8. Optical processes in conjugated materials Full color display- Active matrix x 150 Pixels - 2 inch diagonal Cambridge Display Technology

8.1. Electron-Phonon Coupling E Q Absorption Relaxation effects Emission Ground state Lowest excitation state Excitations

Fluorescence Polyfluorene (F8) Carlos Silva, University of Cambridge - Weak self-absorption - Vibronic structure

Intrachain Exciton Site number Lowest excited state INDO/SCI Exciton size Binding energy  0.3 eV Exciton=electron-hole pair Probability to find the e - and h + at one site

Optical transition versus chain size The “conjugation length” is the length of the oligomer emitting the same luminescence spectrum as the polymer. While the polymer may easily be times longer than a conjugation length, the chain is effectively operating as a sequence of conjugation lengths along a common string. This description is valid for the behaviour of absorption processes; where emission is relevant, the excited state is often more localized. Cornil, J. et al. Chem. Phys. Lett. (1997), 278, 139 1/m (m=number of bonds) absorption emission absorption emission 8.2. Conjugation length

Switch between different structures by applying mechanical force while monitoring the Langmuir monolayer's optical spectra. The figure shows the chemical structures, conformations and spatial arrangements at the air–water interface of the polymer Conformation changes Compression causes a transition from the face-on to the zipper structure, which breaks the conjugation, i.e. decreases the π -conjugation length and generates a large blue shift (34-nm). Kim et al. Nature 411, (2001)

SEKUNDÄR EFFEKT Vridning av ring minskar p z -p z överlapp w < W ENERGI k R R PRIMÄR EFFEKT: Tillför (tar bort) laddning ENERGI k R R Bandgap och Dispersion via Sidogrupper Bandgap och Dispersion via Sidogrupper R R PRIMÄR EFFEKT SEKUNDÄR EFFEKT Gult område: p z densitet

8.3. Influence of Electroactive Substituents Al Need for small energy barriers to optimize hole/electron injection eV eV E eV eV Molecular engineering to modulate the energy of the band edges  n nn

Donor: σ- donor (electronegativity): symmetric destabilization π- donor: asymmetric destabilization Acceptor: σ- acceptor : symmetric stabilization π -acceptor : asymmetric stabilization Note that ”-O-CH 3 ” acts as a globally as a donor. This is the results of a competition between its π- donor and σ- acceptor characters.

8.4. Modulation of the Optical Properties RedYellow-GreenBlue - Molecular backbone - Chain size K. Müllen and co Structure of the conjugated chain

8.4.2 Optical properties and Doping E H L PolaronBipolaron

Electrochromism Neutral Singly charged Doubly charged

Electrochromism in a substituted polythiophene, under electrochemical doping in contact with an electrolyte. The suppression of bandgap absorption in the polymer (with a maximum at 500 nm) due to doping is highly visible; formation of polarons is hardly visible, but the two optical transitions due to bipolarons are found, one peaking at 800 nm and another below 1200 nm. From Peter Åsberg, work in progress, Biorgel, IFM, LiU

Electrochromic Displays on Papers Prof. M. Berggren, Norrköping

Transition Dipole Moment Atomic transition densities 1A g  1B u N = 20 INDO/SCI + -  q K r K =  K 1A g  1B u  q K = 0 K 8.5. Solid State Effect: Exciton Splitting

1 2 E GG EE  GG Cofacial dimer 2  E1  E  tot = 0  tot =  2  H-Aggregate

J-Aggregate E GG EE  GG 2  E1  E  tot = 0  tot =  2 

8.6. Charge and energy transfer in conjugated polymers Organic Solar Cells GlassITO e h Energy transfer LUMO HOMO LUMO Charge transfer

8.6.1.Photoinduced Charge Transfer LUMO HOMO LUMO E Photoinduced ELECTRON transfer LUMO HOMO LUMO E Photoinduced HOLE transfer

Chemical Sensors H L TNTPolymer TNT Land-mine detector (Detection limit : g) Photoinduced Electron Transfer Tim Swager and co, MIT

Polymer / Polymer Interfaces DMOS-PPVMEH-PPVCN-PPV 0.55 eV 0.17 eV H L 0.63 eV 0.44 eV H L

MEH-PPV / CN-PPV Blend Charge transfer Charge transfer J.J.M. Halls, J. Cornil, et al., Phys. Rev. B 60, 5721 (1999) 0.63 eV 0.44 eV H L MEH-PPV CN-PPV

DMOS-PPV / CN-PPV Blend Energy transfer Energy transfer DMOS-PPV CN-PPV 0.55 eV 0.17 eV H L

Charge versus Energy Transfer Penalty to pay to dissociate an exciton on the order of 0.35 eV 0.63 eV 0.44 eV H L MEH-PPVCN-PPV INTRA INTER One-electron levels Excited states GROUND STATE INTRA CN-PPV INTER INTRA MEH-PPV 0.28 eV 0.19 eV Charge transfer at the polymer/polymer interface MEH-PPV / CN-PPV Blend : Charge transfer

Penalty to pay to dissociate an exciton on the order of 0.35 eV 0.55 eV 0.17 eV H L DMOS-PPVCN-PPV INTRA INTER One-electron levels Excited states GROUND STATE INTRA CN-PPV INTER INTRA DMOS-PPV 0.38 eV 0.20 eV Energy transfer towards the CN-PPV chains DMOS-PPV / CN-PPV Blend : Energy transfer