1,3,6,8-Tetraazapyrenes: Synthesis, Solid-State Structures, and Properties as Redox-Active Materials

Synthesis and properties of thienonaphtho[bc]pyridines and thienonaphtho[bc]quinolines

The incorporation of heteroatoms within polycyclic aromatic compounds has gained significant interest due to its potential to effectively alter the inherent physicochemical properties of compounds without the need for profound structural changes. We herein report the development of a modular synthesis of hitherto unknown thienonaphtho[bc]pyridines and thienonaphtho[bc]quinolines in very good yields by Brønsted acid mediated cycloisomerization, permitting selective access to two isomeric products that are isoelectronic to the parent dibenzopyrene. The photophysical and electrochemical properties of the desired compounds were extensively studied and further complemented by DFT calculations.

Synthesis and Properties of 5,7-Diazaullazines

Hitherto unknown 5,8-substituted-pyrimido[4,5,6-ij]pyrrolo[2,1,5-de]quinolizines (5,7-diazaullazines) were prepared by a three-step synthesis via Clauson-Kaas, Sonogashira, and cycloisomerization reactions with diverse functionalization. The properties, including cyclovoltammetry and UV-vis and fluorescence spectroscopy, as well as solvatochromism, were studied for selected derivatives and supported by density functional theory calculations. Results were compared in detail with previously reported 5- and 6-azaullazines, and the impact of introduced nitrogen atoms was analyzed.

Synthesis and Properties of 4- and 10-Benzoyl-1-azapyrenes

A series of 4- and 10-benzoyl-1-azapyrenes were prepared by a combination of Pd-catalyzed cross-coupling reactions and Brønsted-acid-mediated alkyne-carbonyl-metathesis (ACM). The photophysical and electrochemical properties of the products were studied and compared to theoretical results.

Photoinduced asymmetric charge trapping in a symmetric tetraazapyrene-fused bis(tetrathiafulvalene) conjugate

In fused donor-acceptor (D-A) ensembles, rapid charge recombination often occurs because the D and A units are spatially close and strongly coupled. To the best of our knowledge, a long-lived charge separated (CS) state is still elusive in such systems. The results presented here show that symmetric annulation of two tetrathiafulvalene (TTF) donors to a central tetraazapyrene (TAP) acceptor via two quinoxaline units leads to a CS state lifetime of a few ns. A detailed study of the electronic interactions between TTF and TAP units in the ground and excited states was performed and compared with the asymmetric counterpart by cyclic voltammetry, optical absorption and ultrafast transient absorption spectroscopy. The results demonstrate that the photoinduced asymmetric charge trapping between two TTFs significantly stabilizes the CS state, which is also verified theoretically.

Strong signature of electron-vibration coupling in molecules on Ag(111) triggered by tip-gated discharging

Electron-vibration coupling is of critical importance for the development of molecular electronics, spintronics, and quantum technologies, as it affects transport properties and spin dynamics. The control over charge-state transitions and subsequent molecular vibrations using scanning tunneling microscopy typically requires the use of a decoupling layer. Here we show the vibronic excitations of tetrabromotetraazapyrene (TBTAP) molecules directly adsorbed on Ag(111) into an orientational glassy phase. The electron-deficient TBTAP is singly-occupied by an electron donated from the substrate, resulting in a spin 1/2 state, which is confirmed by a Kondo resonance. The TBTAP•- discharge is controlled by tip-gating and leads to a series of peaks in scanning tunneling spectroscopy. These occurrences are explained by combining a double-barrier tunneling junction with a Franck-Condon model including molecular vibrational modes. This work demonstrates that suitable precursor design enables gate-dependent vibrational excitations of molecules on a metal, thereby providing a method to investigate electron-vibration coupling in molecular assemblies without a decoupling layer.

Atomically well-defined nitrogen doping for cross-plane transport through graphene heterojunctions

The nitrogen doping of graphene leads to graphene heterojunctions with a tunable bandgap, suitable for electronic, electrochemical, and sensing applications. However, the microscopic nature and charge transport properties of atomic-level nitrogen-doped graphene are still unknown, mainly due to the multiple doping sites with topological diversities. In this work, we fabricated atomically well-defined N-doped graphene heterojunctions and investigated the cross-plane transport through these heterojunctions to reveal the effects of doping on their electronic properties. We found that a different doping number of nitrogen atoms leads to a conductance difference of up to ∼288%, and the conductance of graphene heterojunctions with nitrogen-doping at different positions in the conjugated framework can also lead to a conductance difference of ∼170%. Combined ultraviolet photoelectron spectroscopy measurements and theoretical calculations reveal that the insertion of nitrogen atoms into the conjugation framework significantly stabilizes the frontier molecular orbitals, leading to a change in the relative positions of the HOMO and LUMO to the Fermi level of the electrodes. Our work provides a unique insight into the role of nitrogen doping in the charge transport through graphene heterojunctions and materials at the single atomic level.

