Fundamentals of Organic Molecules
and Semiconductors
Molecule
2
Periodic Table of the Elements
3
Carbon
•
•
•
•
Carbon is found in every living creature.
Elemental carbon can be black (graphite), or hard and beautiful (diamonds).
Building block of fossil fuels (gasoline and oil)
Basis of organic chemistry
Crystalline forms
Diamond (metastable)
Graphite
Fullerine
Fullerene (C60)
Æ
Æ
Æ
A wide-gap semiconductor
Parallel hexagonal sheets
Semiconducting
Graphite
Diamond -Covalent
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Bonding
•
Covalent (C, Si)
– Electron sharing – 2 or more electrons shared by 2 or more atoms
•
Ionic (GaAs)
– Electron transfer
•
Van der Waals (Bonding between covalent molecules)
– London forces (attraction between molecules with no permanent dipole moment)
– Dipolar attraction (polar molecules)
– Hydrogen bonding (covalently bound hydrogen + negative dipole)
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Covalent Bonding
C-O bond 340 kJ/mol 1.43 Å
C-C bond 360 kJ/mol 1.54 Å
C-H bond 430 kJ/mol 1.11 Å
C=C bond 600 kJ/mol 1.33 Å
C=O bond 690 kJ/mol 1.21 Å
•
Sharing of electrons to achieve stable electron configuration
– Small difference in electronegativity of elements
– Bond energy : 200-400 kJ/mol
– Directional bond; between specific atoms in a specific direction, normally along the
line connecting the two atoms that share a pair of electrons.
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Valence Bond Theory and Hybridization
1) bond formation by overlapping orbitals:
A description of covalent bond formation in terms of atomic orbital overlap is called the valence
bond theory. It gives a localized electron model of bonding: core electrons and lone-pair
valence electrons retain the same orbital locations as in the separated atoms, and the charge
density of the bonding electrons is concentrated in the region of orbital overlap.
2) hybridization of atomic orbitals:
How do a carbon with a s-orbital and three p-orbitals combined with four hydrogen
(s orbitals) form four bonds and all four bonds are found to be 109.5°
: In 1931, Linus Pauling proposed that the wave functions for the s and p atomic orbitals can
be mathematically combined to form a new set of equivalent wave functions called hybrid
orbitals.
: The mathematical process of replacing pure atomic orbitals with reformulated atomic
orbitals for bonded atoms is called hybridization.
In a hybridization scheme, the number of hybrid orbitals equals to the total number of atomic
orbitals that are combined. The symbols identify the numbers and kinds orbitals involved.
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Atomic Orbitals
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sp3 Hybridization
sp3 signifies one s and three p orbitals are combined.
Mixing one s orbital with three p orbitals yields four equivalent sp3 hybrid orbitals.
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sp3 Hybridization
The bonding in methane
Each of the four C-H bonds results from head-on (s) overlap of a singly occupied carbon
sp3 hybrid orbital with a singly occupied hydrogen 1s orbital. Sigma bonds are formed by
head-to-head overlap between the hydrogen s orbital and a singly occupied sp3 hybrid
orbital of carbon.
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sp3 Hybridization
The bonding in Ammonia
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sp2 Hybridization
The molecular geometry is trigonal planar with bond angle = 120°. To explain its geometry,
we can use the following rational. sp2 signifies one s and two p orbitals are combined.
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sp Hybridization
Now consider BeCl2 which has linear molecular geometry determined experimentally.
In hybridization scheme that best describes this compound is that
The combination of one s and one p orbital gives two sp hybrid orbitals oriented 180° apart.
Two unhybridized p orbitals remain and are oriented at 90° angles to the sp hybrids.
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sp3d Hybridization
To described hybridization scheme to correspond to the 5- and 6- electron-group geometries
of VSEPR theory, we need to go beyond s and p orbitals and traditionally this meant
including d orbitals.
We can achieve the five half-filled orbitals and trigonal-bipyramidal molecular geometry
through the hybridization of one s, three p and one d orbitals of valence shell into five sp3d
hybrid orbitals.
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sp3d2 Hybridization
In the same way, we can achieve the six half-filled orbitals and octahedral geometry
through the hybridization of one s, three p and two d orbitals of valence shell into six sp3d2
hybrid orbitals.
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Hybrid Orbitals
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Hybrid Orbitals and Multiple Covalent Bonds
•
Sigma (ρ) bonds are characterized by
– Head-to-head overlap.
– Cylindrical symmetry of electron density about the internuclear axis.
•
Pi (π) bonds are characterized by
– Side-to-side overlap.
– Electron density above and below the
internuclear axis.
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Molecular Orbitals
Molecular orbitals (MOs) are mathematical equations that describe the regions in a
molecule where there is a high probability of finding electrons
Molecular orbitals (MOs) are essentially combinations of atomic orbitals – two types
exist, bonding and antibonding orbitals
Molecular orbitals (MOs) are built up in the same way as atomic orbitals
The hydrogen molecule
Antibonding MO = region of
diminished electron density
Bonding MO = enhanced
region of electron density
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Molecular Orbitals
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Molecular Orbitals for the 2p Electrons
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Carbon-Carbon Single Bonds
The bonding in Ethane
Single bonds are always σ bonds, because σ overlap is greater,
resulting in a stronger bond and more energy lowering.
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Carbon-Carbon Double Bonds
A double bond is made up of one
sigma bond and one pi bond
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Carbon-Carbon Double Bonds
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Carbon-Carbon Double Bonds
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Bonding in Aliphatic hydrocarbons
25
Carbon-Carbon Triple Bonds
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Conjugated System
A chemically conjugated system is a system of atoms covalently bonded with alternating
single and multiple (e.g. double) bonds (e.g., C=C-C=C-C) in a molecule of an organic
compound.
This system results in a general delocalization of the electrons across all of the adjacent
parallel aligned p-orbitals of the atoms, which increases stability and thereby lowers
the overall energy of the molecule.
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Conjugated System
Adjacent, overlapping p orbitals allows for...
•...more resonance
•...more electron delocalization
•...lower electron energy
•...greater stability
Compound
Consequences of p orbital overlap
• Atoms with p orbitals must be planar
• Partial pi bond(s)
• Barrier to rotation
max
165
15,000
217
21,000
256
50,000
290
85,000
334
125,000
364
138,000
ethene
1,3-butadiene
1,3,5-hexatriene
1,3,5,7-octatetraene
1,3,5,7,9-decapentaene
1,3,5,7,9,11-dodecahexaene
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Benzene and Oligo-acene
This kind of structure gives rise to two important
resonance hybrids and leads to the idea that all three
double bonds are delocalized across all six carbon atoms
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Benzene
30
Benzene : Electronic Structure
31
Energy Level and Band Gap
32
Band Gap for Organic Materials
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Some Semiconductors
α−quartertiophene
α-sexithiophene
α -6T
α -4T
S
S
S
S
Tetracene
S
S
S
S
S
S
Pentacene
π-conjugated Molecules (oligomers)
Transport ⇒ π−π* overlap
C60
34