Local aromaticity of the lowest-lying singlet States of [N]acenes (N = 6-9)

The local aromaticities of the six-membered rings in the two lowest-lying singlet states of [n]acenes (n = 6-9) have been assessed by means of three probes of local aromaticity based on structural, magnetic, and electron delocalization properties. Important differences between the local aromaticities of the closed-shell and diradical singlet electronic states are found. Thus, while the inner rings have the largest aromatic character in the closed-shell singlet states, the outer rings become the most aromatic for the diradical singlet states.     

A Unified Mechanism on the Formation of Acenes,Helicenes, and Phenacenes in the Gas Phase

A unified low-temperature reaction mechanism on the formation of acenes, phenacenes, and helicenes—polycyclic aromatic hydrocarbons (PAHs) that are distinct via the linear, zigzag, and ortho-condensed arrangements of fused benzene rings—is revealed. This mechanism is mediated through a barrierless, vinylacetylene mediated gas-phase chemistry utilizing tetracene, [4]phenacene, and [4]helicene as benchmarks contesting established ideas that molecular mass growth processes to PAHs transpire at elevated temperatures. This mechanism opens up an isomer-selective route to aromatic structures involving submerged reaction barriers, resonantly stabilized free-radical intermediates, and systematic ring annulation potentially yielding molecular wires along with racemic mixtures of helicenes in deep space. Connecting helicene templates to the Origins of Life ultimately changes our hypothesis on interstellar carbon chemistry.     

A Unified Mechanism on the Formation of Acenes,Helicenes, and Phenacenes in the Gas Phase

A unified low‐temperature reaction mechanism on the formation of acenes, phenacenes, and helicenes—polycyclic aromatic hydrocarbons (PAHs) that are distinct via the linear, zigzag, and ortho‐condensed arrangements of fused benzene rings—is revealed. This mechanism is mediated through a barrierless, vinylacetylene mediated gas‐phase chemistry utilizing tetracene, [4]phenacene, and [4]helicene as benchmarks contesting established ideas that molecular mass growth processes to PAHs transpire at elevated temperatures. This mechanism opens up an isomer‐selective route to aromatic structures involving submerged reaction barriers, resonantly stabilized free‐radical intermediates, and systematic ring annulation potentially yielding molecular wires along with racemic mixtures of helicenes in deep space. Connecting helicene templates to the Origins of Life ultimately changes our hypothesis on interstellar carbon chemistry.     

Theoretical and experimental studies on circular dichroism of carbo[n]helicenes

The chiroptical properties of a series of carbo[n]helicenes (n = 4-10) were investigated by the state-of-the-art approximate coupled cluster and density functional theory calculations. The theoretical calculation at the RI-CC2/TZVPP//DFT-D2-B97-D/TZVP level nicely reproduced the experimental CD spectra in both excitation energy and rotational strength without any shift or scaling. These calculations afforded the electric and the magnetic transition dipole moment vectors in [n]helicenes, allowing us to discuss the observed rotational strengths as a function of the number of benzene rings. Although the observed CD intensity was not immediately correlated to any of the calculated parameters, the anisotropy (g) factor of the (1)B(b) band and the specific rotation were found inversely proportional to n and nicely correlated with the helical pitch, but discontinuous at n = 6, where the aromatic rings start to overlap. In contrast, the g factor at the (1)B(a) band was rather insensitive to n. It was also revealed that the excitation energies of the (1)B(b) and (1)B(a) bands are inversely proportional to n over the entire range of n examined. The theoretical predictions also enabled us to rectify the erroneous experimental CD spectra of [5]- and [6]helicenes reported earlier, by using the enantiopure samples resolved by chiral HPLC.     

An insight into the local aromaticities of polycyclic aromatic hydrocarbons and fullerenes

In this work we quantify the local aromaticity of six-membered rings in a series of planar and bowl-shaped polycyclic aromatic hydrocarbons (PAHs) and fullerenes. The evaluation of local aromaticity has been carried out through the use of structurally (HOMA) and magnetically (NICS) based measures, as well as by the use of a new electronically based indicator of aromaticity, the para delocalization index (PDI), which is defined as the average of all the Bader delocalization indices between para-related carbon atoms in six-membered rings. The series of PAHs selected includes C(10)H(8), C(12)H(8), C(14)H(8), C(20)H(10), C(26)H(12), and C(30)H(12), with benzene and C(60) taken as references. The change in the local aromaticity of the six-membered rings on going from benzene to C(60) is analyzed. Finally, we also compare the aromaticity of C(60) with that of C(70), open [5,6]- and closed [6,6]-C(60)NH systems, and C(60)F(18).     

