David Reger

Large aza-analogues of curved polycyclic aromatic hydrocarbons with a double-helicene structure present unique features for molecular photonics. We present the preparation and characterization of three such structures. The synthesis of these heterocyclic nanographenes involves only a few high-yield steps that use readily available starting materials. X-ray analysis revealed that each of these new dyes has three conformational isomers: one diastereoisomer in a meso form and two enantiomers in twisted forms [(P,P)] and [(M,M)]. The low energy barriers between the conformers, however, prevent their separation by using chiral HPLC, and the NMR spectra show only one set of signals for each of these curved compounds. Density functional theory (DFT) calculations quantify the small energy difference and the small energy barriers between the chiral and meso forms, which fully supports the experimental results. Their optical absorption lacks any sensitivity to the solvent environment, whereas their fluorescence features exhibit pronounced solvatochromism. This rarely observed solvatofluorochromism of centrosymmetric molecules without either electron-withdrawing groups or -donating substituents was probed by using time-resolved spectroscopy. These studies suggest that, similar to 9,9′-bianthryl, the nonpolar locally excited state shows negligible solvatochromism, whereas the charge-transfer state is sensitive to solvent polarity.

Let′s twist again! In two steps, simple pyrrolo[3,2-b]pyrroles can be transformed into double helical polycyclic aromatic heteroaromatics with low energy barriers between the conformational isomers and fluorescence solvatochromism (see figure). The existence of folded and twisted isomers was unequivocally confirmed by using X-ray crystallography.

A feasible two-step synthesis and characterization of a full series of hexaarylbenzene (HAB) substituted porphyrins and tetrabenzoporphyrins is presented. Key steps represent the microwave-assisted porphyrin condensation and the statistical Diels–Alder reaction to the desired HAB-porphyrins. Regarding their applications, they proved to be easily accessible and effective high molecular mass calibrants for (MA)LDI mass spectrometry. The free-base and zinc(II) porphyrin systems, as well as the respective tetrabenzoporphyrins, demonstrate in solid state experiments strong red- and near-infrared-light emission and are potentially interesting for the application in “truly organic” light-emitting devices. Lastly, they represent facile precursors to large polycyclic aromatic hydrocarbon (PAH) substituted porphyrins. We prepared the first tetra-hexa-peri-hexabenzocoronene substituted porphyrin, which represents the largest prepared PAH-porphyrin conjugate to date.

Hexaarylbenzene (HAB)-substituted (tetrabenzo)porphyrins were found to have useful properties for various applications, such as calibrants for (MA)LDI instruments, as strong red- and near- infrared light emitters, and as precursors to large superbenzene-porphyrin light-harvesting architectures.

With our new home-built circularly polarized luminescence (CPL) instrument, we measured fluorescence and CPL spectra of the enantiomeric pairs of two quasi-isomeric BODIPY DYEmers 1 and 2, endowed with axial chirality. The electronic circular dichroism (ECD) and CPL spectra of these atropisomeric dimers are dominated by the exciton coupling between the main π–π* transitions (550–560 nm) of the two BODIPY rings. Compound 1 has strong ECD and CPL spectra (g_lum=4×10⁻³) well reproduced by TD-DFT and SCS-CC2 (spin-component scaled second-order approximate coupled-cluster) calculations using DFT-optimized ground- and excited-state structures. Compound 2 has weaker ECD and CPL spectra (g_lum=4×10⁻⁴), partly due to the mutual cancellation of electric–electric and electric–magnetic exciton couplings, and partly to its conformational freedom. This compound is computationally very challenging. Starting from the optimized excited-state geometries, we predicted the wrong sign for the CPL band of 2 using TD-DFT with the most recommended hybrid and range-separated functionals, whereas SCS-CC2 or a DFT functional with full exact exchange provided the correct sign.

Chiral light excitement: Fluorescence and circularly polarized luminescence (CPL) spectra of BODIPY DYEmers 1 and 2 were measured with a new home-built CPL instrument (see figure). Compound 1 has strong electronic circular dichroism (ECD) and CPL spectra (g_lum=4×10⁻³) well reproduced by M06-2X and SCS-CC2 calculations. Compound 2 has weaker ECD and CPL spectra (g_lum=4×10⁻⁴), due to the mutual cancellation of μ–μ and m–μ exciton couplings, and to its conformational freedom. The correct CPL sign could be obtained only with SCS-CC2 or M06-HF calculations.

A two-step reaction sequence for accessing meso-(dialkoxyphosphoryl)porphyrins from readily available trans-A₂-type porphyrins was developed. This approach involves bromination and subsequent palladium-catalyzed phosphonylation. Optimal conditions for both steps were identified after exploration of various reaction parameters such as solvent, temperature and catalyst. A series of dialkoxyphosphoryl-substituted A₂B-porphyrins Zn3(a–g) bearing electron-donating, electron-withdrawing or sterically bulky substituents at the meso-aryl groups were prepared in overall yields close to 40 %. These compounds, being air-stable and soluble in most organic solvents, are valuable synthetic intermediates because they can be readily transformed into functionalized trans-A₂BC-type porphyrins through regioselective functionalization at the unsubstituted meso position of the macrocycle. Therefore, this approach offers considerable promise for application to the synthesis of trans-A₂BC-type porphyrins, including water-soluble derivatives, push-pull chromophores and bis(porphyrin)s.

