Cyclic Oligothiophenes Research Articles

Synthesis of penta- and hexa(3,4-thienylene): size-dependent structural properties of cyclic oligothiophenes.

Penta- and hexa(3,4-thienylene)s were synthesized as a potential precursor for thiophene-containing polyarenes, and the structures were determined via X-ray crystallography. The interconversion of thiophene rings is fast in penta(3,4-thienylene), and slow in hexa(3,4-thienylene) reflecting the activation energy for enantiomerization. Size-dependent bathochromic shifts were observed in UV-vis absorption spectra.

Excited-state structural relaxation and exciton delocalization dynamics in linear and cyclic π-conjugated oligothiophenes.

π-Conjugated oligothiophene is considered a chain segment of its polymeric counterpart, whose size and shape can be precisely controlled. Because of its simplified structure, it is possible to understand complex excited-state dynamics of the π-conjugated polymers by employing a bottom-up approach. We review theoretical and experimental aspects of π-conjugated oligothiophenes by summarizing recent works employing time-resolved spectroscopy. The extent of exciton delocalization, which is a prerequisite to efficient charge generation at organic heterojunctions, is described sequentially in model linear and cyclic oligothiophenes, and their analogues. The heterogeneous nature of these systems is highlighted by illustrating the results at both ensemble and single-molecule levels. Exciton dynamics that arise in the polymers are also covered and the signifcance of exciton and charge delocalization in photovoltaic materials is highlighted.

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.

π-Expanded Cyclic Oligothiophene 12-Mers as Semishape-Persistent Macrocycles

π-Expanded Cyclic Oligothiophene 12-Mers as Semishape-Persistent Macrocycles

Facile Synthesis of Thiacalix[n]thiophene Derivatives

Recently, we reported a new synthetic approach toward a ­series of thiacalix[n]thiophene and thiacalix[n]dithienothiophene derivatives, cyclic homologues of sulfur-bridged cyclic oligothiophenes. A palladium-catalyzed coupling reaction of stannyl sulfide (Bu3Sn)2S and dibromothiophene/dibromodithienothiophene derivatives gave the corresponding macrocycles in good yield. Substituents at the β-position can play a key role in effective cyclization. This paper also summarizes the molecular and electronic structures of the novel macrocycles. Unlike conventional calixarenes, they exhibit distinct electron-donating properties. Furthermore, self-assembly and host–guest properties, leading to supramolecular structures, are also briefly discussed.

Defining Cyclic-Acyclic Exciton Transition at the Single-Molecule Level: Size-Dependent Conformational Heterogeneity and Exciton Delocalization in Ethynylene-Bridged Cyclic Oligothiophenes.

Conformational disorder in π-conjugated cyclic systems plays a crucial role in controlling the extent of exciton delocalization in much the same way as that in linear counterparts. However, to date, there have been no detailed spectroscopic investigations on the nature of excitons in π-conjugated cyclic systems at the single-molecule level. Herein, we studied the effect of conformational disorder on the excitonic behaviors of cyclic oligothiophenes composed of 6, 8, 10, and 12 subunits (C-6T, C-8T, C-10T, and C-12T, respectively) by employing single-molecule fluorescence spectroscopy. We found that, due to the cyclic symmetry constraint which suppresses S1-S0 transition, small and rigid C-6T and C-8T exhibit extremely long fluorescence lifetimes, while short lifetimes typical of linear systems are dominant in large, flexible C-10T and C-12T. Two-dimensional correlation maps between fluorescence lifetimes and spectral positions show that, by torsional defects created through continued photoexcitation, fully delocalized cyclic excitons shrink to form acyclic excitons in the case of C-10T, while localized acyclic excitons from initial states are maintained in the case of C-12T. The distribution of linear dichroism values from C-6T to C-10T gradually broadens but narrows in C-12T, suggesting a cyclic-to-acyclic transition in excitonic nature between C-10T and C-12T.

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.

