Competitive photochemical reactivity in a self-assembled monolayer on a colloidal gold cluster

Article abstract of DOI:10.1021/ja003180l

A shell - core cluster 2 produced by depositing a self-assembled monolayer (SAM) of 6-thiohexyl-3-nitro-4-(4′-stilbenoxymethyl)-benzoate (1) on a nanoparticulate gold colloid preserves normal solution-phase photoreactivity of the trans-stilbene and o-nitr

Full text of DOI:10.1021/ja003180l

1464
J. Am. Chem. Soc. 2001, 123, 1464-1470
Competitive Photochemical Reactivity in a Self-Assembled
Monolayer on a Colloidal Gold Cluster
Jun Hu, Jian Zhang, Fang Liu, Kevin Kittredge, James K. Whitesell,* and
Marye Anne Fox*
Contribution from the Department of Chemistry, North Carolina State UniVersity,
Raleigh, North Carolina 27695-8204
ReceiVed August 25, 2000. ReVised Manuscript ReceiVed NoVember 29, 2000
Abstract: A shell-core cluster 2 produced by depositing a self-assembled monolayer (SAM) of 6-thiohexyl-
3-nitro-4-(4′-stilbenoxymethyl)-benzoate (1) on a nanoparticulate gold colloid preserves normal solution-phase
photoreactivity of the trans-stilbene and o-nitrobenzyl ether moieties, but shows quenched fluorescence. Nearest-
neighbor aggregation is weaker for 1 bound to the roughly spherical colloidal gold particle (in cluster 2) than
for analogous stilbenoids bound to a planar gold surface. Unlike the observed photochemical reactivity of
stilbenylalkyl sulfides appended as SAMs on planar gold surfaces, the chromophores present in these shell-
core nanocomposites 2 exhibit efficient trans-to-cis photoisomerization and blocked [2 + 2] photodimerization.
Wavelength selectivity for photoisomerization and photocleavage proved to be elusive, but triplet sensitization
of the geometric isomerization took place without photorelease of the attached group.
Introduction
The design, synthesis, and characterization of surface-
modified nanoparticles are of fundamental importance in
controlling the mesoscopic properties of new materials and in
developing new tools for nanofabrication.1-4 Self-assembled
monolayers (SAMs) of thiolates on colloidal gold clusters are
therefore ideal substrates for investigating nanoparticle micro-
fabrication techniques and the effect of surface-bound reagents
on structure-property relationships.5-7 Although the materials
properties of metal clusters capped by simple alkyl thiols have
been described,8 photochemical characterizations of the supra-
molecular structures of these capped nanoclusters are rare. This
situation may reflect a general belief that the metal cores of
such composite clusters may quench the appended excited states
or otherwise complicate the observed photochemical and
photophysical properties of the photosensitive capping reagent.
Recently we reported the photoreactivity of several chromo-
phores bound as a SAM at the end of a long-chain alkyl spacer
bearing a terminal thiol as an anchor on a planar gold film. In
particular, we observed efficient photoisomerization of stilbene
and photodimerization of attached stilbenes, coumarins, and
anthracenes.9-11 These reports show that it is, indeed, possible
to generate sufficiently long-lived excited states in SAMs
functionalized with photoreactive moieties that effective photo-
Figure 1. Schematic representations of a organic shell-metal core
composite, illustrated with cluster 2.
(1) Fendler, J. H. Chem. Mater. 1996, 8, 1616.
(2) Qin, D.; Xia, Y.; Xu, B.; Yang, H.; Zhu, C.; Whitesides, G. M. AdV.
Mater. 1999, 11, 1433.
(3) Letsinger, R. L.; Elghanian, R. V. G.; Mirkin, C. A. Bioconjugate
Chem. 2000, 11, 289.
(4) Watson, K. J.; Zhu, J.; Nguyen, S.-B. T.; Mirkin, C. A. J. Am. Chem.
Soc. 1999, 121, 462.
(5) Brousseau, L. C.; Zhao, Q.; Shultz, D. A.; Feldheim, D. L. J. Am.
Chem. Soc. 1998, 120, 7645.
chemical modification of the metal-dielectric interface can be
observed. They also demonstrate that photochemistry can be a
useful tool in characterizing the local environment of covalently
attached molecules present as monolayers on metal surfaces.
