Fabrication and fluorescence properties of perylene bisimide dye aggregates bound to gold surfaces and nanopatterns

Article abstract of DOI:10.1039/b210423g

Perylene bisimide dyes with two different imide substituents have been synthesized by sequential imidization reactions to give the disulfide 8 bearing two perylene bisimide dyes. Aggregation properties of this bis-perylene dye were studied by UV/Vis absor

Full text of DOI:10.1039/b210423g

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Fabrication and fluorescence properties of perylene bisimide dye
aggregates bound to gold surfaces and nanopatterns
Ulrike Haas,a,b Christoph Thalacker,a Jo¨rg Adams,*b Ju¨rgen Fuhrmann,b
Silke Riethmu¨ller,a Uwe Beginn,a{ Ulrich Ziener,a Martin Mo¨ller,*a{
Rainer Dobrawaa{ and Frank Wu¨rthner*a{
aDepartment of Organic and Macromolecular Chemistry, University of Ulm, Albert-Einstein-
Allee 11, D-89081 Ulm, Germany
bInstitute of Physical Chemistry, Technical University Clausthal, Arnold-Sommerfeld-Str. 4,
D-38678 Clausthal-Zellerfeld, Germany
Received 23rd October 2002, Accepted 24th January 2003
First published as an Advance Article on the web 19th February 2003
Perylene bisimide dyes with two different imide substituents have been synthesized by sequential imidization
reactions to give the disulfide 8 bearing two perylene bisimide dyes. Aggregation properties of this bis-perylene
dye were studied by UV/Vis absorption and steady-state polarization-dependent as well as time-resolved
fluorescence spectroscopy and the results were compared to those of the symmetrical perylene bisimide dye 4.
In both cases, aggregation is expressed in a strong bathochromic shift of the excitation and the emission spectra
and increasing concentration results in an increase in lifetime from 5 to 8 ns. By means of the thiol linker, self-
assembly of 8 on gold surfaces was accomplished leading to surface-bound dye aggregates. Intense fluorescence
from these dye aggregates was observed on surfaces decorated with hexagonal gold patterns, whereas the
fluorescence is only weak on plain gold substrates.
contrast to other work on self-assembly of alkyl thiols on gold,6
1. Introduction
we do not expect ordering of the alkyl chains in a well-defined
way because their noncovalent interaction is weak and the
structural features are not favorable with large sized perylene
bisimide end groups.
Although the prospects of ‘molecular electronics’ have been
discussed already for several decades1 recent breakthroughs
like the electronic addressing of single molecules in Langmuir–
Blodgett or self-assembled monolayers2 and within thin
organic layers in field effect transistors3 raise our hopes that
some day in the future well-designed organic molecules will be
able to accomplish logical operations in a nanoscopic chip.
Nevertheless the challenges are still tremendous as full
exploitation of the electronic properties given by organic
molecules requires not only optimization of molecular proper-
ties but in addition highly defined organization of these
molecules in space and their connection to the outer world. In
our previous work, we developed methods to deposit noble
metal nanoclusters in defined patterns on surfaces by deposi-
tion, reduction and plasma-treatment of metal-loaded block
copolymer micelles4 as well as highly fluorescent and redox-
active perylene bisimide dyes that form columnar aggregates in
solution and in the solid state.5 In the present work, we
combine these two methodologies and report our first results
on self-assembly of perylene bisimide dye aggregates on a
patterned surface of well-defined gold clusters.
Accordingly, compound 8 seemed to be an ideal candidate as
its tetra-(tert-butylphenoxy)-perylene bisimide fluorophor is
known to form columnar J-type aggregates which exhibit
intense and long-wavelength photoluminescence.5 The choice
for a decyl spacer was mainly based on two arguments, which
are (1) to provide sufficient flexibility for optimization of both
interactions given by the thiol/gold and the perylene/perylene
contacts and (2) to establish a reasonable distance between the
gold surface and the dye aggregate that prevents strong
coupling between their electronic states.
