Wide-range light-harvesting donor-acceptor assemblies through specific intergelator interactions via self-assembly

Article abstract of DOI:10.1002/chem.201103855

We have synthesized two new low-molecular-mass organogelators based on tri-p-phenylene vinylene derivatives, one of which could be designated as the donor whereas the other one is an acceptor. These were prepared specifically to show the intergelator inte

Full text of DOI:10.1002/chem.201103855

FULL PAPER
DOI: 10.1002/chem.201103855
Wide-Range Light-Harvesting Donor–Acceptor Assemblies through Specific
Intergelator Interactions via Self-Assembly
Suman K. Samanta[a] and Santanu Bhattacharya*[a, b]
Abstract: We have synthesized two
new low-molecular-mass organogela-
tors based on tri-p-phenylene vinylene
derivatives, one of which could be des-
ignated as the donor whereas the other
one is an acceptor. These were pre-
pared specifically to show the inter-
organic solvents. Evidence for inter-
AHCTUNGTRENNgGUN elator interactions was acquired from
of four chromophores was built up by
inclusion of two known dyes (anthra-
cene and rhodamine 6G) for the
energy-transfer studies. Interestingly,
an energy-transfer cascade was ob-
served in the assembly of four chromo-
phores in a particular order (anthra-
cene-donor-acceptor-rhodamine 6G),
and if one of the components was re-
moved from the assembly the energy
transfer process was discontinued. This
allowed the build up of a light-harvest-
ing process with a wide range. Excita-
tion at one end produces an emission
at the other end of the assembly.
various spectroscopic, microscopic,
thermal, and mechanical investigations.
Due to the photochromic nature of
these molecules, interesting photophys-
ical properties, such as solvatochrom-
ism and J-type aggregation, were clear-
ly observed. An efficient energy trans-
fer was exhibited by the mixture of
donor–acceptor assemblies. An array
ACHTUNGTRENNUNGgelator interactions at the molecular
level by using donor–acceptor self-as-
sembly to achieve appropriate control
over their macroscopic properties. In-
termolecular hydrogen-bonding, p-
stacking, and van der Waals interac-
tions operate for both the individual
components and the mixtures, leading
to the formation of gels in the chosen
Keywords: chromophores · donor–
acceptor systems · energy transfer ·
gels · reversible reactions
Introduction
important for addressing fundamental questions in the
energy-transfer process but is also of great interest in devel-
oping advanced materials, including optoelectronic devices,
organic light emitting diodes (OLEDs), and solar cells.[8]
Thus, developing light-harvesting systems comprised of suit-
able chromophore arrays holds considerable potential and
challenge for emerging new directions.
Natural photosynthesis occurs with the absorption of sun-
light and the manifestation of a continuous unidirectional
energy transfer process between chromophores plays a
significant role in the efficient light-harvesting process.[1]
The organization of dye assemblies at suitable distances and
in a specific orientation triggered by noncovalent interac-
tions is responsible for the fast migration of excitation
energy.[2] The phenomena of natural-light harvesting are
best understood through synthetically designed molecular
systems in which supramolecular interactions play a signifi-
cant role in the chromophore assembly in appropriate orien-
tations.[3] The supramolecular assemblies used to design arti-
ficial light-harvesting systems include molecular arrays,[4]
The supramolecular self-assembly of low-molecular-mass
gelators (LMMG)[9] gives rise to physical gels that provide
an exceptional opportunity to assist the energy-transfer
process.[10] The self-assembly brought about by dipole–
dipole, hydrogen-bonding, van der Waals, and p-stacking in-
teractions between appropriate chromophoric molecules
provide a novel chromophore assembly.[11] Interesting photo-
physical properties were observed in oligo(p-phenylene vi-
nylene)-based chromophores and research to this end has
ACHTUNGTRENNUNG
supramolecular complexes,[5] dendrimers,[6] and light-
harvesting antennae.[7] This self-assembly process is not only
been spearheaded by Meijer[12] and Ajayaghosh et al.[13]
A
number of cases has been studied by using synthetic donors
and acceptors to modify the efficiency of energy transfer
mostly in binary composites.[14] However, there are relatively
few chromophore arrays containing more than two mole-
cules interacting through noncovalent interactions that lead
to energy transfer mentioned in the literature.[15] Herein, we
discuss the donor–acceptor interactions in terms of hydro-
gen bonding and energy transfer between two new organo-
gelators (D and A, Scheme 1) based on a central tri(p-phen-
[a] Dr. S. K. Samanta, Prof. Dr. S. Bhattacharya
Department of Organic Chemistry
Indian Institute of Science
Bangalore 560012, Karnataka (India)
Fax : (+91)80-23600529
E-mail: sb@orgchem.iisc.ernet.in
[b] Prof. Dr. S. Bhattacharya
Chemical Biology Unit
Jawaharlal Nehru Centre for Advanced Scientific Research
Bangalore 560064, Jakkur (India)
AHCTUNGTREGyNNUN lene vinylene) (TPV) moiety. As per the proposed model
reported earlier,[16] we successfully demonstrate herein an
extended energy-transfer array that depends on the choice
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103855.
