The SF process can be separated into two different categories. The SF is an overall exothermic process if the excited energy level of 2E(T 1 ) < E(S 1 ), as reported in pentacene. In contrast, the SF process is endothermic if 2E(T 1 ) > E(S 1 ), as reported in tetracene. The detailed mechanism of the PSF process in exothermic and endothermic SF has been intensively studied by various forms of ultrafast spectroscopic methods. Zhu et al. employed two-photoemission spectroscopy to examine the process of PSF and reported the direct observation of the T 1 T 1 formation after photoexcitation of singlet S 1 exciton . The measurement of 20 fs of rising time in triplet population both in tetracene and pentacene has been attributed to the formation of multiexciton state. The subsequent calculations performed by Berkelbach et al. showed that in model dimers CT state is not populated but strongly mediate the singlet fission process through second order superexchange mixing . Moreover, a phenomenological model has been constructed to examine the ultrafast process of SF in TIPS and their calculated results supported the coherent superexchange mechanism via an energetically higher-lying CT state . Recent excimer energy decomposition analysis also showed the significant role and geometrical dependence of CT states on S 1 excitation . The role of CT state in the PSF has been further investigated by intramolecular SF in herringbone-packed SF materials and pentacene crystals .