Post on 10-Oct-2020
Conjugated Polymers Part 1
Winter 2016
O
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Properties of Metals and Semiconductors
http://pedia.educdz.com/Encyclopedia_of_Chemical_Physics_and_Physical_Chemistry/a1_3.htm
Ignor Bands are half filled
Fully filled (occupied)
Empty (unoccupied)
Polyacetylene – A Classic Conjugated Polymer
• Two possible structures• Top structure every C is
equivalent all have 1 unpaired electron
• Metallic (unpaired = free electrons)
• Bottom – No unpaired electrons• Results in filled and unfilled pi-
orbitals• Which is correct?• Evidence presented in reading
– Bond-length– Electronic structure
(-CH-)n
(-CH=CH-)n
Consequence of having paired electrons
• HOMO-LUMO level narrows as conjugation length is increased
• Begins to saturate
• We’ll see many experimental examples of this
• Note that it is not possible to ‘close’ gap and form a metal with a neutral species
Polyacetylene: Band Structure
• Pi-band is not continuous• filled and unfilled pi-orbital
and a gap between the frontier orbitals
• HOMOs and LUMOs• Analogous to gap between
the valance and conduction levels in semiconductors
• Polyacetylene and most neutral conjugated polymers are semiconductors!
• Disappointing if the goal is molecular metal
Polyacetylene Synthesis
• Two forms cis and trans• Trans = 2 C’s in repeat unit• Cis = 4 C’s in repeat unit• Low temperature synthesis
leads to cis isomer• Thermal isomerization to
the trans conformation• Characterization?
• Further heating leads to cross-linking and decomposition
Trans Cis
Zeigler-Natta-Type Route
Shirakawa Halogenation
• Chlorination produces a mixture of isomers• Elimination of H-X to make graphite
• Several curious observations:– Film changed color with trace (sub stoichiometric) amount of Cl2– Upon continued reaction becomes clearer
Structure of Polyacetylene
• Half filled not stable• Dimerizes• Bond-lengths alternate• All trans (transoid) form
is most thermodynamically stable
• All trans has the smallest HOMO-LUMO gap
• Degenerate A and B phases
Polyacetylene can be doped!
• Change in the oxidation state that is de-localized through out the molecule
• Large number of redox sites• Both p- and n-type doping
possible• Either gain electron or loose
electron
• Loosing a electron also called gaining a hole
• Acts as a charge carrier• Carriers populate mid-gap
states
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Un-Doped (neutral)
P-doped (oxidized)
I-‐
δ+
δ+
δ+
δ+
δ+
Doping in Polyacetylene
• Polyacetylene has an “A” and “B” form that are degenerate
• Consider mid-chain defect from the A to the B form
• Mobile because of the translational symmetry of the chain
• Results in new molecular orbital
• A non-bonding orbital in the middle of the HOMO-LUMO gap
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Band Picture of Solitons
• Consider charge storage (negative or positive)
• Generates a single new mid-gap state (non-bonding) Nature of state determines charge
• Empty = positive• Occupied = negative• Experimental evidence:
optical properties change with doping concentration
• New low energy transition in polymer
New mid-gap state
Negative Positive
Positive Soliton
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Experimental Evidence: Chemical Doping Increases Conductivity
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Un-Doped (neutral)
Doped
I-‐
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Charge Storage on Conjugated Chain
• Doping is similar to reduction/oxidation• Because of delocalization it changes the band structure of the
polymers (conduction and valance band or pi and pi* bands)
• Solitons occur when the ground state is degenerate (two bond alternating forms are equal in energy)
• Polarons and bipolarons occur when it is non-degenerate
• There are negative, positive and neutral types of solitons• Negative and positive polarons
• Note the spin-charge relationship is different for solitons and polarons
Poly(phenylene vinylene) Ground State Electronic Structure
• In the case of PPV: 8 pi-electrons – 8 pi bands 4 filled, 4 empty
• A and B forms are not degenerate!• Common for all polymers except polyacetylene• Another consequence is the that ground and excited state
have different geometry• Ground state resembles the aromatic A form• Excited resembles the quinoid B form• Ground state is related to bonding; pi (also HOMO)• Excited state is related to antibonding; pi* (also LUMO)
Polarons in Conjugated Polymers
• Polarons occur in any polymer where the A and B phases are not degenerate (Basically aromatic conjugated polymers)
• Mid gap states hybridize to form two new mid gap states
• Electron polaron shown• Polaron is the combination of a neutral and
charged soliton• Both charged and spin = ½ (usual charge
vs. spin arrangement for fermions)• Example: 1 electron reduction of a polymer
with a non-degenerate ground state; a radical anion
a) Scheme of negative and positive polaron in polyphenylene
b) Band diagram of negative polaron
+ -
Bipolarons in Conjugated Polymers
• Bipolarons also occur in any polymer where the A and B phases are not degenerate (basically most other conjugated polymers)
• Bipolaron is the bound state of two solitons of like charge
• Negative bipolaron shown• Charge = +/- 2e and spin = 0• Example: 2 electron reduction of a polymer
with a non-degenerate ground state; a dianion
• All four electrons are in two mid-gap orbitals
• Dication - Two mid gap orbitals are present but unoccupied
a) Shows negative bipolaron
b) Shows band diagram of negative bipolaron
- -
Photo-Doping
• When exposed to energy greater than pi-pi* gap
• Electron can be promoted from pi to pi* by absorption of a photon
• No dopant ions are involved in this process
• A positive and negative site are generated
• They quickly recombine when the source of energy is removed
• May re-emit photon (fluorescence)
• Loose energy non-radiatively
