Macrocyclic Oligothiophenes Research Articles

Resonant Electron Tunneling Induces Isomerization of π-Expanded Oligothiophene Macrocycles in a 2D Crystal.

Macrocyclic oligothiophenes and their π‐expanded derivatives constitute versatile building blocks for the design of (supra)molecularly engineered active interfaces, owing to their structural, chemical, and optoelectronic properties. Here, it is demonstrated how resonant tunneling effect induces single molecular isomerization in a 2D crystal, self‐assembled at solid–liquid interfaces under ambient conditions. Monolayers of a series of four π‐expanded oligothiophene macrocycles are investigated by means of scanning tunneling microscopy and scanning tunneling spectroscopy (STS) at the interface between their octanoic acid solutions and the basal plane of highly oriented pyrolytic graphite. Current–voltage characteristics confirm the donor‐type character of the macrocycles, with the highest occupied molecular orbital and the lowest unoccupied molecular orbital (LUMO) positions consistent with time‐dependent density functional theory calculations. Cyclic STS measurements show the redox isomerization from Z,Z‐8T6A to its isomer E,E‐8T6A occurring in the 2D crystal, due to the formation of a negatively charged species when the tunneling current is in resonance with the LUMO of the macrocycle.

π-Extended Macrocyclic Oligothiophene Heptamer and Tetradecamer: Ringsize Effects on the Physical Properties and Morphological Features

Abstract Synthesis, X-ray structural analysis, redox behavior, and morphological features of π-extended macrocyclic oligothiophene heptamer and tetradecamer are reported. Due to nanophase separation between inner and outer domains, butyl-substituted π-extended macrocyclic oligothiophenes exhibit marked ringsize effects in the solid state. Among macrocycles with the same composition formula, heptamer has a round shape-persistent structure to produce crystals, whereas the corresponding tetradecamer shows morphological diversity to form single crystals, rods, ovals, and balls. The surface structures of small rods, ovals, and balls were investigated by absorption spectra and PXRD analysis. By cyclic voltametric analysis, heptamer and tetradecamer exhibit reversible two-step redox behavior reflecting moderate donor ability, and the first oxidation potentials of heptamer and tetradecamer are roughly consistent with their HOMO levels.

Mechanical Stabilization of Helical Chirality in a Macrocyclic Oligothiophene.

We introduce a design principle to stabilize helically chiral structures from an achiral tetrasubstituted [2.2]paracyclophane by integrating it into a macrocycle. The [2.2]paracyclophane introduces a three-dimensional perturbation into a nearly planar macrocyclic oligothiophene. The resulting helical structure is stabilized by two bulky substituents installed on the [2.2]paracyclophane unit. The increased enantiomerization barrier enabled the separation of both enantiomers. The synthesis of the target helical macrocycle 1 involves a sequence of halogenation and cross-coupling steps and a high-dilution strategy to close the macrocycle. Substituents tuning the energy of the enantiomerization process can be introduced in the last steps of the synthesis. The chiral target compound 1 was fully characterized by NMR spectroscopy and mass spectrometry. The absolute configurations of the isolated enantiomers were assigned by comparing the data of circular dichroism spectroscopy with TD-DFT calculations. The enantiomerization dynamics was studied by dynamic HPLC and variable-temperature 2D exchange spectroscopy and supported by quantum-chemical calculations.

A macrocyclic oligofuran: synthesis, solid state structure and electronic properties.

We report the first π-conjugated macrocyclic system with an oligofuran backbone. The calculated HOMO-LUMO gap is similar to that of the corresponding linear polymer, indicating a remarkable electron delocalization. The X-ray structure reveals a planar conformation, in contrast to the twisted conformation of macrocyclic oligothiophenes. The intermolecular π-π stacking distance is extremely small (3.17 Å), indicating very strong interactions. The macrocycle forms large π-aggregates in solution and shows a tendency toward highly ordered multilayer adsorption at the solid-liquid interface. The face-on orientation of molecules explains the higher hole mobility observed in the out-of-plane direction.

