Principal singlet fission properties of twisted acenes.

Acenes, especially tetracene derivatives, are used as singlet fission materials. The recent synthesis of Dodecaphenyl tetracene (showing end-to-end twist angles of 96-98 degrees) and twisted anthracene derivatives show that the synthesis of twisted linear oligoacenes is possible. Energy calculations and NICS-X-scan studies predict the twisted acenes may be better singlet fission materials compared to their planar analogues.

Nucleus Independent Chemical Shift (NICS) at Small Distances from the Molecular Plane: The Effect of Electron Density.

When close to the molecular plane, the behavior of nucleus independent chemical shift (NICS) as a function of the distance from the molecular plane deviates from its behavior at larger distances. By using a dense grid of NICS-probes (BQs) it is shown that, when close to the molecular plane, maximal (absolute) NICS values are obtained above the atoms. These maxima move towards the center as the grid is elevated until the (absolute) maximum NICS is obtained at the center and stay there when the grid is further elevated. It is shown that this behavior is a result of the current density, which is influenced by the electron density, according to the Biot-Savart law, which, in turn, causes the induced magnetic field measured by the NICS. It is thus concluded that if magnetic aromaticity is studied, the NICS calculations should be carried out at a large enough distance so that only the π-ring current affects the NICS. At distances ≥2 Å, NICS(r)π,zz =A+B*Cr . Using non-linear correlation for obtaining A, B and C and extrapolate to NICS(1)π,zz and NICS(1.7)π,zz is recommended as measures for aromaticity.

Singlet Fission and Aromaticity.

Qualitative aromaticity-based arguments are often used to explain singlet fission (SF) properties in polycyclic conjugated systems. It has been shown by Fowler and collaborators that magnetically induced ring currents are associated with transitions between occupied and unoccupied molecular orbitals (MOs). Since SF has to do with relative energies of electronic states, it was hypothesized that induced currents may indicate SF properties. The quantitative aromaticity of linear oligoacene and several doubly boron-doped anthracenes and phenanthrenes, in their closed-shell singlet, open-shell singlet (where applicable), and triplet electronic states, has been studied using nucleus-independent chemical shift (NICS)-XY scan methods. It is shown that quantitative magnetic aromaticity can be used to identify SF compounds, at least for initial screening. Thus, the induced current features that are indicative of singlet fission (SF) ability have been found to be global current and local current at each ring of the systems. This conclusion was verified for very different systems─a tetracyclic, nitrogen-containing quinone (6), diphenyl benzofurane (7), and cibalackrot (8), all reported to be SF systems. Finally, it is predicted that some isomers of the doubly boron-doped linear[4]phenylene should show SF properties.

Dimorpholinoacetylene and Its Use for the Synthesis of Tetraaminocyclobutadiene Species.

The new diaminoacetylene (DAA) dimorpholinoacetylene (3) was prepared from 1,1-dimorpholinoethene (1) by bromination to form the dibromoketene aminal 2, which upon lithiation afforded 3 through a Fritsch-Buttenberg-Wiechell rearrangement. Heating 3 at elevated temperatures resulted in a complete conversion into the dimer 1,1,2,4-tetramorpholino-1-buten-3-yne (4), which was used for the synthesis of four-membered cyclic bent allene (CBA) transition-metal complexes of the type [(CBA)MLn ] (5-7; MLn =AuCl, RhCl(COD), RhCl(CO)2 ; CBA=1,3,4,4-tetramorpholino-1,2-cyclobutadiene; COD=1,5-cyclooctadiene). The reaction of 3 with tetraethylammonium bromide gave 1,2,3,4-tetramorpholinocyclobutenylium bromide (8), which reacted with bromine to form 1,2,3,4-tetra(morpholino)cyclobutenediylium bis(tribromide) (9). Compound 9 represents the first fully characterized compound containing a tetraaminocyclobutadiene dication and displays a nearly planar C4 N4 core as shown by X-ray diffraction analysis. Detailed quantum chemical calculations were performed to assess the aromaticity of tetraaminocyclubutadiene dications by employing the Nucleus Independent Chemical Shift (NICS) method and current density analysis.

The Aromatic Character of Diindeno[2,1-b:2',1'-h]biphenylene.

