Upconverting-nanoparticle-assisted photochemistry induced by low-intensity near-infrared light: How low can we go?

Article abstract of DOI:10.1002/chem.201500108

Upconverting nanoparticles (UCNPs) convert near-infrared (NIR) light into UV or visible light that can trigger photoreactions of photosensitive compounds. In this paper, we demonstrate how to reduce the intensity of NIR light for UCNP-assisted photochemistry. We synthesized two types of UCNPs with different emission bands and five photosensitive compounds with different absorption bands. A λ=974 nm laser was used to induce photoreactions in all of the investigated photosensitive compounds in the presence of the UCNPs. The excitation thresholds of the photoreactions induced by λ=974 nm light were measured. The lowest threshold was 0.5 W cm^-2, which is lower than the maximum permissible exposure of skin (0.726 W cm^-2). We demonstrate that low-intensity NIR light can induce photoreactions after passing through a piece of tissue without damaging the tissue. Our results indicate that the threshold for UCNP- assisted photochemistry can be reduced by using highly photosensitive compounds that absorb upconverted visible light. Low excitation intensity in UCNP-assisted photochemistry is important for biomedical applications because it minimizes the overheating problems of NIR light and causes less photodamage to biomaterials. Up, up, and away: Upconverting-nanoparticle (UCNP)-assisted photochemistry can be triggered by low-intensity (0.5 W cm^-2) near-infrared (NIR) light when highly photosensitive compounds that absorb upconverted visible light are used (see figure). Low excitation intensity in UCNP-assisted photochemistry is important in biomedical applications because it minimizes the overheating problems of NIR light and causes less photodamage to biomaterials.

Full text of DOI:10.1002/chem.201500108

DOI: 10.1002/chem.201500108
Full Paper
&
Photochemistry
Upconverting-Nanoparticle-Assisted Photochemistry Induced by
Low-Intensity Near-Infrared Light: How Low Can We Go?
Zhijun Chen, Wen Sun, Hans-Jürgen Butt, and Si Wu*[a]
Abstract: Upconverting nanoparticles (UCNPs) convert near-
infrared (NIR) light into UV or visible light that can trigger
photoreactions of photosensitive compounds. In this paper,
we demonstrate how to reduce the intensity of NIR light for
UCNP-assisted photochemistry. We synthesized two types of
UCNPs with different emission bands and five photosensitive
compounds with different absorption bands. A l=974 nm
laser was used to induce photoreactions in all of the investi-
gated photosensitive compounds in the presence of the
UCNPs. The excitation thresholds of the photoreactions
induced by l=974 nm light were measured. The lowest
threshold was 0.5 WcmÀ2, which is lower than the maximum
permissible exposure of skin (0.726 WcmÀ2). We demonstrate
that low-intensity NIR light can induce photoreactions after
passing through a piece of tissue without damaging the
tissue. Our results indicate that the threshold for UCNP-
assisted photochemistry can be reduced by using highly
photosensitive compounds that absorb upconverted visible
light. Low excitation intensity in UCNP-assisted photo-
chemistry is important for biomedical applications because
it minimizes the overheating problems of NIR light and
causes less photodamage to biomaterials.
Introduction
development of new methods that can efficiently induce
photoreactions by NIR light is therefore desirable.
Photochemistry has numerous biomedical applications,
including photoinduced drug delivery,[1] photocontrolled cell
adhesion and migration,[2] and photopatterning of extracellular
matrices.[3] Photoreactions are usually induced by UV light be-
cause only UV light can be efficiently absorbed by most photo-
sensitive compounds. However, UV light is problematic in bio-
medical applications because it may damage biomaterials such
as DNA and because it cannot penetrate deeply into tissue.
