peri-Acenoacenes

Electronic Properties of Graphene Nano-Parallelograms: A Thermally Assisted Occupation DFT Computational Study

In this computational study, we investigate the electronic properties of zigzag graphene nano-parallelograms (GNPs), which are parallelogram-shaped graphene nanoribbons of various widths and lengths, using thermally assisted occupation density functional theory (TAO-DFT). Our calculations revealed a monotonic decrease in the singlet-triplet energy gap as the GNP length increased. The GNPs possessed singlet ground states for all the cases examined. With the increase of GNP length, the vertical ionization potential and fundamental gap decreased monotonically, while the vertical electron affinity increased monotonically. Some of the GNPs studied were found to possess fundamental gaps in the range of 1-3 eV, lying in the ideal region relevant to solar energy applications. Besides, as the GNP length increased, the symmetrized von Neumann entropy increased monotonically, denoting an increase in the degree of the multi-reference character associated with the ground state GNPs. The occupation numbers and real-space representation of active orbitals indicated that there was a transition from the nonradical nature of the shorter GNPs to the increasing polyradical nature of the longer GNPs. In addition, the edge/corner localization of the active orbitals was found for the wider and longer GNPs.

Dibenzannulated peri-acenoacenes from anthanthrene derivatives

A series of dibenzannulated phenyl-annulated [4,2]peri-acenoacenes have been synthesized in three straightforward steps from 4,10-dibromoanthanthrone (vat orange 3). The phenyl bisannulation of [4,2]peri-acenoacene provides extra stability by increasing the overall aromatic character of the molecules, and allows for a 45-80% increase of the molar extinction coefficient (ε) compared to their [5,2]peri-acenoacene isomers. Depending on the substituents attached to the π-conjugated core, some derivatives exhibit strong aggregation in the solid state with association constant (Ka) up to 255 M-1, resulting in a significant broadening of the absorption spectrum and a substantial decrease of the bandgap value (more than 0.3 V) from solution to the solid state. One [4,2]peri-acenoacene derivative was doubly reduced using cesium and the crystal structure of the resulting salt has been obtained. Field-effect transistors showing a temperature-dependent hole mobility have been tested.

Constructing 1-Ethoxyphenanthro[9,10-e]acephenanthrylene for the Synthesis of a Polyaromatic Hydrocarbon Containing a Formal Azulene Unit

peri-Acenoacenes are attractive synthetic targets, but their non-benzenoid isomeric counterparts were unnoticed. 1-Ethoxyphenanthro[9,10-e]acephenanthrylene 8 was synthesized and converted to azulene-embedded 9, which is a tribenzo-fused non-alternant isomeric motif of peri-anthracenoanthracene. Aromaticity and single-crystal analyses suggested a formal azulene core for 9, which showed a smaller highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gap with a charge-transfer absorption band and brighter fluorescence than 8 (quantum yield (Φ): 9 = 41.8%, 8 = 8.9%). The reduction potentials of 8 and 9 were nearly identical, and the observations were further supported by density functional theory (DFT) calculations.

Closed-shell and open-shell dual nature of singlet diradical compounds

Unlike triplet diradicals, singlet diradicals can vary in diradical character from 0 % to 100 % depending on linker units that allow two formally unpaired electrons to couple covalently. In principle, the electronic structure of singlet diradicals can be described as a quantum superposition of closed-shell and open-shell structures. This means that, depending on the external environment, singlet diradicals can behave as either closed-shell or open-shell species. This paper summarizes our progress in understanding the electronic structure of π-conjugated singlet diradical molecules in terms of closed-shell and open-shell dual nature. We first discuss the coexistence of intra- and intermolecular covalent bonding interactions in the π-dimer of a singlet diradical molecule. The intra- and intermolecular coupling of two formally unpaired electrons are related to closed-shell and open-shell nature of singlet diradical, respectively. Then we demonstrate the coexistence of the covalent bonding interactions in the one-dimensional stack of singlet diradical molecules having different diradical character. The relative strength of the interactions is varied with the magnitude of singlet diradical index y 0. Finally, we show the dual reactivity of a singlet diradical molecule, which undergoes rapid [4 + 2] and [4 + 4] cycloaddition reactions in the dark at room temperature. Closed-shell and open-shell nature endow the singlet diradical molecule with the reaction manner as diene and diradical species, respectively.

