How to Bend a Cumulene

Allenes (carbodicarbenes) and [3]cumulenes are linear carbon chains that can be bent when the terminal group has a strong carbene nature. This bending can be quite pronounced in allenes but not in [3]cumulenes. In this study, how N-heterocyclic or cyclic (alkyl)(amino) carbene (NHC and CAAC, respectively) terminal groups can modify the linear structure of [n]cumulenes has been analyzed. A low π acidity of the terminal carbene affects the linearity of [2n]cumulenes. Indeed, it has been found that the NHC [4]cumulene is extremely bent, contrary to classical [4]cumulenes. The predicted NHC [4]cumulene or tricarbodicarbene has two lone pairs and the π electrons are delocalized over the whole molecule. More significantly, DFT calculations have shown that this bent [4]cumulene is very stable, considerably more so than the corresponding [3]cumulene, which has been elusive to synthesize. Remarkably, calculations have shown that all the NHC [2n]cumulenes are more than 25 kcal mol⁻¹ more stable than the [2n−1]cumulenes.     

Unusual Length Dependence of the Conductance in Cumulene Molecular Wires

Cumulenes are sometimes described as “metallic” because an infinitely long cumulene would have the band structure of a metal. Herein, we report the single‐molecule conductance of a series of cumulenes and cumulene analogues, where the number of consecutive C=C bonds in the core is n=1, 2, 3, and 5. The [n]cumulenes with n=3 and 5 have almost the same conductance, and they are both more conductive than the alkene (n=1). This is remarkable because molecular conductance normally falls exponentially with length. The conductance of the allene (n=2) is much lower, because of its twisted geometry. Computational simulations predict a similar trend to the experimental results and indicate that the low conductance of the allene is a general feature of [n]cumulenes where n is even. The lack of length dependence in the conductance of [3] and [5]cumulenes is attributed to the strong decrease in the HOMO–LUMO gap with increasing length.     

Push–Pull Buta‐1,2,3‐trienes: Exceptionally Low Rotational Barriers of Cumulenic CC Bonds and Proacetylenic Reactivity

A variety of asymmetrically donor–acceptor‐substituted [3]cumulenes (buta‐1,2,3‐trienes) were synthesized by developed procedures. The activation barriers to rotation ΔG^≠ were measured by variable temperature NMR spectroscopy and found to be as low as 11.8 kcal mol^?1, in the range of the barriers for rotation around sterically hindered single bonds. The central C?C bond of the push–pull‐substituted [3]cumulene moiety is shortened down to 1.22 Å as measured by X‐ray crystallography, leading to a substantial bond length alternation (BLA) of up to 0.17 Å. All the experimental results are supported by DFT calculations. Zwitterionic transition states (TS) of bond rotation confirm the postulated proacetylenic character of donor–acceptor [3]cumulenes. Additional support for the proacetylenic character of these chromophores is provided by their reaction with tetracyanoethene (TCNE) in a cycloaddition‐retroelectrocyclization (CA–RE) cascade characteristic of donor‐polarized acetylenes.     

An Insight of the Results Provided by Color‐Tuning Studies Made on Ir(III) Complexes: A pi‐Neutral CF3 Group Viewed by Adjusting Energies of pi‐type Molecular Orbitals

The pi‐nature of a CF₃ group can be understood through analysis of its bond orbitals (BOs) mixed into the pi‐type molecular orbitals of CF₃‐substituted Ir(ppy)₂MDPA⁺ complexes (ppy=2‐phenyl‐pyridine and MDPA=methylated 2,2′‐dipyridyl amine). It has been found that, through this natural bond orbital analysis, the parent’s molecular orbitals (MOs) can be stabilized by χρ^*_CF BO via negative hyperconjugation and, simultaneously, destabilized by electron lp(F) BO. Since these two competing pi‐effects are virtually counterbalanced as indicated by the vanishing values of crystal orbital overlap populations, the chemical substitution strategy originated from lowering of HOMO by using this electron‐withdrawing CF₃ group has been found effective in color‐tuning to blue region. Based on reduced shielding effect due to de‐ creased χρ‐electron density, the reported position dependent CF₃‐substitution effects on pi‐type MOs can also be understood through HOMO/LUMO wavefunction analysis.     

Cumulene Rotaxanes: Stabilization and Study of [9]Cumulenes

The stabilization of long [n]cumulenes has traditionally been achieved by placing sterically bulky “protecting groups” at the termini, which shield the reactive carbon chain from unwanted reactions. Herein, we present an alternative strategy: stabilization through threading the sp‐hybridized carbon chain through a phenanthroline‐based macrocycle. The result is stable [9]cumulene rotaxanes that enable the study of properties as a function of length for [n]cumulenes in unprecedented detail, including by quantitative UV/Vis spectroscopy, cyclic voltammetry, and differential scanning calorimetry. The experimental results are supported by DFT calculations.     

