Smectic-A and Hexatic-B Liquid Crystal Phases of Sanidic Alkyl

Article pubs.acs.org/JPCB

Smectic‑A and Hexatic‑B Liquid Crystal Phases of Sanidic AlkylSubstituted Dibenzo[fg,op]naphthacenes Paul J. Repasky,† Deña M. Agra-Kooijman,‡ Satyendra Kumar,*,‡ and C. Scott Hartley*,† †

Department of Chemistry & Biochemistry, Miami University, Oxford, Ohio 45056, United States Department of Physics, Kent State University, Kent, Ohio 44242, United States

‡

S Supporting Information *

ABSTRACT: Despite longstanding interest in liquid crystalline compounds with simple rod- or disc-like shapes (calamitics or discotics), very few examples of the analogous board-shaped, or “sanidic”, liquid crystals exist. A new series of alkyl-substituted dibenzo[fg,op]naphthacenes have been prepared by planarization of o-phenylene precursors through dehydrohalogenation. Their photophysical properties have been studied in dichloromethane. Liquid crystal phase behavior was characterized by polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction. All of the compounds exhibit monotropic liquid crystal phases on cooling from the isotropic phase. The compounds with shorter alkyl (pentyl and heptyl) chains exhibit the uniaxial smectic-A phase analogous to that of simple calamitic mesogens. The compounds with longer alkyl (nonyl, undecyl, and tridecyl) chains exhibit a new smectic liquid crystal phase featuring short-range positional order with an apparent rectangular lattice in the smectic layers, that is, an orthogonal biaxial hexatic-B. The molecular arrangement in this phase likely corresponds to a distorted herringbone packing of the board-shaped structures. Further, the compound with nonyl chains exhibits an underlying smectic-B phase. DFT calculations show that the cores of the mesogens are twisted into C2-symmetric saddle-shaped geometries because of steric interactions along their rims. The liquid crystal phases and their structures are discussed in the context of the compounds’ board-like shapes and intercore interactions.

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INTRODUCTION

Recently, we reported the synthesis and properties of a series of compounds, DBN(OCn) (Chart 1), obtained by photo-

Molecular shape anisotropy is among the most important criteria in the design of new liquid crystals.1 The best examples of this are rod-like (calamitic) and disc-like (discotic) compounds, which comprise the great majority of known mesogens. Investigation of other shapes, such as bent core,2 is an active area of research, with many examples of new and unusual phases. However, small molecules with board-like shapes, one of the simplest possibilities, remain largely unexplored, even though the resulting “sanidic” phases have received extensive theoretical attention3−8 and have long been proposed for applications that exploit, for example, the very fast electrooptical switching of the biaxial nematic9 phase. While uncommon, some examples of small-molecule boardshaped mesogens have been reported. Early examples of metallomesogens from Ghedini,10,11 and others,12 exhibit phases typical of calamitics. In one case, the board-like molecular shape was thought to give rise to macroscopic biaxiality,13 although this property was not further confirmed. Purely organic board-like compounds are typically found to behave as discotics,14,15 albeit occasionally with unusual lamello-columnar phases.16−19 To the best of our knowledge, board-shaped compounds have not yet been shown to exhibit the simple biaxial nematic and (noncolumnar) smectic phases sought for sanidic mesogens. © 2016 American Chemical Society

Chart 1

chemical planarization of an o-phenylene tetramer.20 Unlike other reported dibenzo[fg,op]naphthacenes (DBNs),21−23 these compounds exhibit exclusively the smectic-A (SmA) and -C (SmC) phases, with no evidence of columnar stacking. While these phases are typical of calamitics, some unusual observations suggested that the board-like molecular shapes have a significant effect on their properties: the SmA−SmC phase transition is first-order and is accompanied by a sharp Received: November 9, 2015 Revised: January 20, 2016 Published: February 25, 2016 2829

DOI: 10.1021/acs.jpcb.5b10990 J. Phys. Chem. B 2016, 120, 2829−2837

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The Journal of Physical Chemistry B change in birefringence. Taken together, these observations suggest a “double biaxial” SmC phase, which, unlike the traditional SmC phase of tilted rods, features stronger core− core interactions with restricted rotation about the molecular long axes.18,24 This possibility of strong intercore correlations in smectic phases has great potential for applications in semiconducting films, as such phases should combine the effective interarene charge transport observed for columnar phases with enhanced defect tolerance because of their two-dimensional structures.25 While the results for the DBN(OCn) series are promising, the liquid crystal phases of these compounds occur over only narrow ranges at quite high temperatures (>160 °C). Given that other liquid crystals with PAH cores (e.g., hexabenzocoronenes) often feature alkyl side chains (as opposed to alkoxy),26 we decided to investigate the series of analogous compounds DBN(Cn) (Chart 1). The photophysical properties of these new compounds have been explored in solution. Their liquid crystal phases have been characterized by polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction. The phase behavior is discussed in the context of the molecular structures and DFT calculations of representative intermolecular interactions.

However, the NMR spectra are easily interpreted after photochemical planarization which gave the target compounds DBN(Cn) in moderate, but practical, yields. While we describe these compounds as “board-like”, the DBN cores are in fact distorted considerably from planarity because of steric interactions between the internal side chains and the hydrogen atoms on the opposite sides of the “bay regions”. According to DFT optimization of simplified model compound DBN(C2) (R = Et, B3LYP/6-31G(d)), the core is twisted into a C2-symmetric saddle conformation, shown in Figure 1. This behavior parallels that of the analogous alkoxy-

Figure 1. Geometry of DBN(C2) optimized at the B3LYP/6-31G(d) level.

