EOTD - Polonium
Symbol: Po
Atomic number: 84
Atomic weight: [209]
Colour: Silver
[Xe] 4f14 5d10 6s2 p4
Group number: 16
Group name: Chalcogen
Standard state: solid (298 K)
Classification: Metallic
Oxidation states: 4, 2, 6
•Polonium has more known isotopes than any other element (25
known) with atomic weights ranging from 194-218.
•Weight for weight Po is about 2.5 x 1011 times as toxic as
hydrocyanic acid (HCN)
•Po has been found as a contaminant in tobacco and in uranium
ores
History
•Polonium was the first element discovered by Mme. Curie in
Poland (1989)
•it was discovered while Mme. Curie was seeking the cause of
radioactivity of pitchblende from Joachimshal, Bohemia. An
electroscope showed Polonium separating from Bismuth.
Uses
•A lightweight heat source for thermoelectric power in space
satellites
•Can be mixed or alloyed with beryllium to provide a source of
neutrons
Synthesis, Characterization, and
Electrical Conductivity of
Oligo[aromatic diimidoselenium (IV)]
Ali El-Shekeil, Khalid Y. Abid, and Omar AlShuja’a
Presented by: Danielle Dusome
Background
•One of the most important discoveries in Chemistry in
the 1980’s was the superconductivity of quasi 1-D
organoselenium polymers (first recognized in
organoselenium materials and later in organosulfur
compounds).
•This discovery triggered intense research in the area of
selenium research
•Can be used in brushes for removing dust from photographic
films
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•It was discovered that when conjugated backbone polymers
containing selenium were tested for their room temperature
conductivity that there conductivity was lower then other
chalcogen containing polymers (they generally acted as
insulators instead of conductors)
•Although the above point suggests that selenium-substituted
polymers may not be applicable candidates for conducting
polymers it was found by another group of researchers that
polymers containing N=Se=N moiety may have the ability to
be very good electrical conductors
Experimental
Synthesis of PADIS2, PADIS3, and PADIS4
•1.11 g of SeO2 (0.01mol) in 10 ml of absolute ethanol was
added dropwise to a solution of aromatic diamine (0.01mol) also
in absolute ethanol (15 ml).
•The mixture was refluxed with continuous stirring under a
steam of nitrogen gas.
•The precipitate that formed was suction filtered and washed
several times with absolute ethanol.
•The product was then dried in air for 24 hours.
Objective
Synthesis of PADIS1
•The objective of the work done for this paper is to
prepare poly[aromatic diimidoselenium (IV)] species
and to study their physical properties, including their
electrical conductivity's of their pristine form and 10%
I2-doped polymers.
•1.11 g of SeO2 (0.01mol) in 10 ml of absolute ethanol was
added dropwise to a solution of aromatic diamine (0.01mol) also
in absolute ethanol (15 ml).
The four poly[aromatic diimidoselenium (IV)]
products that were investigated in this report are;
•The mixture was refluxed with continuous stirring under a
steam of nitrogen gas.
Oligo(1,4-phenylene-diimideselenium)
(PADIS1)
Oligo(4,4’-thiodianiline selenium)
(PADIS2)
Oligo([1,1’-biphenyl]-4,4’-diimideselenium)(PADIS3)
Oligo(2,2’-dithiodianiline selenium)
(PADIS1)
•The precipitate that formed was suction filtered and washed
four times with hot ethanol.
•Benzene (300 ml) was then added to the reaction mixture
•The product was then dried in air for 24 hours.
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Why Iodine Doping?
•The iodine acts as a oxidizing agent on the polymer and
removes an electron from the polymer forming a free radical
(this defect in the polymer is called a polaron)
•Upon further oxidation of the polymer the free radical is
removed and a defect called a bipolaron is formed, with
continuous doping a continuous bipolaron system can be
formed
•The formation of polaron and bipolaron defects eases the flow
of electrons through the polymer
Iodine Doping
Method One: (performed to all of the compounds studied)
•10% (wt/wt%) I2 was mixed with the annealed material
•Annealing was then achieved by heating the dry material in a
vacuum oven for 24 hours at 100 º C.
Method Two: Chemical Doping (performed on PADIS1 only)
•The dopant (I2) was dissolved in the minimum amount of
ethanol followed by the addition of an equivalent weight of
PADIS1.
