In situ generation of wavelength-shifting donor-acceptor mixed-monolayer-modified surfaces.

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Energy Transfer
DOI: 10.1002/anie.201002939
In Situ Generation of Wavelength-Shifting Donor–Acceptor
Mixed-Monolayer-Modified Surfaces**
Anthony C. Coleman, Jetsuda Areephong, Javier Vicario, Auke Meetsma, Wesley R. Browne, and
Ben L. Feringa*
Nature traps solar energy by using light-harvesting and
energy-transfer processes that rely on the specific energetic
and spatial arrangement of energy-donor and -acceptor
units.[1]Artificial light-harvesting systems hold considerable
potential in applications as diverse as solar cells[2]and
luminescence-basedsensors.[3]
arrangement and ratio of energy-donor units on surfaces
and interfaces with respect to the acceptor unit is a key
challenge, especially in avoiding phase separation of compo-
nents. Considerable success has been achieved with synthetic
covalently tethered donor–acceptor systems, for example,
dendritic structures,[4]in which energy absorbed by peripheral
donor units is transferred to a central acceptor unit,[5]such as
dendrimers containing pyrene or coumarin donor units and a
perylene acceptor unit,[6]multiporphyrin systems,[7]cyclic
porphyrin hexamer arrays,[8]and donor–acceptor polymers
based on a 4-aminonaphthalimide donor and bidentate Ru
acceptor complex.[9,10]Self-assembly approaches offer advan-
tages over covalent systems in terms of synthesis, as demon-
strated in functionalized polymers,[9,11]Langmuir–Blodgett
films,[12]thin films,[13]microfibers,[14]and in monolayers
composed of mixtures of energy-donor–acceptor molecules
on quartz, indium tin oxide (ITO), and silicon surfaces.[10a,15]
In the latter approach, a recurring challenge is to avoid phase
separation of donor and acceptor units and to control and
optimize the ratio of components immobilized on the
surfaces.
Herein, we report a novel approach to achieving optimum
spatial and energetic arrangement of donor and acceptor
units immobilized on glass and ITO surfaces, in which the
optimum ratio of energy-donor and -acceptor units is
determined by the monolayer itself once formed. We use
the irreversible photochemistry of the bistricyclic aromatic
enylidene(BAE)-basedfluorophores
acceptor unit in situ from the surface-immobilized donor
units themselves (Figure 1). The energy-donor–acceptor
Achieving anoptimum
togeneratethe
system reported here is based on a monolayer of the blue
fluorescent compound 1. Upon irradiation in the presence of
oxygen, 1 undergoes photocyclization followed by oxidation
([Ox]) to form the photostable green fluorescent compound 2
(Figure 1). Once formed, compound 2 acts as a local energy
sink through energy-transfer quenching, thus preventing
further photoconversion of those molecules in proximity.
This approach allows for local self-optimization of the donor–
acceptor ratio.
Details of the preparation of compound 1a are available
as Supporting Information. Compound 1a adopts an anti-
folded structure and is blue fluorescent (fluorescence quan-
tum yield FF=0.48, t?1 ns). Upon irradiation the anti-
Figure 1. A mixed monolayer of donor (1c) and acceptor (2c) is
prepared in situ by irradiation at <400 nm from a preformed mono-
layer containing 1c only. A quartz slide is modified with a monolayer
of donor 1c at 365 nm excitation. The blue fluorescence observed for
1c (A) rapidly changes to the green fluorescence of 1c + 2c (B).
[*] Dr. A. C. Coleman, Dr. J. Areephong, Dr. J. Vicario, A. Meetsma,
Dr. W. R. Browne, Prof. Dr. B. L. Feringa
Center for Systems Chemistry, Stratingh Institute for Chemistry &
Zernike Institute for Advanced Materials
Faculty of Mathematics and Natural Sciences
University of Groningen
Nijenborgh 4, 9747 AG, Groningen (The Netherlands)
Fax: (+31)50-363-4296
E-mail: b.l.feringa@rug.nl
[**] Financial support from the Netherlands Organization for Scientific
Research (NWO-VIDI, W.R.B.) and NanoNed is acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002939.
