Enantio-selective vibronic coupling revealed via dissymmetry of upconverted emission in double-resonance excitation of chiral polymer.

Double-resonance dual-beam spectroscopy, involving vibrational and electronic transitions, is a powerful tool allowing to resolve vibronic coupling in complex molecular systems. Here, introducing resonant three-photon excitation with mid-infrared beam in vibrational resonance, near-infrared beam to connect to a two-photon resonant electronic state, and endowing both beams with circular polarization, we demonstrate enantio-selective vibronic coupling resulting in deep amplitude modulation of coherent upconverted emission. Quantum chemical calculations confirm that helical supramolecular organization of the polymer leads to strong coupling of the local nuclear motion at the chiral sidechains with the delocalized electronic motion of molecular π-system upon its resonant excitation. Time-resolved measurements reveal that local vibrational relaxation is not sensitive to the beam polarization state, which leads to lower dissymmetry of incoherent upconverted fluorescence. Our findings highlight the importance of chiral control of vibronic coupling for achieving deep enantio-selective modulation of coherent upconverted emission for various applications in chiral photonics.