Thesis Abstract and Dissertation Abstracts
Femtosecond Transient Absorption of Donor-Acceptor Systems
Abstract Category: Science
Course / Degree: Science
Institution / University: K.U. Leuven, Belgium
Published in: 2007
The results presented in this thesis concern the photophysical properties of several perylenemonoimide functionalized penthaphenylene systems in solution. The experimental investigation of the excited state dynamics performed by means of the ultrafast laser spectroscopy revealed the occurrence of electron and energy transfer. These processes lead to the formation of excited state intermediates with distinct photophysical properties.
An experimental and systematic study on time-dependent spectral properties of a rigid, extended system consisting of one [PI-(pPh)1-PI], two [PI-(pPh)2-PI] and three [PI-(pPh)3-PI] pentaphenylene units end-capped with perylenemonoimide is presented. These systems are investigated in detail by femtosecond transient absorption and single photon timing experiments and compared to analogous model systems. Exciton-exciton annihilation occurs in both PI-(pPh)1-PI and PI-(pPh)2-PI systems investigated upon high power excitation. These results are in line with the kinetic results obtained for PI-(pPh)3-PI and show that the annihilation becomes faster and more efficient as the exciton coupling increases. The annihilation was found to promote one chromophore into a higher excited singlet state which then rapidly relaxes to S1 via a charge transfer state intermediate. Starting from the higher excited state the PI radical anion is formed even in a low polar environment and decays with a time constant of about 1 ps. The mechanism observed suggests an elegant way to explore reactions in the upper excited states, as in this case an ultrafast charge transfer occurred above the lowest singlet excited state. For the largest system PI-(pPh)3-PI, the experiments demonstrated that a CT-CT annihilation takes place after formation of two independent charge transfer-like states. This novel excited state interaction was observed in a solvent of medium polarity by comparing the transient decay traces recorded with five different excitation powers. Based on quantum-chemical calculations the PI excited state wavefunction was found to significantly spread over the neighboring pentaphenylene skeleton in polar solvents. This leads to a shorter center-to-center separation between the two PI transition dipoles. Both theoretical and experimental results furthermore yield a larger overlap between the excited state emission and absorption spectra upon increasing the solvent polarity.
The excited state properties of PI-(pPh)1 and PI-(pPh)2 are further explored using pump-re-pump-probe and pump-dump-probe transient absorption technique in the visible region. This technique allowed gaining new insights of the dynamics of the excited states. For instance, upon pre-exciting the PI subunit an additional relaxation pathway is revealed in the deactivation process of the pPh. By appropriate selection of wavelength and timing, the extra pulse initially promotes the energy acceptor into an excited state thereby opening a new pathway in transferring the excitation energy from the pPh to the excited state.
Thesis Keywords/Search Tags:
Femtosecond transient absorption, electron transfer, energy transfer, perylene, chromophore, annihilation
This Thesis Abstract may be cited as follows:
Femtosecond Transient Absorption of Donor-Acceptor Systems, PhD thesis, 2007, K.U. Leuven
Submission Details: Thesis Abstract submitted by Eduard Fron from Belgium on 27-Jun-2007 17:06.
Abstract has been viewed 2753 times (since 7 Mar 2010).
Eduard Fron Contact Details: Email: eduard.fron@chem.kuleuven.be
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