Perturbations of the Fully Resolved Electronic Spectra of Large Molecules by the Internal Rotation of Attached Methyl Groups. Influence of Complex Formation

Rotationally resolved S₁←S₀ fluorescence excitation spectra of three methylindoles and their single atom van der Waals complexes with argon have been obtained. Each spectrum is extensively perturbed by the hindered internal rotation of the methyl group. Analyses of these perturbations show that the barriers to such motion are substantially increased by complex formation. Barriers of this type are primarily electronic in origin. Thus, even at the relatively large distances (3.5 Å) found in van der Waals complexes, the attachment of a weakly bound argon atom has a significant effect on the electron distribution in the indole ring.