Plenty of Room on the Top: Pathways and Spectroscopic Signatures of Singlet Fission from Upper Singlet States.

We investigate dynamic signatures of the singlet fission (SF) process triggered by the excitation of a molecular system to an upper singlet state S ( > 1) and develop a computational methodology for the simulation of nonlinear spectroscopic signals revealing the S → TT SF in real time. We demonstrate that SF can proceed directly from the upper state S, bypassing the lowest excited state, S. We determine the main S → TT reaction pathways and show by computer simulation and spectroscopic measurements that the S-initiated SF can be faster and more efficient than the traditionally studied S → TT SF. We claim that the S → TT SF offers novel promising opportunities for engineering SF systems and enhancing SF yields.