Symmetric,Unsymmetrical, and Asymmetric [7]‐, [10]‐, and [13]Helicenes

Fully aromatic helicenes with more than one pitch‐length are illustrious synthetic targets with potential applications in advanced optical devices and nano‐electronics. The task of extending the length of fully conjugated helicenes past one pitch length is challenging. Now, the synthesis of a series of azaoxa[7]‐, [10]‐, and [13]helicenes is described. The synthesis is based on iterative oxidative furan formation between 3,6‐dihydroxycarbazoles and/or 2‐naphthols. The flexibility of the presented method allows the convenient and scalable synthesis of symmetric, unsymmetrical, and asymmetric homo‐chiral structures. The [13]helicenes can be synthetically functionalized both at the termini and the periphery. The full range of helicenes were characterized using NMR and optical spectroscopy (UV/Vis, fluorescence, and CD) along with single‐crystal X‐ray crystallography. The enantiomers of the [13]helicenes are the longest optically pure helicenes isolated to date.     

Symmetric,Unsymmetrical, and Asymmetric [7]‐, [10]‐, and [13]Helicenes

Fully aromatic helicenes with more than one pitch‐length are illustrious synthetic targets with potential applications in advanced optical devices and nano‐electronics. The task of extending the length of fully conjugated helicenes past one pitch length is challenging. Now, the synthesis of a series of azaoxa[7]‐, [10]‐, and [13]helicenes is described. The synthesis is based on iterative oxidative furan formation between 3,6‐dihydroxycarbazoles and/or 2‐naphthols. The flexibility of the presented method allows the convenient and scalable synthesis of symmetric, unsymmetrical, and asymmetric homo‐chiral structures. The [13]helicenes can be synthetically functionalized both at the termini and the periphery. The full range of helicenes were characterized using NMR and optical spectroscopy (UV/Vis, fluorescence, and CD) along with single‐crystal X‐ray crystallography. The enantiomers of the [13]helicenes are the longest optically pure helicenes isolated to date.     

Syntheses and properties of enantiomerically pure higher (n > or = 7) [n-2]triangulanedimethanols and sigma-[n]helicenes

(P)-(+)-Hexaspiro[2.0.0.0. 0.0.2.1.1.1.1.1]pentadecane [(P)-17] as well as (M)-(-)- and (P)-(+)-octaspiro[2.0.0.0.0.0.0.0.2.1.1.1.1.1.1.1]nonadecanes [(M)- and (P)-25]-enantiomerically pure unbranched [7]- and [9]triangulanes-have been prepared starting from racemic THP-protected (methylenecyclopropyl)methanol 6. The relative configurations of all important intermediates as well as the absolute configurations of the key intermediates were established by X-ray crystal structure analyses. This new convergent approach to enantiomerically pure linear [n]triangulanes for n=7, 9 was also tested in two variants towards [15]triangulane. Some of the most prominent and unexpected features of the newly prepared compounds are the remarkable modes of self-assembly of the diols (P)-14, (E)-(3S,3'S,4S,4'S,5R,5'R)-21, (P)-(+)-22, and (E)-31 in the solid state through frameworks of intermolecular hydrogen bonds leading to, depending on the respective structure, nanotube- [(P)-14, (P)-(+)-22, and (E)-31], honeycomb-like structures [(E)-(3S,3'S,4S,4'S,5R,5'R)-21] or a supramolecular double helix [(P)-(+)- and (M)-(-)-22]. Liquid crystalline properties of the esters and ethers of the diols (P)-14, (P)-, and (M)-22 have also been tested. Although all of these [n]triangulanes have no chromophore which would lead to significant absorptions above 200 nm, they exhibit surprisingly high specific rotations even at 589 nm with [alpha](20)(D)=+672.9 (c=0.814 in CHCl(3)) for (P)-(+)-17, +909.9 (c=0.96 in CHCl(3)) for (P)-(+)-25, -890.5 (c=1.01 in CHCl(3)) for (M)-(-)-25, and -1302.5 (c=0.36 in CHCl(3)) for (M)-(-)-39, and the specific rotations increase drastically on going to shorter wavelengths. This outstanding rotatory power is in line with their rather rigid helical arrangement of sigma bonds, and accordingly these helically shaped unbranched [n]triangulanes may be termed "sigma-[n]helicenes", as they represent the sigma-bond analogues of the aromatic pi-[n]helicenes. Density functional theory (DFT) computations at the B3 LYP/6-31+G(d,p) level of theory for the geometry optimization and time-dependent DFT for determining optical rotations with a triplet-zeta basis set (B3 LYP/TZVP) reproduce the optical rotatory dispersions (ORD) very well for the lower members (n=4, 5) of the sigma-[n]helicenes. For the higher ones (n=7, 9, 15) the computed specific rotations turn out increasingly larger than the experimental values. The remarkable increase of the specific rotation with an increasing number of three-membered rings is proportional neither to the molecular weight nor to the number of cyclopropane rings in these sigma-[n]helicenes.     

