# Aryl/fluoroaryl-substituted triphenylene discotic liquid crystals: Synthesis, mesomorphism, and photophysical properties

Jiao He, Yang Chen, Ping Hu, Bi-Qin Wang, Ke-Qing Zhao, Bertrand Donnio

► To cite this version:

Jiao He, Yang Chen, Ping Hu, Bi-Qin Wang, Ke-Qing Zhao, et al., Aryl/fluoroaryl-substituted triphenylene discotic liquid crystals: Synthesis, mesomorphism, and photophysical properties. Journal of Molecular Liquids, 2024, 414, pp.126218. 10.1016/j.molliq.2024.126218. hal-05190257

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# Aryl/fluoroaryl-substituted triphenylene discotic liquid crystals: Synthesis, mesomorphism, and photophysical properties

Jiao He<sup>a</sup>, Yang Chen<sup>a</sup>, Ping Hu<sup>a</sup>, Bi-Qin Wang<sup>a</sup>, Ke-Qing Zhao<sup>a,*</sup>, Bertrand Donnio<sup>b,*</sup>

## ARTICLE INFO

**Keywords:**

S<sub>N</sub>Ar

Discotic liquid crystal

Perfluoroarene

Polar π-interaction

DFT computation

## ABSTRACT

Fluorine aromatic hydrocarbons are ubiquitous optoelectronic functional materials and their efficient synthetic methods are still the subject of rapid, ongoing development. Here, several aryl- and fluoroaryl-substituted triphenylenes (Ar-TP/FAr-TP) were synthesized via either Suzuki-Miyaura cross-coupling or aromatic nucleophilic substitution (S<sub>N</sub>Ar) of perfluoroarenes, respectively, starting indifferently from 2-bromo-3,6,7,10,11-pentakis (hexyloxy)triphenylene (Br-TP). These compounds possess very high thermal stability with initial decomposition temperature over 300 °C, as measured by thermal gravimetric analysis (TGA). Differential scanning calorimetry (DSC), polarized optical microscopy (POM), and small/wide angle X-ray scattering (S/WAXS) revealed that they all self-organize into hexagonal columnar (Col<sub>hex</sub>) mesophase over broad temperature ranges, occurring at room temperature for some of them, with higher stability than the archetypical 2,3,6,7,10,11-hexakis(hexyloxy)triphenylene parent compound. Under the combined influence of the conjugated π-system and core-side arm distortion, the clearing temperatures and mesophase ranges increase from phenyl- (P-TP), naphthyl- (N-TP), to biphenyl-triphenylene (BP-TP). The number of fluorine atoms on the side fluoroaryl group also impacts the columnar mesomorphism: the more fluorine atoms, the higher the clearing temperatures and the wider the Col<sub>hex</sub> phase ranges, concomitantly with increasing arene-fluorarene intermolecular interactions. In addition, these compounds show blue-violet photoluminescence in solution: the largest fluorescent absolute quantum yield is measured for BP-TP (45 %), while the smallest is for 7FN-TP (10 %). Perfluoroaryls tend to lower the LUMO, which results in fluorescence red-shift. The arene-perfluoroarene polar-π interactions improve the physical properties of these FAr-TP discotic liquid crystals.

## 1. Introduction

Organofluorine compounds have been playing increasingly important roles in the biological and pharmaceutical industries [1–3], as well as in the design of advanced functional materials [4–6]. Due to the unique properties of the fluorine atom such as its small size, the ease with which it forms hydrogen bonds, its strong electron-withdrawing effects and many other advantages, the resulting fluoroarene fragments are often introduced into mesogenic molecules in place of hydrogenated aryls to enhance and modulate mesomorphism [7–10]. Further, insertion of fluorine atoms in liquid crystal materials for display devices can reduce viscosity of the materials, lower the driving voltage, enhance dielectric constant, widen mesophase range, and improve the thermal, optical and electrical stability of the materials [7–10]. Therefore, most of the commercial materials used in super twisted nematic

(STN), thin film transistor (TFT), active matrix (AM), ferroelectric liquid crystals (FLCs) contain fluorine-arene fragments in their molecular structures [6–10].

