Cyclooctatetraene Ring Research Articles

β,β-Directly Linked Porphyrin Rings: Synthesis, Photophysical Properties, and Fullerene Binding.

Cyclic porphyrin oligomers have been studied as models for photosynthetic light-harvesting antenna complexes and as potential receptors for supramolecular chemistry. Here, we report the synthesis of unprecedented β,β-directly linked cyclic zinc porphyrin oligomers, the trimer (CP3) and tetramer (CP4), by Yamamoto coupling of a 2,3-dibromoporphyrin precursor. Their three-dimensional structures were confirmed by nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, and single-crystal X-ray diffraction analyses. The minimum-energy geometries of CP3 and CP4 have propeller and saddle shapes, respectively, as calculated using density functional theory. Their different geometries result in distinct photophysical and electrochemical properties. The smaller dihedral angles between the porphyrin units in CP3, compared with CP4, result in stronger π-conjugation, splitting the ultraviolet-vis absorption bands and shifting them to longer wavelengths. Analysis of the crystallographic bond lengths indicates that the central benzene ring of the CP3 is partially aromatic [harmonic oscillator model of aromaticity (HOMA) 0.52], whereas the central cyclooctatetraene ring of the CP4 is non-aromatic (HOMA -0.02). The saddle-shaped structure of CP4 makes it a ditopic receptor for fullerenes, with affinity constants of (1.1 ± 0.4) × 105 M-1 for C70 and (2.2 ± 0.1) × 104 M-1 for C60, respectively, in toluene solution at 298 K. The formation of a 1:2 complex with C60 is confirmed by NMR titration and single-crystal X-ray diffraction.

Linear, Electron-Rich, Homoleptic Rare Earth Metallocene and Its Redox Activity

The first homoleptic sandwich complex of dibenzocyclooctatetraene (dbCOT), representing a large cyclooctatetraene (COT) ligand with two fused benzene moieties, for any metal was accessed through salt metathesis of YCl3 with K2dbCOT in the presence of 2.2.2-cryptand. Single-crystal X-ray diffraction analysis on red-brown [K(crypt-222)][Y(dbCOT)2], 1, revealed a remarkably linear anionic yttrocene complex featuring a centroid-yttrium-centroid angle of 180.0°. The anionic moiety adopts a pseudo D2d geometry, where the carbon atoms of the central COT ring exhibit a staggered geometry. In total, 36 π-electrons are stored on both dbCOT anions, rendering it the largest isolated sandwich complex containing only fused aromatic rings. The solution-state structure of 1 was probed through a series of techniques involving cyclic voltammetry, UV-vis, and 1D and 2D nuclear magnetic resonance (NMR) spectroscopy, including 89Y NMR. The density functional theory (DFT) and natural bond orbital (NBO) analysis uncovered an ionic bonding interaction between the (dbCOT)2- ligands and YIII ion. NICS calculations support the experimentally observed aromatic character of 1, despite the deviation from planarity found in the dbCOT moieties. The cyclic voltammograms allude to the accessibility of a radical oxidation state, dbCOT3-•, based on a quasi-reversible feature. Excitingly, the chemical one-electron reduction of 1 through exposure to potassium graphite yielded a paramagnetic molecule, which was detected by electron paramagnetic resonance (EPR) techniques. Notably, this EPR spectrum is the first one for any sandwich complex containing a COT radical. Remarkably, 1 is thermally stable, and its isolation may provide access to mono- and multinuclear complexes comprising heavier metals with applications in small-molecule activation, single-molecule magnetism, and molecular nanowires.

Triplet conformation in chromophore-fused cyclooctatetraene dyes

Cyclooctatetraene (COT) is predicted to have a planar aromatic triplet state. Using femtosecond-to-microsecond transient absorption spectroscopy, we show that acene chromophore-fused COT dyes also have a planar COT ring in their triplet states, although they are not aromatic.

