Dipolar Photosystems: Engineering Oriented Push–Pull Components into Double- and Triple-Channel Surface Architectures

Article abstract of DOI:10.1002/chem.201600213

Push–pull aromatics are not popular as optoelectronic materials because their supramolecular organization is difficult to control. However, recent progress with synthetic methods has suggested that the directional integration of push–pull components into multicomponent photosystems should become possible. In this study, we report the design, synthesis, and evaluation of double- or triple-channel architectures that contain π stacks with push–pull components in parallel or mixed orientation. Moreover, the parallel push–pull stacks were uniformly oriented with regard to co-axial stacks, either with inward or outward oriented push–pull dipoles. Hole-transporting (p) aminoperylenemonoimides (APIs) and aminonaphthalimides (ANIs) are explored for ordered push–pull stacks. For the co-axial electron-transporting (n) stacks, naphthalenediimides (NDIs) are used. In double-channel photosystems, mixed push–pull stacks are overall less active than parallel push–pull stacks. The orientation of the parallel push–pull stacks with regard to the co-axial NDI stacks has little influence on activity. In triple-channel photosystems, outward-directed dipoles in bridging stacks between peripheral p and central n channels show higher activity than inward-directed dipolar stacks. Higher activities in response to direct irradiation of outward-directed parallel stacks reveal the occurrence of quite remarkable optical gating.

Full text of DOI:10.1002/chem.201600213

DOI: 10.1002/chem.201600213
Full Paper
&
Supramolecular Chemistry |Hot Paper|
Dipolar Photosystems: Engineering Oriented Push–Pull
Components into Double- and Triple-Channel Surface
Architectures
[a, b]
[a]
[a]
Altan Bolag,
Naomi Sakai, and Stefan Matile*
Abstract: Push–pull aromatics are not popular as optoelec-
tronic materials because their supramolecular organization is
difficult to control. However, recent progress with synthetic
methods has suggested that the directional integration of
push–pull components into multicomponent photosystems
should become possible. In this study, we report the design,
synthesis, and evaluation of double- or triple-channel archi-
tectures that contain p stacks with push–pull components in
parallel or mixed orientation. Moreover, the parallel push–
pull stacks were uniformly oriented with regard to co-axial
stacks, either with inward or outward oriented push–pull di-
poles. Hole-transporting (p) aminoperylenemonoimides
dered push–pull stacks. For the co-axial electron-transport-
ing (n) stacks, naphthalenediimides (NDIs) are used. In
double-channel photosystems, mixed push–pull stacks are
overall less active than parallel push–pull stacks. The orienta-
tion of the parallel push–pull stacks with regard to the co-
axial NDI stacks has little influence on activity. In triple-chan-
nel photosystems, outward-directed dipoles in bridging
stacks between peripheral p and central n channels show
higher activity than inward-directed dipolar stacks. Higher
activities in response to direct irradiation of outward-direct-
ed parallel stacks reveal the occurrence of quite remarkable
optical gating.
(
APIs) and aminonaphthalimides (ANIs) are explored for or-
Introduction
cause the synthesis of the required multicomponent architec-
tures has been impossible so far.
Dipolar or push–pull chromophores are characterized by red-
Recently, we became interested in the development of gen-
eral synthetic strategies to build multicomponent surface archi-
tectures with controlled and variable organization. Among the
[
1–4]
shifted absorption and highly polarized excited states.