Generally, the introduction of fluorine atoms in liquid crystal molecules can be of two types: insertion in the flexible chain [6] or substitution on the rigid aromatic core, with completely different outcomes [7]. Insertion in flexible chains of rod-like molecules usually increases nanophase-segregation promoting both mesophase formation and stabilization [11–14]. However, this strategy also has some disadvantages as when the length or number of CF₂ repeating units in the chains becomes too large, the supramolecular organization is drastically affected, even is detrimental to mesophase induction, and/or leads to solubility and purification issues, as well as environmental contamination. In contrast, when the fluorine atoms are substituted on aromatic cores, the corresponding molecules have none or less of these drawbacks; actually,

<footnote><sup>a</sup> College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610066, China</footnote>

<footnote><sup>b</sup> Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), CNRS-Université de Strasbourg (UMR 7504), F-67034 Cedex 2 Strasbourg, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: kqzhao@sicnu.edu.cn (K.-Q. Zhao), bertrand.donnio@ipcms.unistra.fr (B. Donnio).</footnote>
perfluoroarenes incorporated in π-conjugated systems possess strong electron-withdrawing effect that help improving the distribution of electrons in the mesogenic molecule and thereby is useful to tailor the liquid crystal, electronic and optical properties [15,16]. Stronger polar-π interactions in mixed arene- and perfluoroarene-containing mesogens also favor closer intermolecular contacts, resulting in the stabilization of mesophases, and higher charge carrier mobility [17–21].

Organofluorine chemistry for the production of perfluoroarenes is under constant development and evolution [22–30], and “palladium-free” strategies are particularly desirable for low-cost fluorine-based chemicals. For the synthesis of fluorinated discoid molecules (F-DLCs) in particular, transition-metal-catalyzed cross-coupling with combination of Scholl oxidative cyclization [31–34], photocyclodehydrofluorination (PCDHF) [35–37], Yamamoto reaction [38], or electrochemical reaction [39], are commonly used. Very recently, we reported an interesting and fruitful alternative transition-metal-free method for the synthesis of various fused and aryl-substituted F-DLCs, namely the perfluoroarene nucleophilic substitution ($S_NAr$) with arylithium ($ArLi$) [40–42].

Discotic liquid crystals (DLCs) have attracted extensive attentions due to their unique self-assembly behavior, defect self-healing properties, and one-dimensional high-charge carrier mobility [43–54]. Compared with traditional π-conjugated polymers and multi-domain crystalline materials, DLCs show usually higher charge carrier mobilities and have several favorable characteristics that make them easier to process into thin film electronic devices by wet processing techniques. Among them, the archetypical triphenylene (TP) has become one of the most widely studied DLC motifs by virtue of its planar conjugated structure, versatile chemistry and high stability [55,56]. The TP motif

can be easily incorporated into larger π-conjugated polycyclic aromatic cores, in only two steps by combined Suzuki-Miyaura cross-coupling and Scholl oxidative reaction [31–34]. Resulting π-extended poly(hetero) atomic hydrocarbon star-shaped, plate-shaped and butterfly-shaped liquid crystals containing the triphenylene moiety, surrounded by several peripheral flexible alkoxy chains, have been found to exhibit superior liquid crystal performances and higher charge carrier migration rates than other DLCs [57–61]. In fact, the combination of triphenylene discogen with other functional blocks results not only in the mesomorphic behavior modulation, but physical properties are enriched, such as stimuli-responsive luminescence, aggregation-induced luminescence, and many others [62,63]. Triphenylene-based discotic liquid crystals have great potential applications in sensors, organic field-effect transistors (OFETs), organic photovoltaic devices (OPVs) and organic light-emitting diodes (OLEDs).

Here, 2-aryl- and 2-fluoroaryl-triphenylene derivatives (Ar-TP/FAr-TP), with phenyl (P), naphthyl (N) and biphenyl (BP) groups with different degree of aromatic fluorination, were prepared either by Suzuki-Miyaura cross-coupling reaction (Ar-TP) or fluoroaromatic nucleophilic substitution ($S_NAr$) reaction (FAr-TP) in good yields (Scheme 1). All compounds were characterized by ¹H, ¹³C, ¹⁹F NMR, HRMS and elemental analysis. The liquid crystal behavior of the target compounds was studied by polarized optical microscopy (POM), differential scanning calorimetry (DSC) and small- and wide-angle X-ray scattering (S/WAXS). In addition, the photophysical properties of the target compounds were studied by UV-visible absorption spectra and fluorescence emission spectra, with DFT computation results to interpret the experimental outcomes. The structure-properties relationship

<figure><img src="image_4.png" /><figcaption>**Scheme 1.** Synthesis, nomenclature, and yields of Ar-TP and FAr-TP compounds.</figcaption></figure>
between the number and position of fluorine atoms, liquid crystal behavior and optical properties was explored.

