Identity of T* Matters! Improved k_rISC by Modulating Locally Excited Triplet State in TADF Emitters

<b>Abstract:</b><br/>Effective triplet exciton utilization efficiency is an essential factor for constructing low-power operational OLED devices with high luminescence efficiency and durability. Thermally activated delayed fluorescence assists this 100% triplet harvesting by converting dark triplet excitons into radiative singlet excitons via a reverse intersystem crossing process. The enhancement of this rISC rate (<i>k</i>_rISC) is primarily realized by the low energy offset between lowest S & T states; to pursue an extremely large <i>k</i>_rISC, the involvement of high energy locally excited triplet states has become a hot topic in TADF-OLED research in present days. However, the regulation of the ³LE states to match precisely with the ¹CT energy level by suitable molecular design is still under progress.<br/>Here, we demonstrated an effective electronic coupling between ¹CT & ³LE or hybridized triplet state in a series of newly designed and synthesized TADF emitters with strongly twisted D-A structure. The detailed theoretical and experimental investigation reveals that the addition of peripheral donor units to the core D-A backbone incorporates multiple triplet excited states of locally excited or hybridized nature between S₁ and T₁ states, and the close alignment of ¹CT & ³LE states accelerates the spin-flip process and the sizeable radiative rate results in suppressed efficiency roll off with short (ns) delayed lifetime. On the contrary, the isoenergetic alignment of ¹CT and ³CT states realized due to the near orthogonal structure promotes the rISC mechanism. Our work highlights the pivotal role of the electronic nature of the intermediate triplet states in controlling the <i>k</i>_rISC;a thorough photophysical investigation was also performed by manipulating ³LE states by introducing variable donor units to the D-A backbone.