Molecular Packing Structure Research Articles

Homeotropic alignment of non-peripheral octahexyl phthalocyanine in thin film and its photovoltaic properties

The homeotropic alignment of non-peripherally hexyl-substituted phthalocyanine (C6PcH2) molecules in thin films has been investigated. A spin-coated film C6PcH2 was covered with a poly(vinyl phenol) layer, and a thermal treatment through a liquid crystalline phase was carried out. The molecular packing structure and induced homeotropic alignment in the thin films were clarified by the grazing incidence wide-angle X-ray scattering technique. The bulk heterojunction solar cell with the homeotropically aligned C6PcH2 was fabricated, and the photovoltaic properties were discussed by taking the optical properties and carrier mobility in the active layer into account.

Kinetically Controlled Polymorphic Superstructures of Pyrene-Based Asymmetric Liquid Crystal Dendron: Relationship Between Hierarchical Superstructures and Photophysical Properties.

To understand the relationship between kinetically controlled hierarchical superstructures and photophysical properties, pyrene-based asymmetric liquid crystal (LC) dendrons (abbreviated as PD) were newly synthesized by covalently attaching a pyrene moiety (P) at a biphenyl-based LC dendritic group (D). The phase transition behavior of PD has been systematically studied with a combined technique of thermal analysis, microscopy, spectroscopy, and scattering analysis. PD formed two different crystalline structures depending on the cooling rate: a stable crystalline phase (Ks , slow cooling) and a metastable crystalline phase (Kms , quenching). The kinetically controlled molecular packing structures of PD depend on the competition and cooperation of intermolecular physical interactions with nanophase separation. Upon slow cooling, the PD dimer formed by intermoelcular H-bonding constructed a layered hierarchical structure with the help of nanophase separation. Owing to the strong π-π stacking (J-aggregation) with weak H-bondings, the PD dimer in the layer was slightly tilted to give a monoclinic layered structure with a periodic layer d-spacing of 6.6 nm. In contrast, the metastable Kms phase formed by the quenching process showed a significant tilt of the PD dimer in the layer (d-spacing=4.4 nm) due to the weak π-π stacking (H-aggregation) and the strong H-bondings.

Single-crystalline thin-film growth via solution-mediated polymorphic transformation of octahexyl-substituted phthalocyanine and its optical anisotropy

Abstract The fabrication of a single-crystalline thin film of non-peripheral octahexyl-phthalocyanine (C6PcH2) was realized by performing a tetrahydrofuran (THF) vapor treatment on a the spin-coated film. The solution-mediated polymorphic transformation of C6PcH2 was promoted in the THF vapor, and a single-crystalline thin film with α-phase crystal grew in the thin film when setting an appropriate temperature. The thin film exhibited optical anisotropy, such as a high dichroic ratio, and the color of the transmitted light varied from green to transparent depending on the polarization direction. The molecular packing structure on the substrate was determined by X-ray diffraction and polarized absorption measurements.

Domain Structures of Poly(3-dodecylthiophene)-Based Block Copolymers Depend on Regioregularity

Microphase-separation behavior of conjugated–amorphous block copolymers (BCPs) is driven by a complex interplay between Flory–Huggins interaction (χ), liquid crystalline (LC) interaction, and crystallization. Herein, in order to elucidate the influence of LC interaction on the morphology of the BCPs, we report the effects of regioregularity (RR) on the microphase separation and molecular packing structures of poly(3-dodecylthiophene)-block-poly(2-vinylpyridine) (P3DDT-b-P2VP). To decouple the effect of LC interactions from crystallization kinetics, we investigate the morphological behavior of the P3DDT-b-P2VP at above the melting temperature of P3DDT (∼160 °C). Both electron microscopy and X-ray scattering show an abrupt reduction in the domain spacing of both lamellar and cylindrical phases as the RR of P3DDT block increases. Specifically, lower RR (i.e., 85, 79, and 70%) BCPs have larger domain spacings than high RR (94%) by 50% (lamellar) or 80% (cylindrical), even though the overall molecular weights ...

