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

2. 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

3. (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

4. 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

5. these discoveries, based on organic  -conjugated materials, have opened the way to:  plastic electronics and opto-electronics  plastic photonics

6. basic physico-chemical concepts

7.  -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

8. octatetraene

9. 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

10. electron-lattice coupling (1)look at the  backbone: (2) add the  electrons: uneven distribution of  -electron density over the bonds

11. the bonding – antibonding pattern is a reflection of the ground-state geometry HOMO

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

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

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

15. PPV electric field cathode anode 1 1 - - + + 2 2 3 3 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

16. nature of the lowest excited state

17. absorption and emission in oligomers Cornil et al., Chem. Phys. Lett. 247, 425 (1995); 278, 139 (1997) manifestation of strong vibronic coupling

18. INDO/SCI simulations emissionabsorption Cornil et al., Chem. Phys. Lett. 247, 425 (1995); 278, 139 (1997)

19. Kohler et al., Nature 392, 903 (1998) absorption vs. photoconductivity in PPV

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

21. band I: S 1 state Kohler et al., Nature 392, 903 (1998) S 1 is an exciton state

22. band II

23. band III excited state with charge-transfer character: correlation with photoconductivity

24. band IV

25. band V

26. impact of interchain interactions have often been observed to be detrimental to luminescence

27. isolated molecule s o  s 1 s 1  s 0 x polarized mainly along x E s1s1 s0s0 M  M x

28. 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

29. stilbene dimer highly symmetric cofacial configurations R

30. 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

31.  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

32.  “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 + 1 3.88 eV 4.24 eV R = 4 Å S0S0

35. wavefunction analysis INDO/SCI 4 Å S1S1 S 1 = intrachain exciton state

36. 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

37. lower symmetry configurations  lateral translations I / II have no effect III II I  x z y x Y Y Z

38. Side view  strong effect when relative orientations of chain axes (not molecular planes) are different, as in III e.g., spiro-type compounds

39. H-type versus J-type aggregates S1S1 S2S2 S1S1 S2S2 S3S3

40. 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

41. transport in semiconducting  -conjugated oligomers

42. transport processes band-like hopping extended, coherent incoherent motion electronic states of localized charge carriers (polarons) typical residence time on a site:

43. 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)

44. cofacial crystals  influence of intermolecular distance  influence of chain length  influence of lateral displacements PNAS 99, 5804 (2002) INDO calculations

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

46. number of thiophene units splitting (eV) HOMO LUMO d=3.5 Å influence of chain length

47. chain-length evolution E INDO 4 Å interchain transfer integral HOMO LUMO H-1 H L L+1 ethylene

48. influence of lateral displacements along long axis d=4.0 Å splitting (eV) displacement along long axis (A) HOMO LUMO PNAS 99, 5804 (2002)

49. benzene napthalene anthracene tetracene pentacene

50. 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, 3399. c

51. pentacene b a d1 d2 51.7º 6.28 Å 7.71 Å

52. pentacene

53. total bandwidths in oligoacenes from 3D band-structure calculations Y.C. Cheng and R. Silbey (MIT) (eV) HOMOLUMO naphthalene.429.370 anthracene.535.489 tetracene.666.604 pentacene.722.697 Y.C. Cheng et al., J. Chem. Phys.

54. 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

55. Anthony et al., JACS 123, 9482 (2001) ► functionalized pentacenes ► pentacene

56. UPS gas-phase spectrum of pentacene N.E. Gruhn et al. JACS 124, 7918 (2002) INDO simulation experimental spectrum

57. deconvolution of the first ionization energy peak: experimental estimate for : 0.118 eV calculated value (DFT – B3LYP): 0.098 eV JACS 124, 7918 (2002)

58. 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 MO’s: HOMO