Optical and Electrical Properties of Triphenylamine Derivatives for Dye-sensitized Solar Cells and Designing of Novel Molecule

With density functional theory(DFT) method, the optimization of molecular configurations and the calculation of frontier molecular orbitals were achieved for triphenylamine(TPA)-based dye-sensitized solar cell materials at the B3LYP/6-31G(d, p) level. Time-dependent density functional theory(TD-DFT) was applied to calculating the probability of the transition from the ground state to the excited state. And UV-Vis absorption spectra were derived with Franck-Condon approximation. The conjugation length, substitution groups and spatial effects show a slight influence on the dihedral angle of the TPA group. The increase of conjugation length may cause a smaller energy gap as well as a higher highest occupied molecular orbital(HOMO) and a lower lowest unoccupied molecular orbital (LUMO). The introduction of methoxyl group and TPA group could lower the energy gap while the HOMO and LUMO were elevated in energy.     

Photo‐physical Properties of N‐methyl‐3,4‐fulleropyrrolidine and Its Derivatives: A DFT and TD‐DFT Investigation

A series of N‐methyl‐3,4‐fulleropyrrolidine (NMFP) derivatives were designed by selecting different π‐conjugated linkers and electron‐donating groups as D‐π‐A and D‐A systems. The optimised structures and photo‐physical properties of NMFP and its derivatives have been determined using density functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) methods with the B3LYP functional and the 6‐31G basis set. According to the computation analysis, both the π‐conjugated linkers and the electron‐donating groups can influence the electronic and photo‐physical properties of the NMFP derivatives. Our calculated results demonstrated that the electron‐donating groups, with significant electron‐donating ability, had the tendency to increase the highest occupied molecular orbital (HOMO) energy. The π‐conjugated linkers with lower resonance energy decreased the lowest occupied molecular orbital (LUMO) energy and caused a significant decrease in the energy gap (E_g) between the E_HOMO and E_LUMO. A Natural Bond Orbital (NBO) analysis examines the effect of the electron‐donating group, π conjugated linker, and electron‐withdrawing group for these NMFP derivatives. For the NMFP derivatives, a projected density of state (PDOS) analysis demonstrated that the electron density of HOMO and LUMO are concentrated on the electron‐donating group and the π‐conjugated linker, respectively. A TD‐DFT/B3LYP calculation was performed to calculate the electronic absorption spectra of these NMFP derivatives. Both the electron‐donating group and the π‐conjugated linker contribute to the major absorption peaks, which are assigned as HOMO to LUMO transitions and are red‐shifted relative to those of non‐substituted NMFP.     

A Tetraethynyl[5]cumulene

The first example of a new class of carbon‐rich molecules is introduced, namely, a derivative of tetraethynyl[5]cumulene (TE5C). The use of sterically demanding pendent groups is the decisive structural feature to provide a stable product. Whereas triisopropylsilyl groups are seemingly not sufficiently large to afford an isolable cumulene product, switching to the larger tris(3,5‐di‐tert‐butylphenyl)methyl (‘supertrityl’) groups gives a crystalline, stable compound ( [5]TE ). The structural and electronic properties of [5]TE are examined in comparison to its closest known molecular relatives, tetraaryl[5]cumulenes.     

meso‐Cumulenic 2H‐Corroles from meso‐Ethynyl‐3H‐corroles

Alkynyl‐substituted 3H‐corrole 9 a was converted to [3]cumulenic 2H‐corrole 10 a by treatment with trimethylsilyl chloride (TMSCl), and 1,3‐butadiyne‐bridged 3H‐corrole dimer 11 b was transformed into [5]cumulene‐bridged 2H‐corrole dimer 12 b by oxidation with PbO₂. Both 10 a and 12 b were metalated to form Zn^II complexes 10 a‐Zn and 12 b‐Zn . The structures of 10 a‐Zn and 12 b‐Zn show planar conformations with bond‐length alternations that are analogous to those of tetraaryl [n]cumulenes. The cumulenic corrole dimers 12 b and 12 b‐Zn display large NIR absorption bands in the range of 700–1400 nm (maximum ϵ≈1.0×10⁵ m ⁻¹ cm⁻¹) owing to the effective π‐conjugation between the two corrole units through the [5]cumulene bridge.     

