In this paper, we constructed an unified excitonic model to study the coherent dynamics in the PSF process of pentacene and tetracene molecules. With the refined parameters from spectroscopic measurements, we have studied the role of CT state and the coherent effects of the vibronically coupled modes in the molecular crystals. We have performed the calculations of population dy-namics by time non-local quantum master equation. We examined the role of CT state and compare to the calculated results without CT state. We showed that, in the model of pentacene dimer (a prototypical system to study exothermic SF process), the CT state do not play a significant role since the process of the PSF occurs within 100 fs. Instead, the strong vibronic coupling of intramolecular modes interacts with intermolecular motions mediating the PSF process in pentacene. However, the CT state show as a relatively important contribution to mediate the PSF process in tetracene system, which is an prototypical system to study endothermic SF process. Based on our calculations, we show that the CT state has distinct contribution in the process of the PSF and thus, it could play a functional role in the tetracene system, where the site energy of triplet-pair state is comparable to the electronic singlet state. We also believe that the uncovered role of CT state and the coherent effects in the model systems could be further extended to the other molecular crystals capable of SF process. Therefore, we envision that our unified model can be used to decipher general guiding principles to develop new generation of singlet fission materials for photovoltaic applications.