The ability of a series of bridged triarylamines, known as N-heterotriangulenes, to form multilayer-type two-dimensional extended films via a solution-based processing method was investigated using complementary microscopic techniques. The long-range order, crystallinity, and layer thickness were found to be critically dependent on the nature of the substituents attached to the polycyclic backbone. Compounds featuring a carbonyl unit at the bridge position exhibited superior performance as building blocks compared to thioketone-bridged derivatives due to their flatter core structure. Additionally, the nature and length of peripheral substituents significantly influenced the orientation of the aromatic core within highly crystalline films. These findings underscore the importance of a well-designed molecular framework and provide deeper insight into structure formation under two-dimensional confinement for such compounds.
In recent decades, organic semiconductors (OSCs) have emerged as a viable, low-cost, and environmentally friendly alternative to amorphous hydrogenated silicon in modern microelectronics, including applications in radio frequency identification tags, wearable devices, and sensors. Unlike conventional inorganic devices limited to rigid substrates, organic electronics offer compatibility with flexible and elastic supports such as polymers, enabling lightweight or even transparent systems. Early functional OSC layers consisted of polymeric materials like polyacetylene and polythiophene, but charge transport remained hindered by intrinsic disorder in long-chain backbones. Reducing grain boundaries that act as charge traps while establishing high molecular order is now recognized as crucial for efficient charge transport in advanced OSC films. Intermolecular interactions—such as effective π-stacking and hydrophobic effects—are key to achieving a highly crystalline microstructure. For example, elongated alkyl chains attached to the core enhance intermolecular van der Waals (VDW) interactions, promoting electronic coupling between electron-rich molecular regions and acting as molecular fasteners. As a result, charge carrier mobilities in functional 2D organic layers have improved significantly, driving advancements in devices such as organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs).
Numerous hole-transporting (p-type) OSCs, including pentacene derivatives and thiophene-based small molecules, have been successfully applied due to their solution processability and thermal stability. Their synthetic modifiability enables tailored structures that reduce molecular disorder. In contrast, efficient electron transport through electron-deficient (n-type) junctions remains challenging despite its necessity in complementary logic circuits. The instability of radical anions—the primary charge carriers—in air or moisture limits n-type OSC development. Furthermore, large injection barriers between electrode work functions and the semiconductor’s lowest unoccupied molecular orbital (LUMO) impede charge transfer. Although synthesizing electron-deficient groups onto conjugated cores is more difficult than attaching electron-donating ones, recent advances have enabled progress in this area. This study focuses on the synthesis of novel, highly functionalized N-heterotriangulenes and their potential as solution-processable n-type systems capable of forming well-oriented 2D films.
The core unit 1 was synthesized following a procedure reported by Chen and coworkers. Bridging aryl moieties ortho to the central nitrogen atom leads to planarization of the originally propeller-shaped triphenylamine. This results in a nitrogen-centered scaffold amenable to triple lateral functionalization. Solubility was enhanced by replacing halide substituents at para positions with alkyl chains via Negishi, Ullmann, or Sonogashira cross-couplings. The conjugated system can also be extended at bridge positions or the carbonyl oxygen replaced by sulfur using Lawesson’s reagent. Notably, other rigid, conjugated electron-deficient N-heterotriangulenes have been studied previously, and compound 2 has been examined by Müllen and colleagues regarding its solution processability and crystal structure in columnar thin films from dip-coating. However, compounds 2, 4, and 5 may suffer from thermal degradation due to long peripheral chains or unstable C=S units, making vacuum sublimation unsuitable. Instead, a solution-based approach called “solution-epitaxy,” initially developed by Hu and coworkers, was employed. This method utilizes the solvent-water interface as a defect-free substrate for molecular self-assembly, enabling growth close to thermodynamic equilibrium. Compared to traditional deposition techniques like thermal evaporation or dip coating, this strategy avoids diffusion-controlled kinetics in vacuum and facilitates the formation of well-structured, potentially crystalline films up to millimeter-scale areas. Compression of floating molecules into Langmuir-type systems was not pursued due to the lack of amphiphilic character in the target compounds.
