Synthesis and characterization of donor-acceptor chromophores for unidirectional electron transfer

Article abstract of DOI:10.1021/ol063000y

(Chemical Equation Presented) A series of electron-transfer chromophores containing a donor and acceptor linked by an alkyl spacer were synthesized, and their electronic spectra were investigated. By inclusion with amylose, the supramolecularly encapsulat

Full text of DOI:10.1021/ol063000y

ORGANIC
LETTERS
2007
Vol. 9, No. 5
801-804
Synthesis and Characterization of
Donor Acceptor Chromophores for
−
Unidirectional Electron Transfer
M. F. Pepitone,† G. G. Jernigan,‡ J. S. Melinger,‡ and O.-K. Kim*,†
Chemistry DiVision, Electronics Science & Technology DiVision and Institute for
Nanoscience, NaVal Research Laboratory, Washington, DC 20375-5342
oh.kim@nrl.naVy.mil
Received December 12, 2006
ABSTRACT
A series of electron-transfer chromophores containing a donor and acceptor linked by an alkyl spacer were synthesized, and their electronic
spectra were investigated. By inclusion with amylose, the supramolecularly encapsulated chromophores exhibit enhanced fluorescence quenching
with discrete distance dependence and acquire the ability to sustain self-assemblies of a densely packed supramolecular array on a SiOH/Si
substrate.
The key process in photosynthesis is a cascade of energy
transfer (ET) with directionality between arrays of photo-
receptor pigments (bound to a protein matrix) and a
subsequent electron transfer (eT) to a reaction center at a
remote distance.1 A variety of molecular architectures have
been devised to mimic the photosynthetic process, and these
include molecular wires and arrays,2 dendrimers,3 and
supramolecular assemblies.4 Through the investigation of
model donor-acceptor (D-A) chromophores, it has been
recognized that D-A distance is an important parameter for
a long-lived charge-separated state.5 However, due to con-
formational flexibility, the geometrical distance alone cannot
be an effective measure for control of the forward and
backward process. Research efforts have been made to
overcome this problem by rigidifying the chromophore by
incorporating a stiff linker between D-A units.6
This distance-related chain flexibility problem can be
addressed by a supramolecular architecture, which employs
a host polymer such as amylose7 capable of encapsulating
chromophores. Earlier, we developed a supramolecular
technique to align dipoles of nonlinear optical chromophores8
by helical encapsulation with amylose. This supramolecular
assembly provides the chromophores with many functional
† Chemistry Division.
‡ Electronics Science & Technology Division.
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10.1021/ol063000y CCC: $37.00
© 2007 American Chemical Society
Published on Web 02/08/2007

