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.