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

Abstract

The lowest two electronically excited singlet states of indole and its derivatives are labeled as L or L, 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 character in the S 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 is weak, the nature of the S state can still be assigned to be mainly L based on the excited state dipole moment. For 6-fluoroindole, this is not the case anymore, and the L/L nomenclature completely breaks down due to heavily mixed excited states.

Multireference Approaches for Excited States of Molecules

“…Relevant to this mechanism, the effect of the vibronic coupling of 1 L a and 1 L b states has been explored at the CASSCF and DFT/MRCI levels . DFT/MRCI, CC2, and SCS-CC2 methods have also been used to study the character of S 1 and S 2 states of modified 4-, 5-, and 6-fluoroindole . Notably, the original CC2 method fails to describe the mixed character of states predicted by the DFT/MRCI and SCS-CC2 methods.…”

“…states of modified 4-, 5-, and 6-Fluoroindole. 890 Notably, the original CC2 method fails to describe the mixed character of states predicted by the DFT/MRCI and SCS-CC2 methods.…”

“…859 DFT/MRCI, CC2, and SCS-CC2 methods have also been used to study the character of S 1 and S 2 states of modified 4-, 5-, and 6-fluoroindole. 883 Notably, the original CC2 method fails to describe the mixed character of states predicted by the DFT/MRCI and SCS-CC2 methods.…”

Tryptophan as a Template for Development of Visible Fluorescent Amino Acids

“…58 In comparison, however, for both 4-methoxyindole and 4-fluoroindole, the S 1 state was found to be representative of the 1 L b character. 59,60 These results are interesting in that the absorption spectra of 4CN-indole and 4CO 2 H-indole are red-shifted from that of indole and that the CN and COOH groups are strong EWGs, whereas F is a weak EWG and the absorption spectrum of 4-fluoroindole is blueshifted. 46 In addition, the permanent dipole moment of the S 1 state of 4CN-indole was determined to be 9.60 D, which is ca.…”

Theoretical Investigation of Positional Substitution and Solvent Effects on n-Cyanoindole Fluorescent Probes

“…As seen in Figure , the transition dipole moments (TDM) of L b (1 ππ *) and L a (2 ππ *) are nearly orthogonal in a vacuum and there is a higher probability of populating the L a (2 ππ *) state than the L b (1 ππ *). Addition of a weakly electron-withdrawing group such as fluorine on different positions of indole has been found to dramatically influence the orientation of the TDM, the magnitude of the permanent dipole moment, and consequently, the L a /L b nature of the S 1 minima. − As a result, the orientation of the TDM and magnitude of the permanent electric dipole moment have been used to determine the character of the S 1 minima for substituted indoles.…”