The ground-state electronic structure and excited-state dynamics of a fluoreneendcapped butatriene were elucidated. Tetraphenyl-endcapped butatrienes had been known to exhibit unique photoluminescence properties in which fluorescence was completely quenched by ring closure to a fluorene scaffold. Femtosecond time-resolved microspectroscopy on the microcrystalline state and density functional theory calculations revealed that the quenching is derived from the fast nonradiative decay to the ground state and the internal conversion to the dark state.

The synthesis of phenalenyl-endcapped [5]cumulene as acumulene-based singlet biradical and the spin correlation changes of one-dimensional aggregates are described. The high propensity for self-aggregation of phenalenyl rings and the introduction of bulkys ubstituents into the appropriate positions led to the formation of aone-dimensional chain assembly. Single-crystal X-ray structural analysis indicated that the bond length alternation of the cumulene chain increased with decreasing temperature,a long with improved overlapping of the phenalenyl rings.V ariable-temperature Raman spectroscopy and magnetic susceptibility measurements revealed that al ocalized spin pair within the molecule decouples at low temperatures,a nd ac ontinuum spin system involving intraand intermolecular spin-spin interactions emerges in the onedimensional chain.Scheme 1. Phenalenyl-and fluorenyl-endcapped cumulenes.

The synthesis of phenalenyl‐endcapped [5]cumulene as a cumulene‐based singlet biradical and the spin correlation changes of one‐dimensional aggregates are described. The high propensity for self‐aggregation of phenalenyl rings and the introduction of bulky substituents into the appropriate positions led to the formation of a one‐dimensional chain assembly. Single‐crystal X‐ray structural analysis indicated that the bond length alternation of the cumulene chain increased with decreasing temperature, along with improved overlapping of the phenalenyl rings. Variable‐temperature Raman spectroscopy and magnetic susceptibility measurements revealed that a localized spin pair within the molecule decouples at low temperatures, and a continuum spin system involving intra‐ and intermolecular spin–spin interactions emerges in the one‐dimensional chain.

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