The Consequences of Twisting Nanocarbons: Lessons from Tethered

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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The Consequences of Twisting Nanocarbons: Lessons from Tethered Twisted Acenes Published as part of the Accounts of Chemical Research special issue “Advanced Molecular Nanocarbons”. Anjan Bedi and Ori Gidron*

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Institute of Chemistry, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Jerusalem 91904, Israel 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 abovementioned 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.

1. INTRODUCTION Polycyclic aromatic hydrocarbons have dramatically changed the landscape of carbon-based materials in the past decades.1,2 These materials have opened doors to new technologies, with potential applications in many areas of materials science, including energy storage, catalysis, organic electronics, and spintronics.3 Obtaining access to nanocarbons of defined size and shape allows chemists to tailor their electronic, magnetic, and optical properties. An important subclass of polycyclic aromatic hydrocarbons are acenes (Figure 1a), being linearly fused benzenes, which can be considered one-dimensional nanographenes.4 In contrast to their angularly fused analogs, namely, helicenes, parent (unsubstituted) acenes are planar. However, upon substitution, acenes readily twist out of planarity, with the degree of dihedral twist defined as depicted in Figure 1b. For © XXXX American Chemical Society

example, rubrene (5,6,11,12-tetraphenyltetracene), 1, one of the most commonly studied organic semiconductors, is twisted by approximately 44° (Scheme 1).5 Pascal, who coined the term twistacenes (twisted acenes) for this family of molecules showing end-to-end twist (Figure 1a,b),6 stretched the twisting limit by synthesizing a highly twisted pentacene derivative with a 144° twist (2, Scheme 1),7 and this was followed by other highly twisted acenes in recent years.8,9 It is widely acknowledged that twisted acenes are more soluble and more stable compared to parent acenes, and for this reason they are commonly applied as organic semiconductors in organic field-effect transistors (OFETs) and organic light-emitting diodes (OLEDs).10 Twisting can also Received: May 22, 2019

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DOI: 10.1021/acs.accounts.9b00271 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

this Account, we address the effect of twisting on the properties of acenes and their larger analogs, using our recently introduced tethered twisted acenes as a model. For more comprehensive reviews of curved and twisted polycyclic aromatic molecules and cyclophanes, we refer the reader to the excellent publications of Stępień et al.,17 Nuckolls et al.,18 Mayor, Juricek et al.,19,20 Pascal,6 and Bodwell et al.21

2. SYNTHESIS AND STRAIN Enforcing conformational nonplanarity in aromatic rings often poses a great synthetic challenge, as it introduces considerable strain into the molecule. Various strategies have been adopted to meet this challenge.20 Cyclophanes, in which more than two atoms of an aromatic ring are incorporated into a larger ring system, have long been applied to gain access to such highly strained systems.21 For example, the strain energy of [2.2]paracyclophane 3 (Scheme 1), first isolated by Brown and Farthing and synthesized by Cram, is 31 kcal/mol, and the para carbon atoms of the benzene rings are bent 12.6° out of plane.22−24 Bodwell used the large aromatization energy of pyrene to drive the formation of highly bent polycyclic aromatic cyclophanes in which the bend is increased by shortening the alkyl tether.25 In addition, he also synthesized pyranophanes, tethered at the 1,6 positions, which resulted in C2 symmetric, inherently chiral cyclophanes 4 and 5 (Scheme 1).26,27 Recent synthetic developments by the Jasti/Bertozzi, Itami, and Yamago research groups have afforded the synthesis of highly strained cycloparaphenylenes ([n]CPPs) by initially forming an unstrained macrocyclic precursor, followed by reductive aromatization to form the final product. This approach was used to synthesize [5]CPP, which was found to have an overall strain energy of 119 kcal/mol (24 kcal/mol per phenylene unit).28,29 Cycloparaphenylenes opened the door for the recent synthesis of the first carbon nanobelt by Itami, followed by others.30−32 Norton and Houk calculated that acenes can be twisted relatively easily, with the twisting of anthracene to 40° (ca. 13° per ring) requiring less than 10 kcal/mol (ca. 3.3 kcal/mol per ring).33 As mentioned earlier, such twisting is commonly achieved by the addition of substituents to the backbone. Indeed, the substitution of acenes with phenyl rings at the peri positions can lead to significant backbone twist, as demon-

