The lowest excited singlet states of 1-azabicycly[2.2.2]octane: Fluorescence excitation spectroscopy and density functional calculations

Abstract: The lowest excited singlet states of the structurally rigid amines 1-azaadamantane and 1-azabicyclo[2.2.2]octane have been investigated by using fluorescence excitation spectroscopy on samples seeded in supersonic expansions. Based upon the notion that in both species the lowest excited singlet state is a Rydberg state with the ground state of the radical cation as its ionic core, excitation spectra have been analyzed by employing density functional calculations of the equilibrium geometries and force fields of the ground state of the neutral species and its radical cation. A good agreement is obtained between experimentally observed and theoretically predicted frequencies and intensities of vibronic transitions. Subsequent refinements of the geometry of the lowest excited singlet state are shown to account adequately for the minor differences between experiment and the computational results obtained by using the radical cation as a model for the lowest excited singlet state. From our analysis it also becomes apparent that the excited state is in both molecules subject to vibronic coupling with higher-lying excited states, as exemplified by the presence of transitions to non-totally symmetric vibrational levels. The results of the present study enable the determination of mode-specific reorganization energies accompanying ionization of 1-azaadamantane, which are shown to correspond qualitatively well with those determined in resonance Raman studies on the charge transfer transition in the electron donor-acceptor system 1, which contains 1-azaadamantane as the electron donor unit.