Hsu, Chao-HsienChao-HsienHsuLiao, Yi-ChingYi-ChingLiaoCheng, Chu-ChunChu-ChunChengWu, Bo-HanBo-HanWuYang, Chou-HsunChou-HsunYangHsu, Chao-PingChao-PingHsuChen, Bo-HanBo-HanChenYang, Shang-DaShang-DaYangHsu, YulingYulingHsuChu, Li-KangLi-KangChuChiang, Yun-WeiYun-WeiChiangWong, Ken-TsungKen-TsungWongChou, Pi-TaiPi-TaiChou2026-03-022026-03-022026-01-23https://scholars.lib.ntu.edu.tw/handle/123456789/736054Singlet 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.enjournal article10.1021/jacs.5c14851