Nearly monodisperse CdSe quantum dots of 1.8 nm in size were synthesized and capped with a surface monolayer of 1-thioglycerol, The optical properties of thin films made from these matrix-free close-packed quantum dots were studied at different temperatures and electric field strengths and compared with isolated quantum dots of the same size embedded in a PMMA matrix. The broadening and red shift of the optical transitions observed in absorption and excitation spectroscopy for the ensemble of close-packed quantum dots is explained by the formation of collective electronic states between interacting nanocrystals. The reversible collapse of these extended electronic states into localized states was demonstrated for the close-packed quantum dots by applying an external electric field.