RESUMEN
Replacement of sp2-hybridized carbon in polycyclic aromatic hydrocarbons (PAHs) by boron affords electron-deficient π-scaffolds due to the vacant pz-orbital of three-coordinate boron with the potential for pronounced electronic interactions with electron-rich metal surfaces. Using a diboraperylene diborinic acid derivative as precursor and a controlled on-surface non-covalent synthesis approach, we report on a self-assembled chiral supramolecular kagome network on an Ag(111) surface stabilized by intermolecular hydrogen-bonding interactions at low temperature. Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal a flat band at ca. 0.33â eV above the Fermi level which is localized at the molecule center, in good agreement with tight-binding model calculations of flat bands characteristic for kagome lattices.
RESUMEN
Herein we devise and execute a new synthesis of a pristine boron-doped nanographene. Our target boron-doped nanographene was designed based on DFT calculations to possess a low LUMO energy level and a narrow band gap derived from its precise geometry and B-doping arrangement. Our synthesis of this target, a doubly B-doped hexabenzopentacene (B2 -HBP), employs six net C-H borylations of an alkene, comprising consecutive hydroboration/electrophilic borylation/dehydrogenation and BBr3 /AlCl3 /2,6-dichloropyridine-mediated C-H borylation steps. As predicted by our calculations, B2 -HBP absorbs strongly in the visible region and emits in the NIR up to 1150â nm in o-dichlorobenzene solutions. Furthermore, B2 -HBP possesses a very low LUMO level, showing two reversible reductions at -1.00â V and -1.17â V vs. Fc+ /Fc. Our methodology is surprisingly selective despite its implementation of unfunctionalized precursors and offers a new approach to the synthesis of pristine B-doped polycyclic aromatic hydrocarbons.
RESUMEN
Boron-doping has long been recognized as a promising LUMO energy-lowering modification of graphene and related polycyclic aromatic hydrocarbons (PAHs). Unfortunately, synthetic difficulties have been a significant bottleneck for the understanding, optimization, and application of precisely boron-doped PAHs for optoelectronic purposes. Herein, a facile one-pot hydroboration electrophilic borylation cascade/dehydrogenation approach from simple alkene precursors is coupled with postsynthetic B-substitution to give access to ten ambient-stable core- and periphery-tuned boron-doped PAHs. These include large hitherto unknown doubly boron-doped analogues of anthanthrene and triangulene. Crystallographic, optical, electrochemical, and computational studies were performed to clarify the effect of boron-doped PAH shape, size, and structure on optoelectronic properties. Our molecular tuning allowed the synthesis of molecules exhibiting visible-range absorption, near-unity fluorescence quantum yields, and, to our knowledge, the most facile electrochemical reductions of any reported ambient-stable boron-doped PAHs (corresponding to LUMO energy levels as low as fullerenes). Finally, our study describes the first implementation of a precise three-coordinate boron-substituted PAH as an acceptor material in organic solar cells with power conversion efficiencies (PCEs) of up to 3%.
RESUMEN
The linear and nonlinear optical properties of a series of oligomeric squaraine dyes were investigated by one-photon absorption spectroscopy (1PA) and two-photon absorption (2PA) induced fluorescence spectroscopy. The superchromophores are based on two indolenine squaraine dyes with transoid (SQA) and cisoid configuration (SQB). Using these monomers, linear dimers and trimers as well as star-shaped trimers and hexamers with benzene or triphenylamine cores were synthesised and investigated. The red-shifted and intensified 1PA spectra of all superchromophores could well be explained by exciton coupling theory. In the linear chromophore arrangements we also found superradiance of fluorescence but not in the branched systems. Furthermore, the 2PA showed enhanced cross sections for the linear oligomers but only additivity for the branched systems. This emphasizes that the enhancement of the 2PA cross section in the linear arrangements is probably caused by orbital interactions of higher excited configurations.
