Nuclear vibrations play a prominent role in the spectroscopy and dynamics of electronic systems. As recent experimental and theoretical studies suggest, this may be even more so when vibrational frequencies are resonant with transitions between the electronic states. Herein, a vibronic multilevel Redfield model is reported for excitonically coupled electronic two‐level systems with a few explicitly included vibrational modes and interacting with a phonon bath. With numerical simulations the effects of the quantized vibrations on the dynamics of energy transfer and coherence in a model dimer are illustrated. The resonance between the vibrational frequency and energy gap between the sites leads to a large delocalization of vibronic states, which then results in faster energy transfer and longer‐lived mixed coherences.

Observing excitonic effects in squaraine heterodimers, coherently and fluorescence-detected 2D electronic spectroscopy methods show their strengths, weaknesses, similarities and differences.

We measure excitation energy transport in conjugated polymers using direct observation of exciton pair dynamics.

Significance: The excitation energy transfer in light-harvesting complexes is usually studied either by ultrafast bulk spectroscopy or by single-molecule spectroscopy. These methods are to a high degree complementary: Bulk spectroscopy measures ultrafast processes averaged over thousands of complexes, whereas single-molecule spectroscopy observes much slower dynamics in individual complexes. In this work, we combine these approaches using a recently developed ultrafast single-molecule spectroscopy technique. This enables us, for the first time (to our knowledge), to observe ultrafast energy relaxation with a rate around 100 fs in individual light-harvesting complexes. We determine the distribution of the relaxation times, observe changes of the relaxation time in one complex, and find how the relaxation depends on excitation wavelength.