The electron transfer rate of photosystem I is highest when $$\Delta\text{G}$$ is $$\approx$$ to the reorganisation energy. This is because, at this point, the electron transfer rate described in Marcus theory is at its maximum.
If $$\Delta\text{G}$$ become more negative, the energy required to transfer the electrons becomes much higher, and this makes the reaction less favourable. This is because the reaction has entered the Marcus inverted region, and the energy for direct charge recombination falls.