Tullimilli Y. Gopalakrishna

Fully Fused Quinoidal/Aromatic Carbazole Macrocycles with Poly-radical Characters.

While the chemistry of open-shell singlet diradicaloids has been successfully developed in recent years, the synthesis of π-conjugated systems with poly-radical characters (i.e., beyond diradical) in the singlet ground state has been mostly unsuccessful. In this study, we report the synthesis and isolation of two fully fused macrocycles containing four (4MC) and six (6MC) alternatingly arranged quinoidal/aromatic carbazole units. Ab initio electronic structure calculations and various experimental measurements indicate that both 4MC and 6MC have an open-shell singlet ground state with moderate tetraradical and hexaradical characters, respectively. Both compounds can be thermally populated to high-spin excited states, resulting in weak magnetization at room temperature. Our study represents the first demonstration of singlet π-conjugated molecules with poly-radical characters and also gives some insights into molecular magnetism in neutral π-conjugated polycyclic heteroarenes.

Extended Bis(benzothia)quinodimethanes and Their Dications: From Singlet Diradicaloids to Isoelectronic Structures of Long Acenes

Extended bis(benzothia)quinodimethanes and their dications were synthesized as stable species. The neutral compounds mainly have a quinoidal structure in the ground state but show increased diradical character with extension of the central quinodimethane unit. The dications exhibit similar electronic absorption spectra, NMR spectra, NICS values, and diatropic ring currents to their aromatic all-carbon acene analogues and thus can be regarded as genuine isoelectronic structures of pentacene, hexacene, and heptacene, respectively. Our research gave some insights into the design and synthesis of stable longer acene analogues.

Fluorenyl Based Macrocyclic Polyradicaloids

Synthesis of stable open-shell polyradicaloids including control of intramolecular spin-spin interactions is a challenging topic in organic chemistry and materials science. Herein, we report the synthesis and physical characterization of two series of fluorenyl based macrocyclic polyradicaloids. In one series (FR-MCn, n = 4-6), the fluorenyl radicals are directly linked at 3,6-positions; whereas in the other series (MC-FnAn, n = 3-5), an additional ethynylene moiety is inserted between the neighboring fluorenyl units. To access stable macrocyclic polyradicaloids, three synthetic methods were developed. All of these stable macrocycles can be purified by normal silica gel column chromatography under ambient conditions. In all cases, moderate polyradical characters were calculated by restricted active space spin-flip method due to the moderate intramolecular antiferromagnetic spin-spin interactions. The excitation energies from the low-spin ground state to the lowest high-spin excited state were evaluated by superconducting quantum interference device measurements. Their physical properties were also compared with the respective linear fluorenyl radical oligomers (FR-n, n = 3-6). It is found that the geometry, i.e., the distortional angle and spacer (w or w/o ethynylene) between the neighboring fluorenyl units, has significant effect on their polyradical character, excitation energy, one-photon absorption, two-photon absorption and electrochemical properties. In addition, the macrocyclic tetramers FR-MC4 and MC-F4A4 showed global antiaromatic character due to cyclic π-conjugation with 36 and 44 π-electrons, respectively.

Cyclopenta Ring Fused Bisanthene and Its Charged Species with Open-shell Singlet Diradical Character and Global Aromaticity/Anti-Aromaticity

Cyclopenta ring fused bisanthene and its charged species were synthesized. The neutral compound has an open-shell singlet ground state and displays global anti-aromaticity. The dication also exhibits singlet diradical character but has a unique [10]annulene-within-[18]annulene global aromatic structure. The dianion is closed-shell singlet in the ground state and shows global aromaticity with 22 π electrons delocalized on the periphery. These findings prrovide new insight into the design and properties of global aromatic/anti-aromatic systems based on π-conjugated polycyclic hydrocarbons.

Conformationally Flexible Bis(9‐fluorenylidene)porphyrin Diradicaloids

A stable 5,10-bis(9-fluorenylidene)porphyrin (Por-Fl) diradicaloid was synthesized. It shows a quinoidal, saddle-shaped geometry in the single crystal but can be thermally populated to a triplet diradical both in solution and in the solid state. Coordination with the Ni2+ ion (Por-Fl-Ni) does not significantly change the contorted conformation but reduces the singlet-triplet gap. Heat-induced geometric change can explain the observed paramagnetic properties as well as unusual hysteresis in SQUID measurements. On the other hand, protonation (Por-Fl-2H+ ) dramatically changes the conformation while maintains the closed-shell electronic structure. Our studies demonstrate how heat, coordination, and protonation affect the geometry, diradical character, and physical properties of conformationally flexible open-shell singlet diradicaloids.

A Three‐Dimensionally π‐Conjugated Diradical Molecular Cage

π-Conjugated molecular cages are very challenging targets in structural organic chemistry, supramolecular chemistry, and materials science. The synthesis and physical characterizations are reported of the first three-dimensionally π-conjugated diradical molecular cage PTM-C, in which two polychlorotriphenylmethyl (PTM) radicals are linked by three bis(3,6-carbazolyl) bridges. This cage compound was synthesized mainly by intermolecular Yamamoto coupling followed by deprotonation and oxidation. It is stable and its structure was confirmed by X-ray crystallographic analysis. The two carbon-centered PTM radicals are weakly coupled through electronic interactions with the carbazole spacers, as revealed by optical, electronic, and magnetic measurements as well as theoretical calculations.