Charge Injection
• Charge injection occurs at a metal/polymer interface
• Two types of “charge carriers”
• Electrons into pi* orbitals
• holes into pi orbitals• Major mechanism of conduction in
polymer films is thermal hopping, as opposed to tunneling
• Holes and electrons are attracted to one another
• Often recombine to generate a photon - electroluminescence
• Other relaxation pathways do not lead to emission
Polymer Film
Typically 100-300 nm thick
Polyaniline
• The as-synthesized polymer consists of alternating oxidized and reduced forms
• This is the most stable form
• Termed half oxidized or “emeraldine base”
• Insulating
• Becomes highly conductive at low pH
A Closer Look at the Oxidation States
Y = 0.5 (Emeraldine) half oxidized
Y = 1 (Leucomeraldine) fully reduced
Consider three possible Y values
Y = 0 (Pernigraniline) fully oxidized
Can refer to the free base form or salt form
Acid-Base Doping: Polyaniline
• Protonate most basic nitrogens
• Product is not the same as the fully reduced polymer
• Note the charge on the repeat unit
• Number of electrons have not changed
• Acid-doped polyanaline
Chemical Doping: Polyaniline
• Start with the fully reduced polymer
• Add chlorine
• Changes the oxidation state of the polymer
• Total number of electrons have not changed
Photoexcitation: Molecular Semiconductors with Non-degenerate Ground State
• Optical absorption generates charge carriers• Ground state electron is promoted to excited state
• Pi-electron is promoted to pi* level• Formation of positive and negative charge species (overall neutral)
• Bound electron-hole pair or exciton • Often recombine to generate a photon – fluorescence• Other relaxation pathways do not lead to emission
Polymer Film
Describing photoexcitation in conjugated polymers
• Consider excitation with light grater in energy than pi-pi* gap• Generates a vacancy in the pi-band and a new species in the
pi*-band
• Can either be described as pair of charged free (+ and -) polarons, or neutral exciton
– Electron-hole pairs are well screened electron-hole pairs (common in inorganic semiconductors; materials with high dielectric constant and delocalized conduction band)
– Excitons are strongly bound electron-hole pairs (common in molecular semiconductors; materials with low dielectric constants and localized pi* band)
Photoinduced Charge Transfer
• Occurs between donor and acceptor
• Distance dependent• Thermodynamic driving force
• Electrons go downhill
• Holes go uphill
Soluble Semiconducting Polymers• Although all conjugated polymers can be doped (positively), the doped
form is often not stable, difficult to control (and maintain) doping level, not stable
• Polyacetylene is a ‘rigid-rod’ not soluble• Influenced by environmental factors (oxygen, water)• Field move towards soluble, stable semiconducting polymers• A means of developing “plastic electronics”• Poly(phenylene vinylene)s • Poly(thiophene)s • Clarifying some of the terms:
– Valance and conduction bands describes inorganic semiconductors– Pi and pi* levels and HOMO and LUMO levels used to describe
molecular and polymer semiconductors– Strictly speaking: ionization potential and electron affinity are
correct– The term ‘band-gap’ is should only be used in the case when the
species has a true band structure.
Poly(phenylene vinylene) PPV Synthesis
• PPV is insoluble• Soluble precursor
route shown• Wessling and
Zimmerman method
• Generates a soluble polymer precursor
• >100,000 Mw• Precursor can be
deposited from solution or melt-processed
Cl
Cl
S+
S+
S+ S+
S
1) NaOH 2) Acid
Dialysis n
Heat
Counter ions not shown
The Origins of Soluble PPV
• From the Wessling and Zimmerman Method
• Methoxy chains change physical properties, melting temperature
• Partial eliminated product is soluble
• Increasing the entropy of the repeat unit should make rod-like polymers more soluble.
S+
S+
S+
S+
OCH3
H3CO
+
OCH3
H3CO
OCH3
H3CO
OCH3
H3CO
OCH3
H3CO
OCH3
H3CO
OCH3
H3CO
S+
MEH-PPV: A Soluble Conjugated Polymer
• Note that each repeat unit has a branched alkoxy chain• Each is chiral, but atactic • Last step using Wessling Zimmerman Method
MeO
OH
MeO
O
MeO
OCl
ClMeO
O
KOH
R-XR =
HCHO
HCl (g)
Synthesis of Soluble Phenylene Vinylenes
• Dissordered polymer• The chromophore is the light
absorbing species
• Collection of chromophores due to imperfections in the polymer chains, defect etc.
• Soluble• Able to be processed
• Dominated conjugated poly research in 1990’s
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OMEH-PPV
Photophysics: Phenylene Vinylenes
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MEH-‐PPV
• Photons with energy above the absorption edge frequency create excitons
• Ground and excited state have different equilibrium structure
• Electronic couples to Vibronic• Most CPs exhibit a significant
“stokes shift” • Emission energy is lower than
absorption energy– loss of energy to thermal
relaxation in the excited state– Migration to a low energy site
• Some materials exhibit anti-stokes behavior, gain energy from surrounding media or matrix
Photophysics: Phenylene Vinylenes
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MEH-PPV
• Strong light absorbers and emitters
• Exhibits absorption and fluorescence anisotropy (Chains aligned in polyethylene matrices)
• Maximum absorption/emission when chains parallel to incident light
• Note that low energy absorption overlaps with high energy emission
• Vibronic structure in both absorption and emission
X 80
Absorption Emission
Photophysics: Phenylene Vinylenes
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MEH-PPV
• Very significant stokes shift (0.2-0.3 eV)
• Compare with molecular dyes (~0.05 eV)
• Why?• Some from vibrational
relaxation – generally polymers (soluble ones) have more degrees of freedom than small molecules dyes
• Excitons are mobile and can migrate to a region of low energy (defect, or extended conjugation)