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.