Time Dependence of the Molecular Aggregation States of Merocyanine Dye Monolayers at Air/Water Interface

Abstract

Time dependence of the molecular aggregregation states of a neat merocyanine dye monolayer at air/water interface were investigated by in-situ observation of the absorption spectra of the monolayers prepared at various pH's and temperature conditions of the subphase. Jaggregation state was found to be formed immediately after spreading a chloroform solution containing the dye on the interface and to change gradually into the dimeric state. And the J-aggregate form disappeared eventually about one hour after spreading. At a lower pH, the J-aggregation state became less stable and changed to the dimeric form with the higher conversion rate. This suggested that Cd2+ ions might play an important role of bridging the carboxylic groups of the adjacent dyes in the J form and of keeping the Jaggregation state stable. The conversion rate was found nonexponential with respect to time. The temperature effects of the subphase on the rate gave interesting results. At higher temperatures the rate became faster after the J-band intensity had decreased to the one-third of the initial value. On the other hand, the rate showed a minimum value at 26°C in the initial stages. These phenomena may be caused by two competiting thermal effects, one of them being to make the J-aggregate unstable at high temperature and the other being to make it stable due to annealing. The chromophore orientation in the two aggregation states at air/water interface was examined by measuring polarized absorption spectra at the normal and oblique incidence. The transition dipole moment of the chromophore in the dimeric form was found more tilted vertically from the horizontal plane parallel to the interface than that in the J-aggregation form.