Ultrafast Dynamics in Ground and Excited States of Porphyrin Molecules

This thesis describes inter and intramolecular dynamics in porphyrin molecules. To create a basis for understanding the energy and electron transfer processes in photosynthesis, the ultrafast dynamics of monomeric chlorophyll a, chlorophyll b and bacteriochlorophyll a were studied with ultrashort laser pulses. The transient absorption spectra of all three molecules were found to have pronounced contributions from excited state absorption, polarized at an angle relative to the transitions originating in the ground state. This leads to a strongly wavelength dependent absorption anisotropy. Wavelength intervals were identified where the initial (t=0) anisotropy has the expected value of 0.4; these regions can safely be used to monitor energy transfer processes in photosynthetic pigment-protein complexes.

The time resolved measurements further revealed that chlorophyll molecules exhibit ultrafast, ~200 fs, transient holeburning in the ground states, due to solvent-induced very rapid fluctuations of energy levels. In the excited state the chlorophyll molecules display solvation dynamics on the ~5 ps time scale.

Reconstituted bacteriochlorophyll a molecules were used in time resolved measurements for investigating the B800 → B850 energy transfer mechanism in the purple bacterium Rhodospeudomonas acidophila. It was concluded that, in addition to the Förster mechanism of energy transfer, another mechanism, possibly involving the carotenoid molecules of LH2, has to be operative in order to explain the experimental results.

A model of inter and intramolecular processes in photoexcited Zn-porphyrin molecules was proposed on the basis of ultrafast transient absorption and streak camera measurements. The model involves fast vibrational relaxation to the lowest excited state in ~2 ps and a slower cooling process of the vibronic hot porphyrin molecule in ~10-14 ps.