Synthesis, structure and material properties of thiopyranylidene-based asymmetrical squaraines

In this paper, we synthesized a new class of thiopyranylidene-based NIR absorbing asymmetrical squaraines (IPSQ 1–4), and carefully investigated their photophysical, thermal, electrochemical and semiconducting properties. A high hole mobility close to 10−4 cm2/Vs can be realized for IPSQ-3 based organic field effect transistors (OFETs), attributed to the strong intermolecular π-π interactions induced by pyranylidene-type end-groups with large π-system. More interestingly, we find through subtle structural variation, totally different molecular arrangements, herringbone and “brick-wall” -type, can be achieved for IPSQ-1 and -4, respectively, which therefore enables us to study the influence of molecular packing structure on semiconducting properties. For IPSQ-1, the hole mobility was found to be ∼50 times that of IPSQ-4, indicating herringbone-type arrangement is more beneficial to charge carrier transport here. Furthermore, IPSQ-1 also showed a better photovoltaic performance than IPSQ-4. Thus, our findings in this work can help us to understand the implications of tuning structural properties of squaraines, which will be very useful for designing high-performance squaraine-based semiconductors.

Tuning the molecular packing structure of metal-phthalocyanines on graphene by inserting interfacial copper nano grains

Abstract Graphene has been widely used in various organic devices as either electrodes or templates to obtain better device performance. Usually, organic molecules with planar backbone prefer to lie down on graphene leading to molecular π−π stacking perpendicular to graphene substrate. However, for organic field-effect transistors, parallel π−π stacking is superior to the perpendicular ones since it enhances the charge carrier's mobility between source and drain. This triggers studies on tuning the crystalline structures of organic thin film on graphene. For metal-phthalocyanines (MPc) which are widely used in organic field-effect device as active layer, so far MPc thin film with parallel molecular π−π stacking on graphene hasn't been reported. In this study, we report a method to fabricate MPc films with single crystalline structure and molecular π−π stacking parallel to graphene surface. Copper nano grains (CNG) were deposited onto graphene surface prior to the deposition of MPc films. MPc films with parallel π−π stacking mode was obtained on graphene substrate which was covered by CNG with nominal thickness of 1 nm. This work provides a conceptually new approach to induce parallel π−π stacking of MPcs on graphene in that no functionalization of graphene is needed.

Ambipolar carrier transport properties and molecular packing structure of octahexyl-substituted copper phthalocyanine

The charge carrier mobility of a solution-processable low-molecular-weight organic semiconductor material, i.e., 1,4,8,11,15,18,22,25-octahexylphthalocyanine copper complex (C6PcCu), was investigated by the time-of-flight technique. The anomalous ambipolar carrier mobility was discussed from the viewpoint of the molecular packing structure, which was clarified by single-crystal X-ray structure analysis. In the comparison between the molecular packing structures of C6PcCu and its metal-free-type homologue, it was found that the difference in carrier mobility originates from the rotation of the molecule, which is caused by the steric hindrance due to the introduction of a center metal and the interpenetration of the nonperipheral alkyl chains.

Bi-thieno[3,4]pyrrole-4,6-dione based copolymers: 1,2-Bis(2-thienyl)ethene unit vs 1,2-diphenylethene unit

Abstract Bi-thieno[3,4]pyrrole-4,6-dione (bi-TPD) based copolymers containing 1,2-bis(2-thienyl)ethene (P1) and 1,2-diphenylethene (P2) units are synthesized and characterized. Their physicochemical and electrical properties as well as molecular packing structures in thin films have been fully investigated. P1 has higher HOMO energy levels and smaller bandgap than P2, which is attributed to the stronger electron-donating ability of 1,2-bis(2-thienyl)ethene unit. Both polymers form continuous films and adopt face-on and edge-on co-orientation in the aggregation states. The crystallinity of P1 film is higher than that of P2 film. The optimal field-effect mobility is 0.74 cm2V−1s−1 for P1 films and 0.32 cm2V−1s−1 for P2 films. These results demonstrated the merits of 1, 2-bis(2-thienyl)ethene unit and the potential applications of bi-TPD based polymers in high performance polymer transistors.

From Molecular Packing Structures to Electronic Processes: Theoretical Simulations for Organic Solar Cells

AbstractMolecular packing structures in the active layers have a crucial impact on the electronic processes for organic solar cells. To date, however, it is still difficult to probe molecular self‐assembling and packing structures at the atomic level by experimental techniques, which is hindering reliable understanding of the structure–property relationship. Accordingly, theoretical simulations provide a useful tool and are becoming more and more important. Here, recent advances in theoretical simulations for organic solar cells are reviewed. First, a brief introduction of theoretical methodologies, including the strategies of molecular dynamics simulations of active‐layer processing procedures and quantum‐chemical methods for calculating electron transfer processes, is given. Then, the influences of molecular packing structures on charge generation, charge recombination, and charge transport are analyzed and discussed from a theoretical perspective. Finally, prospects and challenges are pointed out for theoretical prediction of the electrical characteristics and photoelectric conversion efficiencies of organic solar cells from molecular structures.