These properties are fully exploited in some of the most effi-
[2]
[6]
cient dye-sensitized solar cells. Their use in other types of or-
ganic optoelectronics is limited because of their tendency to
cause disorder. However, recent studies have shown the poten-
tial of dipolar aromatics to achieve high performance optoelec-
several methods we have developed, self-organizing surface-
initiated polymerization (SOSIP) turned out to be the most ver-
[
7]
satile. Synthetic access to multicomponent architectures of
increasing complexity has been secured with the addition of
[
1]
[8]
tronics depending on their long-range molecular order.
templated stack exchange (TSE). This SOSIP-TSE methodology
[
9]
Push–pull compounds intrinsically prefer to form antiparallel p
stacks by dipole–dipole attraction. The alternative parallel
stacks with uniformly oriented push–pull dipoles generate
quite significant dipolar fields. Although dipoles have been
shown to influence the rate of electron transfer in solution and
has allowed us to build triple-channel photosystems, double-
[
10]
channel photosystems with antiparallel redox gradients in
[
11]
both channels (OMARG-SHJs),
and ion-gated photosys-
[
12]
tems. In this report, SOSIP-TSE is used to elaborate on dipo-
lar photosystems.
[5]
[3]
[13]
in self-assembled monolayers, the applicability of such effects
to more complex systems is quite poorly explored, mostly be-
Amino -perylenemonoimides (APIs) were selected first as
the push–pull component in dipolar stacks because their close
relatives, perylenediimides (PDIs), perform particularly well in
double-channel SOSIP-TSE photosystems. Contrary to common
assumptions in the literature, PDIs in combination with the
[
a] Dr. A. Bolag, Dr. N. Sakai, Prof. S. Matile
Department of Organic Chemistry
University of Geneva, Geneva (Switzerland)
Fax: (+41)22-379-3215
[14]
classical naphthalenediimides (NDIs) act as excellent p trans-
[
15]
porters. Efficient charge separation between perylenemonoi-
E-mail: stefan.matile@unige.ch
Homepage: www.unige.ch/sciences/chiorg/matile
[
3a,13b]
mides and NDIs has also been well established.
Most im-
[
b] Dr. A. Bolag
portantly, covalent face-to-face API dimers have been shown
[
3b]
Present address: Inner Mongolia Key Laboratory for Physics and Chemistry
of Functional Materials
Inner Mongolia Normal University, Hohhot (P. R. China)
to undergo symmetry breaking charge separation. This pro-
cess was identified as important to achieve efficient interchan-
[
16]
nel charge separation in SOSIP. As far as fundamental prop-
erties are concerned, APIs excel with strong absorbance in the
visible range, HOMO–LUMO energy levels that are compatible
Supporting information for this article and ORCID for the corresponding
author are available on the WWW under http://dx.doi.org/10.1002/
chem.201600213.
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with NDIs as n transporters, and strong dipole moments in the
double-channel photosystems as well as their mixed controls
[3]
ground (~5 D) and the S excited state (~20 D).
(Figure 1). Namely, SOSIP uses ring-opening disulfide-exchange
1
[
4,17]
[18]
Smaller homologs of APIs, aminonaphthalimides (ANIs),
polymerization
under mildly basic conditions to grow n-
retain most of the favorable properties described above with
a larger band gap. They were thus included in this study as al-
ternative push–pull components. ANIs are well known for their
transporting p stacks directly on solid oxide surfaces (Figure 2).
[
19]
In TSE, the orthogonal
dynamic–covalent hydrazone ex-
[
20]
change under mildly acidic conditions is then used to grow
a second p stack along the original stack obtained by SOSIP.
The directionality of hydrazone exchange promises synthetic
access to dipolar architectures by the SOSIP-TSE methodology.
Namely, push–pull chromophores with an aldehyde at the
donor side should give outward dipolar architectures such as
photosystem out-1 (Figure 2). Push–pull chromophores with
an aldehyde at the acceptor side should give the complemen-
tary inward dipolar architectures in-1. Co-TSE with both alde-
hydes at the same time should give architectures mixed-1 with-
out strong supramolecular dipolar fields.
[4b,c]
preference to accept electrons from the donor side.
This di-
rectionality has been of interest for the construction of multi-
[4d,e]
step electron-transfer chains.
In this report, parallel and antiparallel APIs and ANIs are en-
gineered into SOSIP-TSE architectures to generate oriented
supramolecular dipolar fields. Coupled with co-axial electron-
transporting channels in double-channel photosystems, we
find that the orientation of parallel stacks does not significantly
affect the photocurrent generation, whereas uncontrolled or
antiparallel orientations give poorer activities. Installed as
a bridge in triple-channel photosystems, oriented dipolar
stacks more significantly affect photoactivity, particularly when
directly irradiated.
