We present the novel observation that Duschinsky mixings can lead to the breakdown of Kasha's rule in a white light phosphor molecule, dibenzo[b,d]thiophen-2-yl (4-chlorophenyl)methanone. Our theoretical analyses show the energy gap between the T 1 and T 2 states (0.48 eV) is too large to allow for any significant population of the T 2 state at room temperature and instead the faster intersystem crossing (ISC) between the S 1 and T 2 states is rather due to strong Duschinsky mixing, leading to the emission from the T 2 state as well. A second-order cumulant-based method has been used for the calculation of the ISC rate, which suggests 2 orders of magnitude faster ISC rates for S 1 ? T 2 compared to those for S 1 ? T 1 . We found that the carbonyl moiety of the S 1 and T 2 states of the molecule is significantly different with respect to bond angle and dihedral angles, engendering large displacements in selective normal modes, thus giving rise to strong Duschinsky mixing. Copyright © 2019 American Chemical Society.