We report herein the photochemical reactivity of a shell-
core nanocluster, Figure 1, in which an outer organic sphere of
6-thiohexyl-3-nitro-4-(4′-stilbenoxymethyl)-benzoate (1) caps a
(6) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal.
Chem. 1995, 67, 735.
(7) Hayat, M. A., Ed. Colloidal Gold: Principles, Methods, and
Applications; Academic Press: San Diego, 1991.
(8) Dai, H. L., Ho, W., Eds. Laser Spectroscopy and Photochemistry on
Metal Surfaces; World Scientific: Singapore, 1995; Parts I and II.
(9) Wolf, M. O.; Fox, M. A. J. Am. Chem. Soc. 1995, 117, 1845.
(10) Li, W.; Lynch, V.; Thompson, H.; Fox, M. A. J. Am. Chem. Soc.
1997, 119, 7211.
(11) Fox, M. A.; Wooten, M. D. Langmuir 1997, 13, 7099.
10.1021/ja003180l CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/25/2001

CompetitiVe Photochemical ReactiVity in a SAM
J. Am. Chem. Soc., Vol. 123, No. 7, 2001 1465
Scheme 1. Photolysis To Release a Bound Reagent from a Covalently Bound Surface
Scheme 2. Wavelength-Sensitive Photolysis of Thiol 1 in
Methylene Chloride
metallic core comprised of colloidal gold of a defined dimension.
This molecule is designed to use the metallic support to
influence nearest-neighbor interactions and to include a photo-
cleavable group that will release, in controlled fashion, the
cluster-derived photoproduct formed under the influence of the
surface bound reagents, Scheme 1. The insertion of a photolabile
group also allows for easy separation of the photoproduct from
the support.
Scheme 3. Synthesis of SAM-Capped Cluster 2
By conducting these photolyses on a capped cluster rather
than as a SAM on a planar surface, we can produce photo-
products in preparative quantities, and, hence, their structures
can be compared with those of photoproducts derived from the
same substrates when irradiated in homogeneous solution. Shifts
in product distribution attained on this metallic cluster, rather
than as a SAM on planar gold, can thus provide valuable
information about local packing differences on these two- and
three-dimensional arrays.
The stilbenoid end group was used as a photochemical probe
of the relative efficiency of trans-cis isomerization4-6 and of
[2 + 2] photodimerization.6,12,13 In particular, the [2 + 2] photo-
reaction is known to be a sensitive probe for the solid-state
packing of stilbenoids in a crystalline solid,14 in a microemul-
sion,15 as an inclusion complex,16 or in a Langmuir-Blodgett
film.17 We thus hoped that these photochemical investigations
of capped metal clusters might provide new insight into the
influence of the orientation of the appended chromophore on
the cluster surface relative to that observed on a flat metal
surface. If so, the extent of photodimerization might clarify the
proximity of chromophores present in the organic shell.
The absorption maximum of the 3-nitrobenzyl ether group
(at 240 nm) is significantly shifted from that of the trans-stilbene
group (at 300 nm). If these separate chromophores were non-
interactive or if the photodissociation and photoisomerization
were to take place through excited-state surfaces with different
multiplicities, thiol 1 might present an excited-state manifold
not directly governed by the Kasha-Vavilov law. If so, selective
excitation at a chosen wavelength could, in principle, allow for
the selective photoreaction of the stilbene or selective photo-
cleavage of the linking group, Scheme 2. We wished, therefore,
to determine whether one can carry out selective photochemistry
with light sources of different wavelengths. Therefore, these
photoreactive shell-core capped clusters may be of interest both
as photoresponsive nanomaterials per se and as new probes for
supramolecular organization.18
Results and Discussion
(12) Binkley, R. W.; Flechtner, T. W. In Synthetic Organic Photo-
chemistry; Horspool, W. M., Ed.; Plenum: New York, 1984; pp 375-417.
(13) Schechter, H.; Link, W. J.; Tiers, G. V. D. J. Am. Chem. Soc. 1963,
85, 1601.
(14) Coates, G. W.; Dunn, A. R.; Henling, L. M.; Ziller, J. W.;
Lobkovsky, E. B.; Grubbs, R. H. J. Am. Chem. Soc. 1998, 120, 3641.
(15) Mishra, B. K.; Valaulikar, B. S.; Kunjappu, T.; Manohar, C. J.
Colloid Interface Sci. 1989, 127, 373.