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folded structure 1a leads to a dihydro photocyclized product,
which is oxidized irreversibly to the more planar green
fluorescent compound 2a (FF0.33, t?6 ns) in the presence
of oxygen (Figure 1). Because of the structural constraints
imposed by cyclization on one side of the molecule, the
deviations from planarity are less pronounced than for 1a and
as a result 2a adopts a helicene-type structure, and as such
chirality is introduced into 2. Furthermore, the cyclization is
fully irreversible and results in a compound (2) that is itself
highly photostable.
Both 1a and 2a were characterized by single-crystal X-ray
analysis.[16]The photocyclization reaction is equivalent to the
photoconversion of cis-stilbene to phenanthrene,[17]and has
been observed for some BAE systems such as bianthrone
upon UV irradiation.[18a,19]Compound 2a was prepared by
preparative photolysis of 1a in CH2Cl2(see the Supporting
Information for the synthesis and characterization of 1a and
2a). Photoconversion of 1 to 2 is accompanied by a bath-
ochromic shift of approximately 100 nm in both absorption
and emission spectra. This shift results in an excellent overlap
of the absorption spectrum of 2a with the emission spectrum
of 1a, which facilitates energy transfer (Figure 2). The
photochemical quantum yield (Fchem) determined for this
photoreaction following 312 nm excitation and 425 nm mon-
itoring is 1.6?10?3, with an iron(III) oxalate/phenanthroline
actinometer system as a reference.[20]
Immobilization of 1c on surfaces was achieved by
immersion of slides overnight in a toluene solution of a
3-aminopropyltriethoxysilane (APTES) derivative of 1a
(Scheme 1, see the Supporting Information for details).
Quartz slides were used as the substrate for photochemical
studies and determination of surface coverage by UV/Vis and
fluorescence spectroscopy, ITO-modified quartz slides were
used for electrochemical studies, and silicon wafers with a thin
SiO2layer (ca. 1.2 nm) for the ellipsometry studies.
The contact angle of water, determined by contact-angle
goniometrystudies using the
increased from q=(31?1)8 8 on unmodified quartz to a
mean contact angle of q=(75.8?1)8 8 upon immobilization
of 1c on quartz. A mean monolayer thickness of 17.3(1) ?
was determined by ellipsometry and is in good agreement
sessile dropmethod,[21]
with similar overcrowded alkene systems on a variety of
surfaces previously reported by our group.[22]The surface
coverage on quartz (3.63?10?10molcm?2determined by UV/
Vis spectroscopy, Figure 3) indicates a high surface packing
density but is consistent with monolayer formation.
Quartz slides modified with 1c were irradiated at 312 nm.
Whereas irradiation of 1a in CH2Cl2resulted in a red shift of
the absorption maximum indicative of cyclization, no change
in the absorption spectrum was observed even upon extended
irradiation of the modified quartz slide. By contrast, the initial
blue fluorescence of 1c rapidly converted within 2 min to
green fluorescence (Figure 2). The emission spectrum of a
modified quartz slide is shown in Figure 3.
Irradiation of slides modified with 1c at 312 nm resulted
in generation of the photocyclized compound 2c (Scheme 1)
within the monolayer of 1c. Following 312 nm excitation of
the modified quartz slide, two emission bands are observed at
420 and 500 nm. The weaker emission band at 420 nm is
attributed to the residual blue emission from the open form of
molecular switch 1c, while the more intense green emission
observed at approximately 500 nm is a result of emission from
photocyclized 2c.[23]The absence of a significant absorption
band at 425 nm from 2c on the modified slides, together with
the dominance in the emission spectrum of the 2c emission
band at 500 nm, indicates that energy transfer (see Figure 1)
involving absorption of incident light by 1c and subsequent
Figure 2. Absorption and normalized emission spectra of 1a (c;
lexc=312 nm) and 2a (a; lexc=425 nm) in CH2Cl2.
Scheme 1. Surface immobilization of the APTES derivative (1c).
NMM=N-methylmorpholine, DMAP=N,N-dimethylaminopyridine.