advantages such as the suppression of intermolecular interac-
tions by the confinement of single molecules in a rigid
environment, which yields a large (orders of magnitude)
increase in the fluorescence quantum yield.9
with an excess (5 equiv) of dibromoalkanes in nitromethane
to afford 1-(bromoalkyl)-1′-alkyl-4,4′-bipyridinium dibro-
mides 6-11. Synthesis of DASP-(R1-V-R) was completed
by refluxing 6-11 with an excess (5 equiv) of DASP 1 in a
4:1 (v/v) MeNO2/MeOH solvent system. After removal of
solvent, the unreacted DASP was removed by washing a 1:1
(v/v) H2O/MeOH solution of the crude product with MeCl2,
followed by recrystallization (see Supporting Information)
to afford 12-17. An initial attempt to synthesize 12-17
involved the formation of an R-bromo-n-alkyl-DASP moiety,
by refluxing DASP with an excess of R,ω-dibromoalkane
in acetonitrile, which was then reacted with a 1-alkyl-4-(4-
pyridyl)pyridinium bromide in a variety of solvent systems.
Unfortunately, isolation and purification of the product was
not achieved via this route. Appending a 4,4′-dipyridyl to
the R-bromo-n-alkyl-DASP moiety was achieved, but the
final N-alkylation was sluggish under a variety of conditions.
Encapsulation of the chromophores was achieved9 by
dilution of DMSO stock solutions of the chromophore (1 ×
10-3 M) and amylose (1 × 10-1 M) with DMSO, followed
by a dropwise addition of H2O with continuous stirring for
14 h at room temperature. A helical encapsulation with
amylose is evidenced by enhanced fluorescence8,9 of the
chromophore (vide infra).
Herein, we report the synthesis and photophysical proper-
ties of a series of 4-[4-(dimethylamino)styryl]pyridine (DASP)-
based chromophores with various D-A distances. The
DASP-based chromophores are studied in solution in an
encapsulation with helical amylose or in the nonencapsulated
(free) state. A combination of absorption, fluorescence, and
time-resolved fluorescence spectroscopy is used to study the
state of the encapsulation relative to the free state in solution
and to characterize the distance dependence of the electron
transfer. We also compare the fluorescence properties of the
chromophores in solution with that of a thin film, which was
made by casting an aqueous solution of the chromophores
onto a silicon substrate. Finally, we describe the film
morphology by atomic force microscopy (AFM).
Scheme 1. Synthesis of DASP-(R1-V-R) 12-17
Scheme 1 depicts the synthesis of the D-A chromophores
[DASP-(R1-V-R)] utilizing a 4-[4-(dimethylamino)styryl]-
pyridine moiety as the donor (D) and a 4,4′-dipyridyl moiety
as the acceptor (A). A simple synthetic approach has been
designed on the basis of quaternizing the pyridines in a
specific order. The first step was the synthesis of 4-[4-
(dimethylamino)styryl]pyridine 1 from literature proce-
dures.10 4,4′-Dipyridyl was then N-alkylated using a modi-
fication of the literature procedures,11 and the resulting 1-n-
alkyl-4-(4-pyridyl)pyridinium bromides 2-5 were reacted
Figure 1. Absorption and fluorescence spectra (a) and excited-
state lifetimes (b) of amylose-encapsulated 13-16 in 10% DMSO.
[13-16] ) 1 × 10-5 M and [amylose] ) 1 × 10-3 M.
Figure 1a shows absorbance and fluorescence measure-
ments for the series of encapsulated DASP-(R1-V-12) 13-
16 in 10% DMSO/H2O. Each of the fluorescence signals
has been scaled to the absorbance at the excitation wave-
length of 500 nm. We note that the fluorescence intensity
(7) (a) Collins, P.; Ferrier, R. Polysaccharides: Their Chemistry;
Wiley: Chichester, 1995; pp 478-523. (b) Lehmann, J. Carbohydrates:
Structure and Biology; Thieme: Stuttgart, 1998; pp 98-103. (c) Noltemeyer,
M.; Saenger, W. J. Am. Chem. Soc. 1980, 102, 2710. (d) Star, A.; Steuerman,
D. W.; Heath, J. R.; Stoddart, J. F. Angew. Chem., Int. Ed. 2002, 41, 2508.
(e) Kim, O.-K.; Je, J.; Baldwin, J. W.; Kooi, S.; Pehrsson, P. E.; Buckley,
L. J. J. Am. Chem. Soc. 2003, 125, 4426.
(8) Kim, O.-K.; Choi, L.-S.; Zhang, H.-Y.; He, X.-H.; Shih, Y.-H. J.
Am. Chem. Soc. 1996, 118, 12220.
(9) (a) Kim, O.-K.; Choi, L.-S. Langmuir 1994, 10, 2842. (b) Kim, O.-
K.; Je, J.; Melinger, J. J. Am. Chem. Soc. 2006, 128, 4532.
(10) Stevens, A. C.; Frutos, R. P.; Harvey, D. F.; Brian, A. A.
Bioconjugate Chem. 1993, 4, 19.
9a
of 16 is comparable to that of DASP-C22 (see Figures S1
and S2, Supporting Information), which has no acceptor. This
suggests that there is a high energy-barrier for eT of 16 over
the longest C12 spacer resulting in a negligible eT. After the
large gap of the fluorescence intensity between the spacer
lengths (R1) C12 and C8, a consistent decrease in the intensity
of DASP-(R1-V-12) persists between the bridge lengths C8
and C4. This is indicative of the distance dependency of
photoinduced eT between D-A (based on the fluorescence
quenching) between C8 and C4.
(11) (a) Tabushi, I.; Yazaki, A. Tetrahedron 1981, 37, 4185. (b) Gomez,
M.; Li, J.; Kaifer, A. E. Langmuir 1991, 7, 1797. (c) Hammarstro¨m, L.;
Almgren, M.; Lind, J.; Mere´nyl, G.; Norrby, T.; A˙ kermark, B. J. Phys.
Chem. 1993, 97, 10083.
The same trend is also reflected in the fluorescence
lifetimes of the encapsulated 13-16, which show a consistent
802
Org. Lett., Vol. 9, No. 5, 2007