Excited‐State Dynamic Planarization of Cyclic Oligothiophenes in the Vicinity of a Ring‐to‐Linear Excitonic Behavioral Turning Point

AbstractExcited‐state dynamic planarization processes play a crucial role in determining exciton size in cyclic systems, as reported for π‐conjugated linear oligomers. Herein, we report time‐resolved fluorescence spectra and molecular dynamics simulations of π‐conjugated cyclic oligothiophenes in which the number of subunits was chosen to show the size‐dependent dynamic planarization in the vicinity of a ring‐to‐linear behavioral turning point. Analyses on the evolution of the total fluorescence intensity and the ratio between 0–1 to 0–0 vibronic bands suggest that excitons formed in a cyclic oligothiophene composed of six subunits fully delocalize over the cyclic carbon backbone, whereas those formed in larger systems fail to achieve complete delocalization. With the aid of molecular dynamics simulations, it is shown that distorted structures unfavorable for efficient exciton delocalization are more easily populated as the size of the cyclic system increases.

Excited-State Dynamic Planarization of Cyclic Oligothiophenes in the Vicinity of a Ring-to-Linear Excitonic Behavioral Turning Point.

Excited-state dynamic planarization processes play a crucial role in determining exciton size in cyclic systems, as reported for π-conjugated linear oligomers. Herein, we report time-resolved fluorescence spectra and molecular dynamics simulations of π-conjugated cyclic oligothiophenes in which the number of subunits was chosen to show the size-dependent dynamic planarization in the vicinity of a ring-to-linear behavioral turning point. Analyses on the evolution of the total fluorescence intensity and the ratio between 0-1 to 0-0 vibronic bands suggest that excitons formed in a cyclic oligothiophene composed of six subunits fully delocalize over the cyclic carbon backbone, whereas those formed in larger systems fail to achieve complete delocalization. With the aid of molecular dynamics simulations, it is shown that distorted structures unfavorable for efficient exciton delocalization are more easily populated as the size of the cyclic system increases.

Relationship between Dynamic Planarization Processes and Exciton Delocalization in Cyclic Oligothiophenes.

In cyclic molecular structures, while the effect of conformational disorder on exciton delocalization is well understood, the impact of dynamic planarization processes remains unclear due to a lack of detailed investigation on the associated exciton dynamics. Thus, we have investigated the exciton delocalization of π-conjugated linear and cyclic oligothiophenes in the course of dynamic planarization processes by time-resolved fluorescence spectra measurements and theoretical calculations. Especially, through a comparative analysis of linear and cyclic oligothiophenes, we found that the evolution of 0-0 and 0-1 vibronic bands is strongly related to the conformations of cyclic molecular systems, reflecting the extent of exciton delocalization. Collectively, we believe that our findings are applicable to various π-conjugated organic materials and will provide new insights into the relationship between exciton delocalization and cyclic molecular conformation.

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.

New cyclic oligothiophenes and their co-oligomers derivatives of molecular engineering interest

The electronic properties of several series of bridged oligomers and their co-oligomers derivatives of five-membered biheterocycles (bithiophene, bipyrrole, bifuran bridged by CO or CS group) in their linear and cyclic structures (planar rings and crowns), have been studied using Density Functional Theory (DFT) calculations. The stability of ring forms of bridged oligo(6)biheterocycles is evidenced. Moreover, when replacing the CO bridging group by the CS one, in all oligomers the HOMO–LUMO energy gap is reduced, due mainly to the strongest stabilization of the LUMO. This energy gap is further reduced with the introduction of bridged bithiophene or bridged bifuran in the cyclic chain of bridged bipyrrole, thus suggesting an improvement of the conductivity properties of such species. The CS bridged co-oligo(3)(bithiophene-bipyrrole) and CS bridged co-oligo(3)(bifuran-bipyrrole) seems to be promising candidates for molecular engineering applications.