(16) Rao, K. S. S. P.; Hubig, S. M.; Moorthy, J. N.; Kochi, J. N. J. Org.
Chem. 1999, 64, 8098.
(17) (a) Whitten, D. G. Acc. Chem. Res. 1993, 26, 502. (b) Whitten, D.
G.; Chen, L.; Geiger, H. C.; Perlstein, J.; Song, X. J. Phys. Chem. B 1998,
102, 10098 and references therein.
Synthesis and Characterization of Shell-Core Nano-
clusters. Thiol 1 was synthesized from the corresponding bro-
mide by trimethylsilylthioxy dehalogenation in THF, followed
by desilylation in situ,19 Scheme 3. The thiolate-capped cluster
2 was synthesized by treating a colloidal suspension of gold
stabilized by tetraoctylammonium bromide with thiol 1.20 This
(18) Turro, N. J. NATO Sci. Ser., Ser. C 1999, 519, 37.
(19) Hu, J.; Fox, M. A. J. Org. Chem. 1999, 64, 4959.
(20) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J.
Chem. Soc., Chem. Commun. 1994, 801.

1466 J. Am. Chem. Soc., Vol. 123, No. 7, 2001
Hu et al.
route is unusual, in that thiolate-capped colloidal gold nano-
particles are usually prepared by treating a dilute HAuCl4
solution with a reducing agent, such as sodium borohydride, in
the presence of a capping thiol.21-23 However, thiol 1 is
susceptible to reductive decomposition, and a ligand exchange
approach was adopted instead. The gold nanoparticles used in
this synthesis contained on average about 500 gold atoms and
exist as roughly faceted spheres contained within cubes with
an edge length of about nine gold atoms. The nanoparticle had
an average diameter in the range of 1.5-3.5 nm, with a
maximum in the distribution at about 2.5 nm.20
A self-assembled monolayer shell of an alkyl thiol on a
colloidal gold core is likely to exhibit organization very similar
to that observed with the corresponding SAM on a flat
polycrystalline gold surface.24 Powder X-ray diffraction and
high-resolution transmission electron microscopy studies have
shown that gold nanoparticles have highly faceted low-energy
surfaces.25 High-resolution X-ray photoelectron spectroscopy
(XPS) studies also demonstrate nearly identical Au-S bonding
in the two systems.26
Cluster 2 displays a remarkably different solubility than its
precursor tetraoctylammonium bromide-stabilized gold colloid.
Whereas most clusters capped by long-chain alkyl thiolates on
stabilized gold colloids form stable suspensions in toluene, the
solubility of cluster 2 was appreciable only in methylene
chloride, and cluster 2 could not be redispersed easily into
solvents such as benzene and chloroform. Thus, the dispersibility
of an organic-capped gold cluster is dominated by the physical
characteristics of the metal-organic interface, i.e., the inter-
actions between the appended chain, especially the end group,
and the solvent. The solubility change accompanying thiol
displacement of the tetraoctylammonium salt indicated that the
displacement proceeded efficiently, producing gold nanoparticles
that are effectively covered by the stilbene-terminated thiolate
units.
Figure 2. 1H NMR spectra in CD2Cl2 of (a) thiol 1 and (b) cluster 2.
The 1H NMR spectrum of cluster 2 in deuterated methylene
chloride at ambient temperature shows three slightly broadened
peaks at high field, which can be assigned to residual tetra-
octylammonium bromide, Figure 2. The signals from thiol 2
are broadened, indicating that the bonded thiol was packed in
a gel-like state on the gold surface such that the motions of the
end groups are severely restricted.27
The structure of cluster 2 was also confirmed by transmission
FT-IR spectroscopy. Although spectral bands characteristic of
thiol 1 could be also observed in cluster 2, peak broadenings
and small resonance frequency shifts were also evident, Figure
3. For example, the broadening of the CH2 peaks at 2850 and
2920 cm-1 was accompanied by the appearance of a CH2 wag
at 1344 cm-1, indicating that the methylene units of the thiolate
linker are not in a perfect crystalline state in the monolayer.28,29
Either the tetraoctylammonium electrolyte remains associated
(21) Turkevich, J.; Stevenson, P. C.; Hillier, P. Discuss. Faraday Soc.
1951, 11, 55.