Synthesis and Properties of Diphenylbenzo[j]naphtho[2,1,8-def][2,7]phenanthrolines

A series of hitherto unknown 5,14-diphenylbenzo[j]naphtho[2,1,8-def][2,7]phenanthrolines, containing a 5-azatetracene and a 2-azapyrene subunit, were prepared by combination of Pd-catalyzed cross-coupling reactions with a one-pot Povarov/cycloisomerization reaction. In the final key step four new bonds are formed in one step. The synthetic approach allows for a high degree of diversification of the heterocyclic core structure. The optical and electrochemical properties were studied experimentally and by DFT/TD-DFT and NICS calculations. Due to the presence of the 2-azapyrene subunit, the typical electronic nature and characteristics of the 5-azatetracene moiety are lost and the compounds are electronically and optically more related to 2-azapyrenes.

Synthesis and Properties of 1-Azapyrenes

A straightforward method for the synthesis of a series of hitherto unknown 1-azapyrenes is reported, which relies on a combination of Pd-catalyzed cross-coupling reactions with Brønsted acid-mediated cycloisomerization reactions. The methodology is highly modular and allows an efficient synthesis of various substituted products in high yields. The structural, electrochemical, and photophysical properties of 1-azapyrenes have been studied by ultraviolet-visible (UV-vis) and fluorescence spectroscopies, cyclic voltammetry, and density functional theory (DFT) calculations.

A BF3-mediated C–H/C–Li coupling of 1,3,7-triazapyrene with 2-thienyllithium in the design of push–pull fluorophores and chemosensors for nitroaromatics

A BF3-mediated strategy of oxidative nucleophilic substitution of hydrogen affords mono- and bis-substituted thiophene-derived 1,3,7-triazapyrene fluorophores of interest in the design of luminescent materials and chemosensors for nitroaromatics.

Effect of tert-butyl groups on electronic communication between redox units in tetrathiafulvalene-tetraazapyrene triads

The electronic effect of tert-butyl groups on intramolecular through-bond interactions between redox units in tetrathiafulvalene-tetraazapyrene (TAP) triads is investigated. The insertion of tert-butyl groups raises the TAP-localised LUMO level by 0.21 eV, in fairly good agreement with 0.17 eV determined by DFT calculations.

Reductive O-triflylation of naphthalene diimide: access to alkyne- and amine-functionalized 2,7-diazapyrenes

A reductive one-pot triflylation and silylation of naphthalene diimide is presented. Post-functionalization of the 1,3,6,8-tetratriflato key compound leads to so far non-accessible UHV processable tetraalkynyl and tetraaminyl 2,7-diazapyrenes.

Azapyrene-based fluorophores: synthesis and photophysical properties

The latest methods for the preparation/functionalization of azapyrenes, and their photophysical/electrochemical properties and possible applications are overviewed.

Synthesis of 2-Azapyrenes and Their Photophysical and Electrochemical Properties

A series of 5,7,9-substituted 2-azapyrenes were synthesized for the first time. The synthesis relies on Brønsted acid promoted benzannulation of alkyne precursors prepared by palladium-catalyzed cross-coupling reactions. The synthetic strategy is efficient and the scope covers a variety of functional groups. The electrochemical behavior and photophysical properties of the products were investigated by UV-vis and fluorescence spectroscopy, cyclic voltammetry, and DFT calculations.

Synthesis of Salts of 1,2,5,6- and 1,4,5,8-Naphthalenetetramine

1,2,5,6-Naphthalenetetramine (1a), its 1,4,5,8-isomer (2a), and their salts are valuable precursors for synthesizing nitrogen-containing arenes and other targets of interest. We describe how salts of tetramines 1a and 2a can be made from simple protected derivatives of 1,5-naphthalenediamine (2d) by sequences of regioselective dinitration, deprotection, and reduction. Various shortcomings of previously reported syntheses of tetramines 1a and 2a can thereby be avoided. In addition, we report structural studies that may help clarify the mechanism of nitration and resolve an earlier controversy about the regioselectivity observed in nitrations of derivatives of 1,5-naphthalenediamine (2d).

Dual role of polyphosphoric acid-activated nitroalkanes in oxidative peri-annulations: efficient synthesis of 1,3,6,8-tetraazapyrenes

Electrophilically activated nitroalkanes play a dual role in the novel reaction with diaminoperimidines, promoting their oxidative peri-annulation to access tetraazapyrenes.

Counter-ligand control of the electronic structure in dinuclear copper-tetrakisguanidine complexes

Decision-making counter-ligands: a bridging redox-active ligand in a dinuclear copper complex could be either neutral (complex type [Cu^II-GFA-Cu^II]) or dicationic (complex type [Cu^I-GFA-Cu^I]), depending on the nature of the counter-ligands X.

Larger π-extended anti-/syn-aroylenediimidazole polyaromatic compounds: synthesis, physical properties, self-assembly, and quasi-linear conjugation effect

Four polyaromatic compounds with 11- or 13-fused rings have been synthesized and their physical properties have been studied.