Monomer synthesis
API 2 was designed as a target molecule for the construction
of the outward dipolar photosystem out-1, and API 3 was de-
veloped for in-1 (Scheme 1). Both target molecules were syn-
Results and Discussion
thesized from perylenedianhydride 4. Following literature pro-
Design
[3,13]
cedures,
the bulky amine 5 was used as a temporary solu-
In the envisioned dipolar double-channel photosystems, one
of the two charge-transporting p stacks is composed of push–
pull aromatics. The intrinsically favored antiparallel stacking re-
sults in mixed stacks without supramolecular dipolar fields
bilizer to facilitate the preparation of monoimide 6 under
harsh conditions. From there, bromination gave monoimide 7,
which was hydrolyzed to give anhydride 8 as a common key
intermediate for the synthesis of both API 2 and API 3.
(
Figure 1b). The intrinsically disfavored parallel stacks of push–
For API 2, anhydride 8 was first reacted with the previously
[
9a]
reported
solubilizing leucine derivative 9. The obtained
imide 10 was then subjected to nucleophilic aromatic substitu-
tion with piperazine (11) to give push–pull API 12, which in
turn was coupled with aldehyde 13 by routine amide bond
formation. API 3 was prepared analogously. Namely, anhydride
8
was first reacted with amine 14. Reaction of the obtained
imide 15 with amine 11 gave API 16, which was elongated
with the leucine solubilizer 17 and finally equipped with alde-
hyde 13.
The ANI aldehydes 18 and 19 were prepared similarly from
commercially available 4-bromo-1,8-naphthalic anhydride. The
details can be found in the Supporting Information.
Figure 1. Definition of antiparallel (b) as well as outward (a) and inward (c)
dipolar double-channel architectures.
pull aromatics have an extended supramolecular dipolar field
(
Figure 1a and c). Rather rich in electrons, push–pull stacks are
Monomer characterization
likely to act as hole-transporting (p) channels. In double-chan-
nel photosystems, parallel push–pull stacks are thus aligned
next to electron-transporting (n) channels. Parallel push–pull
stacks can align along n-transporting channels in two orienta-
tions. Parallel push–pull stacks with positive ends of dipole
moments near the n-transporting partner are referred to as
outward dipolar photosystems (Figure 1a). The complementary
inward dipolar photosystems have the negative ends of their
push–pull dipoles near the n-transporting channel (Figure 1c).
These dipolar double-channel photosystems could so far not
be explored experimentally because synthetic methods for
their construction were not available.
The optoelectronic properties of monomeric APIs were con-
[
3]
firmed to be as expected. Namely, cyclic voltammetry (CV)
and differential pulse voltammetry (DPV) measurements afford-
ed the HOMO and LUMO energy levels in CH Cl (Figure 3a).
2
2
The found values were consistent with the literature and com-
patible with electron transfer from excited APIs to NDIs and
hole transfer from excited NDIs to APIs.
In the absorption and emission spectra of all prepared APIs,
the positive solvatochromism characteristic for push–pull
[
21]
[17]
fluorophores,
including ANIs,
The emission band originates presumably from
a twisted intramolecular charge-transfer (TICT) state, typical for
was fully confirmed (Fig-
[
3a,h]
ure 3c).
[7,8]
The recently introduced SOSIP-TSE methodology
offers all
[
21d]
that is needed to construct outward and inward dipolar
disubstituted amino donor groups.
