MEROCYANINE AGGREGATION IN MESOPOROUS NETWORKS

Nanocrystalline films of TiO2, Al2O3, and ZrO2 were used as hosts for the merocyanine hyl-2-benzothiazolidinylidene)ethylidene)rhodanine (Mc 2) to scrutinize templating effects in the accommodation of the dye wi thin their porous network. Composed of interconnected mesoscopic oxide particles and pores, these films are transparent, allowing for a stra ightforward application of transmission spectroscopy to unravel the op tical features of the incorporated dye species. Apart from H-aggregati on, the formation of two different types of two-dimensional assemblies was witnessed yielding red-shifted absorption bands which were identi fied as J-aggregates, one showing Davidov splitting, the other having a single exciton band. The herringbone packing of the dye molecules in the layered structure of Mc2 sodium salt octahydrate single crystals was taken to model the double-banded J-aggregate structure. On mesosco pic hydroxylated TiO2 anatase films, the structure of the Mc2 assembli es is controlled by the texture of the highly porous substrates as wel l as their surface charge. Furthermore, it responds in a striking fash ion to the presence of solvent in the ambient to which the films are e xposed. Double-banded (herringbone structure) and single-banded (paral lel alignment of the dye) absorption spectra can thus be obtained. The role of solvent is to stabilize one particular aggregate geometry thr ough intercalation into the Mc2 aggregate. Electron injection into TiO 2 from both types of J-aggregates is observed. Laser flash photolysis experiments show that energy transfer from the monomer to the J-aggreg ate is operative prior to charge injection. On hydroxylated Al2O3 and ZrO2 surfaces Mc2 undergoes physisorption and formation of H-aggregate s exhibiting a blue-shifted absorption with regard to the monomer spec trum. Contrary to the results obtained for TiO2 substrates, aggregatio n is hardly influenced by solvent in the ambient. In particular no J-a ggregates can be formed on bare Al2O3 and ZrO2 substrates. However, wh en the porous films are impregnated with concentrated hydroxide soluti ons, H- as well as J-aggregates are formed in humid air. At high humid ity, due to the hygroscopic salt coating, the pores are completely fil led with water, leading to the precipitation of the dye molecules form ing H-aggregates. At lower humidity an air-water interface builds up w ithin the pores an da double-banded J-aggregate spectrum appears. The spectrum is almost identical to the one measured on Mc2 sodium slat oc tahydrate single crystals with a layered organic-inorganic structure. Resonance fluorescence originating from the energetically lower excito n band and internal conversion from the higher to the lower exciton ba nd take place.