Chem 300: Organic Electronic Materials and Devices
Problem Set 1
Name:______________________________________________
Score:_________/50
1. Electron donating and accepting groups are often incorporated into organic structures to tune oxidation and
reduction potentials. These often result in “charge transfer molecules” like the ones below that can pass electron
density from one part of the structure to another.
1 pt each, 8 pts total – Label each indicated building block below as either a donor (D) or acceptor (A). Hint:
The way electrons move during charge transfer frequently resembles the way electron movement is drawn in
resonance structures.
O
O
S
O
N
O
S
N
a.
f.
b.
c.
d.
N
N
N
S
N
N
e.
S
N
h.
g.
2. Use the following CV produced from a mixture of benzophenone and tri-p-tolylamine in equal
concentrations to answer the following questions.
O
N
Benzophenone (BP)
Tri-p-tolylamine (TPTA)
a. 8 pts – Draw the expected relative HOMO/LUMO energies of BP and TPTA. Which structure would be
more easily oxidized? More easily reduced?
BP orbitals
TPTA orbitals
LUMO
HOMO
b. 3 pts – What species would you predominately expect to find at 1.0 V (circle all that apply)?
BP
BP
BP
TPTA
TPTA
TPTA
c. 3 pts – What species would you predominately expect to find at -2.2 V (circle all that apply)?
BP
BP
BP
TPTA
TPTA
TPTA
3. The following combination of absorbance spectra and CVs belong to the same pair of molecules. Use them to
answer the following questions.
a. 6pts – Calculate the E(S+/S) and E(S/S-) for molecules A and B based on the CV.
b. 4 pts – Calculate the Egelec of molecules A and B.
c. 6 pts – Draw lines along the absorbance curves indicating lonset values for each molecule and calculate the
corresponding Egopt values based on the approximate lonset.
3d. 4 pts – Calculate the E(S+/S*) of each molecule base on their E(S+/S) and Egopt values.
4. In Mas-Torrent and Rovira’s review of OFET devices, they report the following as a conductive polymer:
a. 4 pts – As the structure is drawn, CTD-BTZ would be a terrible semiconductor. Circle the part of the structure
that diminishes the polymer’s intramolecular charge transfer ability and explain why it inhibits charge transfer.
C16H33
C16H33
S
S
CDT-BTZ
N
S
N
n
b. 4 pts – How could you alter the structure to improve intramolecular charge transfer (and thereby conductance)
of the polymer from one monomer to the next? Hint: The article that Mas-Torrent and Rovira reference CDTBTZ might provide some insight.
Small molecules vs polymers
Small (single) molecule:
• Definite structure and molecular weight
• Properties usually measured in solution so that no orbital interaction occurs
S
S
N
S
N
N
S
S
N
S
Fe
N
Polymer:
• Repeating units
• Can have a variable number of monomers in the structure
• Reaction conditions can be used to create smaller/larger polymers
• Structural characterization often made in respect to monomer data (more specifics during synthesis talks)
S
S
N
S
S
N
S
n
n
N
Polymer Optics
•
•
Polymers popular in electronics because of the significant
amount of conjugation
• Good charge transfer properties
• Often very narrow optical and electrochemical band
gaps
Conjugated polymers have specific band gaps like small
molecules
• What structural aspect determines HOMO/LUMO
energies and band gaps of polymers?
• What is required for something to be conjugated?