Diversified Photo/Electronic Functions Based on a Simple Chalcone Skeleton: Effects of Substitution Pattern and Molecular Packing

AbstractStructurally simple chalcone derivatives 1–2 are prepared and their diversified emission behaviors are deeply investigated. Two polymorphs (1G: λem = 536 nm, Φf = 0.08, τ = 1.81 ns; 1O: λem = 573 nm, Φf = 0.19, τ = 10.82 ns) with distinctively different emission behaviors are constructed by finely controlling the crystallization conditions of compound 1. 1G exhibits typical amplified spontaneous emission while 1O shows an interesting blue shift under smashing process (1O‐S: λem = 562 nm, Φf = 0.23, τ = 6.42 ns), which is ascribed to their different molecular packing structures and intermolecular interactions. Notably, simply introducing a fluorine substituent effectively endows the crystal with red emission (crystal 2: λem = 598 nm, Φf = 0.16, τ = 18.77 ns). Thus multicolor emissions including green, yellow, orange, and red emissions are obtained based on this simple chalcone skeleton.

Tailoring Phase Transition in Poly(3-hexylselenophene) Thin Films and Correlating Their Crystalline Polymorphs with Charge Transport Properties for Organic Field-Effect Transistors

Poly(3-hexylselenophene) (P3HS) carries attractive advantages over their close analogue poly(3-hexylthiophene) (P3HT), including a stronger intermolecular interaction, a better interchain charge hopping, and a narrower bandgap. However, P3HS is much less studied compared to P3HT. Herein, we report on intriguing reversible phase transition between two different crystalline polymorphs (i.e., form I and II) in P3HS thin films with different molecular weights enabled by alternating thermal and solvent vapor annealing. More importantly, the phase transition kinetics and mechanism as well as the associated changes on molecular packing structures were also scrutinized. The correlation between different P3HS crystalline polymorphs and the resulting field-effect mobilities was explored for the first time. Our study provides an insight into P3HS crystallization and phase transition, thus entailing the use of polyselenophene-based materials for a wide range of optoelectronic applications.

Asymmetric Fullerene Nanosurfactant: Interface Engineering for Automatic Molecular Alignments.

Since the molecular self-assembly of nanomaterials is sensitive to their surface properties, the molecular packing structure on the surface is essential to build the desired chemical and physical properties of nanomaterials. Here, a new nanosurfactant is proposed for the automatic construction of macroscopic surface alignment layer for liquid crystal (LC) molecules. An asymmetric nanosurfactant (C60 NS) consisted of mesogenic cyanobiphenyl moieties with flexible alkyl chains and a [60]fullerene nanoatom is newly designed and precisely synthesized. The C60 NS directly introduced in the anisotropic LC medium is self-assembled into the monolayered protrusions on the surface because of its amphiphilic nature originated by asymmetrically programmed structural motif of LC-favoring moieties and LC-repelling groups. The monolayered protrusions constructed by the phase-separation and self-assembly of asymmetric C60 NS nanosurfactant in the anisotropic LC media amplify and transfer the molecular orientational order from surface to bulk, and finally create the automatic vertical molecular alignment on the macroscopic length scale. The asymmetric C60 NS nanosurfactant and its self-assembly described herein can offer the direct guideline of interface engineering for the automatic molecular alignments.

Triplet Transfer Mediates Triplet Pair Separation during Singlet Fission in 6,13‐Bis(triisopropylsilylethynyl)‐Pentacene

AbstractTriplet population dynamics of solution cast films of isolated polymorphs of 6,13‐bis(triisopropylsilylethynyl) pentacene (TIPS‐Pn) provide quantitative experimental evidence that triplet excitation energy transfer is the dominant mechanism for correlated triplet pair (CTP) separation during singlet fission. Variations in CTP separation rates are compared for polymorphs of TIPS‐Pn with their triplet diffusion characteristics that are controlled by their crystal structures. Since triplet energy transfer is a spin‐forbidden process requiring direct wavefunction overlap, simple calculations of electron and hole transfer integrals are used to predict how molecular packing arrangements would influence triplet transfer rates. The transfer integrals reveal how differences in the packing arrangements affect electronic interactions between pairs of TIPS‐Pn molecules, which are correlated with the relative rates of CTP separation in the polymorphs. These findings suggest that relatively simple computations in conjunction with measurements of molecular packing structures may be used as screening tools to predict a priori whether new types of singlet fission sensitizers have the potential to undergo fast separation of CTP states to form multiplied triplets.