A Chiral Macrocyclic Oligothiophene with Broken Conjugation – Rapid Racemization through Internal Rotation

AbstractA macrocyclic oligothiophene with an integrated pseudo‐para substituted [2.2]paracyclophane has been achieved. The synthetic sequence relies on alternating steps of halogenation‐ and Suzuki‐coupling conditions. By employing a modified Eglinton reaction under high dilution conditions, the macrocycle is closed and the obtained diacetylene is efficiently transferred to the corresponding thiophene. The molecule is fully characterized and its dynamic racemization is analyzed by variable temperature NMR experiments. The racemization barrier hints with 38 kJ/mol at rapid enantiomerization at room temperature by Mislow’s ‘Euclidian rubber glove’ enantiomerization process. Macrocycle formation results in red‐shifted absorption and emission spectra, hinting at increased conjugation through the oligothiophene versus the trough space conjugation through the [2.2]paracyclophane.

Macrocyclic Oligofurans: A Computational Study.

Macrocyclic oligothiophenes ( nCT) have been extensively explored for their unique optical and electronic properties, while their oxygen-containing analogues, macrocyclic oligofurans ( nCF), are not known. In this work, macrocyclic oligofurans bearing 5-16 units (5CF-16CF) are studied theoretically at the B3LYP/6-311G(d) level. We find that small macrocycles (6CF-8CF) exhibit planar or nearly planar geometries, low strain energies, low HOMO-LUMO gaps, and strong π-conjugation as also reflected in their Raman spectra. These findings are in sharp contrast to macrocyclic oligothiophenes of the same size, which are distorted from planarity. Additionally, small macrocyclic furans display significantly lower reorganization energies and ionization potentials compared with their thiophene analogues. Overall, the observed properties highlight the potential of macrocyclic oligofurans to function as p-type organic electronic materials.

A Saturn-Like Complex Composed of Macrocyclic Oligothiophene and C60 Fullerene: Structure, Stability, and Photophysical Properties in Solution and the Solid State.

A Saturn-like 1:1 complex composed of macrocyclic oligothiophene E-8T7A and C60 fullerene (C60 ) was synthesized to investigate the interaction between macrocyclic oligothiophenes and C60 in solution and the solid state. Because the Saturn-like 1:1 complex E-8T7A⋅C60 is mainly stabilized by van der Waals interactions between C60 and the sulfur atoms of the E-8T7A macrocycle, C60 is rather weakly incorporated inside the macro-ring in solution. However, in the solid state the Saturn-like 1:1 complex preferentially formed single crystals or nanostructured polymorphs. Interestingly, X-ray analysis and theoretical calculations exhibited hindered rotation of C60 in the Saturn-like complex due to interactions between C60 and the sulfur atoms. Furthermore, the photoinduced charge transfer (CT) interaction between E-8T7A and C60 in solution was investigated by using femtosecond transient absorption (TA) spectroscopy. The ultrafast TA spectral changes in the photoinduced absorption bands were attributed to the CT process in the Saturn-like structure.

Templated bilayer self-assembly of fully conjugated \u03c0-expanded macrocyclic oligothiophenes complexed with fullerenes

Fully conjugated macrocyclic oligothiophenes exhibit a combination of highly attractive structural, optical and electronic properties, and multifunctional molecular thin film architectures thereof are envisioned. However, control over the self-assembly of such systems becomes increasingly challenging, the more complex the target structures are. Here we show a robust self-assembly based on hierarchical non-covalent interactions. A self-assembled monolayer of hydrogen-bonded trimesic acid at the interface between an organic solution and graphite provides host-sites for the epitaxial ordering of Saturn-like complexes of fullerenes with oligothiophene macrocycles in mono- and bilayers. STM tomography verifies the formation of the templated layers. Molecular dynamics simulations corroborate the conformational stability and assign the adsorption sites of the adlayers. Scanning tunnelling spectroscopy determines their rectification characteristics. Current–voltage characteristics reveal the modification of the rectifying properties of the macrocycles by the formation of donor–acceptor complexes in a densely packed all-self-assembled supramolecular nanostructure.

Macrocyclic Oligothiophene with Stereogenic [2.2]Paracyclophane Scaffolds: Chiroptical Properties from π-Transannular Interactions.

