Crystal Structure and Molecular Packing Behavior of Poly(2,3-diphenyl-1,4-phenylenevinylene) Derivatives Containing Alkyl Side-Chains

Abstract

Phase behavior and crystal structure of a series of poly(2,3-diphenyl-5-alkyl-p-phenylenevinylene) (denoted as DPn-PPV, where n represents the carbon number of the alkyl side-chain, n = 6, 8, 10, 12) were studied using differential scanning calorimetry, one-and two-dimensional (1D and 2D) wide-angle X-ray diffraction (WAXD), and selected area electron diffraction (SAED). The experimental results reveal that DPn-PPV exhibits one crystalline phase at low temperatures. On the basis of 2D WAXD and SAED patterns obtained from the oriented samples, the crystal structures are determined to be orthorhombic for DP6-PPV and monoclinic for DP8-PPV, DP10-PPV, and DP12-PPV. To account for the unusually large unit cell dimensions, we propose that the unit cell of DPn-PPV contains 4 chains (8 chemical repeat units). The complex crystal structure can be attributed to the longitudinal and transverse offsets between the neighboring chains, which shall be mainly due to the requirement of minimizing the steric hindrance caused by the attached pendent groups and maximizing the π−π interaction between the chains. The molecular packing scheme was simulated by using Cerius 2 software, of which the result agrees with the experimental data. The polarized UV−vis absorption and polarized solid-state photoluminescence (PL) property of these polymers was also investigated. The PL spectra indicated that the light emitted from the oriented film was preferentially polarized parallel to the shear direction, implying that DPn-PPV may potentially be useful in linearly polarized luminescence devices.

Structure and Morphology Control in Thin Films of Conjugated Polymers for an Improved Charge Transport

“…The larger domain size is thought to originate from the fibrillar structure of a larger size as a result of more conjugated backbones. Thirdly, the anisotropy of films crucial for device performances could be controlled by the molecular structures of chains [117][118][119].…”

“…As a result, the roughness of films for P3AT with shorter side-chains and more intense conjugation was larger indicating a larger crystallinity, which was the origin of the highest mobility in its film. Besides, a larger dichroic ratio of optical absorption or emission indicating the higher anisotropy of carrier transport could be tuned by the length of side-chains [117,119]. Ren et al reported that with shorter side-chains, the dichroic ratio of films prepared by mechanically shearing turned larger [117].…”

“…Besides, a larger dichroic ratio of optical absorption or emission indicating the higher anisotropy of carrier transport could be tuned by the length of side-chains [117,119]. Ren et al reported that with shorter side-chains, the dichroic ratio of films prepared by mechanically shearing turned larger [117]. They attributed it to the higher T g of PPV with shorter side-chains to obtain orientation due to less relaxing of chains.…”

Ionic Self‐Assembled Derivative of Tetraphenylethylene: Synthesis, Enhanced Solid‐State Emission, Liquid‐Crystalline Structure, and Cu2+ Detection Ability

“…Unfortunately, serious problems associated with most fluorophores are their intrinsic aggregation‐caused quenching (ACQ) effects . To overcome this problem, many molecular strategies have been developed to alleviate the self‐quenching effect . In particular, we have demonstrated that bulky substituents can suppress the continuous packing of conjugated luminogens, which allows the preservation of luminescence efficiency .…”

“…To overcome this problem, many molecular strategies have been developed to alleviate the self‐quenching effect . In particular, we have demonstrated that bulky substituents can suppress the continuous packing of conjugated luminogens, which allows the preservation of luminescence efficiency . Nevertheless, the intrinsic contradiction of high solid‐state efficiency (avoiding aggregation) and mesophase formation (ordered molecular packing) is still difficult to solve fundamentally.…”