Zigzag-Edged Polycyclic Aromatic Hydrocarbons from Benzo[m]tetraphene Precursors

A series of zigzag-edged polycyclic aromatic hydrocarbons (PAHs) (Z1-Z3) were synthesized from 2,12-dibromo-7,14-diphenyl-benzo[m]tetraphene (9) as a versatile building block. Their structures were unambiguously confirmed by laser desorption/ionization time-of-flight mass spectrometry, ¹ H NMR, Raman, and Fourier-transformed infrared (FTIR) spectroscopies as well as scanning tunneling microscopy. The fingerprint vibrational modes were elucidated with theoretical support. The edge- and size-dependent optical properties were characterized by UV-Vis absorption and fluorescence spectroscopy and DFT calculations. Moreover, ultrafast transient absorption spectroscopy revealed distinct modulation of the photophysical properties upon π-extension from Z1 to Z2, the latter having a gulf edge.

Synthesis, Aromaticity, and Application of peri-Pentacenopentacene: Localized Representation of Benzenoid Aromatic Compounds

We report the synthesis and optoelectronic properties of TIPS-peri-pentacenopentacene (TIPS-PPP), a vertical extension of TIPS-pentacene (TIPS-PEN) and a low-band-gap material with remarkable stability. We found the synthetic conditions to avoid the competition between 1,2- and 1,4-addition of lithium acetylide on the large aromatic dione. The high stability of TIPS-PPP is due to the peri-fusion which increases the aromaticity by generating two localized aromatic sextets that are flanked with 2 diene fragments, similar to two fused-anthracenes. Like TIPS-PEN, TIPS-PPP shows the archetypal 2D brickwall motif in crystals with a larger transfer integral and smaller reorganization energy. The high mobility of up to 1 cm²  V^-1  s^-1 was obtained in an organic field-effect transistor fabricated by a wet process. Also, TIPS-PPP was used as a near-infrared (NIR) emitter for NIR organic-light-emitting-diode devices resulting in a high external quantum efficiency at 800 nm.

peri-Acenoacene molecules: tuning of the singlet and triplet excitation energies by modifying their radical character

The energy difference between singlet and triplet excitons, or ΔE_ST, is a key parameter for novel light-emission mechanisms (i.e., TADF or thermally activated delayed fluorescence) or other photoactivated processes. We have studied a set of conjugated molecules (peri-acenoacenes and their heteroatom-doped analogues) to observe the evolution of their excited-state properties upon increasing the system size with and without substitution with a pair of N atoms. Since these molecules exhibit a (ground-state) diradicaloid character, together with marked correlation effects influencing the excited-states formed, we have applied a variety of theoretical methods (FT-DFT, TD-DFT, SF-TD-DFT, CIS, CIS(D), SCS-CC2, SA-CASSCF, and SC-NEVPT2) to bracket the accuracy of the results while concomitantly providing insights into electronic structure. The results show how this chemical strategy (N-doping) largely modifies not only the excited-state energies but also the oscillator strengths and the ΔE_ST values, constituting versatile platforms for fine-tuned photophysical applications.

A Distorted Hybrid Corannulene-Dibenzobistetracene

We have designed and synthesized a new type of distorted nanographene by Diels-Alder and Scholl reactions that contains one dibenzobistetracene (DBT, 1) core and two end-capping corannulene units. Single-crystal X-ray diffraction analyses demonstrate that nanographene 1 contains two [5] helicene subunits with a dihedral angle of 62°, consequently leading to the distorted DBT core. In addition, the photophysical properties and (non)aromaticity of 1 were investigated by the absorption and emission spectra in combination with theoretical calculations.

NBN-Doped Bis-Tetracene and Peri-Tetracene: Synthesis and Characterization

Combining solution‐based and surface‐assisted synthesis, we demonstrate the first synthesis of NBN‐doped bis‐tetracene (NBN‐BT) and peri‐tetracene (NBN‐PT). The chemical structures are clearly elucidated by high‐resolution scanning tunneling microscopy (STM) in combination with noncontact atomic force microscopy (nc‐AFM). Scanning tunneling spectroscopy (STS) characterizations reveal that NBN‐BT and NBN‐PT possess higher energy gaps than bis‐tetracene and peri‐tetracene. Interestingly, NBN‐BT can undergo stepwise one‐electron oxidation and convert into its corresponding radical cation and then to its dication. The energy gap of the NBN‐BT dication is similar to that of bis‐tetracene, indicating their isoelectronic relationship. Moreover, a similar energy gap between the NBN‐PT dication and peri‐tetracene can be predicted by DFT calculations. This work provides a novel synthesis along with characterizations of multi‐NBN‐doped zigzag‐edged peri‐acenes with tunable electronic properties.

