Unusually Short H⋅⋅⋅H Contacts in Intramolecularly Cyclized Helically Fused Anthracenes

The intramolecular coupling of dichloro-substituted helically fused anthracenes using the Yamamoto coupling yielded cyclized products with sterically congested molecular structures. The X-ray analysis and DFT calculations showed that the aromatic framework adopted a nonplanar structure with a twisted conformation about the newly formed single bond, which acts as a chiral axis. Interestingly, the X-ray structure obtained through the Hirshfeld atom refinement revealed short interatomic distances between the inner hydrogen atoms (1.648-1.692 Å), much shorter than the sum of their van der Waals radii. Owing to these unusually short contacts, the 1H NMR spectrum exhibited a significant deshielding (12.5 ppm) and a large nuclear Overhauser effect (44 %). Additionally, the IR spectrum displayed a high-frequency shift of the C-H stretching vibration. These observations, along with the noncovalent interaction plot indicative of a characteristic steric environment, strongly support the presence of steric hindrance. Moreover, dynamic NMR measurement of the mesityl-substituted derivative yielded a barrier to helical inversion of 84 kJ mol-1. The optical properties and crystal packing of the cyclized products are also reported.

A Double Twisted Nanographene with a Contorted Pyrene Core

The mature synthetic methodologies enable us to rationally design and produce chiral nanographenes (NGs), most of which consist of multiple helical motifs. However, inherent chirality originating from twisted geometry has just emerged to be employed in chiral NGs. Herein, we report a red-emissive chiral NG constituted of orthogonally arranged two-fold twisted π-skeletons at a contorted pyrene core which contributes to optical transitions of S0→S1 and vice versa. The thus-obtained NG exhibited a robustness on its redox properties through 2e- uptake/release. The chemical oxidation generated stable radical cation whose absorption covers near-infrared I and II regions. Overall, the contorted pyrene core governs electronic nature of the chiral NG. The twist operation on NGs would be, therefore, a design strategy to alter conventional chirality induction on NGs.

A Curious Case of Two-Photon Absorption in n-Helicene and n-Phenylene, n=6-10: Why n=7 is Different?

n-Helicenes and n-Phenylenes are interesting examples of twisted molecules, where although the atoms are connected through conjugated π ${\pi }$ -bonds, the π ${\pi }$ -conjugation is largely hindered by the twisted nature of the bonds. Such structures provide a unique opportunity to study the effect of twisted π ${\pi }$ -system on non-linear optical properties. In this work, we studied the two-photon absorption in donor-acceptor substituted n-helicenes and n-phenylenes employing the state-of-the-art RI-CC2 method and reported a unique feature we observed in n=7 systems. We found that both 7-helicene and 7-phenylene systems exhibit largest two-photon absorption than other members in their respective classes. Furthermore, using generalized few-state model, we provided a detailed microscopic mechanism of this unique observation involving participation of different transition dipole moment vectors and their relative orientations.

Cyclotrimerization Approach to Symmetric [9]Helical Indenofluorenes: Diverting Cyclization Pathways

Catalytic cyclotrimerization routes to symmetrical [9]helical indenofluorene were explored by using different transition-metal complexes and thermal conditions. Depending on the reaction conditions, the cyclotrimerizations were accompanied by dehydro-Diels-Alder reaction giving rise to another type of aromatic compounds. Structures of both symmetrical [9]helical cyclotrimerization product as well as the dehydro-Diels-Alder product were confirmed by single-crystal X-ray diffraction analyses. Limits of enantioselective cyclotrimerization were assessed as well. DFT calculations shed light on the reaction course and the origin of diminished enantioselectivity.

Infinitenes as the Most Stable Form of Cycloarenes: The Interplay among π Delocalization, Strain, and π-π Stacking