Figure 3. Absorption spectra of a monolayer of 1c on quartz (thick
solid line) and solution spectrum of 1a (thin solid line) in CH2Cl2
(2?10?5m). Normalized emission spectrum of a monolayer of 1c
(a) on quartz, lexc=312 nm.
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energy transfer to 2c occurs without significant conversion of
1c to 2c. Direct excitation of a quartz slide modified with
acceptor 2c at 420 nm resulted in negligible emission, thereby
indicating that an energy-transfer process is responsible for
the observed increased emission at circa 500 nm. Indeed, the
excitation spectrum monitored at 500 nm did not show
significant contributions to the emission above 380 nm (see
the Supporting Information).
Extended irradiation of a slide modified with 1c showed a
gradual loss of emission based on donor 1c with a concom-
itant increase in the emission of acceptor 2c. The residual
emission indicates that the rate of energy transfer required to
inhibit further photochemistry is not less than that required to
quench all fluorescence from 1c.[24]
A mixed monolayer was prepared by deposition of an
equimolar homogeneous solution of 1c and 2c on quartz. The
formation of the monolayer is unlikely to result from self-
assembly as formation of a covalent bond between the
APTES group and the glass surface precludes subsequent
reorganization of the monolayer. Instead the reactivity of the
appended APTES group with the glass surface determines the
rate of monolayer formation. Since the reactivity of the
APTES groups of both 1c and 2c are expected to be identical,
itis expectedthatthe solutionratioof 1cand 2c wouldalsobe
observed for the monolayer formed. The absorption spectrum
of the mixed monolayer shows an approximately 1:1 ratio of
compounds 1c and 2c (see the Supporting Information). By
contrast, the emission spectrum of this mixed monolayer at
lexc312 nm (see the Supporting Information) shows only
emission from 2c. In this case direct excitation at 420 nm
shows a similar emission intensity to when excitation is at
312 nm.
The cyclic voltammograms of 1a, 2a, and 1-ITO are
shown in Figure 4. The results for 1a and 1-ITO are similar
with an irreversible oxidation at approximately +1.4 V;
however, the single return reduction wave is at about
+0.6 V versus the saturated calomel electrode (SCE) for
1-ITO, while in solution this two-electron reduction[25]is
observed as two separate one-electron reductions at 0.5 and
0.8 V vs. SCE. The surface coverage of 1c on the ITO slide
was determined to be 4.2?10?10molcm?2based on the
integrated current of the reduction wave at 0.6 V. This
calculated surface density compares well with that of 3.63?
10?10molcm?2determined by UV/Vis absorption spectrosco-
py. The cyclic voltammetry of 1-ITO is very different from
that of 2a in a 0.1m tetrabutylammonium hexafluorophos-
phate (TBAPF6)/MeCN electrolyte solution, in which two
reversible redox processes are observed at E1=2=0.94 V (DE=
75 mV) and 1.42 V (DE=84 mV). The redox chemistry of
1-ITO is not affected by irradiation with UV light despite
showing intense green emission, further indicating that 1c is
the primary species present in the monolayer and that
amplification of the green emission is a result of energy
transfer and not direct excitation of acceptor 2c formed in situ
upon irradiation.
In systems that undergo self-assembly, intermolecular
interactions (which drive self-assembly and reorganization of
self-assembled monolayers) together with different rates of
surface immobilization can result in phase separation when
two different monolayer-forming compounds (e.g., an energy
donor and an energy acceptor) are assembled simultaneously.
In such cases the in situ formation of the acceptor unit is
advantageous as aggregation of the acceptor is expected to be
unlikely post monolayer self-assembly. In the present study
phase separation is not expected between the donor and
acceptor compounds during monolayer formation, since the
immobilization involves chemical bonding to the surface
through the ATPES unit. However, the formation of a
monolayer with an optimized ratio of donor and acceptor
units is not easily achieved by co-assembly of, for example, 1c
and 2c. The in situ formation of 2c in a monolayer of 1c offers
a major advantage in this regard. It avoids the possibility of a
sufficiently high concentration of 2c being present that would
lead to significant self-quenching of the 2c emission.
