1‐Dy. Ligand‐based mixed‐valency strategy has been used for the first time to improve the magnetic coupling in lanthanide (Ln) SMMs, thus opening up new ways to construct strongly coupled Ln‐SMMs.

Radical-Bridged Heterometallic Single-Molecule Magnets Incorporating Four Lanthanoceniums.

The syntheses and magnetic properties of organometallic heterometallic compounds [K(THF)6 ]{CoI [(μ3 -HAN)RE2 Cp*4 ]2 } (1-RE) and [K(Crypt)]2 {CoI [(μ3 -HAN)RE2 Cp*4 ]2 } (2-RE) containing hexaazatrinaphthylene radicals (HAN⋅3- ) and four rare earth (RE) ions are reported. 1-RE shows isolable species with ligand-based mixed valency as revealed by cyclic voltammetry (CV) thus leading to the isolation of 2-RE via one-electron chemical reduction. Strong electronic communication in mixed-valency supports stronger overall ferromagnetic behaviors in 2-RE than 1-RE containing Gd and Dy ions. Ac magnetic susceptibility data reveal 1-Dy and 2-Dy both exhibit slow magnetic relaxation. Importantly, larger coercive field was observed in the hysteresis of 2-Dy at 2.0 K, indicating the enhanced SMM behavior compared with 1-Dy. Ligand-based mixed-valency strategy has been used for the first time to improve the magnetic coupling in lanthanide (Ln) SMMs, thus opening up new ways to construct strongly coupled Ln-SMMs.

Syntheses, characterizations, and properties of 1,4-phenylene and 4,4′-biphenylene bridged BOBPY dimers

Two novel phenylenes (1,4-phenylene and 4,4′-biphenylene) bridged dimeric BOBPY (Boron complexed [Formula: see text]O-type benzopyrromethenes) were designed and synthesized from the one-pot condensation of pyrrole with formylisoindole and subsequent complexation with diboronic acid. These novel dyes were fully characterized by NMR spectroscopy, HRMS, absorption and fluorescence emission spectroscopy, and cyclic voltammetry. These resulting BOBPY dimers are highly photostable and show intense absorption and emission in the region of 618 to 644 nm with high absorption coefficients and solvent-dependent fluorescence quantum yields. The electronic interactions between BOBPY units are related with length of the linker. And the dimer with short 1,4-phenylene linker was more sensitive to the polarity of the solvents than that of the other dimer, suggesting that the short linkage allowed stronger electronic communication between the two BOBPY units.

Pyrogallate‐Based Metal‐Organic Framework with a Two‐Dimensional Secondary Building Unit

AbstractWe report a metal–organic framework (MOF) with a rare two‐dimensional (2D) secondary building unit (SBU). The SBU comprises mixed‐valent Fe2+ and Fe3+ metal ions bridged by oxygen atoms pertaining to the polytopic ligand 3,3′,4,4′,5,5′‐hexahydroxybiphenyl, which also define the iron‐oxide 2D layers. Overall, the anionic framework exhibits rare topology and evidences strong electronic communication between the mixed‐valence iron sites. These results highlight the importance of dimensionality control of MOF SBUs for discovering new topologies in reticular chemistry, and especially for improving electronic communication within the MOF skeleton.

Pyrogallate-Based Metal-Organic Framework with a Two-Dimensional Secondary Building Unit.

We report a metal-organic framework (MOF) with a rare two-dimensional (2D) secondary building unit (SBU). The SBU comprises mixed-valent Fe2+ and Fe3+ metal ions bridged by oxygen atoms pertaining to the polytopic ligand 3,3',4,4',5,5'-hexahydroxybiphenyl, which also define the iron-oxide 2D layers. Overall, the anionic framework exhibits rare topology and evidences strong electronic communication between the mixed-valence iron sites. These results highlight the importance of dimensionality control of MOF SBUs for discovering new topologies in reticular chemistry, and especially for improving electronic communication within the MOF skeleton.

