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.