Chemistry Reference
In-Depth Information
FIGURE 9.3
Chemical structures of the first series of DLED materials. (a) Second-
generationdendrimerwith peripheral triphenylaminegroups. (b) Third-generation PAdendrimer
with an anthracene core [31]. Dendrimers of lower generations are not shown.
Further reports on PA dendrimers are rare, and device performances are not
particularly remarkable [32]. One possible reason is the strong tendency of the PA
units to flatten in the solid state, causing aggregation and excimer formation [31].
9.2.2.2 E-Stilbenyl-Based Dendrimers An important development in DLED is
the introduction of E-stilbenyl moieties into the dendrimer scaffold by Burn and
coworkers [33]. As this moiety is nearly planar, the entire dendrimer structure remains
relatively flat only until significant crowding takes place at higher generations. The
double bond was constructed usually through Wittig-Horner reaction; alternative
approaches included palladium-catalyzed Heck reaction [34] and Ramberg-
Backlund reaction [35].
In the initial report [33], up to third-generation E-stilbenyl dendrimers with a
distyrylbenzene corewere synthesized to study the generation effect (Figure 9.4). Due
to the meta linkage in the molecular design, the absorption and emission spectra of the
dendrimers were similar to those of the central distyrylbenzene core. On going from
solution to thin film, a red-shifted emission maximum was also observed, but to a
much smaller extent than for PA dendrimer. The devices made with
showed blue
EL, with EQE of 0.01% for
3
, 0.07-0.09% for
4
, and 0.03% for
5
, respectively. The
device stability increased from the first to the third generation; meanwhile, the third-
generation dendrimer had the most similar PL and EL spectrum. The authors thus
nicely demonstrated that dendrimer generation could serve as a new tool to control
intermolecular interactions in DLED.
3
-
5
