Direct, mediated and delayed intramolecular singlet fission mechanism in donor-acceptor copolymers

Donor-acceptor (D-A) extended copolymers have shown great potential to be exploited for intramolecular Singlet Fission (iSF) because of their modular tunability and intrinsic ability to incorporate low-lying charge-transfer (CT) and a triplet-pair (TT) state. While the SF mechanism has been widely debated in homo- and hetero-dimers, little is known about the singlet splitting process in A-D-A copolymer trimers. Unlike traditional two-sites SF, the process of iSF in D-A copolymers involves three molecular units consisting of two A and one D following a A-D-A polymeric chain. This scenario is therefore, different from the homo-dimer analogous in terms of which states (if any) may drive the SF process. In this work, we identify how singlet splitting occurs in prototypical iSF D-A copolymer poly(benzodithiophene-alt-thiophene-1,1-dioxide) (BDT−TDO) by means of wave-packet propagations on the basis of the Linear Vibronic Coupling (LVC) model Hamiltonian. Our results reveal that three different mechanisms drive the S→TT population transfer via antisymmetric and symmetric vibrational motion, including two favorable mechanisms of direct and mediated interactions, as well as a parasitic decay pathway that potentially delays the process. Remarkably, we uncover the interplay between an upper state of marked multi-excitonic character and a low-lying CT state in balancing the splitting efficiency, which anticipates their major role in defining future guidelines for the molecular design of D-A copolymers for iSF.