Next Article in Journal
An In Vitro Catalysis of Tea Polyphenols by Polyphenol Oxidase
Previous Article in Journal
Effects of Dietary Ferulic Acid on Intestinal Health and Ileal Microbiota of Tianfu Broilers Challenged with Lipopolysaccharide
Previous Article in Special Issue
Manipulating the Subcellular Localization and Anticancer Effects of Benzophenothiaziniums by Minor Alterations of N-Alkylation
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Molecules
Volume 28
Issue 4
10.3390/molecules28041721
molecules-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Pyrene-Fused Poly-Aromatic Regioisomers: Synthesis, Columnar Mesomorphism, and Optical Properties

by
Qing Zeng
1,
Shuai Liu
1,
Hang Lin
1,
Ke-Xiao Zhao
1,
Xiao-Yan Bai
1,
Ke-Qing Zhao
1,*,
Ping Hu
1,
Bi-Qin Wang
1 and
Bertrand Donnio
2,*
1
College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610066, China
2
Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), CNRS-Université de Strasbourg (UMR 7504), 67034 Strasbourg, France
*
Authors to whom correspondence should be addressed.
Molecules 2023, 28(4), 1721; https://doi.org/10.3390/molecules28041721
Submission received: 31 October 2022 / Revised: 3 February 2023 / Accepted: 7 February 2023 / Published: 10 February 2023
(This article belongs to the Special Issue π-Conjugated Molecular Systems)
Browse Figures
Versions Notes

Abstract

:
π-Extended pyrene compounds possess remarkable luminescent and semiconducting properties and are being intensively investigated as electroluminescent materials for potential uses in organic light-emitting diodes, transistors, and solar cells. Here, the synthesis of two sets of pyrene-containing π-conjugated polyaromatic regioisomers, namely 2,3,10,11,14,15,20,21-octaalkyloxypentabenzo[a,c,m,o,rst]pentaphene (BBPn) and 2,3,6,7,13,14,17,18-octaalkyloxydibenzo[j,tuv]phenanthro [9,10-b]picene (DBPn), is reported. They were obtained using the Suzuki–Miyaura cross-coupling in tandem with Scholl oxidative cyclodehydrogenation reactions from the easily accessible precursors 1,8- and 1,6-dibromopyrene, respectively. Both sets of compounds, equipped with eight peripheral aliphatic chains, self-assemble into a single hexagonal columnar mesophase, with one short-chain BBPn homolog also exhibiting another columnar mesophase at a lower temperature, with a rectangular symmetry; BBPn isomers also possess wider mesophase ranges and higher mesophases’ stability than their DBPn homologs. These polycyclic aromatic hydrocarbons all show a strong tendency of face-on orientation on the substrate and could be controlled to edge-on alignment through mechanical shearing of interest for their implementation in photoelectronic devices. In addition, both series BBPn and DBPn display green-yellow luminescence, with high fluorescence quantum yields, around 30%. In particular, BBPn exhibit a blue shift phenomenon in both absorption and emission with respect to their DBPn isomers. DFT results were in good agreement with the optical properties and with the stability ranges of the mesophases by confirming the higher divergence from the flatness of DBPn compared with BBPn. Based on these interesting properties, these isomers could be potentially applied not only in the field of fluorescent dyes but also in the field of organic photoelectric semiconductor materials as electron transport materials.

1. Introduction

Pyrene and its derivatives are well known for their remarkable photophysical and electronic properties, such as fluorescence with high quantum yields, excimer-based luminescence, piezo-chromic and mechano-chromic fluorescence, and aggregation-induced emission (AIE) behaviors [1,2,3,4,5,6]. π-Extended pyrene compounds with high luminescent and semiconducting properties have been investigated as electroluminescent materials to be applied in organic light-emitting diodes (OLEDs), transistors (OFETs), and solar cells (OSCs) [7,8,9]. Fluorescence and electronic behaviors of molecules strongly depend on their molecular packing and specific interactions with their environment; hence, their association with liquid crystals has been found highly attractive and promising in the perspective of materials science to, for instance, control these interactions and their processing into thin films.
Of particular interest in this field of research are discotic liquid crystals (DLCs), which usually form columnar nanostructures through self-assembly, and as such represent a unique type of soft material with anisotropic one-dimensional electrical and photoconductive properties [10,11,12,13,14,15,16,17,18,19,20,21]. DLCs display higher charge-carrier mobility when compared with that of amorphous silicon, π-conjugated polymers, and polydomain crystalline materials [18], with advantageous features of easier processing into thin-film electronic devices [10]. In the bulk discotic mesomorphic states, electronic charge carriers can efficiently hop along the one-dimensional conductive columns, while the electronic transport between the columns is blocked, as the saturated flexible chains around the rigid discotic cores naturally act as insulators. Thus, the enhanced charge-carrier transport performances of DLCs rely on this antinomic “rigid–flexible” nature of these molecular structures, which are generally composed of a large, central π-conjugated core to which six to eight peripheral alkyl chains are grafted.
Despite the great qualities of pyrene, surprisingly very few pyrene-containing liquid crystals have been reported so far. Due to its natural pseudo-circular shape, pyrene is intrinsically conducive to the formation of discotic systems, but its insertion within calamitic or bent structures has not been detrimental to mesomorphism induction. At first, 1-functionalized pyrene was reported to produce dimeric liquid crystals, symmetrical and non-symmetrical, and to yield essentially nematic (also including the so-called twist–bend nematic) and smectic phases [22,23,24,25]. Pyrene was also inserted in cyclophanes (via 1,6-positions), i.e., pyrenophanes, also yielding a nematic phase [26,27,28], as well as in polycatenars [29] and, recently, in some bent-core mesogens [30], for which columnar mesophases were systematically induced. Dendrimers bearing apical mono- and disubstituted pyrene moiety [31,32,33,34,35,36,37,38,39] have also been reported to self-organize into columnar and cubic mesophases. Lamello-columnar phases were reported for some pyrene with 1,6- and 1,8-disubstitution [40,41,42].
DLCs based on pyrene can be designed through the selective multi-substitution of the pyrenic core or via its association with classical discotic cores, as strong promoters for columnar-phase induction. As for the first approach, the most commonly used substitution pattern in the design of pyrene-cored X-/star-shaped mesogens is the 1,3,6,8-tetrasubstitution, essentially because of its facile synthetic accessibility [8,43,44,45,46,47] (1–4, Chart 1); however, as exceptions, mesogens with 4,5,9,10-tetrasubsititution [48] and 2,4,5,7,9,10-hexasubstitution [49] have also been reported. In the second approach, pyrene (Py) has been reported to combine with “work-horse” discogens, such as hexabenzocoronene (HBC-Py) [50], triphenylene (TP-Py) [51,52], and phthalocyanine (Pc-Py) [53,54], to co-self-assemble into columnar mesophases, and/or with two-dimensional segregated crystalline assemblies of the different discogens on HOPG solid–liquid interfaces, as observed directly using scanning tunneling microscopy (STM) techniques [50,51,52].
Some of these pyrene-based discotic molecules have been found to exhibit unidimensional long-range (time of flight (TOF)-measured) charge-carrier hopping rate (μ) ranging from 10−5 up to 10−2 cm2·V−1·s−1. In addition, some DLC pyrene compounds have shown high fluorescence quantum yields in both solution and solid states [6,8], were found to possess intriguing mechano-chromic luminescent properties [44] and also to behave as supramolecular organogelators [8], fluorescence sensors [1], as well as fluorescent ferroelectric liquid crystals [55,56].
As seen from the various types of structures reviewed above, the current research on pyrene DLCs mainly focuses on methods of functionalizing pyrene by connecting various functional building blocks with single or acetylenic triple bonds to produce X-/star-shaped compounds (e.g., 1–4, Chart 1), while π-conjugated systems fused via the annulation of aromatic rings together (e.g., 5–8, Chart 1) are very limited due to the synthetic methods and reduced solubility of the resulting large DLCs [1,2,3]. Moreover, the positional/regional isomers of polycyclic aromatic hydrocarbons (PAHs) generally exhibit versatile physical properties, such as solid-state π–π stacking modes, electronic properties, mesomorphism, gelation, fluorescence behavior, and photoconductivity/device performance. Such comparative studies are surprisingly not that common (see below); nevertheless, they could be highly relevant in the field of organic electronics.
Herein, we report the successful implementation of the synthesis of two series of liquid crystal isomers based on pyrene equipped with eight long peripheral alkoxy chains, namely BBPn and DBPn, respectively, through the Suzuki–Miyaura coupling/Scholl oxidative cyclization tandem strategy. Their liquid crystalline behavior and photophysical properties were investigated in great detail, and the structure–property relationship was analyzed using the density functional theory (DFT).

