These two selection rules are weak because the perturbative expansion sums over many L and K states. Thus, it is always likely that some states will satisfy them even for a highly symmetric structure. Physically, however, it is relevant whether the low-order terms satisfy the conditions or not since they contribute the most to the singlet-triplet transition dipole. Thus, for our analysis, we restrict the perturbative contributions in Eq. ( ) to contain only S0, S1, T1, and T2 states (L = 0, 1 and Let us now apply these conditions to BA. Although the T1 minimum has Cs symmetry, we should at least work with C2v point group to be able to distinguish between allowed and forbidden matrix elements. The T1 spin density shown in Figure indicates that BA can be approximately treated as a C2v system. As discussed in the previous section, BA has two T1 minima, with 3 n* (~B1) and 3 * (~A1) characters (see TDDFT results in SI-1). In both minima, S0 is ~A1 and S2 is ~A2 ( 1 n*). At the 3 n* minimum, T2 is a 3 * (~A1) while at 3 * minimum, T2 is an 3 n* (~B1). In C2v, x, y, and z transform as B1, B2, and A1, respectively. In turn, Rx, Ry, and Rz transform as B2, B1, and A2, respectively. For the 3 n* minimum, the first selection rule [Eqs. ] imposes that the only non-null SOC elements in Eq. ( ) are