In conclusion, we have shown that an energy-donor–
acceptor system can be formed in monolayers through in situ
formation of the acceptor unit (2c), which involves photo-
driven isomerization and subsequent oxidation of 1c. The
unique feature of this system is that once formed the energy
acceptor prevents further isomerization of neighboring mol-
ecules through energy-transfer quenching. The system there-
fore allows for wavelength shifting of UV light (<400 nm) to
green light without significant absorption of visible light
(>400 nm). This approachcircumvents completely issues such
as phase separation during assembly of the components and
could see potential application in smart active coatings for
sensor devices.
Experimental Section
Details of synthesis and characterization as well as additional spectra
are available as Supporting Information. UV/Vis measurements in
solution were performed on a Jasco V-630 spectrophotometer using
Uvasol-grade solvents (Merck). Fluorescence spectra were recorded
on a Jasco FP-6200 spectrofluorimeter in 10 mm path length quartz
fluorescence cuvettes. Spectra were corrected between 300 and
600 nm for excitation lamp and photomultiplier sensitivity. Excited-
state lifetime (t) measurements of both 1a and 2a in CH2Cl2solution
Figure 4. Cyclic voltammetry of 1a (top), 2a (middle), and a mono-
layer of triethoxysilane-derivatized 1c on ITO (bottom) in 0.1m
TBAPF6/MeCN electrolyte solution at a scan rate of 0.1 Vs?1. The
voltammograms of 1a and 2a are offset on the current axis by 80 and
40 mA, respectively.
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were measured using an Edinburgh Instruments (TCC900) time-
correlated single photon counter (TCSPC). Fluorescence quantum
yield (FF) values were determined against perylene[26]and 9,10-
diphenylanthracene[27]in argon-purged cyclohexane solution. The
photochemical quantum yield (Fchem) of 1 in CH2Cl2was determined
with the monochromated (5 nm bandwidth) output of the Xe lamp of
the JASCO FP-6200 spectrophotometer as a light source, by using the
method of total absorption at 312 and 365 nm. The iron(III) oxalate/
phenanthroline actinometer system was used as a reference (see the
Supporting Information for details).
Electrochemical measurements were carried out with a Model
760c Electrochemical Workstation (CH Instruments). Analyte con-
centrations were 1.0 mm in anhydrous acetonitrile containing 0.1m
TBAPF6. Unless stated otherwise, a Teflon-shrouded glassy carbon
working electrode (CH Instruments), a Pt wire auxiliary electrode,
andan SCEreferenceelectrodewere employed (calibratedexternally
using 0.1 mm solutions of ferrocene in 0.1m TBAPF6/CH3CN). Cyclic
voltammograms were obtained at sweep rates of between 10 mVs?1
and 10 Vs?1.
Contact angles were determined on a Dataphysics OCA contact-
angle goniometer using the sessile drop method.[21]The contact angles
were determined usingthe relatedSCA20 software. Thecontact angle
was measured at three different locations on each surface and the
results averaged. Spectroscopic ellipsometry of a monolayer of 1c on
a silicon wafer was carried out with a J. A. Woollam VASE
ellipsometer. Measurements were taken at three different locations
on each surface and the results averaged. The functionalized quartz
slides were irradiated at 365 nm using a Spectroline E-Series
handheld UV lamp. Quartz slides were cut into suitably sized
piecesand cleanedusinga piranha solution(3:7 mixture of30%H2O2
in H2SO4) at808 8Cfor30 min,followedbyrinsingwithdoublydistilled
water and methanol and drying at 908 8C for 1 h. The cleaned slides
were modified by placing in a solution of 1c or 2c (or a mixture,
0.1 mm) overnight under argon. After modification the slides were
removed and thoroughly washed with dichloromethane and then
methanol to remove any physisorbed material from the surface.
Silicon wafers for ellipsometry measurements were cleaned and
treated in a similar manner.
Received: May 15, 2010
Published online: August 2, 2010
.
photochemistry · surfaces
Keywords: energy transfer · molecular switches · monolayers ·
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