Three Short Stories about Hexaarylbenzene-Porphyrin Scaffolds

Panchromatic PAH-Porphyrin Hybrids with a Step-Wise Increasing π-System

The rational synthesis of three β-meso-fused porphyrins with a step-wise increasing π-system size is presented. The synthetic route, which introduces a five-membered ring between the macrocycle and an aromatic fragment, is modular in its nature and proceeds straightforwardly. The well-soluble conjugates have intriguing optical properties, namely bathochromically shifted and flattened absorption curves. Density functional theory (DFT) calculations provide insights into the electronic structure and transitions, unveiling small HOMO-LUMO gaps.

Fused Hexabenzocoronene-Porphyrin Conjugates with Tailorable Excited-State Lifetimes

A new clear-cut strategy for fusing N-heterocyclic and carbon-pure systems is introduced en route to a versatile platform of multi-purpose tetrapyrrolic chromophores. In particular, three novel C-C bond-fused porphyrin-hexabenzocoronene (HBC) conjugates were synthesized under oxidative cyclodehydrogenation conditions, starting from tailor-made nickel porphyrin precursors. The fusion of the individual aromatic systems via 5-membered rings led to highly soluble π-extended porphyrins in excellent yields. The resulting porphyrin-HBC conjugates exhibit absorption cross-sections that are of interdisciplinary interest in the ever-growing field of organic photovoltaics and near-infrared (NIR) dyes. Quantum chemical calculations show that the newly formed 5-membered rings induce biradicaloid character in the porphyrin core, which has a strong impact on excited state lifetimes. This is confirmed by a thorough optoelectronic and time-resolved characterization in order to understand these unique features better. Broadened absorption characteristics go hand-in-hand with short-lived excited states with up to six orders of magnitude faster decay rates.

Electron beam-induced demetallation of Fe, Co, Ni, Cu, Zn, Pd, and Pt metalloporphyrins: insights in e-beam chemistry and metal cluster formations

Electron beams are versatile tools for nanoscale fabrication processes, however, the underlying e-beam chemistry remains in its infancy. Through operando transmission electron microscopy investigations, we elucidate a redox-driven cargo release of individual metal atoms triggered by electron beams. The chosen organic delivery molecule, tetraphenylporphyrin (TPP), proves highly versatile, forming complexes with nearly all metals from the periodic table and being easily processed in solution. A comprehensive cinematographic analysis of the dynamics of single metal atoms confirms the nearly instantaneous ejection of complexed metal atoms under an 80 kV electron beam, underscoring the system's broad versatility. Providing mechanistic insights, we employ density functional theory to support the proposed reductive demetallation pathway facilitated by secondary electrons, contributing novel perspectives to electron beam-mediated chemical reaction mechanisms. Lastly, our findings demonstrate that all seven metals investigated form nanoclusters once ejected from TPP, highlighting the method's potential for studying and developing sustainable single-atom and nanocluster catalysts.

Superhelicenes in the gas phase: experimental and computational evidence of stable radical cation dimers

Electrospray-ionization mass spectrometry (ESI-MS) readily produces stable radical cation π-dimers of the superhelicenes. Energy-resolved collision experiments reveal the dissociation of the dicationic dimer into two singly charged superhelicenes. DFT calculations indicate that open-shell dications composed of two radical cations are thermochemically more attractive than the closed-shell dimer formed by a doubly charged and a neutral superhelicene.