The recent successful preparation of infinitene has sparked widespread attention due to its aesthetic appeal and synthetic challenge. Spectroscopic measurements and follow-up computational investigations suggest that infinitene holds fundamental significance and potential applications in chiroptics, optoelectronics, asymmetric synthesis, and supramolecular chemistry. However, unlike other looped polyarenes enriched with sizes and shapes, the infinitene molecule seems, so far, the only known example of this fascinating new form of nanocarbons, whose further exploitation would be considerably limited because of the lack of molecular diversity. Here, we introduce a whole new family of generalized infinitenes with different sizes and topologies. Three types of infinitene structures are rationally designed by joining two units of coronene, kekulene, or their extended analogs. The constructed molecules of varying sizes, each with a large number of possible topoisomers, are systematically studied by DFT calculations. Comprehensive analysis using a simple energy decomposition model uncovers that the stability of infinitenes is governed by the interplay among π delocalization, steric strain, and π-π stacking. While the first two factors are crucial to the stability of smaller infinitenes, the latter is the primary stabilizing interaction for larger infinitenes. Most importantly, we show that larger-sized infinitenes are actually the energetically most favorable form among all known looped polyarenes; their substantial thermodynamic stability surpassing that of circulenes, various carbon nanobelts, and kekulene-like macrocycles renders them promising targets for synthesis. The simulated ¹H NMR, UV-vis, and circular dichroism spectra along with optical rotations for the most stable infinitene species may help their identification in future synthetic efforts.

Effect of Terminal Fluorination on Chiroptical Properties of Carbo[5-8]helicenes: A Systematic Computational Study at the RI-ADC(2) Level

In this article, effects of di-, tetra-, and octafluorination on the structural and chiroptical properties of carbo[5-8]helicenes are reported. Three fluorinated derivatives are designed from each parent carbohelicene by substituting either one, two, or four hydrogens at each terminal ring with fluorine atoms. Excited states properties such as UV-vis and CD spectra of all the six fluorinated carbohelicenes are computed at the ADC(2)/def2-TZVP level, and the results are compared against the results of their respective parent carbohelicene. In addition, CPL properties are also computed at the same level of theory. In the case of carbo[5]helicene (5H), g_CPL decreases with an increase in the degree of fluorination. A similar observation is made in carbo[6]helicene (6H) too, although the value for tetrafluorinated 6H is slightly larger than for difluorinated 6H. Di- and tetrafluorination in carbo[7]helicene (7H), and all types of considered fluorination in carbo[8]helicene (8H) produce improved g_CPL results. Results for fluorescence rate constants are also shown. Results are analyzed in terms of the transition dipole moment vectors and the angles between those.

Enhanced Optical Properties of Azaborole Helicenes by Lateral and Helical Extension

The synthesis and characterization of laterally extended azabora[5]-, -[6]- and -[7]helicenes, assembled from N-heteroaromatic and dibenzo[g,p]chrysene building blocks is described. Formally, the π-conjugated systems of the pristine azaborole helicenes were enlarged with a phenanthrene unit leading to compounds with large Stokes shifts, significantly enhanced luminescence quantum yields (Φ) and dissymmetry factors (g_lum ). The beneficial effect on optical properties was also observed for helical elongation. The combined contributions of lateral and helical extensions resulted in a compound showing green emission with Φ of 0.31 and |g_lum | of 2.2×10^-3 , highest within the series of π-extended azaborahelicenes and superior to emission intensity and chiroptical response of its non-extended congener. This study shows that helical and lateral extensions of π-conjugated systems are viable strategies to improve features of azaborole helicenes. In addition, single crystal X-ray analysis of configurationally stable [6]- and -[7]helicenes was used to provide insight into their packing arrangements.

Synthesis of Aza[n]helicenes (n = 4-7) via Photocyclodehydrochlorination of 1-Chloro-N-aryl-2-naphthamides

A series of aza[n]helicenes (n = 4-7) was synthesized using a photocyclodehydrochlorination of 1-chloro-N-aryl-2-naphthamides as a general synthetic procedure introducing the nitrogen atom to the third ring of the helicene framework. The effect of the nitrogen presence in the helicene skeleton on the physicochemical properties of the prepared aza[n]helicenes was studied and compared to those of the parent carbo-analogues. The insertion of a nitrogen atom into the outer edge of the helicene molecule has a severe impact on certain physicochemical properties such as optical rotation, electrostatic potentials, and intermolecular interactions. On the other hand, some other properties such as UV/vis, fluorescence, and phosphorescence spectra remained almost unaffected when compared to the parent carbohelicenes. A nitrogen atom can be also used for further derivatization, which can lead to further modification of helicene properties, as manifested here in the fluorescence changes induced by protonation.

