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