Probing Charge Management across the π-Systems of Nanographenes in Regioisomeric Electron Donor-Acceptor Architectures

Inspired by light-induced processes in nature to mimic the primary events in the photosynthetic reaction centers, novel functional materials combine electron donors and acceptors, i.e., (metallo)porphyrins (ZnP) and fullerenes (C₆₀), respectively, with emerging materials, i.e., nanographenes. We utilized hexa-peri-hexabenzocoronene (HBC) due to its versatility regarding functionalization and physicochemical properties, to construct three regioisomeric ZnP-HBC-C₆₀ conjugates, which foster geometrical diversity by arranging ZnP and C₆₀ in ortho-, meta-, and para-positions to each other. The corresponding hexaarylbenzene (HAB) motifs, with an interrupted π-system, were also prepared. Transient absorption measurements disclosed the fast population of charge transfer as well as singlet and triplet charge-separated states. With the help of density functional theory (DFT) calculations, we further conceive the communication across the HBCs and HABs. This work reveals the impact of both the geometrical arrangement with respect to through-space versus through-bond interactions and the structural rigidity/flexibility on the charge management across the different π-systems.

Hexabenzocoronene functionalized with antiaromatic S- and Se-core-modified porphyrins (isophlorins): comparison with the dyad with regular porphyrin

The important and perspective molecular building blocks composed of hexaphenylbenzenes (HPBs) or their oxidized derivatives, hexa-peri-hexabenzocoronenes (HBCs), and metalloporphyrins have recently received significant attention of the researchers. In this study, motivated by recent findings, we have addressed the modifications of structures and properties of HBC-porphyrin compounds by using instead of aromatic porphyrins antiaromatic 20π isophlorin derivatives of thiophene or selenophene. We have reported the first comparative computational investigation of the following systems: (i) HBC with one non-metallated aromatic porphyrin, P(N4H2), unit, HBC-P(N4H2), (ii) HBC with one S-core-modified antiaromatic porphyrin (S-isophlorin), PS4, unit, HBC-PS4, and (iii) HBC with one Se-core-modified antiaromatic porphyrin (Se-isophlorin), PSe4, unit, HBC-PSe4. The study has been done employing the B3LYP/6-31G* approach (in the gas phase and in the implicit solvents, benzene and dichloromethane), and comparison with the B3LYP/6-31G** and B3LYP/6-311G* approaches was performed, where relevant. The effects of the core-modified antiaromatic isophlorins on the structures, electronic, and other properties, potentially including reactivity, of the whole building block HBC-isophlorin have been shown to be quite pronounced and to be noticeably stronger than the effects of the original aromatic non-metallated porphyrin. Thus, we have demonstrated theoretically that the complete porphyrin core-modification with other elements, this time with S and Se leading to the formation of the antiaromatic isophlorins, should be considered as a promising way for modifying and tuning structures, electronic properties and reactivity of the hexabenzocoronene-porphyrin(s) building blocks.

Synthesis and Redox Properties of Superbenzene Porphyrin Conjugates

Superbenzene porphyrin conjugates find wide range of applications from nonlinear optical materials to semiconductors. Herein, we report the synthesis and characterization of 5,15-bis(3,5-di-tert-butylphenyl)-10,20-bis(pentaphenylphenyl)phenylporphyrin and its Zinc-metallated complex. Oxidative planarization of 5,15-bis(3,5-di-tert-butylphenyl)-10,20-bis(pentaphenylphenyl)phenylporphyrin and its metallated complex was carried out by using NOBF₄ as an oxidizing agent. The formation of superbenzene porphyrin conjugates validates its Scholl type reactions. The laboratory-synthesized porphyrin conjugates were characterized experimentally using spectroscopic techniques such as ¹H NMR, ¹³C NMR, electron spin resonance, and ultraviolet-visible spectroscopy for structural conformation. In addition, density functional theory calculations were carried out to validate the experimental results. The theoretical and experimental results show that the 4-(pentaphenylphenyl)phenyl ligand increases the stability, optical properties, and rate of planarization of synthesized porphyrins. The conjugates exhibited intense and distant electronic communication between two hexabenzocoronene sites, taking advantage of porphyrin as a π-spacer. The π-radical cation has also been found to be an intermediate in oxidative C-C bond formation. NICS calculations support such a conclusion.

