Photoswitchable NIR-Emitting Gold Nanoparticles

Article abstract of DOI:10.1002/anie.201604290

Photo-switching of the NIR emission of gold nanoparticles (GNP) upon photo-isomerization of azobenzene ligands, bound to the surface, is demonstrated. Photophysical results confirm the occurrence of an excitation energy transfer process from the ligands to the GNP that produces sensitized NIR emission. Because of this process, the excitation efficiency of the gold core, upon excitation of the ligands, is much higher for the trans form than for the cis one, and t→c photo-isomerization causes a relevant decrease of the GNP NIR emission. As a consequence, photo-isomerization can be monitored by ratiometric detection of the NIR emission upon dual excitation. The photo-isomerization process was followed in real-time through the simultaneous detection of absorbance and luminescence changes using a dedicated setup. Surprisingly, the photo-isomerization rate of the ligands, bound to the GNP surface, was the same as measured for the chromophores in solution. This outcome demonstrated that excitation energy transfer to gold assists photo-isomerization, rather than competing with it. These results pave the road to the development of new, NIR-emitting, stimuli-responsive nanomaterials for theranostics.

Full text of DOI:10.1002/anie.201604290

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201604290
German Edition: DOI: 10.1002/ange.201604290
Molecular Switches
Photoswitchable NIR-Emitting Gold Nanoparticles
Sara Bonacchi, Andrea Cantelli, Giulia Battistelli, Gloria Guidetti, Matteo Calvaresi,
Jeannette Manzi, Luca Gabrielli, Federico Ramadori, Alessandro Gambarin, Fabrizio Mancin,
and Marco Montalti*
Abstract: Photo-switching of the NIR emission of gold
nanoparticles (GNP) upon photo-isomerization of azoben-
zene ligands, bound to the surface, is demonstrated. Photo-
physical results confirm the occurrence of an excitation energy
transfer process from the ligands to the GNP that produces
sensitized NIR emission. Because of this process, the excitation
efficiency of the gold core, upon excitation of the ligands, is
much higher for the trans form than for the cis one, and t!c
photo-isomerization causes a relevant decrease of the GNP
NIR emission. As a consequence, photo-isomerization can be
monitored by ratiometric detection of the NIR emission upon
dual excitation. The photo-isomerization process was followed
in real-time through the simultaneous detection of absorbance
and luminescence changes using a dedicated setup. Surpris-
ingly, the photo-isomerization rate of the ligands, bound to the
GNP surface, was the same as measured for the chromophores
in solution. This outcome demonstrated that excitation energy
transfer to gold assists photo-isomerization, rather than com-
peting with it. These results pave the road to the development of
new, NIR-emitting, stimuli-responsive nanomaterials for thera-
nostics.
of application, the development of NPs with tailored emission
in the near-infrared region (NIR) is essential because the
transparency of biological tissues is optimal in this spectral
window.[11]
The photo-isomerization (PI) of azobenzene[12] (AB) has
been widely exploited to control and tune the properties of
materials[13] in order to perform different functions, including
drug controlled release.[8b] A large variety of hybrid nano-
systems that join the unique optical and electronic properties
of gold nanoparticles (GNP) to the photochemical activity of
AB have been developed.[1c,13d,14] Nevertheless, examples of
architectures that combine the well documented NIR emis-
sion of small (d < 2 nm) GNP[15] to the photo-responsivity of
AB are very rare[16] and, in particular, the possibility of
switching the NIR luminescence of GNP upon PI of surface-
bound AB units has never been demonstrated before.
Herein, we report the photophysical and photochemical
properties of a newly synthesized class of luminescent GNP,
stabilized with the AB-containing thiolate A (Scheme 1). Our
results demonstrate that, upon excitation of the ligands,
sensitized NIR emission of the GNP is observed thanks to an
efficient energy transfer (ET) process.[17] As a consequence,
the NIR luminescence of GNP can be switched ON/OFF by
alternating from UV to blue irradiation. Furthermore, the
isomerization state of the NPs can be monitored by ratio-
metric detection of the NIR luminescence (Scheme 1).