2. Results

2.1. Molecular Designing, Synthesis, and Characterization

The chemistry of pyrene is mainly based on electrophilic aromatic substitution reactions, and it can be 1-monosubstituted, 1,6-disubstituted, and 1,3,6,8-tetrasubstituted [1,2,3]. Pyrene-cored dendrimers synthesis needs 1-monosubstituted or 1,6-disubstituted pyrene [31,32,33,34,35,36,37,38,39] as starting materials, while X-/star-shaped pyrene derivatives are usually synthesized from 1,3,6,8-tetrabromopyrene (1–4, Chart 1) [8,43,44,45,46,47]. Recently, Kumar et al. [57] used 1,6-dibromopyrene and disubstituted arylethynylene in Pd-catalyzed coupling reaction in tandem with Scholl cyclopentannulation, to synthesize novel π-extended PAHs (6, Chart 1), which exhibit a wide temperature range of columnar mesophases. Eichhorn et al. and Kaafarani et al. [58,59] reported tetraketopyrene condensation with diamino-terphenylene/diamiono-triphenylene for synthesizing board-shaped quinoxalinophenanthrophenazine derivatives (5, Chart 1), which were also found to exhibit broad columnar mesophases in addition to strong fluorescence properties. Bock et al. [60] reported isomeric dinaphthopyrene–tetracarboxdiimides (7 and 8, Chart 1) that exhibit distinct optical, electrochemical, and mesomorphic properties. They were found to self-assemble into columnar mesophases with hexagonal and rectangular symmetry (depending on the nature of the chains R and R′), the non-centrosymmetric diimides 7 having much larger mesophase ranges than their centrosymmetric counterparts, 8. Recently, our groups have successfully applied the combination of Suzuki cross-coupling [61] and Scholl reaction [62] in tandem together for the construction of various new π-extended aromatic DLC systems, based on triphenylene [63,64], thiophene and fused thiophene [65,66,67,68], benzothienobenzothiophene (BTBT) [69], carbazole [70], fluorene [70], dibenzothiophene [71], naphthalene [72], and pyridine [73].
Molecules 28 01721 ch001 550
Chart 1. Various examples of pyrene-containing columnar polyaromatic liquid crystals: (1) [46,47], (2) [45], (3) [8], (4) [44], (5) [58], (6) [57], and (7,8) [60]; BBPn and DBPn, this work (OR/OR′ = OCnH2n+1, n = 5–12; NH2CHRR′, R = R′ = C5H11, C11H22 and R ≠ R′ [60]).
Chart 1. Various examples of pyrene-containing columnar polyaromatic liquid crystals: (1) [46,47], (2) [45], (3) [8], (4) [44], (5) [58], (6) [57], and (7,8) [60]; BBPn and DBPn, this work (OR/OR′ = OCnH2n+1, n = 5–12; NH2CHRR′, R = R′ = C5H11, C11H22 and R ≠ R′ [60]).
Molecules 28 01721 ch001
To further expand the scope of our synthetic strategy adapted to the construction of novel π-extended DLCs, we initiated the synthesis of new pyrene-based PAHs and investigated the impact of topological isomerism on the liquid crystalline and fluorescence properties. This pair of isomeric structures based on pyrene, i.e., BBPn and DBPn (n = 8, 10, and 12), was designed from commercial starting materials, i.e., 1,8-dibromopyrene and 1,6-dibromopyrene, respectively, and were obtained in two steps from readily synthesized appropriate precursors (Scheme 1). Firstly, 1,8- and 1,6-dibromopyrene were reacted with readily accessible 4,4,5,5-tetramethyl-2-(3′,4,4′,5-tetrakis(alkoxy)-[1,1′-biphenyl]-2-yl)-1,3,2-dioxaborolane by Pd-catalyzed Suzuki–Miyaura cross-coupling to give non-annulated Bn and Dn derivatives in good yields, respectively. Subsequently, Scholl oxidative cyclodehydrogenation promoted by FeCl3 generated the corresponding fused compounds 2,3,10,11,14,15,20,21-octaalkyloxypentabenzo[a,c,m,o,rst]pentaphene (BBPn) and 2,3,6,7,13,14,17,18-octaalkyloxydibenzo[j,tuv]phenanthro[9,10-b]picene (DBPn) in ca. 50–60% overall yields on average. Since the molecular symmetry of both isomers is different (C2v for BBPn and C2h for DBPn), both sets of compounds are expected to exhibit different physical and chemical properties [60]. All target molecules were fully characterized using 1H NMR (Figures S8–S12), 13C NMR (Figures S13–S18), elemental analysis, and HRMS (Figures S19–S30).

2.2. Liquid Crystalline Properties

2.2.1. Thermal Stability and Mesomorphic Properties by TGA, DSC, and POM

All these π-extended pyrene-based compounds showed very high thermal stability (decomposition temperatures > 350 °C, for less than 1% weight loss, in dynamic mode), with no impact on the aromatic cores’ topology and the alkyl chain lengths, as measured with thermal gravimetrical analysis (Figure 1 and Table S1, TGA).
The mesomorphic properties of the BBPn and DBPn compounds were first investigated via polarizing optical microscopy (POM). When the samples were slowly cooled from the isotropic liquid, the formation of a few long linear defects was observed within very large homeotropic zones (Figure 2 and Figure S32), characteristic of columnar mesophases’ optical textures and in agreement with the pseudo-discoid molecular structure. The presence of these large homeotropic areas further indicated that both BBPn and DBPn molecules were orthogonally stacked within long-range ordered columns. BBP8, BBP12, and DBP8 exhibited fine textures upon mechanical shearing only. Quite remarkably, BBPn and DBPn showed an excellent tendency for spontaneous homeotropic alignment in the liquid crystal mesophases, and the shear-controlling orientation of the homeotropic alignment of the discotic columns may represent a truly exceptional feature for their potential use as organic semi-conductive materials with enhanced performances [74].
The phase transition behavior of the BBPn and DBPn derivatives was analyzed via differential scanning calorimetry (DSC, Figure S31), and the phase transition temperatures and enthalpy changes are listed in Table S2. Both BBPn and DBPn exhibit enantiotropic liquid crystalline phases. As the alkoxy chain length increases, the clearing temperatures are found to decrease stepwise for both series, which, combined with a slight increase in the melting temperatures for BBPn and almost invariance for DBPn, leads to a narrowing of the mesophase ranges in both cases (Figure 3 and Figure S31). BBPn exhibit higher clearing points and wider columnar mesophase ranges than their DBPn isomeric counterparts, with the shortest BBPn homolog, BBP8, having the highest clearing point (275 °C) and the widest mesophase range (246 °C). In addition, and as an exception, BBP8 displays another phase transformation at a lower temperature, between the solid crystalline state and the mesophase, which, supported by POM observations (Figure S32), corresponds to another columnar mesophase with a different symmetry. Finally, DBPn all exhibited several crystal-to-crystal phase transformations (Table S2).

2.2.2. Mesophase Characterization with SWAXS

The nature of the mesophases of DBPn and BBPn was ultimately characterized via small- and wide-angle X-ray scattering (SWAXS), and their self-assembly modes within the mesophases were explored (Figure 4 and Figure S33). The main results are reported in Table 1. The SWAXS patterns were not very well developed but presented features characteristic of liquid crystalline mesophases: The octyl and decyl derivatives of both series exhibit only one single but intense and sharp peak in the small angle region, whereas an additional weak reflection could be seen for the dodecyl homologs, with reciprocal spacings in the ratio 1: √3. Supported by their characteristic optical textures (Figure 2 and Figure S32), these features were most readily assigned as the (10) (and (11)) reflection(s) of a hexagonal lattice. These reflections naturally emerge from the segregation between antagonistic domains made of aromatic and aliphatic segments, respectively, and thus define the nature of this interface. The presence of a broad halo with two maxima comprised between ca. 4.3 and 4.5 Å (hch) and between 3.7 and 3.9 Å (hπ) confirmed the fluid nature of the mesophase, the former signal arising from liquid-like lateral distances between the molten chains (hch), whereas the latter and sharper one resulting from the π–π stacking between consecutive large molecular cores (hπ). It can be noticed that BBPn homologs have a sharper π–π signal than their isomeric DBPn, indicating stronger and more efficient, and hence, longer-range columnar stacking for the formers. This is fully consistent with DFT calculations (vide infra), which show that the coplanarity of BBP1 is slightly better than that of DBP1. These observations are reflected in the more extended mesomorphic ranges of BBPn derivatives than for the DPBn ones, consequent to a stronger columnar cohesion. At low temperatures, BBP8 further develops a different mesophase with a reduced symmetry, as deduced above by POM and DSC. The SWAXS pattern exhibits seven sharp, low-angle reflections that were indexed into a rectangular symmetry (Table S3). By convention, the group of highest symmetry, c2mm, corresponding to the centered rectangular lattice is considered; the same wide-angle features as for the Colhex phases above were also displayed (hch and hπ).
Therefore, as expected, both BBPn and DBPn regioisomers behave quite differently, attributed mainly to the different molecular symmetry of their cores (C2v versus C2h, respectively), which affects (i) the distribution of the peripheral chains around the aromatic cores and (ii) the relative flatness of the central rigid nucleus, and thus, the π–π overlap between neighboring cores along the stacking direction. DFT calculations (vide) performed on the methoxy derivatives (BBP1 and DBP1) showed different degrees of twisting between the pyrene core and biphenyl part (dihedral angles in Figure S35): For BBP1, the twist angles are 20.16 and 19.79°, respectively, whereas for DBP1, these twist angles are slightly different, with a value of 21.48° for both angles. Face-to-face interactions between molecules may be affected in some way by this slight divergence from flatness, but they appeared to be more hampered by the chains’ distribution, which was particularly more perturbating in the latter than in the former. Thus, both geometrical parameters are concomitantly likely conductive to stronger intermolecular π–π stackings in BBPn systems than in the DPBn ones and better promoters for the formation of wider and more stable columnar mesophases. A similar observation was made for the isomeric dinaphthopyrene–tetracarboxdiimides (7 and 8, Chart 1) [60]. The molecular symmetry (C2v for BBPn and C2h for DBPn) has, therefore, a non-negligible effect on both their mesophase ranges and, to some extent, at least at short chain lengths, on the nature of the mesophase with the emergence for one specific case of a Colrec mesophase.
As also revealed by DFT calculations (vide infra), the shape of the extended aromatic cores of both isomers deviated from classical disc, which is a priori not ideal with the formation of columnar mesophases organized within hexagonal lattice (Figure 5). Indeed, for the emergence of hexagonal symmetry, columns must have an average circular cross-section and be localized at the nodes of a hexagonal network to allow the homogeneous distribution of the aliphatic chains around the columns into an infinite continuum. Thus here, the core/chain interface, defined by the segregation between the immiscible parts, must take the shape of a cylinder. This is possible if the molecules stack on top of each other with a continuous but random change in the respective orientation of their long molecular axis between neighboring pseudo-ellipsoidal molecules (Figure 5) and possibly some tilts of the cores (ψ, Table 1) with respect to the lattice plane, to generate columns of nearly average circular cross-section (whose resulting equivalent circular cross-sectional diameter can be approximated to Dcyl; Table 1).
The expansion of the lattice area is similar for both compounds and continuously increases with the alkoxy chain length. The surface area required for the peripheral chains in both series is indeed highly compatible with the interface area offered by these π-conjugated core stacks; the calculated ratio q (Table 1) is very close to unity, indicating that the chains are densely packed around the cores. As mentioned above, the aromatic cores are tilted within the columns, as deduced from the slice thickness, slightly larger than the π–π stacking signals (Table 1). However, the presence of the Colrec mesophase for the shortest homolog results from the decrease in the molecular tilt and likely also from the specific chain distribution (with a side deprived of chains), which likely reduces the molecular rotation around the column axis, leading to columns with a more elliptical cross-section, hence the reduction in the mesophase symmetry.

