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SUMMARY:Spectral Diffusion and Stokes’ Shift in Conjugated Polymers: Fro
 m Hopping to Twist - Wichard J.D. Beenken\, Department of Theoretical Phys
 ics I\, Technische Universität Ilmenau\, Germany
DTSTART:20070320T143000Z
DTEND:20070320T153000Z
UID:TALK6112@talks.cam.ac.uk
CONTACT:Chris McNeill
DESCRIPTION:In conjugated polymers\, important organic substances for phot
 o-electronic devices\, spectral diffusion is usually described in the fram
 ework of excitation energy transfer (EET)\, i.e. Förster-type hopping tra
 nsfer\, among so-called spectroscopic units. These spectroscopic units con
 sist of several connected monomers of the polymer chain with common optica
 l transition frequency and dipole. The transfer rate between two spectrosc
 opic units can be calculated as dipole-dipole interaction between the resp
 ective transition (point-dipole approximation). We will present more sophi
 sticated concepts of the excitonic interaction e.g. the line-dipole approx
 imation\, which result in a qualitative and quantitative better agreement 
 with the energy transfer dynamics measured by e.g. anisotropy decay. Besid
 e the EET \, which dynamics of are on a ps-timescale\, we found that there
  may exist other mechanisms\, which result in the observed spectral diffus
 ion and Stokes’ shift as well. These are directly connected with the nuc
 lear relaxation of the polymer structure in the excited states. On timesca
 les just a little bit longer than those characteristic for EET \, torsions
  around the single-bonds connecting the monomeric units of the polymer bec
 ome crucial. On the other hand stretching modes may be able to explain sub
 -ps kinetics\, which have been found in anisotropy decay measurements. We 
 will illustrate these effects by means of quantum-chemical calculations (Z
 INDO-S and TD-DFT) we have performed on oligo-thiophenes.\n\n
LOCATION:Small Lecture Theatre\, Cavendish Laboratory\, Department of Phys
 ics
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