The absorption maxi-
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Figure 2. Schematic, idealized structures of dipolar double-channel architectures with the supramolecular dipolar field in the parallel API stack pointing away
from and toward the NDI stack in photosystems out-1 and in-1, respectively, together with macrodipole-free photosystem mixed-1. The corresponding ANI
photosystems are designated as out-1N, in-1N, and mixed-1N, respectively.
mum in BMI (1-butyl-3-methylimidazolium acetate), an ionic
liquid, was much more redshifted than expected from the
ed with benzaldehyde templates. The resulting SOSIP surface
architectures such as 21 have been characterized previously in
much detail with regard to directional growth from the surface
(microcontact printing), surface roughness (below that of the
underlying ITO surface), long-range organization down to the
molecular level (phase-contrast AFM), self-repair of errors
[22]
rather low dielectric constant (eꢀ15). The lmax =536 nm ap-
peared clearly beyond the lmax =526 nm in acetonitrile or
lmax =531 nm in DMF (eꢀ37), even slightly beyond the l
=
max
5
34 nm in DMSO (eꢀ47). This redshifted absorption in ionic
[
7–9]
liquids could originate from complex 20 with the recently de-
during polymerization, and so on.
In the API series, SOSIP
[17]
scribed ion pair–p interactions operating from both sides.
films with an NDI absorbance at 385 nm of roughly 0.5 were
used (Figure 4). This corresponds to the surface architecture 21
with stacks of nꢀ500 NDIs and a height of ꢀ180 nm
(Scheme 2).
Considering that redshifted absorption has been noticed for
[23]
several push–pull chromophores in ionic liquids,
we con-
clude that ion pair–p interactions could be much more
common and functionally much more important than expect-
ed so far.
The benzaldehyde templates along the central NDI stack of
[
8,9]
21 were removed first with excess hydroxylamine.
The hy-
drazides liberated along the NDI stacks in 22 were then react-
ed with aldehydes 2 and 3. For outward dipolar photosystem
out-1, electrodes 22 were incubated in 25 mm solutions of API
2 in DMSO/AcOH 10:1 at 408C. The increase of API absorption
band demonstrated its incorporation in the photosystem (Fig-
ure 4a). Comparison of the NDI and the API absorbance pro-
vided a qualitative estimation of the TSE yield for the synthesis
of photosystem out-1. Maximal observed TSE yields were 79%
with API 2. The complementary photosystem in-1 was ob-
tained in ꢁ69% by incubating electrode 22 in a solution of
API 3. Co-TSE of electrode 22 in equimolar solutions of API 2
and API 3 gave the mixed-1 in ꢁ69% yield. Considering the
large size of the API stack exchangers compared with the NDI
templates on the surface, these yields were within expecta-
tions for the formation of tightly packed, organized stacks.
Without hydrazone formation, stack exchangers are not bound
Photosystem synthesis
Dipolar photosystems were prepared from the previously re-
[8,9]
ported SOSIP surface architecture 21 (Scheme 2).
As de-
scribed elsewhere in much detail, initiators composed of a cen-
tral NDI, two protected thiols in the main chain, and two pe-
ripheral NDI templates in the side chain were attached on
indium tin oxide (ITO) surfaces through four diphosphonate
bonds. After the formation of monolayers of initiators on ITO,
the thiols were deprotected and deprotonated to initiate the
ring-opening disulfide-exchange polymerization of the propa-
gators from the surface. These propagators contain a NDI to
afford oriented stacks on the central NDIs of the initiators,
strained disulfides above the thiol initiators to produce poly(di-
sulfide)s along the central NDI stacks, and hydrazides protect-
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[17e]
Scheme 1. a) Zn(OAc)
9% (2 steps); d) Zn(OAc)
um hexafluorophosphate (HBTU), TEA, DMF, rt, 30 min, 52%; g) Zn(OAc)
1%; i) HBTU, TEA, DMF, rt, 30 min, 51%; j) 1. Pd(PPh Cl , nBu SnH, AcOH, CH
2
·H
2
O, imidazole, H
2
O, microwave, 1908C, 30 min, 70%;
b) Br
2
, PhCl, 508C, 16 h, 92%; c) 1. KOH, tBuOH, reflux, 15 h; 2. AcOH, rt, 2 h,
7
2
·H
2
O, imidazole, 1008C, 2 h, 80%; e) triethylamine (TEA), DMF, 908C, 18 h, 59%; f) 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluroni-
·H O, imidazole, 1008C, 2 h, 75%; h) TEA, N-methylpyrrolidone (NMP), 908C, 18 h,
2
2
8
3
)
2
2
3
2
Cl
2
, rt, 30 min; 2. HBTU, TEA, DMF, rt, 30 min, 65% (2 steps).
