QUANTUM BEATS OF THE RADICAL PAIR STATE IN PHOTOSYNTHETIC MODELS OBSERVED BY TRANSIENT ELECTRON-PARAMAGNETIC-RESONANCE

Time-resolved electron paramagnetic resonance (EPR) is used to study the transient spin polarization behavior of the light-induced radical pair state in the photosynthetic model system 2-tetraalkylphenylenediamine-Zn porphynn-2-naphthoquinone. The time evolution of the transverse magnetization is monitored at various static magnetic fields. This implies a two-dimensional variation of the signal intensity with respect to both the magnetic field and time axis. Zero quantum coherence between two of the four eigenstates of the radical pair is observed at early times after the laser pulse. The beat frequencies vary significantly with the static magnetic field and extend up to 70 MHz. This model system is the first example where quantum beats have been observed of a correlated radical pair protonated constituents.