Elucidating state-specific dynamics of 1La and 1Lb in cyanoindole derivatives using UV/Vis pump IR detection

Abstract

Indole possesses two close-lying excited states, namely 1La and 1Lb, which are difficult to distinguish using electronic spectroscopy due to the severe spectral overlap. Using UV/Vis pumped IR detection, we demonstrated that two well-separated excited-state vibrational bands are from the 1La and 1Lb states with the indole derivatives having CN substituted on the six-membered rings (4-7CNIs) in ethanol. We observed a population redistribution process between 1La and 1Lb with a time constant of ∼20 ps in 4-7CNIs, which is driven by the excitation-induced solvation relaxation in ethanol. In 3CNI, where the CN is attached on the five-membered ring, only a single peak corresponding to the 1Lb state was detected. Our work shows that with the prolonged nanosecond lifetime and enhanced oscillator strength compared to the ground-state CN stretch, the CN vibrational stretches on the excited states of CNIs can provide unique and benchmarking information on the state-specific dynamics and properties.