A Triplet Label Extends Two‐Dimensional Infrared Spectroscopy from Pico‐ to Microseconds

In conventional two‐dimensional infrared (2D‐IR) spectroscopy, the inherently short vibrational lifetimes limit the time window to observe molecular dynamics typically to tens of picoseconds. The rather complex dynamics of organized molecular systems (e.g., glass formers, polymers, membranes, proteins), however, span a wide range of timescales from femto‐ to microseconds and beyond. Vibrationally Promoted Electronic Resonance (VIPER) 2D‐IR negates the limitations of 2D‐IR spectroscopy, for its signal decays with the electronic lifetime. Here, we present 2‐Isopropylthioxanthone as the first VIPER 2D‐IR probe to exploit intersystem crossing, thereby covering even the microsecond timescale. We achieved the required signal‐to‐noise ratio and resolution by introducing the Fourier‐transform approach to the VIPER 2D‐IR pulse sequence. Now, we are in a position to monitor dynamics via spectral diffusion several orders of magnitude beyond the vibrational lifetime of 2D‐IR labels.