Efficient Singlet Fission via a High-lying 3¹ A_g Dark Intermediate State

An efficient singlet fission (SF) process mediated by high-lying 3 ¹ A g dark intermediate state has been demonstrated in Benzodipyrrolidones (BDPP) skeleton with C 2h structural symmetry based on the ultrafast spectroscopy and theoretical simulation. The proposed new photophysical model highlights that quasi-degeneracy of 3 ¹ A g and bright S 2 (1 ¹ B u ) state enable SF prcoess to occur at ultrafast timescale, and renders the minimum energy loss from photoexcitation. Moreover, the common low-lying 2 ¹ A g dark state turns out to be the optically forbidden S 1 state, and acts as an internal conversion pathway competing with the SF process but is at a disadvantage relative to the latter. These outcomes provide new insights into the understanding of dark state-related photophysics, and also elucidate that more elctronic states beyond 1 ¹ B u and 2 ¹ A g states need to be taken into consideration to present more reasonable molecular photophysics. Furthermore, these results also expand the limited scope of SF materials, which could promote the development of new SF-capable chromophores.