## 2. Experimental

### 2.1. Materials

The reagents used were of analytical purity (AR). The solvents, petroleum ether (PE), dichloromethane (DCM), tetrahydrofuran (THF), ethyl acetate (EtOAc), ethanol (EtOH), nitromethane and N,N-dimethy lformamide (DMF), were purified by classical methods prior to use. The other reagents were not further purified before use.

### 2.2. Compounds characterization

All the final compounds were characterized by elemental analyses (EA), performed on an EA3000 instrument; by NMR spectroscopy, ¹H, ¹⁹F, and ¹³C spectra were measured on an America Bruker Advance; and by high-resolution mass spectrometry (HRMS), recorded on a Fourier Transform ion cyclotron resonance mass spectrometer (7.0 T FTICR-MS) instrument made by IonSpec with MALDI as the ion source.

### 2.3. Mesomorphic properties

For the identification of the liquid crystalline phases, optical textures were observed by an Olympus BH2 polarizing optical microscope (POM), equipped with a MettlerFP82 hot-stage for which temperature was controlled by XPR-201 and MettlerFP90. The materials were sandwiched between untreated glass plates and the observation of “natural” textures was performed on cooling from the isotropic liquid. All pictures of optical textures were observed under crossed polarizing condition. The data acquisition was facilitated by Linksys 32 software (LINKAM, UK). The phase transition temperatures and related enthalpy values were precisely determined using an America TA Discovery DSC Q100 instrument with a heating or cooling rate of 10 °C/min, under a nitrogen atmosphere. 5–10 mg of samples were placed in aluminum crucibles. Prior DSC measurement, the thermal stability was measured on an America TA Discovery TGA Q500 instrument with a heating rate of 10 °C/min. For the characterization of the mesophases, S/WAXS experiments were run on a Rigaku Smartlab with a TCU 110 temperature controller, while the sample temperature was controlled within ±1 K and the X-ray sources (Cu Kα, λ = 0.154 nm) were provided by 40 kW ceramic tubes as well as a scanning speed of 10 °C/min.

### 2.4. Optical properties

UV–Vis absorption spectra were recorded on a Perkin Elmer Lambda 950 spectrophotometer at room temperature. Fluorescence was measured on a HORIBA Fluoromax-4p, and the absolute quantum yields were measured by a HORIB-F-3029 Integrating Sphere, HORIBA, Kyoto, Japan.

## 3. Results and discussion

### 3.1. Molecular design, synthesis, and characterization

The synthesis, molecular structures, nomenclature, and synthetic yields of all compounds are shown in general Scheme 1. Fluorine-containing polycyclic aromatic hydrocarbons (F-PAH), 5FP-TP, 7FN-TP, 9FBP-TP, and 4FBP-TP, were synthesized in one-pot by aromatic nucleophilic substitution reaction (S<sub>NAr</sub>) of perfluoroarenes (perfluorobenzene (C<sub>6</sub>F<sub>6</sub>) [40,42], perfluoronaphthalene (C<sub>10</sub>F<sub>8</sub>) [40], decafluorobiphenyl (C<sub>6</sub>F<sub>5</sub>-C<sub>6</sub>F<sub>5</sub>) and pentafluorobiphenyl (C<sub>6</sub>F<sub>5</sub>-C<sub>6</sub>H<sub>5</sub>) [41]) directly with Li-TP and obtained in satisfactory yields (≈20–40 %); Li-TP were readily obtained from lithium-halogen exchange between 2-bromo-3,6,7,10,11-pentakis(hexyloxy)triphenylene (Br-TP)

[64] and n-BuLi [65–67] at low temperature (Right path, Scheme 1). Additional fluorine-free and some other fluorine-containing triphenylene derivatives with various degrees of fluorination were also prepared, for comparison and evaluation of the effect of arene-perfluoroarene polar-π interactions on the mesomorphism and optical properties, by classical Pd-catalyzed Suzuki-Miyaura cross-coupling reactions between the same Br-TP and commercially available arylboronic acids (Left path, Scheme 1). These couplings proceeded in rather good yields (56–72 %). All 11 target compounds were fully characterized by ¹H-, ¹³C-, ¹⁹F NMR (Figs. S1.1–S1.30), HRMS (Figs. S2.1–S2.11) and elemental analysis. These measurements agree with the proposed structures. All synthetic details, including those for the preparation of precursory Br-TP, and characterization are gathered in supplementary information.