Controlling of Photophysical Behavior of Rhenium(I) Complexes with 2,6-Di(thiazol-2-yl)pyridine-Based Ligands by Pendant π-Conjugated Aryl Groups.

The structure–property correlations and control of electronic excited states in transition metal complexes (TMCs) are of high significance for TMC-based functional material development. Within these studies, a series of Re(I) carbonyl complexes with aryl-substituted 2,6-di(thiazol-2-yl)pyridines (Arn-dtpy) was synthesized, and their ground- and excited-state properties were investigated. A number of condensed aromatic rings, which function as the linking mode of the aryl substituent, play a fundamental role in controlling photophysics of the resulting [ReCl(CO)3(Arn-dtpy-κ2N)]. Photoexcitation of [ReCl(CO)3(Arn-dtpy-κ2N)] with 1-naphthyl-, 2-naphthyl-, 9-phenanthrenyl leads to the population of 3MLCT. The lowest triplet state of Re(I) chromophores bearing 9-anthryl, 2-anthryl, 1-pyrenyl groups is ligand localized. The rhenium(I) complex with appended 1-pyrenyl group features long-lived room temperature emission attributed to the equilibrium between 3MLCT and 3IL/3ILCT. The excited-state dynamics in complexes [ReCl(CO)3(9-anthryl-dtpy-κ2N)] and [ReCl(CO)3(2-anthryl-dtpy-κ2N)] is strongly dependent on the electronic coupling between anthracene and {ReCl(CO)3(dtpy-κ2N)}. Less steric hindrance between the chromophores in [ReCl(CO)3(2-anthryl-dtpy-κ2N)] is responsible for the faster formation of 3IL/3ILCT and larger contribution of 3ILCTanthracene→dtpy in relation to the isomeric complex [ReCl(CO)3(9-anthryl-dtpy-κ2N)]. In agreement with stronger electronic communication between the aryl and Re(I) coordination centre, [ReCl(CO)3(2-anthryl-dtpy-κ2N)] displays room-temperature emission contributed to by 3MLCT and 3ILanthracene/3ILCTanthracene→dtpy phosphorescence. The latter presents rarely observed phenomena in luminescent metal complexes.

S-Shaped Fused Azacorannulene Dimer: Structural and Redox Properties

S-Shaped Fused Azacorannulene Dimer: Structural and Redox Properties

Pyrene-based asymmetric hexaarylbenzene derivatives: Synthesis, crystal structures, and photophysical properties

Hexaarylbenzenes (HAB) and their derivatives have attracted considerable attention due to their fantastic molecular geometry and various applications in materials science. Herein, a set of novel, pyrene-based, asymmetric hexaarylbenzene molecules 3a–c and 10 were synthesized and fully characterized by using 1H/13C NMR, single crystal X-ray diffraction, high-resolution mass spectra (HRMS), UV–Vis and fluorescence spectroscopy, as well as density functional theory (DFT) calculations. The compounds show tunable-emission color from violet (366 nm) to deep blue emission (407 nm) following the order of 10 < 3a < 3b < 3c in solution, and the pyrene-based asymmetric hexaarylbenzenes show a red-shift to the (deep) blue region (in the range from 406 to 466 nm) with a narrow full width at half-maximum (FWHM) emission spectrum in films. On the other hand, as the expanding π-conjugated molecular frameworks decreased in the order of 3c > 3b > 3a > 10 > hexaphenylbenzene (HPB), the asymmetric hexaarylbenzenes exhibited clearer aggregation-induced emission characterestics. In addition, DFT calculations indicated that there is strong electronic communication between the pyrene unit and the neighboring phenyl rings by a through-space charge-transfer process, which would play a curial role in lowering the quantum yield in the molecular aggregation state. Thus, this article provides an efficient synthetic strategy on how to design and synthesise high-performance pyrene-based hexaarylbenzene derivatives with narrow FWHM bands for potential applications in organic electronics.