The enantiomers of a new cyclic oligothiophene, bridged by two pseudo-ortho[2.2]paracyclophanes, were synthesized as a new class of the chiral π-conjugated system. Single-crystal X-ray diffraction analysis revealed a twisted structure for these oligothiophenes induced by a torsion of the cyclophane moieties. The embedding oligothiophenes into the inherent planar chirality provided a significant enhancement in circular dichroism (CD) spectra thanks to the large magnetic/electric transition dipole moments. In the dicationic state, an intramolecular π dimer was formed due to the strong interactions of the oligothiophenes. We further recorded unprecedented wide ranges of CD spectra in 250-1800 nm region. The transitions are reasonably described by the exciton coupling of the oligothiophenes.

The Role of Linkers in the Excited-State Dynamic Planarization Processes of Macrocyclic Oligothiophene 12-Mers.

Linkers adjoining chromophores play an important role in modulating the structure of conjugated systems, which is bound up with their photophysical properties. However, to date, the focus of works dealing with linker effects was limited only to linear π-conjugated materials, and there have been no detailed studies on cyclic counterparts. Herein we report the linker effects on the dynamic planarization processes of π-conjugated macrocyclic oligothiophene 12-mers, where the different ratio between ethynylene and vinylene linkers was chosen to control the backbone rigidity. By analyzing transient fluorescence spectra, we demonstrate that the connecting linkers play a crucial role in the excited-state dynamics of cyclic conjugated systems. Faster dynamic planarization, longer exciton delocalization length, and higher degree of planarity were observed in vinylene inserted cyclic oligothiophenes. Molecular dynamics simulations and density functional theory calculations also stress the importance of the role of linkers in modulating the structure of cyclic oligothiophenes.

Synthesis, structures, and photophysical properties of π-expanded oligothiophene 8-mers and their Saturn-like C₆₀ complexes.

Two isomers of a multifunctional π-expanded macrocyclic oligothiophene 8-mer, E,E-1 and Z,Z-1, were synthesized using a McMurry coupling of a dialdehyde composed of four 2,5-thienylene and three ethynylene units under high dilution conditions. On the other hand, cyclo[8](2,5-thienylene-ethynylene) 2 was synthesized by intramolecular Sonogashira cyclization of ethynyl bromide 5. From STM measurements, both E,E-1 and Z,Z-1 formed self-assembled monolayers at the solid-liquid interface to produce porous networks, and from X-ray analyses of E,E-1 and 2, both compounds had a round shape with a honeycomb stacked structure. E,E-1 formed various fibrous polymorphs due to nanophase separation of the macrorings. E,E-1 and Z,Z-1 in solution exhibited photochromism upon irradiation with visible and UV light, respectively, and this photoisomerization was confirmed by using STM. Furthermore, amorphous films of Z,Z-1 and E,E-1 showed photoisomerization, although single crystals, fibers, and square tubes of E,E-1 remained unchanged under similar conditions. E,E-1 with a 12.5-14.7 Å inner cavity incorporated fullerene C60 in the cavity in solution and the solid state to produce a Saturn-like complex, whose structure was determined by X-ray analysis. 2 also formed a Saturn-like complex with C60 in the solid state. These Saturn-like complexes are stabilized by van der Waals interactions between the sulfur atoms of 8-mer and C60. The complexes exhibited charge-transfer interactions in the solid state. Like E,E-1, Saturn-like complex E,E-1⊃C60 formed small cube and fiber structures depending on the solvent used, whereas those of Saturn-like complex 2⊃C60 were limited due to the rigidity of the macroring of 2.

Inhomogeneity in the excited-state torsional disorder of a conjugated macrocycle.

The photophysics of conjugated polymers has generally been explained based on the interactions between the component conjugated chromophores in a tangled chain. However, conjugated chromophores are entities with static and dynamic structural disorder, which directly affects the conjugated polymer photophysics. Here we demonstrate the impact of chain structure torsional disorder on the spectral characteristics for a macrocyclic oligothiophene 1, which is obscured in conventional linear conjugated chromophores by diverse structural disorders such as those in chromophore size and shape. We used simultaneous multiple fluorescence parameter measurement for a single molecule and quantum-mechanical calculations to show that within the fixed conjugation length across the entire ring an inhomogeneity from torsional disorder in the structure of 1 plays a crucial role in causing its energetic disorder, which affords the spectral broadening of ∼220 meV. The torsional disorder in 1 fluctuated on the time scale of hundreds of milliseconds, typically accompanied by spectral drifts on the order of ∼10 meV. The fluctuations could generate torsional defects and change the electronic structure of 1 associated with the ring symmetry. These findings disclose the fundamental nature of conjugated chromophore that is the most elementary spectroscopic unit in conjugated polymers and suggest the importance of engineering structural disorder to optimize polymer-based device photophysics. Additionally, we combined defocused wide-field fluorescence microscopy and linear dichroism obtained from the simultaneous measurements to show that 1 emits polarized light with a changing polarization direction based on the torsional disorder fluctuations.

