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Due to their hollow space, nanotubes NT can host molecules giving rise to new hybrid molecule@NT systems. A possible class of molecules that can be hosted by nanotubes are π-conjugated oligomers which are attractive due to their electronic and optoelectronic properties[3]. Its drawback are brittleness and low electric conductivity[4]. Encapsulation of dimethyl-quaterthiophene (4TCH3) inside SWCNTs gives rise to a hybrid material, expected to combine the protection of the confining matrix to the photo-physical properties of the encapsulated unit. This hybrid material will be referred hereafter as 4TCH3@NTφ(where φ is the nanotube diameter in Å).The first recent experimental study, based on Raman spectroscopy, brought the proof that the number of encapsulated molecules depend on the size of the confining matrix[5]. Some influence of the diameter of the nanotube on the confinement state of the molecule as well as their interaction with the nanotube is expected. Inelastic neutron scattering, mostly sensitive to the confined molecule, due to the strong cross-section of hydrogen atoms, probes wide Q vector range, which is a very interesting tool in order to obtain a complete picture of the vibrational properties of such system.We are interested on the confinement properties of two hybrid systems: 4TCH3@NT09 and 4TCH3@NT14. Those materials were studied by means of inelastic neutron scattering (INS) at two instruments at the Institute Laue-Langevin, time-of-flight IN4C and Triple Axis IN1-Lagrange spectrometers. The resulting experimental HPDOS can be compared to the DFT-simulated HPDOS, allowing thus the assignment of main vibrational modes. We show that specific vibrational modes undergo to modifications upon a change of the size of the confining matrix, evidencing the influence of encapsulation in such molecules.