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Gelation studies: The gelation efficiency of compounds A
and D was studied in different aliphatic and aromatic hydro-
carbon solvents. It has been observed that aliphatic hydro-
carbon solvents, such as n-hexane, n-heptane, and
n-dodecane, could only gelate A (minimum gelator concen-
tration (MGC) 2.22 mm in n-heptane). However, compound
D could not be gelated in aliphatic hydrocarbons. This may
be due to the presence of four polar OH moieties, which
lead to precipitation on cooling after solubilization in al-
AHCTUNGTREGkNNNU anes. In contrast, in aromatic hydrocarbons such as tol-
uene, compound A did not form a gel but D formed a physi-
cal gel (MGCꢀ19.46 mm in toluene). The MGC was high
for D presumably because of the flexible section between
the OH and the aromatic moiety, which may not allow opti-
mum
Scheme 1. Molecular structures of TPV gelators (A and D) and commer-
cially available dyes (N and R).
hydrogen-bonding interactions between molecules of D. The
van der Waals and aromatic p-stacking interactions are
common for both the molecular assemblies. An interesting
situation arose when the gelation was attempted by using a
mixture of A+D (1:1 molar ratio; denoted as C), which
failed to form a gel in toluene but was able to form a gel in
of the initial donor and final acceptor coupled with TPV
chromophores to manifest an efficient and wide range of
light harvesting. We have reported herein the self-assembly
properties of the gelators alone or their mixtures composed
of the TPV backbone, and the intergelator interactions were
probed by using several physical methods, including a
number of microscopic and spectroscopic techniques. Also,
a cascade energy transfer was observed for assorted chromo-
phore assemblies containing four different luminescent com-
pounds, for example, anthracene, D, A, and rhodamine 6G.
It has been shown that when one of the components was re-
moved from the assembly, the energy transfer process was
discontinued.
A
This demonstrates evidence of interactions between A and
D that lead to the gelation of D with the aid of A in n-hep-
tane. All the gels were translucent in nature (Figure S1 in
the Supporting Information). The toluene gel of D appeared
greenish-yellow in color and a brownish-yellow color was
observed for the n-heptane gel of both A and C under
normal daylight. Under l=365 nm UV light, the toluene gel
of D showed a sky-blue emission, whereas the n-heptane
gels derived from A and C both showed yellow emission.
The gelator D has four hydroxyl groups that can serve as
hydrogen-bonding donors and gelator A has four ester moi-
eties that could act as hydrogen-bonding acceptors. Thus, a
mixture of A and D in 1:1 molar ratio could form a gel; the
gelation also persisted when we varied the ratio to
2:1 (A/D). However, the gelation did not occur when D:A
was mixed in a 2:1 ratio. This again indicates the crucial role
of the complementary hydrogen-bonding motifs between A
and D.
The optimized structures (B3LYP, 6-31G*) of A showed a
syn orientation among the two carbonyl groups in the ester
moieties at one terminal and it was preferred over the anti
orientation (Figure S2 in the Supporting Information). Two
adjacent carbonyl groups in the syn orientation are posi-
tioned perpendicular to each other to avoid dipolar repul-
sion. The syn orientation of the two OH groups in gelator D
was observed to be more stable due to the internal stabiliza-
tion by hydrogen bonding than the anti conformation. The
energy profile diagram shows that A is energetically more
stable due to the greater resonance stabilization energy
compared to D. However, none of the optimized structures
show complete planarity in the conjugated chromophoric
part. Thus in the composite, gelators A and D might adopt a
syn conformation to give better interaction between them
through intermolecular hydrogen-bonding, p-stacking, and
van der Waals interactions.
Results and Discussion
The present investigation includes the synthesis of two new
organogelators based on the TPV moiety. They contain the
same basic backbone and differ only at the termini, and
demonstrate intergelator interactions through hydrogen-
bonding, p-stacking, and van der Waals interactions. This
leads to the build up of an array with an assembly of differ-
ent chromophores for light harvesting, based on their donor/
acceptor properties.
