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 (7AI(2)), 7AI(2)-(h(N)h(C))-(h(N)d(C)), 7AI(2)-(d(N)h(C))-(h(N)d(C)), 7AI(2)-(h(N)h(C))-(d(N)h(C)), and 7AI(2)-(h(N)h(C))-(d(N)d(C)), where It 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 isotoporners 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 7AI(2) dimers except for 7AI(2)-(d(N)h(C))-(h(N)d(C)) in which the excitation is delocalized over the dimer. This indicates that the undeuterated 7AI(2) dimer and its isotopomers that belong to the C-2h symmetry point group should be treated by the weak coupling limit of the exciton theory.