### 3.2. Liquid crystal properties

The optical textures and phase transition behaviors of all compounds were observed via hot stage polarizing microscopy (POM). All POM images were captured during the cooling process at a cooling rate of 1 °C/min after the compounds was heated into the isotropic liquid (Figs. 1 and S3).

Most compounds only show one phase transition, from the isotropic liquid to the mesophase. For P-TP, N-TP and 1FP-TP only, crystallization was observed at lower temperature; o-2FP-TP crystallizes at temperatures below zero degrees Celsius (DSC). When transitioning from the isotropic liquid to the mesophase, the textures of all compounds slowly grow from a leaf-like and eventually exhibit linear defect textures, which are typical optical textures of columnar mesophase [57–61]. In addition, compared with Ar-TP, FAr-TPs exhibit huge area of optical dark field, showing FAr-TP organizing into ordered, 2D columnar assembly with a mean columnar axis perpendicular to the glass substrate surface, which is beneficial to the OLED and OPV solar cells.

Before investigating further the mesomorphism of Ar-TP and FAr-TP compounds, the thermal stability was first tested by thermal gravimetric analysis (TGA) under N₂ atmosphere in dynamic mode. The TGA curves (Fig. 2a) show that all compounds undergo two thermal decomposition processes, an initial thermal decomposition temperature (T<sub>dec</sub>, at weight loss 5 %) above 300 °C (Table S1), probing their excellent thermal stability, and a second one above ca. 450 °C. It is quite obvious that the thermal stability of these discogens is not affected either by the nature of the aryl group or the number of fluorine atoms. In all cases, investigation of the thermotropic liquid crystal properties was performed well below

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** POM textures, observed between crossed polarizers (yellow arrows), of some representative Ar-TP/FAr-TP compounds: (a) P-TP (80 °C), (b) N-TP (100 °C), (c) m-2FP-TP (100 °C), (d) 3FP-TP (110 °C); more POM images can be seen in the supplementary file (Fig. S3).</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 2.** (a) TGA curves of the monosubstituted Ar-TP/FAr-TP derivatives recorded at 10 °C min⁻¹; (b) Phase transition temperature histograms of FAr-TP/Ar-TP series and TP (second heating, rate: 10 °C/min; measurements at different rates and phase diagram on cooling are shown in Fig. S5).</figcaption></figure>

these temperatures.

Differential scanning calorimetry (DSC) are summarized in Fig. 2b (see also Figs. S4 and S5 and Table S2 for details). All phase transitions are consistent with the POM observations. Compared with the parent 2,3,6,7,10,11-hexakis(hychoxy)triphenylene (TP) [68], the extension of the conjugation with aryl groups systematically results in the enhancement of the clearing temperatures and consequently to the widening of the mesophase ranges [31,32,41]. This enhancement is substantial (>50 °C) for triphenylenes containing biphenyl (BP-TP) and naphthyl (N-TP) side groups. The presence of both large conjugated systems must somehow contribute to more efficient π-π stacking and to the improvement of the columnar mesophase stability. Similarly, the presence of fluorinated aryls provides additional fluoroarene-arene interactions, also contributing to efficient π-π stacking between the molecules in the mesophase, and to the enlargement of the mesomorphic range, which increases with the increase of the degree of fluorination. Interestingly, as the number of fluorine atoms increases, crystallization is less favored.

