Heterocyclic [9]Helicenes Exhibiting Bright Circularly Polarized Luminescence.

We describe a concise synthetic strategy for the preparation of heterocyclic [9]helicenes and a simple preparative-scale protocol for the optical resolution of the resulting M- and P-enantiomers. The helicenes were characterized by single-crystal X-ray diffraction along with a range of spectroscopic and computational techniques. A fluorescence quantum yield of up to 65 % was observed, and the chiroptical properties of both M- and P-helicenes revealed large dissymmetry factors. The circularly polarized luminescence brightness reaches up to 17 M-1 cm-1 , as measured experimentally and verified computationally, which makes this the highest circularly polarized luminescence brightness among heterocyclic helicenes. We describe how chiroptical properties (both circular dichroism and circularly polarized luminescence) can be described and predicted using quantum chemical calculations. The synthetic approach also reveals by-products that originate from internal oxidation reactions, presumably mediated by the close proximity of the π-surfaces in the helicene structure.

A Combined Experimental and Theoretical Study of ESR Hyperfine Coupling Constants for N,N,N',N'-Tetrasubstituted p-Phenylenediamine Radical Cations.

A test set of N,N,N',N'-tetrasubstituted p-phenylenediamines are experimentally explored using ESR (electron spin resonance) spectroscopy and analysed from a computational standpoint thereafter. This computational study aims to further aid structural characterisation by comparing experimental ESR hyperfine coupling constants (hfccs) with computed values calculated using ESR-optimised "J-style" basis sets (6-31G(d,p)-J, 6-31G(d,p)-J, 6-311++G(d,p)-J, pcJ-1, pcJ-2 and cc-pVTZ-J) and hybrid-DFT functionals (B3LYP, PBE0, TPSSh, ωB97XD) as well as MP2. PBE0/6-31g(d,p)-J with a polarised continuum solvation model (PCM) correlated best with the experiment, giving an R2 value of 0.8926. A total of 98% of couplings were deemed satisfactory, with five couplings observed as outlier results, thus degrading correlation values significantly. A higher-level electronic structure method, namely MP2, was sought to improve outlier couplings, but only a minority of couples showed improvement, whilst the remaining majority of couplings were negatively degraded.

Utilizing Selective Chlorination to Synthesize New Triangulenium Dyes.

Functionalization of new sites on the triangulenium structure has been achieved by early-stage chlorination with N-chlorosuccinimide (NCS), giving rise to two new triangulenium dyes (1 and 3). By introducing the chlorine functionalities in the acridinium precursor, positions complementary to those previously obtained by electrophilic aromatic substitution on the final dyes are accessed. The chlorination is selective, giving only one regioisomer for both mono- and dichlorination products. For the monochlorinated acridinium compound, a highly selective ring-closing reaction was discovered, generating a single regioisomer of the cationic [4]helicene product. Further investigations into the mechanism of the [4]helicene formation lead to the first isolation of the previously proposed intermediate of the two-step SNAr reaction, key to all aza-bridged triangulenium and helicenium systems. Late-stage functionalization of DAOTA+ with NCS gave rise to a different dichlorinated compound (2). The fully ring closed chlorinated triangulenium dyes 1, 2, and 3 show a redshift in absorption and emission, while maintaining relatively high fluorescence quantum yields of 36%, 26%, and 41% and long fluorescence lifetimes of 15, 12.5, and 16 ns, respectively. Cyclic voltammetry shows that chlorination of the triangulenium dyes significantly lowers reduction potentials and thus allows for efficient tuning of redox and photoredox properties.

Prediction of the standard potentials for one-electron oxidation of N,N,N',N' tetrasubstituted p-phenylenediamines by calculation.

The formal potentials for the reversible one-electron oxidation of N,N,N',N' tetrasubstituted p-phenylenediamines in acetonitrile have been applied as a test set for benchmarking computational methods for a series of compounds with only small structural differences. The aim of the study is to propose a simple method for calculating the standard oxidation potentials, and therefore, the protocol is progressively developed by adding more terms in the energy expression. In addition, the effect of including implicit solvation models (IEFPCM, CPCM, and SMD), larger basis sets, and correlation methods are investigated. The oxidation potentials calculated using the G3MP2B3 approach with IEFPCM resulted in the best fit (R2 = 0.9624), but the slope of the correlation line, 0.74, is far from the optimal value, 1.00. B3LYP/6-311++G(d,p) and TPSSh/6-311++G(2d,p) yielded only slightly less consistent data (R2 = 0.9388 and R2 = 0.9425), but with much better slopes, 1.00 and 0.94, respectively. We conclude that it is important to investigate the basis set size and treatment of electron correlation when calculating oxidation potentials for N,N,N',N' tetrasubstituted p-phenylenediamines.

The best density functional theory functional for the prediction of 1 H and 13 C chemical shifts of protonated alkylpyrroles.