Stimuli-responsive molecular, supramolecular, and nano-
structured systems are drawing increasing attention, in view
of their application in fields of high economic and social
impacts as materials and life sciences.[1] Light[2] offers several
advantages compared to other forms of stimulation (such as
electrical,[3] thermal,[4] pH,[5] pressure,[6] redox).[7] Light
beams, in fact, are minimally invasive, remotely addressable,
and focusable with high spatio-temporal resolution. More-
over, wavelength selection can permit multiplexed detec-
tion.[8] Additionally, multi-functionality can be achieved by
combining photo-activatable and photoluminescent compo-
nents. This approach is typically exploited in nanomedicine to
design theranostic nanoparticles (NPs) suitable both as
luminescent contrast agents for imaging[9] as well as inducible
vectors for the delivery of therapeutic cargos.[10] For this kind
[*] Dr. S. Bonacchi, Dr. A. Cantelli, Dr. G. Battistelli, G. Guidetti,
Dr. M. Calvaresi, J. Manzi, Prof. M. Montalti
Department of Chemistry “G. Ciamician”
University of Bologna
Via Selmi 2, 40126 Bologna (Italy)
E-mail: marco.montalti2@unibo.it
L. Gabrielli, Dr. F. Ramadori, Dr. A. Gambarin, Prof. F. Mancin
Department of Chemical Sciences, Universitꢀ degli Studi di Padova
(Italy)
Scheme 1. Chemical formula of the trans azobenzene tA and of its cis
isomer cA bound to GNP. When the ligands are in the trans form (left,
tA-GNP, ON state) ET from tA to the GNP produces NIR-sensitized
emission upon ligand excitation. Such contributions, owing to sensiti-
zation, are lost upon PI in cA covered NPs (right, cA-GNP, OFF state).
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under
http://dx.doi.org/10.1002/anie.201604290.
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!

Angewandte
Communications
Chemie
The thermodynamically stable,
showed, in CHCl3 solution, the typical absorption band of
AB dyes with maximum at 360 nm and e = 2.7 ꢀ
A
trans isomer, tA
units, as well as between tA and the gold core, were weak in
the ground state.
a
To investigate the influence of the binding to the GNP on
the PI t!c process, tA-GNP were irradiated at 360 nm in the
same conditions used for the reference compound tA. As
shown in Figure 1, the peaks at 314 nm and 448 nm, typical of
cA, were observed at the PSS. Moreover, the same two
isosbestic points at 319 nm and 430 nm, observed during the
PI of the free ligand tA, were maintained during the
irradiation of tA-GNP. In particular, at the PSS, the absorp-
tion spectrum of the NPs (cA-GNP) perfectly matched the
one expected in the case of > 95% t!c conversion. The large
extent of photoswitching was the result of specific engineering
of the ligand, according to studies previously reported for
analogous 4,4’-dialcoxiazobenzene derivatives.[19] Interest-
ingly, the PI quantum yield measured for tA-GNP was
Ft-c = 0.15, a value that matched the one observed for the
reference compound tA.
As far as luminescence is concerned, a broad emission
band in the NIR region, with maximum at about 930 nm, was
observed upon excitation of either tA-GNP or cA-GNP in
CHCl3. The emission spectral profile was, for both of the
samples, independent of the excitation wavelength, in the
300–600 nm range, and consistent with data reported for
similar gold NPs.[15d] To investigate the effect of ligand PI on
the NIR luminescence, we compared the emission spectra of
the same sample of NPs recorded first at the thermodynami-
cally stable state (tA-GNP) and then at the PSS (cA-GNP).
The emission spectra acquired upon direct excitation of the
gold core (lexc,2 = 550 nm, where absorption by the ligands is
negligible) were identical within the experimental error (see
the Supporting Information). On the contrary, upon excita-
tion at 360 nm, a decrease of about 60% of the intensity of the
emission band was observed going from tA-GNP to cA-GNP
as an effect of the t!c PI (Figure 2). These observations
allowed us to conclude that: i) the emission quantum yield of
the gold NP did not change because of the PI, and ii) part of
the excitation energy adsorbed by tA was transferred to the
gold leading to sensitized emission. Sensitization was con-
104 mꢀ1 cmꢀ1.[12a] A gradual decrease of the intensity of this
band was observed, as reported for similar molecules, upon
irradiation at 360 nm, because of t!c PI. At the photo-
stationary state (PSS), the absorbance at 360 nm was
decreased to about 5% of the initial one. This observation
allowed us to conclude that: i) almost complete t!c con-
version occurred at the PSS, and ii) the molar absorption
coefficient of cA at 360 nm was negligible with respect to the
one of tA.