Benzodithiophene‐thiophene‐based photovoltaic polymers with different side‐chains

A series of benzodithiophene‐thiophene‐based alternating copolymers were synthesized with different side‐chains, and their photovoltaic characteristics were examined. The choice of solubilizing side‐chains influences significantly the chain conformation, frontier orbital energy levels, intermolecular organization, and the resulting optical, morphological, and photovoltaic properties. The incorporation of an e‐withdrawing carbonyl group in the side‐chain decreased the highest occupied molecular orbital (HOMO, ca. ?5.4 eV) level and improved the chain planarity through intrachain hydrogen bonding. The shortest π–π stacking distance (3.72 Å) was also measured for the alkylcarbonyl‐substituted BDTCOT:PC₇₁BM blended film by two dimensional grazing incidence X‐ray scattering. With compared to other polymers, the BDTCOT:PC₇₁BM device showed a substantially improved open‐circuit voltage and short‐circuit current density, leading to a 4.66% power conversion efficiency. The side‐chains need to be designed to be multifunctional to induce a deep HOMO level and chain planarity (for interchain ordering) as well as good solution processability. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 854–862     

Five-membered metallacycloalkynes formed from group 4 metals and [n]cumulene (n = 3,5) ligands

In this account [3]- and [5]cumulene complexes of group 4 metallocenes that form five-membered metallacycles are described. These complexes have a “triple bond” despite their five-membered ring structure, showing that they are regarded as 1-metallacyclopent-3-ynes. The molecular structures show their strained alkyne character. These complexes react with transition metals to form alkyne-coordinated bimetallic complexes. They also receive electrophilic attack by protons and boranes resulting in M–C bond cleavage. When a [3]cumulene couples with an alkyne on the metal, the reaction produces seven-membered metallacycloalkynes that have a strained structure showing an interaction between the “triple bond” and the metal center. Hexapentaenes, [5]cumulenes, form conjugated 1-metallacyclopent-3-ynes. The aryl-substituted [5]cumulene complex was reduced by alkali metal to give dianionic species that reacted with protons to give 1-metallacyclopent-3-ene, a cycloalkene, and with iodomethane to give 1-metallacyclopenta-2,3-diene, a cycloallene. The hexapentaene with tert-butyl groups reacts with zirconocene to form an η²-π-coordinated complex in the presence of trimethylphosphine, although it gave a 1-metallacyclopent-3-yne in the absence of the phosphine. The former was transformed into the latter by addition of a phosphine, and vice versa by removing the phosphine, showing a “haptotropic” shift.     

The Pi‐Nature of Methyl Obtained by the Natural Bond Orbital Method: Orbital‐Based Rationalizations of Site‐Dependent Substitution Effects on Fine Color‐Tuning of Luminescence

Both C‐H bonding and antibonding (σ_CH and σ*_CH) of a methyl group would contribute to the highest occupied or lowest unoccupied molecular orbitals (HOMO or LUMO) in methylated derivatives of Ir(ppz)₂ 3 iq (ppz = 1‐phenylpyrazole and 3iq = isoquinoline‐3‐carboxylate). This is found by analysis of HOMO (or LUMO) formed by linear combination of bond orbitals using the natural bond orbital (NBO) method. The elevated level of HOMO (or LUMO) uniformly found for each methylated derivative, indicating the σ_CH‐destabilization outweighs the σ*_CH‐stabilization. To broaden the HOMO‐LUMO gap, methylation at a carbon having smaller contribution to HOMO and/or larger contribution to LUMO is suggested.     

Computational study of substituent effect in para substituted platinabenzene complexes

The electronic structure and properties of the platinabenzene and para substituted platinabenzenes have been investigated using the hybrid density functional mpw1pw91 theory. The substituent effect in structure parameters, frontier orbital energies, aromaticity indexes, and hyperpolarizability has been studied. The calculations show that, in all molecules HOMO → LUMO transition makes the major contribution in the most intense electronic transition.     

Antimicrobial activity and density functional calculations of a supramolecular manganese complex

The density functional theory calculations on the manganese complex ([Mn(8-OHQ)₃] · CH₃OH, 8-OHQ = 8-hydroxyquinoline) is made and its stabilization, molecular orbital composition, orbital energies, and NBO charge distribution have been investigated. The crystal structure of the complex closely resembles the optimized structure. In vitro activities against some antimicrobials (Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, and Bacillus subtilis) are determined. The antimicrobial activities present a positive correlation between the concentrations and bioactivities of the complex. The relationship between bioactivities and molecular orbitals (HOMO and LUMO orbitals) of the complex is also discussed.     

