Electronic and Vibrational Coherence in Photosynthetic and Model Systems

Sammanfattning: Ultrafast optical response and excitation energy dynamics in light harvesting pigment-protein complexes and in a few model systems have been studied both experimentally and theoretically. Femtosecond light pulses were employed to excite the system and monitor the subsequent time evolution of the transient absorption kinetics and anisotropy. A new form of pump-probe technique was developed for performing the spectrally resolved transient absorption measurements using femtosecond pulses from single colour laser source; an appropriate description using density operator formalism was given. It has been shown both experimentally and theoretically that the interaction of broad-band femtosecond light pulses with a system consisting of three quantum levels leads to the appearance of anisotropic coherent transients in pump-probe signal, which are due to creation of optical coherence via tunneling processes. The time-resolved measurements performed on the light harvesting antennae of purple bacteria indicated an extremely short time scale of excitation transfer in these complexes; the process of depolarization of pump-probe signal typically occurs on a ~100 fs time scale but it can be considerably slower on the long wavelength tail of the absorption band at low temperatures due to localization effects. The exciton theory was used to fit the transient absorption spectra of the complexes in order to find the exciton delocalization (or coherence) length. Furthermore the measurements indicated the presence of oscillatory modulation in the pump-probe kinetics due to the vibrational coherence created by the short excitation pulse, which leads to wave-packet motion in the excited state. Numerical simulations based on the density operator theory satisfactorily reproduced experimentally observed wave-length dependent phase shifts and amplitude relaxation of these oscillations. Finally the extensive spectroscopic study of a double-bond bridged porphyrin dimer was undertaken. The experiments showed that the dimer exists in solution in a few different conformations. The CNDO/S calculations and experimental data confirmed that one of these conformers has a particular geometry favouring a common conjugation between the p orbitals of the porphyrin rings and the ethylene bridge; this leads to dramatic changes in the absorption and emission spectra and to ultrafast, viscosity dependent excited state deactivation. These supermolecular properties makes this dimer system conceptually similar to the special pair in the photosynthetic reaction centre.