2.3. Photophysical Properties

Pyrene was one of the very first luminescent materials to be studied [1,2] because of its outstanding fluorescent properties, hence the incessant and intense research activity on pyrene-containing compounds for the development of optoelectronic materials. The photophysical properties, i.e., UV–Vis absorption, fluorescence emission, and fluorescence quantum yields, of these new pyrenic systems (BBP8 and DBP8, chosen as representative compounds) were investigated in different solvents (cyclohexane, dichloromethane, tetrahydrofuran, and N,N-dimethylformamide) in order to explore the effect of solvent polarity on the photophysical properties and in their thin film state (Figure 6, Table 2).
Both BBP8 and DBP8 exhibited strong and wide absorption bands in the 250–500 nm region, independently of the solvent (Figure 6). BBP8 has its maximum absorption at 348 nm, which, according to DFT results of BBP1 (Table S5), can be attributed to the contribution of the following frontier molecular orbitals’ electronic transitions: H-−1→L+1(+63%), H→L+3(+15%), H→L+2(+14%). By contrast, for DBP8, which has its maximum absorption at 372 nm, it may be attributed to the electronic excitations of H−1→L(+48%), H→L+1(+47%), H-3→L(+16%) (see DFT calculations performed on the methoxy model compounds BBP1 and DBP1; Figure S34 and Table S5). Thus, the largest wavelength absorptions of both isomers resulted accordingly from their H→L transition.
The maximum absorption peak of BBP8 is, therefore, red-shifted by about 24 nm with respect to that of DBP8. DFT calculations consistently show that the energy gap between HOMO and LUMO orbitals of BBP1 is larger than that of DBP1, supporting these experimental measurements. Moreover, BBP8 and DBP8 also have additional absorption peaks at 448 nm and 456 nm, respectively, mainly corresponding to the H−0→L+0 transition, which is basically consistent with the calculation results on the model compounds (Tables S4 and S5). At smaller wavelengths than the main absorption peak, the overall absorption signal of DBP8 appear more structured than that of BBP8.
BBP8 and DBP8 both exhibit yellow-green photoluminescence, with a maximum emission peak spanning between 466–474 nm and 473–483 nm, respectively, depending on the solvent (Figure 6). Fluorescence quantum yields (QYs) measured in the different solvents vary between 17.0% and 31.8% for BBP8 and between 22.0% and 36.7% for DBP8 (Table 2), and they are found to decrease with the solvent polarity. These effects on UV–Vis, PL, and quantum yield could be interpreted by the interactions of the excited state of the fluorophores with the solvent molecules [76,77]. In the thin-film state, the fluorescence emission ranges of both compounds are wider than that in the solution state, with the single emission peak around 539 nm and 574 nm for BBP8 and DBP8, respectively. Hence, the maximum emission is strongly red-shifted relative to that in the solution state, of about 65 nm for BBP8 and about 90 nm for DBP8. This strong difference is attributed to the more effective π orbitals overlap in the film state than in the solution and can be interpreted as the excimer emission [8,26,27,28,45]. According to the results, BBPn and DBPn compounds may be potentially interesting for aggregation-induced emission (AIE) materials and could be used in the fields of fluorescent dyes and fluorescent chemical sensors, as well as electron transport materials in the field of organic optoelectronic semiconductors.

2.4. Molecular Aggregation in Solution

Polycyclic aromatic hydrocarbons (PAHs) usually develop a strong tendency to aggregate in organic solvents due to the strong core–core interactions between molecules, which depend on the size and coplanarity of the aromatic nuclei of the PAHs. Both BBPn and DBPn possess large conjugated structures that deviate from planarity, which may affect aggregation in organic solvents. Concentration-dependent 1H NMR was, therefore, chosen to explore the aggregation behaviors of BBP12 and DBP12 in CDCl3 (Figure 7). All ArH signals shifted slightly to a high field with the increase in the concentration of BBP12 (from 3 mg/0.6 mL to 9 mg/0.6 mL, Figure 7a). In other words, with the increase in the concentration, intermolecular π–π face-to-face core interactions are stronger, and molecular aggregation is promoted [78,79], indicating that BBPn isomers have potential application values in the field of gel materials. However, this phenomenon was not observed for the isomeric DBP12, i.e., no variation was observed in the protons’ signals with concentration (Figure 7b). These results further demonstrate the important role played by the aromatic core topology and, here, particularly the higher planarity of BBP12 with respect to that of DBP12 for the aggregation behavior.

2.5. DFT Calculations

Density functional theory (DFT) calculations were used to determine the optimized molecular conformations, frontier molecular orbitals’ distributions, energy levels, and band gaps for these two regioisomers. The corresponding methoxy homologs of BBPn and DBPn were selected to simplify the calculations (Figure S34 and Table S4). It was found that both optimized molecular structures of BBP1 and DBP1 were not perfectly flat, with the biphenyl parts and pyrene skeleton showing some twist angles (Table 3). The coplanarity of BBP1 was found to be slightly better than that of DBP1, which supports the differences observed in the thermal behavior, SAXS/WAXS, and aggregation in organic solvents of both regioisomers (vide supra).
HOMO and LUMO frontiers orbitals are very similar in both cases, as mainly distributed on the pyrene cores in both systems, and thus, as expected, the HOMO–LUMO gaps are also very similar (Figure 8). The HOMO (−4.97 eV) and LUMO (−1.96 eV) of the BBP1 lead to a mild wider energy gap (3.01 eV), while for DBP1, slightly higher HOMO energy level (−4.91 eV) and higher LUMO level (−1.94 eV) were calculated, and, thus, a narrower energy gap (2.97 eV) (Figure 8 and Figure S36, Table S4). The HOMO–LUMO energy gaps from DFT and experimental UV–Vis absorption spectra thus agree reasonably well: BBP1 shows a slightly wider gap value (3.01 eV) than that of DBP1, and thus, the corresponding absorption peak of the former is slightly shorter than that of the latter.

