Remote-control photoswitching using NIR light

Article abstract of DOI:10.1021/ja904746s

(Chemical Equation Presented) Near-infrared (NIR) light is used to toggle photoswitches back and forth between their two isomers even though the chromophores do not significantly absorb this type of light. The reactions are achieved through a "remote control" process by using photon up-converting hexagonal NaYF₄ nanocrystals doped with lanthanide ions. These nanoparticles absorb 980 nm light and convert it to wavelengths that can be used to trigger the photoswitches offering a practical means to potentially achieve 3D-data storage, drug-delivery, and photolithography.

Full text of DOI:10.1021/ja904746s

Published on Web 07/17/2009
Remote-Control Photoswitching Using NIR Light
Carl-Johan Carling,† John-Christopher Boyer,‡ and Neil R. Branda*,†
4D LABS, Department of Chemistry, Simon Fraser UniVersity, 8888 UniVersity DriVe, Burnaby, BC, Canada V5A
1S6, and Department of Chemistry, UniVersity of Victoria, P.O. Box 3065, Victoria, BC, Canada V8W 3 V6
Received June 10, 2009; E-mail: nbranda@sfu.ca
Scheme 1
The use of dithienylethene (DTE) photoswitches to achieve
spatial and temporal control of molecular processes and optoelec-
tronic properties has received significant attention in recent years.1
This attention is well deserved and due to the fact that the
photoswitches can be toggled back and forth between two structur-
ally and electronically different isomers (Scheme 1) having proper-
ties that can be fine-tuned by decorating the molecular backbone
with functional groups so they can be applied to a wide range of
technologies, from optical data storage and processing2 to photo-
release3 and photodynamic therapy.4
One of the major drawbacks of these versatile photoresponsive
systems is the need for ultraviolet and visible light to induce the
ring-closing and ring-opening reactions, respectively. Both types
of light are associated with detrimental effects such as inducing
unwanted side reactions and low penetration into biological tissue.5
More practical systems are those that undergo multiphoton processes
in the near-infrared (NIR) region of the spectrum where the
probability of inducing chemical reactions is reduced and 3D control
can be achieved (a key to photodynamic therapy and volumetric
data storage applications).6 Because photochromic compounds often
have low two-photon-absorbing cross sections,6 a better alternative
is to induce the DTE reactions in a “remote control” process by
using antennae species that absorb NIR light and transfer the
harnessed energy to the photoswitch.
they can emit ultraviolet, violet, blue, green, and red light upon
absorbing NIR (980 nm) light. The “remote control” switching of DTE
1 by UCNPs NaYF4 doped with 0.5 mol % Tm3+ and 30 mol % Yb3+
(NaYF4:TmYb) and NaYF4 doped with 2 mol % Er3+ and 20 mol %
Yb3+ (NaYF4:ErYb) is the focus of our studies. The ring-open isomer
(1a) absorbs UV light,10 which induces ring closing to form 1b. This
light can be generated when a dispersion of the UV-emitting NaYF4:
TmYb nanoparticles absorb NIR light. The ring-opening reaction (1b
f 1a) is triggered by visible light, which can be produced by a
dispersion of the green-emitting NaYF4:ErYb.
DTE 1a and both UCNPs are prepared according to literature
procedures.10 TEM images of the UCNPs demonstrate their nearly
monodisperse particle size (Figure 1a and 1b) of 25.3 ( 1.4 and 25.6
( 1.3 nm for NaYF4:TmYb and NaYF4:ErYb, respectively. Powder
X-ray diffraction10 confirms that the synthesized particles are hexagonal
in phase with no significant impurity phases present. The well-defined
peaks of the X-ray pattern demonstrate the high crystallinity of the
doped NPs, and a crystallite size of ∼23 nm can be determined for
Photon upconverting nanoparticles (UCNPs), in particular hex-
agonal NaYF4 nanocrystals doped with lanthanide ions, are ideal
for “remote control” photoswitching.7 (1) They efficiently absorb
NIR light and convert it to wavelengths that can be used to trigger
appropriate DTE photoswitches. (2) The key electronic excitations
occur in real energy levels, requiring significantly lower intensities
of light than conventional two-photon absorbing dyes. (3) They do
not suffer from photobleaching. (4) They can be coated with
biocompatible polymers and antibodies to decrease toxicity, prolong
circulation time, and increase tissue selectivity, rendering them
particularly useful in biomedical applications.8
In this paper, we demonstrate how DTE photoswitches undergo
their ring-closing and ring-opening reactions using NIR light in
the presence of two types of UCNPs that emit UV and visible light.