Electronic Materials: An Antiaromatic Propeller Made from the Four‐Fold Fusion of Tetraoxa[8]circulene and Perylene Diimides

The synthesis of an antiaromatic tetraoxa[8]circulene annulated with four perylene diimides (PDI), giving a dynamic non-planar π-conjugated system, is described. The molecule contains 32 aromatic rings surrounding one formally antiaromatic planarized cyclooctatetraene (COT). The intense absorption (ϵ=3.35×105 M−1 cm−1 in CH2Cl2) and emission bands are assigned to internal charge-transfer transitions in the combined PDI-circulene π-system. The spectroscopic data is supported by density functional theory calculations, and nuclear independent chemical shift calculation indicate that the antiaromatic COT has increased aromaticity in the reduced state. Electrochemical studies show that the compound can reversibly reach the tetra- and octa-anionic states by reduction of the four PDI units, and the deca-anionic state by reduction of the central COT ring. The material functions effectively in bulk hetero junction solar cells as a non-fullerene acceptor, reaching a power conversion efficiency of 6.4 %.

Electronic Materials: An Antiaromatic Propeller Made from the Four-Fold Fusion of Tetraoxa[8]circulene and Perylene Diimides.

The synthesis of an antiaromatic tetraoxa[8]circulene annulated with four perylene diimides (PDI), giving a dynamic non-planar π-conjugated system, is described. The molecule contains 32 aromatic rings surrounding one formally antiaromatic planarized cyclooctatetraene (COT). The intense absorption (ϵ=3.35×105 M-1 cm-1 in CH2 Cl2 ) and emission bands are assigned to internal charge-transfer transitions in the combined PDI-circulene π-system. The spectroscopic data is supported by density functional theory calculations, and nuclear independent chemical shift calculation indicate that the antiaromatic COT has increased aromaticity in the reduced state. Electrochemical studies show that the compound can reversibly reach the tetra- and octa-anionic states by reduction of the four PDI units, and the deca-anionic state by reduction of the central COT ring. The material functions effectively in bulk hetero junction solar cells as a non-fullerene acceptor, reaching a power conversion efficiency of 6.4 %.

Density functional theory and time‐dependent density functional theory studies on optoelectronic properties of fused heterocycles with cyclooctatetraene

AbstractWe establish a structure–property relation for the system having a cyclooctatetraene (COT) ring fused with four five‐membered heterocyclic rings. The two five‐membered heterocycles selected are furan and selenophene and their fused structure with COT is known as cyclooctatetrafuran (COF) and cyclooctatetraselenophene (COSe), respectively. We found 15 geometrical isomers of each COF and COSe and their structural stability and optoelectronic properties have been evaluated by quantum chemical simulations. The density functional theory (DFT) and time‐dependent DFT simulations were employed for a systematic review of all isomers. Electronic excitations, hole reorganization energies, electron reorganization energies, ionization potentials, and electron affinities, of all the isomers, were reported. Based on our comparative study, it is shown that one of the isomers is suggested as a better charge transport material.

Stability, optical and charge transport properties of saddle-shaped cyclooctatetrathiophene (COTh) isomers: a theoretical study

Design and structure–property relationship studies of the system containing a cyclooctatetraene (COT) ring fused with four 5-membered heterocyclic rings have been reported. The five-membered heterocycle chosen for the present study is thiophene and based on connectivity with COT, cyclooctatetrathiophene (COTh) exists in fifteen geometrical isomers. The systematic reviews of all the isomers of COTh, are carried out by utilizing density functional theory (DFT) and time dependent-density functional theory (TD-DFT) calculations. Electronic excitations, reorganization energies (transport properties of electron and hole), electron affinities (EA) and ionization potentials (IP) of all the isomers are reported. Based on the outcomes of this study, we suggest that one of the isomers, among all fifteen possible isomers, is better suited for applications that take advantage of its charge transport properties.

Graphene-induced planarization of cyclooctatetraene derivatives.

Stable equilibrium compounds containing a planar antiaromatic cyclooctatetraene (COT) ring are promising candidates for organic electronic devices such as organic semiconductor transistors. The planarization of COT by incorporation into rigid planar π-systems, as well as by oxidation or reduction has attracted considerable attention in recent years. Using dispersion-corrected density functional theory calculations, we explore an alternative approach of planarizing COT derivatives by adsorption onto graphene. We show that strong π-π stacking interactions between graphene and COT derivatives induce a planar structure with an antiaromatic central COT ring. In addition to being reversible, this strategy provides a novel approach for planarizing COT without the need for incorporation into a rigid structure, atomic substitution, oxidation, or reduction.

Which isomer is it, 1,2,5,6- or 1,4,5,8-tetrasubstituted cycloocta-1,3,5,7-tetraene? Synthesis of symmetrically tetrasubstituted cycloocta-1,3,5,7-tetraene derivatives.