decrease of the fluorescence lifetime with decreasing D-A
distances (Figure 1b). We note that the nonlinear decay
profiles indicate a somewhat inhomogeneous distribution of
chromophore emitters, perhaps reflecting various encapsula-
tion states in solution. We also observed that in the presence
of amylose the fluorescence intensity of 16 peaks near 10%
DMSO and attenuates with increasing DMSO volume
fractions (Figure S3, Supporting Information). This means
that the degree of the helical encapsulation depends strongly
on the DMSO/H2O mixture ratios and attains the maximum
under the present experimental condition (10% DMSO). A
similar observation was made in our previous work8,9 where
the fluorescence intensity of a nonpolar (OPV) chromophore
increased sharply in a water-rich DMSO mixture. However,
the difference observed between the polar and the nonpolar
chromophores is that the emission band of the polar DASP-
based chromophore, 16, tends to shift more to the blue with
increasing water fraction, whereas that of the nonpolar
counterpart is little affected by the solvent polarity changes.
This is probably due to the environmental sensitivity of the
polar chromophore (see Figure S4 and Note, Supporting
Information).
Information), relative to 100% DMSO where the aggregation
is almost free. In light of the fact that band broadening around
400 nm (aggregation band) is not significant, it is more likely
that the unusually large quenching observed (Figure 2a) is
largely associated with nonradiative deactivation by increas-
ingly polar solvents.12
A preliminary electrochemical study13a shows helical
encapsulation brings about a pronounced effect on the
voltammetric behavior.13b The half-wave potentials (E1/2) and
the peak-to-peak potential gap (∆Ep) for 16 with and without
encapsulation are E1/2 ) 0.135 and 0.215 V and ∆Ep ) 70
and 90 mV, respectively. We interpret that these potential
waves are attributable to oxidation of the D-subunit, and such
a fast redox reaction by the encapsulation is associated with
binding of the D-head, the N,N-dimethylamino group, to a
gold electrode. This is supported by an AFM image showing
a bundle of the encapsulated chromophore onto a gold
substrate while the free counterpart has no features.
In an attempt to modify the photophysical properties of
the donor, an extension of the DASP chromophore was
synthesized by reacting commercially available 4-vinylbenzyl
chloride with triphenylphosphine to yield 4-vinylbenzyl-
triphenylphosphonium chloride following literature proce-
dures.14 A Wittig reaction using NaOEt in EtOH afforded
the E precursor 18 that was subsequently used in a Heck
reaction with 4-bromopyridine hydrochloride to afford
DASSP 19. DASSP was N-alkylated with 1-(R-bromoalkyl)-
1′-hexadecyl-4,4′-bipyridinium dibromides by heating in
NMP to yield a series of DASSP-(R1-V-16) 20-22 (Scheme
2) (see Supporting Information). Full spectroscopic charac-
terization will be reported in a timely manner.
On the other hand, the emission properties of the free
DASP-(R1-V-12) exhibit a markedly different pattern relative
to their encapsulated counterparts (Figure 2a). First, the
Scheme 2. Synthesis of DASSP-(R1-V-R) 20-22
Figure 2. Absorbance and fluorescence spectra (a) and excited-
state lifetimes (b) of free 13-16 in 10% DMSO. [13-16] ) 1 ×
10-5 M and [amylose] ) 0 M.
fluorescence intensities of the free DASP-(R1-V-12) are all
significantly smaller (than the encapsulated counterparts), as
if a larger eT quenching took place. Second, the differences
in the fluorescence quenching among different bridge lengths
of the free chromophores are noticeably smaller, particularly
in the bridge lengths between C6 and C12. Finally, the
fluorescence lifetimes (Figure 2b) are nearly as short as the
instrumental response time (∼40 ps) and virtually indistin-
guishable from each other. These observations suggest the
presence of an additional quenching process that competes
with eT. One possibility is self-quenching due to aggregation
of the free chromophores in the water-rich DMSO mixture.
This is manifested by a measureable blue-shift (up to 15 nm)
of the absorption maximum and a steady and concomitant
decrease in the fluorescence intensity of 16 with increasing
water content in DMSO/H2O mixtures (Figure S5, Supporting
We have also characterized the encapsulated chromophore,
16, in a solid thin film. Self-assembled films on a hydrox-
ylated silicon substrate were prepared by wetting the substrate
with a dilute solution of the encapsulated 16. Fluorescence
spectra taken of these thin films reveal the characteristic line
shape of individual chromophores in solution and thus
(12) (a) Ephardt, H.; Fromherz, P. J. Phys. Chem. 1993, 97, 4540. (b)
Fromherz, P. J. Phys. Chem. 1995, 99, 7188.
Org. Lett., Vol. 9, No. 5, 2007
803

