Theoretical description of photoinduced electron transfer in donor-acceptor supramolecular complexes based on carbon buckybowls

Herein, we explore, from a theoretical perspective, the nonradiative photoinduced processes (charge separation and energy transfer) within a family of donor-acceptor supramolecular complexes based on the electron-donor truxene-tetrathiafulvalene (truxTTF) derivative and a series of curved fullerene fragments (buckybowls) of different shapes and sizes (C30H12, C32H12, and C38H14) as electron acceptors that successfully combine with truxTTF via non-covalent interactions. The resulting supramolecular complexes (truxTTF·C30H12, truxTTF·C32H12, and truxTTF·C38H14) undergo charge-separation processes upon photoexcitation through charge-transfer states involving the donor and acceptor units. Despite the not so different size of the buckybowls, they present noticeable differences in the charge-separation efficiency owing to a complex decay post-photoexcitation mechanism involving several low-lying excited states of different natures (local and charge-transfer excitations), all closely spaced in energy. In this intricate scenario, we have adopted a theoretical approach combining electronic structure calculations at (time-dependent) density functional theory, a multistate multifragment diabatization method, the Marcus-Levitch-Jortner semiclassical rate expression, and a kinetic model to estimate the charge separation rate constants of the supramolecular heterodimers. Our outcomes highlight that the efficiency of the photoinduced charge-separation process increases with the extension of the buckybowl backbone. The supramolecular heterodimer with the largest buckybowl (truxTTF·C38H14) displays multiple and efficient electron-transfer pathways, providing a global photoinduced charge separation in the ultrafast time scale in line with the experimental findings. The study reported indicates that modifications in the shape and size of buckybowl systems can give rise to attractive novel acceptors for potential photovoltaic applications.

Effect of Fusion Manner of Concave Molecules on the Properties of Resulting Nanoboats

A boat-shaped compound, which can be viewed as the fusion of two concave molecules with crossed quinacridone and indolocarbazole throughout, was synthesized and characterized. The investigation determined that the fusion manner of two concave molecules has little influence on the molecular curvature and aromaticity when compared with its congener containing crossed indolocarbazole throughout. The situation of carbonyl groups is critical in adjusting the electronic structure and physicochemical properties due to the fixed position of nitrogen atoms.

A helically twisted ribbon-shaped nanographene constructed around a fenestrindane core

Ten C–C bonds and four cycloheptatriene rings were formed in one single operation, producing a helically twisted ribbon-shaped nanographene in 11% yield.

On-Surface Hydrogenation of Buckybowls: From Curved Aromatic Molecules to Planar Non-Kekulé Aromatic Hydrocarbons

Functionalization of surfaces with derivatives of Buckminsterfullerene fragment molecules seems to be a promising approach toward bottom-up fabrication of carbon nanotube modified electrode surfaces. The modification of a Cu(100) surface with molecules of the buckybowl pentaindenocorannulene has been studied by means of scanning tunneling microscopy, carbon monoxide-modified noncontact atomic force microscopy, time-of-flight secondary mass spectrometry, and quantum chemical calculations. Two different adsorbate modes are identified, in which the majority is oriented such that the bowl cavity points away from the surface and the convex side is partially immersed into a four-atom vacancy in the Cu(100) surface. A minority is oriented such that the convex side points away from the surface with the five benzo tabs oriented basically parallel to the surface. Thermal annealing leads to hydrogenation and planarization of the molecules in two steps under specific C-C bond cleavage. The benzo tabs of the convex side up species serve as a hydrogen source. The final product has an open-shell electron structure that is quenched on the surface.