Ambient Stable Radical Cations, Diradicaloid π-Dimeric Dications, Closed-Shell Dications, and Diradical Dications of Methylthio-Capped Rylenes

Radical cations and dications of π-conjugated systems play vital roles in organic electronic devices, organic conductors, and conducting polymers. Their structures, charge and spin distribution, and mechanism of charge transport are of great interest. In this article, radical cations and dications of a series of newly synthesized methylthio-capped rylenes were synthesized and isolated. Their ground-state structures, physical properties, and solid-state packing were systematically investigated by various experimental methods, such as X-ray crystallographic analysis, UV/Vis/NIR absorption spectroscopy, (spectro-)electrochemistry, nuclear magnetic resonance spectroscopy, electron spin resonance spectroscopy, superconducting quantum interference device, and Raman spectroscopy, assisted by DFT calculations. It was found that all the charged species show an exceptional stability under ambient air and light conditions due to the efficient spin and charge delocalization over the whole rylene backbone. The dication of hexarylene turned out to have an unusual open-shell singlet rather than closed-shell ground state, thus it can be described as a diradical dication. Dimerization was observed for the radical cations and even the dications in crystals due to the strong intermolecular antiferromagnetic spin-spin interaction and π-π interaction, which result in unique magnetic properties. Such intermolecular association was also observed in solution.

Graphene‐like Molecules with Four Zigzag Edges

An efficient synthetic method toward graphene-like molecules (GLMs), having four zigzag edges, is described. They were obtained as stable materials and their structures were confirmed by X-ray crystallographic analysis. They exhibit topology- and size-dependent electronic properties and global aromaticity, which are all different from GLMs having either all-armchair edges, or three zigzag edges, or two armchair/two zigzag edges. They can be reversibly oxidized and reduced into stable charged species, which show fragmental aromatic character to minimize anti-aromaticity. Our studies give some new insights into the electronic structures and properties of a new type of rarely studied GLMs.

B–N–B Bond Embedded Phenalenyl and Its Anions

Despite the great potential of heteroatom-containing polycyclic aromatic hydrocarbons in organic optoelectronics, there are very limited reports on heteroaromatics containing a B-N-B bond in the π-scaffold. Herein, stable 1,9-dibora-9a-azaphenalenyl (DBAP) derivatives, named BNB-embedded phenalenyls, are presented. The DBAP skeleton contains a three-center two-π-electron B-N-B moiety with 12 π-electrons and can be regarded as the isoelectronic structure of the phenalenyl cation. Chemical reduction of the phenyl derivative of DBAP by potassium generated the dianion containing 14 delocalized π-electrons, which can be regarded as the isoelectronic structure of the phenalenyl anion. The dianion is sandwiched and stabilized by two bulky [K([18]crown-6)]+ counterions according to its X-ray structure. However, its monoanion (an isoelectronic structure of the henalenyl radical) generated by mixing equal moles of neutral compound and dianion gave an unusual B-N-B-embedded benzo[cd]fluoranthene dianion, which again was confirmed by X-ray crystallographic analysis. The new dianion containing 20 π-electrons is highly aromatic and is further stabilized by [K([18]crown-6)]+ counterions. An aromaticity driven rearrangement mechanism was proposed for this unusual transformation. Based on X-ray structures and theoretical calculations, the B-N-B moiety in the neutral and anionic DBAP participates in the π-electron delocalization along the whole DBAP skeleton like their phenalenyl cation/radical/anion counterparts, but with more localized character. Therefore, our studies report the first synthesis and characterization of a B-N-B-embedded phenalenyl and its anionic species, which show unique electronic structure and unusual reactivity different from that of their all-carbon phenalenyl analogues.

BODIPY-Based Antiaromatic Macrocycles: Facile Synthesis by Knoevenagel Condensation and Unusual Aggregation-Enhanced Two-Photon Absorption Properties

Two stable boron dipyrromethene (BODIPY)-based antiaromatic macrocycles, Mc-Fur and Mc-Th, were synthesized through a one-pot Knoevenagel condensation reaction between a BODIPY precursor and furan-2,5-dicarboxaldehyde or thiophene-2,5-dicarboxaldehyde, respectively. 1 H NMR spectroscopic characterization of the two macrocycles supported their highly antiaromatic character. The oxidation properties of the two macrocycles were studied through electron spin resonance spectroscopy and UV/Vis absorption spectrophotometry, which suggested the formation of a stable monoradical cation species on first oxidation followed by an aromatic dicationic species on subsequent oxidation. Both molecules have a nearly planar π-conjugated backbone and show a strong tendency to aggregate in solution due to efficient stacking of the antiaromatic macrocycles. Transient absorption and two-photon absorption (TPA) measurements in solution and aggregated states of the macrocycles revealed that aggregation resulted in large enhancement of TPA cross sections and increased excited-state lifetimes, in accordance with the decrease in the antiaromatic character in the aggregated state.