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.

Extended Triangulenium Ions: Syntheses and Characterization of Benzo-Bridged Dioxa- and Diazatriangulenium Dyes.

The very limited class of fluorophores, with a long fluorescence lifetime (>10 ns) and fluorescence beyond 550 nm, has been expanded with two benzo-fused triangulenium derivatives and two cationic [5]-helicene salts. The syntheses of the benzo-bridged dioxa- and diazatriangulenium derivatives (BDOTA+ and BDATA+, respectively) required two different synthetic approaches, which reflect the structural and physiochemical impact on the reactivity of [5]-helicenium precursors. Spectroscopic investigations show that the introduction of the benzo bridge into the triangulenium chromophore significantly redshifts the absorption and emission while maintaining fluorescence lifetimes above 10 ns. The combination of a high quantum yield, long fluorescence lifetime, and emission above 600 nm is possible only if the structural aspects of the triangulenium framework are perfectly harmonized to secure a low rate of nonradiative deactivation. The new benzo bridge may be a general motif to obtain red-shifted derivatives of other dye classes.

Computational Prediction of 1 H and 13 C NMR Chemical Shifts for Protonated Alkylpyrroles: Electron Correlation and Not Solvation is the Salvation.

Prediction of chemical shifts in organic cations is known to be a challenge. In this article we meet this challenge for α-protonated alkylpyrroles, a class of compounds not yet studied in this context, and present a combined experimental and theoretical study of the 13 C and 1 H chemical shifts in three selected pyrroles. We have investigated the importance of the solvation model, basis set, and quantum chemical method with the goal of developing a simple computational protocol, which allows prediction of 13 C and 1 H chemical shifts with sufficient accuracy for identifying such compounds in mixtures. We find that density functional theory with the B3LYP functional is not sufficient for reproducing all 13 C chemical shifts, whereas already the simplest correlated wave function model, Møller-Plesset perturbation theory (MP2), leads to almost perfect agreement with the experimental data. Treatment of solvent effects generally improves the agreement with experiment to some extent and can in most cases be accomplished by a simple polarizable continuum model. The only exception is the NH proton, which requires inclusion of explicit solvent molecules in the calculation.

Tuning Redox Properties and Self-Assembly of Thienoacene-Extended Tetrathiafulvalenes.

Turning on and off associations between molecules by a reversible change in their redox states is a convenient way of controlling self-assembly and hence for advancing supramolecular chemistry. Here we present systematic studies on a selection of extended tetrathiafulvalenes with thienoacene spacers. By cyclic and differential pulse voltammetry and in situ EPR/UV-Vis-NIR spectroelectrochemistry, in combination with computations, we have elucidated how the number and orientations of thiophene rings in the spacer between the two dithiafulvene rings influence both the donor strength and association properties. The radical cations and their associates were found to cover a remarkable large region of the UV-Vis-NIR spectrum, but the appearance of the absorption spectrum of the radical cations as well as of the unassociated dications also depended strongly on the exact molecular structure.

Complexation of Fullerenes by Subphthalocyanine Dimers.

Tweezer-like molecules comprised of two boron subphthalocyanine (SubPc) units were prepared by Sonogashira couplings and investigated using NMR spectroscopy for their ability to bind fullerenes (C60 and C70). The preorganization of the tweezers provided association constants of ca. 103 M-1 in toluene- d8, while a SubPc monomer did not show any association. Nevertheless, the SubPc monomer crystallized with the fullerenes as 2:1 complexes, supporting the favorable tweezer-like design for complexation, which was further corroborated by computations.

The quest for determining one-electron redox potentials of azulene-1-carbonitriles by calculation.

Electrochemical processes drive many chemical and biochemical reactions. Theoretical methods to accurately predict redox potentials are therefore crucial for understanding these reactions and designing new chemical species with desired properties. We have investigated a theoretical methodology using electronic structure methods based on density functional theory and continuum solvation models. These methods have been validated with linear correlation plots comparing theoretical and experimental results for the redox properties of a series of azulene derivatives. The results showed excellent correlations despite only minor structural variations of the azulenes, which support this rather simple theoretical methodology for determining redox potentials of organic molecules. Furthermore, we have estimated the absolute redox potential of the ferrocene/ferrocenium redox couple to be 4.8 ± 0.1 V in dichloromethane, which is slightly lower than previous estimates.

Subphthalocyanine-radiaannulene scaffold - a multi-electron acceptor and strong chromophore.

Two subphthalocyanine (SubPc) units and a perethynylated, alkyne-expanded radiaannulene (RA) were fused together by a four-fold Sonogashira reaction to give a compound exhibiting: (i) four reversible one-electron reductions, the first signalling good acceptor strength of the RA core itself, while the following three are a consequence of the entire scaffold, and (ii) intense light absorption that spans a remarkably broad region.

