Knowledge Bank

The important biological process of charge separation in the photosynthetic reaction center is initiated by photoexcitation of a chlorophyll molecule into the excited singlet state ($S1$). To better understand this process, more insight into the electronic structure and conformation of singlet excited porphyring is essential. Picosecond time resolved pump-probe Resonance Raman scattering can provide this insight by means of probing bond strengths in the molecule in the excited state. For the experiments we selected zn($\Pi$) Octachylporphyrin (ZnOEP) because it has several characteristics beneficial for the experiment. In the $S1$ state the resonance Raman spectrum is dominated by totally symmetric modes at 1564, 1450, 1359, 1261, 1135 and $664cm-1$. Since nontotally symmetric modes are only weakly enbaneed, we conclude that Jahn-Teller distortion is absent in this state. In the lowest triplet state ($T1$) nontotally symmetric modes dominate the spectrum. Jahn-Teller distortion in the triplet state thus appears to be important. The latter conclusion aggress with previous results on triplet states that indicated Jahn-Teller distortion in metalloporphyrins with $D4h$ symmetry, Implications of our results for the efficiency of charge separation in the photosynthetic reaction center will be discussed. (Supported by NIH grant GM25480)