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RESULTS AND DISCUSSION Synthesis and Characterization. For the DBN(Cn) series, we chose to focus on alkyl chains with an odd number of carbon atoms, so as to approximately match the lengths of the previous DBN(OCn) series. The synthesis was similar, as shown in Scheme 1.20 Briefly, we began with triflation of the

substituted core of DBN(OC1).20 The distortion is greater, however, with the average carbon atom in the core of DBN(C2) 0.46 Å from the mean plane, compared to 0.34 Å for DBN(OC1). Presumably this is a consequence of the increased steric demand of a methylene group compared to an oxygen atom. DBNs with this substitution pattern are not, to our knowledge, known. We examined the photophysical properties of DBN(C9) as a representative example. Its UV/vis and fluorescence spectra are shown in Figure 2. Like many

Scheme 1. Synthesis of the DBNsa

Figure 2. UV/vis (blue) and fluorescence (orange) spectra of DBN(C9) as dilute solutions in CH2Cl2.

Reagents and conditions: (a) Tf2O, pyridine, CH2Cl2, −78 °C; (b) 1alkyne, Pd(PPh3)4, CuI, NEt3, DMF; (c) H2, Pd/C, EtOAc; (d) hν, EtOAc. a

polycyclic aromatic hydrocarbons,29 the absorption spectrum features a very weak lowest-energy absorption at 386 nm (ε ≈ 1600 M −1 cm −1 , CH 2 Cl 2 ). The compound is weakly fluorescent, with a very small Stokes shift of 364 cm−1 and well-defined vibronic structure. The fluorescence quantum yield is Φf = 0.0030 in CH2Cl2, measured relative to 9,10diphenylanthracene (excitation at 340 nm), with a lifetime of τ = 9.0 ns as measured by time-correlated single photon counting. The rate of radiative excited-state decay is therefore kr = 3.3 × 105 s−1, and the rate of nonradiative decay is knr = 1.1 × 108 s−1.30

previously reported o-phenylene tetramer 1, giving 2. After optimization, the conditions developed by Panek27 were found to work best for Sonogashira coupling of 2 with various terminal n-alkynes. Standard catalytic hydrogenation gave alkyl o-phenylenes 4(Cn) as precursors to the target compounds. As is typical of o-phenylenes, the NMR spectra of these sterically hindered polyphenylenes are broadened considerably by conformational exchange, complicating characterization.28 2830


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The Journal of Physical Chemistry B The overall forms of the UV/vis and fluorescence spectra of (alkyl-substituted) DBN(C9) are similar to those of the (alkoxy-substituted) DBN(OCn) series and other reported DBNs.31,32 However, the intensity of the excitation to S1 is much lower (1600 vs 13 000 M−1 cm−1 for DBN(OCn)), and the fluorescence quantum yield is 2 orders of magnitude smaller (Φf = 0.0030 vs 0.41). For model compound DBN(C2) (Figure 1), TD-DFT calculations predict the lowest-energy transition to be symmetry-allowed but weak (f = 0.0054, CAM-B3LYP/6311+G(2d,2p)//B3LYP/6-31G(d)). The S1 state is composed of HOMO−1 → LUMO and HOMO → LUMO+1 excitations in roughly equal proportion (38% and 50%, respectively). This closely parallels similar calculations performed on the parent (unsubstituted) DBN compound. The low oscillator strength appears to result from poor spatial overlap of these orbitals and a cancellation of the (opposing) transition dipoles for the two transitions.33 In contrast, the calculations predict a much stronger transition when the ethyl groups are replaced with methoxy groups ( f = 0.14). For the alkoxy-substituted core, the transition is a simple HOMO → LUMO transition (72%). Mesophase Behavior. The phase behavior of the compounds was investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). Each of the compounds in the DBN(Cn) series exhibits monotropic liquid crystal phases; DSC traces are shown in Figure 3 and summarized in Table 1. On cooling

Table 1. Phase Behavior of Compounds DBN(Cn)a compound DBN(C5) DBN(C7) DBN(C9) DBN(C11) DBN(C13)

transition temperatures (°C) [enthalpies, kJ mol−1] heat: cool: heat: cool: heat: cool: heat: cool: heat: cool:

Cr 120 [15] I Cr 91b SmA 106 [4] I Cr 104c I Cr 90 SmA 105 [6] I Cr 105 I SmB 84 [17] Bhex 93 [1] SmA 103 [6] I Cr 107 I Bhex 91 [31] SmA 98 [7] I Cr 106d I Bhex 95 [55] I

a

Temperatures taken from the second heating/cooling cycles using DSC (5 °C/min). bBroad transition. cBimodal peak. dFirst heating cycle (did not crystallize on subsequent cycles).