• Overlapping p orbitals from planar structures
•
•
•
•
Torsional angles caused by steric interactions between aromatic
rings decrease conjugation
Torsional angle: angle between the planes of the atoms in adjacent
rings
Eventually polymers will not behave as conjugated structures
Optical and electrochemical parameters based on longest flat
portion of the polymer
S
S
S
•
Can reduce torsional angle by using smaller rings (above) or fusing
rings together (below)
S
S
Energy
LUMO
S
HOMO
Increasing Conjugation
~36o
~5o
Bulk Materials
• Condensed aggregates of small molecules
• Interactions between molecules cause slight shifts in properties compared to small molecules
• Aggregate modes vary based on sterics and electrostatic interactions
• Very difficult to predict
• Crystal structure confirmed through X-Ray crystallography
S
S
y
S
S
S
S
S
S
S
S
S
S
S
S
x
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
z
S
S
S
Small molecule
orbitals
S
S
S
S
S
S
S
S
S
S
n
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
Polymer orbitals
General Aggregation Modes
H Aggregates: π stacking across the flat edges of conjugated compounds
Shorter conjugated path
• Multiple molecules
behave as a molecule
with less conjugation
LUMO
Energy
y
HOMO
z
monomer
aggregate
J Aggregates: head-to-tail π stacking
Longer conjugated path
• Multiple molecule behave as one long
conjugated molecule
Energy
LUMO
HOMO
monomer
aggregate
Band Theory
• Molecular orbitals mix together in the condensed phase
• Properties of bulk materials differ greatly from properties of individual building blocks
• holes (h+): a positively charged space created when electrons move
•
Several orbitals mix together; orbitals eventually lose the properties of individual orbitals and behave as a group called a “band”
Energy
LUMO
Conduction band (CB): band made of mostly unoccupied orbitals
• 99% chance of not finding electrons in any given orbital
Fermi level: represents the equilibrium point between CB and VB
• 50% chance of finding an electron
HOMO
Valence band (VB): band made of mostly occupied orbitals
• 99% chance of finding electrons in any given orbital
Semiconductor Band Gaps
• Electronic applications need things to pass charge only sometimes
• Need to be able to turn them off and on
CB
Energy
CB
~1 – 3.5 eV
~4 eV
CB
~0.1 eV
VB
VB
VB
Insulator
Semiconductor
• Insulators will never pass charge regardless of applied electrical potential
• Semiconductors pass charge only under certain circumstances (next slide)
• Conductors always pass charge in an electrical circuit
Conductor
Semiconducting Types
•
•
•
•
n-type materials: accept and move electrons; typically low energy CB
p-type materials: donate electrons; creates and moves holes; typically high energy VB
Definitions of n-type and p-type vary by field/application. We will use the definitions above
Charge transfer always occurs from high energy electrons to low energy unoccupied orbitals/bands
• n-type charge transfer
hn
CB
CB
SUMO
LUMO
Donor HOMO
CB
SOMO
HOMO
VB
VB
VB
• p-type charge transfer
CB
CB
CB
SUMO
LUMO
hn
VB
VB
Acceptor
LUMO
HOMO
VB
SOMO
p-n Junctions
•
•
•
•
Ubiquitous in electronic devices
Interface between and electron-rich material and an electron-poor material
Anywhere an electron is passed from one material to another
Exciton: quasiparticle created from an electron/hole pair
e-
exciton
p-type
n-type
p-type
n-type
p-type
n-type
1) p-type material injects electron into n-type material; a hole is created in p-type material
2) Electrons travel through n-type material; holes travel through p-type materials
3) Charges eventually separate and travel through a circuit
Methods of electron transfer
e-
O
Fe
e-
n-type
semiconductor
• Coherent electron transfer
• Also called ”superexchange”
O
D = Donor
A = Acceptor
D–x–x–x–A
• Electrons travel through space
• Typically found in inorganic-organic interfaces
• Very distance dependent
Rate
K = K0•e-βr
• Incoherent electron transfer
• Also called “hopping”
D–x–x–x–A
• Electrons travel through π systems
• Typically found in organic-organic interfaces
O
O
O
O
N
O
K0: Rate constant for system
e: Euler’s number = 2.71828
β: Transfer coefficient
r: distance
O
hn
N
O
O
Rate
K = K0/r
Marcus Theory of Electron Transfer
• Describes relationship between donors and acceptors and the rate of electron transfer
• Electron transfer kinetics based on energetic difference of donor and acceptor
Reaction Coordinate Diagram
R = Reactants
P = Products
DG‡
= activation energy barrier
lEn = reorganization energy; energy
required to shift molecular
geometry from reactant geometry
to product geometry
Energy
Energy
DG0 = Gibbs energy of reaction
R
R
P
P
Reaction Progress
Reaction Progress
• Smaller DG‡ creates faster electron
transfers
• Electron transfer is fastest when
DG0 = – lEn
• When DG0 = – lEn, DG‡ = 0
• If DG0 < – lEn, transfer rate decreases
Marcus Theory Regions
•
One of the most important discoveries of Marcus’ work is the existence of an ”inverted” region
•
Intuitively, a larger DG0 should result in faster electron transfer
Donor
HOMO
Acceptor
LUMO
System 1 has a small DG0
Donor
HOMO
Acceptor
LUMO
System 2 has a larger DG0
•
•
•
•
Electron transfer rate
Energy
DG0 = -lEn
0V
~1.0 V
Prior to Marcus Theory, it was though System 2 should always result in a faster electron transfer no matter what the value of DG0 is
In the “normal Marcus region”, electron transfer rate increases as DG0 increases
In the “inverse Marcus region”, electron transfer rate decreases as DG0 increases
Molecular orbitals must be close enough in energy to enable electron transfer
DG0
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