PHYS 352 |
| The 2,7-dicyano-9,9-spirobifluorene (DCSBF) is studied because its charge transfer properties. DCSBF is donor-acceptor system, in which the aromatic planes of the donor (fluorene) and acceptor (2,7-dicyanofluorene) are oriented perpendicularly to one another. Excitation transfer and charge transfer from localized donor excited state is forbidden in this system due to symmetry. Only hole-transfer is symmetry-allowed from acceptor excited state. Non-radiative charge recombination to the ground state is also symmetry-forbidden. Such molecular geometry results in vanishing Coulomb and exchange interactions, as well as the absence of exciton splitting between the excited chromophores. Therefore, pronounced slowing down of the electron and excitation transfer rates, as well as strong anisotropy of the transient response due to symmetry can be expected. Transient absorption spectra of bichromophoric DCSBF were investigated as a function of polarization between the pump and probe beams.. Upon excitation by UV-light (335 nm) instantaneous formation of local excited state of the acceptor was observed. Subsequently, charge-separation occurred with the time constants less than 300 fs. Reorientation of the excited molecule in solution occurs at a time scale slower that that of the charge separation process. The polarization-dependent measurements which indicates rotational diffusion constants in the range of 50 to 75 ps. In conclusion, despite unfavorable geometry the formation of the charge-separated state occurs rapidly in this system. Accordingly to recent investigation, exited state mixing is very sensitive to the position of higher vibronic levels. This interaction can lead to the electron coupling between the chromophores and diminish the expected symmetry effects in the dynamics of the electron transfer. As a consequence, in further studies of symmetry control of electron transfer in the orthogonal bichormophoric molecules it is essential to control energy offset between vibronic levels of high lying electronic states. |
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PHYS Poster Session - Optical Probes of Dynamics in Complex Environments
7:30 PM-10:00 PM, Wednesday, April 9, 2008 Morial Convention Center -- Hall A, Poster
Division of Physical Chemistry |