3. Materials and Methods

Chemicals: All commercially available starting materials were used directly without further purification. The solvents of air- and moisture-sensitive reactions were carefully distilled from appropriate drying agents before use.
Experimental procedures: Air- and moisture-sensitive reactions were assembled on a Schlenk vacuum line or in a glovebox using oven-dried glassware with a Teflon screw cap under an Ar atmosphere. Air- and moisture-sensitive liquids and solutions were transferred using a syringe. Reactions were stirred using Teflon-coated magnetic stir bars. The elevated temperatures were maintained using thermostat-controlled air baths. Organic solutions were concentrated using a rotary evaporator with a diaphragm vacuum pump.
Analytical methods: 1H/13C-NMR spectra were recorded using a Varian UNITY INOVA 400/100 MHz or Bruker 600 MHz spectrometers in CDCl3, and TMS as the internal standard. High-resolution mass spectra (HRMS) spectra were recorded at the Bruker Fourier Transform High-Resolution Mass Spectrometry (solariX XR) with MALDI as the ion source. Elemental analyses (EA) were performed on a Vario Micro Select (Elementar Company, German). The phase transition temperatures and enthalpy changes were investigated using a TA-DSC Q100 differential scanning calorimeter (DSC) under a N2 atmosphere with a heating or cooling rate of 10 °C/min. Liquid crystalline optical textures were observed on a polarized optical microscope (POM), namely an Olympus BH2 polarized optical microscope, equipped with a Mettler FP82HT hot stages, the temperatures of which were controlled with XPR-201 and Mettler FP90. SAXS experiments were performed on a Rigaku Smartlab (3) X-ray diffractometer equipped with a TCU 110 temperature control unit. The sample temperature was controlled within ±1 °C. The X-ray sources (Cu Kα, λ = 0.154 nm) were provided using 40 kW ceramic tubes. 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 quantum yields were measured with a HORIB-F-3029 Integrating Sphere (HORIBA, Kyoto, Japan). DFT calculations were performed with the B3LYP-D3 method with a base set of 6–311 g (d,p) in Gaussian 09 [80] in the gas phase to obtain the lowest energy conformations of BBP1 and DBP1 and their HOMO/LUMO distributions and energy levels.
General procedure for the synthesis of DBPn and BBPn: Under the protection of argon, 1,6- or 1,8-dibromopyrene (1.0 equiv.), 4,4,5,5-tetramethyl-2-(3′,4,4′,5-tetrakis(alkoxy)-[1,1′-biphenyl]-2-yl)-1,3,2-dioxaborolane (3a/3b/3c, 2.8 equiv.), K2CO3 (30.0 equiv.), Pd(PPh3)4 (20 mol%), and THF/H2O (4/1, 0.01 M) were added to a reaction tube. The resulting solutions were stirred at 70 °C for 48 h. The reaction mixtures were cooled to room temperature and extracted with dichloromethane. The residue was preliminarily purified via silica-gel column chromatography, using dichloromethane/petroleum ether (1:2) mixed solvents as the eluent to obtain white solids (Dn or Bn). Then, a solution of FeCl3 (6.0 equiv.) in CH3NO2 (0.10 M) was added to a stirred solution of Dn or Bn (1.0 equiv.) in CHCl3 (0.0017 M), transferred in a drying tube filled with anhydrous calcium chloride, and the reaction mixtures were stirred at room temperature. The reactions’ progress was tracked every 10 min. After half an hour, the tracking time was reduced to 5 min. After the completion of the reactions was confirmed, methanol was added to terminate the reactions, and the products were extracted with chloroform and distilled water; then, the organic phases were dried with anhydrous MgSO4, filtered to remove MgSO4, and spin-dried in vacuo. The residues were purified using hot-silica-gel column chromatography, and the eluent was a chloroform and petroleum ether (1:2) mixture; finally, the residues were recrystallized with ethyl acetate and ethanol (5:1) to obtain yellow solids (DBPn/BBPn) with a total yield of 53–64%.

4. Conclusions

In summary, two series of π-extended pyrene derivatives bearing flexible alkyl chains of different lengths and differing in the molecular symmetry of the polycyclic aromatic cores were synthesized in good overall yields using the tandem method of Suzuki–Miyaura cross-coupling and Scholl oxidative cyclodehydrogenation reactions, starting from dibromopyrene and appropriate dioxaborolane derivatives. Both sets of isomers formed columnar liquid crystalline mesophases (mainly hexagonal but a rectangular phase was induced for one of the shortest terms) through self-organization. BBPn displayed much broader columnar mesophase ranges with higher thermal stability (higher clearing temperatures) than their DBPn isomeric counterparts, a consequence of the core symmetry, hence the aliphatic chain distribution around the core and their planarity. Both sets also display yellow-green photoluminescence with fluorescence quantum yields between 30% and 40%, modulated by core flatness and chain distribution. DFT calculations were in good agreement with the experimental results and allowed for an understanding and explanation of the difference in their properties. The large homeotropic area alignment behavior of these two types of mesogens on a glass substrate and the column orientation easily controlled by mechanical shearing imply that their potential applications in electronic devices are worth investigating further. Our attempt at understanding the influence of the molecular structure on the π–π interactions and physicochemical properties provides a valid guide to design more alkylated PAHs, with or without heterocyclic constituents, with predictable physical properties and tailorable optoelectronic functions. These isomers could be applied not only in the field of fluorescent dyes but also in the field of organic photoelectric semiconductor materials as electron transport materials. Further investigation of novel DLC isomer systems is currently in progress.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules28041721/s1, General synthetic procedures and characterization; Scheme S1: Preparation of the biphenylboronic ester derivatives; Figures S1: 1H NMR spectrum of 2a; Figure S2: 1H NMR spectrum of 2b; Figure S3: 1H NMR spectrum of 2c; Figure S4: 1H NMR spectrum of 3a; Figure S5: 1H NMR spectrum of 3b; Figure S6: 1H NMR spectrum of 3c; Figure S7: 1H NMR spectrum of DBP8; Figure S8: 1H NMR spectrum of DBP10; Figure S9: 1H NMR spectrum of DBP12; Figure S10: 1H NMR spectrum of BBP8; Figure S11: 1H NMR spectrum of BBP10; Figure S12: 1H NMR spectrum of BBP12; Figures S13–S18: 13C NMR spectrum of BBP8, BBP10, BBP12, DBP8, DBP10, and DBP12; Figures S19–S30: HMRS of B8; Figure S20: HMRS of B10; Figure S21: HMRS of B12; Figure S22: HMRS of D8; Figure S23: HMRS of D10; Figure S24: HMRS of D12; Figure S25: HMRS of DBP8; Figure S26: HMRS of DBP10; Figure S27: HMRS of DBP12; Figure S28: HMRS of BBP8; Figure S29: HMRS of BBP10; Figure S30: HMRS of BBP12; Table S1: Temperatures of decomposition at 1%, 2%, and 5% weight loss for BBPn and DBPn; Table S2: Mesophases, transition temperatures, and enthalpy changes for DBP8/10/12 and BBP8/10/12 (DSC heating/cooling rate is 10 °C/min); Figure S31: DSC traces of BBPn and DBPn at scanning rate of 10 °C/min (heating red, cooling in blue); Figure S32: POM textures of DBPn and BBPn upon cooling from the isotropic liquid; Table S3: Indexation and geometrical parameters of the columnar mesophases of DBPn and BBPn; Figure S33: SAXS patterns of the mesophases of compounds DBPn and BBPn (recorded upon cooling); Figure S34: Molecular structures of model compounds BBP1 and DBP1; Figure S35: Dihedral angles of compounds BBP1 and DBP1; Figure S36: Partial molecular orbital diagram for BBP1 and DBP1 with some selected isodensity (isovalues = 0.02) frontier molecular orbital mainly involved in the electronic transitions; Table S4: List of selected molecular orbital energies for BBP1 and DBP1 and their HOMO–LUMO energy gaps (ΔE); Table S5: Selected calculated excitation energies (ΔE), oscillator strengths (f), main orbital components, and assignment for the BBP1 and DBP1 in THF solution.

Author Contributions

Conceptualization, K.-Q.Z.; methodology, K.-Q.Z.; formal analysis, Q.Z., S.L., H.L., K.-X.Z., X.-Y.B., K.-Q.Z. and B.D.; investigation, Q.Z., S.L., H.L., K.-X.Z. and X.-Y.B.; data curation, K.-Q.Z. and B.D.; writing—original draft preparation, K.-Q.Z. and B.D.; writing—review and editing, K.-Q.Z. and B.D.; supervision, K.-Q.Z.; funding acquisition, K.-Q.Z., P.H. and B.-Q.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant numbers: 51773140, 51973143, 22101193, and 21772135.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

We thank the National Natural Science Foundation of China for funding, the University of Strasbourg (France), and CNRS (Centre National de la Recherche Scientifique) for constant support. We would like to thank Hai-Feng Wang for his contribution to DFT calculations.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

