Excited-state double-proton transfer in the 7-azaindole dimer in the gas phase. 1. Evidence of complete localization in the lowest excited electronic state of asymmetric isotopomers.

The fluorescence excitation and UV-UV hole-burning spectra are measured for deuterated 7-azaindole dimers (7AI2), 7AI2-(h(N)h(C))-(h(N)d(C)), 7AI2-(d(N)h(C))-(h(N)d(C)), 7AI2-(h(N)h(C))-(d(N)h(C)), and 7AI2-(h(N)h(C))-(d(N)d(C)), where h and d indicate the H and D atoms, respectively, and the subscripts N and C designate the imino N-H (or N-D) and C-H (or C-D) bonds at the 3-position. The transitions of the isotopomers have been consistently assigned. It has been found that the excitation is completely localized on one monomer unit in the lowest excited electronic state of asymmetrical 7AI2 dimers except for 7AI2-(d(N)h(C))-(h(N)d(C)) in which the excitation is delocalized over the dimer. This indicates that the undeuterated 7AI2 dimer and its isotopomers that belong to the C2h symmetry point group should be treated by the weak coupling limit of the exciton theory.