Synthesis: Lipophilic TPV derivatives A and D were synthe-
sized in high yields by using
a one-step procedure
(Scheme S1 in the Supporting Information) starting with the
TPV-based bis-aldehyde (1) prepared according to a report-
ed procedure.[17] The aldehyde moieties at the end functional
groups provide an opportunity to attach functionalities
through coupling with active methylene compounds. Thus A
was obtained through a base-catalyzed Knoevenagel re-
ACHTUNGTRENNUNGaction of 1 with dimethyl malonate (DMM). Reduction of A
by using lithium aluminum hydride afforded tetra alcohol D.
1
Each compound was adequately characterized by FTIR, H
and 13C NMR spectroscopy, MALDI-TOF mass spectrosco-
py, and elemental analysis (for details see the Experimental
Section in the Supporting Information).
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Light-Harvesting Donor–Acceptor Assemblies
FULL PAPER
IR spectroscopy studies: FTIR spectral investigations prob-
ing the existence of hydrogen-bonding interaction between
A and D were performed to determine the proximity of
both the gelators in the composite forming the self-assembly.
The IR spectra were recorded for the preformed gel in
n-heptane for A and in toluene for D and were compared
with the corresponding sol in chloroform. When the spec-
trum of gelator A was recorded in chloroform and compared
with the gel samples of A in n-heptane, the vibrational fre-
quencies due to carbonyl group did not show any significant
shift and appeared at 1733 cmÀ1 in both spectra (Figure 1).
(G’’) by an order of magnitude, which indicates a dominant
elastic behavior (stiffness), that is, resistance to flow under
applied stress (Figure 2).[19] The oscillatory frequency re-
Figure 2. Rheological studies showing the oscillatory amplitude sweep ex-
periment of A in the presence of different amounts of D (total concen-
tration fixed at 10 mgmLÀ1 in n-heptane). &: G’ (A), : G’’ (A), : G’
~
*
!
^
(A/D, 2:1), : G’’ (A/D, 2:1), : G’ (A/D, 1:1), N: G’’ (A/D, 1:1).
sponse of the gels reveals that G’ is independent of the ap-
plied angular frequency and much higher than G’’ over an
angular frequency range of 0.5 to 100 radsÀ1 (Figure S4 in
the Supporting Information). It appears that the gel of A in
n-heptane is mechanically more viscoelastic than the gel of
D in toluene. The viscoelasticity of the individual gels were
not abolished in the donor–acceptor composite (D+A), as
shown by the oscillatory rheological measurements. Under
applied angular frequency or shear stress, the solid-like be-
havior (G’) of composite C decreased as the percentage of
D was increased. The yield stress (sy), which is an applied
stress above which the gel starts to flow, also decreased as
the amount of D in the composite was increased.
Figure 1. FTIR spectra of A, D, and C in different solvents showing the
effect of hydrogen-bonding interactions.
À
However, the O H stretching frequency of D appeared at
3360 cmÀ1 in chloroform, whereas D in toluene as a gel
À1
À
showed a broad O H stretching band at 3337 cm . There-
fore, a 23 cmÀ1 shift was observed, which indicates a gel for-
mation induced by hydrogen bonding. Composite C showed
À
a
stretching frequency due to the O H function at
3360 cmÀ1 in chloroform. This shifted to 3306 cmÀ1 when the
gel of C in n-heptane was probed under IR. Thus, a pro-
nounced 54 cmÀ1 shift in the IR stretching frequency in the
gelation of the composite was observed, which indicates the
existence of strong hydrogen-bonding interactions between
A and D in the gel. A lower value in the stretching frequen-
cy may be due to the nature of the hydrogen bonding in the
mixture, in which the hydrogen bonding of the carbonyl
group of A with the OH group of D could be expected.
However, the IR stretching frequency for the carbonyl
group showed a change of approximately 6 cmÀ1. This type
of weak hydrogen-bonding interaction between the terminal
functional groups of the organogelators is necessary to
anchor the formation of the self-assembly.[18] This was fur-
ther assisted by the p–p interactions between the aromatic
backbones of TPV and the van der Waals interactions be-
tween the long lipophilic chains of the gelators (Figure S3 in
the Supporting Information).[17b]
Differential scanning calorimetry (DSC) and polarized opti-
cal microscopy (POM): The thermotropic behavior of the
gels of A and C in n-heptane were investigated to reveal the
influence of the interaction on the self-assembly process.
The gel-to-sol transition temperature (Tm) did not show any
significant changes for either gel (Figure 3). However, a 38C
decrease in the sol-to-gel transition temperature was ob-
served for C compared to A alone. Thus, doping with D did
not significantly alter the thermal stability of A in the com-
posite gel. A small decrease in Tm may be accounted for by
the additional hydrogen-bonding interactions between D
and A.