Synthesis and Absorption Properties of Long Acenoacenes

Acenes, polyaromatic hydrocarbons composed of linearly fused benzene rings have received immense attention due to their performance as semiconductors in organic optoelectronic applications. Their appealing physicochemical properties, such as extended delocalization, high charge carrier mobilities, narrow HOMO-LOMO gaps and partially radical character in the ground state make them very attractive targets for many potential applications. However, the intrinsic synthetic challenges of unsubstituted members such as high reactivity and poor solubility are still limiting factors for their wider exploitation. Herein, we report a simple general synthesis of a new family of angularly fused acenoacenes with improved stability compared to their isoelectronic linear counterparts. The synthesis and comprehensive characterization of pentacenopentacene, pentacenohexacene and hexacenohexacene, with lengths between decacene and dodecacene, are disclosed.

Persistent peri-Heptacene: Synthesis and In Situ Characterization

n‐peri‐Acenes (n‐PAs) have gained interest as model systems of zigzag‐edged graphene nanoribbons for potential applications in nanoelectronics and spintronics. However, the synthesis of n‐PAs larger than peri‐tetracene remains challenging because of their intrinsic open‐shell character and high reactivity. Presented here is the synthesis of a hitherto unknown n‐PA, that is, peri‐heptacene (7‐PA), in which the reactive zigzag edges are kinetically protected with eight 4‐tBu‐C₆H₄ groups. The formation of 7‐PA is validated by high‐resolution mass spectrometry and in situ FT‐Raman spectroscopy. 7‐PA displays a narrow optical energy gap of 1.01 eV and exhibits persistent stability (t_1/2≈25 min) under inert conditions. Moreover, electron‐spin resonance measurements and theoretical studies reveal that 7‐PA exhibits an open‐shell feature and a significant tetraradical character. This strategy could be considered a modular approach for the construction of next‐generation (3 N+1)‐PAs (where N≥3).

Rules of Nucleophilic Additions to Zigzag Nanographene Diones*

Nucleophilic addition of carbon-centered nucleophiles to nanographene ketones represents a valuable late-stage method for the functionalization of zigzag nanographenes, but its use is rare in the chemical literature. Using two model systems, non-Kekulé triangulene-4,8-dione and Kekulé anthanthrone, we identify unexpected regioselectivities and uncover the rules that govern these reactions. Considering the large number of nanographene ketones that have been reported since the pioneering work of Eric Clar, this method enables synthesis and exploration of hitherto unknown functionalized nanographenes.

Large magnetic exchange coupling in rhombus-shaped nanographenes with zigzag periphery

Nanographenes with zigzag edges are predicted to manifest non-trivial π-magnetism resulting from the interplay of concurrent electronic effects, such as hybridization of localized frontier states and Coulomb repulsion between valence electrons. This provides a chemically tunable platform to explore quantum magnetism at the nanoscale and opens avenues towards organic spintronics. The magnetic stability in nanographenes is thus far greatly limited by the weak magnetic exchange coupling, which remains below the room-temperature thermal energy. Here, we report the synthesis of large rhombus-shaped nanographenes with zigzag peripheries on gold and copper surfaces. Single-molecule scanning probe measurements show an emergent magnetic spin singlet ground state with increasing nanographene size. The magnetic exchange coupling in the largest nanographene (C₇₀H₂₂, containing five benzenoid rings along each edge), determined by inelastic electron tunnelling spectroscopy, exceeds 100 meV or 1,160 K, which outclasses most inorganic nanomaterials and survives on a metal electrode.