Compared with UV light, near-infrared (NIR) light is better
suited for biomedical applications. NIR light causes less photo-
damage and it can penetrate deeper into tissue. One approach
to inducing photoreactions by NIR light is based on a simulta-
neous two-photon absorption.[4] For example, to induce the
photoreaction of a photosensitive compound with an absorp-
tion band at l=400 nm, the photosensitive compound may
absorb two l=800 nm photons simultaneously. However,
a two-photon absorption is inefficient even when pulsed lasers
are used because photosensitive compounds typically have
small two-photon absorption cross-sections.[5] A two-photon
absorption only occurs at the focus of a laser because it re-
quires high-intensity light.[5] To induce photoreactions on
a macroscopic scale by a two-photon absorption, the photo-
sensitive compounds must be exposed to high-intensity light
An alternative approach to achieving NIR-light-induced pho-
tochemistry is based on photon upconversion by lanthanide-
doped upconverting nanoparticles (UCNPs). UCNPs convert
NIR light into UV and visible light that can induce photoreac-
tions of normal photosensitive compounds.[6] This process is
referred to as UCNP-assisted photochemistry. Photoreactions,
including photoisomerization,[7] photocleavage,[8] and photopo-
lymerization reactions,[9] have been induced by NIR light
through UCNP-assisted photochemistry. UCNP-assisted photo-
chemistry also has biomedical applications such as drug deliv-
ery[8a,10] and controlled cell adhesion.[11] One advantage of
photon upconversion is that it does not require high-intensity-
pulsed lasers. UCNPs can be excited by continuous-wave NIR
laser diodes with relatively low intensity, thereby enabling pho-
toreactions to be conducted on a macroscopic scale through
UCNP-assisted photochemistry. Photon upconversion is a non-
linear optical process[12] that requires the excitation intensity to
exceed a certain threshold. The reported excitation intensity
for UCNP-assisted photochemistry is typically between several
hundred mWcmÀ2 to several hundred WcmÀ2.[6–11] However, ac-
cording to the “American National Standard for Safe Use of
Lasers”, the maximum permissible exposure of skin to NIR light
with wavelengths from l=650 to 980 nm ranges from 0.2 to
0.726 WcmÀ2.[13] The relatively high-intensity NIR light required
by UCNP-assisted photochemistry may damage cells,[14] tissue
(Figure 1), and other biomaterials.[15] Therefore, the reduction
of the excitation intensity for UCNP-assisted photochemistry to
a medically harmless level is an important objective. However,
methods for achieving such a reduction of the excitation
intensity have not been systematically studied. Such a study is
through
a
time-consuming spot-by-spot process. The
[a] Z. Chen, W. Sun, Prof. Dr. H.-J. Butt, Dr. S. Wu
Max Planck Institute for Polymer Research
Ackermannweg 10, 55128, Mainz (Germany)
E-mail: wusi@mpip-mainz.mpg.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500108.
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Figure 1. Chicken tissue before and after l=974 nm light irradiation for
20 min; l=974 nm light with intensities of 35 and 5 WcmÀ2 caused burn
wounds and shrinking of the tissue; l=974 nm light with an intensity less
than 1 WcmÀ2 did not cause observable burn wounds. The scale bars
represent 1 cm.
essential for advancing the biomedical applications of
UCNP-assisted photochemistry.
Here, we investigated the design of suitable combinations
of UCNPs and photosensitive compounds to reduce the excita-
tion intensity for UCNP-assisted photochemistry. UCNP-assisted
photochemistry requires that the photosensitive compounds
absorb upconverted light from the UCNPs (Figure 2a), that is,
the emission bands of the UCNPs should overlap with the
absorption bands of the photosensitive compounds. We pre-
pared two types of UCNPs with different emission bands and
five photosensitive compounds with different absorption
bands. We systematically studied the UCNP-assisted photo-
chemistry of the photosensitive compounds in the
Figure 2. a) Schematic presentation of UCNP-assisted photochemistry.
UCNPs convert NIR light into UV or visible light that induces photoreactions
of photocleavable compounds. b) Chemical structures of five photocleavable
compounds. The photocleavable bonds are indicated by the arrows.
c,d) TEM images of Tm-UCNP and Er-UCNP. The insets show photographs of
the UCNPs after l=974 nm laser exposure.
presence of the UCNPs. This study provides insights
Table 1. Summary of the photochemical properties of photocleavable compounds.[a]
into the design of materials that are sensitive to low-
intensity NIR light.
