Acenes Research Articles

Manifestations of strong electron correlation in polyacene: Fundamental gap, density of states, and photoconductivity

The quasi-particle (QP) bands of polyacene (PAC) – an infinite linear chain of fused benzene rings – are reconstructed from the QP spectra of oligoacenes (OACs) with N ∈ [ 5 , 30 ] ( N is the number of fused rings) obtained using a computational methodology capable of incorporating the effect of strong non-dynamic electron correlation into the computation of the QP (electrons and holes) energy levels. As the result of mixing with the doubly excited electronic configuration, the fundamental gap in the ground electronic state of PAC widens to ca. 1.8 – 2.2 eV depending on the density functional employed. In the excited singlet electronic state, the band gap closes; thus, leading to photoconductivity of PAC, as well as longer ( N ≳ 9 ) OACs. The calculated optical gap of OACs converges to a finite value of ca. 1 eV in a reasonable agreement with the experiment. The experimentally observed sudden opening of the transmission gap in scanning tunneling spectroscopy of dodecacene ( N = 12) is explained by the effect of strong correlation, which shifts the first maxima of the density of states of the ground and excited states towards larger electron binding energies.

Effect of nitrogen doping and acene cores elongation on charge transport and electronic nature of organic semiconductor materials: A DFT study

With the intention to tune the charge transport nature of preliminary 4,6-di(thiophen-2-yl)pyrimidine (DTP) structure, six novel V-shaped organic semiconductor compounds were designed by nitrogen doping and acene moieties elongation. Initially, the nitrogen atoms were doped in DTP to design 4,6-bis-thiazol-2-yl-pyrimidine (1). Moreover, by ℼ-bridge elongation strategy, 4,6-bis-benzo[b]thiazol-2-yl-pyrimidine (2), 4,6-bis(naphthothiazol-2-yl)pyrimidine (3), 4,6-bis(anthracenothiazol-2-yl)pyrimidine (4), 4,6-bis(tetracenothiazol-2-yl)pyrimidine (5), and 4,6-bis(pentacenothiazol-2-yl)pyrimidine (6) were designed by substituting various oligocenes at both ends. The ground as well as excited state structures were optimized using density functional theory (DFT) and time-dependent DFT at B3LYP/6-31G** and TD-B3LYP/6-31G** levels, correspondingly. We explored their frontier molecular orbitals, electron injection aptitude, photo-stability, ionization energies (IE), electron affinity (EA), and reorganization energies. The bridge elongation significantly elevate the EA while reduce the IE which would results to decrease the injection barrier for electron and hole transport. Furthermore, acene cores elongation expressively decrease the hole and electron reorganization energies as compared to frequently used materials pentacene and tris(8-hydroxyquinolinato)aluminum (mer-Alq3) which revealed that newly designed materials would be proficient to be used in p- and/or n-type semiconductor devices.

A New Path to 9,10-Dihydroacridines and Nitrogen-Doped Polyacenes

A New Path to 9,10-Dihydroacridines and Nitrogen-Doped Polyacenes

A first-principles study on transport properties of polyacene in zigzag graphene nanoribbon: Configuration, length and doping effects

Electronic transport properties of molecular junctions constructed by bridging a polyacene (PA) molecule between two zigzag graphene nanoribbons (ZGNR) are studied based on density functional theory and the nonequilibrium Green function method. It is found that the molecule-electrode coupling strength is related to the PA position with respect to the nanoribbon edge, which gives rise to the configuration dependency of transport properties. Negative differential resistance (NDR) is predicted in the junctions of which the PA molecule aligns with the inner part of the ZGNR. The on-set bias and current peak decrease as the PA molecule moves inward. The origin of NDR is presented by analyzing the transmission spectra, relative voltage-drop rate, and electron density difference of the junctions. The on-set bias is proportional to the energy of the resonance peak of the lowest unoccupied molecular orbital and can be tuned by the PA molecule length or by doping. This work provides a detailed discussion on PA-bridged ZGNR junctions, which may help to understand ZGNR-based molecular junctions and design negative differential resistance devices.