Cover Picture: Inherently Chiral Macrocyclic Oligothiophenes: Easily Accessible Electrosensitive Cavities with Outstanding Enantioselection Performances (Chem. Eur. J. 47/2014)

Cover Picture: Inherently Chiral Macrocyclic Oligothiophenes: Easily Accessible Electrosensitive Cavities with Outstanding Enantioselection Performances (Chem. Eur. J. 47/2014)

Inherently Chiral Macrocyclic Oligothiophenes: Easily Accessible Electrosensitive Cavities with Outstanding Enantioselection Performances

AbstractInvited for the cover of this issue is Francesco Sannicolò and co‐workers at different Italian institutions, who all contributed to the different specific areas of this multidisciplinary work. The image depicts inherently chiral elliptical and triangular oligothiophene macrocycles and their symmetrical resemblance to mysterious crop circles. Read the full text of the article at 10.1002/chem.201404331.

Inherently chiral macrocyclic oligothiophenes: easily accessible electrosensitive cavities with outstanding enantioselection performances.

Linear conjugated oligothiophenes of variable length and different substitution pattern are ubiquitous in technologically advanced optoelectronic devices, though limitations in application derive from insolubility, scarce processability and chain-end effects. This study describes an easy access to chiral cyclic oligothiophenes constituted by 12 and 18 fully conjugated thiophene units. Chemical oxidation of an “inherently chiral” sexithiophene monomer, synthesized in two steps from commercially available materials, induces the formation of an elliptical dimer and a triangular trimer endowed with electrosensitive cavities of different tunable sizes. Combination of chirality with electroactivity makes these molecules unique in the current oligothiophenes literature. These macrocycles, which are stable and soluble in most organic solvents, show outstanding chiroptical properties, high circularly polarized luminescence effects and an exceptional enantiorecognition ability.

Multifunctional π-Expanded Macrocyclic Oligothiophene 6-Mers and Related Macrocyclic Oligomers

Multifunctional π-expanded macrocyclic oligothiophene 6-mer 1, as well as 9- (2) and 12-mers (3), was synthesized using a McMurry coupling reaction as the key step. The 6-mer 1 was converted to cyclo[6](2,5-thienylene-ethynylene) (4) by using a bromination-dehydrobromination procedure. From X-ray analysis, the crystal structures of nonplanar 1 and round-shaped 2 and 4 were elucidated. STM showed that 4 formed a self-assembled monolayer at the liquid/solid interface to produce a hexagonal porous network. Chemical oxidation of 1, 2, and 4 with 1 and 2 equiv of Fe(ClO4)3 produced 1(•+) and 1(2+), 2(•+) and 2(2+), and 4(•+) and 4(2+), respectively. Although oligothiophene radical cations containing β,β-disubstituted thiophenes usually do not form π-dimers, 4(•+) clearly formed a π-dimer owing to its planar, round shape. As for the dications of 1, 2, and 4, 1(2+), which has 34π-electrons, exhibited a large diatropic ring current effect, whereas 34π dication 4(2+) only showed a medium diatropic ring current effect. In contrast to 1(2+) and 4(2+), 52π dication 2(2+) had biradical cationic character instead of Hückel-type cyclic conjugation. Interestingly, 6-mer 1 showed polymorphism and unusual melting point behavior due to the number of stable conformations in the solid state. Single crystals of 1 melted at 176 °C, whereas an amorphous film of 1 crystallized in the temperature range of 80-83 °C to form a lamellarly stacked microcrystalline film, which melted at 139 °C. The polymorphism of 1 was applied to either fluorescence switching or switching of field effect transistor (FET) activity and electrical conductivity.