Compound lmax e@lmax
eF@365 nm eF@470 nm eF@530 nm eF@656 nm
[nm] [mÀ1 cmÀ1] [mÀ1 cmÀ1
]
[mÀ1 cmÀ1
]
[mÀ1 cmÀ1
]
[mÀ1 cmÀ1
]
NB-1
CM-2
CM-3
Ru-4
Ru-5
343
373 32266
453 26666
448
515
5333
19.4
47.5
9.4
35.8
53.8
0
0
33.4
56.5
71.4
0
0
0.7
23.1
138.8
0
0
0
0
42.7
Results and Discussion
8000
6933
To construct systems for UCNP-assisted photo-
chemistry, we used five photocleavable compounds
(Figure 2b); their photoreactions are shown in Figure
S1 in the Supporting Information. Because their ab-
sorption bands are located at different wavelengths
(Figure 3a and Table 1), these photocleavable com-
pounds are sensitive to light of different wave-
[a] Symbols and abbreviations: lmax =absorption maximum, e=absorption coefficient,
F=quantum yield, eF=photosensitivity.
lengths. The nitrobenzyl derivative NB-1 and the coumarin de-
rivative CM-2 are sensitive to l=365 nm light (Figures S2 and
S3 in the Supporting Information). The coumarin derivative
CM-3 and the Ru complex Ru-4 are sensitive to l=365, 470,
and 530 nm light (Figures S4 and S5 in the Supporting Infor-
mation). The Ru complex Ru-5 is sensitive to l=365, 470, 530,
and 656 nm light (Figure S6 in the Supporting Information).
Several photochemical properties of the photocleavable com-
pounds, such as their absorption maximum (lmax), absorption
coefficient (e), quantum yield (F), and photosensitivity (eF),
were measured (Figures S2–S6 and Table S1 in the Supporting
Information); the results are listed in Table 1.
UCNPs are the other component used to construct systems
for UCNP-assisted photochemistry. We prepared Tm-UCNP
(core=NaYF4:0.5 mol% Tm3+:30 mol% Yb3+
, shell=NaYF4)
and Er-UCNP (core=NaYF4:2 mol% Er3+:30 mol% Yb3+, shell=
NaYF4). These UCNPs are two of the most efficient UCNPs
reported to date.[6b,16] The average diameters of both UCNPs,
as measured by TEM, were approximately 50 nm (Figures 2c
and d).
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tation intensities (0.19–2.2 WcmÀ2), the blue emission remains
and the UV emission completely vanishes. The photoreactions
of NB-1 and CM-2 can be induced by the four-photon NIR-to-
UV upconversion that requires greater excitation intensity. The
photoreactions of CM-3, Ru-4, and Ru-5 can be induced by
either the four-photon NIR-to-UV upconversion that requires
greater excitation intensity or the three-photon NIR-to-blue
light upconversion that requires lower excitation intensity.
Therefore, assisted by the Tm-UCNP, the photoreactions of
CM-3, Ru-4, and Ru-5 might require lower excitation intensities
than those required for the photoreactions of NB-1 and CM-2.
We also studied the photon upconversion of Er-UCNPs at
different excitation intensities with a l=974 nm laser (Figur-
es 3c and S8 in the Supporting Information). Er-UCNPs emitted
UV, blue, green, and red light at excitation intensities greater
than 33.8 WcmÀ2. When the excitation intensity was between
0.09 and 2.2 WcmÀ2, only green and red light were emitted.