The decrease in the melting and solidification tempera-
ture was also observed in C when examined by using solid-
phase DSC (Figure S5 in the Supporting Information). In
the heating cycle A and C showed sharp melting behavior
but D showed a broad melting transition. The melting tem-
peratures of A and C did not show any significant changes.
However, the solidification of C from its molten state took
place at a slightly lower temperature (ꢀ28C) than that of A
alone.
Rheological studies: The oscillatory stress amplitude of the
gels of A in n-heptane or D in toluene showed greater
values for the elastic modulus (G’) than the viscous modulus
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S. Bhattacharya and S. K. Samanta
Figure 3. DSC thermograms of the gels of A and C in n-heptane at a
final concentration of 8 mm.
Samples A, D, and their mixtures were optically birefrin-
gent at progressively decreasing temperatures from the iso-
tropic melt under a polarized optical microscope. This indi-
cated the anisotropic growth of the gelator molecules in
three dimensions to give rise to different birefringent tex-
tures.[20] POM images show a greenish-yellow texture with
smaller domain size for D and a bigger red-colored domain
texture of for A (Figure S5 in the Supporting Information).
However, a smaller red-colored domain texture was ob-
served for C when cooled from the isotropic melt. Thus, the
texture in C was governed by D, most likely due to the
hydrogen-bonding interactions, and the color was induced
by A.
Figure 4. AFM topography images of a) D, b) A, and c,d) C. The embed-
ded graph in each image shows the respective cross-section height pro-
file.
paratively thinner fibers with diameters of approximately
250 nm were observed in D, compared with diameters ap-
proximately equal to 1 mm for fibers of A. However, the fi-
brous nature disappeared completely and an aggregated
morphology appeared in C, which indicated the intergelator
interactions.
Microscopic organization: Various microscopic studies were
undertaken to observe the morphology in the individual gel
components and the mixture after self-organization. The lu-
minescent nature of the gelator molecules allowed us to ob-
serve the superstructures created in a thin film on a glass
slide under a fluorescence microscope.[20] When excited by
using UV irradiation (l=340–400 nm), the fibers in the thin
film showed greenish-yellow emission for D and brownish-
yellow emission for A and C (Figure S6 in the Supporting
Information). The presence of three-dimensional aggregates
was clearly observed in all instances. Notably, in the case of
D smaller fibers were observed throughout the sample
along with the presence of rod-like structures that could
form through the assembly of the smaller fibers. Also, a
smaller fiber-like morphology was observed in both A and
C. In C, the color was similar to A, which is a probable con-
sequence of the energy transfer from D to A.
Transmission electron microscopy (TEM) images showed
greater compactness in the aggregated fibers in C compared
with D and A (Figure S7 in the Supporting Information). A
honeycomb-like structure was observed in all instances. The
diameter of the fibers were comparatively uneven in D and
A (50–100 nm) compared with C. Also, the fiber diameter
increased in the composite (ꢀ200 nm) and fibers were or-
ganized almost uniformly throughout the sample.
UV/Vis and fluorescence studies: This class of molecule is
chromophoric, which allowed evaluation of their photophys-
ical properties.[21] The ground-state interactions between A
and D in the solution phase were examined by using
UV/Vis studies in both n-heptane and toluene. The absorp-
tion maxima (lmax) of D and A (1ꢁ10À5 m for both) in n-hep-
tane was observed at l=402 and 442 nm, respectively (Fig-
ure 5a). Interestingly, composite C with a 1:1 molar ratio of
D and A (0.5ꢁ10À5 m each) in n-heptane did not show two
different bands but merged to a single band at l=427 nm,
which indicated that the individual bands were approximate-
ly averaged out in C. On increasing the concentration of D
and A (1ꢁ10À5 m each), the intensity of the absorption band
increased without showing any shift in the lmax. Similar ab-
sorption spectra were obtained in toluene, with a redshift of
6 nm for both D (l=408 nm) and A (l=448 nm); the band
for C appeared at l=431 nm (Figure S8a in the Supporting
Information).
When we increased the concentration of one component
while keeping the other fixed, a stepwise shift in the lmax
with a concomitant increase in the intensity was observed,
probably as a consequence of mutual interactions between
D and A (Figure S9 in the Supporting Information). A plot
of either absorbance or lmax as a function of the ratio of D
to A showed a crossover at the ratio of 1:1 (D:A) in both
situations, irrespective of whether A was titrated with D or
Atomic force microscopy (AFM) images showed the exis-
tence of fibrillar networks in both A and D (Figure 4). Com-
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Light-Harvesting Donor–Acceptor Assemblies
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hydrogen-bonding, p-stacking,
and van der Waals interactions,
and this should thus allow im-
proved energy transfer. The
solution
of
D
in
n-heptane was a light yellow-
ish-green color, but C adopted
the yellow color of A under
normal daylight (Figure S8d in
the Supporting Information).
