Ab initio, DFT, and spectroscopic studies of excited-state structure and dynamics of 9-ethylfluorene

Fluorescence excitation and resonant two-photon ionization spectra were measured for 9-ethylfluorene (EFR) molecule cooled in pulsed supersonic expansion of He in the range of 286.5 ∼ 300 nm. The structures and energies of the global and local minima and the transition states separating them are evaluated with the B3LYP/6-31G(d) and MP2/cc-pVTZ methods. It is found that the vertical excitation energies of fluorene and the EFR conformers can be reliably predicted by the time-dependent DFT method within 8.72%. The vibrational bands above the electronic origin are assigned on the basis of the RCIS/6-31G(d) calculation. Ethyl (or ethene) elimination from the excited neutral and/or ionic molecule is presumed to occur as an activation process along the adiabatic potential energy surface.