to the surface architectures. This dynamic covalent interfacing
by hydrazone formation assures that the orientation of the
push–pull APIs relative to the central NDI stack is as drawn in
outward dipolar photosystem out-1, inward dipolar photosys-
tem in-1, and photosystem mixed-1 (Figure 2).
Interestingly, the absorption spectra of APIs stacked up on
surfaces were different from that of the monomers in solution:
the lowest energy charge-transfer (CT) band became broader
(full width at half maximum=94 nm in DMSO, 123 nm in out-
1) and bent to the higher energy side (Figure 4a). Similar fea-
tures were previously observed for aligned APIs on TiO surfa-
2
+
ces with Li additives, and rationalized with a Stark ef-
[
3c,d,2b]
fect.
Moreover, the excitonic coupling in H aggregates
could contribute to the apparent blueshift of the absorption
maxima, as observed in face-to-face stacked model API di-
[
3b]
mers. The absorption maximum of the outward dipolar pho-
tosystem out-1 was at lmax =532 nm, almost as in DMSO,
whereas that of the inward dipolar photosystem in-1 was sig-
nificantly blueshifted to lmax =520 nm. These results imply the
presence of better p-stacking interactions in in-1 than in out-1.
Preferential p stacking of inward dimers was also found with
[
13c]
covalent perylenemonoimide dimers.
The absorption maxi-
mum of mixed-1 was with lmax =525 nm in between that of di-
polar out-1 and in-1. This finding supported that the content
of APIs 2 and 3 in mixed-1 is roughly equal.
In mixed-1, hydrazones of 2 and 3 could form favorable anti-
[
7b,c,24]
parallel stacks by alternate self-sorting
or form randomly
mixed stacks also containing domains of parallel stacks. With
TSE occurring under kinetic control, incomplete self-sorting
into antiparallel stacks remains possible, and the indistinguish-
able spectroscopic signatures of parallel and antiparallel stacks
did not provide insights into the extent of social self-sorting.
2 2
Scheme 2. a) NH OH, H O, pH 5, >3 h; b) 2 and/or 3, AcOH, DMSO; c) 18
and/or 19, trifluoroacetic acid (TFA), H
DMSO, all at 408C.
2
2
O, DMSO; d) 24 or 25, TFA, H O,
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However, equal incorporation of hydrazones of 2 and 3 assur-
ed that mixed-1 does not contain global supramolecular dipo-
lar fields.
The more soluble ANI aldehydes 18 and 19 were compatible
with TSE conditions that operate under thermodynamic con-
trol, that is, reversible hydrazone exchange in the presence of
water and TFA to accelerate the rate-determining hydrazone
[
20]
hydrolysis.
with good TSE yields of ~70% for both in- and out-1N and
9% for mixed-1N (Scheme 2, Figure 4b). TSE yields under
ANI aldehydes 18 and 19 gave photosystems
8
thermodynamic control with more soluble ANIs and kinetic
control with less soluble APIs were in the same range, suggest-
ing that the process reaches near completion under both con-
ditions. TSE under thermodynamic control could favor alter-
nate self-sorting in mixed-1N, thus maximizing the presence of
antiparallel stacks and minimizing that of randomly mixed
stacks. However, the spectral signature of ANIs was insensitive
to antiparallel and parallel stacking. As with APIs in mixed-1,
the extent of self-sorting into antiparallel stacks of ANIs in
mixed-1N could not be quantified experimentally.
Owing to the extensive overlap with NDI absorption, the CT
band of ANIs appeared as a featureless, less informative broad
shoulder in photosystems 1N (Figure 4b, dashed spectrum).