The tropicity of the title system was studied using NICS(1.7)π,zz-XY-scans and current density analysis, showing a global diatropic loop and local dia- and paratropic loops. This aromaticity picture is very different from the one proposed based on NICS(1) and HOMA ( Org. Lett. 2021, 23, 8794-8798). It is predicted that 1 should be suitable for singlet fission applications. It is concluded that local aromatic indices should not be used for the full analysis of multiring conjugated systems.

Interplay of Aromaticity and Antiaromaticity in N-Doped Nanographenes.

The aromaticity of three nonplanar, fully conjugated aza-nanographenes built around a pyrrolo[3,2-b]pyrrole core is assessed through the application of two different computational procedures-GIMIC and NICS. We examine the calculated magnetically induced current densities (GIMIC) and nucleus-independent chemical shifts (NICS). The structural differences between these three apparently similar molecules lead to significantly different aromatic properties. GIMIC analysis indicates that the peripheral diatropic ring current of 3.9 nA/T for the studied bowl-shaped diaza-nanographene is the strongest, followed by the double [6]helicene which lacks seven-membered rings, and is practically nonexistent for the double [5]helicene possessing seven-membered rings. The biggest difference however is that in the two not-fully-fused molecules, the central pyrrole rings possess a significant diatropic current of about 4.1 nA/T, whereas there is no such current in the diaza-nanographene. Moreover, the antiaromaticity of the seven-membered rings is increasing while moving from double [5]helicene to diaza-nanographene (from -2.4 to -6.0 nA/T). The induced currents derived from NICSπ,zz-XY-scan analysis for all of the studied systems are in qualitative agreement with the GIMIC results. Subtle differences may originate from σ-electron currents in GIMIC or inaccuracy of NICSπ,zz values due to the nonplanarity of the systems, but the general picture is similar.

Flat corannulene: when a transition state becomes a stable molecule.

Flat corannulene has been considered so far only as a transition state of the bowl-to-bowl inversion process. This study was driven by the prediction that substituents with strong steric repulsion could destabilize the bowl-shaped conformation of this molecule to such an extent that the highly unstable planar geometry would become an isolable molecule. To examine the substituents' effect on the corannulene bowl depth, optimized structures for the highly-congested decakis(t-butylsulfido)corannulene were calculated. The computations, performed with both the M06-2X/def2-TZVP and the B3LYP/def2-TZVP methods (the latter with and without Grimme's D3 dispersion correction), predict that this molecule can achieve two minimum structures: a flat carbon framework and a bowl-shaped structure, which are very close in energy. This rather unusual compound was easily synthesized from decachlorocorannulene under mild reaction conditions, and X-ray crystallographic studies gave similar results to the theoretical predictions. This compound crystallized in two different polymorphs, one exhibiting a completely flat corannulene core and the other having a bowl-shaped conformation.

NICS-XY-Scan Predictions of Local, Semi-Global, and Global Ring Currents in Annulated Pentalene and s-Indacene Cores Compared to First-Principles Current Density Maps.

The magnetic response of a set of 20 molecules, their dications and their dianions has been studied by the NICS-XY method, and the results have been compared with first principles current density maps. The molecules have been built from pentalene and s-indacene by single and double annulation of cyclobutadiene, benzene and benzocyclobutadiene in an alternate fashion on both sides of the molecules. The prediction of tropicities obtained by the NICS-XY method are overall consistent with the current density maps. A literal code, developed to give a compact description of the tropicities of currents flowing around rings and bonds cut by the scan trajectory shows that, in most cases, the NICS-XY method leads to an exact match with the current density analyses. Mismatches are generally due to small circulations out of the scan trajectory, and they do not correspond to misinterpretations of the overall tropicities. The dataset provides several cases where the prediction of the overall antiaromatic/aromatic response by the 4n/4n+2 count of π electrons fails.

Reexamination of NICSπ,zz: Height Dependence, Off-Center Values, and Integration.

Applying fine grids of nucleus independent chemical shift (NICS) probes (BQs) at different distances from the molecular plane of aromatic and antiaromatic molecules suggests that, at short distances, NICSπ,zz is maximal (absolute values) off the geometrical center of the systems. In nonsymmetric systems, the center of the induced ring current may be a little off the geometrical center, but the difference is negligible at distances ≥2 Å. At these distances, the change of NICSπ,zz with the distance from the ring follows a power relation. It is shown that the order of diatropicity and paratropicity within a group of molecules depends on the distance from the system (namely, the order of NICS( r)π,zz depends on r). It is thus suggested to use ∫NICSπ,zz as a quantitative measure of aromaticity.

