In this Thesis the results of investigations on the lowest excited triplet state of photosynthetic pigments and some model compounds are presented, partly as reprints of published papers. Most of the experiments are carried out using ODMR ( O
etected of M
esonance), detecting microwave-induced changes in the populations of the spin states of the lowest excited triplet state, via induced changes in the fluorescence emission.
M chlorin free base incorporated as a guest in a n-octane single crystal proved to be a very attractive model for the study of the zero-field-splittings and the decay rate constants of the lowest excited triplet state. Contrary to naturally occurring compounds such as pheophytins and chlorophylls, chlorin can be easily studied without effects of polar solvents and aggregation.. Since chlorin has the same conjugation pathway as the chromophoric group of chlorophyll-a and b and the corresponding pheophy tins, it is interesting to determine its properties. As already has been found for porphin free base by other groups, chlorin exhibits photoinduced rotation of the two central protons at 4.2 K, but at a much slower rate than porphin. In vitro
studies of pheophytins-a and -b have learned that the ODMR technique can be fruitfull in unraveling optical emission spectra of systems containing molecules in different solvated states, which have overlapping optical absorption and emission bands. The results suggest a correlation between the red shift of the fluorescence emission and the spin-lattice-relaxation-rate of molecules in polar environ ments. A exploratory study has been made of the application of the ODMR technique to in vivo
photosynthetic systems such as intact algae. Although hindered by spin-lattice-relaxation at 4.2 K causing a poor signal-to-noise ratio. the technique is promising for the study of whole algae. Since in biological systems both spin-lattice-relaxation and triplet-triplet absorption may cause difficulties in determining the kinetic constants of the lowest excited triplet state, a detailed computer analysis is made of the possible influence of these effects on the measured decay rate constants and populating rates. This analysis indicates the importance of including higher excited singlet and triplet states into the optical pumping cycle for the understanding of ODMR results.