Strong Duschinsky Mixing Induced Breakdown of Kasha's Rule in an Organic Phosphor.

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 T1 and T2 states (0.48 eV) is too large to allow for any significant population of the T2 state at room temperature and instead the faster intersystem crossing (ISC) between the S1 and T2 states is rather due to strong Duschinsky mixing, leading to the emission from the T2 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 S1 → T2 compared to those for S1 → T1. We found that the carbonyl moiety of the S1 and T2 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.