The tri- and di-substituted derivatives 1, 2 exhibit the first
oxidation wave half-wave potentials corresponding to
0
.29 V vs. Fc, which results in a EHOMO value of À5.09 eV
on the basis of the EHOMO energy level of ferrocene as
.8 eV). These results are quite similar to those demonstrated
by mono-substituted TPA 3, only a very slight increase in
HOMO is observed. Additional diphenylethenyl groups
(
4
E
have a more substantial effect on ELUMO, which increases by
approximately 0.1 eV.
When considering the use of an organic material for
hole-transport applications it is important to have an under-
I
standing of its solid state ionization energies (E ). This
understanding can help in identifying suitable partner organic
transport materials and inorganic electrode materials. The
ionization energy was measured by the electron photoemission
in air method (Fig. S4, ESIw) and results are presented in
Table 2, the measurement error is evaluated as 0.03 eV. The
I
measured E values are about 0.3 eV higher as compared with
the HOMO levels found in the CV experiments. The difference
may be caused by different states of the materials investigated
(
solution in CV and solid film in a photoemission method) and
Fig. 2 Normalized absorptions of the films drop-cast from the THF
solutions of 1 (a) and 2 (b).
measurement methods.
In conclusion, we have demonstrated a simple one step
synthesis method to obtain solution processable star-shaped
charge transporting materials with a triphenylamine core
and a varying number of diphenylethenyl sidearms from
commercially available and relatively inexpensive starting
materials. Tris(4-(2,2-diphenylethenyl)phenyl)amine 1 is an
especially promising candidate because it can be handled in
air, requires no high temperature annealing steps, can be
solution deposited, possesses comparatively high mobility,
and could be synthesized in one step from commercially
available and cheap triphenylamine.
indicates that both types of aggregates are formed, but
H-aggregates predominate.
Development of some ordering was also evident in AFM
height images (Fig. 3). The surface of as-cast compound 2
revealed the presence of distinct, rod-shaped features, most
likely corresponding to microcrystallines.
To elucidate the energetic conditions for energy and electron
transfer in dilute solutions, the HOMO/LUMO values were
also determined using cyclic voltammetry (CV) (Table 2).
These EHOMO values do not represent any absolute solid-state
or gas-phase ionization energies, but can be used to compare
different compounds relative to one another. The cyclic
voltammograms of the synthesized compounds in dichloro-
methane show one quasi-reversible and one irreversible
oxidation couples and no reduction waves (Fig. S2 and S3,
ESIw). The electrochemical data are summarized in Table S1
ˇ
This research was funded by a grant (VP1-3.1-SMM-07-K
01-078) from the Research Council of Lithuania.
Notes and references
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(
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¨
1
Fig. 3 Intermittent contact mode AFM image of the spin-cast 2 film.
7
772 Chem. Commun., 2011, 47, 7770–7772
This journal is c The Royal Society of Chemistry 2011