References

  1. Figueira-Duarte, T.M.; Müllen, K. Pyrene-based materials for organic electronics. Chem. Rev. 2011, 111, 7260–7314. [Google Scholar] [CrossRef]
  2. Voskuhl, J.; Giese, M. Mesogens with aggregation-induced emission properties: Materials with a bright future. Aggregate 2022, 3, e124. [Google Scholar] [CrossRef]
  3. Feng, X.; Hu, J.Y.; Redshaw, C.; Yamato, Y. Functionalization of pyrene to prepare luminescent materials-typical examples of synthetic methodology. Chem. Eur. J. 2016, 22, 11898–11916. [Google Scholar] [CrossRef] [PubMed]
  4. Zych, D. Non-K Region Disubstituted Pyrenes (1,3-, 1,6- and 1,8-) by (hetero)aryl groups—Review. Molecules 2019, 24, 2551. [Google Scholar] [CrossRef] [PubMed]
  5. Sagara, Y.; Mutai, T.; Yoshikawa, I.; Araki, K. Material design for piezochromic luminescence: Hydrogen-bond-directed assemblies of a pyrene derivative. J. Am. Chem. Soc. 2007, 129, 1520–1521. [Google Scholar] [CrossRef]
  6. Li, Z.; Li, Y.; Wang, D.; Cui, Q.; Li, Z.; Wang, L.; Yang, H. Unique fluorescence properties of a self-assembling bis-pyrene molecule. Chin. Chem. Lett. 2018, 29, 1645–1647. [Google Scholar] [CrossRef]
  7. De Silva, T.P.D.; Youm, S.G.; Fronczek, F.R.; Sahasrabudhe, G.; Nesterov, E.E.; Warner, I.M. Pyrene-benzimidazole derivatives as novel blue emitters for OLEDs. Molecules 2021, 26, 6523. [Google Scholar] [CrossRef]
  8. Diring, S.; Camerel, F.; Donnio, B.; Dintzer, T.; Toffanin, S.; Capelli, R.; Muccini, M.; Ziessel, R. Luminescent ethynyl-pyrene liquid crystals and gels for optoelectronic devices. J. Am. Chem. Soc. 2009, 131, 18177–18185. [Google Scholar] [CrossRef]
  9. Thiebaut, O.; Bock, H.; Grelet, E. Face-on oriented bilayer of two discotic columnar liquid crystals for organic donor–acceptor heterojunction. J. Am. Chem. Soc. 2010, 132, 6886–6887. [Google Scholar] [CrossRef]
  10. Meijer, E.W.; Schenning, A.P.H.J. Material marriage in electronics. Nature 2002, 419, 353–354. [Google Scholar] [CrossRef]
  11. Laschat, S.; Baro, A.; Steinke, N.; Giesselmann, F.; Hägele, C.; Scalia, G.; Judele, R.; Kapatsina, E.; Sauer, S.; Schreivogel, A.; et al. Discotic liquid crystals: From tailor-made synthesis to plastic electronics. Angew. Chem. Int. Ed. 2007, 46, 4832–4887. [Google Scholar] [CrossRef]
  12. Wöhrle, T.; Wurzbach, I.; Kirres, J.; Kostidou, A.; Kapernaum, N.; Litterscheidt, J.; Haenle, J.C.; Staffeld, P.; Baro, A.; Giesselmann, F.; et al. Discotic liquid crystals. Chem. Rev. 2016, 116, 1139–1241. [Google Scholar] [CrossRef]
  13. Kumar, S. Chemistry of Discotic Liquid Crystals: From Monomer to Polymers; CRC Press: Cleveland, OH, USA, 2011. [Google Scholar]
  14. Bushby, R.J.; Kelly, S.M.; O’Neill, M. (Eds.) Liquid Crystalline Semiconductors: Materials, Properties and Applications; Springer Series in Materials Science, 169; Springer: Dordrecht, The Netherlands, 2012. [Google Scholar]
  15. Sergeyev, S.; Pisula, W.; Geerts, Y.H. Discotic liquid crystals: A new generation of organic semiconductors. Chem. Soc. Rev. 2007, 36, 1902–1929. [Google Scholar] [CrossRef]
  16. O’Neill, M.; Kelly, S.M. Ordered materials for organic electronics and photonics. Adv. Mater. 2011, 23, 566–584. [Google Scholar] [CrossRef]
  17. Eichhorn, S.H.; El-Ballouli, A.O.; Cassar, A.; Kaafarani, B.R. Columnar mesomorphism of board-shaped perylene, diketopyrrolopyrrole, isoindigo, indigo, and quinoxalinophenanthrophenazine dyes. ChemPlusChem 2021, 86, 319–339. [Google Scholar] [CrossRef]
  18. Adam, D.; Schuhmacher, P.; Simmerer, J.; Häussling, L.; Siemensmeyer, K.; Etzbachi, K.H.; Ringsdorf, H.; Haarer, D. Fast photoconduction in the highly ordered columnar phase of a discotic liquid crystal. Nature 1994, 371, 141–143. [Google Scholar] [CrossRef]
  19. Termine, R.; Golemme, A. Charge mobility in discotic liquid crystals. Int. J. Mol. Sci. 2021, 22, 877. [Google Scholar] [CrossRef]
  20. Herod, J.D.; Bruce, D.W. Liquid crystals based on the N-phenylpyridinium cation-mesomorphism and the effect of the anion. Molecules 2021, 26, 2653. [Google Scholar] [CrossRef]
  21. Jochem, M.; Detert, H. 2-{3,5-Bis-[5-(3,4-didodecyloxyphenyl)thien-2-yl]phenyl}-5-(3,4-didodecyloxy-phenyl)thiophene. Molbank 2021, 2021, M1225. [Google Scholar] [CrossRef]
  22. Attard, G.S.; Imrie, C.T. Liquid-crystalline and glass-forming dimers derived from 1-aminopyrene. Liq. Cryst. 1991, 11, 785–789. [Google Scholar] [CrossRef]
  23. Attard, G.S.; Imrie, C.T.; Karasz, F.E. Low molar mass liquid crystalline glasses: Preparation and properties of the a-(4-cyanobiphenyl-4′-oxy)-w-(1-pyreniminebenzylidene-4′-oxy)alkanes. Chem. Mater. 1992, 4, 1246–1253. [Google Scholar] [CrossRef]
  24. Walker, R.; Majewska, M.; Pociecha, D.; Makal, A.; Storey, J.M.D.; Gorecka, E.; Imrie, C.T. Twist-bend nematic glasses: The synthesis and characterisation of pyrene-based nonsymmetric dimers. ChemPhysChem 2021, 22, 461–470. [Google Scholar] [CrossRef] [PubMed]
  25. Perju, E.; Marin, L. Mesomorphic behavior of symmetric azomethine dimers containing different chromophore groups. Molecules 2021, 26, 2183. [Google Scholar] [CrossRef] [PubMed]
  26. Sagara, Y.; Tamoaki, N. Mechanoresponsive luminescence and liquid-crystalline behaviour of a cyclophane featuring two 1,6-bis(phenylethynyl)pyrene groups. RSC Adv. 2017, 7, 47056–47062. [Google Scholar] [CrossRef]
  27. Sagara, Y.; Weder, C.; Tamaoki, N. Asymmetric cyclophanes permit access to supercooled nematic liquid crystals with stimulus-responsive luminescence. Chem. Mater. 2017, 29, 6145–6152. [Google Scholar] [CrossRef]
  28. Sagara, Y.; Muramatsu, T.; Tamaoki, N. A 1,6-Diphenylpyrene-based, photoluminescent cyclophane showing a nematic liquid-crystalline phase at room temperature. Crystals 2019, 9, 92. [Google Scholar] [CrossRef]
  29. Hirose, T.; Takai, H.; Watabe, M.; Minamikawa, H.; Tachikawa, T.; Kodama, K.; Yasutake, M. Effect of alkoxy terminal chain length on mesomorphism of 1,6-disubstituted pyrene-based hexacatenar liquid crystals: Columnar phase control. Tetrahedron 2014, 70, 5100–5108. [Google Scholar] [CrossRef]
  30. Martinez-Abadia, M.; Varghese, S.; Gierschner, J.; Giménez, R.; Ros, M.B. Luminescent assemblies of pyrene-containing bent-core mesogens: Liquid crystals, π-gels and nanotubes. J. Mater. Chem. C 2022, 10, 12012–12021. [Google Scholar] [CrossRef]
  31. Sagara, Y.; Kato, T. Stimuli-responsive luminescent liquid crystals: Change of photoluminescent colors triggered by a shear-induced phase transition. Angew. Chem. Int. Ed. 2008, 47, 5175–5178. [Google Scholar] [CrossRef]
  32. Percec, V.; Glodde, M.; Bera, T.K.; Miura, Y.; Shiyanovskaya, I.; Singer, K.D.; Balagurusamy, V.S.K.; Heiney, P.A.; Schnell, I.; Rapp, A.; et al. Self-organization of supramolecular helical dendrimers into complex electronic materials. Nature 2002, 419, 384–387. [Google Scholar] [CrossRef]
  33. Percec, V.; Glodde, M.; Peterca, M.; Rapp, A.; Schnell, I.; Spiess, H.W.; Bera, T.K.; Miura, Y.; Balagurusamy, V.S.K.; Aqad, E.; et al. Self-assembly of semifluorinated dendrons attached to electron-donor groups mediates their π-stacking via a helical pyramidal column. Chem. Eur. J. 2006, 12, 6298–6314. [Google Scholar] [CrossRef]
  34. Kim, Y.H.; Yoon, D.K.; Lee, E.H.; Ko, Y.K.; Jung, H.T. Photoluminescence properties of a perfluorinated supramolecular columnar liquid crystal with a pyrene core: Effects of the ordering and orientation of the columns. J. Phys. Chem. B 2006, 110, 20836–20842. [Google Scholar] [CrossRef]