Anomalous Ambipolar Transport of Organic Semiconducting Crystals via Control of Molecular Packing Structures.

Organic crystals deposited on 2-dimensional (2D) van der Waals substrates have been widely investigated due to their unprecedented crystal structures and electrical properties. van der Waals interaction between organic molecules and the substrate induces epitaxial growth of high quality organic crystals and their anomalous crystal morphologies. Here, we report on unique ambipolar charge transport of a "lying-down" pentacene crystal grown on a 2D hexagonal boron nitride van der Waals substrate. From in-depth analysis on crystal growth behavior and ultraviolet photoemission spectroscopy measurement, it is revealed that the pentacene crystal at the initial growth stage have a lattice-strained packing structure and unique energy band structure with a deep highest occupied molecular orbital level compared to conventional "standing-up" crystals. The lattice-strained pentacene few layers enable ambipolar charge transport in field-effect transistors with balanced hole and electron field-effect mobilities. Complementary logic circuits composed of the two identical transistors show clear inverting functionality with a high gain up to 15. The interesting crystal morphology of organic crystals on van der Waals substrates is expected to attract broad attentions on organic/2D interfaces for their electronic applications.

Study of copper-phthalocyanine and pentacene film growth on transferred graphene: The influence of polymer residues

Wet transferring of chemical vapor deposition (CVD) graphene with the assistance of poly(methyl methacrylate) (PMMA) usually leaves polymer contaminations onto graphene surface. Knowledge about the influence of PMMA residues on organic film growth properties is crucial for graphene-related organic electronic devices. In this study, we investigate growth properties of coper-phthalocyanine (CuPc) and pentacene on PMMA contaminated graphene substrate. Grain size and morphologies of CuPc and pentacene films were found to be governed by the density of PMMA residues on graphene. Nevertheless, the molecular packing structures of CuPc and pentacene respond differently with the density with PMMA residues. CuPc molecules tend to nucleated into grains with molecular π-π stacking parallel to graphene surface, which is irrespective of the PMMA residues. Whereas the packing behavior of pentacene on PMMA-transferred CVD graphene is sensitive to the density of PMMA residues. Molecular π-π stacking perpendicular to graphene is observed when pentacene was deposited onto as-transferred graphene. Parallel mode was observed only on the substrates without PMMA residues.

Structural analysis of the foldecture derived from racemic peptide foldamers

The molecular packing structure of an elongated parallelogram plate shaped foldecture composed of a 1:1 racemic mixture of 11-helical peptide foldamers was resolved by powder X-ray diffraction (PXRD) analysis. A comprehensive Rietveld refinement procedure compensated for powder texture and identified the principal face of the foldecture. Each foldamer makes head-to-tail intermolecular hydrogen bonds, creating extended chains of single enantiomers that form a network of hydrophobic close contacts with foldamers of both the opposite and the same chiralities. An isosurface for anisotropic microstrain was calculated and found to be smallest along the x-axis, which is parallel to the network of intermolecular hydrogen bonds. Comparison with the single crystal structure found molecular packing motifs to be almost identical—a result infrequently observed in enantiopure foldectures. This is the first powder X-ray diffraction structural analysis of a foldecture composed of multiple components.

Effect of Substituent Position on the Photophysical Properties of Triphenylpyrrole Isomers

The charge distribution, molecular structure, and morphological packing significantly affect the photophysical properties of organic photoluminescent materials. In this work, two triphenylpyrrole isomers, 1,2,5- (TPP1) and 1,3,4- (TPP2), were first synthesized and characterized. Because of their different substituent positions, TPP1 possesses aggregation-caused emission quenching (ACQ) behavior while TPP2 exhibits aggregation-induced emission (AIE). Their different photoluminescent properties were systematically investigated by using UV–vis absorption spectroscopy, fluorescence spectroscopy, density functional theory (DFT) calculations, and single-crystal structure analysis. The results indicate that substituent position of the two phenyl groups predominately affects the charge distribution of the isomers and determines their molecular packing structures, which further cause the different restriction of intramolecular rotation (RIR) capabilities of phenyl rings, thus resulting in different luminescence pr...