Dicyclopenta[ghi,pqr]perylene as a Structural Motif for Bowl-Shaped Hydrocarbons: Synthetic and Conformational Studies

Dicyclopenta[ghi,pqr]perylene (DCPP) is a substructural fragment on the surface of C₇₀ yet not on C₆₀. Unlike its intensely investigated buckybowl cousins, corannulene and sumanene, DCPP is largely ignored due to the lack of synthetic accessibility. This communication describes the first preparation of a DCPP derivative from bay substituted perylene bis(4-(trifluoromethyl)phenyl)methanol as the key cyclization precursor. Further incorporation of indeno substitutions at peri positions was accomplished through Suzuki-Heck benzannulation. DCPP derivatives 4 adopts a planar structure in crystal. On the contrary, indeno DCPP 5 and bis-indeno DCPP 6 adopt the bowl-shaped conformation in both the solid state and solution. Density functional theory (DFT) calculation reveals that the lowest-energy conformations of 4, 5, and 6 are all bowl-shaped. Nevertheless, small bowl-to-bowl inversion barrier for 4 (3.4 kcal/mol) is overcome by the crystal packing force, which leads to its observed planar structure. However, the bowl-shaped structures of 5 and 6 are affirmed by DFT calculation with intermediate and high bowl-to-bowl inversion barriers (10.4 and 18.5 kcal/mol, respectively).

A Polyaromatic Nano-nest for Hosting Fullerenes C60 and C70

A "Janus" type of hexa- cata-hexabenzocoronene with three triptyceno subunits fused symmetrically on the periphery of coronene has been synthesized using a covalent self-assembly strategy. The triptyceno subunits form a nanosized nest on one side of the aromatic plane with space-matching fullerenes such as C₆₀ and C₇₀ to afford shape-complementary supramolecular complexes. The formation of the complexes in solution was confirmed by ¹H NMR and fluorescence titration. Four complexes with C₆₀ or C₇₀ were obtained and studied by single-crystal X-ray diffraction analysis. In the crystal structure, the host shows a proper tunability to adjust its conformation in accordance with the shape of the guest. The different stoichiometric ratios and various stacking patterns of the complexes suggest the diversity of this nonplanar polyaromatic host in complexation with fullerenes.

Bending Carbon Nanoforms for Supramolecular Recognition: A Topological Study on Hemifullerene-Based Aggregates

Buckybowls have risen as appealing fullerene fragment derivatives. Their intrinsic curvature has been exploited in the generation of host-guest supramolecular assemblies, not only through concave-convex complementarity but also through less-known concave-concave staggered arrangements. Whereas the stabilization of bowl-in-bowl dispositions has been ascribed to efficient π-π forces together with favorable dipole-dipole interactions, a detailed analysis on the forces guiding the formation of the staggered arrangements is missing so far. Herein, we present a thorough theoretical characterization of bowl-in-bowl vs staggered hemifullerene-based homodimers and heterodimers with the electron-donor truxTTF molecule, as test cases, under the density functional theory and by means of chemical bonding techniques. Our results clearly reveal strong and localized noncovalent signatures, together with an enhanced orbital interaction, associated with CH-π and sulfur-mediated interactions governing the staggered formation. Bending the fullerene fragment is demonstrated to favor the stabilization in both homo- and heterodimers, in good accord with the depletion in the π-electron density calculated upon increasing the buckybowl curvature. The optimal buckybowl curvature for the highest interaction energy is, however, dependent on the type of supramolecular assembly (bowl-in-bowl vs staggered) and the concave region to which hemifullerene approaches truxTTF. Interestingly, two regimes are found as a function of buckybowl curvature for hemifullerene homodimers: bowl-in-bowl dispositions are calculated more stable at low curvatures whereas staggered dimers prevail for highly curved buckybowls. Our results highlight the potential of discrete CH-π and sulfur-mediated interactions to generate unconventional staggered supramolecular arrangements toward the development of a new and unexplored host-guest chemistry.

Contorted polycyclic aromatic hydrocarbons with cove regions and zig-zag edges

A series of tetrapyrene-fused benzocoronenes was synthesized by a "bottom-up" approach, which offers a facile access to extended polycyclic aromatic hydrocarbons with concave π-surfaces, cove regions and zig-zag edges.

Synthesis of Dibenzo[h,rst]pentaphenes and Dibenzo[fg,qr]pentacenes by the Chemoselective C-O Arylation of Dimethoxyanthraquinones

A convenient method for the syntheses of dibenzo[h,rst]pentaphenes and dibenzo[fg,qr]pentacenes via the ruthenium-catalyzed chemoselective C-O arylation of 1,4- and 1,5-dimethoxyanthraquinones is described. Dimethoxyanthraquinones reacted selectively with arylboronates at the ortho C-O bonds to give diarylation products. An efficient two-step procedure consisting of a Corey-Chaykofsky reaction and subsequent dehydrative aromatization afforded derivatives of dibenzo[h,rst]pentaphenes and dibenzo[fg,qr]pentacenes. Hole-transporting characteristics were observed for a device with a bottom-contact configuration that was fabricated from one of these polycyclic aromatic hydrocarbons.

