Impact of iodine-substitution on the symmetry and room-temperature phosphorescence behavior of thienyl diketone skeleton

Introducing heavy atoms, or replacing atoms with heavier ones, is a routine approach for accelerating spin-flipping photophysical processes. However, predicting its impact on phosphorescence efficiency is not straightforward. Herein, we report an unexpected consequence of bromine-to-iodine substitution in a bromothienyl diketone derivative, TIPS-BrTn, that exhibits outstanding room-temperature phosphorescence (RTP) in cyclohexane solution. Contrary to our expectation, the iodo-congener TIPS-ITn exhibited feeble photoluminescence, which we confirmed as RTP by ultrafast spectroscopy. Further experimental and theoretical studies revealed that, in the T₁ state, an excited-state symmetry breaking occurred on TIPS-ITn while TIPS-BrTn preserved the centrosymmetric geometry. We identified the driving force for the symmetry breaking as an intramolecular two-center three-electron bonding interaction between iodine and carbonyl oxygen in the (n,π*) excited state. Consequently, while the direct T₁-S₀ spin-orbit coupling (SOC) in TIPS-BrTn is symmetry-forbidden and zero, that of TIPS-ITn is non-zero due to the loss of centrosymmetry, thereby accelerating nonradiative T₁-S₀ decay to diminish the RTP. Importantly, the phosphorescence rate constant is not solely dictated by the direct T₁-S₀ SOC; instead, it can be rationalized by the intensity borrowing from higher singlet states. Thus, our work highlights the importance of controlling molecular symmetry, which could suppress the direct T₁-S₀ SOC and lead to a preferential acceleration of radiative decay over nonradiative decay for achieving efficient RTP.