2. Experimental
2.1. General
Solvents and reagents were purchased from Merck unless
otherwise stated and purified and dried according to standard
Our concept is based on the well-established linkage of
dialkyl disulfides to gold surfaces which combines strong
binding with reversibility.6 The latter property is important for
our purpose because it allows structural adjustments by
movement of the thiol groups on the gold surface to optimize
a
second important interaction which is given by the
noncovalent stacking of the perylene bisimide p-systems5 to
afford a surface-bound columnar dye aggregate (Scheme 1). In
{New address: Institute of Technical and Macromolecular Chemistry,
RWTH Aachen, Worringerweg 1, D-52056 Aachen, Germany.
{Institute of Organic Chemistry, University of Wu¨rzburg, Am
Hubland, D-97074 Wu¨rzburg, Germany. E-mail: wuerthner@chemie.
uni-wuerzburg.de; Fax: 149 931 888 4756.
Scheme 1 Concept for the formation of dye aggregates tethered on
surface-deposited gold nanostructures.
DOI: 10.1039/b210423g
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This journal is # The Royal Society of Chemistry 2003
767

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alkaline ethanolic solution of iodine (4.07 g, 0.016 mol in 50 ml
procedures.7 1,6,7,12-Tetra-(tert-butylphenoxy)-perylene-3,4,9,10-
tetracarboxylic acid bisanhydride (1) was synthesized accord-
ing to the literature.5,8 Column chromatography was performed
on silica gel (Merck Silica Gel 60, mesh size 0.2–0.5 mm). Rf
values for analytical thin layer chromatography have been
determined on Merck TLC sheets (Silica Gel 60, F254). NMR
spectra were recorded on a Bruker DRX 400 spectrometer
using TMS as internal standard, mass spectra were taken on
Bruker SSQ 7000 (CI or FAB) and Bruker Reflex III (MALDI-
TOF, positive mode) instruments and AFM measurements
were carried out on a Digital Nanoscope III (Digital Instru-
ments Inc., Santa Barbara, CA).
5% NaOH) by heating for 15 min. The precipitate was isolated
by filtration and recrystallized from ethanol to give 4.43 g
1
(72%) of 6. H NMR (400 MHz, CDCl3, J/Hz): dH 8.20 (d, J
8.1, 4H), 6.96 (d, J 8.0, 4H), 3.99 Hz (t, J 7.5, 4H), 2.70 (t, J 7.3,
4H), 1.84 (mc, 4H), 1.69 (mc, 4H), 1.32 (mc, 24H); 13C NMR
(100 MHz, CDCl3): dC 164.1, 141.2, 125.8, 114.3, 68.8, 39.0,
29.6, 29.3, 29.3, 29.1, 29.1, 28.8, 28.8, 28.4; m/z (FAB, 3-nba):
621 [M1H]1. Elemental analysis calcd. for C32H48N2O6S2: C
61.91, H 7.79, N 4.51. Found: C 62.02, H 7.96, N 4.49.
S,S’-Bis[10-(4’-aminophenyl)decyl]-disulfide (7). Disulfide 6
(1.09 g, 1.75 mmol), graphite (2 g) and hydrazine hydrate (2 g,
0.04 mol) were stirred at 60 uC under argon-gas atmosphere for
8 h during which three additional portions of graphite (each 1 g)
and hydrazine hydrate (each 1 ml, 0.01 mol) were added. The
reaction mixture was heated overnight under reflux and the
product was extracted in a Soxhlet extractor with ethanol.
