Two-dimensional electronic spectra of an aggregating dye: simultaneous measurement of monomeric and dimeric line-shapes.

We present a sequence of two-dimensional electronic spectra of a prototypical cyanine dye, whose spectral properties in aqueous solution are determined by the formation of a monomer-dimer equilibrium. Quantum-chemical methods are utilized to calculate the structure and absorption properties of the two species involved. Our spectroscopic results simultaneously characterize the spectral line-shapes of the two species in terms of underlying dynamic and static disorder, and demonstrate how the two-dimensional technique allows the exploitation of high spectral and temporal resolution in one and the same experiment. The distinctly different spectral relaxation dynamics are quantified in a two-dimensional line-shape analysis, by extracting the time dependent ratios of the diagonal and anti-diagonal peak-widths. Our findings are in line with theoretical considerations, that predict the fluctuational dynamics of an excitonic dimer state to be exchange-narrowed by excitation delocalization.