Figure 1. (a) Structure of twisted acenes: n = 1, anthracene; n = 2, tetracene; n = 3, pentacene, which are prone to back-and-forth twisting between the M and P helicities. (b) The dihedral angle (φ) between A−B−C−D represents the acene twist. (c) Helically locked tethered anthracene, in which a diagonal tether prevents racemization. (d) Schematic representation of tuning the dihedral twist in anthracene by changing the tether length. Reprinted with permission from ref 16. Copyright 2018 American Chemical Society.

affect other fundamental molecular properties: electronic, optical, chiroptical, and magnetic. For example, twisted πconjugated chromophores were recently applied as thermally activated delayed fluorescent materials,11 as nonlinear optical materials,12 and in photovoltaic devices.13 Understanding how twisting affects fundamental molecular properties is important for the design of materials with tailormade properties. However, achieving understanding poses a significant challenge. Twisting of acenes (and other πconjugated backbones) is usually achieved by the addition of substituents, making it difficult to isolate the net effect of aromatic core twisting from substituent effects, which are potentially very large in their own right. For example, while both anthracene and 9,10-diphenylanthracene have a planar anthracene core, the fluorescence quantum yield of the latter is significantly higher.14 An additional issue is the size of the rotational barrier: in many twistacenes the barrier to rotation is low, which prevents their study in the enantiopure form.15 We recently introduced helically locked twisted acenes in their enantiopure form (Figure 1c).16 In our twistacene series, the degree of torsion of the acene core is determined by the length of the tether introduced diagonally across the acene core (Figure 1d), thus allowing us to study the effects of acene twist independently of substituent and racemization effects. In Scheme 1. Structures of Cyclophanes and Twistacenesa

a

1, rubrene (5,6,11,12-tetraphenyltetracene); 2, highly twisted pentacene derivative; 3, [2.2]paracyclophane; 4 and 5, inherently chiral cyclophanes; 6 polysubstituted anthracene, 7, polysubstituted tetracene; 8, single bridge cyclophane; 9, triple decker cyclophane. B


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tether prevents twisting of the acene core and consequent racemization (Figure 2d, red trace).16

strated by Pascal for polysubstituted anthracene 6 and tetracene 7, which display core twisting of 63° and 97°, respectively (Scheme 1).8 An alternative approach, involving the incorporation of acenes into cyclophanes using a diagonal tether, has not been commonly applied. Otsubo et al. and Haenel et al. used this approach to obtain 2,6-tethered naphthalenes as either a single bridge cyclophane, 8, or triple decker cyclophane, 9, respectively, with a dihedral twist of up to 32°.34,35 Very recently, Bodwell et al.36 introduced contractive annulation as a new synthetic approach for strained cyclophanes and demonstrated this approach for the synthesis of asymmetric naphthalenophane. The work of Bodwell, Otsubo and Haenel, inspired us to helically lock pretwisted anthracene by using diagonal tethers of different lengths to achieve various backbone twist angles. For the formation of the tethered twistacene series, Ant-Cn, we found that the preorganization step was crucial to minimize the occurrence of competing intermolecular reactions during ether bridge formation (Figure 2a).16 The precursor,

3. THE EFFECTS OF CORE TWISTING 3.1. Electronic Properties

HOMO and LUMO energy levels and the corresponding gap between them (HLG) determine the color and reactivity of polycyclic aromatic materials and are also major factors (together with morphology) in determining the performance of these materials in electronic devices. The energy levels are tailored by modifying the size, shape, and substituents of the πconjugated backbone. However, the effect of deviation from planarity on these energy levels has seldom been explored. Exploring systems larger than acenes, Bodwell demonstrated that the HOMO level increases as the degree of bending increases in pyrenes.36 Recently, Müllen, Narita, and coworkers demonstrated that the band gap of graphene nanoribbons can be reduced by distorting them from planarity.37 It should be noted that there is a difference between twisting and bending a π-bond: while bending increases the distance between adjacent nodes in one plane and decreases the distance (and therefore increases the overlap) between adjacent nodes in the opposite plane, twisting increases the distance between adjacent nodes in both planes (Figure 3a). 38 Therefore, bending of C nv