Structure, reactivity and aromaticity of acenes and their BN analogues: a density functional and electrostatic investigation

Density functional calculations have been carried out on a series of linearly annelated acenes and their BN analogues. Even though borazine shows aromatic and reactivity behavior parallel with that of benzene, its condensed derivatives show patterns different from those of their hydrocarbon analogues. Nucleus independent chemical shift (NICS) values in acenes suggest that the aromaticity of the inner rings is more than that of benzene, whereas in BN-acenes there is no substantial change in the aromaticity of the individual rings. Molecular electrostatic potential (MESP) is employed to obtain further insights into the bonding and reactivity trends for these systems. The MESP topography patterns of acenes and BN-acenes are substantially different, with BN-acenes showing more localized pi electron features compared to those of acenes. The MESP values at the critical points (CPs) indicate overall lowering of aromaticity in these annelated systems. However, this change is gradual among the BN-acenes.     

The delocalization index as an electronic aromaticity criterion: application to a series of planar polycyclic aromatic hydrocarbons

This work introduces a new local aromaticity measure, defined as the mean of Bader's electron delocalization index (DI) of para-related carbon atoms in six-membered rings. This new electronic criterion of aromaticity is based on the fact that aromaticity is related to the cyclic delocalized distribution of pi-electrons. We have found that this DI and the harmonic oscillator model of aromaticity (HOMA) index are strongly correlated for a series of six-membered rings in eleven planar polycyclic aromatic hydrocarbons. The correlation between the DI and the nucleus-independent chemical shift (NICS) values is less remarkable, although in general six-membered rings with larger DI values also have more negative NICS indices. We have shown that this index can also be applied, with some modifications, to study of the aromaticity in five-membered rings.     

Achieving Adaptive Aromaticity in Cyclo[10]carbon by Screening Cyclo[n]carbon (n=8−24)

Discovery of species with adaptive aromaticity (being aromatic in both the lowest singlet and triplet states) is particularly challenging as cyclic species are generally aromatic either in the ground state or in the excited state only, according to Hückel's and Baird's rules. Inspired by the recent realization of cyclo[18]carbon, here we demonstrate that cyclo[10]carbon possesses adaptive aromaticity by screening cyclo[n]carbon (n=8?24), which is supported by nucleus‐independent chemical shift (NICS), anisotropy of the current‐induced density (ACID), π contribution of electron localization function (ELF_π) and electron density of delocalized bonds (EDDB) analyses. Further study reveals that the lowest triplet state of cyclo[10]carbon is formed by in‐plane ππ* excitation. Thus, the major contribution to the aromaticity from out‐of‐plane π molecular orbitals does not change significantly in the lowest singlet state. Our findings highlight a crucial role of out‐of‐plane π orbitals in maintaining aromaticity for both the lowest singlet and triplet states as well as the aromaticity dependence on the number of the carbon in cyclo[n]carbon.     

Synthesis of [5]-, [6]-, and [7]helicene via Ni(0)- or Co(I)-catalyzed isomerization of aromatic cis,cis-dienetriynes

An original approach to helicene frameworks exploiting atom economic isomerization of appropriate energy-rich aromatic cis,cis-dienetriynes has been developed. The new paradigm provides nonphotochemical syntheses of helicenes based on the easy, convergent, and modular assembly of key cis,cis-dienetriynes and their nickel(0)-catalyzed [2+2+2] cycloisomerization. The potential of the methodology is underlined by the syntheses of the parent [5]helicene (2), 7,8-dibutyl[5]helicene (23), [6]helicene (24), and [7]helicene (25). The approach can be adapted to prepare functionalized helicenes as exemplified by the eight-step synthesis of 7,8-dibutyl-2,3-dimethoxy[6]helicene (34). Density functional theory (DFT) calculations showed that bis[2-((1Z)-1-buten-3-ynyl)phenyl]acetylene (1) and isomeric [5]helicene (2) differ enormously in the Gibbs energy content (DeltaG = -136.6 kcal x mol(-1)) to favor highly the devised intramolecular simultaneous construction of three aromatic rings.     

