Distinguishing packing configurations of molecular dimers using excited-state absorption peaks in two-dimensional electronic spectra.

Packing conformations of molecular aggregates are known to strongly influence the locations and intensities of spectral peaks. Here, we develop the third-order nonlinear spectroscopy signals for a purely electronic model of a molecular dimer, which is a prototype aggregate system. The model-which focuses on excited-state absorption (ESA) pathways in two-dimensional electronic spectra-reveals that orientational averaging leads to diagnostic ESA peak locations for H- and J-dimers. We constructed DNA-templated dimers of cyanine molecules as representative systems and used ultrabroadband two-dimensional electronic spectroscopy measurements to support the predicted signatures arising from the theoretical model. Fitting of steady-state spectra supports the assigned packing conformations. The results elucidate how ESA peaks can be diagnostic spectral signatures of packing conformation. This work lays the foundation for future studies that can include the complicating effects of vibronic states and additional electronic levels.