AbstractThe absorption and emission properties of thiofluorenone (TF, C2vmolecular symmetry) were investigated in 3‐methylpentane at 77 K. By comparison with the well established fluorenone absorption spectrum and in conjunction with polarization measurements the lowest electronic transitions were assigned. They are in order of increasing energies:3A2(π,π*) π1A1(f1.5 ± 10−5),1A2(π,π*) π1A1(f= 1.5 · 10−4), B2(π,π*) ·1A1(f= 6 · 10−3),and B2(π,π*) ·1A1(f= 0.27). It is suggested that the orbitally forbidden, long axis polarized1π,π* transition receives its intensity trough vibronic coupling from B2states. The radiative liefetime, as determined from the emission quantum yield and measured lifetime of TF, compared with the same parameter of thiobenzophenone, allows the assignment of the emission as phosphorescence. The short phosphorescence lifetime (πp(77 K) = 15 μs; πp(77 K) = 0.035) implies a very effective spin‐orbit coupling, a property which is also manifested in the high intersystem crossing quantum y