  35. Shibuya, Y.; Itoh, Y.; Aida, T. Columnar liquid crystalline assembly of a U-shaped molecular scaffold stabilized by covalent or noncovalent incorporation of aromatic molecules. J. Polym. Sci. A Polym. Chem. 2019, 57, 342–351. [Google Scholar] [CrossRef]
  36. Pitto-Barry, A.; Barry, N.P.E.; Russo, V.; Heinrich, B.; Donnio, B.; Therrien, B.; Deschenaux, R. Designing supramolecular liquid-crystalline hybrids from pyrenyl containing dendrimers and arene ruthenium metallacycles. J. Am. Chem. Soc. 2014, 136, 17616–17625. [Google Scholar] [CrossRef]
  37. Kamikawa, Y.; Kato, T. One-dimensional chiral self-assembly of pyrene derivatives based on dendritic oligopeptides. Org. Lett. 2006, 8, 2463–2466. [Google Scholar] [CrossRef]
  38. Kamikawa, Y.; Kato, T. Color-tunable fluorescent organogels: Columnar self-assembly of pyrene-containing oligo(glutamic acid)s. Langmuir 2007, 23, 274–278. [Google Scholar] [CrossRef]
  39. Park, M.; Kang, D.; Choi, Y.; Yoon, W.; Koo, J.; Park, S.H.; Ahn, S.; Jeong, K. Kinetically controlled polymorphic superstructures of pyrene based asymmetric liquid crystal dendron: Relationship between hierarchical superstructures and photophysical properties. Chem. Eur. J. 2018, 24, 9015–9021. [Google Scholar] [CrossRef]
  40. Hirose, T.; Shibano, Y.; Miyazaki, Y.; Sogoshi, N.; Nakabayashi, S.; Yasutake, M. Synthesis and hole transport properties of highly soluble pyrene-based discotic liquid crystals with trialkylsilylethynyl groups. Mol. Cryst. Liq. Cryst. 2011, 534, 81–92. [Google Scholar] [CrossRef]
  41. Yasutake, M.; Fujihara, T.; Nagasawa, A.; Moriya, K.; Hirose, T. Synthesis and phase structures of novel π-acceptor discotic liquid crystalline compounds having a pyrenedione core. Eur. J. Org. Chem. 2008, 2008, 4120–4125. [Google Scholar] [CrossRef]
  42. Hirose, T.; Kawakami, O.; Yasutake, M. Induction and control of columnar mesophase by charge transfer interaction and side chain structures of tetrasubstituted pyrenes. Mol. Cryst. Liq. Cryst. 2006, 451, 65–74. [Google Scholar] [CrossRef]
  43. Hassheider, T.; Benning, S.A.; Kitzerow, H.S.; Achard, M.F.; Bock, H. Color-tuned electroluminescence from columnar liquid crystalline alkyl arenecarboxylates. Angew. Chem. Int. Ed. 2001, 40, 2060–2063. [Google Scholar] [CrossRef]
  44. Gan, K.P.; Yoshio, M.; Kato, T. Columnar liquid-crystalline assemblies of X-shaped pyrene-oligothiophene conjugates: Photoconductivities and mechanochromic functions. J. Mater. Chem. C 2016, 4, 5073–5080. [Google Scholar] [CrossRef]
  45. Hayer, A.; de Halleux, V.; Köhler, A.; El-Garoughy, A.; Meijer, E.W.; Barberá, J.; Tant, J.; Levin, J.; Lehmann, M.; Gierschner, J.; et al. Highly fluorescent crystalline and liquid crystalline columnar phases of pyrene-based structures. J. Phys. Chem. B 2006, 110, 7653–7659. [Google Scholar] [CrossRef] [PubMed]
  46. Sienkowska, M.J.; Monobe, H.; Kaszynski, P.; Shimizu, Y. Photoconductivity of liquid crystalline derivatives of pyrene and carbazole. J. Mater. Chem. 2007, 17, 1392–1398. [Google Scholar] [CrossRef]
  47. Sienkowska, M.J.; Farrar, J.M.; Zhang, F.; Kusuma, S.; Heiney, P.A.; Kaszynski, P. Liquid crystalline behavior of tetraaryl derivatives of benzo[c]cinnoline, tetraazapyrene, phenanthrene, and pyrene: The effect of heteroatom and substitution pattern on phase stability. J. Mater. Chem. 2007, 17, 1399–1411. [Google Scholar] [CrossRef]
  48. Kapf, A.; Eslahi, H.; Blanke, M.; Saccone, M.; Giese, M.; Albrecht, M. Alkyloxy modified pyrene fluorophores with tunable photophysical and crystalline properties. New J. Chem. 2019, 43, 6361–6371. [Google Scholar] [CrossRef]
  49. Kato, S.I.; Kano, H.; Irisawa, K.I.; Yoshikawa, N.; Yamamoto, R.; Kitamura, C.; Nara, D.; Yamanobe, T.; Uehara, H.; Nakamura, Y. 2,4,5,7,9,10-Hexaethynylpyrenes: Synthesis, properties, and self-assembly. Org. Lett. 2018, 20, 7530–7534. [Google Scholar] [CrossRef]
  50. Tchebotareva, N.; Yin, X.; Watson, M.D.; Samori, P.; Rabe, J.P.; Müllen, K. Ordered Architectures of a soluble hexa-peri-hexabenzocoronene-pyrene dyad:  Thermotropic bulk properties and nanoscale phase segregation at surfaces. J. Am. Chem. Soc. 2003, 125, 9734–9739. [Google Scholar] [CrossRef]
  51. Zhang, X.; Wang, H.; Wang, S.; Shen, Y.; Yang, Y.; Deng, K.; Zhao, K.; Zeng, Q.; Wang, C. Triphenylene substituted pyrene derivative: Synthesis and single molecule investigation. J. Phys. Chem. C 2013, 117, 307–312. [Google Scholar] [CrossRef]
  52. Geng, Y.; Chang, S.; Zhao, K.; Zeng, Q.; Wang, C. Self-Assembly of Four-Claw Discotic mesogenic molecules: Influence of core on chirality. J. Phys. Chem. C 2015, 119, 18216–18220. [Google Scholar] [CrossRef]
  53. Tuncel, S.; Kaya, E.N.; Durmuş, M.; Basova, T.; Gürek, A.G.; Ahsen, V.; Banimuslem, H.; Hassan, A. Distribution of single-walled carbon nanotubes in pyrene containing liquid crystalline asymmetric zinc phthalocyanine matrix. Dalton Trans. 2014, 43, 4689–4699. [Google Scholar] [CrossRef]
  54. Kaya, E.N.; Polyakov, M.S.; Basova, T.V.; Durmus, M.; Hassan, A. Pyrene containing liquid crystalline asymmetric phthalocyanines and their composite materials with single-walled carbon nanotubes. J. Porphyr. Phthalocyanines 2018, 22, 56–63. [Google Scholar] [CrossRef]
  55. Anetai, H.; Wada, Y.; Takeda, T.; Hoshino, N.; Yamamoto, S.; Mitsuishi, M.; Takenobu, T.; Akutagawa, T. Fluorescent ferroelectrics of hydrogen-bonded pyrene derivatives. J. Phys. Chem. Lett. 2015, 6, 1813–1818. [Google Scholar]
  56. Anetai, H.; Sambe, K.; Takeda, T.; Hoshino, N.; Akutagawa, T. Nanoscale effects in one-dimensional columnar supramolecular ferroelectrics. Chem. Eur. J. 2019, 25, 11233–11239. [Google Scholar] [CrossRef]
  57. Irla, S.; Pruthvi, M.; Raghunathan, V.A.; Kumar, S. Design and synthesis of extended pyrene based discotic liquid crystalline dyes. Dyes Pigm. 2021, 194, 109574. [Google Scholar] [CrossRef]
  58. Chen, S.; Raad, F.S.; Ahmida, M.; Kaafarani, B.R.; Eichhorn, S.H. Columnar mesomorphism of fluorescent board-shaped quinoxalinophenanthrophenazine derivatives with donor-acceptor structure. Org. Lett. 2013, 15, 558–561. [Google Scholar] [CrossRef]
  59. El-Ballouli, A.O.; Kayal, H.; Shuai, C.; Zeidan, T.A.; Raad, F.S.; Leng, S.; Wex, B.; Cheng, S.Z.D.; Eichhorn, S.H.; Kaafarani, B.R. Lateral extension induces columnar mesomorphism in crucifix shaped quinoxalinophenanthrophenazines. Tetrahedron 2015, 71, 308–314. [Google Scholar] [CrossRef]
  60. Girotto, E.; Ferreira, M.; Sarkar, P.; Bentaleb, A.; Hillard, E.A.; Gallardo, H.; Durola, F.; Bock, H. Plank-Shaped Column-Forming Mesogens with Substituents on One Side Only. Chem. Eur. J. 2015, 21, 7603–7610. [Google Scholar] [CrossRef]
  61. Miyaura, N.; Suzuki, A. Palladium-catalyzed cross-coupling reactions of organoboron compounds. Chem. Rev. 1995, 95, 2457–2483. [Google Scholar] [CrossRef]
  62. Grzybowski, M.; Skonieczny, K.; Butenschön, H.; Gryko, D.T. Comparison of oxidative aromatic coupling and the Scholl reaction. Angew. Chem. Int. Ed. 2013, 52, 9900–9930. [Google Scholar] [CrossRef]
  63. Zhao, K.Q.; Du, J.Q.; Long, X.H.; Jing, M.; Wang, B.Q.; Hu, P.; Monobe, H.; Henrich, B.; Donnio, B. Design of Janus triphenylene mesogens: Facile synthesis, mesomorphism, photoluminescence, and semiconductivity. Dyes Pigm. 2017, 143, 252–260. [Google Scholar] [CrossRef]