Gas-Phase Transformation of Fluorinated Benzoporphyrins to Porphyrin-Embedded Conical Nanocarbons

Geodesic nitrogen-containing graphene fragments are interesting candidates for various material applications, but the available synthetic protocols, which need to overcome intrinsic strain energy during the formation of the bowl-shaped skeletons, are often incompatible with heteroatom-embedded structures. Through this mass spectrometry-based gas-phase study, we show by means of collision-induced dissociation experiments and supported by density functional theory calculations, the first evidence for the formation of a porphyrin-embedded conical nanocarbon. The influences of metalation and functionalization of the used tetrabenzoporphyrins have been investigated, which revealed different cyclization efficiencies, different ionization possibilities, and a variation of the dissociation pathway. Our results suggest a stepwise process for HF elimination from the fjord region, which supports a selective pathway towards bent nitrogen-containing graphene fragments.

A Hexabenzocoronene-Based Helical Nanographene

A synthetic route towards a novel hexabenzocoronene-based helical nanographene motif was developed. A hexaphenylbenzene precursor was therefore designed, which cannot undergo, due to steric restrictions, a complete planarization reaction. This precursor was transformed under oxidative cyclodehydrogenation conditions to a π-extended [5]helicene, which was fully characterized including X-ray diffraction analysis.

Investigation of Cycloparaphenylenes (CPPs) and their Noncovalent Ring-in-Ring and Fullerene-in-Ring Complexes by (Matrix-Assisted) Laser Desorption/Ionization and Density Functional Theory

[n]Cycloparaphenylenes ([n]CPPs) with n=5, 8, 10 and 12 and their noncovalent ring‐in‐ring and [m]fullerene‐in‐ring complexes with m=60, 70 and 84 have been studied by direct and matrix‐assisted laser desorption ionization ((MA)LDI) and density‐functional theory (DFT). LDI is introduced as a straightforward approach for the sensitive analysis of CPPs, free from unwanted decomposition and without the need of a matrix. The ring‐in‐ring system of [[10]CPP⊃[5]CPP]^+. was studied in positive‐ion MALDI. Fragmentation and DFT indicate that the positive charge is exclusively located on the inner ring, while in [[10]CPP⊃C₆₀]^+. it is located solely on the outer nanohoop. Positive‐ion MALDI is introduced as a new sensitive method for analysis of CPP⊃fullerene complexes, enabling the detection of novel complexes [[12]CPP⊃C_{60, 70 and 84}]^+. and [[10]CPP⊃C₈₄]^+.. Selective binding can be observed when mixing one fullerene with two CPPs or vice versa, reflecting ideal size requirements for efficient complex formation. Geometries, binding and fragmentation energies of CPP⊃fullerene complexes from DFT calculations explain the observed fragmentation behavior.

Panchromatic light funneling through the synergy in hexabenzocoronene-(metallo)porphyrin-fullerene assemblies to realize the separation of charges

Here, we present a novel butadiyne-linked HBC-ethynyl-porphyrin dimer, which exhibits in the ground state strong absorption cross sections throughout the UV and visible ranges of the solar spectrum. In short, a unidirectional flow of excited state energy from the HBC termini to the (metallo)porphyrin focal points enables concentrating light at the latter. Control over excitonic interactions within, for example, the electron-donating porphyrin dimers was realized by complexation of bidentate ligands to set up panchromatic absorption that extends all the way into the near-infrared range. The bidentate binding motif was then exploited to create a supramolecular electron donor-acceptor assembly based on a HBC-ethynyl-porphyrin dimer and an electron accepting bis(aminoalkyl)-substituted fullerene. Of great relevance is the fact that charge separation from the photoexcited HBC-ethynyl-porphyrin dimer to the bis(aminoalkyl)-substituted fullerene is activated not only upon photoexciting the HBCs in the UV as well as the (metallo)porphyrins in the visible but also in the NIR. Implicit is the synergetic interplay of energy and charge transfer in a photosynthetic mimicking manner. The dimer and bis-HBC-ethynyl-porphyrin monomers, which serve as references, were probed by means of steady-state as well as time-resolved optical spectroscopies, including global target analyses of the time-resolved transient absorption data.

