Single-Carbon Bridged Pentacene Dimers Enable Efficient Singlet Fission and Quintet State Stabilization

Singlet fission (SF) offers a promising avenue for quantum information science, as it generates spin-entangled triplet pairs with quintet character (⁵TT) upon photoexcitation, enabling access to multilevel spin qubit states beyond the traditional two-level systems. However, the ⁵TT state often decays via several pathways: (1) dissociation into isolated triplets; (2) triplet-triplet annihilation back into the singlet manifold; or (3) spin conversion to lower-multiplicity triplet pair states. These competing relaxation channels pose a major challenge for stabilizing ⁵TT. Here, we introduce a novel molecular design that prolongs ⁵TT lifetime by anchoring two pentacene chromophores to the same carbon (C9) position of a fluorene bridge, yielding FlePc2 and FlePhPc2. This single-point attachment enforces a near-parallel intramolecular geometry, promoting strong through-space spin interactions that hinder dissociation. Field-swept electron spin echo (FS-ESE) measurements reveal dominant ⁵TT signals, indicative of suppressed relaxation pathways. Theoretical calculations predict a substantial binding energy for the reported dimers, accompanied by significant spin density delocalization across both pentacenes, thereby rationalizing ⁵TT stabilization. These findings establish a molecular design principle for kinetically trapping high-spin multiexciton states, paving the way for spin-based quantum technologies.