The energetics of the BA derivatives are given in Table and Table . The S1 The most prominent feature in the state levels of BA and its derivatives is the relative energy gap between 3 n* and 3 * states. In the sequence of increasing electron donation -H < -CH3 < -OCH3 < -N(CH3)2, the 3 n* state is destabilized while the 3 * state is stabilized, as shown in Figure . All four species have a 3 n* and a 3 * triplet minimum. The lowest triplet minimum in BA and MeBA is 3 n*. In MoBA and DMABA, the lowest minimum is 3 *. The inversion occurs around MeBA, where the two minima are nearly degenerated, with 3 n* only 0.01 eV more stable than 3 *. This result nicely agrees with the measurements reported in Ref. . In MoBA, the 3 * minimum is lower than the 3 n* minimum by 0.06 eV, also in excellent agreement with the experimental estimates between 0.04 and 0.11 eV . For BA, MeBA, and MoBA, both 3 n* and 3 * minima are in T1. In DMABA, the 3 n* minimum is in T2. Table . Ground and excitation energies (eV) at the lowest 3 n* and 3 * triplet minima of BA and its derivatives calculated with TD-B3LYP/6-31G(d,p). The state character is also indicated. In BA, MeBA, and MoBA, both minima are T1. In DMABA, they are T1 and T2. The excitation energies are relative to the ground-state minimum energy.