Chemistry Reference
In-Depth Information
FIGURE 9.8
Chemical structures of the first series of truxene-based dendrimers [50].
dendrimers' architecture, makes truxene-based dendrimer a strong emitter even in the
solid state. Finally, derivation of truxene unit is rather simple. The 2, 7, and 12
positions of the truxene can be easily halogenated, allowing various C-C coupling
reactions to take place.
In 2003, we reported the synthesis of the first series of dendrimers incorporating
truxene units [50], as shown in Figure 9.8. The syntheses of dendrimers
11
(G0) and
12
(G1) were based on repetitive Friedel-Crafts acetylation reaction and SiCl
4
-
catalyzed cyclotrimerization reaction. The absorption and emission maxima of the
two dendrimers were not much different from the alkylated truxene monomer. Two
facts account for this observation. First, as computation study has shown, electronic
communication among branches of the truxene is minimum, similar to a 1,3,5-
substituted benzene. Therefore, the contribution of a truxene unit to the conjugation
length is only slightly larger than a fluorene unit. Second, there are significant torsion
angles between the benzene ring and truxene unit due to steric repulsion. Hence, the
conjugation length of dendrimer
12
is only slightly longer than a fluorene unit, despite
the presence of 40 benzene rings.
In thin films, dendrimers
11
and
12
showed structured absorption and emission
bands, only slightly red-shifted compared with those in solution. Hence, the effect of
the alkyl chains and the dendritic architecture is evident. With these results in hand,
our first DLED devices [47] were fabricated with the configuration of ITO/PEDOT/
dendrimer/Ba/Al. The EQE was measured to be 0.16% at 27 V for
11
and 0.16% at
13 V for
12
, respectively. However, the EL emissions were significantly red-shifted in