After evaporation of the solvent, 0.8 g (81%) of 7 were isolated
as a yellow powder. Mp 84–86 uC (87–88 uC)10. 1H NMR (400
MHz, CDCl3, J/Hz): dH 6.69 (d, J 8.5, 4H), 6.58 (d, J 8.5, 4H),
3.79 (t, J 6.6, 4H), 2.61 (t, J 6.5, 4H), 1.65 (t, J 7.4, 4H), 1.60
(mc, 4H), 1.24 (mc, 24H); 13C NMR (100 MHz, CDCl3): dC
152.3, 139.7, 116.3, 115.6, 68.6, 39.1, 29.4, 29.3, 29.3, 29.1, 29.1,
28.8, 28.4, 25.9; m/z (FAB, 3-nba): 561 [M1H]1, 282
2.2. Synthesis
Imide anhydride 3. Perylene bisanhydride 1 (0.55 g, 0.56
mmol), trisdodecyloxyaniline 2 (0.36 g, 0.56 mmol)9 and zinc
acetate dihydrate (0.08 g) were suspended in quinoline (30 ml)
by sonication for 20 min and heated at 180 uC for 2 h under
argon-gas atmosphere. After cooling to room temperature,
the mixture was poured into a mixture of 20 ml conc. HCl
and 130 ml methanol and the red precipitate was collected by
suction filtration. Washing with 2 6 30 ml methanol and
drying in vacuo afforded 0.87 g of a red violet solid which was
purified by column chromatography on silica (eluent: dichloro-
methane–hexane 2:1) to give three fractions containing
perylene dyes, i.e. bisimide 4 (360 mg, 29%, Rf ~ 0.42),5
imide anhydride 3 (170 mg, 19%, Rf ~ 0.36) and remaining
starting material 1 (0.11 g, 20%, Rf ~ 0.26). Compound 3 was
isolated by precipitation with methanol, centrifugation and
drying at 50 uC/1023 mbar. Mp 155–156 uC. 1H NMR
(400 MHz, CDCl3, J/Hz): dH 8.23 (s, 2H), 8.20 (s, 2H), 7.25
(mc, 8H), 6.85 (mc, 8H), 6.39 (s, 2H), 3.97 (t, J 6.6, 2H), 3.88 (t,
J 6.5, 4H), 1.75 (mc, 6H), 1.2–1.4 (m, 90H), 0.85–0.90 (m, 9H);
m/z (MALDI-TOF-MS (Bruker Reflex III, positive mode,
dithranol)): 1611.6 [M1H]1. Elemental analysis calcd. for
[M1H]21
.
Bis-perylenebisimide disulfide 8. A mixture of perylene imide
anhydride 3 (50 mg, 0.03 mmol), aniline 7 (7.25 mg,
0.013 mmol), zinc acetate dihydrate (3.22 mg, 0.015 mmol)
and quinoline (5 ml) was heated at 180 uC for 4 h under argon-
gas atmosphere. The reaction mixture was cooled to room
temperature, poured into 40 ml 1 N HCl and stirred for 20 min.
The resulting precipitate was collected and washed thoroughly
with water and methanol. Twofold column chromatography
(silica; CH2Cl2–hexane 2 : 1) of the dark blue solid afforded a
still impure perylene bisimide dye mixture which was further
purified on a Merck LOBAR MPLC column (Si 60, Size B;
CH2Cl2–hexane 80:20, flow 4 ml min21) to give 5 mg of pure 8
(10%). 1H NMR (400 MHz, CDCl3, J/Hz) dH 8.17 (s, 4H), 8.15
(s, 4H), 7.16 (dd, J 8.8, J 1.4, 16H), 7.06 (d, J 9.0, 4H), 6.91 (d, J
9.0, 4H), 6.78 (m, J 8.7, 16H), 6.33 (s, 4H), 3.91 (t, J 6.6, 8H),
3.82 (t, J 6.5, 8H), 2.60 (t, J 7.5, 4H), 1.69 (mc, 16H), 1.60 (mc,
4H), 1.35 (mc, 20H), 1.19 (mc, 184H), 0.80 (mc, 18H). m/z
(MALDI-TOF-MS (Bruker Reflex III, positive mode, dithra-
C106H133NO12: C 78.92, H 8.31, N 0.87. Found: C 78.74, H
8.21, N 0.98.