However, treatment of the photosystems with phosphate
buffer to assure neutrality of the hydrazone groups caused
a clearly visible broadening or redshift of the CT band of the
ANIs (Figure 4b, solid spectrum). The same buffer treatment of
API photosystems resulted also in a very small redshift of
2 2
Figure 3. a) Cyclic voltammogram of API 2 in CH Cl , and HOMO (bold) and
+
LUMO (dashed) energy levels against vacuum, relative to À5.1 eV for Fc /Fc,
compared with reported values for NDIs and ANIs. b) Schematic structure of
the notional ion pair–p complex 20 between API 2 and BMI. c) Normalized
absorption (left) and emission spectra (right) of 2 (absorption, left to right:
ꢁ
2 nm. The buffer treatment turned out to be important to
obtain consistent and reproducible results in the photocurrent
measurements.
CCl
e=47), BMI (eꢀ15) ; emission, left to right: CCl
DMF, DMSO).
4
(e=2), toluene (e=2), THF (e=8), MeCN (e=38), DMF (e=37), DMSO
[22]
(
4
, toluene, THF, BMI, MeCN,
Photosystem characterization
The dipolar photosystems were evaluated under routine condi-
tions. Namely, the photosystems were used as working electro-
des in the presence of triethanolamine (TEOA) as a mobile
hole acceptor in solution, a Pt wire as a counter electrode, and
Ag/AgCl as a reference electrode. Whereas the activities ob-
tained with this assay are not comparable with data obtained
from optoelectronic devices, they have been shown to reliably
report correct trends under reproducible conditions.
Irradiated with a solar simulator, short-circuit photocurrents
À2
JSC <1 mAcm were obtained with dipolar NDI–API photosys-
tems (Figure 5a). These activities were overall clearly below
the ones observed previously with macrodipole-free NDI–PDI
[
9b,15]
photosystems under similar conditions.
Low incident
photon to current conversion efficiencies (IPCEs) found for
APIs at ꢀ500 nm compared with those of NDIs below 400 nm
indicated that inefficient photoinduced electron transfer from
API to NDI accounts for the weak activity (Figure 4c). This find-
ing could be rationalized by the small LUMO energy difference
between APIs and NDIs (Figure 3a).
Figure 4. a) Absorption spectra before (gray dotted) and after addition of
API 2 (solid, out-1), 3 (dashed, in-1), or 2+3 (dotted, mixed-1) to photosys-
tem 22, compared with 2 in DMSO (gray solid). b) Absorption spectra of
photosystems before (gray dotted) and after TSE with a mixture of ANIs 18
and 19 (dashed), and after an equilibration in phosphate buffer (solid,
mixed-1N) compared with 19 in DMSO (gray solid) as representative exam-
With regard to photocurrent generation, the inward system
in-1 was more active than the outward system out-1 (Fig-
ure 5a). Their normalized action spectra were nearly identical
(Figure 4c). Bimolecular charge recombination efficiencies hBR
ples. c) Normalized action spectra of photosystems out-1 (*), in-1 (
mixed-1 (
). d) Normalized action spectra of photosystems out-1N (*), in-
N ( ), and mixed-1N ( ).
&
), and
&
1
&
&
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rative support that the orientation of parallel stacks in double-
channel photosystems is irrelevant.