Figure 2. (a) Schematic representation of the synthetic concept for tethered twistacenes. The preorganization induced by the substituents is followed by intramolecular tethering. (b) The structure of Ant-Cn. C6 = -(CH2)6-, C5 = -(CH2)5-, C4 = -(CH2)4-, or C3 = -(CH2)3-. (c) X-ray structures of the anthracene core for Ant-C3 and the corresponding end-to-end dihedral angles. The substituents are removed for clarity. (d) Calculated (B3LYP/6-31G(d)) relative energies required to twist the acene cores of Ant-C3 (red trace), AntC4 (orange trace), Ant-C5 (green trace), Ant-C6 (blue trace), and untethered anthracene (purple trace). Adapted with permission from ref 16. Copyright 2018 American Chemical Society.

Figure 3. (a) Representation of two adjacent p-orbitals in planar, bent, and twisted conformations. (b) Calculated (UB3LYP/631G(d)) HOMO and LUMO energy levels of anthracene with different end-to-end dihedral twists.

containing two phenyl substituents at the peri positions, one on each side of the anthracene, is pretwisted by 22° (calculated at the DFT/B3LYP/6-31G(d) level). Using this approach, we were able to synthesize a series of twisted anthracenes whose increasing backbone torsion was achieved using a hexyl to propyl tether (Figure 2b). The X-ray structure of the anthracene core displays backbone twisting of 22−38° (Figure 2c), with the corresponding calculated values being 22−43°, respectively. Enantiomeric conversion between the M and P helicities of Ant-Cn cannot occur easily because the calculated torsional energy (B3LYP/6-31G(d)) is very high (Figure 2d).16 A comparison of twisting from +43° to −43° in nontethered acenes and twistacenes shows that the maximum barrier for the former is low (1.5 kcal/mol), such that helical conversion occurs readily, resulting in racemization (Figure 2d, purple trace). By contrast, the strain energy for twisting AntC3 from +43° to −43° is 50 kcal/mol; thus the presence of a

symmetric cyclophane is expected to result in the formation of a dipole moment.39 However, both bending and twisting are expected to result in π and σ mixing, thus affecting the HOMO and LUMO energy levels. Nuckolls et al. introduced and applied contorted polycyclic aromatic systems as the active materials for organic electronic devices. In particular, helical ribbons of perylene diimides were applied as n-type semiconductors in OFETs,40 and were particularly successful as electron acceptors in organic solar cells41 and photodetectors with ultranarrow band.42 For twistacenes, one might expect that the HLG would increase with loss of planarity, as a result of reduced πconjugation. However, Norton and Houk calculated that the HLG does not increase but even slightly decreases upon core twisting, because the HOMO slightly increases while the LUMO slightly decreases (Figure 3b).33 This occurs because, for fused systems, even extensive end-to-end twisting distorts C


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Figure 4. (a) Experimental UV−vis spectra of Ant-Cn and of the untethered precursor anthracene, 5, measured in chloroform. NT = nontethered. (b) Calculated (TD-DFT/CAM-B3LYP/6-31G(d)) absorption spectra of Ant-Cn. (c) ϕf, kNR, and kf measured for Ant-Cn in chloroform vs the calculated (B3LYP/6-31G(d)) torsion angles. Reprinted with permission from ref 16. Copyright 2018 American Chemical Society.