(22) Cunnane, V. J.; Schiffrin, D. J.; Beltran, C.; Gebltran, G.; Solomon,
T. J. Electroanal. Chem. 1988, 247, 145.
(23) Thomas, J. M. Pure Appl. Chem. 1988, 60, 1517.
(24) Hostetler, M. J.; Murray, R. W. Curr. Opin. Colloid Interface Sci.
1997, 2, 42.
Figure 3. FT-IR spectra recorded as a thin film of (a) thiol 1 and
(b) cluster 2.
(25) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar,
I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.;
Landman, U. AdV. Mater. 1996, 8, 428.
with the surface or the -(CH2)6- alkyl spacer is amorphous at
room temperature.
By comparing the integration intensities of the NMR signals
attributed to the aromatic and aliphatic protons of 2, we estimate
residual tetraoctylammonium bromide present in the capped
(26) Tour, J. M.; Jones, L.; Pearson, D. L.; Lamba, J. J. S.; Burgin, T.
P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am.
Chem. Soc. 1991, 113, 7152.
(27) Badia, A.; Lennox, R. B.; Reven, L. Acc. Chem. Res. 2000, 33,
475.
(28) Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys. Chem. 1992, 43,
437.
(29) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989,
28, 506.

CompetitiVe Photochemical ReactiVity in a SAM
J. Am. Chem. Soc., Vol. 123, No. 7, 2001 1467
Figure 5. Fluorescence emission spectra recorded for (a) thiol 1 and
(b) cluster 2. Inset: spectrum of 2 with gain increased by 100 times.
Figure 4. Absorption spectral monitoring of the 350 nm direct
photolysis of (a) thiol 1 (1 × 10-5 M in CH2Cl2) and (b) cluster 2
(6 × 10-6 M in CH2Cl2).
cluster 2 to be less than 5 mol %. This analysis is also supported
by the qualitative comparison of the intensities of the C-H
vibrations (ca. 3000 cm-1) in the IR spectra of 1 and 2, where
the relative intensities of these two families are virtually
identical. Most likely, the small amount of residual electrolyte
is localized at the edge of the faceted cluster, where covalent
binding of the thiol to the gold surface is weakest. We thus
conclude that the residual tetraoctylammonium bromide is not
a significant inhibitor of close surface packing of thiol 1 in the
equilibrated cluster 2.
IR surface selection rules for SAMs on metal surfaces dictate
that absorptions perpendicular to the surfaces will be enhanced,
whereas parallel ones will be relatively attenuated.30 The relative
intensity of the ester carbonyl stretch (1716 cm-1) in cluster 2
is significantly attenuated, whereas those of the C-O stretching
peaks (1230 and 1272 cm-1) are more intense. Thus, we infer
that the bound thiolate is oriented at an angle from the surface
of the colloidal gold nanoparticle, Figure 1, but that the average
facet size in the gold cluster is smaller than that encountered
upon vapor deposition of gold on a planar support.
Cluster 2 displays a characteristic gold plasmon band in its
absorption spectrum (at about 526 nm, Figure 4), slightly shifted
from the λmax (530 nm) observed for the tetraoctylammonium
bromide-stabilized gold sol before ligand exchange. This small
shift indicates a modest change of the dielectric properties at
the nanoparticle surface. A slightly broadened trans-stilbenoid
absorption peak at 293 nm in the absorption spectrum of 2 is
blue-shifted from the maximum at about 306 nm of the
corresponding thiol 1 in solution. Whitten and co-workers have
reported a 20 nm or so blue-shift in the absorption spectrum of
stilbenoids that are π-stacked in the solid state.17 The smaller
shift observed here indicates a lower density of packing of the
stilbene units in cluster 2. On the other hand, any significant
dipolar interactions between the gold surface and the chromo-
Figure 6. Dependence of fluorescence intensity on time after mixing
of a 1 × 10-5 M solution of 1 in CH2Cl2 after treatment with a solution
of tetraoctylammonium bromide-protected gold cluster. (a) Before
mixing; (b) immediately upon mixing; (c) after 1 min of stirring; (d)
after 3 min of stirring.
phore would stabilize the singlet excited state relative to the
ground state, producing a red-shift in the absorption spectrum.31
It is possible that the smaller-than-expected blue-shift observed
here might represent compensation of these opposing effects.