Compared with the dipolar photosystems in-1 and out-1,
mixed-1 from the API series generated similar photocurrents
with similar charge separation efficiency (IPCE, i.e., similar
action spectrum, Figure 4c), clearly less charge recombination
loss (h , Figure 5b), and clearly higher activation energy (Fig-
BR
ure 5c). In the ANI series, mixed-1N generated significantly
less photocurrent than the dipolar systems in-1N and out-1N
(
Figure 5d). Moreover, mixed-1N generated photocurrent with
more charge recombination loss (Figure 5e) but clearly lower
activation energy (Figure 5 f) than the dipolar in-1N and out-
1
N. These complementary trends are best understood by con-
sidering that the mixed ANI photosystems mixed-1N were pre-
pared under thermodynamic control, whereas the mixed API
photosystems mixed-1 had to be prepared under kinetic con-
trol. The only weakly reduced photocurrent generation of
mixed-1, compared with dipolar in-1 and out-1, with increased
E and reduced h , was thus consistent with less organized ar-
Figure 5. Characteristics of API (a–c) and ANI (d–f) photosystems out-1(N)
o), mixed-1(N) (m), and in-1(N) (i). a,d) Photocurrent generation upon irradi-
a
BR
(
chitectures in more randomly mixed stacks of APIs. The more
significantly reduced activity of mixed-1N with low Ea and
high hBR was consistent with tight stacking interactions be-
tween better self-sorted antiparallel stacks of ANIs.
ation with solar simulator. b,e) Bimolecular charge-recombination efficiencies
upon irradiation with white light (unfilled bars) or above 420 nm (gray bars).
c,f) Activation energies estimated from the temperature dependence of pho-
tocurrent generation. Results are of the SOSIP photosystems with similar ab-
sorbance (a,d) or the average Æerror of data obtained with ꢂ2 independ-
ently prepared photosystems (b,c,e,f). Note that plots are scaled to highlight
overall rather small differences.
Taken together, these findings suggest that the activity of
mixed photosystems depends significantly on their method of
preparation. Moreover, they support that the activity of
double-channel photosystems with well-equilibrated and self-
sorted antiparallel push–pull stacks is significantly lower than
that with parallel stacks.
were determined from the dependence of photocurrents on ir-
[
25]
radiation power. Although the found hBR values were the
same (42%) for both systems upon excitation of APIs at
ꢂ420 nm, full white light irradiation gave rise to a slightly
Dipolar triple-channel photosystems
lower h value of 20% for in-1 compared with 22% for out-
BR
1
(Figure 5b). Activation energies E were determined from the
In double-channel photosystems, parallel stacks of push–pull
components were identified as more powerful than antiparallel
stacks, whereas the orientation of these parallel stacks turned
out to be almost irrelevant. To elaborate on the orientation of
dipolar stacks in triple-channel architectures, photosystems in-
23 and out-23 were designed (Figure 6). Reminiscent of
a
[
15,26]
temperature dependence of photocurrent generation.
A
lower E was found with in-1 than with out-1 (Figure 5c).
a
Taken together, the slightly higher photocurrent generation
of in-1 coincided with slightly lower TSE yield, blueshifted ab-
sorption maxima (Figure 4a), reduced charge recombination,
and shallower charge traps. These overall small changes in
favor of in-1 could indeed originate from the inward oriented
dipolar fields but also from higher charge mobility in better p
stacks, or from other effects. Most importantly, the found dif-
ferences between double-channel photosystems with inward
[
4d,13b,27]
donor–bridge–acceptor type triads,
triple-channel pho-
tosystems could undergo photoinduced charge separation
with electrons and holes in two differently substituted NDIs.
The parallel push–pull stacks placed in between have their di-
polar fields oriented toward the central n channel in in-23 and
toward the peripheral p channel in out-23.
(
in-1) and outward (out-1) oriented parallel API stacks were
very small. Orientation independence has also been reported
Dipolar dyads 24 and 25 were prepared by amide bond for-
mation between ANIs and NDIs with amine donors in the core
(NN-NDIs). Interestingly, the absorption maxima of NN-NDIs in
dyads were slightly (2 nm) shifted to the blue (25) and red (24)
compared with the parent NN-NDI (Figure S5 in the Supporting
Information). An appealing explanation for this finding is the
stabilization and destabilization of the LUMO of NN-NDIs by
the nearby positive and negative ends of the ANI dipoles, re-
spectively. Similar Stark shifts have been observed in other di-
[3g]
previously for APIs in DSSCs.