Enhanced open-circuit voltage in methoxyl substituted benzodithiophene-based polymer solar cells

The open-circuit voltage (V _oc) is one of the important parameters that influence the power conversion efficiency (PCE) of polymer solar cells. Its value is mainly determined by the energy level offset between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor. Therefore, decreasing the HOMO value of the polymer could lead to a high V _oc and thus increasing the cell efficiency. Here we report a facile way to lower the polymer HOMO energy level by using methoxyl substituted-benzodithiophene (BDT) unit. The polymer with the methoxyl functionl group (POBDT(S)-T1) exhibited a HOMO value of–5.65 eV, which is deeper than that (–5.52 eV) of polymer without methoxyl unit (PBDT(S)-T1). As a result, POBDT(S)-T1-based solar cells show a high V _oc of 0.98 V and PCE of 9.2%. In contrast, PBDT(S)-T1-based devices show a relatively lower V _oc of 0.89 V and a moderate PCE of 7.4%. The results suggest that the involvement of methoxyl group into conjugated copolymers can efficiencly lower their HOMO energy levels.     

氟硼二吡咯类pH荧光探针PET光谱特性的理论计算

氟硼二吡咯(BODIPY)类pH荧光探针分子是基于光诱导电子转移(PET)的荧光探针分子, 识别基团氮原子上引入不同取代基可呈现不同的光学灵敏度. 本文应用密度泛函理论(DFT)及含时密度泛函理论(TD-DFT)方法对六种含不同取代基的探针分子进行了几何构型优化及激发态计算, 探讨了不同取代基对PET效应影响. 计算结果表明: 基态时这些探针分子的最高占有分子轨道(HOMO)和最低未占有分子轨道(LUMO)都在荧光母体BODIPY的π, π^*轨道, 而识别基团上氮原子孤对电子所在的轨道为HOMO-1轨道. 但是在激发态, 当氮原子上有两个取代基时, HOMO-1→LUMO跃迁的激发能都小于荧光团的HOMO→LUMO跃迁, 这将有可能产生PET效应并导致荧光熄灭, 而当氮原子上有一个取代基时不会出现这种现象. 通过激发态结构优化可以发现, 无论识别基团氮原子上有一个还是两个取代基, N原子的轨道对称性都发生变化, 由sp³→sp², 孤对电子占据在p轨道上, 其轨道能级升高至荧光团的HOMO和LUMO轨道之间, 将导致不同程度的PET效应, 与实验结果一致.     

Unexpected Formation of a [4]Radialene and Dendralenes by Addition of Tetracyanoethylene to a Tetraaryl[5]cumulene

The use of cumulenes in synthetic transformations offers the possibility to form structurally interesting and potentially useful conjugated molecules. The cycloaddition reaction of a tetraaryl[5]cumulene with the electron‐deficient olefin tetracyanoethylene affords unusual products, including functionalized dendralenes and alkylidene cyclobutanes, as well as a symmetric [4]radialene that shows unique solvatochromism, with λ_max values approaching the near‐IR region. These carbon‐rich products have been investigated spectroscopically and by X‐ray crystallographic analysis (five structures). The cycloaddition reaction sequence has also been explored by mechanistic and theoretical studies. The obtained results clearly demonstrate the potential of [5]cumulenes to serve as precursors for unprecedented conjugated structures.     

Frontier density pattern of dioxins

Ab initio molecular orbital calculations are performed to estimate the electron densities of the highest occupied molecular orbitals (HOMO) of 75 congeners of chlorinated dibenzo-p-dioxins and a nonchlorinated dibenzo-p-dioxin. Electron densities of HOMO on out of plane π orbitals of 12 carbons and two oxygens in the dioxin structure are used as variables in multivariate statistical analysis. Principal component analysis can classify 76 congeners of dioxins according to the principal component scores. All of the most toxic dioxins are involved in the group that has large negative values for both the first and the third principal component scores.     

Distinct Helical Molecular Orbitals through Conformational Lock**

Several theoretical studies have proposed strategies to generate helical molecular orbitals (Hel-MOs) in [n]cumulenes and oligoynes. While chiral even-[n] cumulenes feature Hel-MOs, odd-[n] cumulenes may also present them if the terminal groups lie in different planes. However, the proposed systems have been either experimentally unfeasible or resulted in opposite pseudo-degenerated Hel-MOs. We hereby demonstrate the introduction of a remarkable energy difference between helical orbitals of opposite twist by fixing the torsion angle between the terminal groups in butadiyne fragments. To experimentally lock the conformation of the terminal groups, we designed and synthesized cyclic architectures by combining acetylenes with chiral spirobifluorenes. The high stability of these systems with distinct helical orbitals allowed their isolation and full characterization. In our view, these results constitute a step further in the development of real systems presenting helical molecular orbitals.