Electronic Structure of p-Conjugated Organic Materials

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• Electronic Structure of p-Conjugated Organic
  Materials
• Jean-Luc Brédas
• The University of Arizona
• Georgia Institute of Technology

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1976 polyacetylene (CH)x is
discovered to become highly electrically
conducting following incorporation of electron
donating or accepting molecules redox
reaction sRT 103 S/cm
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(semi)conducting polymers and oligomers
combine in a single material
electrical properties of METALS or SEMICONDUCTORS
mechanical properties of PLASTICS

• lightness
• processability
• tailored synthesis
• flexibility

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2000 Nobel Prize in Chemistry
For the Discovery Development of Conductive
Polymers
Hideki Shirakawa University of Tsukuba
Alan Heeger University of California at Santa
Barbara
Alan MacDiarmid University of Pennsylvania
5
these discoveries, based on organic p-conjugated
materials, have opened the way to

• plastic electronics and
  opto-electronics
• plastic photonics

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basic physico-chemical concepts
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?-conjugated organic compounds

• frontier levels ?-type, delocalized, molecular
  orbitals
• basis for their rich physics
• electron-electron interactions
  electron-lattice coupling
• electron correlation
  strong connection between
• 
  electronic structure
• and
  geometric structure
• ordering of the
• low-lying excited
• states
  charge injection/excitation
• 
  geometry
  modifications

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octatetraene
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electron-electron interactions
electron correlation in polyenes makes 2Ag lt
1Bu
? absence of luminescence
octatetraene
as a result, polyenes and polyacetylene do not
luminesce (this is not the case in polyarylene
vinylenes)
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electron-lattice coupling
(1) look at the ? backbone
(2) add the ? electrons
uneven distribution of ?-electron density over
the bonds
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HOMO
the bonding antibonding pattern is a reflection
of the ground-state geometry
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LUMO
the bonding antibonding pattern is reversed
with respect to the HOMO
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working principle of a conjugated
polymer-based light-emitting diode
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polymer-based light-emitting diodes
R.H. Friend et al., Nature 347, 539 (1990) 397,
121 (1999)
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electroluminescence
electric field
injection
1
-
-
-
2
migration
2
lumo
1
cathode
charge transport
3
recombination
3
hn
4
exciton formation
1
homo
exciton decay
4
anode



PPV
R.H. Friend et al., Nature 397, 121 (1999)
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nature of the lowest excited state
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absorption and emission in oligomers
manifestation of strong vibronic coupling
Cornil et al., Chem. Phys. Lett. 247, 425 (1995)
278, 139 (1997)
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INDO/SCI simulations
emission
absorption
Cornil et al., Chem. Phys. Lett. 247, 425 (1995)
278, 139 (1997)
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absorption vs. photoconductivity in PPV
Kohler et al., Nature 392, 903 (1998)
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INDO/SCI simulation
Kohler et al., Nature 392, 903 (1998)
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band I S1 state
Kohler et al., Nature 392, 903 (1998)
S1 is an exciton state
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band II
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band III
excited state with charge-transfer character
correlation with photoconductivity
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band IV
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band V
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impact of interchain interactions
have often been observed to be detrimental to
luminescence
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isolated molecule
x

• so ? s1
• s1 ? s0

E
s1
polarized mainly along x
M ? Mx
s0
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dimer
Z
X
S0 ? S1

• if, in the S1 state, the e- and the h were to
  evolve on separate chains the S1 ? S0 intensity
  would go down since the transition is polarized
  along x
• the probability of finding h and e- on separate
  chains in S1 can be obtained from the wavefunction

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stilbene dimer highly symmetric cofacial
configurations
R
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no significant wavefunction overlap between the
units? excitation is always localized on a
SINGLE UNIT? luminescence is not affected?
situation in dilute solution or inert matrices

• ? R is large ? 8 Å or higher

?
S0
S1
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R goes below 8 Å
the wavefunctions of the frontier orbitals (HL)
start delocalizing over the two units they are
equally spread for R ? 5 Å
?
S0
S1 / S2
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H - 1 ? L
E
S2
H ? L 1
bg
L 1
bu
L
S1
H ? L
H - 1 ? L 1
3.88 eV
au
H
4.24 eV
ag
H - 1
R 4 Å
S0

