Chloro-substitution in cyclophane-type TIPS-pentacene dimers produces ISC-born triplets after singlet fission

Singlet fission (SF) proceeds via formation of a photoexcited correlated triplet-triplet (1TT) multiexcitonic state to eventually form two independent triplet excitons. However, a possibility exists that the 1(TT) pair intermediates can undergo annihilation to give rise to intersystem crossing (ISC)-born triplets that would decrease the free triplet generation quantum efficiency. In metal-complexes laden with SF chromophores e.g. acenes, it has been shown that heavy metal atom influences the mixing of 1(TT) with the 3(TT) and 5(TT) states leading to ISC triplets although in some cases it is ambiguously absent. In order to fundamentally understand generality of ISC contamination during singlet fission, here we have synthesized a new Chloro-substituted TIPS pentacene cyclophane dimer with ethylene bridges, and compared its SF-dynamics to the nonhalogenated version. Broadband femtosecond-to-nanosecond along with nanosecond-to-microsecond transient absorption spectroscopy reveals formation of 1(TT) state in sub-picosecond timescales while a long-lived triplet state is subsequently observed in both the dimers at room temperature. Notably, the timescale of the 1(TT) dissociation is almost three times slower in the Cl-PC dimer than in the non-halogenated pentacene dimer. Time-resolved EPR spectroscopy at 125 K on the dimers revealed SF-born triplets in non-halogenated dimer while the mixing of 1(TT) state with 3(TT) and 5(TT) states facilitated the formation of ISC-born triplets after SF for the Cl-PC dimer. Electronic structure calculations elaborated on the energetics of the 1(TT) state along with distinct spin-orbit coupling values for both the dimers while highlighting the charge-transfer character of 1(TT) pair multi-excitonic state. Our work therefore emphasizes tuning the dissociation of 1(TT) to two-free triplets generation step in order to overcome the triplet-triplet annihilation leading to ISC, a key dynamics-based design principle which will allow for high quantum efficiency SF process even in the presence of heavy-atoms.