Ni(0) catalyzed cyclotetramerization [(2 + 2 + 2 + 2) cycloaddition] of propargyl alcohol gave D2 symmetric 1,4,5,8-tetrakis(hydroxymethyl)cycloocta-1,3,5,7-tetraene (1) as the major product contrary to what was reported earlier to be the corresponding C2v symmetric isomer, namely 1,2,5,6-tetrakis(hydroxymethyl)cycloocta-1,3,5,7-tetraene. The structure of 1 has been unambiguously assigned on the basis of single crystal XRD data which showed a very interesting "densely" intermolecularly hydrogen bonded solid state supramolecular structure. Tetrabromide (3) prepared from tetra-alcohol (1) reacted with a variety of nucleophiles, namely carbon, nitrogen, sulfur and phosphorus nucleophiles, to give the corresponding derivatives in good yields. The tetraphosphonate derivative (9) underwent the Wittig-Horner reaction with aromatic aldehydes to give the corresponding tetra-styryl derivatives (10a-i). The reaction of tetrabromide with aniline, benzyl amine and R(+)-α-methylbenzyl amine gave the corresponding C2v symmetric pyrroline fused 1,2,5,6-tetra-substituted cycloocta-1,3,5,7-tetraene derivatives, while all the other derivatives belonged to D2 symmetric 1,4,5,8-tetra-substituted cycloocta-1,3,5,7-tetraene. Quantum mechanical calculations support the above conclusion based on the relative stabilities of the two isomers. The fusion of a 5-membered pyrroline ring with the tub-shaped cyclooctatetraene (COT) ring leads to flattening of the tub (shallow tub) making it easier for the ring inversion of COT through a planar intermediate to take place readily resulting in the formation of C2v symmetric pyrroline fused COT derivatives.

Oxidation of a Dithieno[3,4-b:3',4'-d]thiophene Cyclic Dimer Containing a Planar Cyclooctatetraene Ring: Retention of High Antiaromaticity During Reactions.

One-electron and peracid oxidations of dithieno[3,4-b : 3',4'-d]thiophene cyclic dimer, which contains an antiaromatic planar cyclooctatetraene (COT) core, were conducted. The reaction of the cyclic dimer with SbCl5 produced isolable radical cation salts. Density functional theory (DFT) calculations showed that the spin density of the radical cation resides not on the COT ring but on the peripheral sulfur and carbon atoms in the thiophene unit with retention of high antiaromaticity based on the nucleus-independent chemical shift (NICS). The peracid oxidation of the cyclic dimer was found to proceed not on the COT ring but on the bridging sulfur atom in the dithienothiophene moiety. The retention of the high antiaromaticity of the COT ring after the sulfoxide formation was experimentally confirmed based on the relative hardness, and also was theoretically supported by NICS calculations. Interestingly, the DFT calculations suggested that the high antiaromaticity does not enhance the reactivity towards the epoxidation on the COT ring.

A study of the aromaticity of heteroannelated cyclooctatetraene derivatives.

The aromaticity of the rings of thiophene, pyrrole, furan, and benzene annelated cyclooctatetraene (COT) derivatives and of their double charged ions was studied using the graph-theoretical theory of aromaticity. On the basis of topological resonance energy, it was found that the global aromaticity is dependent upon on the arrangement of heteroatoms in the given molecule. Relative stability of these molecules when in different charged states can been explained in terms of the topological charge stabilization rule. We expect that fusing the COT ring with an increasing number of aromatic rings will lead to an increase in the aromaticity of the molecule. According to the bond resonance energy (BRE) and circuit resonance energy (CRE) indices, local antiaromaticity of the COT ring is weakened as the number of fused rings increases, and these changes play a significant role in the global aromaticity of the molecule. For some compounds, our BRE and CRE indices do not predict the same order of magnitude of the local aromatic character of certain rings that the nucleus independent chemical shift (NICS(0) and (NICS(1)) methods predict. Finally, for the available compounds, correlations between the diatropic and paratropic chemical shifts of the protons and our ring current results were analyzed and good agreement was found.

Conformational Planarization versus Singlet Fission: Distinct Excited-State Dynamics of Cyclooctatetraene-Fused Acene Dimers.

