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Helically-Locked Tethered Twistacenes Anjan Bedi, Linda J.W. Shimon, and Ori Gidron J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04447 • Publication Date (Web): 15 Jun 2018 Downloaded from http://pubs.acs.org on June 16, 2018

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Journal of the American Chemical Society

Helically-Locked Tethered Twistacenes Anjan Bedi,† Linda J. W. Shimon,‡ and Ori Gidron*† †

Institute of Chemistry, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Jerusalem, Israel. Chemical Research Support Unit, Weizmann Institute of Science, Rehovot, Israel. Supporting Information Placeholder ‡

ABSTRACT: Twisting linear acenes out of planarity affects their electronic and optical properties, and induces chirality. However, it is difficult to isolate the effect of twisting from the substituent effect. Moreover, many twistacenes (twisted acenes) readily racemize in solution. Here, we introduce a series of twistacenes having an anthracene backbone diagonally tethered by an n-alkyl bridge, which induces a twist of various angles. This allows us to systematically monitor the effect of twisting on electronic and optical properties. We find that absorption is bathochromically shifted with increasing twist, while fluorescence quantum efficiency drops dramatically. The tethered twistacenes were isolated to their enantiomerically pure form, displaying strong chiroptical properties and anisotropy factor (g-value). No racemization was observed even upon prolonged heating, rendering these tethered twistacenes suitable as enantiopure helical building units for π-conjugated backbones. Acenes can be viewed as one-dimensional graphene nanoribbons. They are a unique class of polyaromatic hydrocarbons that consist of only one Clar sextet regardless of the number of annulated rings.1 Consequently, their electronic, optical, and magnetic properties are strongly affected by increasing annulation, which leads to rapidly decreasing HOMO–LUMO gaps and increasing reactivity. They are considered the “brick and mortar” of organic electronics,2 and are embedded in active materials for organic field effect transistors, light emitting diodes, and for singlet fission in organic solar cells.3 Moreover, as higher acenes display a triplet ground state, they are potential candidates for spintronic devices.4 Nevertheless, acenes have low stability and solubility, which limits their application. 5 While parent acenes are planar, they are readily twisted out of planarity upon substitution. Twisted acenes (twistacenes), first introduced by Pascal,6 are more stable and soluble compared with their parent acenes.7,8 Twisting does not drastically hamper πconjugation,9 but it was previously assumed that twisting may significantly change key optical and electronic properties.10 For example, the absorption of twisted decaphenyl-anthracene is bathochromically shifted compared with parent anthracene, while its fluorescence quantum efficiency (ϕf) is reduced.11 However, since different numbers of substituents were always involved in inducing different twisting angles, the effect of twisting on optical and electronic properties could not be studied directly. The fluxional nature of most twistacenes results in back-and-forth twisting between the M and P helicities (Figure 1a), which consequently exist as racemic mixtures. This hinders efforts to obtain enantiomerically pure twistacenes.12 Although the chirality aspect of twistacenes is often overlooked in studies of their solid state morphology, thin films of rubrene were found to have micro domains containing P and M enantiomers.13 Thus, efforts to study specific helicity are confounded both in solution and in the solid state. When enantiopure helical acene backbones are obtained in

their enantiopure form, they can potentially give rise to circularly polarized fluorescent materials, nonlinear optical materials, and recently also spin filters.14,15,16 Thus, a synthetic route to enantiopure twistacenes is of interest both for their potential properties and as a means to isolate and investigate systematically the effects of twisting on those properties. To overcome these challenges, we envisioned a series of acenes whose backbone can be “locked” at a specific helicity and torsion angle. This approach should: (i) allow the isolation of stable M and P enantiomers; and (ii) keep the number of substituents around the acene backbone constant, thus allowing us to monitor the effect of twisting, while circumventing the substituent effect. Backbone locking can be achieved by molecular tethering, such as was previously applied to achieve bending of larger polyaromatic hydrocarbons (pyrenes and perylenes) by Bodwell, Würthner,17,18 as well as for other systems.19 Such tethering is expected to fix the acene backbone in a specific helicity, so preventing racemization (Figure 1b). Here we introduce a family of helically-locked tethered twistacenes having an acene backbone and a propyl- to hexyl- tether (Figure 1c). By varying the tether length, we induce different torsion angles in the acene core while keeping the substitution pattern around the acene backbone constant, so allowing a systematic study of the effect of twisting on different properties. We observe significant changes in key properties, such as absorption, fluorescence lifetime, and fluorescence quantum efficiency, with twist angle. Finally, the M and P enantiomers were isolated, and demonstrated to be stable in their enantiopure form even upon prolonged heating.

