NONCENTROSYMMETRIC AND CENTROSYMMETRIC J-AGGREGATION IN PHOTOMEROCYANINE MONOLAYER STUDIED BY 2ND-HARMONIC GENERATION AND ABSORPTION-SPECTROSCOPY

We found that photomerocyanine isomerized from spiropyran (SP-1822) fo rms two different kinds of J-aggregation by use of second-harmonic gen eration and absorption spectroscopy. First, a J aggregate, which has a n absorption peak at 608 nm in a monolayer at the air/water interface under high surface pressure: since the molecules in the aggregate alig n in a certain direction with the same sense, strong second-harmonic l ight generates in the monolayer. On deposition, the aggregate dissocia tes and the molecules change into monomers. After a while, the molecul es start to form new J aggregates. Promotion of the association by hea ting and the fact that these are no second-harmonic activities of the aggregates suggest that the molecules cancel each other's polarity in the second J aggregate.