the double bond out of planarity to only a small extent (13− 15°), thereby maintaining over 97% of the overlap between adjacent p-orbitals. The reason for the slight HLG decrease can be rationalized by orbital rehybridization arising from changed π and σ interactions leading to the π bond having a greater σ character.43,44 Additionally, helicenes (which are the angularly fused analogs of twistacenes) display a relatively high HLG. High HLG values are found even for long helicenes,45 which renders them less attractive candidates than twistacenes for organic semiconductor applications. Our electrochemical studies of tethered twistacenes reveal that the HOMO level increases only slightly with increasing twist, thus confirming the above-mentioned computational studies. For linear helical porphyrinic oligomers, the HLG was also reduced upon twisting, which was attributed mainly to an increase in the HOMO energy level.46 Norton and Houk have also observed that the already-significant spin contamination for heptacene increases upon twisting, which is an indication that the open shell singlet character should increase upon twisting.33 Overall, twisting linear polycyclic aromatic systems does not significantly reduce π-overlap and only slightly increases their HOMO and decreases their LUMO, resulting in an overall reduction in the HLG.

Figure 5. UV−vis spectra (0.1 mM, 25 °C, EtOH) of naphthalene derivatives. Taken from ref 48. Copyright 2013 American Chemical Society.

of a photon (fluorescence). A third pathway back to the ground state is the relaxation of the system to a triplet state via ISC. The triplet state is usually a long-lived state that decays over longer time scales (phosphorescence). The correlation between the deviation from planarity of aromatic backbones and ISC was previously reported on several occasions. It was previously observed that helicenes display faster ISC rates compared to their planar analogs (phenanthrene), with this finding attributed to increased spin−orbit coupling. Brédas et al. demonstrated that the magnitude of spin−orbit coupling is directly correlated with the degree of deviation from planarity.49 Recently, Hariharan et al. demonstrated a correlation between twisting and ISC, with a decrease in fluorescence quantum efficiency observed for linearly twisted perylene derivatives 12 and 13 (Scheme 2) together with an increase in the ISC.50 They observed that the rate of ISC (KISC) is 40 time faster for the more twisted 13 compared to 12. We found that the excited state behavior of acenes changes considerably with twist. For example, as core twist increases from 22° to 43°, the fluorescence quantum efficiency (Φf) decreases from 30% to 7% (Figure 4c, blue trace) and the rate of fluorescence (Kf) decreases from 0.15 to 0.06 ns−1 (Figure 4c, red trace). The cause of these dramatic changes is yet to be established, but the above-mentioned observed correlation between twisting and the ISC rate may also explain the trend observed for twisted acenes. In this respect, Norton and Houk calculated that, upon twisting anthracene from 0° to 90°, its S0−T1 energy gap decreases from 42 to 38 kcal/mol.33 Overall, there seems to be substantial evidence relating aromatic backbone twist to increased ISC.

3.2. Optical Properties

In accordance with the aforementioned decrease in the HLG, absorption spectra of Ant-Cn show a bathochromic shift as the degree of core twisting increases. We found excellent correlation between the calculated and experimental absorption spectra (Figure 4a,b).16 It should be noted that the dihedral angle between the phenyl substituents and the anthracene core decreases with increasing anthracene twist, resulting in increased π-orbital overlap.47 This serves as an additional factor contributing to the observed bathochromic shift. The π−π* transition consists of a clear vibronic pattern at all the twist angles studied, thus demonstrating that the anthracene backbone remains rigid even when twisted. Yamaguchi et al. observed that 1,8-bis(1-adamantyl)naphthalene, 10, whose backbone is twisted 28.4° out of plane, is bathochromically shifted by 0.51 eV compared to planar 1,8-dimethylnaphthalene 11 (Figure 5).48 Since both structures are substituted with two sp3 carbons in the 1,8 positions, this observation strengthens the conclusion that this bathochromic shift does indeed result from the acene backbone twist. The absorption of a photon from the ground state to the first excited state can be followed by decay back to the ground state achieved through internal conversion or through the emission D


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chiroptical behavior and helicity of enantiopure polythiophenes correlated with their activity in magneto-optic devices.53 Helicenes, which were extensively studied for their chiroptical properties, show modest chiroptical responses; however, by applying symmetry-based rational design principles, the chiroptical properties of helicenes can be improved significantly.54 This can be achieved by designing the chromophores so that their electric and magnetic transition dipole moments align in a parallel fashion, thus maximizing rotational strength (R) according to the equation

