Rigid-rod push-pull naphthalenediimide photosystems

Article abstract of DOI:10.1039/b708449h

Design, synthesis and evaluation of advanced rigid-rod π-stack photosystems with asymmetric scaffolds are reported. The influence of push-pull rods on self-organization, photoinduced charge separation and photosynthetic activity is investigated and turns

Full text of DOI:10.1039/b708449h

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Rigid-rod push–pull naphthalenediimide photosystems†
Naomi Sakai,a Adam L. Sisson,a Sheshanath Bhosale,‡a Alexandre Fu¨rstenberg,b Natalie Banerji,b
Eric Vautheyb and Stefan Matile*a
Received 4th June 2007, Accepted 28th June 2007
First published as an Advance Article on the web 16th July 2007
DOI: 10.1039/b708449h
Design, synthesis and evaluation of advanced rigid-rod p-
stack photosystems with asymmetric scaffolds are reported.
The influence of push–pull rods on self-organization, pho-
toinduced charge separation and photosynthetic activity is
investigated and turns out to be surprisingly small overall.
The design and synthesis of advanced optoelectric nanomaterials
is a topic of current scientific interest.1–18 In this report, we describe
the synthesis and characteristics of push–pull p-octiphenyl 1, a
rigid-rod molecule that carries blue naphthalenediimides (N,N-
NDIs) along the rigid-rod scaffold, a p-donating methoxy at one
rod terminus and a p-accepting amide plus a negative charge at the
other (Fig. 1). Compared to the previous photosystems 2 and 3,16–18
the new push–pull rod 1 contains a macrodipole along the rigid-
rod scaffold. This axial macrodipole was of interest to explore
the possibility of controlling photosynthetic and photophysical
characteristics of 2 and 3 such as stabilization of the symmetry-
breaking2,16 photoinduced charge separation. These effects would
Fig. 1 Notional self-assembly of the blue, asymmetric push–pull rod 1
be amplified with the parallel self-assembly of rod 1 into p-M-
helix 4. This supramolecule might form in lipid bilayers because
of hindered translocation of the anionic carboxylate anchors
across the membrane. In solution, dipole–dipole attraction should
cause antiparallel self-assembly into quadruple p-M-helix 5 with
reduced effects. These high expectations appeared meaningful
considering the extensive evidence for remote control of opto-
electric properties by a-helical macrodipoles. Highlights include
fluorescence quenching in donor–acceptor dyads by a-helical
dipoles,19 amplified dark current along single a-helical dipoles,20
and amplified photocurrents across monolayers of parallel a-
helices on gold.3 Moreover, rigid push–pull rods have been of
use previously to create several variations of voltage-gated ion
channels.21,22
The synthesis of rod 1 is summarized in Scheme 1. The p-
sexiphenyl 6 was envisioned as an ideal building block to attach
the p-attracting and p-donating rod termini 7 and 8, respectively,
and the blue N,N-NDIs 9 along the rigid-rod scaffold. Rapid
access to p-sexiphenyl 6 from the commercially available biphenyl
10 and tert-butyl bromoacetate 11 has been reported previously.22
Last year, we also synthesized N,N-NDI 9 starting with oxidation
of pyrene 12, reaction of the resulting dichlorodianhydride first
into parallel p-helix 4 in lipid bilayer membranes and antiparallel p-helix
5 in solution; for p-donor D and p-acceptor A with anionic anchor, please
see Scheme 1. The symmetric blue and red controls 2 and 3 have been
described.18–20
with amines 13, 14 and then with 15 to give 16, followed by
chemoselective removal of Alloc.16
Rod terminus 7 was synthesized from benzaldehyde 17, which
was converted into iodinated compound 18 in four steps.23 After
acidic deprotection, benzaldehyde 19 was reacted first with tert-
butyl bromoacetate 11, followed by oxidation. Coupling of the
resulting acid 20 with C-protected glycine 21 gave the desired
p-accepting amide 22 with a protected carboxylate anchor. Pina-
colboronate 23 was obtained by Pd-catalyzed conversion of aryl
iodide 22.24 It was directly converted to the pull-fragment 7 with
KHF2 to benefit from increased stability, easier purification and
higher reactivity of potassium trifluoroborates.25–27
The push-fragment 8 was synthesized from phenol 24. The
previously reported ortho-iodination gave pure regioisomer 25,28
which was subjected to Williamson ether synthesis with tert-butyl
bromoacetate 11. The obtained aryl iodide 26 was transformed
via boronate 27 into the desired, air-stable and solid potassium
trifluoroborate 8.
