On the Additivity of Molecular Fragment Dipole Moments of 5‐Substituted Indole Derivatives

Abstract

The estimate of the magnitude and the orientation of molecular electric dipole moments from the vector sum of bond or fragment dipole moments is a widely used approach in chemistry. However, the limitations of this intuitive model have rarely been tested experimentally, particularly for electronically excited states. Herein, we find rules for a number of indole derivatives by using rotationally resolved electronic Stark spectroscopy and ab initio calculations. Based on a natural-bond-orbital analysis, we discuss whether the vector additivity rule can be applied in a given electronic state. From a comparison of the experimental data with ab initio calculations, we deduced that the additivity model does not apply when the flow of electron density from the substituent is opposed to that inside the chromophore.

Rotationally resolved electronic spectroscopy of 3-cyanoindole and the 3-cyanoindole–water complex

“…For 5-cyanoindole we found a good additivity of the dipole moments of indole and of the cyano fragment in the electronic ground state. 61 In this molecule, the indole dipole and the dipole of the cyano fragment which is attached to the benzenoid ring are nearly aligned. For 3-cyanoindole, the individual dipoles form a large angle, and the cyano fragment is attached to the pyrrolic ring.…”

Structures, dipole moments and excited state lifetime of isolated 4-cyanoindole in its ground and lowest electronically excited singlet states

“…For 3-cyanoindole, for which the lowest state is of 1 L b character, we found a decrease from 5.89 D to 5.39 D upon electronic excitation, 40 while for 5-cyanoindole (lowest excited singlet state is 1 L a ) an increase of 1 D is found. 66 Thus, the change of permanent dipole moment upon excitation of the singlet state can be used as a sensitive indicator of the electronic nature of this excited state.…”

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

“…Additionally, we compare the results to those of 5FI for which the nature of the lowest excited singlet state was ascertained to be 1 L b . For 5FI, a small decrease in the permanent dipole moment of 0.3 D upon excitation to the S 1 state has been reported …”

“…38 For 5FI, a small decrease in the permanent dipole moment of 0.3 D upon excitation to the S 1 state has been reported. 40 Consequently, a small perturbation of the lowest electronically excited state due to an increased amount of LUMO ← HOMO character, as it is observed for 4FI, only affects the TDM orientation, whereas the change in the permanent dipole moment upon excitation and thus, the electronic nature of the excited state remains 1 L b -like. For 6FI, theory predicts a heavily mixed character of both electronically excited states.…”