Modulation of the L_a/L_b 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 L_a or L_b, 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 L_a 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 L_a is weak, the nature of the S₁ state can still be assigned to be mainly L_b based on the excited state dipole moment. For 6-fluoroindole, this is not the case anymore, and the L_a/L_b nomenclature completely breaks down due to heavily mixed excited states.