Molecular design of flexible liquid crystal oligomers stabilising the chiral frustrated phases

ABSTRACTChirality induces structural frustration in liquid crystal systems, producing various kinds of chiral frustrated phases, for example, twist grain boundary (TGB) phases, blue phases (BPs) and dark conglomerate (DC) phases. Almost all molecules exhibiting these frustrated phases have a rigid shape. Especially, a bent–core unit is regarded as a key structure for BPs and DC phases. This paper describes that some flexible liquid crystal oligomers being far from a rigid bent–core molecule stabilise these phases. The LC oligomers have a supermolecular structure in which mesogenic units are connected via flexible spacers. By designing intermolecular interactions, they can exhibit various molecular packing structures in the liquid-crystalline phases as follows: chiral dimers inducing TGB phases, U-shaped and T-shaped oligomers stabilising BPs and achiral liquid crystal trimers exhibiting DC phases. I discuss how the designed liquid crystal oligomers produce the chiral frustrated phases.

Quantitative Correlation between Carrier Mobility and Intermolecular Center-to-Center Distance in Organic Single Crystals

Charge transport properties of organic semiconductors critically depend on their molecular packing structures. Controlling the charge transport by varying the molecular packing and understanding their structure–property correlations are essential for developing high-performance organic electronic devices. Here, we demonstrate that the charge carrier mobility in organic single-crystal nanowires can be modulated with respect to the intermolecular center-to-center distance by applying uniaxial strain to the cofacially stacked crystals. Monotonic changes in charge carrier mobility (from 0.0196 to 19.6 cm2V–1s–1 for 6,13-bis(triisopropylsilylethylnyl) pentacene (TIPS-PEN)) were observed under a wide range of strains from −16.7% (compressive) to 16.7% (tensile). Furthermore, the measured values of charge carrier mobility were in good agreement with theoretical calculations based on charge localized hopping theory. These results provide a definitive relationship between intermolecular packing arrangement and cha...

Crystal structure of 1-(4-hydroxybenzoyl)-2,7-dimethoxynaphthalene: Contribution of classical and non-classical hydrogen bonding interactions to the molecular packing structure

Crystal structure of 1-(4-hydroxybenzoyl)-2,7-dimethoxynaphthalene, C 19 O 4 H 16 , is reported from the viewpoint of characteristics in the helical alignments and difference compared to the crystal structure of a structural isomeric compound. The asymmetric unit of title compound contains two conformers A and B. Furthermore, the molecules in crystal exhibit atropisomerism brought about by molecular stereogenic axis of carbon-carbon bond between the carbonyl moiety and the naphthalene ring. Therefore, a pair of R - and S -enantiomeric molecules exists for each conformer. The two pairs of the enantiomeric molecules are related by two-fold helical axis in the asymmetric unit of P2 1 /c space group, exhibiting the number of molecules is eight, Z = 8. Single molecular structure of title compound shows non-coplanarly accumulated aromatic-rings structure. The molecular packing structure is mainly stabilized non-classical hydrogen bonds involving C-H…O hydrogen bonds and C-H…π ones, however O-H…O=C classical hydrogen bonds are solely formed between same configured conformers. Comparison with the spatial alignment of an isomeric homologue, 2-hydroxy-1-(4-methoxy benzoyl)-7-methoxynaphthalene, has clarified that substitution position of hydroxy group determines not only direction of classical hydrogen bonds but also total feature of molecular packing, i.e. , the homologous compound, which has hydroxy group at 2-position of naphthalene core forms intramolecular O-H…O=C classical hydrogen bond, and O-H…OMe classical hydrogen bonds between opposite enantiomeric isomers. The presence/absence and direction of the predominantly strong classical hydrogen bonds govern balance of interactions of other less effective classical and non-classical hydrogen bonds in molecular packing. In homologous compound, each of non-classical hydrogen bonds between same signed enantiomeric isomers and those between opposite enantiomeric isomers demonstrates almost same distances. In title compound, both types of non-classical hydrogen bonds formed by conformer A are imbalanced. The imbalanced non-classical hydrogen bonds are adjusted and reinforced by non-classical hydrogen bonds between conformers A and B.