Giant Conjugated Macrocycles: Synthesis and Applications

Conjugated giant macrocyclic oligothiophenes 2Tn (n=2-4), 5Tn (n=2-6), and 6Tn (n=2-5) composed of 2,5-thienylenes, ethynylenes, and vinylenes with 24π to 180π electron systems have been synthesized using a modified McMurry coupling reaction as the key step. For the synthesis of cyclo[n](2,5-thienylene-ethynylene)s nCTE (n=10 and 12) composed of 2,5-thienylenes and ethynylenes, a bromination-dehydrobromination then double elimination procedure was employed. X-ray analyses of 2T2 (24π), 5T2 (60π), and 6T2 (72π) revealed unique molecular and packing structures, reflecting their planar cyclic frameworks having medium to large inner cavities. Interestingly, giant macrocycles self-aggregate in the solid state to form various nanostructures reflecting their nanophase separation, and the two-photon absorption properties of 6Tn (n=2-5) show that increasing π-conjugation leads to an increase in the two-photon absorption cross section with magnitudes as high as 100,000 GM being realized. Furthermore, the giant macrocycle 6T5 behaves as a synthetic cyclic pigment comparable to the natural light-harvesting system.

Fully conjugated macrocycles composed of thiophenes, acetylenes, and ethylenes

Fully conjugated π-expanded macrocyclic oligothiophenes with 24π to 180π electron systems have been synthesized using a modified McMurry coupling reaction as the key step. For the synthesis of cyclo[n](2,5-thienylene-ethynylene)s composed of thiophenes and acetylenes, bromination-dehydrobromination and double elimination procedure were employed. X-ray analyses of macrocyclic oligothiophenes with 24π, 60π, and 72π systems revealed unique molecular and packing structures, reflecting planar cyclic frameworks with medium to large inner cavities. All the giant macrocycles exhibit multistep reversible redox behavior with a fairly low first-oxidation potentials, reflecting their cyclic conjugation. The doping of macrocycles with iodine forms semiconductors owing to their π-donor properties and π–π stacking ability. Interestingly, 90π, 108π, and 120π systems self-aggregate in the solid state to form red nanofibers or petal structure, whereas 150π to 180π systems form red nanoparticles. The structures of fibrous aggregates and particles have been established by scanning electron microscopy (SEM) and atomic force microscopy (AFM) analyses. Furthermore, two-photon properties of 72π, 108π, 144π, and 180π systems show that the increasing π-conjugation leads to an increase in the two-photon absorption cross-section with magnitudes as high as 100 000 GM in the visible spectral region.

Giant macrocycles composed of thiophene, acetylene, and ethylene units

Fully conjugated π-expanded macrocyclic oligothiophenes with 60π–180π electron systems have been synthesized using modified McMurry coupling reaction as a key step. X-ray analyses of 60π and 72π systems revealed unique molecular and packing structures, reflecting planar cyclic structures with large inner cavities. All giant macrocycles exhibit multi-step reversible redox behavior with fairly low first oxidation potentials, reflecting their cyclic conjugation. Doping of macrocycles with iodine forms semiconductors owing to their π-donor properties and π–π stacking ability. Interestingly, 90π and 120π systems self-aggregate in the solid state to form red nanofibers. The structures of fibrous aggregates have been established by SEM and AFM analyses. Furthermore, two-photon properties of 72π, 108π, 144π, and 180π systems show that the increasing π-conjugation leads to an increase in the two-photon absorption cross-section with magnitudes as high as 100,000 GM in the visible spectral region.

Self-assembly and Nanostructure Formation of Multi-functional Organic π-Donors

Abstract Self-assembly of tetrathiafulvalenes, their conjugated oligo-mers, and giant macrocyclic oligothiophenes in solution leads to important solvatochromic, thermochromic, electrochromic, and switching properties. Furthermore, based on the self-assembly in solution, various nanostructures can be constructed in the solid state to produce nanowires, nanoparticles, nanotubes, tapes, ribbons, and microcrystals. This article highlights the self-aggregation of organic π-donors in solution and the formation of nanostructures from these π-donors in the solid state.