Electronic Structure of -Conjugated Organic Materials Jean-Luc Brédas The University of Arizona...
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Transcript of Electronic Structure of -Conjugated Organic Materials Jean-Luc Brédas The University of Arizona...
Electronic Structure of -Conjugated Organic Materials
Jean-Luc BrédasJean-Luc Brédas
The The UUniversity of niversity of AArizonarizonaGeorgia Georgia Institute ofInstitute of Tech Technologynology
1976: polyacetylene (CH)x
is discovered to become highly electrically highly electrically conductingconducting following incorporation of electron
donating or accepting molecules
redox reaction
RT ~ 103 S/cm
C
H
C
H
C
H
C
H
C
H
C
H
C
H
(semi)conducting polymers and oligomers
combine in a single material
electrical propertiesof METALSMETALS or
SEMICONDUCTORSSEMICONDUCTORS
mechanical properties
of PLASTICSPLASTICS
lightness processability tailored synthesis flexibility
2000 Nobel Prize in Chemistry2000 Nobel Prize in Chemistry
“For the Discovery & Development of Conductive Polymers”
Alan HeegerUniversity of California
at Santa Barbara
Alan MacDiarmidUniversity ofPennsylvania
Hideki ShirakawaUniversity of Tsukuba
these discoveries, based on organic -conjugated materials, have
opened the way to:
plastic electronics and opto-electronics
plastic photonics
basic physico-chemical concepts
-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
change in electronic structure
octatetraene
electron-electron interactions
electron correlation in polyenes makes 2Ag < 1Bu
absence of luminescence
as a result, polyenes and polyacetylene do not luminesce (this is not the case in polyarylene vinylenes)
octatetraene
electron-lattice coupling
(1) look at the backbone:
(2) add the electrons:
uneven distribution of -electron density over the bonds
the bonding – antibonding pattern is areflection of the ground-state geometry
HOMO
LUMO
the bonding – antibonding pattern is reversed with respect to the HOMO
working principle of a conjugated polymer-basedlight-emitting diode
R.H. Friend et al., Nature 347, 539 (1990); 397, 121 (1999)
polymer-based light-emitting diodes
n
PPV
electric field
cathode
anode
1
1
- -
+ +
2
2
33
injection
migration
recombination
electroluminescence
exciton formation
R.H. Friend et al., Nature 397, 121 (1999)
4
4
h
exciton decay
-
+
1
charge transport
lumo
homo
nature of the lowest excited state
n
absorption and emissionin oligomers
Cornil et al., Chem. Phys. Lett.247, 425 (1995); 278, 139 (1997)
manifestation of strong vibronic coupling
INDO/SCI simulations
emissionabsorption
Cornil et al., Chem. Phys. Lett.247, 425 (1995); 278, 139 (1997)
Kohler et al.,
Nature 392, 903 (1998)
absorption vs. photoconductivity in PPV
n
INDO/SCI simulation
Kohler et al.,
Nature 392, 903 (1998)
band I: S1 state
Kohler et al.,Nature 392, 903 (1998) S1 is an exciton state
band II
band III
excited state with charge-transfer character: correlation with photoconductivity
band IV
band V
impact of interchain interactions
have often been observed to be detrimental to luminescence
isolated molecule
so s1
s1 s0
x
polarized mainly along x
Es1
s0
M Mx
dimer
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
Z
X S0 S1
stilbene dimer
highly symmetric cofacial configurations
R
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
R goes below 8 Å
S0 S1 / S2
the wavefunctions of the frontier orbitals (H;L) start delocalizing over the two units
they are equally spread for R 5 Å
“band”-like formation for lowest excited state bottom of band is OPTICALLY FORBIDDEN
