Angewandte
Chemie
“fuzzily”[16] organized LBL assembly[14,15] leaves no doubt that
the anionic Au-7 will ion-pair with the cationic OPE-NDI 2,
cationic Au-7-2 will ion-pair with the anionic OPE-NDI 3, and
so on (Figure 3). Photocurrent density saturated at about 8
layers with OPE-NDI architectures generated LBL, whereas
*
zipper saturation occurs at 20 layers (Figure 3a, versus &).
The maximal short-circuit current of Au-7(-2-3)n was half of
that generated by Au-1(-2-3)10. LBL assembly should be
irresponsive to capping because any eventual OPE interdigi-
tation is functionally irrelevant. This is exactly what was
found for the LBL assembly of OPE-NDI dyads, that is, Au-7-
&
2-1-(2-3)n (Figure 3a, ). Mismatched POP-NDI architec-
tures were not only irresponsive to different modes of
assembly but also to all capping experiments (Figure 3b).
The mass deposited per layer was determined electro-
mechanically with a quartz crystal microbalance (QCM)
(Figure 3c). Equal quantities were found to deposit for zipper
and LBL assembly without saturation. This suggested that
photocurrent saturation occurs because charges start to
recombine before reaching the electrodes, and not because
the growth of the assembly stops. The observed “critical
thickness” thus relates to charge mobility; that is, supra-
molecular organization. Better critical thickness of OPE
zippers (about 20 layers) compared to LBL assemblies (circa
10 layers) and POP zippers (circa 8 layers) is thus consistent
with the long-range organization of topologically matching
OPE-NDI zipper architectures (Figure 3).
Excellent organization of OPE-NDI zipper architectures
was confirmed with smooth surfaces in atomic force micro-
scopy (AFM) images (Figure 3d), smoother than those of
LBL and POP controls (Supporting Information, Figures S9–
11) as well as LBL and BHJ photosystems in the litera-
ture.[3,8,14] Different to LBL[14] and related approaches,[8]
surface roughness did not significantly increase with multi-
layer thickness. AFM images further revealed steps of 1.5 nm
height as expected for zipper assembly, and occasional clumps
of unknown origin but seen already for Au-1. Access to low
surface roughness has been proposed to be important in
molecular optoelectronics, including high photovoltaic effi-
ciency.[3] Indeed, current–voltage curves of OPE-NDI zippers
Au-1-(2-3)7 revealed a fill factor (FF) of 61% (Supporting
Information, Figure S7). As in the thick films prepared “top-
down” in optimized BHJ organic solar cells,[3] this FF was
consistent with high charge mobility in OPE-NDI architec-
tures. When considering plasmon resonance quenching on
gold by factors up to 280, the reported short circuit currents
are quite high.[18]
Figure 4. Transient absorption and action spectra of OPE-POP sys-
tems. a) Transient absorption spectra recorded 0.8 (dotted) and 10 ps
(solid) after excitation of OPE-NDI 2 at 400 nm (top) and 0.1 (dotted),
1, 3.5 and 10 ps (solid) after excitation of 2 at 520 nm (bottom); inset:
*
*
DA at 415 nm with time after excitation at 400 ( ) and 520 nm ( ).
*
b) Action spectrum of OPE-NDI zipper Au-1-(2-3)7 ( ) compared to
absorption spectra of 2 in methanol with 2% triethylamine (solid line).
22). The appearance of OPE bleaching at 415 nm together
with the broad NDI radical anion band around 600 nm
indicated that, independent of the initially excited chromo-
phore, very fast electron transfer takes place from OPE to
+
NDI, and that the formed charge separated state (OPEC –
NDICÀ pair) of 2 is relatively long-lived (flash photolysis
lifetime t4 = 270 ns; Supporting Information, Table S4). This
finding was important as it demonstrates the presence of the
+
OPEC –NDICÀ pair; that is, supramolecular n/p-heterojunc-
tions. The faster charge separation observed upon OPE
excitation than upon NDI excitation further demonstrated
that electron rather than energy transfers from OPE to NDI
(Figure 4a and inset). For POP-NDI, both photoinduced
charge separation and charge recombination were slower and
influenced by unusual and more complex triplet contributions
arising from heavy-atom effects (lifetime t4 = 2.5 ms; Support-
ing Information, Table S4).
In summary, we report synthetic access to ordered,
oriented multicomponent surface architectures, and that
topological matching is the key to get there. The obtained
photosystems excel with efficient photocurrent generation,
smooth surfaces, and perfect responsiveness to functional
probes for the existence of operational intra- and interlayer
recognition motifs. Homologous photosystems with mis-
matched POP scaffolds or “fuzzy”[16] organizations made by
standard layer-by-layer assembly are less functional. These
results demonstrate that highly ordered and oriented supra-
molecular organization is achievable only through careful
design, and that it matters for function. Access to surface
architectures with long-range organization will be essential
for future molecular optoelectronics, including OMARG-
SHJ solar cells or molecular logic devices.
The action spectrum of OPE-NDI zipper Au-1-(2-3)7
revealed that this photoactivity is due not only to organization
but also to the contribution of both NDIs and OPEs to the
*
light harvesting (Figure 4b, ). Moreover, the observed
bathochromic absorption of OPE in the action spectum
implies co-planar orientation of phenyl groups. OPE plana-
rization was expected for the zipper architecture, and should
give rise to higher charge mobility of the p-semiconduc-
tor.[27,28]
Femtosecond fluorescence and transient absorption spec-
troscopy provided further support for the high photoactivity
of OPEs (Figure 4a; Supporting Information, Figures S17–
Angew. Chem. Int. Ed. 2009, 48, 6461 –6464
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Kel, Oksana