Pull-fragment 7, push-fragment 8 and chromophores 9 were
coupled sequentially to p-sexiphenyl 6. Suzuki coupling of p-
sexiphenyl 6 first with push-fragment 8 gave p-septiphenyl 28 in
42% conversion yield, subsequent attachment of the pull-fragment
7 at the other terminus gave the final push–pull rod 29. The lateral
tert-butyl esters in 29 were cleaved with TFA without damage to
the terminal benzyl esters. The liberated carboxylic acids along the
rigid-rod scaffold of 30 were reacted with eight N,N-NDI amines
aDepartment of Organic Chemistry, University of Geneva, Geneva, Switzer-
land. E-mail: stefan.matile@chiorg.unige.ch; Fax: (+41 22) 379 5123;
Tel: (+41 22) 379 6523
bDepartment of Physical Chemistry, University of Geneva, Geneva, Switzer-
land
† Electronic supplementary information (ESI) available: Experimental
section. See DOI: 10.1039/b708449h
‡ Current address: Monash University, Clayton, Australia
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Scheme 1 (a) Four steps, see ref. 23. (b) HCl (aq), THF, 88%. (c) 1. Cs2CO3, DMF, 91%; 2. NaClO2, NaH2PO4, H2O2, 91%. (d) HATU, iPr2NEt, quant.
(e) Pinacolborane, PdCl2(dppf), Et3N, CH3CN, 1 h, reflux. (f) KHF2, MeOH–H2O, 2 h, rt, 47% from 22. (g) AgOTf, I2, CHCl3.27 (h) Cs2CO3, DMF, 92%.
(i) Pinacolborane, PdCl2(dppf), Et3N, CH3CN, 1 h, reflux, 88%. (j) KHF2, MeOH–H2O, 2 h, rt, quant. (k) PdCl2(dppf), Et3N, MeOH–THF 1 : 1, 30 min,
reflux, 25%, conversion yield 42%. (l) PdCl2(dppf), Et3N, MeOH–THF 5 : 1, 30 min, reflux, 49%, conversion yield 66%. (m) TFA, CH2Cl2, 30 min, rt.
(n) HATU, Et3N, 2,6-di-tBu-pyridine, DMF, rt, 24 h, 75% from 29. (o) HBr, AcOH, TFA, thioanisole, pentamethylbenzene, 90 min, rt, quant. (p) Five
steps, see ref. 22. (q) Four steps, see ref. 16. (r) Bu3SnH, Pd(PPh3)2Cl2, CH2Cl2, rt, 30 min, 86%, see ref. 16.
9, and the obtained conjugate 31 was fully deprotected with HBr–
AcOH to give target molecule 1.
charge separation might be the result of an increased concentration
of p-stacked N,N-NDI chromophores in p-helix 5. Moving from
MeOH to TFE, the quantum yield of push–pull system 1 (Ufl =
3 × 10−4) did not change significantly. This independence on
partial disassembly of p-helix 5 into 1 suggested that indeed the
macrodipole contributes to photoinduced charge separation in
monomer 1.
The hypsochromic shoulder of the band around 600 nm in the
absorption spectrum of the push–pull system 1 ↔ 5 was indicative
of face-to-face p-stacking of the N,N-NDI chromophores.2,16
The bisignate CD Cotton effect of this transition supported
self-organization with M-helicity (Fig. 2C).16,17 Strong in iso-
propanol, intermediate in methanol (MeOH) and weak in 2,2,2-
trifluoroethanol (TFE), both effects exhibited identical solvent
dependence (Fig. 2B). Unrelated to solvent polarity (Fig. 2A),
this trend could reflect self-assembly of rod 1 into antiparallel
p-helix 5 in isopropanol and MeOH but not as much in TFE.
The early fluorescence dynamics were monitored using the
fluorescence up-conversion technique (400 nm excitation, 100 fs
pulses). In all cases, fluorescence decay was highly non-
exponential, and at least 5 exponential functions were needed to
reproduce the fluorescence decay. The average lifetime of push–
pull system 1 (sav = 20–50 ps) was 4- (in MeOH) to 15-times
(in TFE) shorter than that of the symmetric control 2 (sav
=
200–310 ps). Particularly, the extent of ultrafast decay within the
first 15 ps for push–pull system 1 (89% in MeOH, 76% in TFE)
compared to 2 (66% in MeOH, 56% in TFE) could be interpreted
as accelerated charge separation in push–pull photosystem 1
(Fig. 3A).