2
The excitation thresholds for the H9/2–4I15/2 transition (l=
2
409 nm, three-photon process[18]), the H11/2–4I15/2 transition (l=
4
520 nm, two-photon process[18]), the S3/2–4I15/2 (l=540 nm,
4
two-photon process[18]), and the F9/2–4I15/2 transition (l=
653 nm, two-photon process[18]) were 2.2, 0.09, 0.04, and
0.09 WcmÀ2, respectively (Table 2). Thus, at low excitation in-
tensities (0.09–2.2 WcmÀ2), the green and red emissions
remain and the blue emission completely vanishes. Among the
five investigated photocleavable compounds, the photoreac-
tions of NB-1 and CM-2 cannot be induced by the two-photon
NIR-to-green light upconversion and the photoreaction of
CM-3 induced by this upconversion should be inefficient be-
cause of the low photosensitivity of CM-3 in the green-light
wavelength region (Table 1); however, the photoreactions of
Ru-4 and Ru-5 can be induced by the two-photon NIR-to-
green light upconversion. Therefore, the required excitation
intensities for the photoreactions of Ru-4 and Ru-5 assisted by
the Er-UCNP should be lower than those for the photo-
reactions of the other compounds.
Figure 3. a) UV/Vis absorption spectra of five photocleavable compounds.
b,c) Emission spectra of Tm-UCNP and Er-UCNP excited by a l=974 nm
laser with different excitation intensities. The excitation intensity was con-
trolled by a laser driver. The emission intensities of Tm-UCNP and Er-UCNP
are normalized at l=470 and 540 nm, respectively.
We measured the emission spectra of the Tm-UCNPs at dif-
ferent excitation intensities with a l=974 nm laser (Figures 3b
and S7 in the Supporting Information). The Tm-UCNPs emitted
both blue and UV light at excitation intensities greater than
2.2 WcmÀ2. When the excitation intensity was between 0.19
and 2.2 WcmÀ2, only blue light was emitted. The relative inten-
sity of the emissions depends on the excitation intensity.[7a,12,17]
To study UCNP-assisted photochemistry, dispersions of the
UCNPs were mixed with solutions of the photocleavable com-
pounds. The concentrations of the UCNPs (360 mgmLÀ1) and
the photocleavable compounds (3.7”10À6 molLÀ1) in the mix-
tures were constant. To measure the apparent threshold for
UCNP-assisted photochemistry, the mixtures were exposed to
l=974 nm light with different intensities for 1 h. The mixtures
were then analyzed by UV/Vis absorption spectroscopy (Fig-
ures 4 and 5). The lowest excitation intensity that induced
a spectral change was defined as the apparent threshold of
UCNP-assisted chemistry for a given photocleavable com-
pound/UCNP mixture. The measured apparent threshold might
be dependent on the irradiation time and the ratio between
the UCNPs and the photocleavable compounds. These
parameters were fixed in our experiments.
1
The excitation thresholds for the I6–3F4 transition (l=340 nm,
five-photon process[17,18]), the 1D2–3H6 transition (l=360 nm,
four-photon process[17,18]), the 1D2–3F4 transition (l=450 nm,
four-photon process[17,18]), and the 1G4–3H6 transition (l=
470 nm, three-photon process[17,18]) were 5.5, 2.2, 0.64, and
0.19 WcmÀ2, respectively (Table 2). Thus, at relatively low exci-
Table 2. Summary of the photon upconversion in Tm-UCNP and Er-UCNP.
Nanoparticle
Tm-UCNP
Transition (l [nm][a])
Upconversion threshold [WcmÀ2
]
1I6–3F4 (340)
1D2–3H6 (360)
1D2–3F4 (450)
1G4–3H6 (470)
2G9/2–4H15/2 (409)
2H11/2–4I15/2 (520)
4S3/2–4I15/2 (540)
4F9/2–4I15/2 (653)
5.5
2.2
0.64
0.19
2.2
0.09
0.04
0.09
The apparent thresholds for mixtures of photocleavable
compound/Tm-UCNP were measured (Figure 4). Exposure of
NB-1/Tm-UCNP and CM-2/Tm-UCNP to l=974 nm light
(35 WcmÀ2, 1 h) did not induce any spectral change, indicating
that their apparent thresholds were greater than 35 WcmÀ2.
