Multiexciton Dynamics Depending on Intramolecular Orientations in Pentacene Dimers: Recombination and Dissociation of Correlated Triplet Pairs

Pentacene dimers bridged by a phenylene at ortho and meta positions [denoted as <i>o</i>-(Pc)₂ and <i>m</i>-(Pc)₂] were synthesized to examine intramolecular orientation-dependent multiexciton dynamics, especially focusing on singlet fission (SF) and recombination from correlated triplet pairs [(TT)]. Absorption and electrochemical measurements indicated strong intramolecular couplings of <i>o</i>-(Pc)₂ relative to <i>m</i>-(Pc)₂. Femtosecond and nanosecond TA measurements successfully demonstrated efficient SF in both dimers. In contrast, the dissociation process from the (TT) to the individual triplets [(2 × T)] was clearly observed in <i>m</i>-(Pc)₂, which is in sharp contrast to a major recombination process in <i>o</i>-(Pc)₂. Time-resolved electron spin resonance (TR-ESR) measurements demonstrated that the recombination and dissociation proceed from the quintet state of ⁵(TT) in <i>m</i>-(Pc)₂. The rate constant of the SF was 2 orders of magnitude greater in <i>o</i>-(Pc)₂ than that in <i>m</i>-(Pc)₂ and was rationalized by enhanced electronic coupling between adjacent HOMOs of the Pc units