Going more into detail, the absorption spectra recorded at
different irradiation times (see the Supporting Information)
showed two isosbestic points at 320 nm and 429 nm. This
behavior demonstrated that only the two species tA and cA
were present in the solution, while no side photo-products
were formed upon irradiation. Moreover, the absorption
spectrum at the PSS strongly resembles cA. This spectrum
showed two peaks at 314 nm and 448 nm (e = 1.0 ꢀ
104 mꢀ1 cmꢀ1 and e = 3.0 ꢀ 103 mꢀ1 cmꢀ1 respectively). The
measured PI quantum yield was Ft-c = 0.15, as reported for
analogous AB derivatives.[12a] No fluorescence was observed
either for tA (lexc = 360 nm) or for cA (lexc = 480 nm).
As far as the NPs are concerned, the absorption spectrum
of the tA-coated nanoclusters tA-GNP (average diameter of
the gold core 1.7 nm, estimated formula Au144tA60) presented,
in CHCl3, both the band at 360 nm of the tA chromophore
and the weak surface plasmon resonance band of the gold
core,[18] with the latter one dominant in the region above
550 nm (Figure 1). More precisely, the absorption spectrum of
tA-GNP matches the one calculated for Au144tA60 as the
linear combination of the spectra of tA and of a reference
sample of CH3(CH2)11SH-stabilized GNP (Figure 1). This
spectral matching, and thus the lack of spectral perturbation,
showed that electronic interactions between adjacent tA
Figure 1. Continuous lines: absorption spectra of the thermodynami-
cally stable tA-GNP in CHCl3 (red) of the photo-isomerized NPs cA-
GNP (green) and of reference CH3(CH2)11SH stabilized GNP (black)
Dashed lines: linear combinations of the absorption spectrum of the
reference GNP with those of the ligands tA (red) and cA (green).
Inset: Absorption spectra of a CHCl3 solution of tA-GNP during
irradiation at 360 nm.
Figure 2. Continuous lines: excitation spectra of the thermodynami-
cally stable tA-GNP in CHCl3 (black) and of the photo-isomerized NPs
cA-GNP (red). Dashed lines: luminescence spectra (lexc =360 nm) of
the thermodynamically stable tA-GNP in CHCl3 (black) and of the
photo-isomerized NPs cA-GNP (red).
2
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!

Angewandte
Communications
Chemie
firmed by the excitation spectrum of the NIR emission
GNP (Figure 3), with a decay constant k = 0.42 ꢁ 0.03 minꢀ1.
(Figure 2), where the band corresponding to the absorption of
tA was clearly detectable in the tA-GNP. In contrast, the
typical absorption band at 450 nm of the cis form cA was not
observed in the excitation spectrum of the photo-isomerized
NPs cA-GNP (Figure 2), indicating a poor contribution due
to sensitized emission.
These results confirmed that the rate of the t!c PI process for
tA, bound to the GNP surface, was identical to one measured
for the molecule free in solution.
Upon irradiation of cA-GNP at 480 nm, c!t PI occurred
and about 70% of cA was converted into tA at the PSS. To
further investigate the reversibility of the system, we per-
formed a series of PI cycles by detecting the GNP emission
Although the mechanism of excitation ET toward GNP is
still debated,[20] the poor sensitization observed in the case of
cA-GNP is consistent with the following observations: i) cA
electronic transition centered at 450 nm is forbidden (n-p),[12a]
and thus its contribution to the overall excitation efficiency of
the cA-GNP is minor. ii) MD simulation (see the Supporting
Information) demonstrated that a major fraction of AB
molecules are closer to the metal core in tA-GNP than in cA-
GNP. iii) Much shorter excited state lifetimes have been
reported for the cis than for the trans form of AB.[12a] As
a consequence, independent of the model,[20] ET efficiency is
expected to be much lower for the same ET constant rate in
the case of cA with respect to tA (Supporting Information).
