Boron-doped double [6]carbohelicenes: a combination of helicene and boron-doped π-systems.

Helicenes, featuring unique helical structures, have a long history as three-dimensional polycyclic aromatic hydrocarbons (PAHs). Incorporation of heteroatoms into helicenes may alter their electronic structures and achieve unexpected physical properties. Here, we disclose fusion of boron-doped π-systems onto helicenes as an efficient strategy to design boron-doped carbohelicenes. Two boron-doped double [6]carbohelicenes were synthesized, which possess the C58B2 and C86B2 polycyclic π-skeletons containing two [6]helicene subunits, respectively. The C86B2 molecule thus represents the largest-size helicene-based boron-doped PAH. A thorough investigation reveals that the helicene moieties and boron atoms endow the polycyclic π-systems with delocalized electronic structures, and well-tunable ground-state and excited-state photophysical properties. It is notable that the C58B2 molecule displays excited-state stimulated emission behavior and amplified spontaneous emission (ASE) properties in not only the blend films with various doped concentrations but also the pure film. To our knowledge, it is the first example of ASE-active [n]helicene (n ≥ 6), and moreover, such robust ASE performance has rarely been observed in PAHs, demonstrating the promising utility of boron-doped carbohelicenes for laser materials.

Diradicaloid Boron-Doped Molecular Carbons Achieved by Pentagon-Fusion.

Molecular carbons (MCs) are molecular cutouts of carbon materials. Doping with heteroatoms and constructing open-shell structures are two powerful approaches to achieve unexpected and unique properties of MCs. Herein, we disclose a new strategy to design open-shell boron-doped MCs (BMCs), namely by pentagon-fusion of an organoborane π-system. We synthesized two diradicaloid BMC molecules that feature C24 B and C38 B π-skeletons containing a pentagonal ring. A thorough investigation reveals that such pentagon-fusion not only leads to their local antiaromaticity, but also incorporates an internal quinoidal substructure and thereby induces open-shell singlet diradical states. Moreover, their fully fused structures enable efficient π conjugation, which is expanded over the whole frameworks. Consequently, some intriguing physical properties are achieved, such as narrow energy gaps, very broad light absorptions, and superior photothermal capability, along with excellent photostability. Notably, the solid of the C38 B molecule exhibits absorption that covers the range of 300-1200 nm and an efficiency of 93.5 % for solar-driven water evaporation, thus demonstrating the potential of diradicaloid BMCs as high-performance organic photothermal materials.

A Combination of B- and N-Doped π-Systems Enabling Systematic Tuning of Electronic Structures and Properties.

Doping heteroatoms into polycyclic aromatic hydrocarbons (PAHs) may alter their structures and thereby physical properties. This study reports the construction of B/N-codoped PAHs via combining the B- and N-doped π-systems. Two π-extended B/N-codoped PAHs were synthesized through the Mallory photoreaction. Both feature a C48 BN2 π-skeleton, which is assembled by linearly fusing three substructures including B-doped and sp2 -hybridized N-doped π-moieties and one pyrene unit. In comparison to the pristine B-doped analog, their intramolecular charge transfer (ICT) states are distinctly modulated by the fused N-doped π-system and the further incorporated cyano group, leading to their tunable optical properties, as revealed by detailed theoretical and experimental analysis. Furthermore, these three molecules have sufficient Lewis acidity and can coordinate with Lewis base to form Lewis acid-base adducts, and notably, such intermolecular complexation can further dynamically modulate their ICT transitions and thereby photophysical properties, such as producing blue, green and red fluorescence.

Photonic Modulation Enabled by Controlling the Edge Structures of Boron-Doped Molecular Carbons.