Also, under l=365 nm UV
light, D showed sky-blue emis-
sion and A showed green emis-
sion. Mixture
C
exhibited
green emission, which again in-
dicates an energy transfer from
D to A. On titration of D
(1ꢁ10À5 m) with A in n-hep-
tane, a progressive disappear-
ance of the l=457 nm band
was observed, which indicates
greater extent of energy trans-
fer (Figure 6a). Concentration-
dependent emission spectra
Figure 5. a) Absorption and b) emission spectra of D, A and C in n-heptane (excitation at l=380 nm, the con-
centration of C indicates D/A is 1:1). c) Emission of C on excitation at l=380 nm (c) and direct excitation
of A at l=420 nm (a) illustrating amplified emission. d) Emission spectra of C (1ꢁ10À5 m) in n-heptane
(c) and toluene (a).
D was titrated with A. The absorption maxima in C did not
show two different peaks at the positions of individual
chromophores of D and A, but they appeared to be almost
a single component, which indicated the existence of mutual
interactions between D and A in the ground state.
showed an optimum concentration of 5ꢁ10À5 m of C re-
quired for maximum energy transfer in n-heptane, at which
the l=457 nm peak disappeared completely (Figure 6b).
The effect of temperature was also reflected in the aggre-
gation patterns of D and A in n-heptane. When a solution
of either D or A (2ꢁ10À5 m) was cooled from 65 to 108C,
The emission maximum of D (1ꢁ10À5 m) in n-heptane ap-
peared at l=457 nm with a hump at l=484 nm; for A the
maximum was at l=512 nm with a hump near l=550 nm
(Figure 5b). A good overlap between the emission band of
the donor (D) and the absorption band of the acceptor (A)
and a higher extinction coefficient of the acceptor indicates
a possible transfer of resonant excitation energy from the
donor to the acceptor (Figure S8b in the Supporting Infor-
mation).[22] When mixture C (1ꢁ10À5 m) was excited at
l=380 nm, it showed a sharp decrease in the fluorescence
emission at l=457 nm and an increase in intensity at
l=512 nm. This indicates an energy transfer from D to A
rather than direct excitation of the acceptor. Evidence of
energy transfer was also obtained by comparing the emis-
sion of A excited directly at l=420 nm and the emission of
C excited at l=380 nm (Figure 5c). The intensity of C excit-
ed at l=380 nm was greater than that of directly excited A,
which indicates that the excitation energy is transferred
from D to A. In toluene, D (1ꢁ10À5 m) showed emission
maxima at l=463 and 489 nm, A (1ꢁ10À5 m) showed peaks
at l=526 and 564 nm, and C (1ꢁ10À5 m) showed peaks at
l=465 and 497 nm and a broad hump at l=524 nm (Fig-
ure S8c in the Supporting Information).
The extent of energy transfer was more pronounced in n-
heptane than in toluene, as observed from the ratio of emis-
sion intensity at l=457 and 512 nm (Figure 5d). Because C
forms a gel in n-heptane and not in toluene, the proximity
between D and A should be greater in n-heptane due to
Figure 6. a) Titration of D with increasing quantities of A and b) emission
spectra of incremental concentrations of C in n-heptane. lex =380 nm for
both graphs.
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S. Bhattacharya and S. K. Samanta
Figure 7. Absorption and emission spectra showing temperature-dependent aggregation of a,d) D (20 mm); b,e) A (20 mm); and c,f) C (20 mm of D +
20 mm of A) in n-heptane.
the intensity of the absorption maxima decreased, which in-
dicates the appearance of an aggregated state (Figure 7).[17b]
Interestingly, when C (2ꢁ10À5 m) was subjected to tempera-
ture variation, the intensity decreased, with a redshift of
about 20 nm of the band that indicated a J-type aggrega-
tion[23] between D and A. This was also observed in the
emission spectra under variable temperature conditions. A
monomeric emission was observed at higher temperatures
for D and A in n-heptane. The emission maximum showed
an intensity loss upon a decrease in temperature and finally
this reached an aggregated state showing emission at higher
cates that the effect of the solvent polarity was more pro-
nounced in the excited state than in the ground state. Mo-
notonous redshifts of the emission maxima of about 10 nm
for D and about 35 nm for A were observed upon changing
the solvent from n-heptane to chloroform (Figure 8). How-
ever, an abrupt change in the emission spectra, with a con-
comitant redshift and intensity loss, was observed in ethanol
and acetonitrile for both D and A, most likely due to the
self-aggregation process. Also, the redshift in the emission
maxima was greater in A than in D, which suggests that the
excited state of A is more sensitive to the solvent polarity
than that of D. This could be explained from the calculated
frontier molecular orbitals of the energy-minimized struc-
tures, which show that the lowest unoccupied molecular or-
bitals (LUMOs) are more polar than the corresponding
highest occupied molecular orbitals (HOMOs). It is clear
from the molecular structure of the gelators that through
conjugation with the end functional group (ester) is possible
only for A due to the presence of sp2 carbons as linkers, and
this is not possible for D (sp3-hybridized linkers). Therefore,
electron delocalization is restricted in the central TPV core
in D. Thus, the calculated LUMO showed insignificant delo-
calized orbital contribution in the sp3 centers in D but a sig-
nificant delocalized orbital contribution in the sp2 centers of
A. This indicates that the LUMO of A is more polar than
the LUMO of D. Thus, in a polar solvent, the LUMO of A
should be more stabilized and this leads to a decrease in the
HOMO–LUMO gap, which should result in an increase in
the wavelength of the emission maximum (the redshifts).
