In this work, Kukura and co-workers have proposed that the ultrafast PSF in pentacene and its derivatives are realized by the degeneration of two potential energy surfaces between singlet and triplet-pair states. The PSF process in rubrene and its coherent generation of triplet-pair state are further studied by transient absorption spectroscopy and the associated coherence is examined by vibrational analysis after exponential fitting procedure . The results demonstrated that the PSF is induced by a symmetry-breaking mode. This vibronic coherence has also been reported in tetracene and hexacene . Deng et al. used femtosecond transient infrared spectroscopy to observe transient spectral evolution of the ring-stretching modes of hexacene in order to investigate the structural dynamics of the intermediate electronic states in SF . In combination with ab initio calculations, they assigned these experimentally observed ring-stretching vibrational modes as the reaction coordinates. Recently, the coherent two-dimensional electronic spectroscopy has been applied to study the PSF process in pentacene film . In the study, intramolecular vibrational modes are identified and has been considered for the mediating of PSF process from singlet to triplet-pair state. A vibronic dimer model has been constructed on the basis of intramolecular vibrations to fit the time scale of PSF . However, other studies favoured the CT state for the mediating of transfer from S 1 to T 1 T 1 state . Frontiera and co-workers used femtosecond stimulated Raman spectroscopy to study crystalline pentacene after photoexcitation at 400 nm. Their study showed two-excited Raman peaks for each of the ground state peaks, which were assigned to intermediates with charge transfer character . Musser and co-workers studied the role of CT state in intramolecular singlet exciton fission by chemical engineering of the distinct CT intermediate . Moreover, ab-initio calculations showed the importance of the CT state, which works as the intermediate bridge between singlet and triplet-pair states . Apparently, despite the enormous theoretical and experimental efforts, the understanding of the PSF, i.e., the dynamics of the formation of (T 1 T 1 ), the role of intermolecular vibrations and the CT states in the SF materials remained elusive.