Through Bridge Spin Coupling in Homo- and Heterobimetallic Porphyrin Dimers upon Stepwise Oxidations: A Spectroscopic and Theoretical Investigation.

We have described copper(II)-iron(III) and copper(II)-manganese(III) heterobimetallic porphyrin dimers and compared them with the corresponding homobimetallic analogs. UV-visible spectra are very distinct in the heterometallic species while electrochemical studies demonstrate that these species, as compared to the homobimetallic analog, are much easier to oxidize. Combined Mössbauer, EPR, NMR, magnetic and UV-visible spectroscopic studies show that upon 2e-oxidation of the heterobimetallic complexes only ring-centered oxidation occurs. The energy differences between HOMO and LUMO are linearly dependent with the low-energy NIR band obtained for the 2e-oxidized complexes. Also, strong electronic communication between two porphyrin rings through the bridge facilitates coupling between various unpaired spins present while the coupling model depends on the nature of metal ions used. While unpaired spins of Fe(III) and the porphyrin π-cation radical are strongly antiferromagnetically coupled, such coupling is rather weak between Mn(III) and a porphyrin π-cation radical. Moreover, the coupling between two π-cation radicals are much stronger in the 2e-oxidized complexes of dimanganese(III) and copper(II)-manganese(III) porphyrin dimers as compared to their diiron(III) and copper(II)-iron(III) analogs. Furthermore, coupling between the unpaired spins of a π-cation radical and copper(II) is much stronger in the 2e-oxidized complex of copper(II)-iron(III) porphyrin dimer as compared to its copper(II)-manganese(III) analog. The Mulliken spin density distributions in 2e-oxidized homo- and heterobimetallic complexes show symmetric and asymmetric spread between the two macrocycles, respectively. In both the 2e-oxidized heterobimetallic complexes, the Cu(II) porphyrin center acts as a charge donor while Fe(III)/Mn(III) porphyrin center act as a charge acceptor. The experimental observations are also strongly supported by DFT calculations.

Strong Electronic Communication in Linearly Elongated Rylenes Featuring Tunable Bridges.

A modified synthetic pathway towards perylene‐perylene dimers and a facile purification method to obtain the regioisomerically pure syn‐ and anti‐isomers are reported. In addition, a novel perylene‐naphthalene heterodimer with 30 conjugated π‐electron pairs was designed and synthesized on the basis of a previously described precursor and the resulting regioisomers were separated from each other. Thereby, the opto‐electronic properties of the linearly elongated chromophores could be investigated regarding the differences in length of their aromatic system and the configuration of the isomers. Further tuning of their energy gaps was realized via protonation and methylation of the dibenzimidazole‐bridging unit. Extraordinary red‐shifts of the absorption maxima of 62 nm for the methylated and 92 nm for the protonated perylene‐perylene anti‐isomer could be achieved. Moreover, the maxima for the syn‐isomer could be shifted bathochromically by 87 and 113 nm, respectively.

Cobalt coordination controlled carbon nanospheres formation and inclusion of amorphous Co3O4 and AuNPs: strongly enhanced oxygen evolution reaction with excellent mass activity.

Carbon nanospheres integrated with AuNPs and amorphous Co3O4 were fabricated by making use of cobalt coordination with AuNP surface ligands, which exhibited an enhanced oxygen evolution reaction (OER) with excellent mass activity. Co2+ coordination with AuNP surface functional molecules significantly influenced the nanostructure formation and OER activity. Nanospheres of carbon with an optimum concentration of AuNPs and Co3O4 (2) showed strong OER activity. 2 exhibited a high current density (358 mA cm-2 at an applied potential of 1.59 V) and required a low overpotential (256 mV) to generate a geometric current density (10 mA cm-2) compared to commercial RuO2 (363 mV). Importantly, 2 showed high mass activity (1352.5 mA mg-1), 14 times higher than RuO2 (93.87 mA mg-1). The low Tafel slope (52.4 mV dec-1) and charge transfer resistance along with large double layer capacitance (Cdl = 20.1) of 2 suggest strong electronic communication between the catalyst and the electrode surface and facilitated fast charge transport. Chronoamperometric studies confirmed the excellent stability of the catalyst. The present work demonstrates that the electrocatalytic activity of earth-abundant amorphous metal oxides can be strongly enhanced by integrating metallic nanoparticles (NPs) and optimizing nanostructures.