wavelengths (lꢀ550 nm). At higher temperatures,
C
showed an emission maximum at l=506 nm with a small
hump at l=457 nm. Upon cooling, there was an intensity
loss in the spectrum with a concomitant redshift (ꢀ15 nm)
of the emission band; this finally reached an aggregated
state. This suggests that, upon cooling, the mixture first
formed a primary J-type self-assembly followed by a sec-
ACHTUNGTRENNUNGondary self-aggregation. It may be noted that the l=457 nm
band disappeared at lower temperatures, which indicates an
increase in the energy transfer efficiency due to the associa-
tion of D to A in close proximity.
Solvatochromism was observed for both D and A depend-
ing on the polarity of the organic solvents. The absorption
maximum showed a redshift of about 8 nm for D and about
10 nm for A upon changing the solvent from n-heptane to
chloroform (Figure S10 in the Supporting Information). The
effect of solvent-induced shifts was more pronounced in the
emission maxima than in the absorption maxima. This indi-
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Light-Harvesting Donor–Acceptor Assemblies
FULL PAPER
(10:1:1:1 molar ratio) exhibited
only one emission maximum at
l=565 nm when excited at
l=356 nm. Thus each new ad-
dition of an individual chromo-
phore led to
a quantitative
energy transfer to the higher
wavelength, which finally accu-
mulated at that of R. The cor-
responding excitation spectra
at the respective emission
maxima is shown in Figure S12
(see the Supporting Informa-
tion). Interestingly, in this
light-harvesting assembly that
contains N-D-A-R, excitation
at any point in the whole spec-
tral region showed
a single
emission at R (Figure 9b). A
remarkable shift in lmax (l
ꢀ210 nm) was obtained when
the assembly was excited at
l=356 nm. This resulted in the
emission at
R (l=566 nm),
which indicates efficient light
harvesting. However, the emis-
sion intensity could be tuned
depending on a multitude of
factors, including the excitation
wavelength of the donor, the
distance from the donor to the
final acceptor (R) in the se-
Figure 8. Solvatochromic effect of a) D and b) A under fluorescence spectroscopy. [D]=[A]=1ꢁ10À5 m in
each case, lex =380 nm for D and 420 nm for A. DFT B3YLP/6-31G* level single point energy calculations of
D and A in n-heptane using PCM solvent model. For simplicity in computation, the n-hexadecyloxy side-
chains were replaced with methoxy groups. Arrows indicate the charge delocalization.
However, this effect would be less pronounced in D, which
clearly matches with the experimental observations.
quence, the overlap integral, the quantum efficiency of the
donor, the extinction coefficient of the acceptor, and the in-
teraction between the donors and R.[22] Thus, the emission
intensity of R increased on approaching the excitation wave-
length from N to R. Interestingly, the color of the solution
shifted accordingly on successive addition of different chro-
mophores in that particular order. Visual examination
showed the solution of N to be colorless, N+D to be green-
ish-yellow, N+D+A to be deep yellow, and N+D+A+R to
be reddish-orange under normal daylight (Figure 9c). When
the respective mixtures were observed under l=365 nm UV
light, they showed a blue emission for N, a sky-blue emis-
sion for N+D, a green emission for N+D+A, and a red-
dish-orange emission for N+D+A+R. Note that an almost
quantitative efficiency in energy transfer was obtained in
solution only at 5ꢁ10À5 m. This is due to the efficient p-
stacking interactions between the molecules aided by van
der Waals and hydrogen-bonding interactions between D
and A in n-heptane. This can be anticipated from the obser-
vation that R alone precipitated readily from n-heptane
within a few minutes, whereas it remained in solution along
with the assembly for a number of days.
