Molecular engineering of mesomorphic fluorene-bridged triphenylene

arrays to form 2D columnar mesophases.1,2 Nematic LC materials are nevertheless considered ... potential industrial applications, as negative birefrin...
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Molecular engineering of mesomorphic fluorene-bridged triphenylene triads: Thermotropic nematic/columnar mesophases, and p-type semi-conducting behavior Yao Yang, Hu Wang, Hai-feng Wang, Chun-Xia Liu, Ke-Qing Zhao, BiQin Wang, Ping Hu, Hirosato Monobe, Benoît Heinrich, and Bertrand Donnio Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00083 • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on June 29, 2018

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Crystal Growth & Design

Molecular engineering of mesomorphic fluorene-bridged triphenylene triads: Thermotropic nematic/columnar mesophases, and p-type semi-conducting behavior Yao Yang†, Hu Wang†, Hai-Feng Wang†, Chun-Xia Liu†, Ke-Qing Zhao†,*, Bi-Qin Wang†, Ping Hu†, Hirosato Monobe‡,*, Benoît Heinrich§, and Bertrand Donnio§,* †

College of Chemistry and Material Science, Sichuan Normal University, Chengdu 610066, China. E-mail: [email protected] ‡ Inorganic Functional Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 5638577, Japan. E-mail: [email protected] §

Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), CNRS-Université de Strasbourg (UMR 7504), 67034 Strasbourg, France. E-mail: [email protected]

ABSTRACT: Discotic polyaromatic liquid crystals exhibiting 2D columnar mesophases are actively investigated for their potential utilisation as one-dimensional organic semiconductors, due essentially to their long-range order self-organization and high-charge carrier transport ability. The much less reported nematic-containing discotic materials have however been recently found to be of industrial importance too. In this context, highly conjugated fluorene-bridged triphenylene triads have been designed with the aim of inducing room temperature nematic phases. In our approach, the length of the alkyl chains bore by the central fluorene unit was changed in order to constrain the space filling, to modulate the intermolecular π-π interactions, and to control the tendency of the molecular triads to stack into columns, and the subsequent aggregation of the columns in specific ways. This strategy was successful, and in addition to the naturally expected two-dimensional columnar mesophases, most triads exhibit the more elusive columnar nematic phase, as only the orientational-type long-range order is preserved. Photoluminescence, measured in both solution and thin films, revealed strong blue emission, characteristic of the fluorene moiety, with impressively high quantum efficiency. Charge carrier mobility measurements revealed novel p-type molecular semiconductors with hole transport mobility in the range of 0.22x10-3 cm2 V-1 s-1 in the mesophases (Colrec, Colhex and NCol) and up to 5x10-4 cm2 V-1 s-1 in the isotropic phase. Reasons for the occurrence of these mesophases and their supramolecular structures are discussed along with the effects on the semiconducting properties.

INTRODUCTION Nematic (N) phases of discotic mesogens are not so common and much less reported than their two-dimensional discotic columnarforming LC analogous materials.1 The main reason for the rare observation of discotic N phase lies essentially in the difficulty of their design, as most discotic mesogens tend to stack into columns, which eventually will aggregate into essentially planar arrays to form 2D columnar mesophases.1,2 Nematic LC materials are nevertheless considered now as quite relevant due to their lower viscosity and easier alignment tendency, crucial for many potential industrial applications, as negative birefringent materials, for the fabrication of optical compensation films widening the viewing angles of liquid crystal displays (LCDs), optical sensors, detectors and so on.1,3 These promising results have since stimulated revived research activity in this area and particularly in the molecular engineering of semiconducting nematic materials. Understanding the correlations between the molecular structure and the emergence of the N phase in discotic-like materials remains therefore an essential challenge of great relevance for both fundamental and applied sciences. Up to now, there are only a few examples of discotic molecules known to display a nematic phase. After the pioneering work on hexakis(phenylethynyl)aryl derivatives with either benzene, naphthalene or triphenylene central ring,4-6 more recent works include some hexakis(4-ethyleneoxidebenzoate)triphenylenes mixed with alkali salts,7 phenylene ethynylene macrocycles,8,9 phenylethynylbenzene derivatives,10,11 as well as truxenes, thiatruxenes, and oxatruxenes.1 LC systems with structures deviating

