Investigation of two cyanine dyes at normal and high pressure by UV/Vis spectroscopy

Abstract

Two cyanine dyes, pseudoisocyanine chloride (PIC) and DYE A, were investigated in dilute and more concentrated aqueous solutions. Experiments by pressure variation reveal the volume contractions, ΔV_D, of both dyes, which are discussed with reference to their different molecular structures. Additionally, the changes of entropy for dimerisation, ΔS_D, in dilute solutions (c<10⁻⁴ mol L⁻¹) along with further thermodynamic data such as ΔG_D and ΔH_D, were obtained from temperature dependent measurements. Detailed analysis of the absorption spectra of both dyes in terms of resolution provided a twisted arrangement of the interacting molecules within the H-aggregates. For more concentrated solutions of PIC (c>10⁻³ mol L⁻¹), the delocalisation lengths of the excitons, N_del, are calculated and compared with the values determined from the temperature dependence of the half-width of the J-bands, Δν_1/2,J. The same values of 20–100 are found for N_del from both different methods and high-pressure experiments as well. In the case of DYE A, N_del values based on the estimation from Δν_1/2,J were found to be smaller and less sensitive to temperature than for PIC. The effects of c₀, p and T on N_del are discussed. Furthermore, we present for the first time a pressure–concentration–state diagram for the appearing/vanishing J-band of aqueous solutions of PIC. From the high-pressure experiments it is concluded that the density of the J-aggregates is higher than that of the H-aggregates, corresponding to a more compact arrangement of the molecules within the aggregates.