Conformational behavior and stacking interactions of contorted polycyclic aromatics

Computational studies of non-covalent dimers of saddle-shaped molecules unveil widely varying conformations and stacking configurations.

Fused π-Extended Truxenes via a Threefold Borylation as the Key Step

On the basis of a threefold borylated truxene, which is accessible in high yields by iridium-catalyzed borylation under CH-activation, fused π-extended truxenes have been synthesized by a two-step method of first Suzuki-Miyaura cross-coupling reaction and subsequent condensation reaction. The mild condensation method tolerates the presence of a variety of functional groups, such as nitro, fluoro, or carboxyl moieties. Furthermore, by using this approach, N- and S-heteroarene analogues become accessible for the first time, as well as larger structures that represent derivatives of precursors for fullerene C60 or buckybowls. The attached tert-butyl groups make all derivatives sufficiently soluble to allow full spectroscopic and electrochemical investigations. Postfunctionalization of selected derivatives for further synthetic applications of the compounds is also presented.

Controlling electronic effects and intermolecular packing in contorted polyaromatic hydrocarbons (c-PAHs): towards high mobility field effect transistors

Classical MD simulations followed by DFT calculations for computationally screened contorted polyaromatic hydrocarbons reveal encouraging OFET potential.

Fullerene recognition with molecular tweezers made up of efficient buckybowls: a dispersion-corrected DFT study

In 2007, Sygula and co-workers introduced a novel type of molecular tweezers with buckybowl pincers that have attracted the substantial interest of researchers due to their ideal architecture for recognizing fullerenes by concave-convex π∙∙∙π interactions (A. Sygula et al., J. Am. Chem. Soc., 2007, 129, 3842). Although in recent years some modifications have been performed on these original molecular tweezers to improve their ability for catching fullerenes, very few improvements were achieved to date. For that reason, in the present work a series of molecular tweezers have been devised and their supramolecular complexes with C60 studied at the B97-D2/TZVP//SCC-DFTB-D and B97-D2/TZVP levels. Three different strategies have been tested: (1) changing the corannulene pincers to other buckybowls, (2) replacing the tetrabenzocyclooctatetraene tether by a buckybowl, and (3) adding methyl groups on the molecular tweezers. According to the results, all the three approaches are effective, in such a way that a combination of the three strategies results in buckycatchers with complexation energies (with C60) up to 2.6 times larger than that of the original buckycatcher, reaching almost -100 kcal mol(-1). The B97-D2/TZVP//SCC-DFTB-D approach can be a rapid screening tool for testing new molecular tweezers. However, since this approach does not reproduce correctly the deformation energy and this energy represents an important contribution to the total complexation energy of complexes, subsequent higher-level re-optimization is compulsory to achieve reliable results (the full B97-D2/TZVP level is used herein). This re-optimization could be superfluous when quite rigid buckycatchers are studied.

π-extended [12]cycloparaphenylenes: from a hexaphenylbenzene cyclohexamer to its unexpected C2-symmetric congener

Based on a π-extended [12]CPP, two different precursors for the bottom-up synthesis of CNTs were synthesized. The congested hexaphenylbenzene mode of connectivity of the two macrocycles reveals an improved oxidative cyclodehydrogenation over previous reported strategies.

The synthesis of π-extended [12]cycloparaphenylene (CPP) derivatives from a kinked triangular macrocycle is presented. Depending on the reaction conditions for reductive aromatization, either a hexaphenylbenzene cyclohexamer or its C₂-symmetric congener was obtained. Their structures were confirmed by NMR spectroscopy or X-ray crystallographic analysis. With the support of DFT calculations, a mechanistic explanation for the unexpected formation of the oval shaped bis(cyclohexadiene)-bridged C₂-symmetric macrocycle is provided. The here employed congested hexaphenylbenzene mode of connectivity in conjunction with a non-strained precursor improves oxidative cyclodehydrogenation toward the formation of ultrashort carbon nanotubes (CNT)s. Thus, this strategy can pave the way for new conceptual approaches of a solution-based bottom-up synthesis of CNTs.