Figure 4. Phase characterization of DBN(C5). (a) Characteristic focal conic POM texture in the SmA phase at 90.9 °C. (b) Conoscopic pattern of a homeotropically aligned SmA sample at 96.9 °C. (c) X-ray diffraction pattern in the SmA phase at 98.9 °C. (d) Integrated intensity vs q scan corresponding to (c); the positions of the reflection peaks and d-spacings were determined from Lorentzian fits.

accompanied by a relatively large transition enthalpy (31 and 55 kJ/mol). Compound DBN(C9) exhibits a much smaller 1 kJ/mol transition enthalpy for the SmA−Bhex transition, suggesting subtle changes and a weakly first-order phase transition, a conclusion also supported by the X-ray diffraction results shown in the Supporting Information. The structural changes at this transition arise from the development of longrange bond-orientational and short-range positional orders. At the lower-temperature Bhex−SmB transition, a much higher transition enthalpy of 17 kJ/mol is measured, as the positional order becomes long range. As we show later, the sharpening of X-ray diffraction peaks from the in-plane structure and higher harmonics of the reflection from layers of the SmB phase support these conclusions. XRD patterns confirm the assignments of the SmA phase, with oriented sharp reflections at low scattering vectors q (= 4π sin θ/λ = 4π/d) arising from the thickness d of the smectic layers. The two sets of low- and high-q peaks are at 90° to each other, confirming that there is no molecular tilt. There is no indication of columnar stacking in the XRD patterns, which

Figure 3. DSC traces of compounds DBN(Cn) (5 °C/min, exothermic up). Solid lines: second heating/cooling cycles. For DBN(C11) and DBN(C13) the first heating cycles are shown (dashed lines), as the intensity of the Cr−I intensity was weak (and scan rate dependent) or unobserved on subsequent cycles.

below the isotropic liquid (I) phase, all compounds except DBN(C13) exhibit the SmA phase, in which the molecules are organized in layers with their long axes parallel to the layer normal. The phase was identified from its characteristic focalconic fan texture observed by POM, shown in Figure 4a for DBN(C5), and by XRD (see below). The phase was confirmed to be uniaxial by conoscopy in homeotropically aligned samples. For compounds DBN(C11) and DBN(C13), the hexatic-B (Bhex) phase is obtained below the SmA and I phases, respectively (see below). In both cases, the transition is 2831


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The Journal of Physical Chemistry B would have manifested as a peak at ∼3.4 Å arising from π−π stacking. The pair of diffuse arcs in the high-q region corresponds to d = 4.3 Å and can be attributed to the average lateral separation between the aliphatic chains. The diffuse scattering confirms liquid-like order within the smectic layers and overall rotational freedom of molecules about their long axes. This is consistent with the untilted structure, that is, the SmA phase. The values of d calculated from the strongest peaks in different phases are shown in Table 2. The smectic layer Table 2. Molecular Lengths and d-Spacings Observed in Different Phases Determined from the Locations of the Bragg Reflections d (Å) compound

PM7 length (Å)

SmA

DBN(C5) DBN(C7) DBN(C9)

21 26 31

20.2, 4.29 23.8, 4.30 26.8, 4.35

DBN(C11)

36

30.4, 4.40

DBN(C13)

41

Bhex

29.7, 14.8, 9.91, 4.46, 4.25 35.2, 17.6, 11.7, 4.46, 4.26 40.8, 20.4, 13.6, 4.46, 4.25

SmB

Figure 5. POM images of DBN(C9) in a cell with homogeneous alignment layers. (a) Focal-conic texture in the SmA phase at 96.9 °C, (b) the mosaic texture of the rectangular Bhex phase, and (c) the dark texture in homeotropically aligned Bhex phase which shows (d) weak birefringence at increased light intensity.

30.0, 21.2, 15.0, 4.48, 4.27

thickness in the SmA phase is comparable to the molecular lengths of the fully extended molecular conformations (estimated using the semiempirical PM7 method34), also confirming the orthogonal orientation of the molecular long axes with respect to the smectic layers. The slight discrepancy is attributed to the thermal fluctuations and to the orientational order parameter that is less than 1 for the molecules with respect to the layer normal. Effectively, the layer spacing dSmA = L⟨cos θ⟩, where L is the fully extended length of the molecule and θ is the average tilt fluctuation of the molecule with respect to the layer normal. For DBN(C11), for example, ⟨cos θ⟩ ∼ 0.85, determined35 from the azimuthal intensity profile of the large angle peak corresponding to 4.3 Å. Whereas DBN(C5) and DBN(C7) crystallize below their SmA phases, more interesting phase behavior is obtained for the higher homologues. On cooling DBN(C9) from the SmA phase, the POM texture first changes from focal conic to mosaic in the Bhex phase, followed by a transformation to a dark texture ∼2 °C below the transition, as shown in Figure 5. Presumably this texture corresponds to homeotropic alignment (with the smectic layer normal parallel to the surface normal). Shearing of the higher temperature texture on a glass slide gives rise to the same dark texture and homeotropic alignment. On increasing the light intensity, a faint fan-like texture was observed within the dark texture (Figure 5d). This suggests that there is very weak birefringence within the smectic layers of the Bhex phase arising from an in-plane rectangular lattice. Compounds DBN(C11) and DBN(C13) also exhibit this phase with the dark texture, on cooling from the I and SmA phases, respectively. For compound DBN(C 9), the oriented SmA Bragg reflections transform into reflection rings on cooling the sample to the Bhex phase at 87.0 °C, with a Bragg peak at d = 29.7 Å and its second and third order multiples at d = 29.7/2 = 14.9 Å and 29.7/3 = 9.9 Å, respectively, as shown in Figure 6. The rings indicate somewhat randomly oriented smectic domains. At first inspection, the reflection at the higher q region near the d ∼ 4.3 Å peak appeared to be a single diffuse