Their apparent thresholds are high because photocleavage of
Er-UCNP
[a] Emission wavelength.
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Figure 4. UV/Vis absorption spectra of a) NB-1/Tm-UCNP, b) CM-2/Tm-UCNP, c) CM-3/Tm-UCNP, d) Ru-4/Tm-UCNP, and e) Ru-5/Tm-UCNP before and after
irradiation with l=974 nm light. The irradiation time was fixed at 1 h. The light intensity was systematically changed to determine the apparent threshold of
the UCNP-assisted photochemistry. f) Apparent thresholds of the UCNP-assisted photochemistry for photocleavable compound/Tm-UCNP mixtures.
NB-1 and CM-2 can only be triggered by the four-photon NIR-
to-UV upconversion, which requires high-intensity NIR light.
Our result is also in accordance with the reported result that
high-intensity NIR light (255 WcmÀ2) was required for the
cleavage of a similar nitrobenzyl derivative through UCNP-as-
sisted photochemistry.[8b] The apparent thresholds for CM-3/
Tm-UCNP, Ru-4/Tm-UCNP, and Ru-5/Tm-UCNP were 35, 5, and
1 WcmÀ2, respectively. Photoreactions of these three com-
pounds can be triggered by the three-photon NIR-to-blue light
upconversion, which requires relatively low-intensity NIR light.
A comparison of the apparent thresholds with the photosensi-
tivities of these three compounds at l=470 nm (Table 1) sug-
gests that the apparent threshold is lower if the compound is
more photosensitive at the emission wavelength of the UCNPs.
The apparent thresholds of photocleavable compounds with
Er-UCNP were also measured (Figure 5). The apparent thresh-
olds for NB-1/Er-UCNP, CM-2/Er-UCNP, and CM-3/Er-UCNP were
greater than 35 WcmÀ2. The apparent thresholds for Ru-4/Er-
Figure 5. UV/Vis absorption spectra of a) NB-1/Er-UCNP, b) CM-2/Er-UCNP, c) CM-3/Er-UCNP, d) Ru-4/Er-UCNP, and e) Ru-5/Er-UCNP before and after irradiation
with l=974 nm light. The irradiation time was fixed at 1 h. The light intensity was systematically changed to determine the apparent threshold of the UCNP-
assisted photochemistry. f) Apparent thresholds of the UCNP-assisted photochemistry for photocleavable compound/Er-UCNP mixtures.
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UCNP and Ru-5/Er-UCNP were 10 and 0.5 WcmÀ2, respectively.
Ru-4/Er-UCNP and Ru-5/Er-UCNP exhibit lower apparent
thresholds because their photoreactions can be triggered by
the two-photon NIR-to-green light upconversion, which
requires relatively low-intensity NIR light. A comparison of the
apparent thresholds with the photosensitivities of Ru-4 and
Ru-5 at l=530 and 656 nm (Table 1) suggests that the appar-
ent threshold is lower if the compound is more photosensitive
at the emission wavelengths of the UCNPs. Exposure of all
photocleavable compounds to l=974 nm light in the absence
of UCNPs did not change the absorption spectra of the com-
pounds (Figures S9–S13 in the Supporting Information). This
control experiment confirms that the photoreactions shown in
Figures 4 and 5 were induced by upconversion.
Figure 6. a) Schematic model and b) UV/Vis absorption spectra of Ru-5/Er-
UCNP when a 0.5 mm-thick tissue is placed between the l=974 nm light
(0.72 WcmÀ2) and Ru-5/Er-UCNP.
UCNP (Figure 6a). The l=974 nm light at an intensity of
0.72 WcmÀ2 was still able to cleave Ru-5 after passing through
the tissue (Figures 6b and S14 as well as Table S3 in the Sup-
porting Information). Thus, we envision that UCNP-assisted
photochemistry with low excitation intensity can be used in
deep-tissue biomedical applications.