Synthesis of Aza[n]phenacenes (n = 4-6) via Photocyclodehydrochlorination of 2-Chloro-N-aryl-1-naphthamides

A novel methodology for the synthesis of aza[n]phenacenes was successfully developed utilizing photocyclodehydrochlorination reaction of 2-chloro-N-aryl-1-naphthamides. In these key intermediates, the factors influencing the photoreaction were studied. The target aza[n]phenacenes were obtained by triflation or chlorination from prepared phenanthridinones, followed by hydrogenation. The introduction of a nitrogen atom into a phenacene skeleton induced changes in the physicochemical properties. The important properties of prepared aza[n]phenacenes (n = 4-6) were studied experimentally and by density functional theory calculations and were compared to those of their carbo analogues. Furthermore, some important features of the crystalline aza[n]phenacenes were investigated, including intermolecular interaction in the crystal lattice and the increased solubility or decreased melting points.

S-Shaped Double Helicene Diimides: Synthesis, Self-Assembly, and Mechanofluorochromism

Herein we present a synthesis of an S-shaped double helicene with fused imide moieties, achieving a contorted aromatic diimide (DHDI) with good fluorescence properties in both solution and the solid state. DHDI demonstrates distinct mechanofluorochromism from yellow to green emission under grinding of its crystalline powder.

Synthesis of Terminally Fluorinated [7]Helicenes and Their Application to Photochemical Domino Reactions

The intramolecular Diels-Alder reactions of helicenes deform their π-conjugated screw-shaped skeletons. In particular, terminally tetrafluorinated [7]helicene (F₄ -[7]helicene) undergoes a photoinduced Diels-Alder reaction followed by a photoinduced double fluorine atom transfer. Herein, we thoroughly investigated this photochemical domino process by decreasing the level of fluorine substitution. F₃ -[7]Helicenes bearing two fluorine atoms at the dienophile terminal underwent photoinduced Diels-Alder reactions, but the whole domino process became slow. F₂ -[7]Helicene, which is difluorinated only at the dienophile terminal, was also photolabile. As a result, two fluorine atoms were sufficient for the photochemical domino reaction to occur. X-ray crystallographic analysis revealed that F₂ -[7]helicene was less compressed than F₄ -[7]helicene, indicating that terminal polyfluorination enhanced the intramolecular arene-fluoroarene stacking interactions and thus promoted the transformations.

A General Synthetic Approach and Photophysical Properties of Regioselectively Fluorinated [5]- and [6]-Helical Bispiroindenofluorenes

A first series of fluorinated [n]helical compounds (n=5 and 6) with the dihydroindenofluorene scaffold was prepared in 5 or 9 (octafluorinated dihydroindenofluorene) steps and their photophysical properties were determined. Rh-catalyzed intramolecular [2+2+2] cyclotrimerization of triyndiols, which were prepared in a modular fashion from simple starting material such as fluorinated haloarylcarbaldehydes, to the intermediate [n]helical dihydroindeno[2,1-c]fluorene-5,8-diols was the crucial synthetic step and proceeded with high efficacy. Their further transformation gave the desired selectively fluorinated bispirodihydroindeno[2,1-c]fluorenes. Their absorption and emission spectra were recorded. The fluorescence quantum yields were up to 92 % and the emission maxima were red-shifted in comparison with their non-fluorinated counterparts (386-413 nm).

Columnar Stacking of Partially Fluorinated [4]Helicenes: C-H⋅⋅⋅F Interactions Change the Stacking Orientation

The partial fluorination of polycyclic aromatic hydrocarbons often produces a layered crystal packing, where fluorinated aromatic surfaces are stacked over nonfluorinated aromatic surfaces. Herein, we report the synthesis and crystal packing of partially fluorinated [4]helicenes with steric congestion resulting from H and F atoms in the fjord region. F₆ -[4]Helicene forms head-to-tail columnar stacks consisting of an alternate arrangement of perfluorinated and nonfluorinated naphthalene moieties. With decreasing fluorine content, aromatic stacking switched from arene-fluoroarene (Ar^H -Ar^F ) hetero-stacking to Ar^H -Ar^H /Ar^F -Ar^F homo-stacking with the help of intermolecular C-H⋅⋅⋅F contacts in the fjord region. As a result, head-to-head columnar stacks appear. Therefore, the conventional Ar^H -Ar^F stacking motif is not always applicable to F_n -[4]helicenes with twisted π-surfaces.

