dr. Alexei Halpin: Two-dimensional spectroscopy of molecular excitons in a model dimer system

Abstract

The physics of molecular excitons has been the subject of many recent studies using two-dimensional electronic spectroscopy (2DES), particularly in the context of light harvesting in photosynthesis. Since the spectra for multichromophoric aggregates can be congested, and particularly so at room temperature, we present a study on a model dimer comprised of identical chromophores with a well defined electronic coupling strength, such that the evolution of coherences between vibronic excitons can be unambiguously extracted from the 2D spectra. In the experiments we observe oscillatory spectral features corresponding to such interexciton coherence, which are damped on a sub-100 fs timescale. These results, analyzed in the framework of a vibronic exciton model, indicate that the signals observed earlier in photosynthetic proteins likely reflect vibrational motion in isolated pigments, and not delocalized quantum coherence. While long-lived vibronic coherences are of questionable biological relevance at face value, we conclude with a discussion of initial findings using coherently-controlled 2DES, which could assist in unraveling the importance of vibronic coherence in excitation energy transfer in an experimentally direct fashion 30.