Near-IR-triggered, remote-controlled release of metal ions: A novel strategy for caged ions

Article abstract of DOI:10.1002/anie.201405462

A ligand incorporating a dithioethenyl moiety is cleaved into fragments which have a lower metal-ion affinity upon irradiation with low-energy red/near-IR light. The cleavage is a result of singlet oxygen generation which occurs on excitation of the photosensitizer modules. The method has many tunable factors that could make it a satisfactory caging strategy for metal ions. Metal ions on demand: Near-IR irradiation of a designer ligand results in singlet-oxygen-mediated fragmentation with the consequent release of metal ions. The modular nature of the "cage" may herald a new class of agents that could supply chemical effectors on demand.

Full text of DOI:10.1002/anie.201405462

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Angewandte
Communications
DOI: 10.1002/anie.201405462
Caged Compounds
Near-IR-Triggered, Remote-Controlled Release of Metal Ions: A
Novel Strategy for Caged Ions**
Ahmet Atilgan, Esra Tanriverdi EÅik, Ruslan Guliyev, T. Bilal Uyar, Sundus Erbas-Cakmak, and
Engin U. Akkaya*
Abstract: A ligand incorporating a dithioethenyl moiety is
cleaved into fragments which have a lower metal-ion affinity
upon irradiation with low-energy red/near-IR light. The
cleavage is a result of singlet oxygen generation which occurs
on excitation of the photosensitizer modules. The method has
many tunable factors that could make it a satisfactory caging
strategy for metal ions.
a limited focal region for the two-photon techniques, and
possible requirements for redesign of the cages.
In recent years, a cleavage reaction has been investigated
which is dependent on the generation of singlet oxygen
through photosensitization of dissolved molecular oxygen.[7]
The process has some similarities to photodynamic action but
with one key difference: generated singlet oxygen reacts with
an electron-rich alkene (dialkylthio- or dialkoxy-substituted)
in the designed microenvironment, resulting in the cleavage
of the molecule into two fragments at the site of the electron-
rich alkene.[8] The reaction is efficient and can be driven by
very low-energy light (l = 650–900 nm).[9] This use of low-
energy light could be a major advantage if the fragmentation
event is linked to a biological process.
In our design, we wanted to use low-energy radiation to
remotely release biologically relevant metal ions on demand.
Zn2+ is an important target for caging, as changing concen-
trations of labile zinc ions are involved in a number of
pathological conditions, including the development of pros-
tate cancer and Alzheimerꢀs disease.[1c,10] In addition, a mod-
ular design was employed so that with minimal synthetic
changes to the modules the type of the metal ion to be caged/
released and the excitation wavelength could be changed.
Our proof of principle design (Figure 1) incorporates
symmetric structures to simplify the synthetic procedure.
With this in mind, brominated di-styryl bodipy units
(bodipy = boron dipyrromethene), with absorption maxima
in the red region of the electromagnetic spectrum and meso-
azido substituents, were prepared (Supporting Information).
Through the azido moieties, these units can undergo “click”
reactions with the specially crafted alkynyl-substituted
dithioethenyl unit (Figure 2). Zn2+ ions should preferentially
coordinate to the nitrogen donor atoms rather than the sulfur
donors of the “labile” linker, and the click reaction itself
potentially contributes two new N-donor atoms on both sides
of the linker (Figure 1). In this case, the copper(I) catalyst is
strongly deactivated by the ligand and standard click reaction
conditions were ineffective. An alternative copper(I) com-
plex, developed by ꢁzÅubukÅu et al.[11] for difficult reactions
of this type, was used with satisfactory results.
T
he remote manipulation of molecular or ionic concentra-
tions at will, especially in well-defined compartments, such as
cells, organelles, or in vivo in tissues, is very important, as it
provides an unparalleled capability to control biochemical
processes.[1] Caged compounds, in principle, have such
a potential.[2] However, as most uncaging processes involve
breaking a covalent bond, there is a strict lower limit for the
photonic energy of the light suitable for photochemical
uncaging.[3] For nitrobenzyl derivatives and related moieties
the lowest-energy limit is approximately l = 360 nm.
Although for some other molecules the lowest-energy limit
can be pushed back to l = 400–450 nm, there is a penalty in
the form of significantly decreased reaction quantum yields
and diminished conversion efficiencies.[4] Unfortunately, this
requirement for UV or blue-light excitation limits the
applicability severely as a result of potential photodamage
to cells, high scatter, and light absorption in biological media
resulting in very poor tissue penetration.[5] The problem can
be circumvented by a few techniques, such as two-photon
excitation, X-ray photolysis, or by incorporating upconverting
nanoparticles (UCNP).[6] Each of the three techniques have
advantages and disadvantages, and although they offer
palliative solutions, they may also introduce limitations/
problems of their own, such as the toxicity of UCNPs,
[*] A. Atilgan, Dr. E. Tanriverdi EÅik, T. B. Uyar, Dr. S. Erbas-Cakmak,
Prof. Dr. E. U. Akkaya
UNAM- National Nanotechnology Research Center
Bilkent University
06800 Ankara (Turkey)
E-mail: eua@fen.bilkent.edu.tr
Dr. E. Tanriverdi EÅik
Department of Chemistry, Gebze Institute of Technology
41400 Kocaeli (Turkey)
In our design, the photosensitizer is an integral part of the
cage molecule and is not added externally. The advantage of
this strategy is obvious in terms of reaction efficiency.