A dedicated experimental setup, suitable for simultane-
ously detecting light transmitted and emitted by the NPs
samples during irradiation (Figure 3, inset), was developed in
order to demonstrate, definitively, the correlation between
the PI process and the NIR luminescence changes.
intensity upon excitation either at lexc,1 = 360 nm or lexc,2
=
550 nm (I1 and I2 respectively). Thus, we calculated the
concentration-independent ratiometric signal I1/I2, which is
potentially useful for theranostic applications (Figure 4, red
Figure 4. NIR luminescence intensity ratio upon excitation at lexc,1
(lexc,1 =360 nm for the red tracks and lexc,1 =480 nm for the blue
tracks, intensity I1 is measured at 780 nm) and lexc,2 =550 (intensity I2
at 780 nm) of tA-GNP in CHCl3. Squares show the emission intensity
at the beginning (green) and at the end (red) of each irradiation cycle
at 360 nm (t!c PI).
lines). Regeneration of the trans form was achieved by
irradiation at lexc,1 = 480 nm (Figure 4, blue lines). As shown
in Figure 4, the NIR emission decreased upon irradiation/
excitation at 360 nm. In particular, starting from tA-GNP, an
intensity decrease of about 70% was observed in the first
240 s, because of the isomerization of tA and the consequent
loss of the sensitized emission. During the following 240 s of
irradiation/excitation at 480 nm, the NIR emission intensity
increased slightly, because only poor sensitization of the gold
emission occurred. After that, 360 nm excitation was restored
and a strong increase of the ratiometric signal, with respect to
the final value of the previous UV irradiation cycle, was
observed.
To summarize, the NIR emission of the GNP can be
reversibly switched from ON (Figure 4, green squares) to
OFF (Figure 4, red squares) by alternating between UV and
Vis irradiation. Moreover, the I1/I2 value at lexc,1 = 360 nm and
lexc,2 = 550 nm can be clearly used as an isomerization state
indicator.
Figure 3. Changes in the photophysical properties of tA-GNP during
irradiation at 360 nm in CHCl3. The setup used for the measurements
is shown in the inset. The fraction of irradiation light (I0) transmitted
by the sample It is used to measure the absorbance in real time
(A=ꢀlog I0/It, red line). Luminescence at 780 nm is also measured in
real-time (I, blue line). The absorbance changes of a CHCl3 solution of
tA during irradiation at 360 nm were also measured (green dots).
Absorbance and emission data recorded for tA-GNP
during irradiation were compared with the absorbance values
measured for tA in the same conditions (Figure 3). The good
overlap between the normalized absorbance and lumines-
cence plots for tA-GNP confirmed the correlation between
the PI process and the decrease of the luminescence intensity
of the GNP.
Both absorbance and luminescence plots could be fitted
with an exponential decay with a rate constant k = 0.43 ꢁ
0.03 minꢀ1. Interestingly, absorbance changes measured for
the reference compound tA showed the same kinetics as tA-
In conclusion, we demonstrated that PI of the AB
derivative tA bound to GNP occurred efficiently in our
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
These are not the final page numbers!

Angewandte
Communications
Chemie
Chem. 2015, 127, 11619 – 11623; d) W. A. Velema, J. P. van der
Berg, W. Szymanski, A. J. M. Driessen, B. L. Feringa, ACS
Chem. Biol. 2014, 9, 1969 – 1974; e) W. A. Velema, J. P. van der
Berg, M. J. Hansen, W. Szymanski, A. J. M. Driessen, B. L.
Feringa, Nat. Chem. 2013, 5, 924 – 928.
system. The conversion of tA-GNP into cA-GNP took place
almost quantitatively (> 95%) with a quantum yield identical
to the one measured for tA in solution (upon irradiation at
360 nm). Regeneration of 70% of tA-GNP was, in turn,
achieved by irradiation of cA-GNP at 480 nm. Photolumi-
nescence measurements showed that tA-GNP emitted in the
[3] S. Taccola, F. Greco, E. Sinibaldi, A. Mondini, B. Mazzolai, V.
Mattoli, Adv. Mater. 2015, 27, 1668 – 1675.
[4] C. A. Figg, A. Simula, K. A. Gebre, B. S. Tucker, D. M.
Haddleton, B. S. Sumerlin, Chem. Sci. 2015, 6, 1230 – 1236.
[5] M. Kanamala, W. R. Wilson, M. Yang, B. D. Palmer, Z. Wu,
Biomaterials 2016, 85, 152 – 167.
[6] Y. Wang, X. Tan, Y.-M. Zhang, S. Zhu, I. Zhang, B. Yu, K. Wang,
B. Yang, M. Li, B. Zou, S. X.-A. Zhang, J. Am. Chem. Soc. 2015,
137, 931 – 939.
[7] K. Miyamae, M. Nakahata, Y. Takashima, A. Harada, Angew.
Chem. Int. Ed. 2015, 54, 8984 – 8987; Angew. Chem. 2015, 127,
9112 – 9115.
