Thin films consisting of 20 layers of 1,4-bis(4-hydroxybutyl)-8,11,15,18,22,25-hexakisdecyl phthalocyaninato copper (CuAmPc) molecules have been deposited onto pre-cleaned glass slides by the Langmuir-Blodgett (LB) technique. The optical absorption of these films were studied in the range 1.4-4.0 eV in air and also in the presence of NO2 gas at 100 ppm level. Prolonged exposures have resulted in dramatic changes in the visible absorption band with the consequence of disappearance of Davydov splitting. Such effects are caused by the domination of local interaction between the central metal atoms and NO2 molecules resulting in a distorted herring-bone structure. This leads to the broadening of the energy levels a(2u) and e(g). Gas adsorption and desorption processes have been described by exponential functions, both characterised by a single time constant.