Figure 1. Schematic representation of (a) non-tethered anthracene core, which is prone to back-and-forth twisting between the M and P helicities and (b) helically-locked tethered anthracene. (c) The tethered twistacenes described in this work. Cn = n-propyl to nhexyl. In order to twist (rather than bend) the acene core, the tethering is applied diagonally via the 1,5 positions of anthracene. To the best of our knowledge, tethering of the 1,5 positions was not reported prior to this work.20 To prevent undesired deplanarization of the carbons at the 1,5 positions, the bridge is connected from the ortho position of the anisole.18 To this end, the iodo-substituted 1,5 positions of anthraquinone 1 were coupled with o-anisole boronic

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acid 2. The addition of 3,5-bis(trifluoromethyl)phenyllithium 4 (produced in situ from 3,5-bis(trifluoromethyl)bromobenzene and butyllithium) was followed by deoxygenation with NaI and TMSCl to form 5.21 The methoxy groups were then cleaved with BBr3 to afford 6 (in both syn and anti conformations).22 Finally, Williamson ether synthesis of 6 with dibromoalkanes of different lengths (npropane to n-hexane) resulted in a series of tethered anthracenes Ant-Cn (n = 3–6; Scheme 1) as a mixture of M and P enantiomers. Scheme 1. Synthesis of Ant-Cna

aConditions: (a) 2, Pd(PPh ) 3 4 (i.e., Pd[P(C6H5)3]4), Na2CO3, dioxane/water, 46 %; (b) i. 4, THF, reflux; ii. NaI, (CH3)3SiCl, 0 °C RT, 25 %; (c) BBr3, 0 °C, 55 %; (d) CH2Br(CH2)n-2CH2Br (n=3–6), KOH, DMSO, 12-24 %.

While the non-tethered anthracenes 4 and 5 were found to be prone to photooxidation, the tethered products, Ant-Cn, were significantly more stable, with the highest stability observed for Ant-C3.23 All compounds were fully characterized by NMR (see SI Section S3). The bridging methylene ether protons display two different chemical shifts, as expected for diastereotopic protons, and are shifted upfield from 3.5 ppm (Ant-C6) to 2.9 ppm (AntC3) upon twisting. All Ant-Cn compounds display the expected C2 symmetry in the NMR timescale. Variable temperature NMR (VTNMR) shows no significant changes (no broadening) upon heating to 120 °C, indicating the conformational stability of the tethered acenes. This is in contrast to the non-tethered 5, in which arene rotation can be observed upon heating (SI Section S4). The X-ray structures, which were solved for all members of the Ant-Cn series, confirm the expected 1,5 tethered structure with varying degree of twist (Figure 2). The systematic increase in twist angles affirms the effect of the shorter tether on the anthracene core, with the torsion angle increasing from 23° for Ant-C6 to 38° for Ant-C3. As we aimed to monitor the twisting of the anthracene core, it was important to verify the absence of significant deplanarization of the sp2 carbons in the 1 and 5 positions. Indeed, the C4-C1-C1 angle (Figure 2a) changes by merely 3° from Ant-C6 (10°) to Ant-C3 (13°). The 13C-1H coupling supports this observation, with JC-H for the 2–4 positions on the anthracene core in the range of 157–163 Hz for all Ant-Cn, indicating no pyramidalization in these carbons (Table S2, see SI). 24

Figure 2. X-ray structures of (a) Ant-C6, (b) Ant-C5, (c) Ant-C4, (d) Ant-C3. Hydrogens are omitted for clarity. The anthracene core for each structure is depicted beneath each X-ray structure, with the substituents removed for clarity. Acenes have a typical UV-Vis absorption spectrum composed mainly of β and p (1La state) bands.25 Upon twisting, an increase in the intensity of the p band is clearly observed. This increase can be explained by the p band being vibronically allowed, and therefore increasing with decreasing symmetry (Figure 3a). Such a symmetry decrease is expected upon twisting the anthracene backbone. 10 Additionally, a bathochromic shift, from 407 nm for untethered 5 to 427 nm for Ant-C3, is observed as twisting increases. It was previously noted that distortion of π-conjugation results in a decrease of the LUMO for curved aromatics.26 Indeed, while the calculated HOMO shows only small variations, as also verified experimentally by cyclic voltammetry measurements (Figure S61 and Table S1, see SI), the calculated (B3LYP/6-31G(d)) LUMO systematically decreases from -2.07 for 5 to -2.24 for Ant-C3 (Table S1, see SI). Time-dependent density functional theory (TDDFT) calculations (CAM-B3LYP/6-31G(d)) support the observed experimental trends of increased p-band intensity and bathochromic shift with increasing twist angle (Figure 3b). In addition to the abovementioned trends, the vibronic spectra, which is almost featureless for 5, become more distinct for the tethered molecules, as expected for a more rigid backbone. Figure 3c describes the change in photophysical properties that occurs with increasing twist of the anthracene backbone. The fluorescence quantum efficiency (ϕf) decreases from 0.3 for AntC6 to 0.07 for Ant-C3 (Figure 3c). This decrease can be rationalized by the decrease in the rate of radiative decay (Kf) with increasing torsion angle from 0.15 ns-1 for Ant-C6 to 0.06 ns-1 for Ant-C3.27 The rate of non-radiative decay (KNR) increases from 0.35 ns-1 for Ant-C6 to 0.81 ns-1 for Ant-C3 (See Table S1 in the SI for full details). Similar trends of bathochromically shifted absorption and decrease in ϕf were previously reported for twisted decaphenylanthracene.11 However, in that case, the effect could also have stemmed from increasing substitution. Our current study leads us to conclude that the decrease in ϕf is not related to the fluxional nature of different substituents, but is likely to be related