Synthesis of thiophene-fused porphyrin dimers as effective π-extended helical chromophores.

We synthesized thiophene-fused porphyrin dimers as effective π-extended helical chromophores. The porphyrin dimers exhibit a red-shifted absorption with the edge extending up to 1100 nm, implying strong electronic communication over the two porphyrin moieties through the thiophene-fused structure. Importantly, their racemic inversion barriers can be modulated by the central metal ions.

Visible light promoted porphyrin-based metal-organic adduct

Supramolecular adducts formed by a commercial porphyrin derivative and silver nanoparticles have been obtained using exclusively light as an external trigger that is able to promote the formation of the plasmonic nanostructures. In particular, a water-soluble porphyrin, i.e. tetrakis(4-carboxyphenyl)porphyrin, has been used. It has been thoroughly characterized by means of UV-vis and fluorescence spectroscopy in order to explain its peculiar behavior when illumined with visible photons. Herein we demonstrate that, by means of light illumination, it is possible to tune the porphyrin aggregation state. Furthermore, when the monomeric form of the organic macrocycle is induced and a controlled amount of AgNO[Formula: see text] is simultaneously dissolved, it is possible to promote the formation of silver nanostructures using visible light. The proposed approach allowed porphyrin derivatives/Ag nanoparticles hybrid nanostructures to be obtained without using a chemical reducing agent: the porphyrin derivative simultaneously acts as reducing agent when irradiated by visible light and as a capping agent for the silver nanostructures. The organic/inorganic adduct was characterized by means of steady-state fluorescence that highlights a strong energetic or electronic communication between the two species. XRD and SEM investigations evidence that silver nanoparticles are formed without using any reducing agent.

Quadrupolar Cyclopenta[Hi]Aceanthrylene-Based Electron Donor-Acceptor-Donor Conjugates: Charge Transfer Versuscharge Separation

Cyclopenta[hi]aceanthrylene (CPA) has been functionalized at two of its positions with electronically-inert trimethylsilylethynyl (1), as well as electron donating 4-ethynyl-N,N-dimethylaniline (2), ethynyl Zn(II)phthalocyanine (3), and ethynyl Zn(II)porphyrin (4) units. Consistent with X-ray crystal structures of 2 and 4, analyses of absorption and fluorescence of 2-4 prompt to strong electronic communication between CPA and the peripheral units, affording quadrupolar electron donor-acceptor-donor charge transfer conjugates. By virtue of their quadrupolar/dipolar charge transfer characters in the excited state, 2-4 exhibit fluorosolvatochromism. Transient absorption spectroscopy confirmed delocalized quadrupolar ground states and formation of weakly solvent stabilized quadrupolar singlet excited states. The latter transform into strongly stabilized dipolar excited states before deactivating to the ground state in 2 and give rise to a fully charge separated state in 3 and 4.

Enhancing the Acceptor Character of Conjugated Organoborane Macrocycles: A Highly Electron‐Deficient Hexaboracyclophane

AbstractWe introduce a new boron‐doped cyclophane, the hexabora[16]cyclophane B6‐FMes, in which six tricoordinate borane moieties alternate with short conjugated p‐phenylene linkers. Exocyclic 2,4,6‐tris(trifluoromethyl)phenyl (FMes) groups serve not only to further withdraw electron density but at the same time sterically shield the boron atoms, resulting in a macrocycle that is both highly electron‐deficient and stable. The optical and electronic properties are compared with those of related linear oligomers and the electronic structure is further evaluated by computational methods. The studies uncover unique properties of B6‐FMes, including a low‐lying and extensively delocalized LUMO and a wide HOMO–LUMO gap, which arise from the combination of a cyclic π‐system, strong electronic communication between the closely spaced borons, and the attachment of electron‐deficient pendent groups. The binding of small anions to the electron‐deficient macrocycle and molecular model compounds is investigated and emissive exciplexes are detected in aromatic solvents.