Perhaps the most striking spectroscopic change resulting from
the assembly of thiol 1 in cluster 2 is the quenching of its
fluorescence. Thiol 1 in methylene chloride displays a typical
stilbenoid fluorescence spectrum with a broad emission at 370
nm, Figure 5, curve a. In cluster 2, this emission is quenched,
Figure 5, curve b. This result is as expected if the attachment
of 1 to the colloidal gold cluster either enhances the rate of
photoreaction (vide infra), increases the rate of intersystem
crossing to a reactive but nonemissive triplet, or facilitates
nonradiative decay of the bound singlet state.
It is also possible that some of the observed fluorescence
quenching may derive from trivial reabsorption of the emission
from 1 by the suspended colloidal gold clusters. The gold
clusters do strongly absorb in the same spectral region of the
emission of 1. If trivial absorption were significant, fluorescence
quenching would be immediate upon mixing of a solution of
the thiol 1 with the tetraoctylammonium-protected gold cluster.
Instead, the fluorescence intensity of such a solution weakens
over a period of several minutes, Figure 6, showing that the
fluorescence emission efficiency decreases as the thiol becomes
(30) (a) Blanke, J. F.; Vincent, S. E.; Overend, J. Spectrochim. Acta,
Part A 1976, 32, 163. (b) Aguila, A.; Murray, R. W. Langmuir 2000, 16,
5949. (c) Hostetler, M. J.; Wingate, J. E.; Zhong, C. J.; Harris, J. E.; Vachet,
R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall,
G. D.; Glish, G. L.; Morter, M. D.; Evans, N. D.; Murray, R. W. Langmuir
1998, 14, 17.
(31) Nordlander, P. In Laser Spectroscopy and Photochemistry on Metal
Surfaces; Dai, H.-L., Ho, W., Eds.; World Science: Singapore, 1995; Part
I, Chapter 9, p 347.

1468 J. Am. Chem. Soc., Vol. 123, No. 7, 2001
Hu et al.
bound to the cluster. Thus, direct quenching through bonds
formed during binding of the fluorophore to the cluster is the
dominant route for fluorescence quenching.
Because fluorescence can be considered as deriving from an
oscillating excitation dipole, the emission from an excitation
dipole near a metal surface is modeled by classical dipole
antenna theory,31 and an image electron-hole pair can be
induced by generation of an excited state near the surface of
the metal. When the excited dipole is perpendicular to the metal
surface, the image dipole and antenna dipole are 180° out of
phase and cancel. When the excitation dipole is parallel to the
surface, the image dipole reinforces the antenna excitation. The
observed fluorescence quenching in cluster 2 supports the
assertion that the thiol units are attached such that the trans-
stilbene moieties are oriented at an angle from the surface such
that there is a substantial electronic component roughly per-
pendicular to the surface of the metal particle. This implies that
the singlet excitation dipole of trans-stilbene will extend along
the axis that links the center of the two phenyl rings,32 producing
substantial fluorescence quenching.
Figure 7. Reaction profile of the solution photolysis of 3 (0.03 M in
CH2Cl2) obtained by H NMR peak integration: (a) the sum of cis/
trans-bromide 3; (b) the sum of trans-bromide 3 and trans-4-hydroxy-
stilbene; (c) the sum of cis-bromide 3 and cis-4-hydroxystilbene.
1
the o-nitrobenzyl linkage was suppressed upon triplet sensitiza-
tion, although 1 did slowly disappear after prolonged irradiation.
As expected from extrapolation of results obtained for stilbene
itself and for 1 in dilute solution, no photodimerization product
could be observed, even after extensive photolysis.
Direct Observation of Photochemical Reactions on
Colloidal Gold Nanoparticles. The direct photolysis of thiol
1 was first examined in methylene chloride. When monitored
by absorption spectroscopy, the absorption of 1 at 306 nm
decreased as the absorption below 277 nm rose, with an
isosbestic point at 277 nm, Figure 4. Such absorption changes
are typical for trans-cis photoisomerization of stilbenoids.17
The growth of absorption beyond 345 nm, together with an
isosbestic point at about 550 nm, indicates photodetachment of
the o-nitrobenzyl ether moiety,33 thus affecting directly the
position and intensity of the plasmon band as the identity of
the bound chemical species is altered. When the photolysis was
Direct irradiation of cluster 2 revealed photoreactivity of its
stilbene group very similar to that in its precursor 1. As shown
in Figure 3, the trans-stilbene absorption peak at about 300 nm
decreases upon irradiation at 350 nm with simultaneous growth
in intensity below 270 nm with an isosbestic point at 270 nm,
indicating production of the cis-isomer.17 It is interesting that a
large shift of the gold plasmon band (from 530 to 550 nm) was
observed as the photoreaction proceeded. Because the surface
plasmon band is very sensitive to changes in properties at the
metal/dielectric interface, the observed shift indicates a steep
change in dielectric36 that can be plausibly attributed to the
photodetachment of the highly polar 4-hydroxystilbene group
from the cluster surface.