Compared with API photosystems, ANI photosystems gener-
ated overall more photocurrent (Figure 5a, d). More efficient
electron transfer from ANI to NDI was evident from the compa-
rable action and absorption spectra (Figure 4d, b). These re-
sults could be explained by a larger LUMO energy difference
between ANI and NDI compared with API and NDI (DELUMO
ꢀ
0.77 versus 0.34 eV, Figure 3b). The difference in activity be-
tween ANI photosystems with inward (in-1N) and outward
out-1N) oriented parallel stacks was overall even smaller than
[
5b]
polar systems.
(
These small shifts of the NN-NDI absorption maxima were
conserved in triple-channel photosystems in-23 and out-23
(Figure 7a). They were obtained from SOSIP architecture 22 by
with API photosystems (Figures 4d, 5d–f). These consistent
trends with different push–pull components provided corrobo-
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Figure 6. Schematic, idealized structure of dipolar triple-channel architectures with macrodipoles in the ANI stack pointing away from and toward the central
NDI stack in photosystems out-23 and in-23, respectively.
hydrazone formation with aldehydes 24 and 25 (Scheme 2,
Figure 6). Only moderate 40–50% TSE yields could be achieved
for these photosystems, probably owing to the large size of
the aldehydes (Figure 7a).
Upon solar irradiation, the outward triple-channel photosys-
tem out-23 generated significantly more photocurrent than
the inward system in-23 (Figure 7b). This finding was remark-
able because ANIs in out-23 are oriented in the opposite way
from those found in most donor–bridge–acceptor type tri-
[4d,13b]
ads.
Apparently, the distances between neither the
donors (NN-NDI and HOMO of ANI) nor the acceptors (NDI and
LUMO of ANI) influenced activity significantly (Figure 6). More-
over, dipole-induced acceleration or deceleration of charge
separation could be excluded as a likely origin for the differen-
ces in activity because the nearly identical normalized action
spectra demonstrated that the relative contributions from each
of the chromophores to photocurrent generation are similar in
the two systems (Figure 7c). However, charge recombination
was significantly affected by the orientation of dipolar ANIs
Figure 7. a) Absorption spectra before (gray dotted, 22) and after reaction of
ANI-NN-NDI 24 (dashed, in-23) and 25 (solid, out-23) with photosystem 22.
b) Photocurrent generation by out-23 (o) and in-23 (i) upon irradiation with
a solar simulator. c) Normalized action spectra of photosystems out-23 (*)
and in-23 (
3 (o) and in-23 (i) with white light (unfilled), ꢂ420 nm (gray) or ꢂ520 nm
black filled). Results are of photosystems with similar absorbance, error bars
&
). d) Bimolecular charge recombination upon irradiation of out-
2
(
(
Figure 7d). Whereas charge-recombination efficiencies hBR
were almost the same for both systems upon excitation of
only NN-NDIs at ꢂ520 nm, they were clearly higher for in-23
when ANIs were also irradiated. This quite remarkable “optical
gating” suggested that the charge-separated state of in-23 is
destabilized by wrongly oriented strong excited-state dipoles
of ANIs, whereas ground-state dipoles are too weak to result in
a detectable effect. The complementary stabilization of the
charge-separated state by strong supramolecular dipolar fields
with excited-state push–pull dipoles (Figures 6 and 7d) should
then account for the high activity of the matched triple-chan-
nel photosystem out-23 (Figure 7b).
refer to curve fit.
Conclusion
In summary, the SOSIP-TSE strategy allowed us to dissect the
inherent and supramolecular properties of dipolar compounds
in photocurrent generation. Oriented in a parallel manner in
double-channel photosystems, the inward or outward direction
of push–pull dyes did not matter much for photocurrent gen-
eration. In mixed systems, dipolar dyes in random or antiparal-
lel orientations gave reduced activities. Thus, in double-chan-
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nel systems, the effect of dipoles is primarily on the supra-
molecular organization, which in turn determines photocurrent
generation, while their direct influence on charge separation
or recombination is minimal.
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Published online on May 20, 2016
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www.chemeurj.org
9014
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim