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Whereas the vibrational structures of the ground singlet (S0) and the lowest-lying triplet (T1) electronic states in 9H\u2013FL 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\u2013FL, 1H\u2013FL, 2H\u2013FL, 3H\u2013FL, and 4H\u2013FL via the computation of the Franck\u2013Condon factors. Whereas the S0 state in 9H\u2013FL is unbound, the 1H\u2013FL, 2H\u2013FL, 3H\u2013FL, and 4H\u2013FL isomers form stable anions and have calculated adiabatic electron affinity values of 0.990, 1.282, 0.569, and 1.561\u00a0eV. The adiabatic origins of the T1 states in 1H\u2013FL, 2H\u2013FL, 3H\u2013FL, 4H\u2013FL, and 9H\u2013FL lie at 2.350, 1.706, 2.271, 1.790, and 2.777\u00a0eV above their respective anionic states, yielding singlet\u2013triplet energy splittings \u0394EST of 1.360, 0.421, 1.701, 0.233, and 2.780\u00a0eV. The negative ion photoelectron spectra representing the electron detachment to form the S0 and T1 states exhibit pronounced 0\u20130 transitions that are suggestive of the subtle geometric changes between the anions and corresponding neutrals. The computed thermochemistry data reveal that the 9H\u2013FL has the strongest sp3 C\u2013H bond dissociation energy and is the least acidic.<\/jats:p>","DOI":"10.1063\/5.0353580","type":"journal-article","created":{"date-parts":[[2026,10,9]],"date-time":"2026-10-09T10:46:26Z","timestamp":1791542786000},"update-policy":"https:\/\/doi.org\/10.1063\/aip-crossmark-policy-page","source":"Crossref","is-referenced-by-count":0,"title":["Fluorene radical anion isomers: Computed photodetachment spectra, singlet\u2013triplet energy splittings, and computational thermochemistry"],"prefix":"10.1063","volume":"165","author":[{"ORCID":"https:\/\/orcid.org\/0009-0008-4088-4660","authenticated-orcid":false,"given":"Wyatt W.","family":"Tunstall","sequence":"first","affiliation":[{"name":"Department of Chemistry, Tennessee Tech University 1 , 1 William L. Jones Dr., Cookeville, Tennessee 38505,","place":["USA"]},{"name":"Department of Chemical Engineering, Tennessee Tech University 2 , 1 William L. 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