The prediction of 13 C chemical shifts can be challenging with density functional theory (DFT). In this study 39 different functionals and three different basis sets were tested on three neutral alkylpyrroles and their corresponding protonated species. The calculated shielding constants were compared to experimental data and results from previous calculations at the MP2. We find that the meta-hybrid functional TPSSh with either the Pople style basis set 6-311++G(2d,p) or the polarization consistent basis set pcSseg-1 gives the best results for the 13 C chemical shifts, whereas for the 1 H chemical shifts it is the TPSSh functional with either the 6-311++G(2d,p) or pcSseg-2 basis set. Including an explicit solvent molecule hydrogen bonded to NH in the alkylpyrroles improves the results slightly for the 13 C chemical shifts. On the other hand, for 1 H chemical shifts the opposite is true. Compared to calculations at the MP2 level none of the DFT functionals can compete with MP2 for the 13 C chemical shifts but for the 1 H chemical shifts the investigated DFT functionals are shown to give better agreement with experiment than MP2 calculations.

A Fully Conjugated Planar Heterocyclic [9]Circulene.

Fully aromatic [n]circulenes have only been known to encompass up to eight aromatic rings (n = 8), with no reports of endeavors in the synthesis of higher-order analogues (n > 8). Herein we present the first [9]circulene, formally a diazatrioxa[9]circulene, along with a tetrahydro-diazatetraoxa[10]circulene. The key transformation, for construction of the macrocyclic framework, is a simple high-yielding dimerizing condensation between 3,6-dihydroxycarbazole and glyoxal. Single crystal X-ray analysis reveals the [9]circulene to be perfectly planar and containing elongated benzene rings, which is induced by strain to accommodate planarity. Alternating bond lengths in the solid state indicate contribution from a [9]radialene resonance structure in the [9]circulene π-system. The central nonaromatic rings of both circulenes have paratropic ring currents, as evident by nucleus independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations, which can be attributed to induced paratropicity from the surrounding aromatic rings.

Core carbo-mer of an Extended Tetrathiafulvalene: Redox-Controlled Reversible Conversion to a carbo-Benzenic Dication.

carbo-Benzene is an aromatic molecule devised by inserting C2 units within each C-C bond of the benzene molecule. By integrating the corresponding carbo-quinoid core as bridging unit in a π-extended tetrathiafulvalene (exTTF), it is shown that a carbo-benzene ring can be reversibly formed by electrochemical reduction or oxidation. The so-called carbo-exTTF molecule was thus experimentally prepared and studied by UV-visible absorption spectroscopy and cyclic voltammetry, as well as by X-ray crystallography and by scanning tunneling microscopy (STM) on a surface of highly oriented pyrolytic graphite (HOPG). The molecule and its oxidized and reduced forms were subjected to a computational study at the density functional theory (DFT) level, supporting carbo-aromaticity as a driving force for the formation of the dication, radical cation, and radical anion. By allowing co-planarity of the dithiolylidene rings and carbo-quinoidal core, carbo-exTTFs present a promising new class of redox-active systems.

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).

Toward Redox-Active Indenofluorene-Extended Tetrathiafulvalene Oligomers-Synthesis and Studies of Dimeric Scaffolds.

The promotion of mixed-valence interactions between redox-active, π-conjugated scaffolds is of interest when developing new conducting or electrochromic materials as well as in the construction of redox-controlled supramolecular assemblies. In this work, dimeric structures of the redox-active indenofluorene-extended tetrathiafulvalene (IF-TTF) unit were synthesized in a stepwise protocol. The synthesis relied on the development of a new unsymmetrical IF-TTF building block by a combination of phosphite-mediated and Horner-Wadsworth-Emmons reactions for introduction of the dithiafulvene units. The redox properties were studied by cyclic voltammetry, where it was observed that a first one-electron oxidation, corresponding to a mixed-valence state, occurs at a significantly lower potential when the IF-TTF unit is incorporated into a dimer, compared to a monomer analogue. This result indicates that locking the redox-active IF-TTF units in close proximity promotes intramolecular mixed-valence interactions. A computational study was also conducted, supporting the involvement of intramolecular interactions.

Synthesis of radiaannulene oligomers to model the elusive carbon allotrope 6,6,12-graphyne.

Graphyne allotropes of carbon are fascinating materials, and their electronic properties are predicted to rival those of the "wonder material" graphene. One allotrope of graphyne, having rectangular symmetry rather than hexagonal, stands out as particularly attractive, namely 6,6,12-graphyne. It is currently an insurmountable challenge, however, to design and execute a synthesis of this material. Herein, we present synthesis and electronic properties of molecules that serve as model compounds. These oligomers, so-called radiaannulenes, are prepared by iterative acetylenic coupling reactions. Systematic optical and redox studies indicate the effective conjugation length of the radiaannulene oligomers is nearly met by the length of the trimer. The HOMO-LUMO gap suggested by the series of oligomers is still, however, higher than that expected for 6,6,12-graphyne from theory, which predicts two nonequivalent distorted Dirac cones (no band gap). Thus, the radiaannulene oligomers present a suitable length in one dimension of a sheet, but should be expanded in the second dimension to provide a unique representation of 6,6,12-graphyne.