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

H

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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-­‐  

δ+  

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δ+  

δ+  

δ+  

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)