Scheme 2. Core Twisted Aromatics 12 and 13 and the Rate of Intersystem Crossing Measured for Each50

R = |μe ||μm | cos θ

(1)

where μe and μm are the electric and magnetic transition dipole moments, respectively, and θ is the angle between the μe and μm vectors. Unlike long helicenes, twistacenes often have low racemization barriers, which prohibit their study in enantiopure form. Therefore, until recently, the effect of twisting on the chiroptical properties of acenes was not explored. In a combined computational and experimental study, we investigated the effect of twisting on the chiroptical properties of acenes, using enantiomers of helically locked twistacenes AntCn that were separated using chiral HPLC.47 The electronic circular dichroism (ECD) spectra of Ant-C3 is displayed in Figure 6a (red trace). As the twist angle increases from 22° to 43°, the Cotton effect increases for the π−π* (1La) transition (Figure 6b). The spectral changes observed upon twisting are more pronounced in ECD spectra than in UV/vis spectra. For example, although the extinction coefficient (ε) is only slightly affected by increasing the twist from 22° to 43°, Δε increases by 76%. The anisotropy factor, gabs, is 0.015 for Ant-C6 and generally increases with increasing twist (Figure 6b).47 This anisotropy factor is thus larger than the corresponding values for the longer [5]helicene ((4−9) × 10−3) and for the larger [6]helicene (4.8 × 10−3) and is among the highest reported for small helical molecules.55 Similar effects on chiroptical properties are observed for the oxidized species. Upon oxidation (chemical or electrochemical), the stable radical cation of Ant-Cn can be formed.56 DFT calculations demonstrate that the gradual increase in backbone twist with increasing length persists in the radical cations, thus allowing us to study the effect of twisting on the

The discovery of a method to control the ISC rate would have significant consequences for several key optical factors. Controlling the gap between the triplet and excited singlet energy levels can lead to reverse ISC, a process that enables a theoretical internal quantum efficiency of 100% for all-organic materials and leads to efficient OLEDs without the need to include expensive transition metals, in a process known as thermally activated delayed fluorescence (TADF).11 In addition, acenes are considered benchmark materials for singlet fission, in which the absorption of one photon to the first excited state (S1) results in two molecules in the triplet state (Tn). Attaining singlet fission is expected to lead ultimately to more efficient organic solar cells.51 However, a prerequisite of its attainment is to engineer the singlet−triplet energy gap so that Tn < 2S1. Therefore, obtaining additional control of the S1 to Tn energy levels can result in the design of more efficient active materials for both photovoltaic and light emission applications, and the backbone planarity of the candidate should also be considered in this respect. 3.3. Chiroptical Properties

Twisting polycyclic aromatic molecules out of planarity results in the elimination of inversion symmetry, consequently giving rise to new chiroptical properties. Helically chiral π-conjugated materials have been applied as spin-filters in spintronic devices and in circularly polarized light responsive OLED detectors (CP-OLEDs).52 Recently, Swager et al. found that the

Figure 6. (a) ECD spectra of M and P enantiomers (solid and dashed lines, respectively) of Ant-C3 in the neutral (blue) and oxidized (green) states, measured in acetonitrile. Adapted with permission from ref 56. Copyright 2019 The Royal Chemical Society. (b) Computational (blue) and experimental (red) Cotton effects (solid lines) and anisotropy factors (dashed lines) for Ant-Cn. Adapted with permission from ref 47. Copyright 2019 Wiley. (c) Schematic representations of electric (μe, blue) and magnetic (μm, red) transition dipole moments of the 1La transition for anthracene at a 30° twist per benzene ring (top) and the 1Bb transition of [6]helicene (bottom). E


