The interest in intersystem crossings (ISC) or spin crossovers has been intensified by the advent of femtosecond time-resolved spectroscopic studies showing that ISC may occur at the picosecond timescale. In the case of xanthone, 1 the 1 nπ * -→ 3 ππ * ISC takes place within ∼ 2.0 ps right after an ultrafast 1 ππ * -→ 1 nπ * internal conversion (IC). Wolf et al. have also obtained a circumstantial experimental evidence of the population transfer to triplet states on the timescale of 3.5±0.3 ps in the photo-dynamics of thymine. El-Sayed's rule forbids ISC between singlet and triplet (doublet and quartet, triplet and quintet, etc.) states with the same occupation scheme.