slightly from conventional disc, such as triphenylene twins,12-15 non-uniformly alkyl-chains substituted phthalocyanines16 and dibenzotetraaza[14]annulenes,17 cyclo[6]aramides,18 and shapepersistent aromatic oligoamides,19 soft spacer-connected triphenylene-pentaalkynylbenzene dyads,20 as well as some polycatenar mesogens, with unsymmetrical substitution or lateral chains21-23 have also been reported to exhibit a nematic mesomorphism. Very recently, new materials for molecular electronics have shown high performances in conjunction with the appearance of nematic properties. In particular, high charge carrier mobility has been measured for a metallomesogen based on a porphyrin-core dendrimer, with value in the nematic state comparable to the highest ones in columnar mesophases with positional long-range order.24 In this line, higher hole mobility and BHJ solar cell efficiency has been found for the nematic terms of a series of triazatruxene-based dumbbell-shaped molecules.25 After this concise overview of nematogenic, discotic molecular architectures, it seems that molecular conformations deviating slightly from disc-shape and unsymmetrical space filling are important driving forces for the preferential emergence of the N mesophase with respect to 2D columnar phases. Indeed, for the hexabenzoate triphenylenes,7 the conformational variation of the benzoate periphery related to triphenylene core, or for the discotic twinned molecules, covalently bonded by a rigid connector (diacetylene, thiophene, biphenylene),12-15 the somewhat disturbed π-π intermolecular interactions and the unfilled void space, all may reduce the tendency of aggregation of columns in regular 2D arrays. Accordingly, we report here a strategy to construct new and original mesogenic triad architectures with potential electronic func-

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tion and promotion of nematic mesomorphism (Scheme 1). Specifically, the molecules associate fluorene, as a typical building block of semi-conducting materials, and triphenylene (TP), as a common discogen, the former segment constituting the central rigid bridge of the triads, and TP, the two lateral wings. Nematic mesomorphism could then be induced through the optimization of peripheral alkyl chains, as they modulate face-to-face stacking ability and space-filling since the dialkyl side-groups bore by the fluorene core and those on TP closest to the bridge were anticipated to play a particular role, they were varied independently from the other ones on TP. This design was successful, as depending on the chain volume fraction and chain distribution, and the consequent changes in the molecular structures, the columnar N phase (NCol) was induced in most triads, either from frozen roomtemperature state or above 2D columnar mesophases (Colhex and Colrec),. RESULTS AND DISCUSSION Molecular design and Synthetic procedures. TP is maybe the most emblematic disk-like mesogenic core, due to its near-ideal disc-like shape and its propensity to stack face-to-face to form cylindrical columnar mesophases when substituted with sufficiently long peripheral alkyl chains.26-28 TP is also a very versatile model system for designing complex and elaborated liquidcrystalline structures with multiple properties. For example, one or several of the radial chain(s) can be used as a spacer to introduce other conjugated aromatic ring structures, which leads to a wide range of oligomeric derivatives exhibiting columnar mesomorphism.29-36 An alternative route is the attachment of other aromatic rings directly to the TP core through a σ-bond, though it might at first sight fuse both rings into a new one. In fact, σ-bonds authorize free rotation, which largely restores the integrity of both types of aromatic components, and for instance confers to the simple phenylene substituents an average cylindrical shape characteristic of rod-like mesogens.37 Accordingly, phenyl substituents connected at the bay positions ‘1’ strongly hamper face-toface stacking and mesomorphic properties, due the discrepancy between both the natural spacings of rod-like and disk-like mesogens (typically 4.5 Å against 3.5 Å, respectively), while the substitutions in peripheral positions ‘2’ (dyad) and ‘2,7’ (triad) promote the induction of columnar-like mesomorphism.38 For such dyads, and for triads with the disc-like unit in the centre, the peripheral rod-like segments could just insert between alkyl chains respecting the optimal self-organization of both moieties. Lateral arrangement and stacking should on the contrary conflict for triads with the rod-like segment in the centre, as examined hereafter for a series of dialkylfluorene units connected at both ends to TP species. In this study, the molecular triads were synthesized in one single step by Suzuki cross-coupling between the commercially available dialkyl fluorene-2,7-diboronic acids and the readily accessible 3,6,7,10,11-pentakis(alkoxy)triphenylen-2-yl triflates (Scheme 1, 2a-c in Supporting Information), the latter as versatile alternative to bromide homologs.38 The combination of two different fluorene-2,7-diboronic acids (Scheme 1, R1 = CnH2n+1, n = 1, 8) and three triphenylene triflates (Scheme 1, R2 = CoH2o+1, o = 1, 6, 8/R3 = CmH2m+1, m = 6 or 8) resulted in six novel triads, thereafter abbreviated as FTP-nom. These six new triads were obtained in good yields and fully characterized by 1H NMR and HRMS. All synthetic details and structural characterizations are given in the electronic supplementary information. Scheme 1. Synthesis, molecular structures of fluorene-bridged triphenylene (FTP-nom) triads, nomenclature and yields (Details in Supp. Info).

