π out electrons, the S 0 →T 1 excitation must correspond to an π in → π out, * type orbital transition (Figure (a)), while the S 0 →Q 1 is a double-electron excitation with the form of π in /π out → π in, */π out, *, which is characterized by the four BOs (Figure corresponds to short π bonds (relatively strong), while the particle BO corresponds to long π bonds (relatively weak), which is why the former has lower energy than the latter. Green and blue correspond to positive and negative phases, respectively. The electron transitions corresponding to S 0 →T 1 and S 0 →Q 1 excitations are indicated by the arrows.