Additionally, the modular design of the system allows us to
choose a sensitizer that could absorb in any region of the
spectrum, especially in the near-IR region, and is independent
of the ligand design (Figure 2).
Dr. R. Guliyev, Prof. Dr. E. U. Akkaya
Department of Chemistry, Bilkent University
06800 Ankara (Turkey)
[**] Funding from TUBITAK (113T043) is gratefully acknowledged. R.G.
and T.B.U. acknowledge support from TUBITAK in the form of
postdoctoral and doctoral scholarships.
The ligand design, in principle, can be modified by placing
alternative donor groups, chelating groups, or even carbox-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201405462.
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Figure 1. Structural formula of cage ligands (1 and 2), the correspond-
ing caged Zn2+ compounds (1+Zn2+ and 2+Zn2+), and the reporter
molecule 3. PS=photosensitizer unit.
ylate moieties (for alkaline and alkaline-earth cations), on the
thiol substituents. The current design (PS1) is anticipated to
work optimally at an excitation wavelength of l = 660 nm. To
follow the progress of the uncaging reaction, that is, the
release of Zn2+ ions, we chose to employ a bodipy-based
(BOD) fluorescent reporter of zinc(II), with a dipicolylamine
(DPA) ligand (3; DPA-BOD). This reporter molecule has
a rapid and strong response to increasing zinc concentrations
with an increasing emission intensity.[12] The binding strength
and stoichiometry of compounds 1 and 2 to Zn2+ ions was
studied with isothermal titration calorimetry (ITC) by using
a model compound (page S17, Figure S47, Supporting Infor-
mation). The model compound clearly shows a 1:1 binding
stoichiometry, and a binding constant of 2.6 ꢂ 105 mÀ1 was
calculated. Additionally, a + 2 charged species with the
correct m/z ratio was identified for the 1:1 zinc(II) complex
(page S17, Figures S44,S45, Supporting Information).
Figure 3. Working principle to activate the caged Zn2+ compound
(1+Zn2+) by light of any required energy. hn=incident light absorbed
by the PS.
dissolved molecular oxygen follows to generate singlet
oxygen, which undergoes addition to the double bond to
form a reactive dioxetane ring. Thermal ring opening at room
temperature results in the cleavage products. The ligand itself
is cleaved into two pieces which will have significantly lower
affinity for Zn2+ ions. The net result is a release of metal ions.
Based on the above strategy, the uncaging experiment was
then carried out by the irradiation of the caged Zn2+ complex,
which was prepared by the addition of one equivalent of Zn2+
ions to a solution of ligand 1 in the presence of the reporter
molecule 3. The solution was irradiated with filtered broad-
band white light where wavelengths shorter than l = 400 nm
had been removed. Aliquots of the irradiated solution were
taken at four-minute intervals and the fluorescence of the
Zn2+ reporter molecule was determined (Figure 4).
With increasing irradiation time, the emission intensity of
the probe 3 increased steadily, demonstrating the release of
Zn2+ from the molecular cage. Alternative possibilities that
might lead to an emission increase at l = 510 nm were
experimentally tested and eventually eliminated. For exam-
ple, in the absence of irradiation, no change in the emission
intensity of the probe is detected when mixed with the caged
Zn2+ species, attesting to the higher affinity of the cage for
Zn2+ ions compared to the probe 3. In the absence of the
probe itself, no change in the emission is detected in the l =
510 nm region of the spectrum upon irradiation of the cage
compound at l = 660 nm. Additional control experiments
also demonstrated that the singlet oxygen produced on
excitation does not interfere with the photoluminescence
The zinc(II) release process (Figure 3) is initiated by
absorption of a photon of light, in this case, it is a photon of
red light (l = 660 nm). The photosensitizer undergoes rapid
excitation to access the triplet excited state as a result of the
heavy atoms in the system (in this case two bromine atoms)
which facilitate intersystem crossing. Energy transfer to
Figure 2. Modular design and synthetic route to prepare the proposed
cage ligands (1 and 2).
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Figure 5. Emission spectra showing the fluorescence response of
compound 3 (DPA-BOD) after uncaging of 2+Zn2+ (5.0 mm each) by
light irradiation (recorded in CH3CN). Time values indicate the
duration of irradiation with the Xe lamp (wavelengths of higher energy
than l=400 nm filtered out). Compound 3 initially exhibits low
fluorescence (quenched due to the active PET process). Irradiation
results in the photolysis of the 2+Zn2+ complex and is followed by the
enhanced emission intensity of 3 (lex =490 nm). Highest intensity
emission intensity represents the maximum emission intensity of
3 (DPA-BOD) obtained by the addition of 1.0 equivalents of zinc(II)
ions in the form of the triflate salt.
