We report the first synthesis of solvent-free pentacene fluids at room temperature together with observation of singlet fission (SF). Three pentacenes with different number of ethylene glycol (EG) side chains (n) were employed (denoted as (EG)n-Pc-(EG)n: n = 2, 3, and 4). The morphologies of these pentacenes largely depend on the lengths of EG chains (n). (EG)3-Pc-(EG)3 and (EG)4-Pc-(EG)4 indicate fluid compounds at room temperature, whereas (EG)2-Pc-(EG)2 is a solid compound. Microscopic clustering with short-range interactions between pentacene chromophores was confirmed in X-ray diffraction profiles of solvent-free fluids. Such a structural trend is an important origin of SF and consistent with the steady-state spectroscopic results. To one’s surprise, femtosecond transient absorption spectroscopy demonstrated that SF occurred in thin films prepared from solvent-free fluids of (EG)3-Pc-(EG)3 and (EG)4-Pc-(EG)4 in spite of such excessive EG chains.
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