Investigations of Low-Symmetrical Tetraaryltetrabenzoporphyrins Produced by Mixed Condensation Reactions

Within the past decade, tetraaryltetrabenzoporphyrins (TATBPs) have gained rising attention due to their potential in various fields of materials science and medicinal chemistry. However, this class of compounds still lacks in structural diversity, especially in the case of low-symmetrical compounds. Herein, mixed condensations were utilized to generate TATBPs with different substituents either in the meso-positions or the periphery of the macrocycle with total yields of 55-58%. The separation of crude mixtures was achieved by feasible chromatographic purification. The influence of symmetry on the electronic properties of TATBPs was studied by optical spectroscopy, electrochemistry, and X-ray diffraction.

A Comprehensive Study on Tetraaryltetrabenzoporphyrins

Tetraaryltetrabenzoporphyrins (TATBPs) show, due to their optoelectronic properties, rising potential as dyes in various fields of physical and biomedical sciences. However, unlike in the case of porphyrins, the potential structural diversity of TATBPs has been explored only to little extent, owed mainly to synthetic hurdles. Herein, we prepared a comprehensive library of 30 TATBPs and investigated their fundamental properties. We elucidated structural properties by X-ray crystallography and found explanations for physical properties such as solubility. Fundamental electronic aspects were studied by optical spectroscopy as well as by electrochemistry and brought in context to the stability of the molecules. Finally, we were able to develop a universal synthetic protocol, utilizing a readily established isoindole synthon, which gives TATBPs in high yields, regardless of the nature of the used arylaldehyde and without meticulous chromatographic purifications steps. This work serves as point of orientation for scientists, that aim to utilize these molecules in materials, nanotechnological, and biomedical applications.

Porphyrin-rylene-diimide-hexabenzocoronene triads

Hexa-peri-hexabenzocoronenes (HBCs), substituted by two different chromophores in an ortho arrangement, were synthesized for the first time and investigated towards their photophysical properties. Rylene-diimide dyes, in particular naphthalene-diimides (NDIs) and perylene-diimides (PDIs), were attached to the HBC as the first, and porphyrins as the second chromophore. Therefore, porphyrin-NDI-HPB as well as porphyrin-PDI-HPB precursors were accessed and converted into the respective HBC derivatives applying oxidative Scholl conditions. UV-vis absorption and fluorescence emission spectroscopy showed interesting photophysical behavior such as energy transfer processes. Particularly, the porphyrin-PDI-HBC displays pronounced absorption features between 330–650 nm, which cover a wide range of the visible spectrum.

Multiple-Porphyrin Functionalized Hexabenzocoronenes

Porphyrin-hexabenzocoronene architectures serve as good model compounds to study light-harvesting systems. Herein, the synthesis of porphyrin functionalized hexa-peri-hexabenzocoronenes (HBCs), in which one or more porphyrins are covalently linked to a central HBC core, is presented. A series of hexaphenylbenzenes (HPBs) was prepared and reacted under oxidative coupling conditions. The transformation to the respective HBC derivatives worked well with mono- and tri-porphyrin-substituted HPBs. However, if more porphyrins are attached to the HPB core, Scholl oxidations are hampered or completely suppressed. Hence, a change of the synthetic strategy was necessary to first preform the HBC core, followed by the introduction of the porphyrins. All products were fully characterized, including, if possible, single-crystal XRD. UV/Vis absorption spectra of porphyrin-HBCs showed, depending on the number of porphyrins as well as with respect to the substitution pattern, variations in their spectral features with strong distortions of the porphyrins' B-band.

