LAboratoire de Spectrochimie Infrarouge et Raman – UMR 8516
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Séminaire du Pr. Hiroshi Miyasaka

Osaka University, Graduate School of Engineering Science and Center for Quantum Science and Technology under Extreme Conditions, Division of Frontier Materials Science, Japan
Femtosecond dynamics of electron transfer processes in photoconductive poly(N-vinylcarbazole) amorphous solid

Poly(N-vinylcarbazole) (PVCz) is one of the most well-known organic photoconductive materials. Although much more efficient materials have been developed in recent years, PVCz and its derivatives are still regarded as representative references and widely utilized as building units in advanced functional systems. For the quantitative evaluation of the carrier photo-generation yield in PVCz and other organic photoconductors, the conventional scheme based on the Onsager model has been applied. The key process is the rapid increase in the interionic distance during thermalization after photoinduced charge separation (CS). Typical interionic distance (r0) after thermalization was estimated to be ca. 20-30 Å for PVCz and related organic systems.

On the other hand, direct detection of the photo-primary processes in PVCz by picosecond transient dichroism measurements revealed that the hole escapes from the initial ion pair with a rate constant of (1 ns)-1, in competition with geminate charge recombination. This phenomenon is followed by the sequential hole hopping process. Thermalization (the cooling process of the vibrational excess energy) occurs in ca. 10 ps in the condensed phase. If the interionic distance increased up to ca. 20 Å ( >4-5 carbazolyl (Cz) units) in this short time, no dichroism signal of Cz+ could remain in the ns regime. In addition, the increase of interionic distance from the initial CS state to 20-30 Å is an endothermic process of 0.3-0.5 eV owing to the Coulombic attraction in the nonpolar environment. The rapid (ca. 10 ps) increase of interionic distance via charge shift reactions with such a large energy gap is quite difficult to rationally explain from the viewpoint of photoinduced electron transfer processes.

It is however worth recalling that the conventional model has quantitatively well explained the experimental results on the electric field dependence of carrier generation yields in many organic photoconductors. Accordingly, clear elucidation of the two conflicting models described above is very important, not only for the comprehensive understanding of the carrier photo-generation phenomenon but also for rationally designing advanced organic photoconductive materials and other systems related to electron transport. Along this line, we have investigated the photoprimary processes in PVCz solid films doped with an electron acceptor by means of femtosecond time-resolved dichroism measurement in the visible region and of transient absorption spectroscopy in the near-infrared (NIR) region. From these measurements, it was observed that the cationic state of the Cz group undergoes very rapid delocalization in sub-ps to several ps time region as shown in Scheme below, leading to the reduction of the Coulombic attraction between the cation and the counter anion. The role of the delocalization in the subsequent hole migration reaction will be discussed in detail.

seminaire-Miyasaka

Le séminaire aura lieu le 28.11.2013 à 10h30
salle de réunion du LASIR, sous-sol bâtiment C5