The predictive power of aromaticity: quantitative correlation between aromaticity and ionization potentials and HOMO-LUMO gaps in oligomers of benzene, pyrrole, furan, and thiophene.

Aromaticity is a central and ubiquitous concept in organic chemistry, and is used extensively to explain various phenomena. Yet, aromaticity cannot be observed or measured as a property in its own right and, to date, only qualitative and semi-quantitative relationships have been described between aromaticity and an observable property. We now demonstrate for the first time a robust quantitative relationship between the HOMO-LUMO gap and adiabatic ionization potential of a polycyclic aromatic hydrocarbon oligomer - both measurable physical quantities - and its aromaticity, as quantified by the Nucleus Independent Chemical Shift (NICS) index. The agreement found for a range of structurally and electronically diverse oligomeric systems of varying lengths is so well-behaved as to enable accurate prediction of the properties of longer members of the respective oligomer family. The established correlation allows for preliminary screening of compounds geared towards functional use.

The Planar Cyclooctatetraene Bridge in Bis-Metallic Macrocycles: Isolating or Conjugating?

A minor modification of the reported procedure for the synthesis of a corrole dimer that is fused by the cyclooctatetraene (COT) unit, (H3tpfc)2COT, allowed for its isolation in 18% yield. Of the two redox isomers that this interesting macrocycle does form, the current focus is on the reduced form, in which each subunit resembles that of monomeric corroles with a trianionic N4 coordination core. The corresponding bis-gallium(III) complex was prepared as an entry into the potentially rich coordination chemistry of (H3tpfc)2COT. Both X-ray crystallography and DFT calculations disclosed that the COT moiety is essentially planar with very unusual nonalternating C-C bonds. The same holds true for the bis-gallium(III) complexes [(Ga-tpfc)2]COT(py)2 and [(Ga-tpfc)2]COT(py)4, obtained with one and two pyridine molecules coordinated to each metal ion, respectively. The electronic spectra of both the free base and the gallium(III) complexes display an extremely low energy band (λmax at 720-724 nm), which points toward extensive π delocalization through the COT bridge. This aspect was fully addressed by examining the interactions between the two corrole subunits in terms of electrochemistry and DFT calculations of the oxidized and reduced macrocycle. The new near-IR bands that appear upon both oxidation (λmax 1250 nm) and reduction (λmax 1780 nm) serve as additional supporting evidence for this conclusion.

Magnetic criteria of aromaticity.

This review describes the current state of magnetic criteria of aromaticity. The introduction contains the fundamentals of ring currents in aromatic and antiaromatic systems, followed by a brief description of experimental and computational tools: NMR, diamagnetic susceptibility exaltation, current density analyses (CDA) and nucleus independent chemical shifts (NICS). This is followed by more comprehensive chapters: NMR - focusing on the work of R. Mitchell - NICS and CDA - describing the progress and development of the methods to their current state and presenting some examples of representative work.

The Impact of Antiaromatic Subunits in [4n+2] π-Systems: Bispentalenes with [4n+2] π-Electron Perimeters and Antiaromatic Character.

Three series of stable, neutral, π-extended bispentalene derivatives, with two pentalenes fused to a central benzene or naphthalene moiety, have been prepared through a modified double carbopalladation cascade reaction. While these chromophores feature skeletons with [4n+2] π-electron perimeters, the two 8 π-electron pentalene subunits strongly influence bonding and spectral properties. (1)H NMR spectra showed large upfield shifts of the protons in the pentalene moieties, comparable to antiaromatic monobenzopentalenes. Further investigations on magnetic ring currents through NICS-XY-scans suggest a global paratropic current and a local diatropic current at the central benzene ring in two of the series, while the third series, with a central naphthalene ring, showed more localized ring currents, with stronger paratropic ring currents on the pentalene moieties. X-ray diffraction analyses revealed planar bispentalene cores with large double- and single-bond alternation in the pentalene units, characteristic for antiaromaticity, and small alternation in the central aromatic rings. In agreement with TD-DFT calculations, both optical and electrochemical data showed much smaller HOMO-LUMO energy gaps compared to other neutral, acene-like hydrocarbons with the same number of fused rings. Both experimental and computational results suggest that the molecular properties of the presented bispentalenes are dominated by the antiaromatic pentalene-subunits despite the [4n+2] π-electron perimeter of the skeletons.

Tetraazaacenes containing four-membered rings in different oxidation states. Are they aromatic? A computational study.

