We first observed a unique pyrene (Py) emission from 10-(1-(6-hexyl)pyrenyl)decanoic acid (1,6-HPDA) Langmuir-Blodgett (LB) films. In the fluorescence spectra of the 1,6-HPDA LB films, the normal bands of monomer and excimer emission of Py were absent but the new structured emission band with peaks at 422, 448, and 473 nm was observed. Besides the intensity, the shape of the characteristic structured emission band at shorter wavelengths than normal excimers did not changed essentially on lowering the Py concentration in the LB monolayer from 100 to 1 mol%. The spectroscopic studies on HPDA LB films of a variety of Py concentrations suggested that a novel excimer responsible for the structured new emission would be efficiently formed by photo-excitation of a Py-Py pre-associated dimer in different manners than those of normal excimer formations. (c) 2005 Elsevier B.V. All fights reserved.