Quantum Beats and Phase Shifts in Two-Dimensional Electronic Spectra of Zinc Naphthalocyanine Monomer and Aggregate

The origin of quantum coherence in two-dimensional (2D) electronic spectra of molecular aggregates and light-harvesting complexes still remains an open question. In particular, it could be challenging to distinguish between electronic and vibrational coherences for a coupled system, where both degrees of freedom can be simultaneously excited. In this Letter, we examine quantum beats in the 2D spectra of zinc naphthalocyanine (ZnNc) aggregate and monomer, and compare their characteristic features in terms of the frequency and relative phase of diagonal and off-diagonal amplitude oscillations. The long-lasting oscillating components (>1 ps) at 600–700 cm–1 observed in both the aggregate and monomer are found to be attributed to the vibrational coherence. The wide phase variations of the 2D spectral amplitude oscillations are observed not just in the aggregate but also in the monomer state. This suggests that the unusual 90° phase shift may be attributed to neither quantum population-to-coherence transfer nor vibronic exciton coupling.