A two-step reaction sequence for the preparation of A₂B-type porphyrin phosphonate diesters from readily available A₂-type porphyrins was developed. These compounds are valuable synthetic intermediates because they can be readily transformed into functionalized A₂BC-type porphyrin phosphonate diesters through selective functionalization at the unsubstituted meso position.

The application of porphyrinoids in biomedical fields, such as photodynamic therapy (PDT), requires the introduction of functional groups to tune their solubility for the biological environment and to allow a coupling to other active moieties or carrier systems. A valuable motif in this regard is the pentafluorophenyl (PFP) substituent, which can easily undergo a regiospecific nucleophilic replacement (S_NAr) of its para-fluorine atom by a number of nucleophiles. Here, it is shown that, instead of amino-substitution on the final porphyrinoid or BODIPY (boron dipyrromethene), the precursor 5-(PFP)-dipyrrane can be modified with amines (or alcohols). These dipyrranes were transformed into amino-substituted BODIPYs. Condensation of these dipyrranes with aldehydes gave access to trans-A₂B₂-porphyrins and trans-A₂B-corroles. By using pentafluorobenzaldehyde, it was possible to introduce another para-fluorine atom, which enabled the synthesis of multifunctionalized tetrapyrroles. Furthermore, alkoxy- and amino-substituted dipyrranes were applied to the synthesis of A₃B₃-hexaphyrins. The polar porphyrins that were prepared by using this method exhibited in vitro PDT activity against several tumor cell lines.

Agents of change: An alternative approach to amino-substitution of porphyrinoid or BODIPY compounds is achieved by modification of the PFP-dipyrrane precursor with a range of different amines or alcohols. These dipyrranes are transformed to specifically substituted corroles, porphyrins, hexaphyrins, and BODIPYs. Selected porphyrins are tested for their photodynamic therapy activity.

The efficient and selective catalytic reduction of CO₂ is a highly promising process for both of the storage of renewable energy as well as the production of valuable chemical feedstocks. In this work, we show that the addition of an ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate, in an aprotic electrolyte containing a proton source and FeTPP, promotes the in situ formation of the [Fe⁰TPP]²⁻ homogeneous catalyst at a less negative potential, resulting in lower overpotentials for the CO₂ reduction (670 mV) and increased kinetics of electron transfer. This co-catalysis exhibits high Faradaic efficiency for CO production (93 %) and turnover number (2 740 000 after 4 hour electrolysis), with a four-fold increase in turnover frequency (TOF) when compared with the standard system without the ionic liquid.

The potential of attraction: The presence of an imidazolium ionic liquid has a significant effect on the Fe porphyrin-catalyzed reduction of CO₂ to CO in the presence of a proton source. The ionic liquid cation interacts with the reduced porphyrin leading to a significantly lowered overpotential and substantially higher catalytic activity in the reduction of CO_2.

Stille, Suzuki–Miyaura and Negishi cross-coupling reactions of bromine-functionalised borylated precursors enable the facile, high yielding, synthesis of borylated donor–acceptor materials that contain electron-rich aromatic units and/or extended effective conjugation lengths. These materials have large Stokes shifts, low LUMO energies, small band-gaps and significant fluorescence emission >700 nm in solution and when dispersed in a host polymer.

The perfect couple: Cross-coupling enables the high yielding synthesis of borylated donor–acceptor materials that contain electron-rich aromatics units and/or extended effective conjugation lengths. These materials have large Stokes shifts, low LUMO energies, small band-gaps and significant fluorescence emission >700 nm in solution and in the solid state.

Azafullerenes are as yet the only synthetically available heterofullerenes. Herein, we present plausible reaction pathways towards pentaaryl azafullerenes, focusing on the reactivity of hydro-azafullerene intermediates and their regiochemistry. The X-ray structure of a β′-hydro-tetraaryl adduct is presented for the first time. The reactivity of dihydro-azafullerene adducts is demonstrated here through H-abstraction in mass spectrometric experiments. Moreover, hydride abstraction and subsequent hydroxylation is possible, as well as deprotonation followed by alkylation.

Regioselectivity in hydro-azafullerene reactions: The reactivity of hydro-azafullerene intermediates en route to pentaaryl derivatives has been explored. Experimental results have shown that both hydride and proton abstraction occur preferentially at the α′-positions of dihydro-azafullerenes.

Dynamic covalent chemistry (DCC) is a vibrant research area that has extended its reach from supramolecular chemistry into fields as diverse as bioconjugation, materials science and systems chemistry. Because DCC crucially rests on reversible organic reactions, considerable effort has been invested in the development of new reactions of this type and the identification of new methods for facilitating existing processes. Our group has recently reported that orthoester exchange is a previously overlooked reversible reaction that is in principle suited for many applications of DCC. This microreview gives an overview on our ongoing work on orthoester exchange, and puts it into the context of previous studies on the related dynamic covalent chemistry of esters and acetals.

This microreview discusses the fundamental aspects of three related reversible covalent reactions and their uses in dynamic combinatorial libraries and the self-assembly of stimuli-responsive molecules and materials.