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Varying the Interpentacene Electronic Coupling to Tune Singlet Fission

Papadopoulos, Ilias, Zirzlmeier, Johannes, Hetzer, Constantin, Bae, Youn J., Krzyaniak, Matthew D., Wasielewski, Michael R., Clark, Timothy, Tykwinski, Rik R., Guldi, Dirk M.
Journal of the American Chemical Society 2019 v.141 no.15 pp. 6191-6203
absorption, electron paramagnetic resonance spectroscopy
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.