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 o-(Pc)(2) and m-(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 o-(Pc)(2) relative to m-(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 X T)] was clearly observed in m-(Pc)(2) which is in sharp contrast to a major recombination process in o-(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 m-(Pc)(2). The rate constant of the SF was 2 orders of magnitude greater in o-(Pc)(2) than that in m-(Pc)(2) and was rationalized by enhanced electronic coupling between adjacent HOMOs of the Pc units.