Exclusive occurrence of photoinduced energy transfer and switching of its direction by rectangular π-extension of nanographenes

As structure defined cutouts of the graphene lattice, nanographene molecules have gained plenty of attention because of their high potential for versatile applications in organic electronics and energy conversion devices and as ideal model systems for the better understanding of intrinsic structure-property correlations of graphenes. In this study, well-defined nanographenes with sp2 carbon networks of different sizes, hexa-peri-hexabenzocoronene (HBC) and its rectangularly π-extended version, a short graphene nanoribbon (GNR), have been covalently functionalized with photoactive porphyrin molecules. On the basis of their spectroscopic studies, the photodynamics of the porphyrin-linked nanographenes was found to be influenced substantially by the size of the nanographenes. Photoexcitation of the porphyrin-HBC linked system led to exclusive energy transfer (EnT) from the first singlet excited state (S1) of the nanographene to the porphyrin, whereas opposite selective EnT occurred from the first and second singlet excited states (S1 and S2) of the porphyrin to the nanographene in the porphyrin-GNR linked system. In particular, ultrafast efficient EnTs from both the S2 and S1 states of the porphyrin to GNR mimic the corresponding ultrafast EnTs from the S2 and S1 states of carotenoids to chlorophylls in light-harvesting systems of natural photosynthesis. Such unique photophysical properties will be useful for the rational design of carbon-based photofunctional nanomaterials for optoelectronics and solar energy conversion devices.

Porphyrin-Hexaphenylbenzene Conjugates via Mixed Cyclotrimerization Reactions

Mixed cyclotrimerization reactions of diarylacetylenes (tolans) were applied to generate a library of multiple porphyrin-hexaphenylbenzene (HPB) architectures. Successful reactions, which could be influenced by the ratio of tolan starting materials, were conducted using dicobaltoctacarbonyl as a catalyst. Separation of the reaction products was performed by chromatographic and crystallization techniques. The physical properties were investigated with respect to the number of porphyrins per HPB and their substitution pattern.

Solvent and Temperature Effects on Dynamics and Chiroptical Properties of Propeller Chirality and Toroidal Interaction of Hexaarylbenzenes

Because of the unique interaction of radial aromatic blades, propeller-shaped hexaarylbenzenes (HABs) attract much research interest and find various practical applications. By introducing a small point-chiral group at the tip of aromatic blade(s), HAB becomes propeller-chiral to exhibit strong Cotton effects. Because of the dynamic nature of propeller chirality, the chiroptical properties of HAB critically responded to minute changes in the environment. Using a series of chiral HABs with one to six ( R)-1-methylpropyloxy substituent(s) introduced at the blade tip, we elucidated how the smallest chiral auxiliary at the HAB periphery progressively and cooperatively boosts the overall chiroptical properties and also how subtle changes in temperature and solvent structure affect the propeller dynamics and thus the chiroptical responses. The unique features of propeller-chiral HABs further enabled us to switch on/off their circularly polarized luminescence.

Ruthenium(II)-Catalyzed Microwave-Promoted Multiple C-H Activation in Synthesis of Hexa(heteroaryl)benzenes in Water

A series of hexa(heteroaryl)benzenes were synthesized by the Ru(II)-carboxylate-catalyzed multiple C-H activation of benzenes carrying pyridyl, pyrimidyl, or pyrazolyl directing groups using N-heteroaryl bromides as coupling partners. The reactions proceeded with high selectivity under microwave irradiation in water. Iterative penta-arylation could be implemented via activation of C-H bonds of generated intermediates by cascade chelation assistance of in situ installed pyridyl groups. This strategy provides multidentate ligands for selective complexation of transition metals and potential building of photoredox systems.

Synthesis of a porphyrin-hexaarylbenzene-hexabenzocoronene triad

The synthesis of a triad consisting of porphyrin and hexabenzocoronene (HBC) units, which are connected via a central hexaarylbenzene (HAB) core, is presented. The ortho substitution pattern at the core results in a close proximity of the two chromophores, which influences their properties such as the intensity ratio of UV-vis absorption bands.