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

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.


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.
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 com-parative 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).

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 (Figures 2 and S32), characteristic of columnar mesophases' optical textures and in agreement with the pseudo-discoid molecular structure. The 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 (Figures 2 and 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 (Figures 3 and 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).
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 (Figures 3 and 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).

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 (Figures 4 and 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 (Figures 2 and 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

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 (Figures 4 and 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 (Figures 2 and 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 (h ch ) and between 3.7 and 3.9 Å (h π ) confirmed the fluid nature of the mesophase, the former signal arising from liquid-like lateral distances between the molten chains (h ch ), whereas the latter and sharper one resulting from the π-π stacking between consecutive large molecular cores (h π ). 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 Col hex phases above were also displayed (h ch and h π ).    Therefore, as expected, both BBPn and DBPn regioisomers behave quite differently, attributed mainly to the different molecular symmetry of their cores (C 2v versus C 2h , 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 (C 2v for BBPn and C 2h 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 Col rec mesophase. a Temperature of experiment ( • C); b molecular volume (Å 3 ) and density (g cm −3 ) calculated from partial volumes of reference substances: V mol = V ar + V ch , the sum of the volume of the aromatic part, V ar (from reference compounds), and the volume of the chains, V ch = n ch × (nV CH2 + ∆V CH3 ), where n ch is the number of chains per molecule, i.e., n ch = 8; n is the number of methylene groups per chain; V CH2 is the volume of one methylene unit; and ∆V CH3 is the volume contribution of the end methylene (as a function of temperature in • C); see reference [75]; ρ = MW/(N A × V mol ); c lattice parameter, a (Å) and area, A = a 2 √ 3/2 (Å 2 ) for Col hex and a, b, and S = a × b = 2 × A for Col rec ; d columnar slice thickness, h mol = V mol /A (Å), and stacking distance, h π (Å), average π-π stacking distance between molecules determined from S/WAXS pattern, and h ch is average distance between molten chains; e out-of-plane tilt angle of mesogen cores inside columns ( • ), ψ = arcos(h π /h mol ); f aromatic volume fraction, χ core = V ar /V mol , with V ar ≈ 665 Å 3 ; g diameter (Å) of equivalent circular cross-sectional area A core , D cyl = √ (4 × A core /π), with A core = χ core × A; h cylinder area per chain: s ch = πD cyl × h mol /n ch (Å 2 ); note that the calculation of the interface in the Col rec phase (BBP8), the cross-section of the column was considered circular for facilitating the calculation (see references [65,67]); i chain-packing ratio: q = s ch /σ ch , σ ch (Å 2 ) is the cross-sectional area of a molten chain (see [75]).
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 crosssection 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 D cyl ; Table 1).

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.
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 crosssection, hence the reduction in the mesophase symmetry.

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). (a) the elongated shape of the aromatic core roughly approximates an ellipse of thickness h mol (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 h mol 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, s ch , and average circular diameter, D cyl ; (c) top view and (d) side view of the pseudo-cylindrical columns arranged in hexagonal lattice of parameter a and with molecular slice thickness h mol compared with π-stacking distance, h π .
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 πconjugated 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 Col rec 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.

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. a Absorption (λ abs ) and emission (λ em ) 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, λ ex = 350 nm.
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.

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 1 H NMR was, therefore, chosen to explore the aggregation behaviors of BBP12 and DBP12 in CDCl 3 (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.
Molecules 2023, 28, x FOR PEER REVIEW 13 of 20 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.

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 1 H 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.

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

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). Table 3. Ball and stick model of optimized molecular structures of BBP1 and DBP1. Red: oxygen; dark gray: carbon; light gray: hydrogen.

Compd.
Top Views Side Views

BBP1
Molecules 2023, 28, x FOR PEER REVIEW 14 of 20 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). Table 3. Ball and stick model of optimized molecular structures of BBP1 and DBP1. Red: oxygen; dark gray: carbon; light gray: hydrogen.

DBP1
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) (Figures 8 and 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.
Molecules 2023, 28, x FOR PEER REVIEW 14 of 20 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) (Figures 8 and 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.

DBP1
Molecules 2023, 28, x FOR PEER REVIEW 14 of 20 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) (Figures 8 and 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.
Molecules 2023, 28, x FOR PEER REVIEW 14 of 20 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) (Figures 8 and 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.
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 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.

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

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: 1 H/ 13 C-NMR spectra were recorded using a Varian UNITY IN-OVA 400/100 MHz or Bruker 600 MHz spectrometers in CDCl 3 , 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 N 2 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.

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.