Blue Light Emitting Polyphenylene Dendrimers with Bipolar Charge Transport Moieties

Abstract

Two light-emitting polyphenylene dendrimers with both hole and electron transporting moieties were synthesized and characterized. Both molecules exhibited pure blue emission solely from the pyrene core and efficient surface-to-core energy transfers when characterized in a nonpolar environment. In particular, the carbazole-and oxadiazole-functionalized dendrimer (D1) manifested a pure blue emission from the pyrene core without showing intramolecular charge transfer (ICT) in environments with increasing polarity. On the other hand, the triphenylamine-and oxadiazole-functionalized one (D2) displayed notable ICT with dual emission from both the core and an ICT state in highly polar solvents. D1, in a three-layer organic light emitting diode (OLED) by solution processing gave a pure blue emission with Commission Internationale de l'Éclairage 1931 CIE xy = (0.16, 0.12), a peak current efficiency of 0.21 cd/A and a peak luminance of 2700 cd/m 2 . This represents the first reported pure blue dendrimer emitter with bipolar charge transport and surface-to-core energy transfer in OLEDs.

Blue Single‐Layer Organic Light‐Emitting Diodes Using Fluorescent Materials: A Molecular Design View Point

“…Several pyrene‐based dendrimers with different peripheral chromophores have also been incorporated in SL‐OLEDs. For example, in 57 , the central pyrenyl core is linked to four 3,5‐bis(trifluoromethyl)‐phenyl units, [ 54 ] whereas it is linked to eight oxadiazole cores thanks to well suited oligophenyl links in 58 [ 55 ] and 59 [ 77 ] (Figure 8, left).…”

“…The EL properties of these OSCs were investigated in either TE‐SL 57 , [ 54 ] SP‐SL 58, [ 55 ] or SP‐SL 59 . [ 77 ] The device performances are low with CE max only reaching 0.004 cd A −1 (TE‐SL 57 ), 0.04 cd A −1 (SP‐SL 59 [ 77 ] ) or 0.14 cd A −1 (SP‐SL 58 [ 55 ] ) despite low LUMO levels (−2.86 eV for 57 , −2.46 eV for 58, and −2.52 eV for 59 ). The three devices emit light at high V on : 9.7 V with 59 , 8.6 V with 57 and 7 V with 58 , translating a bad charge injection and showing that manipulation of the LUMO energy is not sufficient to reach high performance devices.…”

“…In 2016, Müllen's group also designed dendrimers with a central pyrene core decorated alternatively by four donor and four acceptor units ( 77 , Figure 10). [ 77 ] The acceptor unit is an oxadiazole and the donor unit is a carbazole. 77 possesses the same central core as that of 25–27 , dendrimers decorated by eight donor branches (Figure 3) and as that of 58 [ 55 ] and 59 [ 77 ] (Figure 8) with eight acceptor branches.…”

“…[ 77 ] The acceptor unit is an oxadiazole and the donor unit is a carbazole. 77 possesses the same central core as that of 25–27 , dendrimers decorated by eight donor branches (Figure 3) and as that of 58 [ 55 ] and 59 [ 77 ] (Figure 8) with eight acceptor branches. The UV–vis absorption spectrum of 77 presents in THF three main bands at i) 299 nm, characteristic of the carbazole absorption (also observed for 26 [ 55 ] ), ii) 319 nm, assigned to the absorption of the oxadiazole units (as observed for 58 [ 55 ] and 59 [ 77 ] ) and finally iii) a band centered at 392 nm, which originates from π–π* transitions of the central pyrene core (found in all the above mentioned dendrimers).…”

“…The reason of the absence of ICT is due to the lack of electron donation from carbazole to the central core consistent with similar HOMO levels between carbazole (−5.52 eV) and the core (−5.50 eV). [ 77 ]…”