Control over topological edges of molecular carbons (MCs) is of importance for achieving diverse molecular topologies and desirable physical properties. However, it remains very challenging for heteroatom-doped MCs due to the synthetic difficulty. Herein, we report control over the edge structures of boron-doped MCs (BMCs) via the sequential cyclization strategy. Three BMC molecules that feature the C56 B2 or C84 B2 polycyclic π-skeletons with selective cove/fjord or cove/bay edges, respectively, were synthesized through the rational combination of Mallory photoreaction and Scholl reaction. We not only obtain the largest boron-doped π-system reported so far, but also disclose that fine control of their edges and length greatly affects electronic structures and thereby photonic properties of BMCs, such as tunable aromaticity, decreased band gaps, as well as redshifted absorptions and fluorescence. Remarkably, the C56 B2 molecule exhibits stimulated emission behavior and amplified spontaneous emission property, both of which have never been reported for pristine boron-doped π-systems, thus demonstrating the potential of BMCs as optical gain materials for laser cavities.

Ribbon-Type Boron-Doped Polycyclic Aromatic Hydrocarbons: Conformations, Dynamic Complexation and Electronic Properties.

Molecular ribbons (MRs), one-dimentional topological polycyclic aromatic hydrocarbons (PAHs), are of importance for synthetic chemistry and material sciences. Herein, we disclose an effective strategy to develop boron-doped MRs, i.e. photochemical cyclization on conjugated organoboranes for rapid π-extension. A series of ribbon-type boron-doped PAHs that own multiple cove edges were synthesized using Mallory photoreaction in solution. Two of them feature isomeric C68 B2 π-skeletons with 2.2 nm in length, thus representing a new kind of boron-doped MRs. The boron atoms endow them with sufficient Lewis acidity, and notably, the formed Lewis acid-base adducts based on boron-doped MRs display the photo-induced dual-dissociation behavior in the excited state and thus photochromism property. Moreover, despite of the highly contorted topological conformations, their potential utility as organic semiconductor was demonstrated by fabrication of organic field-effect transistors.

PO-containing dibenzopentaarenes: facile synthesis, structures and optoelectronic properties.

Incorporation of heteroatoms into polyarenes has been developed as an effective approach to alter their intrinsic structures and properties. Herein, we designed and synthesized two PO-containing dibenzopentaarene isomers (5a and 5b) and studied their structures and properties, along with those of dibenzopentaarenes containing six-membered Si- and B-heterocycles (3 and 4). These heterocyclic polyarenes have similar frameworks to well-known heptazethrene, and thus can be regarded as members of the heteroatom-doped zethrene system. The heterocycles greatly affect not only the molecular and packing structures but also the electronic structures and properties. Notably, while compounds 3 and 4 adopt almost planar geometries, 5a possesses a clearly curved conformation, leading to its brick-type slipped and dense π-π stacking mode. Moreover, the electron-withdrawing PO groups endow 5a and 5b with simultaneously lowered lowest unoccupied molecular orbital (LUMO)/highest occupied molecular orbital (HOMO) levels, whereas the p-π conjugation of the B atoms in 4 leads to its smaller energy gap and thus remarkably red-shifted absorption and fluorescence bands by over 80 nm, though all of these molecules possess similar closed-shell structures. This study thus deepens the understanding of heteroatom-doping effects, which may be expanded to develop other heteroatom-doped zethrene materials.

Tuning Diradical Properties of Boron-Containing π-Systems by Structural Isomerism.

Tuning diradical character is an important topic for organic diradicaloids. Herein, we report the precise borylation enabling structural isomerism as an effective strategy to modulate diradical character and thereby properties of organic diradicaloids. We synthesized a new B-containing polycyclic hydrocarbon that has the indeno[1,2-b]fluorene π-skeleton with the ß-carbons bonding to two boron atoms. Detailed theoretical and experimental results show that this bonding pattern leads to its distinctive electronic structures and properties in comparison to that of its isomeric molecule. This molecule has the efficient conjugation between boron atoms and π-skeleton, resulting in downshifted LUMO and HOMO levels. Moreover, it exhibits smaller diradical character and thereby inhibited diradical properties, such as significantly blue-shifted light absorption, larger energy bandgap and weak para-magnetic resonance. Notably, this B-containing polycyclic hydrocarbon possesses much stronger Lewis acidity and its Lewis acid-base adducts display enhanced diradical character, demonstrating the positive effects of Lewis coordination on modulating diradical performance.