10-(p-Nitrophenoxy)-decylbromide (5). Potassium carbonate
(89.44 g, 0.65 mol) and potassium iodide (1.55 g, 0.009 mol)
were added to a solution of p-nitrophenol (10.0 g, 0.07 mol)
and 1,10-dibromodecane (43.15 g, 0.14 mol) in cyclohexanone
(500 ml) and the resulting suspension was heated to reflux
under a nitrogen-gas atmosphere for 5 h. The hot solution was
filtered and the residue was extracted with 3 6 50 ml of
cyclohexanone. The combined organic layers were concen-
trated in vacuo at 140 uC/0.2 Torr and the highly viscous brown
residue was treated with a mixture of ethanol–methanol (1:10)
to give 50 g of a yellowish powder after standing overnight.
Column chromatography of this product on silica with hexane–
nole)): 3748 [M1H]1, 1874 [M1H]21
.
2.3. Preparation of gold-patterned surfaces.
Polystyrene[1350]-block-poly-(2-vinylpyridine)[400] was dissolved
in 5 ml toluene at a concentration of 5 mg ml21 and the mixture
was stirred for 24 h before 6.21 mg HAuCl4 was added
corresponding to a loading of ca. 30% of the pyridine units.
Into this solution glass wafers (DESAG, Type D 263, D ~ 0,
175; cleaved into sizes of 1 6 2 cm and purified by sonication in
acetone, Millipore water and isopropanol) were dipped to a
depth of 1 cm and pulled out at a speed of 10 mm min21 with a
Wilhelmy balance. Thus prepared densely packed gold-loaded
micelles were exposed to a hydrogen plasma (TePla 100,
0.2 mbar, 100 W, 25 min) which removes the polymer and
reduces the gold to give a two-dimensional hexagonal lattice of
5–7 nm gold particles at a distance of 80 nm according to
AFM. The surface was further activated with an oxygen
plasma (0.2 mbar, 100 W, 10 min) and, to avoid any unspecific
adsorption of the dyes by means of dipolar interactions with
the glass wafers, treated in a glove box with a 1023 mM
solution of octadecyltrichlorosilane in dry toluene for 12 h and
rinsed several times with toluene before they were immersed
into a 1.9 6 1024 M solution of 8 in dichloromethane for 12 h.
Finally, these solutions were rinsed with dichloromethane and
1
ethylacetate (20 : 1) afforded 19.2 g (74%) of pure 5. H NMR
(400 MHz, CDCl3, J/Hz): dH 8.19 (d, J 9.3, 2H), 6.95 (d, J 9.4,
2H), 4.06 (t, J 6.5, 2H), 3.28 (t, J 7.3, 2H), 1.82 (mc, 2H), 1.48
(mc, 2H), 1.32 (mc, 12H); 13C NMR (100 MHz, CDCl3): dC
164.1, 141.2, 125.8, 114.3, 68.8, 33.9, 32.7, 30.3, 29.2, 28.8, 28.6,
28.0, 25.8, 22.7; m/z (FAB, 3-nba): 358 [M1H]1. Elemental
analysis calcd. for C16H24NO3Br: C 53.64, H 6.75, N 3.91.
Found: C 53.50, H 6.74, N 3.73.
S,S’-Bis[10-(4’-nitrophenyl)decyl]-disulfide (6). Bromide
5
(7.15 g, 0.02 mol), and thiourea (1.52 g, 0.02 mol) were
heated in ethanol (15 ml) for 3 h under reflux. The mixture was
cooled and a 9% aqueous NaOH solution (15 ml) was added.
The mixture was heated to reflux for another 2 h, cooled to
room temperature, acidified by addition of 1 N HCl and
extracted with ether (3 6 30 ml). The combined organic layers
were washed with water (2 6 15 ml) and dried over sodium
sulfate. After filtration and evaporation of the solvent 5.7 g of
an orange oil was obtained that was further reacted with an
768
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synthetic procedure has been recently reported by Panetta
acetone, and subsequently dried in a stream of nitrogen. The
photoluminescence was characterized by a Zeiss-Axiophot
Fluorescence Microscope and a Spex Fluorolog 2 fluorescence
spectrometer. In the same way a continuously gold-coated
silicon wafer (Crystec silicon S 3214, polished D ~ 3, (100),
p-type boron; coated with 15 nm titanium and 50 nm gold by
vacuum sublimation) was treated with 8.