[8] a) D. Blꢁger, S. Hecht, Angew. Chem. Int. Ed. 2015, 54, 11338 –
11349; Angew. Chem. 2015, 127, 11494 – 11506; b) W. A. Velema,
W. Szymanski, B. L. Feringa, J. Am. Chem. Soc. 2014, 136, 2178 –
2191; c) M. Baroncini, S. OꢂAgostino, G. Bergamini, P. Ceroni,
A. Comotti, P. Sozzani, I. Bassanetti, F. Grepioni, T. M.
Hernandez, S. Silvi, M. Venturi, A. Credi, Nat. Chem. 2015, 7,
634 – 640; d) V. Balzani, G. Bergamini, P. Ceroni, Angew. Chem.
Int. Ed. 2015, 54, 11320 – 11337; Angew. Chem. 2015, 127, 11474 –
11492.
[9] Z. Tian, A. D. Q. Li, Acc. Chem. Res. 2013, 46, 269 – 279.
[10] a) R. Lehner, X. Wang, M. Wolf, P. Hunziker, J. Controlled
Release 2012, 161, 307 – 316; b) C. Stoffelen, J. Voskuhl, P.
Jonkheijm, J. Huskens, Angew. Chem. Int. Ed. 2014, 53, 3400 –
3404; Angew. Chem. 2014, 126, 3468 – 3472.
NIR region upon direct excitation of the gold core (lexc,2
550 nm) and that, upon excitation of the tA ligand (lexc,1
=
=
360 nm), an ET process from the molecular ligand to the
metal core occurred, producing sensitized NIR emission.
Thanks to this process, the ratio of the emission intensity,
measured in the two different excitation conditions (either
lexc,1 or lexc,2) could be used to detect the isomerization state of
the NPs. Ratiometric detection offers several advantages with
respect to single-wavelength detection, including concentra-
tion-independent responses. Surprisingly, the lack of any
effect of the binding to GNP on the PI rate of tA suggested
that the ET process to gold assisted the PI rather than
competing with it.
Finally, although the typically modest luminescence
quantum yield of GNP[15d,21] needs to be improved to make
them competitive with other systems for in vivo applica-
tions,[11c,e,22] we believe that the results we reported here pave
the road to the development of new NIR-emitting stimuli-
responsive nanomaterials for combined diagnostics and
therapeutics.
[11] a) V. Shanmugam, S. Selvakumar, C. S. Yeh, Chem. Soc. Rev.
2014, 43, 6254 – 6287; b) D. M. Yang, P. A. Ma, Z. Y. Hou, Z. Y.
Cheng, C. X. Li, J. Lin, Chem. Soc. Rev. 2015, 44, 1416 – 1448;
c) M. Montalti, A. Cantelli, G. Battistelli, Chem. Soc. Rev. 2015,
44, 4853 – 4921; d) M. Montalti, L. Prodi, E. Rampazzo, N.
Zaccheroni, Chem. Soc. Rev. 2014, 43, 4243 – 4268; e) E.
Rampazzo, F. Boschi, S. Bonacchi, R. Juris, M. Montalti, N.
Zaccheroni, L. Prodi, L. Calderan, B. Rossi, S. Becchi, A.
Sbarbati, Nanoscale 2012, 4, 824 – 830; f) D. Genovese, S.
Bonacchi, R. Juris, M. Montalti, L. Prodi, E. Rampazzo, N.
Zaccheroni, Angew. Chem. Int. Ed. 2013, 52, 5965 – 5968; Angew.
Chem. 2013, 125, 6081 – 6084.
Acknowledgements
The research leading to these results has received funding
from the European Union Seventh Framework Programme
under grant agreement no. 604391 Graphene Flagship. We
gratefully acknowledge financial support from ERC
(“MOSAIC” Starting Grant 259014).
Keywords: gold · luminescence · nanoparticles · near infrared ·
stimuli-responsive compounds
[12] a) H. M. D. Bandara, S. C. Burdette, Chem. Soc. Rev. 2012, 41,
1809 – 1825; b) S. Castellanos, A. Goulet-Hanssens, F. Zhao, A.
Dikhtiarenko, A. Pustovarenko, S. Hecht, J. Gascon, F. Kapteijn,
D. Bleger, Chem. Eur. J. 2016, 22, 746 – 752; c) S. Fredrich, R.
Goestl, M. Herder, L. Grubert, S. Hecht, Angew. Chem. Int. Ed.