The mesophases of FAr-TP/Ar-TP were characterized by small- and wide-angle X-ray scattering (S/WAXS) at several temperatures (Fig. S6, Tables S3 and S4), and the self-assembly mode within the mesophases was explored. The X-ray patterns for most compounds are quasi identical, exhibiting essentially one sharp and intense reflection in the small-angle range, and an additional small peak, indexed as (21), observed only for 7FN-TP and 4FBP-TP. The presence of one small-angle

reflection only renders the mesophase identification uncertain as either mesophase with hexagonal or a square lattice can be considered. However, supported by the characteristic optical textures observed by POM (presence of large, optically extinct dark regions due to optically uniaxial mesophases), and considering their structural similarity to related compounds [31], a Col<sub>hex</sub> phase mesophase can nevertheless be attributed for all of the compounds. The position of this main small-angle reflection is quasi-invariant with temperature, i.e. the intercolumnar distance is temperature-independent, but vary slightly with the nature of the side-group (Table S4, Fig. S7). The presence of only one reflection may be due to fuzzy aromatic-aliphatic interfaces [69], which suggest that the pending group must be partly immersed within the aliphatic continuum by crossing this interface [41].

In the large-angle region, a strong broad scattering halo corresponding to the lateral distances between molten aliphatic tails (h<sub>ch</sub> ≈ 4.20 Å), confirming the liquid-like nature of the mesophases, and a very intense and sharp signal, corresponding to extended π-π intracolumnar stacking interactions between consecutive aromatic cores (h<sub>π</sub> ≈ 3.55 Å). The sharpness of the wide-angle signal corresponding to the stacking periodicity h<sub>π</sub> reveals a long correlation range of stacking, ξ, of ca. 30 up to 110 Å, as calculated by the Debye-Scherrer formula which corresponds to regular and robust columnar stacks of ca. 8–27 molecules (Fig. 3, Table S4). The stacking correlation is generally increased upon increasing fluorination of the pending aryls (Fig. 3).

From DFT calculations (see below, Fig. 5), the optimized molecular conformation of all compounds is rather non-planar, with a substantial angle between the pending aryl group and the triphenylene core, the dihedral angle (Table S5) varying significantly from 40.63° for P-TP and up to ca. 48° for 5FP-TP, although with a slight decrease for intermediate degrees of substitution down to 35°, from ca. 39° for BP-TP increasing up to 42° for the partially fluorinated homolog and 47.34° for the fully fluorinated homolog, 9FBP-TP, and from 42.39° for N-TP to ca. 48° for 7FN-TP (Table S5). It can be seen that the larger the pending aryl moiety, the larger the dihedral angle (Fig. 3, P-TP ≈ BP-TP < N-TP). For both fluorinated biphenylene and naphthalene derivatives, and for 5FP-TP, fluorination contributes to a large increase of this angle, likely due to the proximity of fluorine atom with the alkoxy chain in the *meta* position of the triphenylene ring, as compared to their non-fluorinated counterparts, and particularly to phenyl-triphenylene P-TP derivatives whose angle remains more or less in the same range, not greatly affected by the degree of fluorination. Overall, such a molecular distortion from planarity induced by the pending group contributes to the slight increase of the molecular thickness, h<sub>mol</sub>, obtained by dividing the molecular volume by the columnar cross section (Fig. 3, Table S4); the h<sub>mol</sub> values,

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Variation of dihedral angles, molecular thickness, h<sub>mol</sub>, and stacking correlation length, ξ, as a function of the Ar/FAr side groups in Ar-TPs.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Schematic supramolecular organization (side and top views of columns, a, and lattice view, b) in $Col_{hex}$ phase of Ar-TP/FAr-TP compounds (black disc: Ar/FAr-TPs, blue disc: triphenylene moiety TP, green spheres: aromatic side-groups, Ar/FAr). Arrows indicate vertical fluctuations of TP cores with respect to lattice plane; aliphatic chains are not shown for clarity.</figcaption></figure>

ranging from 3.58 to 4.06 Å, are nevertheless very close (or only slightly larger than) to the stacking distance between consecutive cores (3.50 Å ≤ $h_π$ ≤ 3.57 Å). This implies that the triphenylene mesogens pile up in the columns with only small fluctuations with respect to the lattice plane (blue arrows in Fig. 4a) [41], which decrease substantially with fluorination (the dihedral values are consistent with such small fluctuations) (Fig. 3). As for the pending aryl fragments, as they are partly blended with the aliphatic chain's continuum separating the columns, they allow the molecules to wedge into the structure. The random distribution of the pending segments within the columns, facilitated by these molecular distortions, contributes to the overall cylindrical shape of the columns [70]. Such an arrangement is likely further stabilized by the presence of arene-fluoroarene intermolecular interactions. The hexagonal columnar phase thus emerges from the long-range order, almost parallel stacking of the central rigid discoidal molecular units into cylindrical columns, whose main axes are located at the nodes of the hexagonal array, surrounded by homogeneously distributed molten chains (Fig. 4).

