Quantitative investigations of thermal and photoinduced J- and H-aggregation of hydrophobic spirooxazines in binary solvent through UV/vis spectroscopy

The aggregation of two hydrophobic spirooxazine dyes has been investigated in water-acetonitrile binary solvent and in presence of sodium dodecylsulfate (SDS). For the thermochromic molecule 1, aggregation is spontaneous and occurs when a large amount of water is added to an acetonitrile solution of the dye. There exists an optimum value of the water-acetonitrile molar ratio giving rise to a sharper H-aggregate visible absorption spectrum. Kinetic analysis of the H-aggregate formation has shown that there is an increase in the sticking probability of the monomers, as the aggregates grow. A subtle interplay between H-and J-aggregation was witnessed by adding small amounts of SDS. This change is so sensitive that this system could be the basis of a new chemosensory technique for SDS assessment. Excess of surfactant induces the collapse of the aggregates and the re-dissolution of the dye under the form of a merocyanine-SDS ion-pair. For the spirooxazine 2, aggregation must be photo-induced. Comparison of the spectral evolutions of the two dyes in the presence of SDS has shown that the opening of the closed spirooxazines paves the way to the merocyanine-SDS ion-pair formation. Reversibility of the aggregation has been suggested to occur when the aggregate size is still small. Finally, reversibility has been checked experimentally by precipitate re-dissolution and spirooxazine recovery. Possible structures and reactivity of the H-and J-aggregates have been deduced.