Quantifying QX → QY Energy Relaxation in Chlorophyll a

Abstract

Chlorophyll a is a prototypical porphyrinoid chromophore featuring complex electronic structure, which plays a central role in oxygenic photosynthesis. In particular, the properties of the lowest-energy Q-band govern light absorption, ultrafast energy transfer, and charge separation processes. However, the precise time scale of intramolecular energy relaxation between its two lowest electronic states, QX and QY, a key step in chlorophyll photophysics, eluded accurate quantification. Here we directly measure this relaxation time in chlorophyll a and determine a definitive time scale of 25 fs. We used sub-10 fs pulses and a specially developed polarization-control scheme for two-dimensional electronic spectroscopy to selectively isolate population-transfer pathways. This disentangles ultrafast energy relaxation from the spectral congestion, pulse overlap signals, and coherent artifacts. The measured time scale establishes that the vibronically-mixed QX state functions as a transient intermediate, rapidly funneling excitation into the lowest-energy QY state, consistent with the non-adiabatic vibronic coupling model. This result resolves a long-standing ambiguity in chlorophyll photophysics and provides an experimental benchmark for vibronic mixing models describing vibroelectronic properties of porphyrinoid chromophores and intramolecular energy relaxation. Furthermore, the presented experimental approach will be indispensable for determining ultrafast photophysical and photochemical processes in various molecular systems and will help to unravel hitherto-hidden molecular mechanisms.