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Journal of the American Chemical Society directly to twisting of the anthracene backbone. Similarly, the bathochromic shift is not a result of increasing substitution, but rather arises from the increasing twist of the acene core, which might be related to increasing rate of intersystem crossing.10 To verify that the abovementioned trends are not significantly affected by the oxygen atoms, the derivative of Ant-Cn, for which oxygen atoms are replaced with methylene groups were investigated computationally (TD-DFT). The same trends are also obeserved for these derivatives (the increase in intensity and bathochromic shift for the p-band upon increasing twist), indicating that the photophysical properties are not significantly influenced by the nature of the tether and likely to result directly from the acene twist (Figure S66, see SI).

barriers not exceeding 1.5 kcal mol-1, twisting of Ant-C3 in that range requires nearly 50 kcal mol-1. Thus, the tethering prevent twisting of the acene core to the opposite helicity. This is in contrast with various non tethered acenes, for which the helicity readily inverts, resulting in racemization.19b In order to investigate the chiroptical properties of twistacenes, Ant-C5 was isolated to its M and P enantiomers using preparative chiral HPLC. The electronic circular dichroism (ECD) spectra of Ant-Cn show relatively strong Cotton effects for the p band (e.g., Δε = 30 M-1cm-1 for Ant-C5), whose vibronic pattern is clearly observed, in contrast to the ECD for helicenes. The anisotropy factor (g factor) for Ant-C5 is 2×102 (Figure 4b, inset) which is considerably higher than the values measured for [6]helicene and [5]helicene (4-9×10-3) or for larger hexapole helicene (4.8×10-3).28 The compounds remain enantiomerically pure even upon heating to 100 °C for 48 h, with no apparent change in the ECD spectrum (Figure S65, see SI). To the best of our knowledge, this is the first example of enantiomerically-stable helical anthracene core.

Figure 4. (a) Calculated (B3LYP/6-31G(d)) relative energies required to twist the acene cores of Ant-Cn and of the untethered precursor anthracene, 5 (torsion angle of ABCD positions). (b) Electronic circular dichroism (ECD) spectra for the M and P enantiomers of Ant-C5 measured in chloroform. Inset: g-factor for Ant-C5 measured in chloroform.

Figure 3. (a) Experimental UV-Vis spectra of Ant-Cn and of the untethered precursor anthracene, 5, measured in chloroform. (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. The calculated torsion energies (B3LYP/6-31G(d)) for Ant-Cn demonstrate that twisting between the M and P helicities requires very high energies (Figure 4a). For example, while the non-tethered acene 5 can twist back and forth in the range of +43° to -43° with

In conclusion, we have introduced tethered twistacenes as conformationally stable acenes, with controlled degrees of twisting. This family of molecules allows us to monitor the effect of backbone twisting on different properties. We found that upon twisting of the anthracene backbone: (i) the p band increases in intensity, and decrease in energy; (ii) the fluorescence quantum efficiency decreases; (iii) the fluorescence decay rate decreases; and (iv) the non-radiative decay rate increases. The resulting enantiomers of Ant-C5 were isolated and exhibited a relatively intense g-factor with no racemization even upon prolonged heating. We believe that these systems can serve as enantiopure helical conjugated building units possessing controlled degrees of



twisting. We are currently investigating the effect of backbone helicity on various electronic, optical, and magnetic properties of these molecules.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures; spectral data; Xray structural data, details of electrochemical measurements and computational details (PDF).

AUTHOR INFORMATION Corresponding Author *[email protected]

ACKNOWLEDGMENT This research was supported by the Israel Science Foundation (grant No. 1789/16). A.B. is supported by a PBC fellowship. We thank Dr. Benny Bogoslavsky for his help in recording X-ray structures.

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