How vibrational excitation shakes up the UV/VIS spectrum: A 2D-VE study.

Recently, powerful experimental techniques have emerged that use infrared vibrational excitation to modulate UV/VIS spectra for applications such as subensemble-selective photochemistry in VIPER 2D-IR spectroscopy, vibrationally enhanced multiplexing in fluorescence imaging, single molecule vibrational spectroscopy, and IR-induced modulation of photocurrents in optoelectronic devices. Although these approaches rely on IR induced modulation of electronic spectra, they do not directly measure it. Here, we combine ultrafast two-dimensional vibrational-electronic (2D-VE) spectroscopy and theoretical spectroscopy to directly probe how IR pre-excitation impacts the UV/VIS spectrum, using the dye coumarin 6 as an example. We find that IR excitation does not simply shift the electronic absorption band but produces complex spectral changes arising from multiple vibronic transitions. Simulations reproduce the experimental spectra and reveal that dominant contributions originate from changes near the 0-0 transition rather than from a single red-shifted M-0 transition. In addition, oscillatory features in the 2D-VE spectra are observed and assigned to zero-quantum coherences between vibrational modes, demonstrating that vibrational coherences can strongly influence signal amplitudes in VIPER-type experiments. Finally, the correlation between vibrational and electronic frequencies enables the separation and analysis of molecular subensembles, as illustrated for hydrogen-bonded and free coumarin 6 in mixed solvents. These results provide a detailed microscopic picture of how vibrational excitation modifies electronic spectra and offer important insight for understanding and optimizing vibrationally promoted electronic resonance techniques.