Positional isomerism effect of spirobifluorene and terpyridine moieties of “(A)n–D–(A)n” type electron transport materials for long-lived and highly efficient TADF-PhOLEDs

Abstract

Combining rigid twisted spirobifluorene with two strongly electron-withdrawing terpyridine moieties to form a “(A)_n–D–(A)_n” structure is an effective way to achieve electron transport materials (ETMs) with high triplet energy, suitable frontier orbital levels, excellent thermal stability and electrochemical stability for long-lived and highly efficient organic light-emitting diodes (OLEDs), 2,2′-di([2,2′:6′,2′′-terpyridin]-4′-yl)-9,9′-spirobi[fluorene] (22-TPSF) and 2,7-di([2,2′:6′,2′′-terpyridin]-4′-yl)-9,9′-spirobifluorene (27-TPSF), both of which are better than the conventional ETM 1,3,5-tris(N-phenylbenzimidazol-2-yl-benzene (TPBi). In addition, the crystal packing mode in their single crystals undergoes a significant transformation from the sandwich arrangement of 22-TPSF into the brick wall arrangement of 27-TPSF, causing a big difference in electron transport mobility, which changes from 0.012 to 0.104 cm² V⁻¹ s⁻¹ as calculated through density functional theory. This variation leads to a phenomenon where the 22-TPSF based devices display a lower maximum external quantum efficiency of 22.9% and a shorter half-life (T₅₀) of 173 925 hours at an initial luminance of 100 cd m⁻² than the 27-TPSF based devices. These findings highlight the great potential of the ETM structured as “(A)_n–D–(A)_n” using the terpyridine and spirobifluorene moieties; moreover, the positional isomerism effect allows remarkable tuning of the electron transport mobility and makes an obvious influence on OLED performance and lifetime.