Varying the Interpentacene Electronic Coupling to Tune Singlet Fission

We have designed and used four different spacers, denoted A-D, to connect two pentacenes and to probe the impact of intramolecular forces on the modulation of pentacene-pentacene interactions and, in turn, on the key steps in singlet fission (SF), that is, the ¹(S₁S₀)-to-¹(T₁T₁) as well as ¹(T₁T₁)-to-⁵(T₁T₁) transitions by means of transient absorption and electron paramagnetic resonance measurements. In terms of the ¹(S₁S₀)-to-¹(T₁T₁) transition, a superexchange mechanism, that is, coupling to a higher-lying CT state to generate a virtual intermediate, enables rapid SF in A-D. Sizeable electronic coupling in A and B opens, on one hand, an additional pathway, that is, the population of a real intermediate, and changes, on the other hand, the mechanism to that of hopping. In turn, A and B feature much higher ¹(T₁T₁) quantum yields than C and D, with a maximum value of 162% for A. In terms of the ¹(T₁T₁)-to-⁵(T₁T₁) transition, the sizable electronic coupling in A and B is counterproductive, and C and D give rise to higher ⁵(T₁T₁)-to-(T₁ + T₁) quantum yields than A and B, with a maximum value of 85% for D.