Exploring a Route to Cyclic Acenes by On‐Surface Synthesis

AbstractA route to generate cyclacenes by on‐surface synthesis is explored. We started by synthesizing two tetraepoxycyclacenes by sequences of Diels–Alder cycloadditions. Subsequently, these molecules were deposited onto Cu(111) and scanning‐tunneling‐microscopy(STM)‐based atom manipulation was employed to dissociate the oxygen atoms. Atomic force microscopy (AFM) with CO‐functionalized tips enabled the detailed characterization of the reaction products and revealed that, at most, two oxygens per molecule could be removed. Importantly, our experimental results suggest that the generation of cyclacenes by the described route might be possible for larger epoxycyclacenes.

The Longest Acene Diimide To Date

The Longest Acene Diimide To Date

To [n]Acenes and Beyond!

To [n]Acenes and Beyond!

Evaluation of Aromaticity for Open-Shell Singlet Dicyclopenta-Fused Acenes and Polyacenes Based on a Magnetically Induced Current.

The aromaticity of dicyclopenta-fused acenes (DPAs) and polyacenes (PAs) of increasing size has been studied by evaluation with the GIMIC method at the DFT level of the magnetically-induced currents (MICs), and by analyzing their spatial distributions. For these open-shell singlet molecules, spin-restricted and -unrestricted treatments provide very different MICs, the latter ones providing the most reliable solution. These MICs and the differences between spin-restricted and -unrestricted treatments are interpreted in terms of the bond current strengths and the current gradients, which indicate the bond aromaticity and enable the spatial distributions of the diatropic and paratropic currents to be analyzed, respectively. In particular, they allow the rationalization of the MICs in correlation with the odd-electron density distributions and their diradical characters. These calculations demonstrate that 1) in increasingly large PAs the bond current strengths get smaller and smaller than in benzene and get almost similar in the central and terminal rings, 2) for DPAs the MICs increase from dominant paratropic currents and antiaromaticity in the small compounds to diatropic currents and aromaticity in the larger ones, and 3) in the largest DPAs, the central rings are characterized by large diatropic currents and the terminal five-membered rings, for which the odd-electron densities are localized by weak ones.

The Consequences of Adding a Twist to Acenes

Key words acenes - polycyclic aromatic hydrocarbons - circular dichroism - fluorescence

A New Twist on Acene Synthesis

A New Twist on Acene Synthesis

First-principles study on atomistic and electronic structures of boron–nitrogen and boron–phosphorous nanoribbons, nanorings, and nanotubes

We computationally study the atomistic and electronic structures of boron–phosphorous (BP) nanoribbons (NRBs), nanorings (NRGs), and nanotubes (NTBs) by comparison with similar boron-nitride (BN) nanostructures. We consider polyacene (PAC)-type NRBs. First-principles calculations demonstrate that BN PAC-type finite NRBs conserve the flatness of the NRB plane whereas BP NRBs result in bendings along the NRB plane. Rolling of an NRB (head to tail) produces an NRG. Accordingly, a specific NRG with a “magic ring number” is produced in the BP system whereas BN-NRGs freely produced various ring numbers as well as cyclacene systems. Stacking of NRGs further produces an NTB whose electronic characteristics are determined by the chiral index. However, the band-edge states of heteroatom NTBs are controlled by the difference in the on-site energies rather than by the chiral index.

Combinations of 1,2- and 1,4-Additions to an Acene Dione

Key words polycyclic aromatic hydrocarbons - anthanthrenes - Michael addition - organic semiconductors

Conductance Superposition Rule in Carbon Nanowire Junctions with Parallel Paths

Using first-principles calculations, we investigate conductance of molecular junctions consisted of single and double polyacene (PA) molecules bridging different carbon nanowire electrodes, including armchair carbon nanotubes (CNT) and zigzag graphene nanoribbons (GNR). Doubling the PA molecule enhances the junction conductance, except in the junction where a molecule contacts armchair CNT with nonvertical edges. Elongating the PA molecules change junction conductance. The different conductance scaling behaviors among various junctions are governed by the interface between the molecule and the electrode, the molecular length, and the edge states of zigzag GNR.