et al.;10 however, this method involves one step with a very low
yield of only 20%. Accordingly, we applied another sequence
involving the intermediate 6 and a specific reduction of the
nitro group by hydrazine–graphite9b which provided the
desired disulfide in a better yield of 58% (based on 5). Finally,
imide anhydride 3 and aniline 7 were reacted in boiling
quinoline in the presence of zinc acetate to afford perylene
disulfide 8. Owing to the formation of numerous side-products
and the difficult handling of the amorphous high-molecular
weight compound 8, several chromatographic purification
steps became necessary and the pure product could only be
isolated in 10% yield.
2.4. Fluorescence spectroscopy
Corrected fluorescence spectra were taken at room temperature
with a Spex Fluorolog 2, using photon-counting technique. The
wavelengths of the intensity maxima were chosen as excitation
and emission wavelengths for the emission and excitation
spectrum of each sample if not stated otherwise.
The polarized spectra were obtained using a Glan-Thomson
and a Glan-Taylor polarizer on the excitation and emission
side of the spectrometer, respectively. The fluorescence
anisotropy r was calculated from the measured I according to
3.2. Fluorescence of solutions
In our previous work, we applied steady-state absorption and
fluorescence spectroscopy to characterize the aggregation of
perylene bisimide 4 in solution.5 In these studies we found that
no aggregation of the p-systems takes place for concentrations
v1023 M in ‘good’ solvents like dichloromethane which can
interact with the conjugated p-systems by strong dispersion and
electrostatic interactions, whereas in ‘bad’ solvents like
methylcyclohexane (MCH) a well-defined transition from
non-interacting chromophores to columnar aggregates takes
place within the concentration range from 1026 to 1024 M.
Additionally, we noted a significant bathochromic shift of the
fluorescence spectra upon aggregation and an essentially
unchanged fluorescence intensity of these J-type aggregates.
A similar concentration-dependent behavior was now found
for the new compound 8 by optical spectroscopy. As for 4, no
changes took place in dichloromethane but a strong batho-
chromic shift of the fluorescence maximum was observed
upon increasing the concentration from 1026 to 1024 M in
MCH (Fig. 1) indicating similar interaction energies and an
identical aggregate structure for both assemblies.
For a more detailed insight into the fluorescence properties
of these aggregates, time-resolved and polarization-dependent
spectra were recorded for both compounds 4 and 8 at different
concentrations in MCH (Table 1). From the time-resolved
spectra, the fluorescence decay times could be deduced
assuming a monoexponential decay behavior. Remarkably,
both compounds 4 and 8 show an increase of the fluorescence
lifetime upon increasing concentration. Most fluorophors
usually behave in the opposite way either due to fluorescence
quenching by energy dissipation through collisions at higher
dye concentration or new relaxation pathways within the dye
aggregates.11 However, the long fluorescence lifetimes for the
aggregated chromophores 4 and 8 are in good accordance with
our earlier observation of an undiminished fluorescence
intensity of aggregated perylene bisimide 4.5 Our results for
the non-aggregated dyes are also consistent with lifetime data
for similar dyes which have been reported by other groups.12
Presumably, both observations are related to high rigidity of
the chromophores within the aggregate which exhibit less
vibrational deactivation pathways and/or changes in the dyes’
Ihv
Ivv{Ivh
Ihh
Ihv
Ivvz2Ivh
Ihh
r~
with the first index corresponding to the polarization of the
excitation light (v: vertical, h: horizontal) and the second index
corresponding to the polarization of the analyzer in the
emission light path.
Time-resolved measurements were obtained using the time-
correlated single photon counting technique. A nF 900 nano-
second flash lamp (Edinburgh Instruments Ltd.) with hydrogen
as filling gas served as light source. The wavelength selection
was achieved by using appropriate interference filters. From
the fluorescence decay profiles, the lifetime was calculated
assuming a mono-exponential decay law.