• band-like formation for lowest excited state
• bottom of band is OPTICALLY FORBIDDEN
• from the ground state

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wavefunction analysis
4 Å
?
S1
INDO/SCI
S1 intrachain exciton state
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charge-transfer excited state
located a few tenths of an eV above S1
CT state can be the lowest in energy when two
chains of a different chemical nature are in
interaction
J.J.M. Halls et al., Phys. Rev. B 60, 5721 (1999)
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lower symmetry configurations? lateral
translations I / II have no effect
I
? x
z
y
x

• III II

Y
Z
Y
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? strong effect when relative orientations of
chain axes (not molecular planes) are
different, as in III
e.g., spiro-type compounds
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H-type versus J-type aggregates
S3
S2
S2
S1
S1
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how to avoid solid-state luminescence quenching

• separate the chains by means of bulky
  substituents or through encapsulation
  (channels, dendritic boxes,)

• use highly delocalized conjugated chains

• promote a finite angle between the long chain axes

• reach a brickwall-like architecture with
  molecular materials

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transport in semiconducting p-conjugated
oligomers
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transport processes
band-like hopping
extended, coherent incoherent
motion electronic states
of localized charge carriers

(polarons)
typical residence time on a site
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charge-transport processes in the bulk
correspond to electron-transfer reactions
t electronic coupling l reorganization energy
Marcus-Jortner electron-transfer theory
JACS 123, 1250 (2001) - Adv. Mat. 13, 1053
(2001) 14, 726 (2002) Proc. Nat. Acad. Sci. USA
99, 5804 (2002)
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cofacial crystals
INDO calculations

• influence of intermolecular distance
• influence of chain length
• influence of lateral displacements

PNAS 99, 5804 (2002)
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influence of intermolecular distance
HOMO
splitting (eV)
LUMO
distance (?)
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influence of chain length
d3.5 Å
HOMO
splitting (eV)
LUMO
number of thiophene units
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chain-length evolution
ethylene
interchain transfer integral
INDO
4 Å
E
L1
L
HOMO
LUMO
H
H-1
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influence of lateral displacements along long axis
d4.0 Å
HOMO
splitting (eV)
LUMO
displacement along long axis (A)
PNAS 99, 5804 (2002)
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benzene
napthalene
anthracene
tetracene
pentacene
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herringbone packing
from benzene to pentacene
c
b
85.2º
7.44 Å
49.7º
7.71 Å
a
a
d2
d1
d2
d1
6.92 Å
6.28 Å
b
c
 benzene G. E. Bacon et al. Proc. R. Soc. London
Ser. A. 1964, 279, 98 naphthalene   V. I.
Ponomarev et al. Kristallografiya, 1976, 21, 392
anthracene  C. Pratt Brock et al. Acta
Crystallogr., Sect. B (Str. Sci), 1990, 46, 795
tetracene and pentacene D. Holmes et al. Chem.
Eur. J. 1999, 5, 3399.
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pentacene
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pentacene
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total bandwidths in oligoacenes
from 3D band-structure calculations Y.C. Cheng and R. Silbey (MIT) (eV) from 3D band-structure calculations Y.C. Cheng and R. Silbey (MIT) (eV) from 3D band-structure calculations Y.C. Cheng and R. Silbey (MIT) (eV)
HOMO LUMO
naphthalene .429 .370
anthracene .535 .489
tetracene .666 .604
pentacene .722 .697
Y.C. Cheng et al., J. Chem. Phys.
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reorganization energy l
cost in geometry modifications to go from a
neutral to a charged oligomer and vice versa
the lower the reorganization energy terms l, the
higher the electron transfer rate
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? pentacene
? functionalized pentacenes
Anthony et al., JACS 123, 9482 (2001)
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UPS gas-phase spectrum of pentacene N.E.
Gruhn et al. JACS 124, 7918 (2002)
experimental spectrum
INDO simulation
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JACS 124, 7918 (2002)
deconvolution of the first ionization energy
peak experimental estimate for l 0.118
eV calculated value (DFT B3LYP) 0.098 eV
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calculated (DFT B3LYP) reorganization
energies pentacene 0.098
eV functionalized pentacenes 0.143-0.145
eV TPD 0.290 eV pentacene provides for a
rigid macrocyclic backbone and highly delocalized
frontier MOs
HOMO