  64. Zhao, K.Q.; Jing, M.; An, L.L.; Du, J.Q.; Wang, Y.H.; Hu, P.; Wang, B.Q.; Monobe, H.; Heinrich, B.; Donnio, B. Facile transformation of 1-aryltriphenylenes into dibenzo[fg,op]tetracenes by intramolecular Scholl cyclodehydrogenation: Synthesis, self-assembly, and charge carrier mobility of large pi-extended discogens. J. Mater. Chem. C 2017, 5, 669–682. [Google Scholar] [CrossRef]
  65. Ma, T.; Zhong, Y.J.; Wang, H.F.; Zhao, K.Q.; Wang, B.Q.; Hu, P.; Monobe, H.; Donnio, B. Butterfly-like shape liquid crystals based fused-thiophene as unidimensional ambipolar organic semiconductors with high mobility. Chem. Asian J. 2021, 16, 1106–1117. [Google Scholar] [CrossRef] [PubMed]
  66. Ma, T.; Wang, H.F.; Zhao, K.Q.; Wang, B.Q.; Hu, P.; Monobe, H.; Heinrich, B.; Donnio, B. Nonlinear nonacenes with a dithienothiophene substructure: Multifunctional compounds that act as columnar mesogens, luminophores, pi gelators, and p-type semiconductors. ChemPlusChem 2019, 84, 1439–1448. [Google Scholar] [CrossRef]
  67. Zhao, K.C.; Du, J.Q.; Wang, H.F.; Zhao, K.Q.; Hu, P.; Wang, B.Q.; Monobe, H.; Heinrich, B.; Donnio, B. Board-like fused-thiophene liquid crystals and their benzene analogs: Facile synthesis, self-assembly, p-type semiconductivity, and photoluminescence. Chem. Asian J. 2019, 14, 462–470. [Google Scholar] [CrossRef]
  68. Lin, H.; Zhao, K.X.; Jing, M.; Long, X.H.; Zhao, K.Q.; Hu, P.; Wang, B.Q.; Lei, P.; Zeng, Q.D.; Donnio, B. Synthesis, self-assembly and optical properties of some rigid π-bridged triphenylene dimers. J. Mater. Chem. C 2022, 10, 14453–14470. [Google Scholar] [CrossRef]
  69. Liu, C.X.; Wang, H.; Du, J.Q.; Zhao, K.Q.; Hu, P.; Wang, B.Q.; Monobe, H.; Heinrich, B.; Donnio, B. Molecular design of benzothienobenzothiophene-cored columnar mesogens: Facile synthesis, mesomorphism, and charge carrier mobility. J. Mater. Chem. C 2018, 6, 4471–4478. [Google Scholar] [CrossRef]
  70. Hang, J.F.; Lin, H.; Zhao, K.Q.; Hu, P.; Wang, B.Q.; Monobe, H.; Zhu, C.; Donnio, B. Butterfly mesogens based on carbazole, fluorene or fluorenone: Mesomorphous, gelling, photophysical, and photoconductive properties. Eur. J. Org. Chem. 2021, 2021, 1989–2002. [Google Scholar] [CrossRef]
  71. Deng, W.J.; Liu, S.; Lin, H.; Zhao, K.X.; Bai, X.Y.; Zhao, K.Q.; Hu, P.; Wang, B.Q.; Monobe, H.; Donnio, B. Ditriphenylenothiophene butterfly-shape liquid crystals. The influence of polyarene core topology on self-organization, fluorescence and photoconductivity. New J. Chem. 2022, 46, 7936–7949. [Google Scholar] [CrossRef]
  72. Zhu, X.M.; Bai, X.Y.; Wang, H.F.; Hu, P.; Wang, B.Q.; Zhao, K.Q. Benzo[g]chrysene discotic liquid crystals: Synthesis, columnar mesophase and photophysical properties. Acta Chim. Sinica 2021, 79, 1486–1493. [Google Scholar] [CrossRef]
  73. Dai, J.; Zhao, K.Q.; Wang, B.Q.; Hu, P.; Heinrich, B.; Donnio, B. Liquid crystal ionic self-assembly and anion-selective photoluminescence in discotic azatriphenylenes. J. Mater. Chem. C 2020, 8, 4215–4225. [Google Scholar] [CrossRef]
  74. Shoji, Y.; Kobayashi, M.; Kosaka, A.; Haruki, R.; Kumai, R.; Adachi, S.; Kajitani, T.; Fukushima, T. Design of discotic liquid crystal enabling complete switching along with memory of homeotropic and homogeneous alignment over a large area. Chem. Sci. 2022, 13, 9891–9901. [Google Scholar] [CrossRef]
  75. Donnio, B.; Heinrich, B.; Allouchi, H.; Kain, J.; Dele, S.; Guillon, D.; Bruce, D.W. A generalized model for the molecular arrangement in the columnar mesophases of polycatenar mesogens. crystal and molecular structure of two hexacatenar mesogens. J. Am. Chem. Soc. 2004, 126, 15258–15268. [Google Scholar] [CrossRef]
  76. Reichardt, C. Solvatochromic Dyes as Solvent Polarity Indicators. Chem. Rev. 1994, 94, 2319–2358. [Google Scholar] [CrossRef]
  77. Diaz, M.S.; Freile, M.L.; Gutierrez, M.I. Solvent effect on the UV/Vis absorption and fluorescence spectroscopic properties of berberine. Photochem. Photobiol. Sci. 2009, 8, 970–974. [Google Scholar] [CrossRef]
  78. Martin, R.B. Comparisons of Indefinite Self-Association Models. Chem. Rev. 1996, 96, 3043–3064. [Google Scholar] [CrossRef]
  79. Kastler, M.; Pisula, W.; Wasserfallen, D.; Pakula, T.; Müllen, K. Influence of Alkyl Substituents on the Solution- and Surface-Organization of Hexa-peri-hexabenzocoronenes. J. Am. Chem. Soc. 2005, 127, 4286–4296. [Google Scholar] [CrossRef]
  80. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G.A.; et al. Gaussian 09, Revision E.01; Gaussian, Inc.: Wallingford, CT, USA, 2009. [Google Scholar]
Molecules 28 01721 sch001 550
Scheme 1. Synthetic route, nomenclature, and overall yields to the pyrene-containing BBPn and DBPn isomeric derivatives.
Scheme 1. Synthetic route, nomenclature, and overall yields to the pyrene-containing BBPn and DBPn isomeric derivatives.
Molecules 28 01721 sch001
Molecules 28 01721 g001 550
Figure 1. TGA curves for synthesized pyrene-based PAHs.
Figure 1. TGA curves for synthesized pyrene-based PAHs.
Molecules 28 01721 g001
Molecules 28 01721 g002 550
Figure 2. Representative POM photomicrographs (upon cooling from isotropic liquid with a rate of −5 °C/min): (a,b) BBP8, before and after mechanical shearing at 255 °C; (c,d) BBP12, before and after mechanical shearing at 220 °C; (e,f) DBP8, before and after mechanical shearing at 130 °C and 175 °C.
Figure 2. Representative POM photomicrographs (upon cooling from isotropic liquid with a rate of −5 °C/min): (a,b) BBP8, before and after mechanical shearing at 255 °C; (c,d) BBP12, before and after mechanical shearing at 220 °C; (e,f) DBP8, before and after mechanical shearing at 130 °C and 175 °C.
Molecules 28 01721 g002
Molecules 28 01721 g003 550
Figure 3. Thermotropic mesophases diagram of DBPn and BBPn; Cr: crystalline phase, Colrec: rectangular columnar phase, Colhex: hexagonal columnar phase.
Figure 3. Thermotropic mesophases diagram of DBPn and BBPn; Cr: crystalline phase, Colrec: rectangular columnar phase, Colhex: hexagonal columnar phase.
Molecules 28 01721 g003
Molecules 28 01721 g004 550
Figure 4. Representative SWAXS patterns of the Colhex mesophases of BBPn (top) and DBPn (bottom) at different temperatures.
Figure 4. Representative SWAXS patterns of the Colhex mesophases of BBPn (top) and DBPn (bottom) at different temperatures.
Molecules 28 01721 g004
Molecules 28 01721 g005 550
Figure 5. Model for the supramolecular organization in the Colhex phase of BBPn or DPBn molecules: (a) the elongated shape of the aromatic core roughly approximates an ellipse of thickness hmol (both BBPn or DPBn molecules substantially deviated from planarity, hence this apparent average tilt ψ, visualized by black arrows, with respect to lattice plane; A, ψ, and hmol defined in Table 1); (b) the continuous orientational changes in the stacked cores distributed the space-demanding radiating alkyl chains over the whole stack periphery and design average cylindrical columns of reduced statistical interface area per chain, sch, and average circular diameter, Dcyl; (c) top view and (d) side view of the pseudo-cylindrical columns arranged in hexagonal lattice of parameter a and with molecular slice thickness hmol compared with π-stacking distance, hπ.