In a preliminary study, we also illustrate the concept of “remote
control” photorelease using the DTE architecture. A recent publica-
tion9 describes the use of UCNPs and a DTE for optical memory
applications. Although effective quenching of the UCNP emission
by one of the DTE isomers was achieved, “remote control”
switching was not demonstrated, likely due to the use of the less-
efficient upconverting LaF3 nanoparticles, a lower laser power, and/
or the occurrence of an energy-transfer process, which produces a
state with different properties from our case.
Figure 1. Emission spectra of colloidal CHCl3 solutions of (a) NaYF4:TmYb
and (b) NaYF4:ErYb UCNPs when excited with a 980 nm laser diode.11 The
light used to trigger photoswitch 1 is highlighted. TEM images are shown as
insets. Changes in the UV-vis absorption spectra (left) and cropped photographs
(right) of acrylate films (12 × 8 × 1 mm) containing (c) 1a + NaYF4:TmYb,
and (d) 1b + NaYF4:ErYb as they are irradiated with 980 nm light. The stripes
observed in the right panel of (c) and (d) correspond to the direction of the
beam of the 980 nm laser.10 The irradiation in (c) was carried out two times
with perpendicular orientations. The small squares in all images are the cut-
out holes in the sample holder through which the absorbance was measured.
We chose to use NaYF4 nanoparticles because they are some of
the best UCNPs known to date, and depending on their composition
† Simon Fraser University.
‡ University of Victoria.
9
10838 J. AM. CHEM. SOC. 2009, 131, 10838–10839
10.1021/ja904746s CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. (a) Scheme illustrating the ring-opening and release reactions of bicyclic compound 2b as it is irradiated with visible or NIR light. (b) Changes
in the UV-vis absorption spectra (left) and cropped photographs (right) of an acrylate film (8 × 7 × 1 mm) containing 2b + NaYF4:ErYb as it is irradiated
with 980 nm light. The stripe observed in the right panel corresponds to the direction of the beam of the 980 nm laser.
both types of UCNPs from the line broadening of the diffraction peaks,
which correlate well to the TEM data.
(Figure 2b) showing that “remote control” switching also operates
in this system. When probed using UV-vis absorption spectros-
copy, in addition to the decrease in the absorption band at 550 nm,
which signifies ring opening (2b f 2a), an increase in an absorption
band at 400 nm is observed. This increase can be attributed to the
appearance of dithienylfulvene 3 through the spontaneous release
from 2a. Irradiation of this film with 365 nm light results in no
observable spectral changes supporting the success of the release
even in the relatively solid matrix since any remaining ring-open
isomer would be converted back to its ring-closed counterpart (2b).
Our demonstration of “remote control” switching using the
versatile DTE architecture and upconverting doped nanocrystals
offers new opportunities in photodynamic therapy and 3D data
storage. In these studies we have demonstrated that either ring-
closing or ring-opening reactions can be triggered by the judicial
choice of organic chromophore and UCNP.
DTE 1a absorbs UV light (365 nm) and undergoes efficient
conversion to its ring-closed counterpart (1b) as illustrated by the
changes in the UV-vis absorption spectrum10 and the visual color
change of a CH3CN solution from colorless to red. Visible light
(>450 nm) triggers the reverse reaction and regenerates the original
spectrum. These wavelengths correspond to those emitted by the
UCNPs, which have upconversion emission spectra that match those
reported previously.7 Of the five emissions observed for NaYF4:
TmYb ranging from ultraviolet to near-infrared (Figure 1a), the
highest energy emission is appropriate for photocyclization (1a f
1b). In the case of NaYF4:ErYb, violet, green, and red emissions
are observed (Figures 1b), with the second one appropriate to induce
ring opening (1b f 1a).12
To ensure that the UCNPs and DTE remain in close enough
proximity to promote reabsorption of the light emitted from the
nanocrystals by the organic chromophore, we chose to demonstrate
“remote control” switching by casting both components in a
polymer composite material comprised of cross-linked poly(ethyl-
eneglycol)dimethacrylate and toluene, which provides a flexible
environment for the photoreactions of 1.13
When a pale yellow film containing 1a and an excess of NaYF4:
TmYb is irradiated with 980 nm diode laser light, the film only
changes to a red color along the path of the beam of light (Figure
1c), which can be attributed to the photocyclization reaction (1a
f 1b). This reaction is supported by the changes in the absorption
spectrum, which match those for a sample of 1a irradiated with
UV light.10 Similarly, a film containing 1b and NaYF4:ErYb
undergoes decolorization only along the path of the light (Figure
1d). The ring-opening reaction (1b f 1a) is also supported by
spectral changes that match those for a sample of 1b irradiated
with visible light. The fact that the reactions of the DTE chro-
mophore are triggered by absorbing the light generated by the
UCNPs is demonstrated by irradiating a film containing 1b but no
NaYF4:ErYb, which undergoes almost no change in its color or its
UV-vis spectrum (see Figure S5 in the Supporting Information).