Because the singlet state is effectively quenched by inter-
actions with the gold core, it is likely that the geometric
isomerization can take place from either the singlet or triplet
manifold, as is typical for many simple olefins.35 When the
isomerization takes place through the triplet manifold, it will
dominate over a lower quantum yield process. The lower
quantum efficiency observed for all conversions in 2 than in 1
thus makes it difficult to rule out completely any singlet state
involvement.
1
scaled to preparative quantities and monitored by H NMR
spectroscopy, the bromide precursor 3, instead of thiol 1, was
employed since 1 and 3 presented virtually identical excited-
state profiles. The chemical shifts of the two trans-stilbene vinyl
hydrogens in 1 (7.02 and 6.96 ppm, JAB ) 16 Hz) gradually
converted into those characteristic of the cis form (7.09 and
7.06 ppm, JAB ) 8.8 Hz),34 and the benzylic proton signals of
the o-nitrobenzyl ether moiety (4.4 ppm) disappeared. The
spectral bands assigned to both the cis- and trans-stilbenoids
were present in the NMR spectrum of the bulk photolysis of
trans-3 at 350 nm. The photoinduced production of the cis-
isomer was unambiguously established by the isolation of the
cis-3 and a mixture of both geometric isomers of 4-hydroxy-
stilbene. We could observe no photodimerization product, within
the limits of detection of this method (<5%). Therefore, the
photoreaction of thiol 1 proceeds as shown in Scheme 2 through
competing geometric isomerization and photocleavage.
It is also interesting that the trans-to-cis isomerization is
observed in cluster 2, whereas the analogous trans-to-cis
geometric photoisomerization is blocked in self-assembled
monolayer 4. The latter includes a similar stilbenoid bound at
The photoisomerization of the stilbene group in 1 is
more efficient than the photodissociation of the o-nitrobezyl
1
moiety. By monitoring the course of the reaction by H NMR
spectroscopy, the progress of the two photoreactions (geometric
isomerization and benzyllic cleavage) could be deconvoluted,
Figure 7. Under constant irradiation of dilute solutions (0.03
M in stilbene) at 350 nm, the photoisomerization reaches a
photostationary state after about 25 min of irradiation, whereas
the competing dissociation required about 60 min to complete.
A much cleaner geometric photoisomerization was observed
when the reaction was triplet photosensitized with 1,4-dibromo-
naphthalene (ET ) 58.1 kcal/mol).35 The photodissociation of
an angle similar to that observed in 2,9 but on planar gold with
larger metal facets, better packing of the trans-stilbene groups
sterically blocks the isomerization. In contrast, the corresponding
cis-isomer of 4 is less well-packed and can photoisomerize
easily. It seems likely, therefore, that the surface disorder
(32) Kawski, A.; Kubicki, A. Acta Phys. Pol. A 1991, 79, 457.
(33) Pillai, V. N. R. In Organic Photochemistry; Padwa, A., Ed.; Marcel
Dekker: New York, 1987; Vol. 9.
(35) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photo-
chemistry, 2nd ed.; Marcel Dekker: New York, 1993.
(34) Fisher, T. H.; Schultz, T. P. Magn. Reson. Chem. 1991, 29, 966.
(36) Lazarides, A. A.; Schatz, G. C. J. Phys. Chem. B 2000, 104, 460.

CompetitiVe Photochemical ReactiVity in a SAM
Scheme 4. Triplet Sensitization of Thiol 1
J. Am. Chem. Soc., Vol. 123, No. 7, 2001 1469
colloidal gold. Spectroscopic and solubility experiments indicate
that the self-assembled array on cluster 2 has structural order
similar to, but not identical with, that observed with long-chain
alkyl thiolate SAMs on bulk polycrystalline gold. Thus, the end
groups are less tightly packed in 2 than in the analogous stilbene
4 bound to larger planar gold facets. Fluorescence of the stilbene
moiety in thiol 1 was quenched in cluster 2, indicating strong
electronic interaction between the excited stilbene and the metal
surface.