Figure 4. Emission spectra showing the fluorescence response of
compound 3 (DPA-BOD) after uncaging of 1+Zn2+ (5.0 mm each) by
light irradiation (recorded in CH3CN). Time values indicate the
duration of irradiation with the Xe lamp (wavelengths of higher energy
than l=400 nm filtered out). Irradiation results in the photolysis of
the Zn2+ complex 1+Zn2+ and is followed by an increase in emission
intensity due to the formation of the 3+Zn2+ complex (lex =490 nm).
Highest intensity emission trace represents the maximum emission
intensity of 3 (DPA-BOD) obtained by the addition of 1.0 equivalents
of zinc(II) ions in the form of the triflate salt.
signal detected upon Zn2+ binding to the reporter compound
3 (Figure S8).
The efficiency of the uncaging process was then compared
with an established o-nitrobenzyl-containing caged Zn2+
compound[14] (which is structurally similar to a well-studied
cage ligand[4a]) under the same conditions. Excitation at l =
360 nm of the o-nitrobenzyl caged compounds results in
a similar photoluminescence signal enhancement in the
reporter molecule (Figure 6) as detected for the symmetric
cage system (1 + Zn2+), but less than the asymmetric system
(2 + Zn2+). In other words, better results were obtained in
terms of percent Zn2+ release under lower energy irradiation
(l = 660 nm), when compared to that of o-nitrobenzyl cages
The singlet oxygen quantum yield of the current system
using PS1 as the photosensitizer dyes was determined to be
0.14 (Supporting Information). Despite the modest efficiency,
application of this photosensitizer resulted in impressive
uncaging of zinc(II) ions. Replacement of the chromophore
with a more efficient photosensitizer is anticipated to
generate more effective uncaging by molecular cleavage.
We next sought to demonstrate the ability of an energy-
transfer cassette to affect the uncaging process. To that end,
asymmetric compound 2 was synthesized (Figure 1). In this
molecule, the dye PS2, with a shorter wavelength absorption
maxima (l = 600 nm) and a larger extinction coefficient, is
employed. In molecule 2, PS2 is the primary excitation target,
and as it does not contain heavy atoms, it is not expected to
undergo efficient intersystem crossing. However, from the
PS2 moiety, excited-state energy transfer (EET) is possible.[13]
When EET takes place to the other chromophore module
(PS1), singlet oxygen is generated as a result of the heavy
atom effect of the bromine atoms on that bodipy core. This
again leads to the cleavage of the alkene, releasing fragments
of the original ligand which have a lower binding affinity for
Zn2+ ions, and uncaging Zn2+ ions which are then reported by
the reporter compound 3 (Figure 5). Using a broadband
excitation source, we were able to excite the photosensitizer
both directly and by energy transfer, which is expected to
yield a higher efficiency in the uncaging process. The propor-
tional increase in emission intensity is higher under these
conditions, demonstrating that Zn2+ release can be coupled to
intramolecular energy transfer. This experiment shows that
the antenna effect can be utilized for more effective uncaging
and metal-ion release.
Figure 6. Emission spectra showing the fluorescence response of
compound 3 after uncaging of one equivalent of o-nitrobenzyl
Zn2+ cage complex (5.0 mm each) by light irradiation (recorded in
CH3CN). Compound 3 (DPA-BOD) initially exhibits no fluorescence
(quenched due to the active PET process). As the solution was
irradiated at l=360 nm (for 10 min) the increase in the emission
intensity levels off, which can be interpreted as the completion of
photolysis of the Zn2+ cage compound.
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Bandara, T. P. Walsh, S. C. Burdette, Chem. Eur. J. 2011, 17,
under UV irradiation. The percentage of zinc(II) released
following irradiation can be quantified. The area underneath
the emission band for one equivalent of Zn2+ ions in the
presence of the reporter unit 3 can be set to correspond to
100% free Zn2+ ions. By comparison, the ratios of the
emission band areas will yield the relative amounts of Zn2+
ions released. By this method, 31% Zn2+ ion release is
calculated for compound 1 + Zn2+, whereas asymmetric
compound 2 + Zn2+ leads to 40% free Zn2+. Under compa-
rable conditions, that is, when the emission intensity increase
has leveled off in acetonitrile, 29% of the o-nitrobenzyl caged
zinc compound (Figure 6) releases its bound Zn2+ ions which
are reported by the fluorescent Zn2+ probe. These values do
not imply incomplete reactions as some fraction of the Zn2+
may not be available to the reporter compound as a result of
residual affinity of the cleaved fragments for Zn2+ ions.
In conclusion, we report a novel modular design for caging
ligands and red to near-IR light-triggered uncaging of
Zn2+ ions from that cage. This is the first example of metal-
ion uncaging using more penetrating lower-energy red to
near-IR light without using upconversion, X-rays, or two-
photon techniques. There is substantial potential for the
application of systems of this type in biological systems
including model organisms (in vivo), and further work may be
also expected to yield designer cages for therapeutic appli-
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Received: May 20, 2014
Revised: June 30, 2014
Published online: August 11, 2014
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Keywords: caged compounds · photochemistry ·
photosensitizers · singlet oxygen · zinc
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