Stabilized Z-Shaped Acenes

Stabilized Z-Shaped Acenes

Theoretical study on the diradical characters and third-order nonlinear optical properties of cyclic thiazyl diradical compounds

Using the spin-unrestricted density functional theory method, the relationship between the diradical characters (y) and the longitudinal second hyperpolarizabilities (γ) has been investigated for several cyclic thiazyl compounds (TZs) and polyacenes (PAs) involving up to six fused rings (N=6). It turns out that y strongly depends on the substitution position of the nitrogen and sulfur atoms in the five-membered rings of TZs and that its increase with N depends on the substitution positions. The γ values of TZs show an enhancement for intermediate y values and are significantly larger than those of PAs of similar size.

Size dependences of the diradical character and the second hyperpolarizabilities in dicyclopenta-fused acenes: relationships with their aromaticity/antiaromaticity

Using long-range corrected density functional theory, the relationships between the electronic, magnetic, and nonlinear optical properties are drawn for two families of organic compounds, the dicyclopenta-fused acenes (DPAs) and the polyacenes (PAs), containing up to N = 12 fused rings. First, the longitudinal second hyperpolarizability (γ) of singlet DPAs is significantly enhanced with increasing system size, in comparison to PAs. This behavior is associated with an increase in the longitudinal spin polarization between the terminal five-membered rings of DPAs and is consistent with previous studies where γ is maximized for intermediate diradical character. The size dependence of the diradical character is also found to cause a hump in the γ/N evolution for singlet DPAs around N = 8. In fact, in the case of singlet PAs, the diradical characters y(0) and y(1), the various magnetic properties and the γ/N values vary monotonically with N, whereas for singlet DPAs, the shielding, the magnetizability, and the γ/N values exhibit extrema near N = 8 due to the appearance of transversal spin polarization in the middle six-membered rings in addition to the longitudinal spin polarization between the terminal five-membered rings. Moreover, it is shown that for singlet DPAs the longitudinal spin polarization (characterized by y(0)) is associated with the antiaromaticity (N ≤ 3) and the slight- or non-aromaticity (N ≥ 4) of the terminal five-membered rings, whereas the appearance of transversal spin-polarization (characterized by y(1)) is associated with the decrease in the aromaticity in the inner six-membered rings as shown for large PAs. Therefore, the exceptional behaviors in singlet DPAs for small N (N < 9) are caused by the increase in diradical character y(0) correlated with the anti-aromaticity or the slight-/non-aromaticity of terminal rings and the corresponding emergence of a global aromatic character. Such a relationship between the aromaticity/antiaromaticity and the diradical character is useful for designing real open-shell NLO molecules through the control of their diradical characters.

The low-lying excited states of neutral polyacenes and their radical cations: a quantum chemical study employing the algebraic diagrammatic construction scheme of second order

The vertical excited electronic states of linearly fused neutral polyacenes (PACs) and their radical cations have been investigated using the algebraic diagrammatic construction scheme of second order (ADC(2)). While strict ADC(2) (ADC(2)-s) correctly reproduces trends for mainly singly excited states, in extended ADC(2) (ADC(2)-x) the description of doubly excited states is critically improved. It is shown that a combined application of strict and extended ADC(2) nicely reveals the importance of doubly excited configurations in the description of the excited state spectrum of the neutral PACs. In contrast to general expectation, our calculations of the lowest excited states of the radical cations of the PACs employing unrestricted ADC(2)-s and ADC(2)-x identified one B1 g electronic transition whose excitation energy increases with increasing molecular size. Thorough analysis of this electronic state and the involved molecular orbitals relates this unusual phenomenon to an increasing energy gap between the doubly occupied molecular orbital, out of which an electron is excited, and the singly occupied orbital, into which it is excited.

ChemInform Abstract: Electronic Structure of Higher Acenes and Polyacene — The Perspective Developed by Theoretical Analyses

AbstractReview: 57 refs.

Electronic structure of higher acenes and polyacene: The perspective developed by theoretical analyses

The hypothetical polymer obtained by linear annelation of benzene units, polyacene (PAC) (C4H2)n, has received considerable attention over the last 50 years. This interest is due to the unusual electronic structure that is assumed to result in usual physical properties. The review summarizes the theoretical investigations of PAC research. The most recent computational analyses available in the literature are based on density functional theory (DFT) for PAC and on the complete active space self-consistent field (CASSCF) method for oligoacenes and suggest an undistorted symmetrical structure with an antiferromagnetic (AFM) coupling of electrons.

HOMO and LUMO Trends in Conjugated Acenes

HOMO and LUMO Trends in Conjugated Acenes