Conformational Switching by Vibrational Excitation of a Remote NH Bond

Here we describe an unprecedented reversible interconversion between two conformational states, accomplished by selective vibrational excitation of a bond remotely located in relation to the isomerizing fragment. In contrast to previous studies reporting conformational changes via vibrational excitation of a nearby OH group, in this study the transformations were successfully achieved by excitation of a distant NH stretching coordinate. The syn and anti forms of monomeric 6-methoxyindole, which differ in the orientation of the methoxy group, were trapped in low-temperature inert matrixes and characterized spectroscopically. These forms could be selectively shifted in both directions by using near-IR excitations tuned at the frequency of the first NH stretching overtone. The observed isomerization proves the possibility of efficient vibrational relaxation to carry the energy deposited at the NH stretching coordinate into the reactive C-O torsional mode localized on the methoxy group four bonds away from the excited NH moiety.