We recently demonstrated how visible light could be used to
trigger the release of a small molecule from a DTE derivative by
inducing the ring-opening reaction of a “locked” species.3 The
process is based on the creation of an unstable ring-open compound
that spontaneously undergoes a reverse Diels-Alder reaction to
generate a fulvene and a dienophile as illustrated by analogous
compounds shown in Figure 2a. We now demonstrate that we can
trigger the release using NIR light and UCNPs.
Acknowledgment. This research was supported by the Natural
Sciences and Engineering Research Council (NSERC) of Canada,
the Canada Research Chairs Program, the University of Victoria,
and Simon Fraser University.
Supporting Information Available: Synthesis and characterizations
of chromophores and UCNPs, switching in solid-state forms. This
material is available free of charge via the Internet at http://pubs.acs.org.
References
(1) Tian, H.; Yang, S. Chem. Soc. ReV. 2004, 33, 85. Tian, H.; Wang, S. Chem.
Commun. 2007, 781. Irie, M. In Molecular Switches; Feringa, B. L., Ed.;
Wiley-VCH: Weinheim, Germany, 2001; p 37. Irie, M. In Photochromic
and Thermochromic Compounds; Crano, J. C., Guglielmetti, R. J., Eds.;
Plenum: New York, 1999; Vol. 1, p 207.
(2) Irie, M. Chem. ReV. 2000, 100, 1685.
(3) Lemieux, V.; Gauthier, S.; Branda, N. R. Angew. Chem., Int. Ed. 2006,
45, 6820.
(4) Sud, D.; Wigglesworth, T. J.; Branda, N. R. Angew. Chem., Int. Ed. 2007,
46, 8017.
(5) Goeldner, M.; Givens, R. Dynamic Studies in Biology: Phototriggers,
Photoswitches and Caged Biomolecules; Wiley-VCH: Weinheim, Germany,
2005. Stolika, S.; Delgado, J. A.; Pe´rez, A.; Anasagasti, L. J. Photochem.
Photobiol. B 2000, 57, 90.
(6) Mikhailov, I. A.; Belfield, K. D.; Masunov, A. E J. Phys. Chem. A 2009,
113, 7080.
(7) Shan, J.; Chen, J.; Meng, J.; Collins, J.; Soboyejo, W.; Friedberg, J. S.; Ju,
Y. J. Appl. Phys. 2008, 104, 094308/1. Nyk, M.; Kumar, R.; Ohulchanskyy,
T. Y.; Bergey, E. J.; Prasad, P. N. Nano Lett. 2008, 8, 3834. Qian, H.-S.;
Zhang, Y. Langmuir 2008, 24, 12123. Heer, S.; Kompe, K.; Gu¨del, H. U.;
Haase, M. AdV. Mater. 2004, 16, 2102.
(8) Kumar, M.; Guo, Y.; Zhang, P. Biosens. Bioelectron. 2009, 24, 1522. Yu,
M.; Li, F.; Chen, Z.; Hu, H.; Zhan, C.; Yang, H.; Huang, C. Anal. Chem.
2009, 81, 930. Ungun, B.; Prud’homme, R. K.; Budijono, S. J.; Shan, J.;
Lim, S. F.; Ju, Y.; Austin, R. Opt. Express 2009, 17, 80.
(9) Zhou, Z.; Hu, H.; Yang, H.; Yi, T.; Huang, K.; Yu, M.; Li, F.; Huang, C.
Chem. Commun. 2008, 4786.
(10) See Supporting Information for details.
(11) The transitions responsible for the luminescence are given in the Supporting
Information.
(12) The inset of Figure S3b in the Supporting Information shows a digital
photograph of the total upconversion luminescence of a dispersion of
NaYF4:ErYb when stimulated with a 980 nm diode laser.
(13) See Figure S4 in the Supporting Information. Other solid-state versions
(bulk gels and solvent-free films) of the composite material were prepared
and exhibit similar photochemistry as shown in Figure S6.
Dithienylfulvene 3 is synthesized using our original methods with
some minor modifications.10 The ring-closed Diels-Alder product
(2b) is produced as a mixture of four stereoisomers after treating
3 with an excess of diethyl dicyanofumarate and exposing the
unstable intermediate (2a) to UV light. When an acrylate film
containing 2b and NaYF4:ErYb is exposed to 980 nm light in an
analogous manner as for 1b, similar color changes can be observed
JA904746S
9
J. AM. CHEM. SOC. VOL. 131, NO. 31, 2009 10839