All N-heterotriangulenes exhibit good solubility in common organic solvents such as dichloromethane, chloroform, toluene, and acetonitrile, making them ideal candidates for interfacial self-assembly at concentrations between 0.025 and 0.5 mg/mL. Compounds 2, 3, and 5 formed continuous, closed film structures observable via optical microscopy (OM), whereas compound 4 showed discontinuous, flake-like agglomerates despite appearing closed to the naked eye. Interestingly, the only structural difference between 2 and 4 is the replacement of oxygen with sulfur at the bridge position, yet the resulting film morphologies differ drastically. At the standard concentration of 0.25 mg/mL, no isotropic 2D growth was observed for compound 4. Both carbonyl-bridged compounds 2 and 3 displayed preferred in-plane growth, clearly visible in OM images. Cracks observed in the microstructure, particularly in films of 2 and 5, are likely due to mechanical stress during transfer from water surface to silicon oxide substrates. Film thickness and morphology were determined using non-contact atomic force microscopy (AFM) at these disrupted areas. Compound 2 exhibited a base film thickness of 8.14 nm, while compound 3 showed 17.43 nm. Additional layered structures were identified on top of the base film, suggesting monomolecular or bimolecular layers depending on molecular orientation. Considering the maximum molecular extension of 2 (3.79 nm) and 3 (1.52 nm), these step-like features align with a lying-down configuration. The dodecyl chains in 2 remain flexible in the solid state, contributing to a rougher morphology (RMS roughness = 12.7 Å). Narrow crevices in compound 2’s film may arise from toluene, an aromatic component in the original solution, which participates in π-stacking and accumulates during film formation. AFM images of compounds 3–5, prepared without toluene, do not show such features. After deposition, residual solvent evaporates gradually, leaving minor gaps approximately 2.5 nm deep.
Compound 3 also displays directional growth, but with nearly double the base film thickness of compound 2. Due to shorter perfluorinated n-propyl chains compared to dodecyl chains in 2, the surface roughness is lower (10.1 Å). Replacing carbonyl bridges with thiocarbonyls eliminates preferential growth; instead, film formation is restricted to an ultrathin base layer of only 3.29 nm (compound 4) and 5.96 nm (compound 5). This corresponds to just a few molecular layers. Despite longer chains, compound 4 shows higher RMS roughness (8.2 Å) than compound 5 (6.3 Å), indicating chain length dominates over substituent bulkiness in determining surface roughness. No continuous layered growth was observed beyond the initial base film in thiocarbonyl-bridged compounds, which lack consistent structural organization. The absence of molecular steps results in significantly lower roughness for both 4 and 5.
To gain deeper insight into microstructure and molecular orientation, angle-dependent near-edge X-ray absorption fine structure (NEXAFS) measurements were conducted.PANK2 Antibody site Absorption intensity from core-level transitions (C1s) into unoccupied orbitals was recorded across grazing incidence angles (20°) to normal incidence (90°), with p-polarized photons.Phospho-SQSTM1/p62 Antibody Technical Information The intensity Iv depends on experimental angle θ, polar angle φ, and azimuthal angle α according to Iv = cos²θ cos²α + sin²θ sin²α cos²φ + 2 sinα cosα sinθ cosθ cosφ. This equation allows determination of the orientation of the rigid aromatic backbone relative to the substrate. When the average orientation is preferential and θ exceeds the magic angle (54.7°), the * transition intensity increases with θ, while the higher-energy *-transition decreases (linear dichroism effect). The opposite trend occurs when θ < 54.7°. Thus, qualitative information about preferred alignment—potentially crystalline—is obtainable. For highly aligned films with twofold or higher symmetry, azimuthal angle α can be deduced from Iv = cos²θ cos²α + sin²θ sin²α cos²φ. C K-edge spectra of compounds 2 and 3 revealed strong dichroic effects, confirming preferential molecular orientation. For compound 2, the region from 284 eV to ~287 eV corresponds to low-energy *-resonances of carbon double bonds in the core. While * (C=C) intensities vary slightly with incidence angle, changes are unambiguous. Transitions from nitrogen-bound aromatic carbons (~286 eV) and carbonyl groups (~286.8 eV) behave similarly under different angles due to their alignment within the rigid, planar core. A sharp peak at ~288.PMID:35108568 4 eV arises from *(C–H) transitions in saturated dodecyl chains, showing strong intensity when the chain axis is perpendicular to the backbone (θ ≈ 90°). Quantitative analysis of the *(CH) resonance at 288.4 eV yields an average tilt angle of 72 ± 2° relative to the surface normal, indicating the C12 substituents stand nearly upright. Weak absorption at 293.3 eV (attributed to *(C–C) backbone transitions) at normal incidence and enhanced intensity at grazing incidence further support this conclusion. For the core unit, the *-orbital polar angle φ is estimated near the magic angle (54.7°), explaining minimal intensity variation across rotation angles. This implies the core plane is tilted ~35° from the surface normal (Table 1).