Both the stilbene and the o-nitrobenzyl moieties were
photochemically active in cluster 2 upon direct photolysis, but
selective activation through excitation wavelength selection of
either reaction (geometric isomerization of the stilbene or
photodeprotection of the phenolic group) to the exclusion of
the other proved to be elusive. However, triplet sensitization
permitted selective excitation of the stilbene moiety without
photodissociation of the o-nitrobenzyl linkage.
Both direct and sensitized irradiation led to geometric
isomerization, in contrast to the unidirectional (cis-to-trans)
isomerization observed with 4. Photodimerization of the trans-
stilbene units in cluster 2 was not observed, implying that the
stilbene units in 2 are more loosely packed than in a typical
L-B film or in a solid crystal in which face-to-face π-stacking
of the stilbene moieties facilitates [2 + 2] cycloaddition at the
olefiinic π bond.
characteristic of cis-4 similarly characterizes the appended
stilbene units in trans-2.
Likewise, [2 + 2] photodimerization could be observed with
4 on planar gold, but not with cluster 2. The quantum yield for
photocycloaddition of 4 is very low compared with that for
geometric isomerization. It can be observed, in fact, only
because of the absence of a photorelease pathway such as that
available to 2. It is not surprising that the photorelease route in
2 precludes the possibility of observing the sterically disfavored
pericyclic photoreaction.
The efficiencies of both photoisomerization and photo-
dissociation in cluster 2 were significantly lower than those in
the soluble precursor 1, Scheme 4. There are two probable
causes for this reduction of photoefficiency. Suppression of
geometric isomerization may arise either from steric hindrance
associated with tight monolayer packing or from metal-based
nonradiative quenching. The isomerization of the trans-stilbene
moiety to the corresponding cis-isomer does involve a large
geometric change which would be difficult were the stilbene
units in 2 as closely packed in the cluster 2 as on a larger planar
facet of SAM 4 on gold. Contrasting expectations from the
broadened 1H NMR spectrum, which indicates a gel-like packed
state, and from the observation of trans-to-cis isomerization,
which requires loose packing, provide a rationale for the
observation that the photoisomerization could be retarded.
However, were packing the dominant factor, the photodetach-
ment of the o-nitrobenzyl group should be relatively facile, since
it is less affected by packing. In fact, the release of the relatively
large bound chromophore from the capped cluster surface is
presumably sterically favorable. Retardation of both the photo-
isomerization and the photodissociation thus requires a signifi-
cant role for surface quenching of the chromophore by the metal
core.3
As in the solution-phase reaction, triplet-sensitized photolysis
of cluster 1 gave no photodetachment product in 20 min of
irradiation, Scheme 4. After prolonged irradiation, a very
complex mixture was formed, although neither photodetachment
nor [2 + 2] photocycloaddition products could be isolated. The
absence of the cycloaddition product likely indicates that the
stilbene units in cluster 2 are less well-packed than in the solid
state or in a compressed Langmuir-Blodgett (L-B) film, where
the π-stacked stilbenoid units of the “shell” in cluster 2 are
apparently only loosely associated, but not nearly as tightly
π-stacked as in the crystalline solid or in a L-B film.
Inclusion of a photocleavable linker between a metallic
surface and a reactive chromophore provides several advan-
tages: an optical route for surface patterning, a photochemical
method for release of a bound reagent from a covalently bound
metal surface, and a new technique for exploiting local
organization in choosing among competing photochemical
pathways.
Experimental Section
General Procedures and Techniques. Unless specified in the
synthetic procedures, all chemical reagents (Aldrich) were used as
received. H2AuCl4 was purchased from Strem and tetraoctylammonium
bromide from Fluka. Spectroscopic grade solvents from Fisher or
Aldrich were used both for spectral measurements and for photochemi-
cal reactions. Pure water (Milli-Q) used in the colloidal gold synthesis
had a resistance of higher than 4 MΩ. Tetrahydrofuran (THF), benzene,
toluene, and diethyl ether were distilled from sodium benzophenone
ketyl. Ethylene glycol was distilled from sodium metal, and methylene
chloride was distilled from calcium hydride under argon prior to use.
Deuterated solvents obtained from Isotech or Aldrich were used as
received.
