The Consequences of Twisting Nanocarbons: Lessons from Tethered Twisted Acenes

Abstract

Conspectus The properties of polycyclic aromatic hydrocarbons are determined by their size, shape, and functional groups. Equally important is their curvature, since deviation from planarity can affect their optical, electronic, and magnetic properties and also induce chirality. Acenes, which can be viewed as one-dimensional nanocarbons, are often twisted out of planarity. Although twisting is expected to affect the above-mentioned properties, it is often overlooked. This Account focuses on helically locked twistacenes (twisted acenes) having different twist angles and the effect of twisting on their electronic and optical properties. Various synthetic approaches to inducing backbone twist in acenes are discussed, with a focus on the introduction of a diagonal tether across the core, as this minimizes confounding substituent effects. Using such tethered acenes as our model, we then discuss the effects of twisting the aromatic core on twistacene properties. Electronic properties. Increasing the degree of twist only slightly affects the HOMO and LUMO energy levels. Twisting leads to a small increase in the HOMO level and a decrease in the LUMO level, which produces an overall decrease in the HOMO–LUMO gap. Optical properties. As the degree of twist increases, a slight bathochromic shift is observed in the absorption spectra, in accordance with the decrease in the HOMO–LUMO gap. The fluorescence quantum efficiency and the fluorescence lifetime also decrease. This is likely to be related to an increasing rate of intersystem crossing, which arises from increased spin–orbit coupling. In addition, computational studies indicate that the S0–T1 energy gap decreases with increasing twist. Chiroptical properties. Increased twisting results in a larger Cotton effect and anisotropy factor, with the anisotropy factors of Ant-Cn being higher than those of longer helicenes. The parallel orientation of electric and magnetic transition dipole moments in twistacenes underlies this behavior and renders them as excellent chiroptical materials. The same trend is observed for the radical cations of twistacenes, which absorb in the NIR spectral region. Conjugation and delocalization. Twisting the anthracene radical cation up to 40° (13° per benzene ring) does not significantly affect spin delocalization, with the EPR spectra of twistacene radical cations showing that only slight localization occurs. This is in line with computational studies, which show only a small decrease in π-overlap for large acene twist. Overall, modifying the length of the tether in diagonally tethered acenes allows chemists to control core twist and to induce chirality. Twisting affects key optical, electronic, and chiroptical properties of acenes. Consequently, controlling the twist angle can improve the future design of nanocarbons with desired properties.

Multi‐Stage Redox Systems Based on Dicationic P‐Containing Polycyclic Aromatic Hydrocarbons

“…S13). This is most likely related to the non-planar character of the -core of 6[OTf]2 , [15] and is in line with the specific NICS(0) value noted for the antiaromatic P-rings (Fig. S12).…”

“…This first communication highlights the great potential of these novel organophosphorus derivatives for further "viologene-like" switching applications, taking advantage of the fluorescence of 2-6[OTf]2 in solution and in the solid-state. [18] In addition, the simplicity of the synthetic approach paves the way toward the preparation of a virtually unlimited panel of redox-active Pderivatives based on (twisted) acenes, helicenes, nanographenes etc [9,15,19] 3460 3480 3500…”

Highly Twisted Aryl‐Anthraquinodimethanes: Synthesis, Characterization, and Fluorescence Sensing of TNT

“…Numerous reaction conditions were attempted, including the use of various solvent systems (THF/water, dioxane/water, toluene/water, and toluene/ ethanol/water), different bases (K 2 CO 3 , Ba(OH) 2 , CsF, K 3 PO 4 , and NaOAc), and catalysts (Pd(PPh 3 ) 4 , Pd(PPh 3 ) 2 Cl 2 /PPh 3 ). The major challenge in this synthesis is the significant deborylation of the starting material, 9-anthraceneboronic acid (6), which resulted in the formation of anthracene as the main product. It is also interesting to observe that a few sets of conditions facilitated the formation of partially cross-coupled products.…”

“…Carbon nanomaterials based on polycyclic aromatic hydrocarbons (PAHs) have captured considerable attention in recent years, owing to their challenging synthesis, intriguing properties, and promising applications in advanced electronic and luminescent devices. [1][2][3][4][5][6] Of many PAH structures, 9,10-anthraquinodimethane (AQ) serves as a useful molecular building block for the generation of novel organic π-conjugated materials. [7][8][9][10] Upon substitution, the AQ moiety usually prefers a non-planar, butterfly-shaped molecular conformation.…”

A DFT study to probe homo-conjugated norbornylogous bridged spacers in dye-sensitized solar cells: an approach to suppressing agglomeration of dye molecules

“…However, there are reports that strained systems can induce lower HLG with the raising HOMO level. 49 The decrease in the HLG in strain systems can be rationalized by orbital rehybridization developing from the alteration in s & p orbital interactions. 43 In general, oxa-norbornadiene and oxa-sesquinorbornatriene spacers (1, 3 & 5) the -C-O bond lengths are relatively shorter than the norbornadiene and sesquinorbornatriene (2, 4 & 6) -C-Cbond lengths and hence can cause more strain than the former systems (1, 3 & 5).…”