NEXAFS spectra of compound 3 show higher overall intensity in the pre-edge region compared to post-edge features, reflecting the dominance of the electron-rich core over shorter perfluorinated chains. Resonant transitions resemble those of compound 2, consistent with shared backbone architecture. Strong dichroism in the low-energy region indicates φ < 54.7°, meaning the *-orbitals are tilted toward the surface. In the *-region, resonances at 292.1 eV (CF₂), 296.1 eV (C–C), and 298.3 eV (CF₃) show varying intensities depending on incidence angle, confirming preferential orientation. The distinct behavior—strong C–F absorption at normal incidence and weak C–C absorption, reversed at grazing incidence—has been documented by Gland and coworkers. In contrast to compound 2, the C=C *-transitions in 3 yield clearer linear dichroism suitable for quantitative analysis, yielding a polar angle of 37° ± 3°. An azimuthal angle of ~90° is derived assuming a single domain is probed. The anisotropy of the silicon oxide substrate is not considered significant, as film formation occurs exclusively at the water-solvent interface without further structural rearrangement during transfer. Therefore, the reference direction for spectral analysis is defined by the electric field vector E (polarization direction). Consequently, both oxygen-containing compounds form 2D-extended films with well-defined molecular orientations, as confirmed by NEXAFS linear dichroism. In contrast, sulfur-containing compounds 4 and 5 show only weak or negligible dichroic effects, indicating disordered film structures. Compound 5, in particular, exhibits nearly identical absorption intensities across all incidence angles, suggesting no preferential orientation. This behavior is expected when θ approaches the magic angle, where dichroism vanishes. Detailed structural insights into the crystalline nature of films from compounds 2 and 3 were obtained via selected area electron diffraction (SAED). Clean diffraction patterns confirm high crystallinity, as evidenced by OM, AFM, and NEXAFS. The absence of Debye-Scherrer rings indicates minimal polycrystalline content. For compound 2 (Figure 6a), no loss of crystallinity or change in crystal structure was observed upon transitioning from 1D nanostructures to 2D thin films, consistent with previous reports. Electron diffraction under normal incidence produces highly anisotropic spot patterns due to columnar stack alignment along the preferential growth direction. From main reflection positions, an intercolumnar spacing of 2.06 nm is derived, slightly less than the 2.14 nm measured in 1D structures. Equatorial 00L reflections correspond to columnar order, yielding a molecular period of 0.47 nm. Assuming a 45° tilt angle along the stacks, the π-stacking distance is calculated as 0.33 nm. A similar electron diffraction pattern is observed for compound 3 (Figure 6b), with unit cell parameters of comparable magnitude. Intercolumnar spacing is slightly larger (2.24 nm), and the molecular period within columns is 0.54 nm, leading to a π-stacking distance of 0.38 nm assuming the same 45° tilt angle. Notably, these values differ significantly from those obtained via X-ray diffraction (XRD) on bulk crystals of compound 3 (a = 6.6 Å, b = 12.6 Å, c = 17.8 Å), highlighting the influence of 2D-confined growth conditions. Table 1 summarizes the structural characterization data for all four N-heterotriangulenes. The discussion reveals that strong π-interactions among the electron-rich core units promote long-range supramolecular organization in solution. Directional and isotropic 2D growth is favored in carbonyl-bridged derivatives with non-bulky para-substituents. The crystal structure of compound 3 shows a short intermolecular distance (dC = 3.44 Å) between adjacent molecules, supporting close packing. Quantum mechanical geometry optimizations suggest that sulfur-containing compounds adopt an angulate core due to increased electron density and steric demand at the bridge site, disrupting efficient π-stacking. This explains the lack of directional growth and preferred orientation in compounds 4 and 5. Additionally, 2D film formation was not observed in N-heterotriangulenes S2 and S3 bearing bulky substituents without acetylene spacers at para positions, likely due to steric hindrance preventing coplanarity. Molecules tend to aggregate into one-dimensional fibers or needles. For compound 3, the average alignment of the electron-rich core was derived from NEXAFS data fitting. Although compound 2 lacks significant dichroism in core-related transitions, the clean SAED pattern and well-defined orientation of long alkyl chains indicate preferential arrangement. The *-orbitals are likely near