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-pi-acceptor-pi-donor (D-pi-A-pi- 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-ditert-butyl-9H-carbazol-9-yl for compounds TzTz-PCz2 and TzTzPtbCz2, respectively. In dilute CH2Cl2 solutions, both compounds display intense (PL quantum yield = 60-74%) and short-lived (tau = 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 (lambda em = 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 pi-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 lambda EL,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.