Figure 5. Model for the supramolecular organization in the Colhex phase of BBPn or DPBn molecules: (a) the elongated shape of the aromatic core roughly approximates an ellipse of thickness hmol (both BBPn or DPBn molecules substantially deviated from planarity, hence this apparent average tilt ψ, visualized by black arrows, with respect to lattice plane; A, ψ, and hmol defined in Table 1); (b) the continuous orientational changes in the stacked cores distributed the space-demanding radiating alkyl chains over the whole stack periphery and design average cylindrical columns of reduced statistical interface area per chain, sch, and average circular diameter, Dcyl; (c) top view and (d) side view of the pseudo-cylindrical columns arranged in hexagonal lattice of parameter a and with molecular slice thickness hmol compared with π-stacking distance, hπ.
Molecules 28 01721 g005
Molecules 28 01721 g006 550
Figure 6. Photophysical properties of BBP8 and DBP8: UV–Vis absorption, fluorescence emission in solution (c = 1 × 10−5 mol/L) in various solvents (cyclohexane, CH2Cl2, THF = tetrahydrofuran, and DMF = N,N-dimethylformamide) and fluorescence emission in film of (a) BBP8 and (b) DBP8.
Figure 6. Photophysical properties of BBP8 and DBP8: UV–Vis absorption, fluorescence emission in solution (c = 1 × 10−5 mol/L) in various solvents (cyclohexane, CH2Cl2, THF = tetrahydrofuran, and DMF = N,N-dimethylformamide) and fluorescence emission in film of (a) BBP8 and (b) DBP8.
Molecules 28 01721 g006
Molecules 28 01721 g007 550
Figure 7. Concentration-dependent 1H NMR in CDCl3 of (a) BBP12 and (b) DBP12.
Figure 7. Concentration-dependent 1H NMR in CDCl3 of (a) BBP12 and (b) DBP12.
Molecules 28 01721 g007
Molecules 28 01721 g008 550
Figure 8. Comparison of HOMO–LUMO energy levels of BBP1 and DBP1 (isovalues = 0.02), as models for compounds BBPn and DBPn, respectively.
Figure 8. Comparison of HOMO–LUMO energy levels of BBP1 and DBP1 (isovalues = 0.02), as models for compounds BBPn and DBPn, respectively.
Molecules 28 01721 g008
Table
Table 1. Compounds and mesophase parameters and geometrical data.
Table 1. Compounds and mesophase parameters and geometrical data.
CpdsT aVmol bρba/b cA chmol dhchhπ (ξ) dψ eχcore fDcyl gsch hq i
BBP8Colrec8725051.01353.80/
24.05		646.953.874.293.71170.26514.7722.451.00
Colhex12025700.98727.03632.824.064.323.74230.25914.4423.041.01
Colhex24028670.88527.23641.984.464.343.84300.23213.7724.150.98
BBP10Colhex8029430.98928.20688.604.274.323.70300.22614.0823.621.06
Colhex16031350.92828.39698.224.494.393.77330.21213.7324.211.03
Colhex23033380.87228.43700.174.774.513.87360.19913.3224.951.01
BBP12Colhex7533820.97130.56809.044.184.413.71270.19714.2323.361.05
Colhex16036120.90930.67814.564.434.373.78290.18413.8124.031.02
Colhex20037370.87830.79821.324.554.533.88310.17813.6424.371.01
DPB8Colhex12025700.98726.43604.934.254.283.71290.25914.1223.551.03
Colhex16526700.95026.47606.814.404.273.76310.24913.8723.971.02
DPB10Colhex11030100.96728.28692.814.344.363.70320.22113.9623.791.05
Colhex15031090.93628.55706.064.404.393.72320.21413.8723.961.03
DPB12Colhex12535110.93530.38799.304.394.323.72320.18913.8823.931.04
Colhex14035530.92430.67814.564.364.303.76300.18713.9323.851.03
a Temperature of experiment (°C); b molecular volume (Å3) and density (g cm−3) calculated from partial volumes of reference substances: Vmol = Var + Vch, the sum of the volume of the aromatic part, Var (from reference compounds), and the volume of the chains, Vch = nch × (nVCH2 + ΔVCH3), where nch is the number of chains per molecule, i.e., nch = 8; n is the number of methylene groups per chain; VCH2 is the volume of one methylene unit; and ∆VCH3 is the volume contribution of the end methylene (as a function of temperature in °C); see reference [75]; ρ = MW/(NA × Vmol); c lattice parameter, a (Å) and area, A = a2√3/2 (Å2) for Colhex and a, b, and S = a × b = 2 × A for Colrec; d columnar slice thickness, hmol = Vmol/A (Å), and stacking distance, hπ (Å), average π–π stacking distance between molecules determined from S/WAXS pattern, and hch is average distance between molten chains; e out-of-plane tilt angle of mesogen cores inside columns (°), ψ = arcos(hπ/hmol); f aromatic volume fraction, χcore = Var/Vmol, with Var ≈ 665 Å3; g diameter (Å) of equivalent circular cross-sectional area Acore, Dcyl = √(4 × Acore/π), with Acore = χcore × A; h cylinder area per chain: sch = πDcyl × hmol/nch (Å2); note that the calculation of the interface in the Colrec phase (BBP8), the cross-section of the column was considered circular for facilitating the calculation (see references [65,67]); i chain-packing ratio: q = sch/σch, σch (Å2) is the cross-sectional area of a molten chain (see [75]).
Table
Table 2. Spectroscopic parameters of BBP8 and DBP8 in various solvents and thin films a.
Table 2. Spectroscopic parameters of BBP8 and DBP8 in various solvents and thin films a.
SolutionFilm
CompoundsSolventλabsελemQYλem
BBP8C6H12280
344
420
446		7.40 × 104
8.90 × 104
4.36 × 104
5.25 × 104		466
495		17.0539
CH2Cl2282
348
422
448		7.83 × 104
9.58 × 104
4.66 × 104
5.95 × 104		474
500		25.2
THF280
346
422
448		8.00 × 104
9.83 × 104
4.78 × 104
6.13 × 104		470
498		26.3
DMF282
348
424
450		6.82 × 104
8.32 × 104
4.04 × 104
5.23 × 104		474
498		31.8
DBP8C6H12252
280
330
352
370
428
452		4.74 × 104
4.06 × 104
4.87 × 104
4.89 × 104
6.48 × 104
2.66 × 104
3.16 × 104		473
503		22.0574
CH2Cl2254
282
324
354
372
434
456		8.40 × 104
7.31 × 104
7.86 × 104
8.05 × 104
11.63 × 104
4.26 × 104
5.78 × 104		483
508		26.9
THF254
282
326
354
372
432
456		7.90 × 104
6.91 × 104
7.41 × 104
7.72 × 104
11.29 × 104
4.10 × 104
5.61 × 104		479
506		27.8
DMF240
282
326
356
372
434
458		2.94 × 104
3.15 × 104
3.50 × 104
3.64 × 104
4.78 × 104
2.31 × 104
2.68 × 104		483
509		36.7
a Absorption (λabs) and emission (λem) wavelengths in nm; molecular absorption coefficient (ε, L mol−1 cm−1); absolute quantum yields, QY, (%) measured in corresponding solvent at 1 × 10−5 mol/L, λex = 350 nm.
Table
Table 3. Ball and stick model of optimized molecular structures of BBP1 and DBP1. Red: oxygen; dark gray: carbon; light gray: hydrogen.
Table 3. Ball and stick model of optimized molecular structures of BBP1 and DBP1. Red: oxygen; dark gray: carbon; light gray: hydrogen.
Compd.Top ViewsSide Views
BBP1Molecules 28 01721 i001Molecules 28 01721 i002
DBP1Molecules 28 01721 i003Molecules 28 01721 i004
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Zeng, Q.; Liu, S.; Lin, H.; Zhao, K.-X.; Bai, X.-Y.; Zhao, K.-Q.; Hu, P.; Wang, B.-Q.; Donnio, B. Pyrene-Fused Poly-Aromatic Regioisomers: Synthesis, Columnar Mesomorphism, and Optical Properties. Molecules 2023, 28, 1721. https://doi.org/10.3390/molecules28041721

AMA Style

Zeng Q, Liu S, Lin H, Zhao K-X, Bai X-Y, Zhao K-Q, Hu P, Wang B-Q, Donnio B. Pyrene-Fused Poly-Aromatic Regioisomers: Synthesis, Columnar Mesomorphism, and Optical Properties. Molecules. 2023; 28(4):1721. https://doi.org/10.3390/molecules28041721

Chicago/Turabian Style

Zeng, Qing, Shuai Liu, Hang Lin, Ke-Xiao Zhao, Xiao-Yan Bai, Ke-Qing Zhao, Ping Hu, Bi-Qin Wang, and Bertrand Donnio. 2023. "Pyrene-Fused Poly-Aromatic Regioisomers: Synthesis, Columnar Mesomorphism, and Optical Properties" Molecules 28, no. 4: 1721. https://doi.org/10.3390/molecules28041721

Article Metrics

Back to TopTop