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

1976: polyacetylene (CH) x highly electrically conducting is discovered to become highly electrically conducting following incorporation of electron donating or accepting molecules redox reaction  RT ~ 10 3 S/cm

(semi)conducting polymers and oligomers combine in a single material electrical properties METALS of METALS orSEMICONDUCTORS mechanical properties PLASTICS of PLASTICS  lightness  processability  tailored synthesis  flexibility

2000 Nobel Prize in Chemistry “For the Discovery & Development of Conductive Polymers” Alan Heeger University of California at Santa Barbara Alan MacDiarmid University of Pennsylvania Hideki Shirakawa University 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 2A g < 1B u  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 a reflection of the ground-state geometry HOMO

LUMO the bonding – antibonding pattern is reversed with respect to the HOMO

working principle of a conjugated polymer-based light-emitting diode

R.H. Friend et al., Nature 347, 539 (1990); 397, 121 (1999) polymer-based light-emitting diodes

PPV electric field cathode anode injection migration recombination electroluminescence exciton formation R.H. Friend et al., Nature 397, 121 (1999) 4 4 h exciton decay charge transport lumo homo

nature of the lowest excited state

absorption and emission in 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

INDO/SCI simulation Kohler et al., Nature 392, 903 (1998)

band I: S 1 state Kohler et al., Nature 392, 903 (1998) S 1 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 s o  s 1 s 1  s 0 x polarized mainly along x E s1s1 s0s0 M  M x

dimer if, in the S 1 state, the e - and the h + were to evolve on separate chains: the S 1  S 0 intensity would go down since the transition is polarized along x the probability of finding h + and e - on separate chains in S 1 can be obtained from the wavefunction Z X S 0  S 1

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  S0S0 S1S1

 R goes below 8 Å  S0S0 S 1 / S 2 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 bgbg bubu L + 1 L auau agag H H - 1 S2S2 S1S1 H - 1  L H  L + 1 H  L H - 1  L eV 4.24 eV R = 4 Å S0S0

wavefunction analysis INDO/SCI 4 Å S1S1 S 1 = 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 S 1

lower symmetry configurations  lateral translations I / II have no effect III II I  x z 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 S1S1 S2S2 S1S1 S2S2 S3S3

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:

charge-transport processes in the bulk: charge-transport processes in the bulk: correspond to electron-transfer reactions correspond to electron-transfer reactions Marcus-Jortner electron-transfer theory 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 PNAS 99, 5804 (2002) INDO calculations

influence of intermolecular distance d HOMO LUMO distance (Ǻ) splitting (eV)

number of thiophene units splitting (eV) 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

influence of lateral displacements along long axis d=4.0 Å splitting (eV) 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 d1 d2 85.2º 6.92 Å 7.44 Å a b d1 d2 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, c

pentacene b a d1 d2 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) HOMOLUMO naphthalene anthracene tetracene pentacene 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

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) INDO simulation experimental spectrum

deconvolution of the first ionization energy peak: experimental estimate for : eV calculated value (DFT – B3LYP): eV JACS 124, 7918 (2002)

calculated (DFT – B3LYP) reorganization energies: pentacene: eV functionalized pentacenes: eV TPD:0.290 eV pentacene provides for a rigid macrocyclic backbone and highly delocalized frontier MO’s: HOMO