Finely tuning electronic and steric structures of spiro[fluorene-9,9′-xanthene] (SFX)-based emitters by isomerization strategy towards efficient electroluminescence

Abstract

Considering the advantages of spiro[fluorene-9,9′-xanthene] (SFX) on facilitating radiative transition and suppressing concentration quenching, a pair of isomers, namely F-BT-F and X-BT-X, were designed and synthesized with benzo[c][1,2,5]thiadiazole (BT) as an acceptor core to bridge fluorene moieties (F-BT-F) or xanthene moieties (X-BT-X) of SFXs. The pair of isomers with D-A-D architecture presented almost the same energy gap in chloroform solution, providing a delicate model to probe the electronic and steric differences between binary structure of SFX. Single-crystal diffractions reveal that SFX-donor units are in antiparallel arrangement on the both sides of the BT core, and X-BT-X molecules have more discrete and ordered packing compared to F-BT-F in crystal phase. Photophysical, electrochemical measurements and DFT calculations manifest that the pair of emitters possess excellent photoluminescence quantum yields up to 95%, and the X-BT-X shows weaker charge-transfer character compared with the F-BT-F, attributed to the strong electron-donating nature and partly interrupted conjugation pathway of xanthene-end relative to fluorene-end. In doped organic light-emitting devices (OLEDs), the F-BT-F and X-BT-X displayed nearly uniform electroluminescence (EL) with the emission peaks at 515 and 518 nm, as well as the maximum external quantum efficiencies (EQEs) approaching 5.0%. In non-doped OLEDs, however, the X-BT-X had more remarkable EL performance with the emission peak at 525 nm, maximum current efficiencies of 14.02 cd A−1, maximum power efficiencies of 8.81 lm W−1 and maximum EQE of 4.25%, exceeding the efficiency values of 13.75 cd A−1, 7.71 lm W−1, and 4.08% of F-BT-F based device. By illuminating the electronic and steric features of xanthene moiety, this work opens a new door to construct high-efficiency and low-cost SFX-based fluorescence emitters.