Modulation of the La/Lb Mixing in an Indole Derivative: A Position-Dependent Study Using 4-, 5-, and 6-Fluoroindole.

The lowest two electronically excited singlet states of indole and its derivatives are labeled as La or Lb, based on the orientation of the transition dipole moment (TDM) and the magnitude of the permanent electric dipole moment. Rotationally resolved electronic Stark spectroscopy in combination with high level ab initio calculations offers the possibility to determine these characteristics and thus the electronic nature of the excited states. In the present contribution this approach was pursued for the systems 4- and 6-fluoroindole and the results compared to the previously investigated system 5-fluoroindole. Changing the position of the fluorine atom from 5 to 4 or 6 is accompanied by an increasing amount of La character in the S1 state. This dramatically influences the orientation of the TDM and erases its ability to be a reasonable identifier of the nature of the excited states for both molecules. However, for 4-fluoroindole, where the influence of the La is weak, the nature of the S1 state can still be assigned to be mainly Lb based on the excited state dipole moment. For 6-fluoroindole, this is not the case anymore, and the La/Lb nomenclature completely breaks down due to heavily mixed excited states.