Effects of asymmetric acceptor and donor positioning in deep blue pyridyl-sulfonyl based TADF emitters

Abstract

In this work, we report synthesis and photophysical properties of deep-blue emitting donor-acceptor (D-A) and donor-acceptor-donor (D-A-D) thermally activated delayed fluorescence (TADF) molecules, where the molecules designed using carbazole as a donor (D) and a pyridyl (a)-sulfonyl (A) based bifunctional group as an acceptor. The work reveals how structural changes favor reverse intersystem crossing (rISC) by forming emissive charge transfer (CT) state, which is thoroughly investigated in different donor and asymmetric acceptor positions. Three comparison sets of regioisomers are investigated. 2,5-substituted pyridine derivatives in Set-1, are D-Aa, D-aA and D-Aa-D structures with asymmetric acceptor systems, revealing that the donor nearer to the pyridine group substantially controls the TADF properties In Set-2, modified the D-Aa-D structures reveal how ortho and meta positioned a relative to A (keeping the carbazole at meta to the A) affects the emission properties, deactivating TADF and promotion triplet-triplet annihilation. In the final set, 2,4-substituted pyridyl-sulfonyl derivatives show that the positioning of the donor far from the pyridine group has minimal influence. This final set of molecules show superior optical and physical properties though, indicating the importance of correct positioning between D, a, and A.