The apparent thresholds of UCNP-assisted photochemistry
(Figures 4 f and 5 f) are higher than the thresholds of the corre-
sponding upconverted emissions that can induce the photo-
reactions (Table 2). This phenomenon is understandable. For
example, exposure of Ru-4/Tm-UCNP to l=974 nm light with
an intensity of 5 WcmÀ2 for 1 h induced an observable photo-
reaction (Figure 4d). The l=974 nm light at an intensity of
0.19 WcmÀ2 is sufficient to excite the NIR-to-blue light upcon-
version (Figure 3b), which may induce the photoreaction of
Ru-4. However, the intensity of blue light upconverted from
l=974 nm light at an intensity of 0.19 WcmÀ2 is only approxi-
mately 0.4% that of blue light upconverted from l=974 nm
light at an intensity of 5 WcmÀ2 (Figure S7 in the Supporting
Information). Thus, achieving the same effect of blue light up-
converted from l=974 nm light at 5 WcmÀ2 for 1 h requires
250 h at 0.19 WcmÀ2. Therefore, no influence of l=974 nm
light at 0.19 WcmÀ2 on the absorption spectrum of Ru-4/Tm-
UCNP was observed within our experimental timeframe. Addi-
tionally, l=974 nm light at 5 WcmÀ2 also excites l=450 and
360 nm emissions that contribute to photocleavage of Ru-4.
The irradiation time and the UCNP concentration may influ-
ence the measured apparent threshold of UCNP-assisted pho-
tochemistry. The irradiation time in our experiments was 1 h,
which is comparable to the irradiation time for photocontrol-
led drug delivery and cell adhesion based on UCNP-assisted
photochemistry.[6b,8a,10,11] The concentration of the UCNPs in
our experiments was 360 mgmLÀ1, which is comparable to the
concentrations of UCNPs used in drug delivery.[6b,8a,10a–c,e,f]
Higher concentrations of UCNPs in biomedical applications
may result in toxicity.[6b] Moreover, the photocleavable chromo-
phores used in this work exhibit numerous potential biomedi-
cal applications.[3a–c,4,5,6b,8a,10f,19] Thus, we believe our results can
provide guidance for the design of materials that are sensitive
to low-intensity NIR light for biomedical applications.
Conclusion
We measured the thresholds of UCNP-assisted photochemistry
for five different photocleavable compounds combined with
Tm- or Er-UCNPs. The threshold of the UCNP-assisted photo-
chemistry depends on the emissive properties of the UCNPs
and on the photochemical properties of the photocleavable
compounds. To design photosensitive materials that are sensi-
tive to low-intensity NIR light by using UCNPs, our results sug-
gest that 1) the absorption wavelengths of the photosensitive
compounds should overlap the wavelengths of the upconvert-
ed emissions that can be excited by low-intensity NIR light and
2) the photosensitive compounds should be highly photosensi-
tive at the emission wavelengths of the UCNPs. The threshold
of
UCNP-assisted
photochemistry
for
Ru-5/Er-UCNP
(0.5 WcmÀ2) is lower than the maximum permissible exposure
of skin (0.726 WcmÀ2). We expect that not only the com-
pounds used in this work but also other visible-light-sensitive
compounds[19,20] should be suitable for UCNP-assisted photo-
chemistry induced by low-intensity NIR light. The excitation in-
tensity for UCNP-assisted photochemistry can be reduced even
further by 1) grafting the photosensitive compounds onto the
surface of the UCNPs, 2) using UCNPs with improved upcon-
version efficiency, and 3) using surface plasmons to enhance
upconversion. Low excitation intensity in UCNP-assisted
photochemistry can prevent photodamage to biomaterials,
which is important in biomedical applications.
