Bulkyl diphenylamine functionalized spiro[fluorene-9,9′- xanthene] based solution-processible small molecule host in red PhOLEDs for investigating structure-property relationship

A novel diphenylamine/spiro[fluorene-9,9′-xanthene] (SFX) hybrid (SFX27DPA) was constructed successfully by incorporating diphenylamine into SFX via Pd-catalyzed C–N cross-coupling to ingeniously develop state-of-the-art models of solution-processible small molecule host for uncovering the structure-property relationship of molecular structure change in the application of PhOLEDs, which is vital to rational cumulative enhancement in device performance by purposive and feasible structure modifications. SFX27DPA shows outstanding thermal stability (T_d, 425 °C) compared to those of SFXDPA ((Spiro[fluorene-9,9′-xanthen]-2-yl)-diphenylamine) (T_d, 338 °C) and SFX-Cz ((2-carbazolyl-spiro[fluorene-9,9′-xanthene]) (T_d, 345 °C) by increasing the molecular weight but owns weak morphological stability due to its low melting point (T_c, 60.5 °C). The HOMO (−5.28 eV), LUMO (−2.08 eV), and energy gap (E_g, 3.20 eV) were obtained by cyclic voltammetry measurements (CVs). The high T₁ (T₁, 2.98 eV) and narrow energy gap (E_g, 3.20 eV) are beneficial to achieving high efficient device. The compound not only possesses a high triplet energy level but also exhibits good solubility in common solvents. Two SFX-based compounds (SFX27DPA, SFX-Cz) for red PhOLEDs have been fabricated successfully by solution process for deep researching the relationship between molecular structure and device performance. An enhanced performance of device based on SFX27DPA with maximum external quantum efficiencies (EQEs) of 10.9% under lower driving voltage was obtained compared to that of SFX-Cz-based red PhOLEDs (1.5%). This investigation offers good reference for future molecular design to achieve high performance through accumulative structure modification.