Contorted polycyclic aromatics

CONSPECTUS: This Account describes a body of research in the design, synthesis, and assembly of molecular materials made from strained polycyclic aromatic molecules. The strain in the molecular subunits severely distorts the aromatic molecules away from planarity. We coined the term "contorted aromatics" to describe this class of molecules. Using these molecules, we demonstrate that the curved pi-surfaces are useful as subunits to make self-assembled electronic materials. We have created and continue to study two broad classes of these "contorted aromatics": discs and ribbons. The figure that accompanies this conspectus displays the three-dimensional surfaces of a selection of these "contorted aromatics". The disc-shaped contorted molecules have well-defined conformations that create concave pi-surfaces. When these disc-shaped molecules are substituted with hydrocarbon side chains, they self-assemble into columnar superstructures. Depending on the hydrocarbon substitution, they form either liquid crystalline films or macroscopic cables. In both cases, the columnar structures are photoconductive and form p-type, hole- transporting materials in field effect transistor devices. This columnar motif is robust, allowing us to form monolayers of these columns attached to the surface of dielectrics such as silicon oxide. We use ultrathin point contacts made from individual single-walled carbon nanotubes that are separated by a few nanometers to probe the electronic properties of short stacks of a few contorted discs. We find that these materials have high mobility and can sense electron-deficient aromatic molecules. The concave surfaces of these disc-shaped contorted molecules form ideal receptors for the molecular recognition and assembly with spherical molecules such as fullerenes. These interfaces resemble ball-and-socket joints, where the fullerene nests itself in the concave surface of the contorted disc. The tightness of the binding between the two partners can be increased by creating more hemispherically shaped contorted molecules. Given the electronic structure of these contorted discs and the fullerenes, this junction is a molecular version of a p-n junction. These ball-and-socket interfaces are ideal for photoinduced charge separation. Photovoltaic devices containing these molecular recognition elements demonstrate approximately two orders of magnitude increase in charge separation. The ribbon-shaped, contorted molecules can be conceptualized as ultranarrow pieces of graphene. The contortion causes them to wind into helical ribbons. These ribbons can be formed into the active layer of field effect transistors. We substitute the ribbons with di-imides and therefore are able to transport electrons. Furthermore, these materials absorb light strongly and have ideal energetic alignment of their orbitals with conventional p-type electronic polymers. In solar cells, these contorted ribbons with commercial donor polymers have record efficiencies for non-fullerene-based solar cells. An area of interest for future exploration is the merger of these highly efficient contorted ribbons with the well-defined interfaces of the ball-and-socket materials.

Tailoring buckybowls for fullerene recognition. A dispersion-corrected DFT study

The shape of a buckybowl plays a fundamental role in the enhancement of fullerene recognition. Compounds whose structure possesses flaps at the rim of the bowl show an enhanced ability.

New advances in nanographene chemistry

This review discusses recent advancements in nanographene chemistry, focusing on the bottom-up synthesis of graphene molecules and graphene nanoribbons.

Metal-catalyzed annulation reactions for π-conjugated polycycles

The progress of the metal-catalyzed annulation reactions toward construction of various π-conjugated polycyclic cores with high conjugation extension is described. This article gives a brief overview of various annulation reactions promoted by metal catalysts including C-H bond functionalization, [2+2+2] cycloaddition, cascade processes, ring closing metathesis, electrophilic aromatization, and various cross-coupling reactions. A variety of conjugated polycycles with planar, bowl-shaped, and helical structures have been constructed in high efficiency and selectivity.

Facile bottom-up synthesis of coronene-based 3-fold symmetrical and highly substituted nanographenes from simple aromatics

A facile and efficient self-sorting assemble (CSA) strategy has been paved for bottom-up construction of the 3-fold symmetrical and highly substituted hexa-cata-hexabenzocoronenes (c-HBCs), the trithieno analogues, and larger disc-shaped PAHs from simple chemicals using benzylic carbons as tenon joints and a novel FeCl3-mediated AAA process as a key step. The structures of the as-prepared c-HBCs and related NGs were clearly identified by spectral analyses and X-ray crystallographic studies. Moreover, these can be envisaged to serve as new launching platforms for the construction of larger and more complex π-conjugated molecules and supramolecular architectures because of the modifiable and symmetrical decorations.