Hexa-peri-benzocoronene with two extra K-regions in an ortho-configuration

There are three possible isomers for hexa-peri-hexabenzocoronene (HBC) with two extra K-regions, but only two of them have been reported, namely with the meta- and para-configurations. Herein, we describe the synthesis of HBC 4 with two extra K-regions in the ortho-configuration, forming a longer zigzag edge compared with the other two isomers. The structure of 4 was validated by laser desorption/ionization time-of-flight mass analysis and nuclear magnetic resonance spectra, as well as Raman and infrared spectroscopies supported by density functional theory calculations. The optical properties of 4 were investigated by UV/vis absorption and ultrafast transient absorption spectroscopy. Together with the analysis of aromaticity, the influence of the zigzag edge on the π-conjugation pathway and HOMO–LUMO gaps of the three isomers were investigated.

We reported the synthesis of hexa-peri-benzocoronene (HBC) with two extra K-regions adopting an ortho-configuration. The systematical study provides deep insights about the effect of zigzag edge on the π-conjugated pathway and molecular design.

Synthetic Tailoring of Graphene Nanostructures with Zigzag-Edged Topologies: Progress and Perspectives

Experimental and theoretical investigations have revealed that the chemical and physical properties of graphene are crucially determined by their topological structures. Therefore, the atomically precise synthesis of graphene nanostructures is essential. A particular example is graphene nanostructures with zigzag-edged structures, which exhibit unique (opto)electronic and magnetic properties owing to their spin-polarized edge state. Recent progress in the development of synthetic methods and strategies as well as characterization methods has given access to this class of unprecedented graphene nanostructures, which used to be purely molecular objectives in theoretical chemistry. Thus, clear insight into the structure-property relationships has become possible as well as new applications in organic carbon-based electronic and spintronic devices. In this Minireview, we discuss the recent progress in the controlled synthesis of zigzag-edged graphene nanostructures with different topologies through a bottom-up synthetic strategy.

Modulating the ground state, stability and charge transport in OFETs of biradicaloid hexahydro-diindenopyrene derivatives and a proposed method to estimate the biradical character

Biradicaloid compounds with an open-shell ground state have been the subject of intense research in the past decade. Although diindenoacenes are one of the most developed families, only a few examples have been reported as active layers in organic field-effect transistors (OFETs) with a charge mobility of around 10-3 cm2 V-1 s-1 due to a steric disadvantage of the mesityl group to kinetically stabilize compounds. Herein, we disclose our efforts to improve the charge transport of the diindenoacene family based on hexahydro-diindenopyrene (HDIP) derivatives with different annelation modes for which the most reactive position has been functionalized with (triisopropylsilyl)ethynyl (TIPS) groups. All the HDIP derivatives show remarkably higher stability than that of TIPS-pentacene, enduring for 2 days to more than 30 days, which depends on the oxidation potential, the contribution of the singlet biradical form in the ground state and the annelation mode. The annelation mode affects not only the band gap and the biradical character (y 0) but also the value of the singlet-triplet energy gap (ΔE S-T) that does not follow the reverse trend of y 0. A method based on comparison between experimental and theoretical bond lengths has been disclosed to estimate y 0 and shows that y 0 computed at the projected unrestricted Hartree-Fock (PUHF) level is the most relevant among those reported by all other methods. Thanks to their high stability, thin-film OFETs were successfully fabricated. Well balanced ambipolar transport was obtained in the order of 10-3 cm2 V-1 s-1 in the bottom-gate/top-contact configuration, and unipolar transport in the top-gate/bottom-contact configuration was obtained in the order of 10-1 cm2 V-1 s-1 which is the highest value obtained for biradical compounds with a diindenoacene skeleton.

Cove-Edged Nanographenes with Localized Double Bonds

The efficient synthesis and electronic properties of two large-size cove-edged nanographenes (NGs), CN1 and CN2, are presented. X-ray crystallographic analysis reveals a contorted backbone for both molecules owing to the steric repulsion at the inner cove position. Noticeably, the dominant structures of these molecules contain four (for CN1) or six (for CN2) localized C=C double bonds embedded in nine (for CN1) or twelve (for CN2) aromatic sextet rings according to Clar's formula, which is supported by bond length analysis and theoretical (NICS, ACID) calculations. Furthermore, Raman spectra exhibit a band associated with the longitudinal CC stretching mode of olefinic double bonds. Owing to the existence of the additional olefinic bonds, both compounds show a small band gap (1.84 eV for CN1 and 1.37 eV for CN2). They also display moderate fluorescence quantum yield (35 % for CN1 and 50 % for CN2) owing to the contorted geometry, which can suppress aggregation in solution.