A set of flapping acene dimers fused with an 8π cyclooctatetraene (COT) ring showed distinct excited-state dynamics in solution. While the anthracene dimer showed a fast V-shaped-to-planar conformational change within 10 ps in the lowest excited singlet state, reminding us of extended Baird aromaticity, the tetracene dimer and the pentacene dimer underwent intramolecular singlet fission (SF) in different manners: A fast and reversible SF with a characteristic delayed fluorescence (FL), and a fast and quantitative SF, respectively. Conformational flexibility of the fused COT linkage plays an important role in these ultrafast dynamics, demonstrating the utility of the flapping molecular series as a versatile platform for designing photofunctional systems.

Conformational Planarization versus Singlet Fission: Distinct Excited‐State Dynamics of Cyclooctatetraene‐Fused Acene Dimers

AbstractA set of flapping acene dimers fused with an 8π cyclooctatetraene (COT) ring showed distinct excited‐state dynamics in solution. While the anthracene dimer showed a fast V‐shaped‐to‐planar conformational change within 10 ps in the lowest excited singlet state, reminding us of extended Baird aromaticity, the tetracene dimer and the pentacene dimer underwent intramolecular singlet fission (SF) in different manners: A fast and reversible SF with a characteristic delayed fluorescence (FL), and a fast and quantitative SF, respectively. Conformational flexibility of the fused COT linkage plays an important role in these ultrafast dynamics, demonstrating the utility of the flapping molecular series as a versatile platform for designing photofunctional systems.

Stabilizing a different cyclooctatetraene stereoisomer

An unconventional cis-cis-cis-trans or (Z,Z,Z,E) structure B of cyclooctatetraene (COT) is calculated to lie only 23 kcal/mol above the well-known tub-shaped (Z,Z,Z,Z) isomer A; one example of this type of structure is known. The barrier for B returning to A is small, 3 kcal/mol. However, by suitable choice of substituents, the (Z,Z,Z,E) isomer can be made to lie in energy below the tub-shaped structure. Steric, clamping, and electronic strategies are proposed for achieving this. In the steric strategy, the C8H4(CH3)2(C( t Bu)3)2 structure B is predicted to lie 21 kcal/mol below structure A, which is separated from form B only by a small barrier. A simple clamping strategy, effective for COT planarization, does not influence the A/B isomerization much. But, if the clamping group is aromatic (a fused benzene, pyrrole, thiophene, furan), the subtle interplay of potential aromaticity with clamping can be used to confer persistence if not stability on the (Z,Z,Z,E) isomer. An electronic strategy of a different kind, push-pull substitution on the COT ring, was not very effective in stabilizing the B form. However, it led us to vicinal amine-borane-substituted normal COTs that proved to be quite good at activating H2 in a frustrated Lewis pair scenario.

Twisted Polycyclic Aromatic Hydrocarbon with a Cyclooctatetraene Core via Formal [4+4] Dimerization of Indenofluorene

A polycyclic aromatic hydrocarbon having a twisted cyclooctatetraene (COT) core was prepared by a formal [4+4] dimerization of an in situ generated indeno[2,1-<i>a</i>]fluorene derivative. The compound consists of two indenofluorene frameworks connected with two double bonds and adopts a twisted geometry to avoid intramolecular steric repulsion. It exhibits a long-wavelength absorption band with a maximum at 634 nm extending to about 850 nm, which is consistent with the moderate antiaromaticity of the twisted COT ring as indicated by the NICS value (+11.9). A mechanism for cycloaddition reaction is proposed in terms of stepwise bond formations due to the diradical character of the indenofluorene framework, on the basis of the completely different reaction product obtained from the extended benzo analogue.

Cyclooctatetraenophanes: A Computational Study.

DFT (ωB97X-D, B3LYP-D3, M06-2x, and B3LYP) along with MP2 computations were performed on four cyclophanes composed of two or three cyclooctatetraene (COT) rings connected by two, four, or eight ethylene bridges. Both COT rings in cyclophanes with two ethylene bridges (2) and with four bridges in the 1, 2, 5, and 6 positions (6) are in a tub conformation. However, the cyclophane with the four bridges in the 1, 3, 5, and 7 positions (7) is notable for the near planar geometry of the COT rings. The triple-decker cyclophane 8 has planar top and bottom COT rings, while the central ring is puckered with alternating carbon positions up and down. The nature of the COT rings, especially their antiaromatic character, is assessed using NICS and bond alternation and the distance between the centers of the COT rings.