Enhancing the Acceptor Character of Conjugated Organoborane Macrocycles: A Highly Electron-Deficient Hexaboracyclophane.

We introduce a new boron-doped cyclophane, the hexabora[16 ]cyclophane B6-F Mes, in which six tricoordinate borane moieties alternate with short conjugated p-phenylene linkers. Exocyclic 2,4,6-tris(trifluoromethyl)phenyl (F Mes) groups serve not only to further withdraw electron density but at the same time sterically shield the boron atoms, resulting in a macrocycle that is both highly electron-deficient and stable. The optical and electronic properties are compared with those of related linear oligomers and the electronic structure is further evaluated by computational methods. The studies uncover unique properties of B6-F Mes, including a low-lying and extensively delocalized LUMO and a wide HOMO-LUMO gap, which arise from the combination of a cyclic π-system, strong electronic communication between the closely spaced borons, and the attachment of electron-deficient pendent groups. The binding of small anions to the electron-deficient macrocycle and molecular model compounds is investigated and emissive exciplexes are detected in aromatic solvents.

Supramolecular Assemblies for Electronic Materials

This work presents a synergy between organic electronics and supramolecular chemistry, in which a host-guest complex is designed to function as an efficacious electronic material. Specifically, the noncovalent recognition of a fullerene, phenyl-C61 -butyric acid methyl ester (PC61 BM), by an alternating perylene diimide (P)-bithiophene (B) conjugated macrocycle (PBPB) results in a greater than five-fold enhancement in electron mobility, relative to the macrocycle alone. Characterization and quantification of the binding of fullerenes by host PBPB is provided alongside evidence for intermolecular electronic communication within the host-guest complexes.

Two-Step Charge Separation Passing Through the Partial Charge-Transfer State in a Molecular Dyad.

Charge separation (CS) in molecular systems usually takes place in weakly coupled donor-acceptor dyads where an electron charge moves from the donor to the acceptor in the local excited state of a chromophore. Herein, we present a two-step charge-separation process in a newly synthesized diketopyrrolopyrrole-pyrrolopyrrole (DPP-PP) dyad (AD), which starts from the initial photoexcited bright exciton and goes through a partial charge-transfer (CT) state before finally reaching the charge-separated (CS) state. The evolving CT character in the excited state is demonstrated through the complementary use of transient absorption, broad-band fluorescence upconversion, and transient impulsive stimulated Raman spectroscopy. The bright exciton state of the dyad relaxes to a partial CT state with 1 and 20 ps during solvent and structural fluctuations in toluene, respectively, and with 700 fs for the solvent fluctuations occurring in tetrahydrofuran. This is evident from the characteristic excited-state absorption spectra and the reduced fluorescence intensity observed on the adiabatic potential energy surface. AD in THF additionally evolves to the diabatic potential energy surface of the CS state, whose absorption spectrum converges to that of a DPP anion for which fluorescence is completely quenched. The trend of shifting for certain vibrational frequencies also supports the proposed CT dynamics and mechanism; furthermore, it gives quantitative insight into the CT characters of the bright state (0.1 e) and intermediate partial CT state (0.5 e), as determined by the linear relationship that exists between the vibrational frequency of the marker modes and the CT character. We have found that as the structure of the bridge between donor and acceptor enables an intermediate level of electronic communication, the charge-separation can rapidly occur through a distinct partial charge-transfer state. It seems that the exceptionally strong electronic communication at positions 2 and 5 of the pyrrolo[3,2-b]pyrrole core is a crucial element for this charge-transfer mechanism, which could be applied to organic photovoltaics or light-emitting diodes requiring efficient charge separation.