Rectifying behavior of [60]fullerene charge transfer complexes: A theoretical study

New type of molecular diodes based on charge transfer complexes of fullerene[60] with organic donors has been proposed and studied theoretically. Current–voltage curves and the rectification ratios (RR) for three different molecular diodes were calculated using direct ab initio method at M06/LACVP(d) level of theory in the range from −2 to +2 V. The highest RR of 73.6 was determined for the complex of C60 with cyclic oligothiophene at 0.3 V (C60–CT8–Au). Other molecular diodes show lower RR, however, all complexes show RR higher than 1 at all voltages. The asymmetric evolutions and alignment of the molecular orbitals with the applied bias were found to be essential in generating the molecular diode rectification behavior. It seems that large donor–acceptor distance in C60–CT8 complex is crucial for the high RR in C60–CT8–Au compared to other complexes.

Self-assembled cyclic oligothiophene nanotubes: Electronic properties from a dispersion-corrected hybrid functional

The band structure and size scaling of electronic properties in self-assembled cyclic oligothiophene nanotubes are investigated using density functional theory. In these unique tubular aggregates, the \ensuremath{\pi}-\ensuremath{\pi} stacking interactions between adjacent monomers provide pathways for charge transport and energy migration along the periodic one-dimensional nanostructure. In order to simultaneously describe both the \ensuremath{\pi}-\ensuremath{\pi} stacking interactions and the global electronic band structure of these nanotubes, we utilize a dispersion-corrected Becke three-parameter Lee-Yang-Parr-D (B3LYP-D) hybrid functional in conjunction with all-electron basis sets and one-dimensional periodic boundary conditions. Based on our B3LYP-D calculations, we present simple analytical formulae for estimating the fundamental band gaps of these unique nanotubes as a function of size and diameter. Our results on these molecular nanostructures indicate that all of the oligothiophene nanotubes are direct-gap semiconductors with band gaps ranging from 0.9 to 3.3 eV, depending on tube diameter and oligothiophene orientation. These nanotubes have cohesive energies of up to 2.43 eV per monomer, indicating future potential use in organic electronic devices due to their tunable electronic band structure and high structural stability.

Theoretical investigation of charge injection and transport properties of novel organic semiconductor materials—cyclic oligothiophenes

By employing the Marcus electron transfer theory coupled with two-state model, the Brownian diffusion assumption and density functional theory (DFT), we have investigated the charge injection and transport properties for three types of the cyclic oligothiophenes (A, B and C in Fig. 1), and their corresponding linear molecules (linear-A, linear-B and linear-C in Fig. 1). By indentifying 13 distinct nearest-neighbor hopping pathways based on the crystal structures of the molecules reported (A and B), we predicted the electronic coupling matrix elements for a wide variety of charge transfer pathways using the “energy splitting in dimer” (ESD) method and their carrier mobility. The theoretical results indicate that they possess large hole carrier mobility, importantly, very outstanding properties of electron transport. The major reason should be that (1) the closed ring structure restricts the rotation of the thiophene rings in charge transfer process, (2) the introduction of the alkynyl and double bonds weakens the coulomb repulsion between lone electron pairs of sulfur atoms, the ring stain and the steric effect resulting from the butyls, more importantly, (3) their introduction stabilizes LUMO level, in result to decrease electronic organization energy ( λ e ) and thereby improve their ability of electron transport. The temperature dependences of the carrier mobility were theoretically investigated for molecules A and B and analyzed within the hopping mechanism.

Molecular and electronic structure of cyclo[10]thiophene in various oxidation states: polaron pair vs. bipolaron

The molecular structure of a cyclic oligothiophene, C10T, has been determined by single-crystal X-ray structure analysis. The exclusive syn-conformation of all thiophene units as confirmed in the solid state and the ring strain in this macrocycle result in its unusual and optoelectronic properties. This does not only apply to neutral C10T but also to its oxidized states, as demonstrated by absorption and ESR spectroscopy, supporting the formation of a polaron-pair structure upon oxidation of C10T to C10T2(·+) as has been discussed for linear oligothiophenes. To the best of our knowledge, C10T2(·+) represents an unambiguous example comprising a two-polaron structure (polaron-pair) of a thiophene-based conjugated macrocycle.

