Varying the inter-pentacene electronic coupling to tune singlet fission

We have designed and used four different spacers to connect two pentacenes and probed the impact of the spacer on the key steps in singlet fission (SF), that is, the 1(S1S0) to 1(T1T1) transition, as well as the 1(T1T1) to 5(T1T1) transition. In terms of 1(S1S0) to 1(T1T1) transition, a super-exchange mechanism, that is, coupling to a higher-lying CT state to generate a virtual intermediate, enables rapid SF in A–D. Strong electronic coupling in A and B opens, on one hand, an additional pathway, that is, population of a real intermediate, and changes, on the other hand, the mechanism to that of hopping. In turn, A and B features much higher 1(T1T1) quantum yields than C and D with a maximum value of 162% for A. In terms of 1(T1T1) to 5(T1T1) transition, the strong electronic coupling in A and B is counterproductive and, in turn, C and D give rise to higher 5(T1T1) / (T1+T1) quantum yields than A and B with a maximum value of 85% for D.