Browsing by Author "Chen, Er-Qiang"

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Crystal Structure and Molecular Packing Behavior of Poly(2,3-diphenyl-1,4-phenylenevinylene) Derivatives Containing Alkyl Side-Chains
(2013-01-08)
Ren, Xiang-Kui
;
Wu, Yu-Chun
;
Wang, Shao-Jie
;
Jiang, Shi-Dong
;
Zheng, Jun-Feng
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 pi-pi 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.
Preservation of Photoluminescence Efficiency in the Ordered phases of Poly(2,3-diphenyl-1,4-phenylenevinylene) via Disturbing the Intermolecular pi-pi Interactions with Dendritic Aliphatic Side Chains
(2012-06-12)
Wu, Yu-Chun
;
Ren, Xiang-Kui
;
Chen, Er-Qiang
;
Lee, Hsun-Mei
;
Duvail, Jean-Luc
To understand the relation between the solid-state phase structures and the photophysical properties of poly(2,3-diphenyl-1,4-phenylenevinylene) (DP-PPV) derivatives, three DP-PPV derivatives, P1-P3, were designed, synthesized via Gilch polymerization and characterized. Among the polymers, PI is a reported highly emissive poly(2,3-diphenyl-5-hexyl-p-phenylenevinylene), and P2 and P3 are novel DP-PPV derivatives, which are purposely designed to bear hydrophobic and hydrophilic Percec-type dendrons as side chains. The bulkiness and hydrophobic hydrophilic natures of the side chains show strong effects on photophysical properties of the polymers. The solutions and as-casted films of P1-F3 all show remarkably high photoluminescence (PL) efficiency (Phi(PL)) (>80% in chloroform solution, and >63% for the as-casted films). However, Phi(PL) of P1 and P3 decrease significantly to 30% after cooled their polymer melts to room temperature. Through the phase behavior analysis by differential scanning calorimetry (DSC), and phase structure analysis by wide-angle X-ray diffraction (WAXD), the decrease of Phi(PL) can be elucidated and attributed to the ordering of the solid-state structures of PI and P3. To our surprise, Phi(PL) of P2 is preserved even in an ordered solid-state phase, and it is insensitive to the structural ordering. Structural analysis of P2 revealed that the aliphatic dendritic side chains of P2 effectively disturbing the intermolecular pi-pi interactions among the conjugated backbones, which allows the preservation of Phi(PL) in the environment with ordered packing of DP-PPV molecules. The results of time-resolved PL decay experiments also confirmed that P2 possesses long-lived decay time because of excitons confined more effectively for emissive relaxation.