### 3.3. Photophysical properties

UV-visible absorption and fluorescence emission spectra of FAr-TP/ Ar-TP were recorded in diluted THF (10⁻⁵ mol/L, Fig. 5). They show simple and similar absorption spectra with one higher energy band and the maximum absorption peak around 272–286 nm (Fig. 5a,c), and another additional second strongest bands about 336/310 nm (Fig. 5a).

<figure><img src="image_9.png" /><figcaption>**Fig. 5.** UV/Vis absorption (a) and fluorescence (b) spectra in THF micro-molar solution (same color code for compound names for absorption and emission), and variations of $λ_{abs,max}$ and $λ_{em,max}$ (c) as a function of the side groups in Ar-TP/FAr-TP compounds (QY, quantum yields of emission in brackets).</figcaption></figure>

The molar extinction coefficients range between 6- to 14 × 10⁴–10⁵ and ca. 2–4 × 10⁴ L cm⁻¹ mol⁻¹, for the large and the small band respectively, with some fluctuations depending on the nature of the side-block and the degree of fluorination (Table S5). The high energy band around 280 nm is due to π-π* electronic transition from the polybenzene structure and simultaneously, the low energy absorption band at 336 nm (and 310 nm for highly fluorinated compounds) is associated to the HOMO-1 to LUMO, electronic transition and intramolecular charge transfer (ICT) transition from TP to (fluoro)aryl moieties [41]. Although, UV-visible absorption spectra are somehow quite similar for all compounds, their fluorescence emission spectra are in contrast very different (Fig. 5b,c, Fig. S8). The emission in THF is concentrated in the blue-violet band with a maximum fluorescence emission wavelength ($λ_{em}$) from 394 nm to 490 nm. In the non-fluorinated system, the small $λ_{em}$ redshift is likely caused by the expansion of the π-conjugated system
(λ<sub>em</sub>: P-TP < N-TP < B-TP, Fig. 5c). However, this redshift is more important for the fluorinated compounds, particularly for the biphenylene and naphthalene derivatives with respect to their nonfluorinated homologs, in consistency with increased dihedral angle, whereas this shift is smaller in the P-TP compounds. In contrast, the emission is quite localized around a mean value of around 400 nm in the P-TP family, irrespective of the degree of fluorination.

For FAr-TP, owing to the introduction of the highly fluorinated aryl,
the conjugation is extended in LUMO and the energy levels are lowered.
At the same time, the fluorine atom, as strong electron-withdrawing
group, notably enhances the ICT effect, so the electron delocalization
is larger for the excited state, leading to the red-shift of λem. Addition-
ally, the absolute fluorescence quantum yields were measured in THF.
The quantum yields range from 9.6 % to 44.6 % (Fig. 5c), with lower
absolute fluorescence quantum yields for perfluoroaryl-TP due to the
stronger intermolecular interaction in solution. In general, non-
fluorinated Ar-TP compounds (P-TP, BP-P and N-TP) exhibit good
photophysical properties, higher potential application value in the fields
of organic luminescent materials.

3.4. DFT study

For further understanding the effects of fluorine atoms and π-conjugating systems on the molecular electronic structure and photophysical properties, DFT computations at the B3LYP/6-31G (d) theoretical level were performed (Fig. 6, Table S6) [71]. The optimized conformations of these molecules are all distorted non-planar conformations, and there are different distorted dihedral angles between triphenylene and adjacent aryl units. For example, a large twisted dihedral angle can be caused by the large volume naphthalene unit (N-TP, 42.39°) due to the steric hindrance of the side aryl. However, when the naphthalene changes to a biphenyl (BP-TP, 39.30°) with greater degree of freedom, the dihedral angle is significantly reduced. It is noted that the 5FP-TP, 7FN-TP, and 9FBP-TP display the greatest dihedral angles (48.63 to 47.34°); but surprisingly, 4FBP-TP just shows moderate distortion (42.00°). Obviously, when the fluorine atom is far away from the triphenylene core (1FP-TP, 2FP-TP, 3FP-TP), the π-electron density is dispersed due to electron-withdrawing effect, which makes the molecule more planar. But when the fluorine atom is close to the triphenylene core (5FP-TP, 7FN-TP), the repulsions between the electron cloud of F/Ar and the triphenylene/methoxy aggravate the dihedral distortion. These computational results seem to be consistent with the experimental ones: 3FP-TP exhibits the highest clearing temperature, the most stable columnar mesophase, due to the smallest dihedral distortion and high arene-perfluoroarene interaction, followed by the 1FP-TP and both 2FP-TP homologs.