QTAIM N-center delocalization indices as descriptors of aromaticity in mono and poly heterocycles

The implementation of the n-center electron delocalization indices, n-DIs, and n-order electron localization indices, n-LIs, within the framework of the quantum theory of atoms in molecules, QTAIM, is performed. n-DIs are shown to be very useful to study the local aromaticity in monocyclic and polycyclic compounds. Total and pi n-DIs from n=4 to 7 were computed for a series of typical 4, 5, 6, and 7-center aromatic and antiaromatic rings. For n>or=5 the pi n-DI accounts for the 95% of the total n-DI and can be employed alone to measure the aromaticity. A scaling factor on the n-DIs is required in order to compare the aromaticity of [5c-6e] and [6c-6e] rings, the same correction allows to estimate the relative aromatic stabilization of polycyclic compounds using the sum of its values for individual rings. This is called Effective Scaled Electron Delocalization, ESED. The comparison with other aromaticity indices reflects a good correlation between ESED and both resonance energies, and HOMA indices. The most important differences between scaled pi n-DIs and NICS(0) indices are found for compounds that contain rings with different number of centers or pi electrons.     

A naphthalene with unusual bond alternation made by annelation with bicyclo[2.1.1]hexene units: aromaticity and reactivity

An entirely new naphthalene having one six-membered ring, whose bond lengths are equalized, and another ring, whose bond lengths are alternated, has been prepared via one-electron oxidation of a benzene annelated with three bicyclo[2.1.1]hexene units. The structure has been determined by X-ray crystallography and the aromaticity of two benzene rings estimated by theoretical calculations. Although the former six-membered ring has a higher degree of aromaticity, it has been found to be more reactive toward singlet oxygen owing to the greater strain contained in it. These results have been interpreted by means of theoretical calculations.     

Compressing a Non-Planar Aromatic Heterocyclic [7]Helicene to a Planar Hetero[8]Circulene

This work describes a synthetic approach where a non-planar aromatic heterocyclic [7]helicene is compressed to yield a hetero[8]circulene containing an inner antiaromatic cyclooctatetraene (COT) core. This [8]circulene consists of four benzene rings and four heterocyclic rings, and it is the first heterocyclic [8]circulene containing three different heteroatoms. The synthetic pathway proceeds via a the flattened dehydro-hetero[7]helicene, which is partially a helicene and partially a circulene: it is non-planar and helically chiral as helicenes, and contains a COT motif like [8]circulenes. The antiaromaticity of the COT core is confirmed by nucleus independent chemical shift (NICS) calculations. The planarization from a helically π-conjugated [7]helicene to a fully planar heterocyclic [8]circulene significantly alters the spectroscopic properties of the molecules. Post-functionalization of the [7]helicenes and the [8]circulenes by oxygenation of the thiophene rings to the corresponding thiophene-sulfones allows an almost complete fluorescence emission coverage of the visible region of the optical spectrum (400–700 nm).     

Structure and vibrational spectra of some 8-oxa[5]helicenes

A systematic study has been conducted on the geometrical and electronic structure, heliomeric conformations of a series of 8-oxa[5]helicenes based on density functional theory (DFT) computations. A complete vibrational analysis has also been attempted for one of the 8-oxa[5]helicenes (molecule 5) on the basis of experimental infrared spectra in the far and mid infrared regions (60-3100 cm(-1)) and density functional theory computations using B3LYP/6-31G** method characteristic bands of the molecule identified. The approximate mean plane angle between the terminal rings A and E in the presently studied molecules are found to have values between 48.64° and 59.46°. This angle is much larger than the corresponding angle between the terminal rings in benzo[c]phenanthrene (～27°) and partially reduced benzo[c]phenanthrene (34.6-46.0°) and indicates that the presence of oxygen-containing six-membered ring provides a greater helicity to the molecules. Detailed quantum chemical study on molecule 4 shows the existence of two enantiomeric forms M- and P- of almost equal energy separated by a potential barrier of 15.55 kcal/mol. It is expected that similar 8-oxa[5]helicines (molecules 3, 5 and 6) may also exist in two enantiomeric forms.     

Applying the conjugated circuits method to Clar structures of [n]phenylenes for determining resonance energies

We consider the aromaticity of biphenylene and structurally related linear or angular [n]phenylenes for which the direct application of the model of conjugated circuits does not offer valid expressions for resonance energy and aromaticity. We located the cause of this problem as being due to Kekulé valence structures in which neighboring benzenoid rings are connected by two CC double bonds. By restricting the selection of Kekulé valence structures to those that contribute to Clar structures of such systems, we were able to show that linear and angular [n]phenylenes have approximately similar resonance energies, with angular [n]phenylenes being slightly more stable due to second order contributions arising from disjoint conjugated circuits. Expressions for resonance energies of [n]phenylenes up to n = 8 are listed and recursion expressions for higher n values are outlined.