To establish a light-harvesting array, we chose two known
dyes, that is, anthracene (N) and rhodamine 6G (R; Scheme
1), because their emission and absorption spectra provide a
significant overlap along a particular order of the probes
and the two complementary gelators, N-D-A-R (Figure S11a
in the Supporting Information). We have already shown that
due to the presence of ground-state interaction between D
and A, the individual absorption maximum merged into a
single absorption maximum. However, this kind of ground-
state interaction was not observed in any combination of the
four chromophores except D and A only (Figure S11b in the
Supporting Information).
Figure 9a showed a stepwise addition of all the four chro-
mophores together to achieve a stepwise energy transfer in
the assembly. When N (5ꢁ10À4 m) was excited at l=356 nm,
it showed an emission maximum at l=400 nm. Addition of
D (5ꢁ10À5 m) showed effective quenching of the emission at
l=400 nm and resulted in new emission maxima at l=461
and 486 nm. Due to the weak emissive property of N, a ten-
fold excess was used relative to D. This indicated transfer of
the excitation energy from N to D. A ternary mixture of N+
D+A (10:1:1) showed an emission maximum at l=518 nm
and the mixture of all four chromophores N+D+A+R
To establish the participation of each chromophore in the
assembly as a stepping stone for energy transfer, we per-
formed a number of control experiments. Concentrations of
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S. Bhattacharya and S. K. Samanta
Figure 10. Emission spectra showing a) individual chromophores and the
N+D+A+R mixture, b) the stepwise energy transfer, c) the energy
transfer efficiency from N to D, A, and R, and d) termination of energy
transfer with elimination of D or A from the assembly in n-heptane at
l=356 nm excitation. [N]=1ꢁ10À4 m and [D]=[A]=[R]=1ꢁ10À5 m in
each case.
emission when excited at l=356 nm. A mixture of all four
chromophores showed loss in the emission intensity for N,
D, and A and accumulated at R, which was more prominent
at increased concentrations (Figure S13c in the Supporting
Information). The stepwise energy transfer was also demon-
strated at this lower concentration. Chromophore N alone
(1ꢁ10À4 m) showed an emission maximum at l=400 nm
when excited at l=356 nm in n-heptane (Figure 10b). Addi-
tion of D (1ꢁ10À5 m) to the solution of N resulted in a de-
crease in the intensity of N at l=400 nm and the emission
intensity accumulated at l=455 nm. The ternary complex
after addition of A (1ꢁ10À5 m) led to a further loss in fluo-
rescence emission intensity at l=400 and also at 455 nm,
and showed a maximum emission at l=508 nm due to A.
Finally, addition of R (1ꢁ10À5 m) extracted the energy from
the assembly, leading to an intensity loss of A at l=508 nm.
However, due to the low emissive nature of R in n-heptane,
the emission intensity was not so prominent at this concen-
tration. Furthermore, it should be mentioned that a com-
plete energy transfer from N to D did not lead to amplified
emission because of the low emissive nature of N. Also, the
ratio of D/A and A/R was 1:1 in both cases, which indicates
that an excess of the acceptor is required for complete
energy transfer.
Figure 9. a) Stepwise energy transfer in the assembly of N-D-A-R excited
at l=356 nm. b) Emission spectra of the N-D-A-R mixture at different
wavelengths. c) The same solutions under normal daylight and l=365 nm
UV light. [N]=5ꢁ10À4 m and [D]=[A]=[R]=5ꢁ10À5 m in n-heptane in
each case. (A colored version of Figure 9c is available in the Supporting
Information).
5ꢁ10À4 m for N and 5ꢁ10À5 m for D, A, and R were chosen
because the variable-concentration emission spectra of the
mixtures of N+D, D+A, or A+R showed a complete
energy transfer at these concentrations (Figure 6b and Fig-
ure S13 in the Supporting Information). Also, the variable-
concentration emission spectra of the mixture of all four
chromophores showed an efficient energy transfer at the
above concentrations. The rate of energy transfer (kET; 3.6ꢁ
1013 mÀ1SÀ1 for N to D, 1.7ꢁ1014 mÀ1SÀ1 for D to A and 4.7ꢁ
1014 mÀ1SÀ1 for A to R) was calculated by using the Stern–
Volmer constants (kSV; 1.3ꢁ105 mÀ1 for N+D, 2.5ꢁ105 mÀ1
for D+A, 4.7ꢁ105 mÀ1 for A+R) and the individual life-
times (Figure S14 and Table S1 in the Supporting Informa-
tion).[24]
For the control experiment, fivefold diluted solutions
were used to gain a better understanding of the role of each
chromophore. The individual emission band due to each of
the four chromophores was clearly distinguishable in the as-
sembly in these diluted solutions. When the individual chro-
mophores were excited at l=356 nm they showed the re-
spective emission spectra due to the residual absorbance at
l=356 nm (Figure 10a). Chromophore R showed negligible
Thus, a mixture of N and D shows an emission at D (l=
455 nm) upon excitation at l=356 nm (Figure 10c). A lesser
extent of energy transfer was also observed from N to A
due to the spectral overlap of the emission of N and absorp-
tion of A. However, a negligible energy transfer from N to
R was observed due to the lack of spectral overlap. The effi-
ciency of energy transfer should be reduced if one or two
chromophores are removed from the N-D-A-R array. Thus,
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Light-Harvesting Donor–Acceptor Assemblies
FULL PAPER
elimination of A from the assembly showed discontinuation
of the energy transfer to R and the emission accumulated at
l=455 nm (Figure 10d). A mixture of N+D+A showed
emission at A (l=508 nm). However, elimination of D re-
vealed a lesser extent of energy transfer from N to R due to
the spectral overlap between N and A, as mentioned earlier.