Synthesis and Properties of Subphthalocyanine-Tetracyanobutadiene-Ferrocene Triads.

A series of boron subphthalocyanine-tetracyanobutadiene-ferrocene (SubPc-TCBD-Fc) triads was synthesized by subjecting SubPcs with a ferrocenylethynyl substituent at either the axial or peripheral position to a [2 + 2] cycloaddition reaction with tetracyanoethylene followed by retroelectrocyclization. The ferrocenylethynyl unit was introduced at the axial position (at the boron atom) by a simple aluminum chloride-mediated alkynylation reaction, while functionalization at the SubPc periphery was accomplished by a Sonogashira coupling reaction. The conversion of one alkyne unit into a TCBD unit in combination with the location of the resulting TCBD-Fc moiety was found to have a strong influence on the optical and redox properties, which is ascribed to very different ground-state interactions between the individual donor/acceptor systems. The first electrochemical oxidation could thus be anodically shifted by as much as 0.4 V from the strongest donor molecule (with most unperturbed ferrocene character) to the poorest donor molecule (with strongly perturbed ferrocene character). Six redox states could be reached reversibly for the SubPc-TCBD-Fc triads, -3, -2, -1, 0, + 1, + 2, and for one compound the formation of a tetraanion persistent at the time scale of slow scan voltammetry was observed.

Acetylenic scaffolding with subphthalocyanines - synthetic scope and elucidation of electronic interactions in dimeric structures.

Boron subphthalocyanines (SubPcs) are powerful chromophoric heterocycles that can be synthetically modified at both axial and peripheral positions. Acetylenic scaffolding offers the possibility of building large, unsaturated carbon-rich frameworks that can exhibit excellent electron-accepting properties, and when combined with SubPcs it presents a convenient method for preparing interesting chromophore-acceptor architectures. Here we present synthetic methodologies for the post-functionalization of the relatively sensitive SubPc chromophore via acetylenic coupling reactions. By gentle AlCl3-mediated alkynylation at the axial boron position, we managed to anchor two SubPcs to the geminal positions of a tetraethynylethene (TEE) acceptor. Convenient conditions that allow for stepwise desilylations of trimethylsilyl (TMS) and triisopropylsilyl (TIPS) protected SubPc-decorated acetylenes using silver(i) fluoride were developed. The resulting terminal alkynes were successfully used as coupling partners in metal-catalyzed couplings, providing access to larger acetylenic SubPc scaffolds and multiple chromophore systems. Moreover, conditions allowing for the conversion of a terminal alkyne into an iodoalkyne in the presence of SubPc were developed, and the product was subjected to cross-coupling reactions affording unsymmetrical 1,3-butadiynes. The degree of interactions between two SubPc units as a function of the acetylenic bridge was studied by UV-Vis absorption spectroscopy and cyclic voltammetry. A TEE bridging unit was found to strongly influence the reductions and oxidations of the two SubPc units, while a more flexible bridge had no influence.

Thieno-Fused Subporphyrazines: A New Class of Light Harvesters.

Boron subphthalocyanines comprised of three isoindole units bridged by aza-linkages are attractive light harvesters on account of their intense low-energy absorptions. Herein, we present a class of related compounds, in which one or two isoindole units are substituted for thieno[3,4-c]pyrrole units - thieno-fused subporphyrazines. Such changes have remarkable consequences for the optical properties, as was revealed by combined experimental and theoretical studies. Thus, we find that the lowest-energy absorptions cover a much broader region with a significantly redshifted end-absorption and without compromising the absorption intensities. Thieno-fused subporphyrazines also underwent more readily oxidation and reduction, indicating an increased HOMO energy and decreased LUMO energy. In addition, they were found to readily co-crystallize with Buckminsterfullerene, C60 . Altogether, these findings render this new class of chromophores attractive candidates for light-harvesting applications.

Expanded Indacene-Tetrathiafulvalene Scaffolds: Structural Implications for Redox Properties and Association Behavior.

Redox-controlled dimerization of tetrathiafulvalene (TTF) derivatives is an important tool for achieving reversible assemblies. Herein, the synthesis and characterization of indacene-TTF hybrids, in which the central core is fused to naphtho or benzothieno units, are reported. The orientation of these units has a strong influence on the redox properties, with regard to the number of electrons involved in the first oxidation event and the ability of the oxidized forms to associate. The formation of mixed-valence and π-dimer complexes is turned off for those systems in which geometrical constraints provide nonplanar π-systems. Introduction of bulky substituents presents another way of preventing association, as revealed by studies of indenofluorene and diindenoanthracene scaffolds with (triisopropylsilyl)ethynyl groups. Horner-Wadsworth-Emmons reactions present a general synthetic protocol to access superextended TTFs, by using diones of polycyclic conjugated hydrocarbons as substrates.