Bent π-Conjugated System Composed of Two Dibenzocyclooctatetraene Units: Multifunctional Properties of Dynamic Molecular Tweezers in Solution and the Solid State

Abstract A series of bent π-conjugated compounds, 2a, 2b, 3a, and 3b, composed of two tub-shaped dibenzocyclooctatetraene (DBCOT) units with two cyano substituents on each cyclooctatetraene (COT) ring were designed and synthesized by using improved synthetic procedures. The anti- and syn-conformers of 2a, 2b and 3a, 3b, which coexist in equilibrium in solution, were independently observed by using 1H NMR spectroscopy at room temperature, and the signals of the anti- and syn-forms coalesced at 130 °C to give signals assigned to a D2h symmetric molecule. The ratio of the anti- and syn-isomers of these dynamic molecular tweezers (DMTs) were dependent on the position of the cyano substituents on the COT rings due to the difference in either torsional strain or dipole moments between the anti- and syn-isomers (2a, 2b: syn &amp;gt; anti; 3a, 3b: syn &amp;lt; anti). The dynamic guest incorporation ability of these DMTs in solution was investigated by using 1H NMR and UV–vis spectroscopic titration experiments and evaluated on the basis of the calculated affinity for a guest molecule. We found that 2a, 2b and 3a, 3b exhibited unique aggregation-induced emission in solution and crystalline-state emission induced by incorporated solvents in the crystal lattices of the DMTs. The emission wavelength and quantum yield of the aggregation-induced and crystalline-state emissions depended on their restricted flexibility and syn–anti ratio. Theoretical calculations confirmed that the dihedral angle between the central and terminal benzene rings had the largest effect on the emission wavelength in the crystalline state.

(η4-Cyclooctatetraene)(η8-cyclooctatetraene)iodidotantalum(V)

The title complex, [Ta(η4-C8H8)(η8-C8H8)I], lies across a crystallographic mirror plane that includes the TaV atom and the iodide ligand. One cyclo­octa­tetra­ene (cot) ring is η4-coordinating and is bis­ected by the mirror plane. The fold angle between the plane of the coordinating butadiene portion and the middle plane of the ring is 27.4 (4)°. An additional minor fold angle of 9.3 (7)° exists between the final plane in the ring and the middle plane. The other cot ring is η8-coordinating and is also cut by the mirror plane. In this case, the ring is disordered over the mirror plane, and one position is modeled with appropriate restraints and constraints with respect to distances, angles and displacement parameters (the second position is generated by symmetry). This ring is nearly planar, with an r.m.s. deviation of only 0.05 Å when all eight C atoms are included in the calculation. Pairs of inter­molecular η8-cot rings are parallel stacked and slightly off center, with a centroid–centroid distance of 3.652 Å. No other significant inter­molecular inter­actions are observed. The compound is of inter­est as the first structurally characterized mixed halogen–cot complex of the group 5 metals and contains the longest terminal Ta—I distance [3.0107 (5) Å] reported to date.

The Electronic Structure of Heteroannelated Cyclooctatetraenes and their UV-Vis Absorption Spectra

The electronic structure and the UV-Vis absorption spectra of heteroannelated cyclooctatetraene derivatives are studied by density functional theory and by its non-stationary variant, the time-dependent density functional theory. The cyclooctatetraene ring is shown to be planar in all of the molecules considered, except the annelated fluorene and dibenzothiophene derivatives, and exhibits an antiaromatic character according to the magnetic and structural aromaticity criteria. The double ionization of annelated cyclooctatetraene molecules (quasi-circulenes) causes a change in the aromatic properties of cyclooctatetraene, which generally gaines aromaticity by double reduction and becomes non-aromatic upon double oxidation. The time-dependent density functional theory calculations enable the interpretation of electronic absorption spectra of recently synthesized quasi-circulenes and to predict the spectra of new, hypothetical molecules, which are important for the general theoretical understanding of hetero[8]circulene spectra.

Dithieno[3,4-b:3′,4′-d]thiophene-Annelated Antiaromatic Planar Cyclooctatetraene with Olefinic Protons

The design and synthesis of a new planar cyclooctatetraene (COT) with protons directly connected to the COT ring was attained by monoannelation with dithieno[3,4-b:3',4'-d]thiophene. The planar structure of the COT core was unambiguously confirmed by X-ray crystallography. The magnetic antiaromaticity of the COT core was found to be higher than that of the previously synthesized planar COTs with olefinic protons, according to the results of (1)H NMR and absorption spectra as well as NICS calculations.