ARCHITECTURE OF J-AGGREGATES STUDIED BY PRESSURE-DEPENDENT ABSORPTIONAND FLUORESCENCE MEASUREMENTS

The spectral shift of absorption and fluorescence bands as well as the fluorescence decay of four J-aggregate forming 5,5',6,6'-tetrachlorob enzimidacarbocyanine dyes have been investigated in solution under hig h pressure up to 50 kbar. The investigated dyes differ only in their 1 ,1' and 3,3' nitrogen substituents which do not influence the chromoph ore itself. Whereas the spectral behaviour of the monomeric dyes is co mparable to that of aromatic and polyenic compounds, the J-aggregates show some unusual spectral features. Two types of J-aggregates can be distinguished. One type exhibits a linear dependence of the absorption and fluorescence maximum on pressure with vanishing Stokes shift over the whole pressure range. The second type is characterised by a limit ed shift of the absorption band to longer wavelengths whereas the Stok es shift increases even at pressures where the absorption energy remai ns constant. The spectral behaviour of the first type is consistent wi th the well-known chain-shaped or brickstone-like structure of J-aggre gates. As one dye of the second type is known to form optically active , Davydov-split J-aggregates the pressure-dependent spectral behaviour of such dyes corroborates the assumption that their J-aggregates cons ist of helical cylindrical micelles.