A symmetrical tetraazaacene incorporating a central cyclobutadiene ring was calculated in different oxidation (hydrogenation) states, displaying different tautomers and conformers. Geometries, thermodynamics, and electronic properties were computed, and the aromaticity of all these species was calculated on a per ring basis by NICS-scans and NICS-X-scans. The results unveil unexpected and fascinating insights into the complex aromaticity of those compounds, including a formally aromatic (!) cyclobutadiene ring.

The NICS-XY-scan: identification of local and global ring currents in multi-ring systems.

Nucleus-independent chemical shift (NICS)-based methods are very popular for the determination of the induced magnetic field under an external magnetic field. These methods are used mostly (but not only) for the determination of the aromaticity and antiaromaticity of molecules and ions, both qualitatively and quantitatively. The ghost atom that serves as the NICS probe senses the induced magnetic field and reports it in the form of an NMR chemical shift. However, the source of the field cannot be determined by NICS. Thus, in a multi-ring system that may contain more than one induced current circuit (and therefore more than one source of the induced magnetic field) the NICS value may represent the sum of many induced magnetic fields. This may lead to wrong assignments of the aromaticity (and antiaromaticity) of the systems under study. In this paper, we present a NICS-based method for the determination of local and global ring currents in conjugated multi-ring systems. The method involves placing the NICS probes along the X axis, and if needed, along the Y axis, at a constant height above the system under study. Following the change in the induced field along these axes allows the identification of global and local induced currents. The best NICS type to use for these scans is NICSπZZ , but it is shown that at a height of 1.7 Šabove the molecular plane, NICSZZ provides the same qualitative picture. This method, namely the NICS-XY-scan, gives information equivalent to that obtained through current density analysis methods, and in some cases, provides even more details.

Aromatic stabilization energy and magnetic properties in fulvalenes: is there a connection between these two aromaticity indices?

The nucleus-independent chemical shift (NICS) and Coulombic energy of 15 j,k-fulvalenes (j, k = 3, 5, 7, 9, 11) were investigated. It was found that in all of the hetero derivatives (j ≠ k) charge transfer between the rings occurs. It occurs when the one ring contains 4n + 1 and the other 4n + 3 π electrons, forming two rings containing numbers of π electrons closer to 4n + 2, and also when both rings contain either 4n + 3 or 4n + 1 π electrons, forming a partially aromatized ring and a partially antiaromatized ring. Both types of charge transfer are associated with aromatic stabilization energy. The NICS values are consistent with the above-described partial aromatization and antiaromatization. A semiquantitative relationship between the aromatic stabilization energy and NICS is given.

Concurrence between current density, nucleus-independent chemical shifts, and aromatic stabilization energy: the case of isomeric [4]- and [5]phenylenes.

The 17 isomers of the [4]- and [5]phenylenes have been studied with three different computational levels of current-density analysis (CDA) and by calculation of the out-of-plane contribution to nucleus-independent chemical shifts (NICS(πzz)). Current-density maps for these isomeric phenylenes are typically dominated by strong paratropic ring currents in four-membered rings. The relative energies of the isomers, which differ only through the effects of differential strain and aromaticity, were computed at the B3LYP/6-311G* computational level. It was found that the three levels of CDA correlate well among themselves and with NICS(πzz). The latter correlation is improved when the ring sum ΣNICS(πzz) for each isomer is correlated to the ring-current sum ΣJ extracted from CDA. The strain-corrected relative energies of the isomers correlate linearly with ΣNICS(πzz). In particular, the compatibility of different summed quantities with easily computed Hückel-London ring currents suggests a simply calculated measure for dealing with global aromaticity of polycyclic systems.

Evidence for fully conjugated double-stranded cycles.

A chain of circumstantial evidence for the existence of the first fully conjugated, double-stranded cycles is presented. The products have the structure of the belt-region of fullerene C(84)(D(2)) and carry either four hexyl chains or four phenyl groups. The unsubstituted parent cycle is also presented. The chain of evidence is mainly based on mass spectrometric analysis and trapping reactions, the latter being supported by quantum mechanical calculations. It is also of importance that the phenyl-substituted and unsubstituted products cannot undergo a [1,5] hydrogen shift, the only reasonable side-reaction that recently could not be excluded for the alkyl-substituted analogue. It is concluded that the fully aromatic targets truly exist in the gas phase. Whether they can be generated in solution under the applied conditions cannot yet be firmly decided; theoretical evidence speaks against.