In addition, the frontier molecular orbital electron cloud distribu-
tions of HOMO/LUMO and energy gaps were also calculated by DFT. For
the HOMO, the electron cloud distributions are surprisingly consistent:

they are localized on the highly conjugated triphenylene unit; for the
LUMO, the electron clouds shift from the central TP core to the side
aromatic ring. In addition, for FAr-TP, due to the strong electron-
withdrawing ability of the fluorine atoms, it forms an intramolecular
D-A system between triphenylene and FAr, so the ICT phenomenon
becomes more obvious with the increasing substitution of the fluorine
atoms. For more details, in the LUMO of 7FN-TP and 9FBP-TP, the
electron clouds are almost completely separated from the triphenylene
cores and located on the perfluoroaryl arms.

Finally, we investigated the HOMO/LUMO and energy gaps of these
functional molecules, evaluating their optical, electronic and semi-
conducting properties. The most heavily fluorinated compounds 7FN-TP
and 9FBP-TP, have the lowest LUMO energies and narrowest energy
gaps (3.45 eV), which agrees with the highest red-shifted fluorescent
emissions with peaks at 470 and 455 nm, respectively (Fig. 5c). It is
demonstrated that the perfluoroaryl substitution strategy can be used for
the molecular designing of functional material, to modify the lumines-
cent colors.

4. Conclusion

Series of 2-fluoroaryl- and 2-aryl-3,6,7,10,11-pentakis(hexyloxy)tri-
phenylene (FAr-/Ar-TP) discogens were synthesized by S<sub>N</sub>Ar or Suzuki-
Miyaura cross-coupling reactions. The number of fluorine atom and the
size of aryl group affect the self-assembly behavior and photophysical
property. The extension of the π-conjugated system increases the
clearing temperatures and broaden the mesophase ranges. All com-
pounds exhibit ordered Col<sub>hex</sub> mesophase with regular intracolumnar
π-π stacking. Compounds BP-TP and N-TP with larger intramolecular
steric hindrance exhibit weakened molecular aggregation and less stable
Col<sub>hex</sub> mesophase. In homologous compounds of 2-phenyl-TP, the
introduction of fluor atom on phenyl group improves the stability of the
Col<sub>hex</sub> mesophase. When fluor atom on phenyl do not significantly affect
the phenyl-triphenylene core distortion, the more fluor atom on phenyl,
the higher the clearing temperatures and the wider the mesophase
range. This is due to arene-perfluoroarene polar-π interactions beneficial
to the stability of the columnar mesophase. All compounds show
excellent photophysical properties in THF solution: they exhibit blue-
violet photoluminescence, with absolute fluorescence quantum yield
up to 45 %. These aryl-triphenylene DLCs have potential applications in
liquid crystal semiconductors, light-emitting devices and other fields.

5. Author statement

All authors have accepted the responsibility for the content of the
manuscript, and consented to its submission, reviewed all the results,
and approved the final version of the manuscript. All authors contrib-
uted to the writing of the paper, the measurements and the analysis of
the results.

<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Optimized structures, dihedral angles values (in °) between triphenylene and aryl side group, frontier molecular orbital electron cloud distributions (HOMO and LUMO) and energy band gaps values (in eV) in FAr-TP/Ar-TP.</figcaption></figure>
CRediT authorship contribution statement

**Jiao He:** Investigation, Formal analysis. **Yang Chen:** Investigation, Formal analysis. **Ping Hu:** Funding acquisition. **Bi-Qin Wang:** Funding acquisition. **Ke-Qing Zhao:** Writing – review & editing, Writing – original draft, Supervision, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Ber-trand Donnio:** Writing – review & editing, Writing – original draft, Investigation, Formal analysis, Data curation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.

Acknowledgements

YC and JH contributed equally to this work. We thank the National Natural Science Foundation of China for funding (51773140, 51973143 and 21772135). BD thanks the CNRS and the University of Strasbourg for constant support.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.molliq.2024.126218.

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