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Accounts of Chemical Research oxidized species. The ECD spectra for Ant-Cn•+ display positive transitions for the M enantiomer, similar to the π−π* transition in their neutral states (Figure 6a, blue trace), with the lower energy transitions at 500 and 600−750 nm clearly observed. The vibronic spacing for the HOMO − 1 → SOMO transitions of around 0.17 eV (∼1400 cm−1, corresponding to a CC stretch) indicate that the structure of the acene remains rigid also in the radical cation state.56 The Cotton effect for this low-energy transition shows a similar trend to that found for the neutral species. Unlike helicenes, which require a redox center (organometallic or organic) to act as chiroptical switches, twisted acenes can potentially function as chiroptical switches without the need for an additional redox center. As discussed above, twisting affects the chiroptical properties of both the neutral and oxidized (radical cation) species. To understand the origin of these effects, we computationally investigated the chiroptical properties of neutral parent acenes, anthracene to pentacene with various dihedral twists.47 Upon twisting anthracene from 5° to 30° per ring, μe decreases by approximately 5% while μm increases by 500%, resulting in a significant increase in the overall Cotton effect. The most significant difference, however, between twistacenes and helicenes was found to be the relative orientation between μm and μe: while in [n]helicenes (from [3] to [10] rings) the transition dipole moments are not parallel to each other, for twistacenes, these vectors always adopt a parallel or antiparallel orientation, regardless of the twist angle (Figure 6c). As the two vectors adopt a parallel orientation, the cos(θ) component in eq 1 reaches its maximal value. It is important to note that, since the diagonal tether imposes a C2 symmetric structure, regardless of the acene core twist, decreasing tether length also decreases the dihedral angle with the phenyl substituents, with this angle also potentially affecting chiroptical properties. To isolate the net effect of acene core twisting, we investigated the effect of twisting the phenyl while keeping the anthracene planar. We concluded that, whereas the observed bathochromic shift largely results from the phenyl twist, the chiroptical properties largely arise from the anthracene twist. Overall, recent computational and experimental results indicate that continuous twisting of the polycyclic aromatic core leads to a systematic increase in the Cotton effect, and therefore twistacenes can serve as excellent chromophores for chiroptical devices.

Figure 7. (a) EPR signals from Ant-Cn•+ obtained by electrochemical oxidation of the neutral parent molecules in acetonitrile solution (colored lines) compared with the simulated spectrum (black lines). (b) The change in hyperfine coupling vs anthracene twist angle for hydrogen pairs H2−H4 located on each of the terminal anthracene rings (for clarity, shown for only one terminal ring), R = 3,5bis(trifluoromethyl)benzyl. Reproduced from ref 56 with permission from The Royal Society of Chemistry.

for acene cation radicals. Three pairs of weak hyperfine coupling constants revealed by simulation data were assigned to the three pairs of hydrogens on each terminal ring.64 Subtle decreases in the hyperfine values, indicating slight increase of localization (i.e., loss of delocalization) as the twist angle increases (Figure 7b). This trend most likely arises from twisting leading to a reduction in the extent of p-orbital overlap in the π-conjugated system. Distal hydrogens H2 and H3 are most affected by twisting, whereas the more centrally located H4 atom is the least affected (Figure 7b). Interestingly, notable changes to hyperfine coupling constants with increasing twist angle are observed solely for a twist angle greater than 40° (13° per benzene ring). The relatively unchanging hyperfine constants found at smaller twist angles suggest that twisting of up to 13° per benzene ring could serve as a design principle for avoiding significant loss of delocalization in future nonplanar aromatic systems.

4. SUMMARY AND OUTLOOK Twisting polycyclic aromatic molecules out of planarity gives rise to changes in the electronic, optical, chiroptical, and magnetic properties. Using molecular tethering of pretwisted anthracene, we introduced a family of twistacenes characterized by different twist angles, allowing us to systematically monitor the effect of twisting on some of these properties. While research into these molecules is ongoing, the following properties are clearly affected.