2016, 55, 1208 – 1212; Angew. Chem. 2016, 128, 1226 – 1230;
d) C.-L. Lee, T. Liebig, S. Hecht, D. Bleger, J. P. Rabe, ACS Nano
2014, 8, 11987 – 11993; e) J. Moreno, M. Gerecke, L. Grubert,
S. A. Kovalenko, S. Hecht, Angew. Chem. Int. Ed. 2016, 55,
1544 – 1547; Angew. Chem. 2016, 128, 1569 – 1573.
[13] a) W. Feng, W. Luo, Y. Feng, Nanoscale 2012, 4, 6118 – 6134;
b) F. D. Jochum, P. Theato, Chem. Soc. Rev. 2013, 42, 7468 –
7483; c) W.-P. Lin, S.-J. Liu, T. Gong, Q. Zhao, W. Huang, Adv.
Mater. 2014, 26, 570 – 606; d) M. Dçbbelin, A. Ciesielski, S. Haar,
S. Osella, M. Bruna, A. Minoia, L. Grisanti, T. Mosciatti, F.
Richard, E. A. Prasetyanto, L. De Cola, V. Palermo, R. Mazzaro,
V. Morandi, R. Lazzaroni, A. C. Ferrari, D. Beljonne, P. Samori,
Nat. Commun. 2016, 7, 11090.
[1] a) E. Borrꢁ, J. F. Stumbꢁ, S. Bellemin-Laponnaz, M. Mauro,
Angew. Chem. Int. Ed. 2016, 55, 1313 – 1317; Angew. Chem. 2016,
128, 1335 – 1339; b) G. S. Kumar, D. C. Neckers, Chem. Rev.
1989, 89, 1915 – 1925; c) J. Zhang, J. K. Whitesell, M. A. Fox,
Chem. Mater. 2001, 13, 2323 – 2331; d) A. P. Blum, J. K. Kam-
meyer, A. M. Rush, C. E. Callmann, M. E. Hahn, N. C. Gianne-
schi, J. Am. Chem. Soc. 2015, 137, 2140 – 2154; e) L. D. Zarzar, J.
Aizenberg, Acc. Chem. Res. 2014, 47, 530 – 539; f) J.-M. Lehn,
Angew. Chem. Int. Ed. 2015, 54, 3276 – 3289; Angew. Chem.
2015, 127, 3326 – 3340; g) M. Montalti, G. Battistelli, A. Cantelli,
D. Genovese, Chem. Commun. 2014, 50, 5326 – 5329; h) H.
Kumari, S. R. Kline, S. R. Kennedy, C. Garvey, C. L. Raston,
J. L. Atwood, J. W. Steed, Chem. Commun. 2016, 52, 4513 – 4516;
i) G. O. Lloyd, J. W. Steed, Nat. Chem. 2009, 1, 437 – 442.
[14] a) G. K. Joshi, K. N. Blodgett, B. B. Muhoberac, M. A. Johnson,
K. A. Smith, R. Sardar, Nano Lett. 2014, 14, 532 – 540; b) G. L.
Hallett-Tapley, C. D’Alfonso, N. L. Pacioni, C. D. McTiernan, M.
Gonzalez-Bejar, O. Lanzalunga, E. I. Alarcon, J. C. Scaiano,
Chem. Commun. 2013, 49, 10073 – 10075; c) D. T. Valley, M.
Onstott, S. Malyk, A. V. Benderskii, Langmuir 2013, 29, 11623 –
[2] a) S. V. Aradhya, L. Venkataraman, Nat. Nanotechnol. 2013, 8,
399 – 410; b) Y. Wu, Y. Xie, Q. Zhang, H. Tian, W. Zhu, A. D. Q.
Li, Angew. Chem. Int. Ed. 2014, 53, 2090 – 2094; Angew. Chem.
2014, 126, 2122 – 2126; c) S. J. Wezenberg, K.-Y. Chen, B. L.
Feringa, Angew. Chem. Int. Ed. 2015, 54, 11457 – 11461; Angew.
4
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!

Angewandte
Communications
Chemie
11631; d) Y. Q. Yan, J. I. L. Chen, D. S. Ginger, Nano Lett. 2012,
[18] a) N. K. Chaki, Y. Negishi, H. Tsunoyama, Y. Shichibu, T.
