Architecture of J‐aggregates studied by pressure‐dependent absorption and 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′-tetrachlorobenzimidacarbocyanine dyes have been investigated in solution under high pressure up to 50 kbar0 The investigated dyes differ only in their 1,1′ and 3,3′ nitrogen substituents which do not influence the chromophore itself. Whereas the spectral behaviour of the monomeric dyes is comparable 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 wim vanishing Stokes shift over the whole pressure range. The second type is characterised by a limited shift of the absorption band to longer wavelengths whereas the Stokes shift increases even at pressures where the absorption energy remains constant. The spectral behaviour of the first type is consistent with the well-known chain-shaped or brickstone-like structure of J-aggregates. 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 mat their J-aggregates consist of helical cylindrical micelles.