Figure 6. X-ray diffraction patterns of compound DBN(C9) in (a) the SmA phase at 96.9 °C, (b) the Bhex phase at 87.0 °C, and (c) the SmB phase at 79.1 °C. (d) Integrated intensity vs q plots shown for the patterns in (a)−(c).

ring. However, the integrated intensity vs q plots revealed two superimposed, relatively broad peaks corresponding to distances of 4.46 and 4.25 Å. These reflections are associated with the packing of the molecules within the smectic layers. The presence of two reflections indicates either centered rectangular or distorted hexagonal packing. The somewhat diffuse nature of these rings arises from short-range in-plane positional order, confirming that this is a Bhex phase and not the more-ordered SmB. The layer thickness in the Bhex phase is somewhat larger than in the SmA. In the Bhex phase, the effect of thermal fluctuations on molecular conformation should become less pronounced, as manifested in the relatively sharp high-q reflections. Thus, the d-spacing is effectively equal to the 2832


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The Journal of Physical Chemistry B

The presence of three pairs of high-q reflections, joined by dashed lines 1, 2, and 3, indicates a two-dimensional hexagonal organization. However, slightly different values of the d spacings calculated from these pairs suggest a weakly distorted hexagonal lattice. These reflections in the high-q region (∼4.4 Å) form a quasi-hexagon (red dashed line), which suggests (crystal) SmB structure. Unfortunately, standard powder crystallography analysis cannot be used to determine the structure in the present case because (a) there are fewer than ∼15 peaks, which such methods typically require, and (b) the sample is partially aligned and the true relative peak intensities cannot be determined. To determine the precise nature of inplane packing, additional information from experiments with the incident X-ray beam normal to the smectic planes is necessary. Such experiments on freely suspended films are planned and will be reported separately. It is worthwhile to compare the mesophase behavior of the DBN(Cn) series with that of the previously reported DBN(OCn). The most obvious difference is that exchange of the alkoxy groups for alkyl groups depresses the clearing points for side chains of equal length (by 67−96 °C), as would be expected on the basis of structure−property trends in calamitic liquid crystals.1 More importantly, while the alkoxy-substituted DBN(OCn) compounds give, for the higher (decyloxy and dodecyloxy) homologues, the tilted SmC phase, the alkylsubstituted DBN(Cn) give only untilted SmA and SmB phases. This difference is consistent with the idea that alkoxy groups are more likely to give tilted smectic phases because of the torque introduced by “outboard dipoles” attached to the core.40,41 As was the case for the DBN(OCn) compounds, there is no evidence for columnar stacking for any of the DBN(Cn) compounds. This is in contrast with other reported mesogens based on the same DBN core21−23 that exhibit exclusively columnar phases. The key structural difference is that these other examples feature more (6−8) side chains radially distributed around the core. For the present series, there is little interface curvature42 between the alkyl and aromatic blocks. In fact, they are structurally analogous to biaryl-based calamitics fused side-to-side (i.e., they are “rafts” of 4,4′-biaryl “logs”). Thus, it is not all that surprising that they exhibit phases similar to those of calamitics. To better understand the interactions underlying the organization of the DBN core in the Bhex or columnar liquid crystal phases, we examined the energetics of DBN dimers using DFT calculations. As we were interested in qualitative trends intrinsic to the aromatic core, only gas-phase dimers of the parent compound DBN(H) were considered, as shown in Figure 8. Their energies were examined at the B97-D/ TZV(2d,2p) level, a dispersion-corrected method which is very effective in modeling aromatic stacking.43 The DBN(H) dimers were not themselves optimized: they comprise the separately optimized cores at a fixed separation of 3.5 Å, chosen to best approximate the expected separation in a typical liquid crystal.26 The dimer geometries were then varied in two ways: by displacing by r perpendicular to the junction points of the side chains in DBN(Cn) (and DBN(OCn)), as would be expected, in general, in a smectic layer (in addition to edge-toface interactions), and by rotating by θ, as would be expected in a columnar stack. For most PAHs (including benzene itself), perfect cofacial stacking is not the preferred interaction geometry.44 Instead, displacement or rotation yields more stable dimers. The

molecular length. Similar increased layer spacing has been observed for the Bhex phase below the SmA in calamitics.36 Side and top views of a possible in-plane molecular organization of the Bhex phase are depicted in Figures 7a and

Figure 7. (a) Possible in-plane molecular organization of sanidic molecules in the Bhex phase; the boxes represent the molecular envelopes of the board-like molecules and the dark ellipses the ends of the aliphatic chains. (b) Molecular arrangement viewed from the top with a rectangular (nearly hexagonal) packing of the aliphatic chains.

7b, respectively. The molecular long axes are perpendicular to the smectic layer. Because two aliphatic chains are confined to a common core, packing of the chains leads to a hybrid lattice.37 The chain packing (sub)lattice is rectangular, approaching hexagonal symmetry.38 We believe that the most likely packing of the molecules is a herringbone structure, which is accommodated by molecules with effective cross-sectional dimensions of 4.25 × 10.15 Å (an equivalent packing can be constructed for dimensions of 4.45 × 9.98 Å). While further experiments on single domains (such as in freely suspended film samples) are needed to clarify the exact molecular arrangement, the calculated geometries of DBN(Cn) are in good agreement with this proposed structure: the molecules are indeed expected to be approximately 10 Å wide, and the 4.25 Å thickness is consistent with the nonplanar DBN core (Figure 1). It could also be argued that the two relatively sharp peaks at high-q arise from a distorted hexagonal arrangement caused by tilting of the molecules with respect to the layer normal (i.e., as in the smectic-C, -F, or -I phases). However, in all cases here the measured smectic layer spacing is comparable to the estimated molecular length of the compounds in their fully extended, all-anti conformations (Table 2). Further, for the compounds that exhibit both the SmA and Bhex phases (DBN(C9) and DBN(C11)), the layer spacing in the Bhex is longer than in the SmA phase. These results unequivocally rule out the tilted hexagonal structure. The proposed Bhex phase structure is also consistent with the POM textures (Figure 5b− d). Like the SmA phase, a typical Bhex phase would be expected to give either mosaic or optically isotropic (homeotropic alignment) textures.39 The observed mosaic and weakly birefringent “dark” textures imply that the phase is biaxial, albeit with low in-plane birefringence. On further cooling of DBN(C9), the diffraction pattern, shown in Figure 6c, shows oriented reflections in the smallangle region. Bragg peaks at 30.0 Å, and its second- and thirdorder multiples at d = 30.0/2 = 15.0 Å and 30/3 = 9.98 Å, indicate a highly condensed layered structure. Several additional peaks suggest a structure with more order than the Bhex phase. Additionally, in the high-q region of Figure 6c, the pattern clearly shows a hexagonal pattern parallel to the smectic layers. 2833


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plane lattice, and the SmB phase, as indicated by polarized microscopy and X-ray diffraction. The occurrence of this phase can be rationalized by the flat interface curvature between the alkyl and aromatic blocks in these compounds, as DFT calculations suggest that the DBN core itself interacts favorably through either rotated or displaced geometries.

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EXPERIMENTAL SECTION Synthesis. General. Unless otherwise noted, all starting materials, reagents, and solvents were purchased from commercial sources and used without further purification. Anhydrous DMF was purchased in a sealed bottle over molecular sieves. NMR spectra were measured for CDCl3 solutions using a Bruker Avance 500 MHz NMR spectrometer. Chemical shifts are reported in δ (ppm) relative to TMS, with the residual solvent protons used as internal standards. Highresolution ESI mass spectra were obtained from the Ohio State Mass Spectrometry and Proteomics Facility. High-resolution MALDI mass spectra were obtained from the University of Akron Mass Spectrometry Center. Triflated o-Phenylene Tetramer 2. To a stirred suspension of 120 (1.00 g, 2.28 mmol) in CH2Cl2 (25 mL) was added pyridine (1.42 mL, 17.6 mmol). The reaction mixture was cooled to −78 °C. Once the majority of product had dissolved, triflic anhydride (2.2 mL, 13 mmol) was added dropwise. The reaction mixture was allowed to slowly warm to room temperature. After 18 h, the reaction mixture was washed with brine and the organic layer dried (MgSO4), filtered, and concentrated. Purification by flash chromatography (7:3 hexanes:CH2Cl2) gave 2 as a clear oil that solidified to white crystals (684.3 mg, 31%); mp 121.0 °C. 1H NMR (500 MHz, CDCl3) broadened by conformational exchange, see Supporting Information. HRMS (ESI) calcd for C28H12Cl2F2NaO12S4 (M + Na+) 988.8295; found 988.8303. General Procedure for the Sonagashira Coupling and Hydrogenation of 2 with 1-Alkynes. Compound 2, Pd(PPh3)4 (0.40 equiv), and CuI (0.20 equiv) were added to a Schlenk vacuum tube which was evacuated and backfilled with argon. Anhydrous DMF (5 mL), NEt3 (20 equiv), and alkyne (6.6 equiv) were added sequentially, and the reaction mixture was thoroughly degassed by three freeze−pump−thaw cycles. The tube was sealed and placed in a 50 °C oil bath for 24 h. The reaction mixture was then cooled, partitioned between Et2O and water, washed with brine (3×), dried (MgSO4), filtered, concentrated, and purified by flash chromatography. This product was then dissolved in EtOAc and treated with Pd/C (10% Pd loading, 0.40 equiv). A balloon of H2(g) was placed on the reaction vessel, H2 was allowed to flow freely through the vented vessel for 3 min, and then the vent was removed. The reaction was stirred vigorously for 4 h. The suspension was filtered through Celite, washing the solids with EtOAc (3×). The solution was then concentrated and purified by flash chromatography. o-Phenylene Tetramer 4(C5). The synthesis followed the general Sonogashira coupling procedure with 2 (250 mg, 0.258 mmol), CuI (20 mg, 0.11 mmol), Pd(PPh3)4 (60 mg, 0.052 mmol), NEt3 (800 μL, 5.73 mmol), 1-pentyne (300 μL, 3.04 mmol), and DMF (5 mL). Note: the oil bath temperature was reduced to 37 °C because of the low boiling point of 1-pentyne. Purification by flash chromatography (4:1 hexanes:CH2Cl2) gave a colorless crystalline solid (135 mg) that was hydrogenated with Pd/C (60 mg, 0.056 mmol Pd). Purification by flash chromatography (9:1 hexanes:CH2Cl2) gave 4(C5) as a

Figure 8. Energetics of DBN dimers. The dimers are held at a fixed separation of 3.5 Å and then either displaced by r or rotated by θ. Dimer energies were calculated at the B97-D/TZV(2d,2p) level.

DBN(H) dimer is greatly stabilized by rotation by θ ≥ 30°, with a flat energy surface that is consistent with the DBN core’s suitability for disordered columnar phases.21−23 However, a similar stabilization of the dimer can also be achieved by lateral displacement of r ∼ 1−3 Å. Taken together, these calculations suggest that the DBN core itself has little intrinsic preference for either lamellar or columnar packing. Thus, other effects should have a strong effect on their phase behavior, such as the side-chain placement and associated interface between alkyl and aromatic blocks. Thus, the packing in the biaxial Bhex phase of DBN(C9), DBN(C11), and DBN(C13) is directly attributable to their board-like shapes. This phase has not, to our knowledge, been observed previously, despite the occurrence of the (uniaxial) Bhex and SmB phases for many calamitics. This behavior distinguishes the present compounds from other reported examples of board-shaped liquid crystals discussed above, which exhibit columnar phases and nematic/smectic phases similar to those of calamitics. Perhaps the best analogy to the phase structure observed here is the biaxial SmA phase observed for binary mixtures of board-shaped metallomesogens with trinitrofluorenone, which experience strong intercore interactions.45 Here, interactions between the DBN cores are strong enough that a related phase structure is observed for the neat materials. Interestingly, 4,4′-dialkylbiphenyls, such as 4,4′-dipentylbiphenyl, are known to exhibit the conventional hexatic SmB phase.46,47 The DBN(Cn) series therefore represents the first step in probing the effect of systematically increasing the molecular biaxiality by laterally extending the rigid cores. Efforts to synthesize the next members of this series are ongoing.48

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CONCLUSIONS In summary, a new DBN(Cn) series of dibenzo[fg,op]naphthacene-based mesogens have been synthesized through photochemical dehydrohalogenation of an o-phenylene precursor. DFT calculations predict that the core is twisted into a saddle conformation. The compounds all exhibit monotropic liquid crystal phases. The shorter homologues exhibit conventional uniaxial smectic-A phases whereas the longer homologues exhibit an unusual hexatic-B phase, with a rectangular in2834


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DBN(C5). The general procedure was followed using 4(C5) (90 mg, 0.14 mmol) in EtOAc (20 mL). Recrystallization (EtOAc) gave white crystals (30 mg, 0.051 mmol, 38%). 1H NMR (500 MHz, CDCl3): δ 8.59 (d, J = 8.2 Hz, 2H), 8.55 (d, J = 8.5 Hz, 2H), 8.21 (s, 2H), 7.83 (d, J = 8.1 Hz, 2H), 7.51 (d, J = 8.2 Hz, 2H), 3.5−3.3 (m, 4H), 2.86 (t, J = 7.7 Hz, 4H), 2.11−2.02 (m, 4H), 1.81 (m, 4H), 1.6−1.3 (m, 16H), 1.1−0.9 (m, 12H). 13C NMR (125 MHz, CDCl3): δ 139.9, 138.0, 129.8, 129.5, 129.2, 128.7, 128.2, 127.7, 126.9, 126.4, 123.7, 120.3, 36.7, 36.4, 32.4, 32.1, 31.6, 31.4, 22.8, 22.7, 14.2, 14.1. HRMS (ESI) calcd for C88H108Na (2M + Na+) 1187.8349; found 1187.8355. DBN(C7). The general procedure was followed using 4(C7) (125 mg, 0.163 mmol) in EtOAc (20 mL). Recrystallization (EtOAc) gave white crystals (47 mg, 42%). 1H NMR (500 MHz, CDCl3): δ 8.60 (d, J = 8.2 Hz, 2H), 8.55 (d, J = 8.5 Hz, 2H), 8.21 (s, 2H), 7.83 (d, J = 8.1 Hz, 2H), 7.51 (d, J = 8.2 Hz, 2H), 3.39 (t, J = 8.1 Hz, 4H), 2.86 (t, J = 7.7 Hz, 4H), 2.06 (qu, J = 7.7 Hz, 4H), 1.81 (qu, J = 7.5 Hz, 4H), 1.5−1.2 (m, 32H), 1.0−0.8 (m, 12H). 13C NMR (125 MHz, CDCl3): δ 139.9, 138.0, 129.9, 129.5, 129.2, 128.7, 128.2, 127.6, 126.9, 126.4, 123.7, 120.3, 36.7, 36.4, 32.5, 32.0, 31.9, 31.8, 30.2, 29.45, 29.38, 29.3, 22.7, 14.1. HRMS (MALDI) calcd for C52H70 (M+) 694.5478; found 694.548. DBN(C9). The general procedure was followed using 4(C9) (142 mg, 0.161 mmol) in EtOAc (20 mL). Recrystallization (EtOAc) gave white crystals (52 mg, 40%). 1H NMR (500 MHz, CDCl3): δ 8.62 (d, J = 8.2 Hz, 2H), 8.57 (d, J = 8.4 Hz, 2H), 8.22 (s, 2H), 7.83 (d, J = 8.1 Hz, 2H), 7.53 (d, 8.3 Hz, 2H), 3.5−3.3 (m, 4H), 2.86 (t, J = 7.7 Hz, 4H), 2.1−2.0 (m, 4H), 1.80 (qu, J = 7.5 Hz, 4H), 1.3−1.5 (m, 48H), 0.96−0.85 (m, 12H). 13C NMR (125 MHz, CDCl3): δ 139.9, 138.0, 129.9, 129.5, 129.2, 128.7, 128.2, 127.6, 126.9, 126.4, 123.7, 120.3, 36.7, 36.4, 32.5, 31.9, 31.7, 30.2, 29.73, 29.66, 29.5, 29.4, 22.7, 14.1. HRMS (MALDI) calcd for C60H86 (M+) 806.6730; found 806.672. DBN(C11). The general procedure was followed using 4(C11) (178 mg, 0.179 mmol) in EtOAc (20 mL). Recrystallization (EtOAc) gave white crystals (64 mg, 39%). 1H NMR (500 MHz, CDCl3): δ 8.64 (d, J = 8.2 Hz, 2H), 8.56 (d, J = 8.5 Hz, 2H), 8.21 (s, 2H), 7.87 (d, J = 8.1 Hz, 2H), 7.53 (d, 8.2 Hz, 2H), 3.5−3.4 (m, 4H), 2.86 (t, J = 7.3 Hz, 4H), 2.1−2.0 (m, 4H), 1.80 (m, 4H), 1.5−1.2 (m, 64H), 1.0−0.8 (m, 12H). 13C NMR (125 MHz, CDCl3): δ 139.9, 138.0, 129.9, 129.5, 129.2, 128.7, 128.2, 127.6, 126.9, 126.4, 123.7, 120.3, 36.7, 36.4, 32.5, 32.0, 31.7, 30.2, 29.7, 29.5, 29.4, 22.7, 14.1. HRMS (MALDI) calcd for C68H102 918.7982; found 918.800. DBN(C13). The general procedure was followed using 4(C13) (160 mg, 0.145 mmol) in EtOAc (20 mL). Recrystallization (EtOAc) gave white crystals (85 mg, 57%). 1H NMR (500 MHz, CDCl3): δ 8.58 (d, J = 8.2 Hz, 2H), 8.53 (d, J = 8.2 Hz, 2H), 8.19 (s, 2H), 7.81 (d, J = 8.1 Hz, 2H), 7.50 (d, J = 8.2 Hz, 2H), 3.5−3.3 (m, 4H), 2.85 (t, J = 7.6 Hz, 4H), 2.11−2.01 (m, 4H), 1.81 (m, 4H), 1.50−1.37 (m, 80H), 0.98−0.85 (m, 12H). 13 C NMR (125 MHz, CDCl3): δ 139.8, 138.0, 129.8, 129.5, 129.2, 128.7, 128.2, 127.6, 126.9, 126.4, 123.7, 120.3, 36.7, 36.4, 32.5, 31.99, 31.98, 31.7, 30.2, 29.8, 29.5, 29.4, 22.7, 14.2. HRMS (MALDI) calcd for C76H118 (M+) 1030.9234; found 1030.938. Photophysical Characterization. UV−vis and fluorescence spectroscopy were performed using spectrometric-grade CH2Cl2 used without further purification. Fluorescence quantum yield and lifetime measurements were performed on solutions that had been sparged with nitrogen. Quantum yields