Complexes of C60 with Cyclic Oligothiophenes: A Theoretical Study

Complexes of C60 and cyclic and linear oligothiophenes containing 8 and 12 repeating units have been modeled at the M05-2X/6-311G**//M05-2X/6-31G* level of theory. BSSE-corrected binding energies of neutral donor-acceptor complexes vary from 5 to 12 kcal/mol depending on the complex and donor type. Inclusion complexes formed by C60 and cyclooligothiophenes containing 8 repeating units were found to be the most stable ones. Only weak charge transfer from oligothiophene to C60 fragment (<0.1 electron) is observed in the ground state, whereas complete electron transfer from oligothiophene fragment to C60 has been found in the excited state. One electron oxidation or reduction increases binding energies of "tight" complexes and decreases donor-acceptor interaction for "loose" complexes. In the case of cation radicals, positive charge is totally concentrated at the oligothiophene fragment, whereas in anion radicals, negative charge is located at the C60 moiety. Calculations demonstrated that plane oligothiophene conformation and weak binding in a donor-acceptor complex favor the photoinduced charge-carrier formation.

Giant Cyclo[n]thiophenes with Extended π Conjugation

Dual roles and nanorings: A new family of cyclic oligothiophenes with up to 35 thiophene units is synthesized by using platinum complexes in a dual role as template and as reactive center. The ease of ionization and aggregation of the new compounds indicates that they have promising properties for use as organic electronic materials of increased structural complexity.

Copper‐Mediated Aryl‐Aryl Couplings for the Construction of Oligophenylenes and Related Heteroaromatics

Abstractmagnified imageEfficient methods for the syntheses of biaryls by the copper‐mediated homocoupling of arylmetals are summarized. First, a smooth transmetallation between arylzinc halides and copper(II) salts has been employed for the selective homocoupling of arylzinc halides to produce either biphenylenes or tetraphenylenes. This reaction has been applied to the synthesis of cyclic oligothiophenes. Second, an efficient method for the homocoupling of aryl halides by the electron‐transfer (ET) oxidation of Lipshutz cuprates [Ar2Cu(CN)Li2] with organic electron acceptors has been developed. The ET oxidation of cuprates with 1,4‐benzoquinones proceeds smoothly to afford the corresponding coupling products in moderate to high yields. The ET oxidation of cuprates has been applied to the synthesis of either thiophene‐ or benzene‐fused 10‐membered ring cyclophanes. For the synthesis of macrocyclic cyclophanes, a linear CCuC structure of Lipshutz cuprates should be maintained in the dimetallacyclic intermediates to efficiently produce large‐ring cyclophanes. Moreover, nonaphenylenes and dodecaphenylenes have been synthesized using the ET oxidation of cuprates with duroquinone. Interestingly, hexadodecyloxynonaphenylene exhibits different nanostructures between fibrous materials in solution and cast films on a surface. Films of this compound can detect vaporized nitroaromatics such as dinitrobenzene and trinitrobenzene.

Tubular Aggregates of Cyclic Oligothiophenes. A Theoretical Study

The geometries of neutral, mono-, and dioxidized tubular aggregates of cyclo[8]thiophenes containing up to 5 repeating units were fully optimized at the MPWB1K/3-21G* level of theory. Calculated interplane distances between macrocycles were found to be close to 3.1 A for neutral and charged aggregates. The binding energies between macrocycles in neutral intermediates were in the range of 40-45 kcal/mol, increasing for monocations and dropping strongly for dicationic species due to electrostatic repulsion between polarons. It was established that there exists a noticeable interaction between pi-orbitals of individual macrocycles in tubular aggregates as follows from decreasing of the band gap with a number of repeating units in aggregates and the polaron delocalization toward tube axes in oxidized species. A polaron pair is the most stable dicationic state for all studied molecules according to the calculations. A singlet polaron pair is more stable than a triplet one. The energy difference between singlet and triplet states is growing smaller with the size of the system, becoming zero for the pentamer corresponding to a completely dissociated bipolaron.