Synthesis and structural data of tetrabenzo[8]circulene

In 1976, the first attempted synthesis of the saddle-shaped molecule [8]circulene was reported. The next 37 years produced no advancement towards the construction of this complicated molecule. But remarkably, over the last six months, a flurry of progress has been made with two groups reporting independent and strikingly different strategies for the synthesis of [8]circulene derivatives. Herein, we present a third synthetic method, in which we target tetrabenzo[8]circulene. Our approach employs a Diels-Alder reaction and a palladium-catalyzed arylation reaction as the key steps. Despite calculations describing the instability of [8]circulene, coupled with the reported instability of synthesized derivatives of the parent molecule, the addition of four fused benzenoid rings around the periphery of the molecule provides a highly stable structure. This increased stability over the parent [8]circulene was predicted by using Clar's theory of aromatic sextets and is a result of the compound becoming fully benzenoid upon incorporation of these additional rings. The synthesized compound exhibits remarkable stability under ambient conditions-even at elevated temperatures-with no signs of decomposition over several months. The solid-state structure of this compound is significantly twisted compared to the calculated structure primarily as a result of crystal-packing forces in the solid state. Despite this contortion from the lowest-energy structure, a range of structural data is presented confirming the presence of localized aromaticity in this large polycyclic aromatic hydrocarbon.

A comparative study on the performance of subphthalocyanines and corannulene derivatives as receptors for fullerenes

One for the other! Subphthalocyanines functionalized at the peripheral region show a remarkable affinity towards C₆₀ and C₇₀.

Electron acceptors based on functionalizable cyclopenta[hi]aceanthrylenes and dicyclopenta[de,mn]tetracenes

We report the synthesis and selective functionalization of two externally fused cyclopenta-fused polycyclic aromatic hydrocarbons (CP-PAHs) and demonstrate their electron accepting behavior. 2,7-Bis(trimethylsilyl)cyclopenta[hi]aceanthrylene (1) and 2,8-bis(trimethylsilyl)dicyclopenta[de,mn]tetracene (4) were prepared in a one-pot, palladium-catalyzed cross-coupling of (trimethylsilyl)acetylene and either 9,10-dibromoanthracene or 5,11-dibromotetracene, respectively. The trimethylsilyl groups were selectively converted into bromides via substitution with N-bromosuccinimide to create universal partners (2 and 6) for metal-catalyzed cross-coupling reactions. To demonstrate the utility of the halogenated CP-PAHs, we successfully employed a Sonogashira cross-coupling between the CP-PAHs and a phenylacetylene derivative. The resulting compounds (3 and 7) were found to be highly conjugated between the CP-PAH core and the substituents, as demonstrated by large bathochromic shifts in the absorption spectra as well as density functional theory calculations. Ethynylated CP-PAHs 3 and 7 were found to possess low optical bandgaps (1.52 and 1.51 eV, respectively) and displayed two reversible reductions. We further demonstrated the fullerene-like electron-accepting behavior of 3 through solution-phase fluorescence quenching of the prototypical electron donor, poly(3-hexylthiophene).

Conjugated circuits currents in hexabenzocoronene and its derivatives formed by joining proximal carbons

We report on calculated CC bond currents for a dozen derivatives of hexabenzocoroenene in which one or more proximal carbon atoms at the molecular periphery have been bridged. The approach that we use is graph-theoretical in nature, following our outline of this method in 2003, which is based on finding all conjugated circuits in all Kekulé valence structures of these molecules. To the π-electrons having 4n + 2 π-electrons are assigned anticlockwise π-electron currents and to conjugated circuits having 4n π-electrons are assigned π-electron currents. One may summarize the results reported in this work by stating that CC bond currents in the compounds considered decrease on going from peripheral rings to the central ring of the molecule, and also that CC bond currents decrease by insertion of bridges to proximal peripheral benzenoid rings.