Relationship Between Spectroscopic and Structural Properties of Poly(meta/para Phenylene)
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CH3
CH3
CH3
CH3
OCH3
OCH3
OCH3
para phenylene
meta phenylene
Figure.1: PMPP motives EXPERIMENTAL DETAILS The vapour deposition of PMPP occurred under high vacuum (∼ 10-6 torr) at a rate of 15 Å/s. The thickness of polymeric layers was fixed at approximately 1000 Å. Polymeric layers were deposited on precleaned, indium tin oxide ITO (50-100 Ω / ) and spectrosil glass substrates were used for absorption measurements. EPR measurements at the X band were performed with a Brucker ESP-300E spectrometer using an Oxford ESR-900 gas-flow-type cryostat for temperature variation. The surface morphology of PMPP films was imaged with a Nanoscope III atomic force microscope in the contact mode, their mean roughness (R were calculated from the raw data using the Nanoscope software. UV-Vis spectroscopy was performed on PMPP thin films deposited on spectrosil glasses and scanned over the spectral range 190-900 nm in an Uvikon 941 Plus UV-Vis spectrophotometer (scan rate 500 nm/min). RESULTS EPR spectroscopy Figure.2 shows two EPR spectra of PMPP corresponding to long and short chainlength.
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Figure.2: Electron paramagnetic resonance spectra for (a) long (30 monomer units) and (b) short (9 monomer units) chainlengths showing (A) organic signal and (B) the Cu2+ signature.
Each of spectrum is formed mainly by two signals; one sharp located at almost 335 mT (A) and the second one is displaced to lower fields (B). Moreover, we can observe an increase of the peak to peak amplitude of the signal (A), relatively to the (B) signal, when decreasing chainlength. The evolution of the (A) signal with different physical parameters (light and temperature) has been investigated in order to identify the nature of paramagnetic species in PMPP. The three chainlengths present the same behavior. We have recorded under different conditions the EPR spectra. No significant difference between the spectra obtained at 300 K under illumination of visible light and in the dark (fig.3). 15000
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Figure.3: EPR spectra recorded (o) under illumination and ( ) in the dark (at T = 300 K and in vacuum) The magnetic susceptibility χ deduced from numerical calculation on the EPR spectra increase linearly with the inverse of temperature (fig.4).
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Figure.4: PMPP magnetic susceptibility vs. 1000/T
Optical spectroscopy The PMPP structural and electronic properties was studied showing particularly that PMPP forms an amorphous material [10] with a bandgap increasing with the inverse of the chainlength as summarized in table.I. Chainlength 9 16 30 (monomer units) Optical
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