The emission intensity of the mixture of N, D, and A ap-
peared at l=508 nm due to the continuation of the array.
Also, the mixture of D, A, and R showed greater emission
intensity when excited at l=380 nm due to the high emis-
sive nature of D.
formation). Lifetimes of the individual compounds are listed
in the Table S2 (see the Supporting Information). Lifetimes
of the assembly of N, D, A, and R were also recorded in so-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
the emission was recorded at l=400 nm. Addition of D to
the solution of N induced a decrease in the lifetime of N
with the average lifetime (tav) of 3.3 ns, which indicated an
energy transfer efficiency of only 8% (Table S3 in the Sup-
porting Information). However, at this concentration the
energy transfer reached its optimum and, therefore, addition
of A into this mixture (N+D) did not result in any further
decrease in the lifetime of N. However, in a thin film, the as-
sembly showed a progressive decrease in the lifetime of N
(emission recorded at l=420 nm), which indicated a contin-
uous unidirectional energy transfer from N to R. In the thin
film, N alone showed a bi-exponential decay with the aver-
age lifetime of 2.3 ns, which showed an energy transfer of
9% upon addition of D (N+D), 13% upon addition of A
(N+D+A), and up to 43% upon addition of R (N+D+A+
R; Table S4 in the Supporting Information). This indicates
that the process of energy transfer occurs through the Fçr-
ster pathway, in which the excitation energy of N migrates
to R through D and A. Although evidence of energy trans-
fer was observed in both solution and a thin film, the effi-
ciency of the energy transfer is greater in the thin film.[15,24c]
Therefore, all the results obtained herein supported the
construction of an effective energy transfer assembly. The
energy transfer occurs from N to R via D and A when the
first donor (N) in the assembly is excited. Figure 12 shows a
Energy transfer was also observed in the solid state, in a
thin film prepared on a quartz plate using a front-face ge-
ACHTUNGTRENNUNG
ometry.[24c] When excited at l=356 nm, N exhibited an
emission maximum at l=420 nm (Figure 11a). The mixture
Figure 11. a) Emission and b) excitation spectra of the assembly of N, D,
A, and R in the solid state.
of N and D (10:1 molar ratio) showed an emission maxi-
mum at l=528 nm. The ternary complex of N, D, and A
(10:1:1) showed an emission at l=568 nm. However, a mix-
ture of all the four chromophores (10:1:1:1) quenched the
emission due to the non-emissive nature of R in the solid
state. Thus R acted as an energy sink in the solid state. The
corresponding excitation spectra at their respective emission
maxima showed that the excitation maximum was located
near
l=380 nm (Figure 11b). However, the excitation spectra
gradually became broader on progressive doping of D and
A with N.
To probe the cascade energy transfer, lifetime measure-
ments were performed in n-heptane by using 5ꢁ10À4 m N
and 5ꢁ10À5 m D, A, and R (Figure S15 in the Supporting In-
Figure 12. Proposed model depicting the phenomenon of energy transfer
through donor–acceptor interactions between the chromophores.
model that depicts the process of energy transfer through
the association of four chromophores. The individual chro-
mophores are luminescent in nature. When they are brought
together, they show energy transfer in a particular order de-
pending on their spectral overlap. Also, elimination of any
chromophore from the assembly leads to the disruption of
the cascade energy transfer.
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S. Bhattacharya and S. K. Samanta
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Acknowledgements
We thank the Department of Science and Technology (J.C. Bose fellow-
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IISc and the Raman Research Institute for various instrumental facilities,
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Received: December 12, 2011
Revised: July 13, 2012
Published online: October 16, 2012
Chem. Eur. J. 2012, 18, 15875 – 15885
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15885