THE LOWEST EXCITED SINGLET-STATES OF 1-AZAADAMANTANE AND 1-AZABICYCLO[2.2.2]OCTANE - FLUORESCENCE EXCITATION SPECTROSCOPY AND DENSITY-FUNCTIONAL CALCULATIONS

The lowest excited singlet states of the structurally rigid amines 1-a zaadamantane and 1-azabicyclo[2.2.2]-octane have been investigated by using fluorescence excitation spectroscopy on samples seeded in supers onic 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 analyz ed by employing density functional calculations of the equilibrium geo metries and force fields of the ground state of the neutral species an d its radical cation. A good agreement is obtained between experimenta lly observed and theoretically predicted frequencies and intensities o f vibronic transitions. Subsequent refinements of the geometry of the lowest excited singlet state are shown to account adequately for the m inor differences between experiment and the computational results obta ined by using the radical cation as a model for the lowest excited sin glet state. From our analysis it also becomes apparent that the excite d 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 energie s accompanying ionization of 1-azaadamantane, which are shown to corre spond qualitatively well with those determined in resonance Raman stud ies on the charge transfer transition in the electron donor-acceptor s ystem 1, which contains 1-azaadamantane as the electron donor unit.