Fluorene radical anion isomers: Computed photodetachment spectra, singlet–triplet energy splittings, and computational thermochemistry

Fluorene (FL) possesses a characteristic π-electron conjugation, resulting in its diverse utilization as a molecular scaffold in fields such as organic electronics, biochemistry, combustion science, and the synthesis of electroluminescent materials. Whereas the vibrational structures of the ground singlet (S0) and the lowest-lying triplet (T1) electronic states in 9H-FL have previously been investigated using techniques such as matrix-isolation Fourier-transform infrared spectroscopy, electron attachment to FL isomers remains largely unexplored. In this work, geometries and vibrational frequencies from the density functional theory B3LYP/6-311++G (2d, 2p) level of theory are utilized to predict the vibrational structures of the S0 and T1 states in 9H-FL, 1H-FL, 2H-FL, 3H-FL, and 4H-FL via the computation of the Franck-Condon factors. Whereas the S0 state in 9H-FL is unbound, the 1H-FL, 2H-FL, 3H-FL, and 4H-FL isomers form stable anions and have calculated adiabatic electron affinity values of 0.990, 1.282, 0.569, and 1.561 eV. The adiabatic origins of the T1 states in 1H-FL, 2H-FL, 3H-FL, 4H-FL, and 9H-FL lie at 2.350, 1.706, 2.271, 1.790, and 2.777 eV above their respective anionic states, yielding singlet-triplet energy splittings ΔEST of 1.360, 0.421, 1.701, 0.233, and 2.780 eV. The negative ion photoelectron spectra representing the electron detachment to form the S0 and T1 states exhibit pronounced 0-0 transitions that are suggestive of the subtle geometric changes between the anions and corresponding neutrals. The computed thermochemistry data reveal that the 9H-FL has the strongest sp3 C-H bond dissociation energy and is the least acidic.