Among the studied samples, Ru-5/Er-UCNP exhibited the
lowest apparent threshold (0.5 WcmÀ2); its threshold is lower
than the maximum permissible exposure of skin
(0.726 WcmÀ2). Thus, the use of l=974 nm light with an inten-
sity of 0.5 WcmÀ2 can minimize photodamage to tissue
(Figure 1). Moreover, we studied the feasibility of using low-in-
tensity NIR light to induce photocleavage of Ru-5 in deep
tissue in the presence of Er-UCNP. For this purpose, a piece of
tissue was inserted between a l=974 nm laser and Ru-5/Er-
Experimental Section
Materials: Ytterbium(III) acetate hydrate (99.9%), thulium(III)
acetate hydrate (99.9%), yttrium(III) acetate hydrate (99.9%),
erbium(III) acetate hydrate (99.9%), 2,2’-bipyridine (98%), 1-octa-
decene (technical grade, 90%), oleic acid (technical grade, 90%),
ammonium fluoride (99.99%), ruthenium(III) chloride trihydrate
(technical grade), 2,2’-biquinoline (98%), and 2,2’:6’,2’’-terpyridine
(98%) were purchased from Sigma–Aldrich. All solvents and other
chemicals were purchased from Sigma–Aldrich or Fisher Scientific.
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The synthesis of Tm-UCNP was reported in our previous work.[8a]
Er-UCNP,[7a] NB-1,[21] CM-2,[22] CM-3,[22] Ru-4,[23] and Ru-5[24] were syn-
thesized according to the procedures described in the literature.
Mixtures of the photocleavable compounds and the UCNPs were
prepared by adding dispersions of the UCNPs (20 mL, 45 mgmLÀ1
in cyclohexane) to solutions of the photosensitive compounds
(2.5 mL, 3.75”10À6 m in THF/H2O (99:1, v/v)).
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General methods: UV/Vis absorption spectra were recorded on
a Lambda 900 spectrophotometer (Perkin–Elmer). TEM images
were collected on a JEOL JEM1400 transmission electron micro-
scope. Upconversion photoluminescence measurements were per-
formed on a Spex Fluorolog II (212) spectrometer. A diode laser
with a wavelength of l=974 nm (device type P976MF, PhotonTec
Berlin GmbH) coupled with a 105 mm (core) fiber was used as the
excitation source. The diode laser was equipped with an adjustable
fiber collimator (Changchun New Industries Optoelectronics Tech-
nology). The output power of the diode laser was controlled by
a tabletop laser driver (device type ds11-la12v08-pa08v16-t9519-
254-282, OsTech GmbH i.G.). The output power density of the
diode laser was measured by using an optical power meter (model
407A, Spectra-Physics Corporation) and an NIR indicator (model F-
IRC1, Newport). To measure the apparent threshold of UCNP-assist-
ed photochemistry, photocleavable compound/UCNP mixtures
were exposed to l=974 nm light with a given intensity for 1 h.
The UV/Vis absorption spectrum of the photocleavable compound/
UCNP mixture was then measured to determine if the light intensi-
ty was sufficient to induce the photoreaction. To measure the pho-
tosensitivity, the solutions of the photocleavable compounds (3.7”
10À6 molLÀ1) were irradiated by LEDs at l=365, 470, 530, and
656 nm (device types LCS-0365-07-22, LCS-0470-03-22, LCS-0530-
15-22, and LCS-0656-03-22, Mightex Systems). The photoreactions
were studied by UV/Vis absorption spectroscopy (Figures S2–S6 in
the Supporting Information) and the photosensitivities of the reac-
tions were calculated (Table S1 in the Supporting Information). The
output power densities of the LEDs were controlled by an LED
controller (device type SLC-MA04-MU, Mightex Systems).
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Acknowledgements
Z.C. and W.S. were supported by the CSC program. We thank
H. Menges for measuring the upconverting luminescence
spectra. This work was partly supported by the Deutsche For-
schungsgemeinschaft (DFG, WU 787/2-1).
Keywords: nanoparticles
photochemistry · upconversion
· near infrared · photocages ·
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Received: January 11, 2015
Published online on May 12, 2015
Chem. Eur. J. 2015, 21, 9165 – 9170
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