Direct Detection of Hole Migration along the Polymer Chain: Poly(N-vinylcarbazole) in 1,2-Dichloroethane Solution As Revealed by Picosecond Transient Absorption and Dichroism Measurements

Abstract

Picosecond transient absorption spectroscopy and transient dichroism measurements were applied for the direct elucidation of photoinduced electron-transfer dynamics (charge separation, charge recombination, and hole-transfer reactions) in poly(N-vinylcarbazole) (PVCz)−electron acceptors as well as the corresponding monomer model system N-ethylcarbazole−electron acceptors in 1,2-dichloroethane solution. The measurement of the dichroism of the ion-pair absorption in the monomer model system demonstrated that the time constants of the rotational relaxation of the anion and the cation were identical with each other, indicating that the mutual geometry of the ions in the pair was maintained at least during the rotational relaxation process. In the polymer systems, the decay of the dichroism signal of the cation was much faster than that of the anion, indicating that the rapid hole-migration process diminished the dichroism signal of the cation of the carbazolyl (Cz) groups in the PVCz chain. The electron-transfer dynamics in PVCz in 1,2-dichloroethane solution was well-described by the simple scheme that the cation continuously migrates along pendant Cz moieties in a PVCz chain with the charge recombination at the initial position of the charge separation. The time constant for the hole transfer from the cation state of Cz moiety (Cz+) to neighboring Cz's was obtained to be ca. 500 ps, although the activation energy for the hole-transfer reaction from Cz+ in the initial ion pair (which was produced by the excitation of the ground state charge transfer complex) to the neighboring Cz's was estimated to be ≫10kBT by the usual theories of electron transfer. By integrating the present results with those in other solutions and in other aromatic vinyl polymers, the factors regulating the rapid hole-transfer process in PVCz were discussed.