By employing these techniques, solutions of 4 and 8 in
methylcyclohexane or dichloromethane were measured in
standard 10 mm fluorescence cells. In general, all measure-
ments were performed under equilibrium oxygen concentra-
tion. To determine the quenching effect of oxygen on the
fluorescence, one series of lifetime measurements was carried
out for 8 after the methylcyclohexane solutions have been
thoroughly degassed by three freeze-pump-thaw cycles. As can
be seen from Table 1, the fluorescence lifetime is only slightly
increased in the deoxygenated samples compared to the air-
saturated solutions and, therefore, the impact of oxygen
quenching can be neglected.
3. Results and discussion
3.1. Synthesis
In the first step of our synthesis, bisanhydride 1 was reacted
with one equivalent of trisdodecyloxyaniline 29 to give a
mixture of 3 and 4 which could be easily separated by column
chromatography (Scheme 2). For the preparation of the second
aniline component 7 bearing the disulfide group, a two-step
Table 1 Characterization of the concentration-dependent perylene bisimide fluorescence of 4 and 8 in methylcyclohexane by steady-state and time-
resolved fluorescence spectroscopy
lmax(emission)/nm
tf/1029
s
Anisotropy r
Concentration/M
4
8
4
8a
4
8
1027
1026
1025
1024
590
592
634
642
617
611
615
627
643
629
4.8
5.0
6.3
8.1
—
5.1 (6.5)
5.7 (7.3)
6.5 (8.0)
8.3 (9.1)
—
0.08
0.08
0.05
0.01
0.13
0.05
0.02
0.02
0.01
—
1026 in CH2Cl2
b
aValues in parentheses for degassed samples after three freeze-pump-thaw cycles. bFor comparison data for fully monomeric dyes have been
recorded in dichloromethane.
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Scheme 2 Synthesis of perylene disulfide 8. (a) Propionic acid, 140 uC, 2 h, 19% 3, 29% 4. (b) Thiourea, EtOH, 80 uC, 3 h. (c) aq. NaOH, 80 uC, 2 h.
(d) Iodine, aq. NaOH–EtOH, 80 uC, 15 min., 72% (based on 5). (e) N2H5OH, graphite, EtOH, 20 h, 81%. (f) Quinoline, Zn(OAc)2 6 2H2O, 5 h,
180 uC, 10%.
diffusion is fast and causes depolarization even for dilute
solutions where energy transfer processes are absent owing to
the large distance between the dyes. In the present case,
anisotropy values of 0.05 (for 8) and of 0.08 (for 4) are observed
for the least concentrated solutions in MCH. These values may
be attributed to the depolarization times for the non-
aggregated dyes because only a slightly increased value is
observed for 4 in dichloromethane where aggregation is fully
absent. It is reasonable to relate the lower value for 8 to
intramolecular energy transfer processes between the two
chromophores that are linked by the long aliphatic disulfide
bridge. Upon increasing the concentration of dyes 4 and 8 in
MCH, either the anisotropy may increase because the
rotational diffusion should be much slower for high molecular
weight aggregates, or the anisotropy may decrease if energy
migration takes place within the aggregated dyes. The data in
Table 1 reveal that the latter effect prevails as both compounds
show complete fluorescence depolarization. These results
Fig. 1 Normalized excitation (e, at lem ~ 615 nm) and emission spectra
(a–d, at lex ~ 580 nm) of 8 in MCH. Concentration a: 1027 M, b:
1026 M, c: 1025 M, d: 1024 M, e: 1027 M. The excitation spectra at
higher concentration exhibit the same wavelength dependence and are,
therefore, not shown.
indicate the formation of extended dye aggregates that exhibit
very efficient energy migration.
