Orientational and Solvent Couplings of Molecules at Interfaces by Interface-Specific Two-Dimensional Vibrational-Electronic (i2D-VE) Spectroscopy

Vibronic coupling (VC) is especially important in excited state systems and results in the overlap of both electronic and vibrational adiabatic potential energy surfaces. Previously, two-dimensional vibrational-electronic spectroscopy has been developed for VC for studies in bulk systems. Here we demonstrate interface-specific 2D vibrational-electronic spectroscopy (i2D-VE) to directly study interactions of vibrational and electronic motions of azo-dye molecules at the air/water interfaces. After laying the theoretical framework for i2D-VE, we used it to quantify relative orientations of the coupled vibrational and electronic dipoles. We further examined time-dependent i2D-VE spectra for interfacial water coupling with vibrational and electronic transitions. Comparisons with simulated i2D-VE spectra correlated with experimental data to confirm the relationship between the interfacial solvation structure and VC therein. These results open venues to track the orientational coupling, dynamic solvent coupling, and structural evolution of photoinduced excited states at interfaces and surfaces.