12, 2530 – 2536; e) C. Raimondo, B. Kenens, F. Reinders, M.
Mayor, H. Uji-i, P. Samori, Nanoscale 2015, 7, 13836 – 13839;
f) N. Crivillers, S. Osella, C. Van Dyck, G. M. Lazzerini, D.
Cornil, A. Liscio, F. Di Stasio, S. Mian, O. Fenwick, F. Reinders,
M. Neuburger, E. Treossi, M. Mayor, V. Palermo, F. Cacialli, J.
Cornil, P. Samori, Adv. Mater. 2013, 25, 432 – 436; g) C.
Raimondo, N. Crivillers, F. Reinders, F. Sander, M. Mayor, P.
Samori, Proc. Natl. Acad. Sci. USA 2012, 109, 12375 – 12380;
h) C. Raimondo, F. Reinders, U. Soydaner, M. Mayor, P. Samori,
Chem. Commun. 2010, 46, 1147 – 1149.
Tsukuda, J. Am. Chem. Soc. 2008, 130, 8608 – 8610; b) H. Qian,
R. Jin, Nano Lett. 2009, 9, 4083 – 4087; c) L. M. Tvedte, C. J.
Ackerson, J. Phys. Chem. A 2014, 118, 8124 – 8128.
´
[19] a) J. Dokic, M. Gothe, J. Wirth, M. V. Peters, J. Schwarz, S.
Hecht, P. Saalfrank, J. Phys. Chem. A 2009, 113, 6763 – 6773;
b) C. Dri, M. V. Peters, J. Schwarz, S. Hecht, L. Grill, Nat.
Nanotechnol. 2008, 3, 649 – 653.
[20] a) T. L. Jennings, M. P. Singh, G. F. Strouse, J. Am. Chem. Soc.
2006, 128, 5462 – 5467; b) A. Samanta, Y. D. Zhou, S. L. Zou, H.
Yan, Y. Liu, Nano Lett. 2014, 14, 5052 – 5057.
[15] a) R. Khandelia, S. Bhandari, U. N. Pan, S. S. Ghosh, A.
Chattopadhyay, Small 2015, 11, 4075 – 4081; b) Z. K. Wu, R. C.
Jin, Nano Lett. 2010, 10, 2568 – 2573; c) G. L. Wang, T. Huang,
R. W. Murray, L. Menard, R. G. Nuzzo, J. Am. Chem. Soc. 2005,
127, 812 – 813; d) M. Montalti, N. Zaccheroni, L. Prodi, N.
OꢂReilly, S. L. James, J. Am. Chem. Soc. 2007, 129, 2418 – 2419;
e) K. Saha, S. S. Agasti, C. Kim, X. N. Li, V. M. Rotello, Chem.
Rev. 2012, 112, 2739 – 2779.
[21] Y. Cheng, G. Lu, Y. He, H. Shen, J. Zhao, K. Xia, Q. Gong,
Nanoscale 2016, 8, 2188 – 2194.
[22] a) Z. Guo, S. Park, J. Yoon, I. Shin, Chem. Soc. Rev. 2014, 43, 16 –
29; b) L. Yuan, W. Lin, K. Zheng, L. He, W. Huang, Chem. Soc.
Rev. 2013, 42, 622 – 661.
[16] Y. Negishi, U. Kamimura, M. Ide, M. Hirayama, Nanoscale 2012,
4, 4263 – 4268.
[17] E. Rampazzo, S. Bonacchi, R. Juris, M. Montalti, D. Genovese,
N. Zaccheroni, L. Prodi, D. C. Rambaldi, A. Zattoni, P.
Reschiglian, J. Phys. Chem. B 2010, 114, 14605 – 14613.
Received: May 3, 2016
Revised: June 9, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Molecular Switches
Reversible photo-isomerization of azo-
benzene ligands bound to the surface of
gold nanoparticles (GNP) induces ON/
OFF switching of the metal core NIR
luminescence. An excitation energy
transfer process from the ligands to the
GNP, which produces sensitized NIR
emission, is shown to be the basis of the
phenomenon.
S. Bonacchi, A. Cantelli, G. Battistelli,
G. Guidetti, M. Calvaresi, J. Manzi,
L. Gabrielli, F. Ramadori, A. Gambarin,
F. Mancin, M. Montalti*
&&&& — &&&&
Photoswitchable NIR-Emitting Gold
Nanoparticles
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!