Investigation of spectral shifts of monomeric and J-aggregated cyanine dyes at high pressure by UV/Vis spectroscopy

The solvent-induced and pressure-induced shifts (ν and Δν, respectively) of the absorption spectra of four different cyanine dyes (PIC, DYE A, C and D) in monomeric and J-aggregated form were investigated by UV/Vis spectroscopy. For the monomers, it has been confirmed that the refractive index, n, is of major influence when interpreting the spectral shifts. Quantitative analysis of the shifts of the monomeric dyes was performed in terms of Bayliss-parameters, F(n) = (n2 − 1)/(2n2 + 1), in the case of solvent variation and F′(n) = ρ/ρ0(n2 − 1)/(2n2 + 1) in the case of pressure variation. The respective shift coefficients of the monomers, dν/dF(n), were found to be the same within experimental error (−3700 ± 800 cm−1). The same is valid for dΔν/dF′(n), measured in ethane-1,2-diol (−2000 ± 200 cm−1). Additionally, the pressure coefficients, dΔν/dp, were evaluated for both the monomeric and the J-aggregated dyes for better comparison to literature data. In contrast to the monomer values (−20 cm−1 kbar−1), the pressure coefficients for the J-aggregates obtained from measurements in water (−30 cm−1 kbar−1) are larger, in agreement with recent literature values. The different pressure behaviours of the monomers, compared to that of the J-aggregates, were attributed to the electronic interactions and changes in the specific arrangement of the molecules within the aggregates. For the pressure coefficients of the J-aggregates, a linear correlation has been found between dΔν/dp and an approximate dipolar coupling constant, J*, derived from the peak positions of the absorption spectra. Moreover, by extrapolation of the dΔν/dp values to J* = 0, the mean value equal to the same quantity for the monomers was found within experimental error. Comparison of this finding with recent literature values gave rough agreement. A supplemented form factor analysis of light scattering data for two representative J-aggregated dyes (PIC and DYE A) in aqueous solution at atmospheric pressure is consistent with rod-, worm- and coil-like aggregate structures, depending on the length of the aggregates.