Femtosecond transient absorption of donor-acceptor systems

Abstract

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. In Chapter 1 theoretical aspects of photo-induced electron and excitation energy transfer are described with the focus on intramolecular interactions. The relevant concepts and equations related to electron transfer are briefly outlined to serve as a basis for interpretation. Particular attention is given to the Förster model for energy transfer with the theoretical investigations of dipole-dipole interaction and its boundary conditions. Chapter 2 is devoted to a brief literature review of the photophysical properties of a few rigid organic π-conjugated donor-acceptor systems. Several examples of dynamic processes taking place in π-conjugated systems were presented, including charge transport, charge recombination and various electronic interactions. The elucidation of the details of excited-state dynamics in molecular models is essential in understanding the photophysics of the natural and synthetic multichromophoric systems. Chapter 3 addresses the femtosecond spectrometer setup, detection principles and the different methods used to obtain the experimental data. The generation, amplification and conversion as parts of the femtosecond system are briefly outlined. It is shown that the improvements made to the apparatus have boosted its accuracy and flexibility as a spectroscopic tool for sensitive investigations including biological samples. The data analysis section receives a particular consideration due to the complexity of the data resulting from a substantial number of competing processes involved. A novel spectroscopic tool has been implemented by incorporating a third independent spectrally and temporally tunable femtosecond pulse in the conventional transient absorption. To achieve this, a part of the regenerative amplifier output was branched off and routed into a second OPA were the pulses were spectrally tuned to the desired wavelength. A second delay line was mounted at this OPA exit to allow for the necessary timing relative to the first excitation pulse. Successively, the pulse was adjusted in power, polarized under required orientation, routed and directed into the sample where all three pulses overlap. Collecting the data with the CCD camera ensures that within a single measurement the evolution of spectrally different excited state species are monitored at the same time and under the same conditions. Three-pulse transient absorption was used to intervene in the photoinduced dynamics of two molecular systems by modifying either the excited or ground state population. Chapter 4 presents 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. These systems are investigated in detail by femtosecond transient absorption and single photon timing experiments and compared to analogous model systems. The obtained results here are compared to those obtained for PI-(pPh)1 and PI-(pPh)2. 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. In Chapter 5 the excited state properties of PI-(pPh)1 and PI-(pPh)2 are explored using pump-re-pump-probe and pump-dump-probe transient absorption technique in the visible region. Although the data are complex, this 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. The results presented demonstrate the complexity of the excited state properties and dynamics in perylene end-capped phentaphenylenes. These open the opportunity for studying other complex photophysical systems using similar experimental approaches.status: publishe