suggest that the chromophores remain inside the amylose
helix after film formation (Figure 3).
Figure 4. Tapping mode AFM image of encapsulated supramo-
lecular assembly 16 on a SiOH/Si surface.
Figure 3. Fluorescence spectra (left) and excited-state fluorescence
lifetime (right) of amylose-encapsulated 16: 10% DMSO solution
([16] ) 1 × 10-5 M and [amylose] ) 1 × 10-3 M) and solution-
cast supramolecular thin film on a SiOH/Si surface.
directional order, at least three important criteria must be
met: (1) rigidified helices by inclusion of a guest dye, (2)
substrate-specific functionality (e.g., N,N-dimethylamino
group) at the chromophore terminus for interface binding,
and (3) intermolecular hydrogen-bonding capability of the
host helix.
In conclusion, we have synthesized a series of D-A pair
chromophores, and the chromophores were encapsulated with
helical amylose. Photoinduced electron transfer (eT) of the
chromophores was investigated based on the fluorescence
quenching with respect to the encapsulation. For the encap-
sulated chromophores, the eT quenching shows a clear
dependence on the D-A distance along the helical axis. For
the free chromophores, the fluorescence quenching has a
mixed contribution both from eT and self-quenching due to
aggregation and/or solvent deactivation. The AFM image of
the supramolecules as a self-assembled thin film reveals
densely packed crystalline bundles. This occurs only when
the amylose is included with a guest dye, and no features
appear with the guest or host by itself.
Remarkably, there is an order of magnitude increase in
the excited-state lifetimes of the thin-film chromophores on
the silicon substrate (Figure 3), relative to that in solution.
Further, the fluorescence decay from the film is nearly
described by a single exponential, which suggests there is a
more homogeneous encapsulation of chromophores in the
film than in the solutions. If the chromophores were to release
out of the helix upon film formation, then it is expected that
their aggregation would significantly quench the emission
and produce much shorter fluorescence decay times.
Atomic force microscopy (AFM) images of the thin-film
chromophores (Figure 4) reveal a high packing density,
whose geometric configuration is about 14 nm tall and 50
nm wide. Given that the length scale of the amylose helix is
about 5 nm, the film shown most likely contains more than
one layer. We suggest that the first layer of the film in contact
with the surface is oriented with the terminal dimethylamine
binding to the surface. We believe that for the amylose-based
supramolecular system to self-assemble into a film with
Acknowledgment. We are grateful to the Office of Naval
Research for partial funding support.
(13) (a) Cyclic voltammetry of 16 with and without amylose in 25%
DMSO: 300 µL of sample + 100 µL of NaClO4 (0.100 M); potential range
-1.0 to +0.40 V vs Ag/AgCl, 3 M KCl; scan rate, 5 mV s-1. (b) However,
two reduction waves (E1/2) of the A-subunit (viologen), which are supposed
to appear around -0.3 and -0.7 V, respectively, were not observed upon
sweeping the potential down to -1.0 V, regardless of encapsulation of the
chromophore. At present, it is not clear whether it is due to a high polar aq
solution (25% DMSO), low concentration, or the orientation of the D-head
to the electrode surface (bringing the A-subunit far away).
Supporting Information Available: Experimental details
for synthetic procedures, NMR spectra of 1, 3-18, and 20-
22, and spectroscopic (absorption and emission) data. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(14) Bazan, G. C.; Oldham, W. J.; Lachicotte, R. J.; Tretiak, S.; Chernyak,
V.; Mukamel, S. J. Am. Chem. Soc. 1998, 120, 9188.
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