Extending El Sayed's Rule by ¹(n,π*)-³(n,π*) Rotation via Intramolecular Secondary Interaction Enables...

The development of purely organic chiral room temperature (RT) phosphorescent emitters is in the limelight. However, the key parameters governing the polarized luminescence are difficult to predict and rationalize since the phosphorescence emission is rarely obtained in solution, despite the introduction of heavy atoms (Br, I) or carbonyl groups, and engineering of total angular momentum, for example, singlet/triplet states corresponding to n-π*/π-π* transitions (or vice-versa), as prescribed by El-Sayed's rule. Here, we show that intramolecular secondary interactions can induce an efficient ISC between formally identical ¹(n,π*) and ³(n,π*) states in chiral thionated phthalimide derivatives. This unprecedented type of inter-system crossing (ISC) extends the usual thinking of the El-Sayed principle and enables the realization of circularly polarized (CP) RT phosphorescence (RTP) in solution with g_lum of up to 4 × 10^-3. These results bring additional molecular design guidelines to reach CP-RTP in solution and offer new insights into the subtle relationships between excited states of different spin multiplicities that enable higher CP phosphorescence intensity.