Targeting Molecular Subpopulations of a Hydrogen-Bonded Catalysis Complex: Insights from 2D Vibrational Electronic Spectroscopy

Abstract

Selective electronic excitation of specific molecules or subpopulations within a mixture is challenging when their ultraviolet/visible (UV/VIS) spectra are strongly overlapping. Vibrationally Promoted Electronic Resonance (VIPER) excitation addresses this challenge by combining the selectivity of infrared (IR) excitation with UV/VIS excitation, which triggers the desired processes. The IR excitation modulates the UV/VIS spectrum of the selected subpopulation via vibronic couplings, shifting it into resonance with a subsequent off-resonant UV/VIS pump pulse. Here, we demonstrate that 2D vibrational electronic (2D-VE) spectroscopy is ideally suited to measure such IR-induced UV/VIS modulations. As a case study, we investigate the dynamic equilibrium of Schreiner’s thiourea catalyst binding a coumarin substrate. We find the substrate’s carbonyl (CO) stretching vibration to be vibronically coupled to the electronic transition of the coumarin and also to show distinct vibrational frequencies in the bound and free state, rendering it an excellent handle for future VIPER studies. Moreover, we show that direct vibronic coupling is not the sole pathway to subpopulation-selective excitation: vibrational energy transfer (VET) from the catalyst to the substrate through hydrogen bonds can likewise modulate the UV/VIS spectrum, providing an alternative mechanism for selective excitation. We further demonstrate that IR excitation of Fermi resonances generates additional UV/VIS modulations, which must be considered when planning VIPER-type experiments. Selective excitation of molecular subpopulations through vibronic couplings opens new possibilities to disentangle the kinetics of distinct species involved in photoreactions and advance our understanding of complex photocatalytic systems.