Orientation and conformation of two [6]carbohelicenes in stretched polystyrene and a thermoresponsive polyaspartate

Two functionalized [6]carbohelicenes, one of which was also available in its two enantiomeric pure forms, were oriented in stretched polystyrene in CDCl₃ , and in a recently introduced chiral thermoresponsive lyotropic polyaspartate (poly(benzyl)_0.5 (phenethyl)_0.5 -L-aspartate) in C₂ D₂ Cl₄ . From the resulting ¹ H,¹³ C residual dipolar couplings, the helical pitch of a methylated [6]carbohelicene was determined and found to be in agreement with theoretical predictions and existing crystal structures (d(C2,C2') ≈ 4.3 Å). For a second [6]carbohelicene with para-methoxyphenyl substituents, a clear conformational preference of the substituents was observed. The orientational properties of the two helicene enantiomers in the chiral polyaspartate are very similar, but both drastically change around 306 K. We suggest this behavior is due to an unusual phase transition in the liquid crystal.

Magnetically-induced current density investigation in carbohelicenes and azahelicenes

A computational study of the magnetically-induced current (MIC) density has been carried out for a variety of ortho fused polycyclic aromatic molecules at the density functional theory level with the gauge including magnetically induced current (GIMIC) method. With this method, the aromatic character of each ring in a homologous series of carbohelicenes with an increasing number of fused benzene rings is assessed and compared with other aromaticity criteria such as the Nucleus Independent Chemical Shift [NICS(0), NICSzz(0)] and Bond Length Alternation (BLA) parameters. All criteria indicate that the two outer rings are the most aromatic ones [i.e. higher induced current, more negative NICS(0) and NICSzz(0) values, and smaller BLA values]. For the large helicenes (n > 10), the current drops along the following four rings and then rises again. Additionally, we have proven that this behavior is not due to a difference of the local magnetic field coming from a difference of orientation of the ring with respect to the external magnetic field (oriented along the helical axis). Upon fusing additional benzene rings to form hexa-peri-hexabenzo[7]helicene, some rings (B, D, and F) are a lot less aromatic (even non-aromatic) than the others. The NICS(0) and NICSzz(0) values exaggerate this behavior because they are all positive values, which is a signature of antiaromaticity. Then, when substituting one, three, or four benzene rings with pyrrole ones to form mono-aza-[7]-helicene, tri-aza-[7]-helicene, and tetra-aza-[7]-helicene, remarkable changes in the electronic structures of the helicenes are observed. Indeed, the induced currents are always smaller in the pyrrole rings than in the benzene ones. This has been further investigated using the streamline and the color map representations, which indicate that the diatropic current density passing through the plane cutting the C-N bonds in the pyrrole rings is stronger but is more localized than the current density passing through the plane cutting the C-C bonds in the benzene rings. This gives a positive but smaller total induced current for the pyrrole rings than for the benzene ones. For these systems, the NICS(0) and even more the NICSzz(0) values are not fully reliable to probe the local aromaticity, contrary to the induced current. Indeed, the NICSzz(0) values for the tri-aza-[7]-helicene molecule range from -14.23 to 1.14 ppm, which cannot lead to the same conclusion as the induced current values (10.03 to 12.87 nA T-1).

Preparation and Physicochemical Properties of [6]Helicenes Fluorinated at Terminal Rings

The first racemization-stable helicene derivatives fluorinated at terminal rings, 1,2,3,4-tetrafluoro[6]helicene (6) and 1,2,3,4,13,14,15,16-octafluoro[6]helicene (15), were synthesized via the Wittig reaction followed by oxidative photocyclization in an overall yield of 41% of 6 and 76% of 15. The changed electronic structure in fluorinated helicenes was reflected in a slight shift of UV-vis absorption, fluorescence excitation, and emission spectra maxima when compared to unsubstituted [6]helicene. Cyclic voltammetry revealed a moderate decrease in the HOMO-LUMO gap with increasing fluorination. The specific rotation of tetrafluoro[6]helicene 6 enantiomers was found to be approximately 25% lower than that of unsubstituted [6]helicene. The theoretical study of the racemization barrier suggested a reasonable shift toward higher energy with increasing fluorination. The increasing fluorination also significantly affected the intermolecular interactions in the crystal lattice. The observed CH···F interactions led to the formation of 1D-molecular chains in the crystal structures of both fluorinated helicenes.

Dihydrogen contacts observed by through-space indirect NMR coupling

"Through-space" indirect spin-spin couplings between hydrogen atoms formally separated by up to 18 covalent bonds have been detected by NMR experiments in model helical molecules. It is demonstrated that this coupling can provide crucial structural information on the molecular conformation in solution. The coupling pathways have been visualised and analysed by computational methods. The conformational dependence of the coupling is explained in terms of orbital interactions.