Molecular orientation in mixed LB films containing photochromic molecules

Orientation of photochromic molecules in mixed Langmuir-Blodgett (LB) films that were subjected to ultraviolet irradiation has been investigated by polarised spectroscopy. The photochromic molecules (spiropyran (SP) and spirooxazine (SO)) were incorporated as guests in matrices of arachidic acid (AA), poly (methyl methacrylate) (PMMA) and poly (isobutyl methacrylate) (PIBM). The films were deposited in the dark (dark-deposited) and also while the Langmuir film on the water surface was subjected to ultraviolet irradiation (UV-deposited). Orientation of the open form of the chromophores was strongly influenced by the nature of the matrix and by the presence or absence of ultraviolet irradiation. In the case of LB films formed from either chromophore and PIBM, the dipole moments of the open forms exhibited large tilt angles relative to the substrate normal for both the dark-deposited and UV-deposited films. With AA or PMMA as the matrix, the tilt angles were large for the dark-deposited films, but the UV-deposited films displayed considerably smaller tilt angles. The nature of the matrix species and its Langmuir film is suggested as the basis for inteipreting the observations.