•The resulting mixture was refluxed for 20 minutes, then
filtered and dried
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Physical Measurements
Characterization
•Melting points- determined using an electrothermal melting
point apparatus
Results
•IR Absorption Spectra- were recorded using KBr pellets on a
Mattson 5000 spectrophotometer
•Selenium content- measured using a Perkin Elmer 2330 atomic
spectrophotometer and measured by titration after digestion of
the Gould Method
•UV-visible Absorption Spectra- were measured in DMSO with
a Cecil CE 599 automatic scanning spectrophotometer
Electrical Conductivity
Physical Properties
•Polymers were put into compact disk form (~ 1mm in thickness
and ~ 1 cm in diameter) by subjecting a specific amount of powder
to pressure
•A homemade circuit was used in electrical measurements with a
constant voltage of 10 V
•All measurements were taken in a vacuum and the system was
allowed to stabilize for one hour prior to readings to eliminate any
accumulated electrostatic surface charge
•Temperature changes were limited to 2ºC min-1 to assure that the
specimen environment was at thermal equilibrium during
measurements
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IR Spectra
•The main IR band assignments made in the experiment were all
consistent with the suggested structures of the synthesized
polymers
•All four of the compounds contained aromatic C=C vibrations
occurring in the region from 1484 to 1620 cm-1
•The four compounds also all contained C-N single bond stretch
between 1185 and 1302 cm-1
•PADIS2 and PADIS4 differed from the other polymers in that
they showed a C-S single bond stretch in the regions of 693 and
677 cm-1 respectively
•The IR spectra of PADIS4 differed from the rest in that it
contained a S-S stretch at 464 cm-1
UV-visible Spectra
•The UV-visible spectra showed both π-π* transitions and n-π*
transitions for all four of the polymers synthesized in this paper
•The changes in UV absorption of the chemically doped
PADIS1 was used to confirm the addition of I2 to the selenium
atom since it is known that addition of auxochromes, such as I2
raise the energy of n-π* transitions and lower the energy of
π-π* transitions
•π-π* transitions are transitions of electrons from bonding to
anti-bonding orbitals and n-π* transitions are transitions of
electrons from non-bonding orbital of one atom to a antibonding orbital of another atom
π-π* transitions
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n-π* transitions
[1H] NMR Spectra
•The [1H] NMR spectra of the PADIS materials in the DMSO
showed aromatic protons of the para-substituted rings as
multiplet bands at δ=6.9-7 ppm.
•A few terminal NH2 groups were seen as a broad singlet at
δ=3.35-4 ppm.
•The presents of terminal amines in the spectra confirms that
the synthesized polymers are short chain oligomers, this is
consistent with the low values of inherent viscosity
Electrical Conductivity
•The conductivity of all of the polymers were relatively close
with maximum conductivity's corresponding to maximum
temperatures around 10-7 (S/cm) which falls into the region of
semiconductors
•The electrical conductivity of both the PADIS1 and PADIS3
polymers was lowered by the I2 doping by the annealing method
which is a contrary result to the usual action of a dopant on the
electrical conductivity of a polymer
•The electrical conductivity of the doped PADIS2 was similar to
that of the annealed material with a slight increase in
conductivity
(the conductivity of PADIS4 was not measured due to the low melting point of the
polymer)
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Why did doping by the annealing
method cause conductivity decreases?
The unusual decrease in electrical conductivity by doping with
I2 may be the result of one or more of the following:
Conclusion
•All of the poly[aromatic diimidoselenium (IV)] polymers desired
in this study were successfully synthesized and the structures of
the polymers was confirmed by IR spectroscopy and [1H] NMR.
•1.There may be a change in the bulk energy band structure
and, consequently, the bulk energy gap
•The doping of I2 to the Selenium atom was confirmed using UVvisible spectroscopy
•2.The crystallinity of the oligomers may be destroyed as the
dopant molecules affect the stacking morphology
•The synthesized poly[aromatic diimidoselenium (IV)] were
determined to be short chain oligomers due to the results of the
[1H] NMR and the low viscosity of the compounds.
•3. The dopant may destroy defects (i.e.destroy polaron and/or
bipolarons) through which the flow of electrons on the
oligomer chain is possible
Effects of Chemical Doping on
PADIS1
•The chemical doping of PADIS1 with I2 increased the
electrical conductivity of the oligomer at 25ºC to a conductivity
of about 10-7 S cm-1 with no further increases with increased
temperature
•This can be explained in that the chemical doping lead to good
interaction between the dopant and the PADIS1 to form Se-I-I
units (some I2 charge transfer complexes may have been
formed causing an instant enhancement of the electrical
conductivity to its maximum)
•The electrical conductivity of the oligomers is in the region of
that of semiconductors and is highest at high temperatures
•Doping of I2 by the annealing method decreased the
conductivity of the PADIS and doping by the chemical method
initially increased the conductivity at low temperature but the
conductivity no longer increased with increases in temperature
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Why would we want conducting
polymers?
•Conducting polymers can be used as adhesives that allow
electrical currents to pass through from one conducting object
to another.
•Conducting polymers can be used to coat the inside of the
plastic casing of electrical devices, specifically computers to
causes radiation given off by the device to be absorbed.
• There is the possibility that due to the biocompatibility of
some conducting polymers these polymers may be used to
transport electrical signals through the body, i.e. act as artificial
nerves.
•Modern planes are made of lightweight components that make
then vulnerable to damage from lighting bolts. By coating
aircraft with a conducting polymer the electricity can be
directed away from the vulnerable internals of the aircraft.
References
El-Shekeil, A., K.Y., Abid, and O. Al-Shuja, 2001. Synthesis,
characterization, and electrical conductivity of Oligo[aromatic
diimidoselenium(IV)]. Journal of Inoganic and Oranometallic
Polymers. 11:217-234.
Bruice, P.Y. Organic Chemistry (third edition); Prentice Hall;
New Jersey, 2001, p
1118.
Ebbings, D.D. and S.D. Gammon, 2001 General Chemistry
(seventh edition); Houghton Mifflin Company: Boston. p 10901092.
Merlic, C.A., WebSpectra, 2000. Introduction to IR Spectra,
June 22, 2000.
Pratt, C.M. Conducting Polymers,1995. February 22, 1996.
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