Fig. 2 Relative absorption of hypsochromic shoulder/absorption maxi-
mum in the absorption spectra (A, ᭹) and amplitude A in the CD spectra
of 1 (A, ᭺) in, with decreasing dielectric constant, MeCN, MeOH (red),
TFE (blue), isopropanol (purple) and THF. (C) Original CD spectra in
isopropanol (purple), MeOH (red) and TFE (blue).
In MeOH, push–pull system 1 (Ufl = 4 × 10−4) was 20-times
less fluorescent than the symmetric control 2 (Ufl = 9 × 10−3).
The same weak emission was found for 1 in vesicles. This effect
could imply more efficient photoinduced charge separation along
the macrodipole in monomeric 1. Alternatively, dipole–dipole
attraction might promote antiparallel self-assembly, and increased
Fig. 3 Intensity-normalized time profiles of (A) fluorescence decay and
(B) decay of the transient N,N-NDI•− absorption of 1 (᭹) and 2 (᭺) in
TFE. For full data sets, see ESI.†
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The existence of photoinduced charge separation was confirmed
by the transient absorption of the N,N-NDI•− radical anion at
510 and above 670 nm following excitation with a 50 fs laser pulse
at 610 nm.16 According to the decay kinetics of the N,N-NDI•−
radical anion, the lifetime of the photoinduced charge-separated
species of push–pull system 1 (MeOH: s = 50 ps, TFE: s = 40 ps,
Fig. 3B) was quite similar to that of the blue control 2 (MeOH: s =
65 ps,16 TFE: s = 50 ps) but clearly, about 10-times shorter than
that of the red control 3 (MeOH: s ≈ 500 ps18). This suggested that,
in contrast to accelerated charge separation, scaffold asymmetry
in push–pull photosystem 1 did not affect charge recombination
much.
The delivery of functional nanoarchitecture to bilayer mem-
branes is an underrecognized and underexplored process that
often determines the finally observed apparent activities.29,30 For
membrane delivery, concentrated solutions in various solvents
(and eventual additives) are simply added to the vesicle sus-
pension, and the functional compounds are hoped to reach
the membrane before precipitation. To determine the solvent
that best delivers photosystem 1, large egg yolk phosphatidylcho-
line vesicles (EYPC LUVs) were labeled with N-(7-nitrobenz-2-
oxa-1,3-diazol-4-yl)-1,2-dihexadecanoyl-sn-glycero-3-phospho-
ethanolamine (NBD-PE: kex 463 nm, kem 535 nm). Quenching of
lipid-bound NBD by FRET to N,N-NDIs indicates the relative
quantity of delivered photosystem 1. With the disassembling TFE
as best (᭹) and the assembling MeOH as worst transporter
(᭺), significant differences were found in the ability of solvent
additives to deliver photosystem 1 to EYPC LUVs. Controls with
monomeric N,N-NDIs 9a (ꢀ)16,18 confirmed that poor delivery by
MeOH is compound-specific and not intrinsic (Fig. 4A).
4 ↔ 5 exhibited no cooperativity under the present conditions.17
Different to both symmetric controls, this finding suggested that
the introduced asymmetry has a negative effect on the formation
of active self-assembly. Facilitated antiparallel self-assembly into
the hydrophobic p-helix 5 by dipole–dipole attraction before
reaching the membrane could explain both poor activity and
lack of cooperativity. Alternatively, these results could also be
explained by hindrance of parallel self-assembly into 4 by dipole–
dipole repulsion in the membrane. Moreover, vectorial control
of partitioning by the terminal carboxylate might be lacking or
opposite to expectations. Overall, we caution that a number of
pathways to the functional system are conceivable to account
for the reduced activity and cooperativity found with push–pull
photosystems.
In summary, the introduction of asymmetry in photoactive
rigid-rod p-stack nanoarchitecture is found to reduce fluorescence,
to accelerate photoinduced charge separation, but to have little
effect on charge recombination. These overall favorable properties,
however, were not reflected in higher photosynthetic activity
in lipid bilayer membranes. Hindered formation of the active
suprastructure might account at least in part for these overall
complex and surprisingly small effects. We conclude that vectorial
self-assembly of asymmetric photosystems in lipid bilayers will be
very challenging.
We thank D. Jeannerat, A. Pinto and S. Grass for NMR mea-
surements, P. Perrottet, the group of F. Gu¨lac¸ar and C. A. Schalley
for MS measurements, H. Hagemann for access to the Xe lamp,
one referee for helpful suggestions, and the Swiss NSF for financial
support. S. B. is a fellow of the Roche Research Foundation.
Notes and references
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Fig. 4 Intensity-normalized concentration dependence for (A) membrane
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