Density Functional Theory (DFT) calculations. In order to collect some information about the overall molecular conformation of this family of triads, DFT calculations were performed. For saving calculation time and maintaining the essential molecular parameters of the triads, FTP-111 (R1 = R2 = R3 = Me) was used as model compound instead of FTP-116), and the molecular structure was optimized using B3LYP/6-311++**. The dihedral angle (DA) between TP and fluorene blocks was first set to 0°, and then the molecular structure was optimized, and the relative energy calculated. Then, a gradual 10°-rotation between 0° to 180° was implemented around the inter-ring C-C bond connecting both blocks (torsion angle), and the molecular structure optimized and relative potential energy computed again for each torsion angle value. The results, summarized in Figure S38, show that molecular conformations with the lowest energy, and thus the more stable ones, are obtained with DA values of 50° and 130°, respectively (Figures S40 and S42), whereas those with highest energy have DA values of 0 and 180°(Figures S39 and S43); in the situation of DA = 90o (Figure S41), where TP and fluorene cores are perpendicular to each other, the molecule has an intermediate relative energy in between the lowest and highest ones. Intramolecular hydrogen bonding of aromatic C-H...O=C (carbonyl oxygen) were reported to be capable of locking planar conformation, and thus to extend conjugation, in small molecules and polymers’ backbones, also supported by single crystals or DFT computational analysis.39,40 For the most stable state molecular conformations, the intramolecular hydrogen interactions CH...O between ortho-OMe (TP) and H-Ar (fluorene) appear too weak (too far) to lock the conformation. Furthermore, after comparing with literature data39,40 our triads, with the ortho ether group (TP-OR) close to fluorene core, show less degree of polarity, and the negative charge on the oxygen is small (Figure S44), while the reported examples with locked planar conformations (ortho carbonyl functional groups) are more polar and possess higher negative charge on the oxygen atom, therefore nontraditional hydrogen bonding CH***O can form. These theoretical results are supported by our experimental results as these triads possess good solubility in organic solvents. The non-planar conformations and the relatively flexible triad cores augur well for low melting temperatures and induction of mesomorphic properties (see below). Thermal behaviour and liquid-crystalline properties. The thermal behaviour and mesomorphism of the synthesized triads were first investigated by polarizing optical microscopy (POM,

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Crystal Growth & Design

Figures 1 and S1) and differential scanning calorimetry (DSC, Figure S2). No sign of decomposition upon heating was detected for any member of the series in the temperature range of study (up to 300°C), and all compounds cleared reversibly in the isotropic liquid phase. FTP-116, which possesses the shortest alkyl chains, is not mesomorphic, and shows instead a single crystalline phase, with a rather high melting temperature (Table 1, Figure S3). As the chain-volume fraction (χVch, Table 2) was increased, mesomorphism was systematically induced in all the other triads, with the clearing temperatures decreasing accordingly more or less consistently from FTP-166 to FTP-888 (Table 1). Characteristic fluid and highly birefringent optical textures of liquid crystals could be easily recognized (fan-like domains for the columnar phases, and Schlieren for the N phases, Figure 1 and S1). All the mesomorphic compounds but FTP-188 show a columnar phase (a columnar phase was also formed on cooling for FTP-888, but could be only detected during SAXS acquisition, see below), whilst a N phase was seen for all terms, except for FTP-816.

FTP-866

Colrec 78 (-35) Cr 118 (26) NCol 143 (3) I

I 142 (-3) NCol 99 (-1) Colrec

FTP-188

NCol 159 (2) I

I 158 (-1.5) NCol

FTP-888

Colrecb

I 117 (-1.5) NCol (-) Colrec[b]

- (-) NCol 119 (1.5) I

a

Cr, Cr’: crystalline phases; Colhex: hexagonal columnar phase; Colrec: rectangular columnar phase; NCol: columnar nematic phase (see text); I: isotropic liquid; Glass transition temperatures were not detected by DSC. bThe Colrec mesophase was detected by SAXS only. SAXS investigation and supramolecular organisation. SAXS patterns were recorded in all the mesophases and exhibited extended diffuse signals in the wide-angle range, corresponding to several molecular contributions, and broad and/or sharp signals in the small-angle range, confirming the assignment of the mesophases made above from POM. Two representative SAXS patterns are displayed below, namely of FTP-816 in the Colhex, and of FTP-166 in the N phase (Figure 2), whereas the other are shown in Figure S4.

Figure 1. Representative POM textures of the N phase (recorded on cooling from the isotropic liquid, clockwise, from top left): FTP-166 at 187°C, FTP-866 at 125°C, FTP-188 at 80°C, and FTP-888 at 100°C. Thus, most triads exhibit two mesophases: a columnar phase at low temperature, and an N phase between the columnar and isotropic phases, whose transitions were also detected by DSC (Figure S2). On cooling, none of these mesomorphous triads crystallize (no transition detected in DSC), and all the mesophases freeze in a glassy state, whilst hardening and retaining the optical texture of the adjacent mesophase. Crystallization occurs nevertheless in the particular case of FTP-866 for subsequent heating from the frozen Colrec mesophase, and is followed at higher temperature by the melting of the just crystallized sample to the NCol phase (see DSC and SAXS pattern in SI). Similar case of polymer-like cold crystallization behaviour competing with self-assembly into mesophase was already reported for other molecular systems of comparable architecture.25 Table 1. Mesophases, transition temperatures and enthalpy changes for FTP-nom triads (heating and cooling rate of 5-10 o C·min-1)a FTP-nom

2nd heating/°C (∆H, kJ/mol)

1st cooling/°C (∆H, kJ/mol)

FTP-116

Cr -14 (∼2) Cr’ 241 (58) I

I 222 (-54) Cr’ -16 (∼−2) Cr

FTP-166

Colhex 169 (3) NCol 193 (2.5) I

I 192 (-2) NCol 167 (-3) Colhex

FTP-816

Colhex 155 (12) I

I 152 (-12) Colhex

Figure 2. SAXS patterns in the Colhex phase of FTP-816 at 118°C and in the N phase of FTP-166 at 189°C; (10) is the Miller indice of the fundamental reflection from the hexagonal lattice, D is a short-range periodicity in the lattice plane; D⊥ and D// are shortrange periodicities of the lateral arrangement of twin columns in the N phase; hch: liquid-like lateral distances between molten chains; hfl, hπ: average distances between fluorene segments and stacked TP units (see text). For a comprehensive analysis of the supramolecular organisation, the features of the lateral arrangement of fluorene segments were inferred from single-crystal structures of simpler model molecules, such as 2,7-dibromo-9,9-dioctyl-9H-fluorene (Figure S541). In these structures, the fluorene segments close-pack parallel to each other, herringbone-like with nearly orthogonal ring planes, and with a respective longitudinal shift required by the chains on carbon 9 (spacing and shift are of respectively 5.2 Å and 4.0 Å).

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The effect of melting to a mesophase will mainly enhance conformational and structural disorder, and for instance change the apparent cross-section of the rings to oval, affecting only marginally the average spacing and longitudinal shift. These packing features were hence considered for the triad cores, and the selfassembling processes which could combine them to the stacking properties of TP were envisaged. Actually, both the packing features of the fluorene and the stacking distance of TP can easily be respected, but at the cost of a lower overlap between TP rings (Figure 3), a higher overlap resulting in important distortions of the arrangement of the fluorene cores. The SAXS patterns of the columnar mesophase of FTP-166 and FTP-816 are in agreement with this ‘twin’ column model. Specifically, the location of the scattering signal from average spacing of TP (hπ ≈ 3.7-3.75 Å, at around 100°C, Table S1) is similar to hexa(hexyloxy)-triphenylene (TP-66),42 but the very low intensity of the signal indicates indeed a low overlap between successive TP rings. Consistently, a further semi-diffuse scattering signal is observed at 5.0 Å for the lateral arrangement of the fluorene cores, hfl, while the molten state of peripheral chains is trivially attested by the broad scattering hch. In the small-angle range of the pattern, the unique very strong reflection was attributed to (10) of a hexagonal lattice, the absence of higher-order reflections being explained by the irregular interfaces between columns of lowoverlapped TP and aliphatic periphery.43 This lattice was found to involve a half-twin column, from the agreement between hπ and the columnar slice thickness, hmol, i.e. the ratio of (half)-molecular volume and lattice area (Tables 2 and S1). Such fractional molecular stack per lattice and the hexagonal symmetry imply equal distribution of fluorene cores and chains between neighboring TP columns. At the local-range, twin columns have nonetheless a given orientation and their arrangement in staggered rows leads to a short-range periodicity that could explain the presence and position in the pattern of a broad reflection D (Figure 3). Such an apparently undifferentiated periphery of cores and chains at the long-range lattice level has recently been encountered for a similar triad with triazatruxene rings connected to a thienopyrroledione core.25 The reason is obviously that the orientation of twin columns frequently changes in the lattice plane, so that cores and chains appear equally distributed in all crystallographic directions. This mechanism is equivalent to the permutation of entire columns or column segments reported for systems self-associating in columns of different nature and which also led to Colhex phases of fractional molecular stacks.30,44

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NCol (30)

Vmol = 3204 Å3 (ρ = 1.00); χVch = 0.696; Hmol ≈ 3.64 Å

FTP-188 NCol (80)

Smol ≈ 881 Å2

FTP-188 NCol (130)

Vmol = 3323 Å3 (ρ = 0.96); Hmol ≈ 3.77 Å Smol ≈ 839 Å2 Vmol = 3442 Å3 (ρ = 0.93); Hmol ≈ 4.11 Å

FTP-888 Colrec (30)

a = 31.26 Å; b = 29.70 Å; A = 928 Å2 (Z = 1); Smol = 928 Å2; a/b = 1.052

FTP-888 NCol (80)

Smol ≈ 881 Å2

FTP-888 NCol (100)

