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Is quantum coherence responsible for the surprising efficiency of natural light harvesters? If so, how do such systems avert the loss of coherence due to interaction with their warm, wet and noisy environments? The answer to these important questions rests in the fruitful interplay between electronic and vibrational degrees of freedom, leading to enhanced excitation energy transport and long-lasting superpositions. Here we report experimental and theoretical verification of exciton-vibrational (vibronic) coupling as the origin of long-lived coherences in an artificial light harvester. We investigate a macroscopically aligned bi-tubular J-aggregate (C8O3), for which polarization controlled 2D spectroscopy delivers a clean, uncongested optical response. This allows us to explain the longevity of the observed coherences (1.2 ps at room temperature) by vibronic coupling and to rule out other explanations based upon correlated fluctuations.
The discussed vibronic coupling mechanism is functionally relevant, as it describes interactions in the electronically excited state and hence bears potential for enhancing energy transfer. For the biologically relevant donor-acceptor system of carotenoid and bacteriochlorophyll, we show that vibronic coupling is responsible for the ultrafast electronic population transfer on a 40 fs timescale.
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