True- to Sky-Blue Emitters Bearing the Thiazolo[5,4-d]thiazole Electron Acceptor for Single and Tandem Organic Light-Emitting Diodes

A series of emitters displaying efficient photoluminescence (PL) and electroluminescence (EL) into the true- to sky-blue regions is herein described and employed as electroluminescent materials for the fabrication of efficient blue organic light-emitting diodes (OLEDs). The compounds possess linear, yet a twisted, donor−π–acceptor−π–donor (D−π–A−π–D) architecture, where D and A are a (substituted) carbazole (Cz) and thiazolo[5,4-d]thiazole (TzTz) moiety as the electron donor and accepting unit, respectively. D = 9H-carbazol-9-yl and 3,6-di-tert-butyl-9H-carbazol-9-yl for compounds TzTz-PCz2 and TzTz-PtbCz2, respectively. In dilute CH2Cl2 solutions, both compounds display intense (PL quantum yield = 60–74%) and short-lived (τ = ca. 0.8–2.0 ns) luminescence arising from an intramolecular charge-transfer (1ICT) excited state and falling into the sky-blue to the greenish-blue region (λem = 448–502 nm). Compared to the previously reported TzTz-TPA parental emitters that features two triphenylamine (TPA) donor groups, the highly twisted nature and less extended π-conjugation of the donor–acceptor TzTz-PCz2 and TzTz-PtbCz2 counterparts herein presented enable to widen the HOMO–LUMO energy gaps and warrant blue shifting of the emission. Finally, OLED devices fabricated by employing the herein proposed emitters are presented as well with both single- and tandem-device architectures. The so-prepared OLEDs display true-blue to sky-blue EL peaking at λEL,max = 460 nm (TzTz-PCz2) and 467–470 nm (TzTz-PtbCz2) and achieve maximum external quantum efficiency and luminance of 10.2% and 44,851 cd m–2 (for TzTz-PtbCz2) in the case of a tandem device, respectively, with a mitigated roll-off efficiency. These results demonstrate that the tandem device employing these TzTz donor–acceptor blue emitters raises the feasibility of fluorescent compounds to compete with other potential emitters in practical applications.