3.4. Spin Delocalization

In polycyclic aromatic systems, charge delocalization is directly related to molecular morphology,57 in general, and to deviation from planarity, in particular. Twisting, in turn, affects aromaticity,58,59 energy levels,60 and π-orbital overlap.33,61 Although understanding and controlling these effects is prerequisite to designing nonplanar polycyclic aromatic compounds, the absence of a suitable experimental system previously precluded their systematic experimental study. Recently, this situation changed with the synthesis of the Ant-Cn•+ series, which provides a suitable model for electron paramagnetic resonance (EPR) spectroscopy investigations into how charge localization is affected by twist angle.62,63 All Ant-Cn•+ series members exhibited a signal during in situ EPR electrochemistry measurements in which acetonitrile served as the solvent and tetrabutylammonium perchlorate as the electrolyte (Figure 7a).56 Data analysis indicated localization of the electron primarily to the central ring, as is usual

4.1. Frontier molecular orbitals

Upon twisting, rehybridization of σ and π orbitals results in a small but consistent increase in the HOMO levels and decrease in the LUMO levels, leading to an overall decrease in the HLG. 4.2. Reduced Fluorescence Quantum Efficiency and Increased Intersystem Crossing

Twisting of Ant-Cn molecules significantly reduces their fluorescence quantum efficiency, with a subsequent decrease in fluorescence lifetime. Previous studies on twisted and bent aromatics demonstrated an increase in the ISC rate, most likely F


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Accounts of Chemical Research due to increased spin−orbit coupling, which can account for this effect.

chiral organic semiconductors, curved aromatic systems, and furancontaining materials for organic electronics.

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4.3. Chiroptical Properties

ACKNOWLEDGMENTS This research was supported by the Israel Science Foundation (Grant No. 1789/16). A.B. is supported by a PBC fellowship, Council for Higher Education, Israel.

Increased twisting results in a continuous increase in the Cotton effect and anisotropy factor. The anisotropy factors of Ant-Cn are considerably greater than those of longer helicenes. The reason was found to be the parallel orientation of the twistacenes’ electric and magnetic transition dipole moments, with this orientation rendering them excellent chiroptical materials.

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4.4. Spin Delocalization

Twisting the anthracene radical cation up to 40° does not significantly affect spin delocalization, with only slight localization found at twist angles greater than 13° per benzene ring. Many unanswered questions still remain. For example, how would cross tethering (two tethers, oriented in a diagonal manner rather than one), which allows D2 symmetry (rather than C2 symmetry), affect the properties of such systems, and would these effects follow the same trend with greater twist angle? The next step is to introduce extendable groups to tethered twistacenes, allowing the introduction of helically chiral conjugated polymers with tunable twist, as well as donor−bridge−acceptor systems with a tunable bridge. In conclusion, when designing a polycyclic aromatic molecule, the effect of backbone twist should not be overlooked. Indeed, it can be exploited to generate desirable properties for the future application of these materials in optical, electronic, and magnetic devices.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Anjan Bedi: 0000-0001-6418-4409 Ori Gidron: 0000-0002-7037-0563 Notes

The authors declare no competing financial interest. Biographies Anjan Bedi received his M.Sc. in Chemistry from IIT-ISM, Dhanbad, India, and his Ph.D. from IISER Kolkata, India, under the supervision of Dr. Sanjio S. Zade. In 2014, he joined the group of Prof. Guy Koeckelberghs as an F+ postdoctoral fellow at KU Leuven, followed by a postdoctoral position in Prof. Satish Patil’s group at the IISc, Bangalore. Since 2017, he is a postdoctoral researcher at the Hebrew University of Jerusalem, on a PBC fellowship in the group of Dr. Ori Gidron. His current research focuses on the helically twisted polyaromatics. Ori Gidron received his M.Sc. in Chemistry from the Weizmann Institute of Science (2007) under the supervision of Professor M. van der Boom. He then joined the group of Professor M. Bendikov at the Weizmann Institute of Science and received his Ph.D. in 2012, for which he was awarded the Dov-Elad Prize for Excellence in Chemical Research. In June 2013, he accepted a Marie Curie (IEF) Postdoctoral Fellowship to work in the group of Professor F. Diederich at ETH Zurich on supramolecular assemblies of carbonrich materials. In 2015, he became a senior lecturer at the Hebrew University of Jerusalem. In 2018, he received the Thieme Chemistry Journal Award for Excellent Young Researcher. His main interests are G


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