clear oil (90 mg, 53% over two steps). 1H NMR (500 MHz, CDCl3) broadened by slow conformational exchange, see Supporting Information. HRMS (ESI) calcd for C44H56Cl2Na (M + Na+) 677.3657; found 677.3671. o-Phenylene Tetramer 4(C7). The synthesis followed the general Sonogashira coupling procedure with 2 (298 mg, 0.308 mmol), CuI (20 mg, 0.11 mmol), Pd(PPh3)4 (60 mg, 0.052 mmol), NEt3 (750 μL, 5.73 mmol), 1-heptyne (260 μL, 2.00 mmol), and DMF (5 mL). Purification by flash chromatography (4:1 hexanes:CH2Cl2) gave a colorless crystalline solid (180 mg) that was hydrogenated (150 mg) with Pd/C (80 mg, 0.0564 mmol Pd). Purification by flash chromatography (9:1 hexanes:CH2Cl2) gave 4(C7) as a clear oil (125 mg, 63% over two steps). 1H NMR (500 MHz, CDCl3) broadened by slow conformational exchange, see Supporting Information. HRMS (ESI) calcd for C52H72Cl2Na (M + Na+) 789.4909; found 789.4919. o-Phenylene Tetramer 4(C9). The synthesis followed the general Sonogashira coupling procedure with 2 (250 mg, 0.258 mmol), CuI (20 mg, 0.11 mmol), Pd(PPh3)4 (60 mg, 0.052 mmol), NEt3 (750 μL, 5.4 mmol), 1-nonyne (332 μL, 2.02 mmol), and DMF (5 mL). Purification by flash chromatography (4:1 hexanes:CH2Cl2) gave a colorless crystalline solid (201 mg) that was hydrogenated (170 mg) with Pd/C (83 mg, 0.0780 mmol Pd). Purification by flash chromatography (9:1 hexanes:CH2Cl2) gave 4(C9) as a clear oil (142 mg, 74% over two steps). 1H NMR (500 MHz, CDCl3) broadened by slow conformational exchange, see Supporting Information. HRMS (ESI) calcd for C60H88Cl2Na (M + Na+) 901.6161; found 901.6161. o-Phenylene Tetramer 4(C11). The synthesis followed the general Sonogashira coupling procedure with 2 (250 mg, 0.258 mmol), CuI (20 mg, 0.11 mmol), Pd(PPh3)4 (60 mg, 0.052 mmol), NEt3 (724 μL, 5.2 mmol), 1-undecyne (332 μL, 1.68 mmol), and DMF (5 mL). Purification by flash chromatography (4:1 hexanes:CH2Cl2) gave a colorless crystalline solid (215 mg) that was hydrogenated (215 mg) with Pd/C (94 mg, 0.088 mmol Pd). Purification by flash chromatography (9:1 hexanes:CH2Cl2) gave 4(C11) as a clear oil (178 mg, 70% over two steps). 1H NMR (500 MHz, CDCl3) broadened by slow conformational exchange, see Supporting Information. HRMS (ESI) calcd for C68H104Cl2Na (M + Na+) 1013.7413; found 1013.7422. o-Phenylene Tetramer 4(C13). The synthesis followed the general Sonogashira coupling procedure with 2 (340 mg, 0.351 mmol), CuI (20 mg, 0.11 mmol), Pd(PPh3)4 (60 mg, 0.052 mmol), NEt3 (724 μL, 5.2 mmol), 1-tridecyne (0.5 mL, 2 mmol), and DMF (5 mL). Purification by flash chromatography (4:1 hexanes:CH2Cl2) gave a colorless crystalline solid (320 mg) that was hydrogenated (320 mg) with Pd/C (128 mg, 0.12 mmol Pd). Purification by flash chromatography (9:1 hexanes:CH2Cl2) gave 4(C13) as a clear oil (280 mg, 72% over two steps). 1H NMR (500 MHz, CDCl3) broadened by slow conformational exchange, see Supporting Information. HRMS (ESI) calcd for C76H120Cl2Na (M + Na+) 1125.8665; found 1125.8671. General Procedure for the Photochemical Dehydrochlorination of 4. A quartz tube was charged with a solution of 4 in EtOAc. Argon was bubbled vigorously through the solution for 30 min before the reactor tube was sealed. The solution was subjected to UV irradiation (medium pressure mercury lamps) for 18−48 h, then concentrated, and washed with cold EtOAc. 2835



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ACKNOWLEDGMENTS P.J.R. and C.S.H. acknowledge support from the National Science Foundation (CHE-1306437). Optical polarized microscopy, conoscopy, and X-ray diffraction work of D.M.A.-K. and S.K. were supported by the division of Basic Energy Sciences, Office of Science of the US Department of Energy, under Grant DE-SC-0001412.

were determined by reference to 9,10-diphenylanthracene in cyclohexane (Φf = 0.91) which was cross-checked with quinine bisulfate in 0.5 M H2SO4(aq) (Φf = 0.54). Five solutions of varying concentration were prepared for each sample, and good fits (R2 ≥ 0.99) were obtained in all cases. The absorbance of all sample solutions was kept below 0.10 cm−1 to avoid the innerfilter effect. Measurements were performed at room temperature with both samples and standards excited at the same wavelength. Fluorescence lifetimes were determined by timecorrelated single-photon counting. Optical Microscopy and X-ray Diffraction. The phases exhibited on heating/cooling the compounds were identified via optical microscopy (Olympus BX51), which was carefully executed by cooling (5 °C/min) the sample mounted on a microscope slide in a hot stage (Mettler FP90) from the isotropic phase. Optical textures at specific temperatures were captured by a SPOT Insight CCD camera. For oriented samples, the slide and coverslip were surface treated with Quilon H and rubbed polyimide (SE7492) for homeotropic and homogeneous molecular orientations, respectively. The transition temperatures were determined from differential scanning calorimetry scans (DSC Q20) with a heating/cooling rate of 5 °C/min under a nitrogen atmosphere. Optical conoscopy49−51 was performed in slightly sheared homeotropically aligned samples by inserting a Bertrand lens before the analyzer in the polarizing microscope setup to discriminate uniaxial and biaxial phases.52 For definitive identification of the phases, X-ray diffraction (XRD) on the samples was performed using a Rigaku Screen Machine (Copper anode, λ = 1.542 Å) with Mercury 3 CCD detector of resolution 1024 × 1024 pixels (size: 73 × 73 μm2). Each sample was contained in a flame-sealed 1.0 mm quartz capillary and placed in hot stage (Linkam-CAP) for temperature control with 0.05 °C precision. Computational Chemistry. Semiempirical calculations of molecular lengths were performed at the PM7 level using MOPAC2012, v. 15.026M.53 So as to best approximate the effective length of a molecule packed within a smectic layer, the lengths were taken as the distance between terminal hydrogen atoms for side chains on the same side of the DBN core. DFT calculations were performed using Gaussian 09, rev. B.01.54 All energy minima were verified to have 0 imaginary frequencies by vibrational frequency analysis. Geometries are provided in the Supporting Information.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b10990.

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Article

Computational chemistry data; fluorescence lifetime measurement; complete liquid crystal phase characterization; NMR spectra of synthesized compounds (PDF) Computational geometries (TXT)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.K.). *E-mail: [email protected] (C.S.H.). Notes

The authors declare no competing financial interest. 2836


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Smectic-A and Hexatic-B Liquid Crystal Phases of Sanidic Alkyl

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