the magic angle, implying a core tilt of ~35.3° from the surface normal. With a film thickness of 8.14 nm and molecular extension of 3.79 nm, this corresponds to 3–4 monolayers. In contrast, compound 3’s base film consists of ~12 monolayers, based on a tilt angle of 53°, film thickness of 17.43 nm, and molecular extension of 1.52 nm. The significant difference in base film thickness and RMS roughness underscores the impact of peripheral side chains. The vertically aligned dodecyl chains in compound 2 enhance vertical order through extensive VDW interactions, stabilizing thinner films. However, hydrophobicity between top-layer chains and the observed mono- and bilayer growth contribute to higher RMS roughness (12.7 Å). Shorter fluorinated chains in compound 3 lead to lower RMS roughness (10.1 Å) and greater base film thickness. The crystal structure of 3 reveals strong hydrogen bonding between aromatic C–H and CO groups (O···HAr = 2.52 Å) and between C–F and aromatic H atoms (F···HAr = 2.58 Å), resulting in a shape-persistent, homogeneous supramolecular network. The hydrophilicity of the perfluorinated groups favors a lying-down configuration during film formation, contrasting with the hydrophobic dodecyl chains in compound 2. Comparing SAED patterns of compound 3 with XRD data from bulk crystals reveals significant discrepancies in unit cell parameters. This suggests that 2D-confined growth at the water surface differs fundamentally from bulk crystal formation via solvent diffusion. To resolve this, molecular dynamics (MD) simulations were performed on a 6 nm liquid slab of 7402 water molecules using 2D periodic boundary conditions. Fifty-eight molecules of compound 3 were introduced at random positions above the upper surface. After 1 ns simulation at 300 K, a 2D liquid-like state with partial ordering was achieved, demonstrating early-stage preference for columnar stacking (Figure 8c). The columnar stacks deviate from the stair-like packing seen in XRD structures. Instead, the face-to-face orientation of core units aligns with the crystal structure of compound 2 in both 1D fibers and 2D films, explaining the similar electron diffraction patterns of 2 and 3. Thus, both carbonyl-bridged N-heterotriangulenes exhibit a strong tendency toward similar binding motifs despite differing side chains. This highlights the role of 2D-confined environments in guiding self-assembly. Sufficiently strong π-interactions and the absence of interacting solid substrates drive gradual, consistent self-assembly into columnar structures, breaking the three-fold symmetry of the parent molecule. Incorporating thin films of compounds 3 and 5 into bottom-gate/top-contact OFET devices unexpectedly yielded p-type behavior, with current amplification of several orders of magnitude (up to ~10⁴) under negative gate voltage. Such hole transport in solution-processed n-type semiconductors is rare and sensitive to oxygen and moisture diffusion. Similar observations have been reported before. Charge-carrier mobilities reached the upper end of 10⁻³ cm² V⁻¹ s⁻¹. Although threshold voltages exceeded –20 V, switching behavior differed: device 5 showed instantaneous drain current increase above –20 V, a desirable trait in layered organic electronics, while device 3 exhibited gradual amplification. OFETs based on compounds 2 and 4 showed neither p-type nor n-type behavior, possibly due to long alkyl chains obstructing charge transport. In summary, we successfully fabricated 2D-extended thin films of four soluble N-heterotriangulene derivatives at the liquid-liquid interface, with thicknesses ranging from 5 to 20 nm—indicative of multilayer systems. Thioketone-bridged compounds lacked directional growth and preferred orientation, as confirmed by NEXAFS. In contrast, carbonyl-bridged compounds formed long-range ordered, crystalline layered systems, verified by AFM, SAED, and clear dichroic effects in NEXAFS. Compound 2 maintains a similar crystal structure to previously reported 1D nanofibers, with a strongly tilted core and surface-normal-aligned side chains. Compound 3 shows exceptional crystallinity, with a more inclined core favoring a lying-down configuration. Importantly, unit cell parameters diverge from XRD data, emphasizing the influence of 2D growth conditions. Therefore, both core and peripheral substituents, along with dimensionality and preparation method, play vital roles in designing ultrathin crystalline layers. From a practical standpoint, our work offers a powerful toolkit for precise design and unambiguous structural elucidation of high-quality 2D N-heterotriangulene systems for potential applications in layered organic electronics.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com