Air- and moisture-sensitive reactions were performed under an
atmosphere (nitrogen or argon after a Drierite tower). Glassware was
oven-dried at 140 °C or flame-dried if necessary. Reactions were
monitored by thin-layer chromatography (TLC) carried out on 0.25
mm E. Merck silica gel plates (60F-254). Baxter silica gel 60 (230-
400 mesh ASTM) was used for flash column chromatography. Melting
points are uncorrected. 1H NMR (300 MHz) and 13C NMR (75.5 MHz)
spectra were recorded on a Varian UNITY Plus-300 spectrometer at
room temperature with an internal standard (1H, 7.24 ppm and 13C,
77.0 ppm). IR spectra were recorded on a Nicolet 510P FT-IR as thin
films on NaCl plates. High-resolution mass spectra were obtained on
a VG ZAB-E4F instrument using EI or CI ionization. Absorption spectra
were monitored with an HP diode array spectrophotometer and a
Shimadzu 3101PC spectrophotometer. Emission spectra were recorded
on a Shimadzu RF-5301PC spectrofluorometer.
Synthesis of 6-Mercaptohexyl-3-nitro-4-(4′-stilbenoxymethyl)-
benzoate 1. Thiol 1 was prepared by following a procedure reported
by Hu and Fox19 from the corresponding bromide (see Supporting
Information).
Synthesis of Cluster 2. The shell-core composite 2 was synthesized
by treating a tetraoctylammonium bromide-stabilized gold colloidal
suspension20 with thiol 1 (0.20 g, 0.39 mmol) in toluene. The resulting
Conclusions
We have prepared a photodetachable thiol linker for covalent
surface modiciation of colloidal gold. Using this linker, we have
constructed a densely covered shell-core nanoparticle in which
a trans-stilbenoid is present as a capping group on size-selected

1470 J. Am. Chem. Soc., Vol. 123, No. 7, 2001
Hu et al.
suspension was stirred under N2 in the dark for 10 h. The capped
nanoparticles precipitated and were collected by membrane filtration
on a Millipore HTTP 0.4 µm filter. The dark solid precipitate was
washed with copious amounts of toluene and ethanol to remove the
electrolyte and unreacted thiol. A dark amorphous solid was obtained,
which was resuspended in methylene chloride.
trans-isomer in the presence of traces of I2. Photoreactions of cluster
2 were monitored directly in an NMR tube by 1H NMR spectroscopy.
In the triplet-sensitized reactions, 1,4-dibromonaphthalene (20 equiv)
was added to the reaction solutions to ensure that direct absorption of
light by the substrate would be negligible.
Photolysis Reactions. Photolyses were carried out with phosphor-
coated low-pressure mercury arcs with emission centered at 350 or 300
nm in a Rayonet photochemical reactor (Southern New England
Ultraviolet) at ambient temperature. When the progress of the reaction
was monitored spectroscopically, a methylene chloride solution of thiol
1 or cluster 2 was placed in a Pyrex cell and degassed by bubbling Ar
for 10 min before irradiation. For synthetic photoconversions, a solution
of 1 or bromide 3 (7.4 mM in 50 mL of CH2Cl2) was placed in a Pyrex
tube under Ar. The resulting solution was irradiated at 350 nm in a
Rayonet photochemical reactor. After removal of solvent, the solid
residue was separated by flash chromatography on silica gel to give
4-hydroxystilbene and 1 or 3 as mixtures of cis- and trans-stereoisomers.
The trans-isomers were identified by comparing their 1H NMR spectra
with those of authentic samples. The formation of the cis-isomers was
inferred from the product 1H NMR spectrum for the olefinic protons19,37
and confirmed by their complete reconversion to the corresponding
Acknowledgment. This work was supported by the U.S.
Department of Energy, Office of Science, Chemistry Division.
We thank our colleagues at North Carolina State University for
sharing their research instruments and space while our laboratory
renovations were completed.
Supporting Information Available: Synthesis and charac-
terization of 6-bromohexyl-3-nitro-4-methyl-benzoate, 6-bromo-
hexyl-3-nitro-4-bromomethyl-benzoate, and 6-bromohexyl-3-
nitro-4-(4′-stilbenoxymethyl)-benzoate (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
JA003180L
(37) Friedrich, K.; Hennig, H. G. Chem. Ber. 1959, 92, 2944.