3.3. Fluorescence of adsorbed dye molecules
Based on these encouraging results, we now addressed the
covalent attachment of dyes 8 by means of the disulfide
functional groups to gold surfaces. For this purpose two
different substrates were chosen, one consists of a wafer which
was continuously covered with a 50 nm thick gold layer and the
other one contains a hexagonal lattice of 5–7 nm gold
nanoparticles at a distance of 80 nm. The latter substrate
was prepared from metal-loaded block copolymer micelles by
dip-coating and plasma treatment according to Scheme 3.4,13
HOMO/LUMO energies upon aggregation. Additional experi-
ments with degassed samples led to the same conclusions and
confirmed earlier observations that fluorescence quenching by
oxygen is of minor importance for this class of dyes.12
Likewise conclusive are the results obtained from fluores-
cence depolarization (Table 1). For excitation into the S0–S1
transition (500–600 nm), we expect a positive fluorescence
anisotropy value owing to the parallel orientation of the
absorption and emission transition dipole moments. According
to the theory, a maximum value of 0.4 is possible if no
depolarization takes place neither by energy transfer processes
to neighboring dyes nor by rotational diffusion. In reality, for
low viscosity solvents and small fluorophors rotational
Treatment of both surfaces by immersion into a 2 6 1024
M
solution of 8 in dichloromethane afforded gold-bound perylene
bisimide dyes as confirmed by fluorescence microscopy.
In control experiments with wafers that did not contain gold
but otherwise were treated in the same fashion with 8, no
770
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Scheme 3 Loading of polystyrene-poly(2-vinylpyridine) block copolymers with tetrachloroauric acid to form reversed micelles (top left) which can
be deposited by dip-coating on glass wafers to give hexagonal gold patterns after hydrogen plasma treatment (bottom left). 2 6 2 (mm)2 AFM image
of the gold-decorated surface with a cross section (right).
fluorescence was observed. To confirm that this fluorescence
originates from the perylene bisimide fluorophore, both
substrates were investigated by fluorescence spectroscopy
(Fig. 2).
of dyes 8 to continuous gold surfaces takes place in a rather
irregular way. On the other hand, the more bathochromically
shifted emission maximum for the patterned surface suggests
J-type aggregated chromophores as proposed in Scheme 1.
Although the reason for the different aggregation behavior of
dye 8 on plain and nano-structured gold surfaces remains to be
explored, these results are of interest to distinguish different kinds
of metal-coated surfaces by simple fluorescence measurements.
In both cases, we observed the characteristic features of the
excitation and emission bands of the perylene bisimide dye, the
details are, however, quite distinct. Thus, the fluorescence is
much more intense in the case of the patterned surface despite
the fact that the amount of surface-bound dye is much lower
owing to the large areas between the gold particles. It is likely
that the reduced intensity for the continuously coated gold
surface is due to fluorescence quenching by energy levels of the
gold surface whilst the energetic states of the gold clusters are
less destructive. Additionally, for the patterned surface the
emission maximum is centered at 635 nm, whereas the emission
maximum for the continuously coated surface is at only 605 nm.
Because the latter value is very close to the fluorescence
maximum of non-aggregated dyes, we assume that the linkage
4. Conclusion
We have successfully synthesized an intensely fluorescent
perylene bisimide dye that can be covalently linked with high
specificity to gold surfaces by means of a thiol functional
group. Owing to the capability of this dye to form fluorescent
aggregates that emit at higher wavelength than the parent dye,
information on the supramolecular ordering of the dyes on the
surface becomes available from fluorescence spectra. Our
results imply that the new dye 8 is well-suited to characterize
surface patterns of deposited noble metals because it fluoresces
in the aggregated as well as in the non-aggregated state and at
considerably higher wavelength than most other fluorophors.14
Accordingly, fluorescence quenching of dye 8 by electronic
states of the metals is reduced. Our future work will focus on
the assembly of well-defined arrays of these dyes on nanoscopic
metal lines and grids to gain insight into energy and electron
transfer processes within such superstructures.
Acknowledgements
This work was supported by the Deutsche Forschungsgemein-
schaft within the framework of the Sonderforschungsbereich
569 at the University of Ulm.
References
Fig. 2 Normalized excitation and emission spectra of thiol-linked
monolayers of 8 on a continuous gold surface (solid line) and on 4–6 nm
gold nanoparticles (dashed line). lex ~ 472 nm, lem ~ 680 nm.
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