Vmol = 3584 Å3 (ρ = 0.98 g/cm3); χVch = 0.728; hmol = 3.86 Å Vmol = 3717 Å3 (ρ = 0.95 g/cm3); Hmol ≈ 4.22 Å Smol ≈ 859 Å2 Vmol = 3770 Å3 (ρ = 0.94 g/cm3); Hmol ≈ 4.39 Å

a

a, b, A, Z: rectangular lattice parameters, lattice area and number of molecular stacks per lattice; Smol: area per molecular stack in the columnar plane (Colhex, Colrec phases: Smol = A/Z; NCol phase: same calculation assuming either Colhex (d10 ≈ D⊥; only for FTP-166) or Colrec (d11 ≈ D⊥, d21 ≈ D//; otherwise) localrange structures); Vmol, ρ: molecular volume and density; χVch: aliphatic volume fraction; hmol = Vmol/Smol, Hmol = Vmol/Smol: molecular slice thicknesses in Colhex/Colrec and in NCol phases, respectively.

Table 2. Molecular, structural and geometrical parameters of the FTP-nom triads and of the mesophases (at T in °C).a

FTP-166 Colhex (98) FTP-166 NCol (189) FTP-816 Colhex (118)

a = 20.63 Å; A = 369 Å2 (Z = 1/2); Smol = 737 Å2 Vmol = 2794 Å3 (ρ = 0.98 g/cm3); χVch = 0.633; hmol = 3.79 Å Smol ≈ 726 Å2 Vmol = 2975 Å3 (ρ = 0.92 g/cm3); Hmol ≈ 4.10 Å a = 20.61 Å; A = 368 Å2 (Z = 1/2); Smol = 736 Å2 Vmol = 2950 Å3 (ρ = 0.96 g/cm3); χVch = 0.648; hmol = 4.01 Å

FTP-866 Colrec (25)

a = 36.28 Å; b = 21.33 Å; A = 774 Å2 (Z = 1); Smol = 774 Å2; a/b = 1.701

FTP-866 NCol (134)

Smol ≈ 767 Å2 Vmol = 3276 Å3 (ρ = 0.94); Hmol ≈ 4.27 Å

FTP-188

Smol ≈ 880 Å2

Vmol = 3029 Å3 (ρ = 1.01 g/cm3); χVch = 0.679; hmol = 3.91 Å

Figure 3. Self-organisation models of triad molecules in the various columnar phases (NCol, Colhex, and Colrec). Frame: schematic representation of the mesogenic core of FTP-nom triads, composed of two triphenylene rings (TP, blue discs) connected by 9,9-dialkylfluorene cores (green rectangles); side view of a twincolumn model respecting the packing features of both the fluorene cores and the stacking distance of TP rings (columnar axis along n), but to the cost of low overlap of TP rings; section view of halftwin column, displaying herringbone arrangement of fluorene moieties and the face-to-face stacking of TP, respectively characterized by average distances hfl and hπ. In-plane arrangements (and 2D lattices) of twin columns in staggered rows with either

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Crystal Growth & Design

frequent changes of the row direction (Colhex, with half-twin column per lattice), unique row direction and only changes of the twin column orientation within rows (Colrec, with entire twin column per lattice), or with random orientation (NCol, characterized by short-range periodicities perpendicular and parallel to the direction of staggered twin columns [d10 is the fundamental periodicity of the hexagonal lattice and D, a short-range periodicity from local-range arrangement; d11 and d21 are periodicities from the rectangular lattice; ξ schematizes the columnar domain size (see text)]. A rectangular columnar mesophase, Colrec, was found for two compounds with higher aliphatic fractions: FTP-866 and FTP888. The most striking feature, compared to the Colhex phase, is the considerable enhancement of hπ that evidences high TP rings overlap. Signal position and thus TP spacing remained on the contrary unchanged, but hfl slightly moved from 5.0 to 5.2 Å, possibly through the presumed distortion of the lateral arrangement (see above). The small-angle range was composed of a weak first-order reflection with a trailing wing and of a second weak and broadened reflection, indexed to (11) and (21) of a rectangular lattice, while reflection (20) fell within the trailing profile (Figure S4). Consistently, the lattice area matches an entire twin column and leads to hmol values in agreement with hπ. The lattice geometry (a/b