from the ground state
E bg
bu
L + 1
L
au
ag
HH - 1
S2
S1
H - 1 LH L + 1
H LH - 1 L + 1
3.88 eV4.24 eV
R = 4 ÅS0
+
* higher energy
* LARGE oscillatorstrength
+* lower energy
* NO oscillator strength
Kasha’s model
S1
S0S0
S1
S2
isolatedchain
interactingchains
wavefunction analysis
INDO/SCI
1
2 34 5
6
7
8 910 11
12
13
14 1516 17
18
19
20 2122 23
24
25
26 27
28 4 Å
S1
S1 = intrachain exciton state
charge-transfer excited state
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)
located a few tenths of an eV above S1
lower symmetry configurations
lateral translations I / II have no effect
III II
I
xz y
x
Y
Y
Z
Side view
strong effect when relative orientations of chain axes (not molecular planes) are different, as in III
e.g., spiro-type compounds
H-type versus J-type aggregates
S1
S2
S1
S2
S3
• 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
how to avoid solid-state luminescence quenching
transport in semiconducting -conjugated oligomers
transport processes
band-like hopping
extended, coherent incoherent motion electronic states of localized charge carriers (polarons)
typical residence time on a site:
W s
W10
32
(eV)
15-
charge-transport processescharge-transport processes in the bulk: in the bulk: correspond tocorrespond to electron-transfer reactionselectron-transfer reactions
Marcus-Jortner electron-transfer theory
kT
GSS
kTt
hk
s
s
sET
4
)'(exp
!')exp(
4
14 20
'
'21
22
iS
t = electronic coupling = reorganization energy
JACS 123, 1250 (2001) - Adv. Mat. 13, 1053 (2001); 14, 726 (2002) Proc. Nat. Acad. Sci. USA 99, 5804 (2002)
cofacial crystals
influence of intermolecular distance influence of chain length influence of lateral displacements
S
S
S
S
S
S
PNAS 99, 5804 (2002)
INDO calculations
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.8 5
distance (Å)
Sp
litti
ng
(eV
)
HOMO
LUMO
influence of intermolecular distance
dHOMO
LUMO
distance (Ǻ)
split
tin
g (e
V)
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1 2 3 4 5 6 7 8
Number of thiophene unit
Sp
litti
ng
(eV
)
HOMO
LUMO
number of thiophene units
split
tin
g (e
V)
HOMO
LUMO
d=3.5 Å
influence of chain length
chain-length evolution
E
INDO
4 Å
interchain transfer integral
HOMO
LUMO
H-1
H
L
L+1
ethylene
C C
H
H
H
H
0.00
0.05
0.10
0.15
0.20
0.25
0 2 4 6 8 10 12 14 16 18 20
X shift (Å)
Sp
litt
ing
(eV
)
HOMO
LUMO
influence of lateral displacements along long axis
d=4.0 Å
split
tin
g (e
V)
displacement along long axis (A)
HOMO
LUMO
PNAS 99, 5804 (2002)
benzene
napthalene
anthracene
tetracene
pentacene
herringbone packing:
a
c
b
from benzene to pentacene
d1d2
85.2º
6.92 Å
7.44 Å
a
b
d1d2
49.7º
6.28 Å
7.71 Å
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.
c
pentacene
b
a
d1d2
51.7º
6.28 Å
7.71 Å
pentacene
total bandwidths in oligoacenes
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.
reorganization energy reorganization energy
the lower the reorganization energy terms , the higher the electron transfer rate
cost in geometry modifications to go from a neutral to a charged oligomer and vice versa
kT
GSS
kTt
hk
s
s
sET
4
)'(exp
!')exp(
4
14 20
'
'21
22
iS
Anthony et al., JACS 123, 9482 (2001)
► functionalized pentacenes
► pentacene
UPS gas-phase spectrum of pentacene N.E. Gruhn et al. JACS 124, 7918 (2002)
INDOsimulation
experimental spectrum
deconvolution of the first ionization energy peak: experimental estimate for : 0.118 eV
calculated value (DFT – B3LYP): 0.098 eV
JACS 124, 7918 (2002)
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 backboneand highly delocalized frontier MO’s:
HOMO