Photoreleasable Protecting Groups Triggered by Sequential Two-Photon Absorption of Visible Light: Release of Carboxylic Acids from a Linked Anthraquinone- N -Alkylpicolinium Ester Molecule

Article abstract of DOI:10.1021/acs.jpca.8b00657

A photoreleasable protecting group activated by sequential absorption of two visible photons is designed, synthesized, and tested. Specifically, an anthraquinone-based chromophore is covalently attached to an N-alkylpicolinium ester. Photolysis of this linked system results in the clean release of a corresponding carboxylic acid. Through product analysis, laser flash photolysis, and steady-state UV-vis spectroscopy, it is demonstrated that carboxylate ion release is affected by sequential absorption of two photons. The initial photochemical step results in reduction of an anthraquinone chromophore to the corresponding hydroquinone. The latter either reacts with O₂ to regenerate the starting material, or absorption of a second photon causes an electron transfer to the picolinium group triggering C-O bond scission and release of the carboxylate.

Full text of DOI:10.1021/acs.jpca.8b00657

Street N.W.,
States
Washington,
DC 20036
Published
Photoreleasable

Street N.W.,
of Visible
Washington,
DC 20036
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Published
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Street N.W.,
Molecule
Washington,
DC 20036
Matthew
Published
D. Thum,

on MarchS1tr4ee,t2N0.W18.,
Washington,
DC 20036
Published

Street N.W.,
site and published as an
Washington,
to the manuscript text an
DC 20036
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Published

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The Journal of Physical ChemistPryage 2 of 22
hν1
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D+•
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M*
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X
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kd[AQH•]
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OH
O
AQH•
AQH2

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The Journal of Physical ChemistPryageO6 of 22
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Sodium Dithionite
D2O/CD3OD
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O
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1) 447 nm 60 min
OH
2) O
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3*
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O
N
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Cl
hν1, 447 nm
O
O
OH
OH
H
H2O2
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PAQ-1
PAQ-13*
λmax = 490 nm
O2
O
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Ph
Ph
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hν2, 447 nm
PET
-H+
OH
N
OH
O
N
H
N
Cl
Cl
H
N
Disproportionation
O
O
PAQH•-1
PAQ-1
OH
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PAQH-1
λ
max = 390 nm
λ
max = 410 nm
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PAQ-0
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Photoreleasable Protecting Groups Triggered by Sequential Two-
Photon Absorption of Visible Light: Release of Carboxylic Acids
from a Linked Anthraquinone-N-Alkylpicolinium Ester Molecule
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Matthew D. Thum and Daniel E. Falvey*
University of Maryland, College Park, Maryland 20742, United States
ABSTRACT: A photoreleasable protecting group activated by sequential absorption of two visible photons is designed, synthe-
sized and tested. Specifically, an anthraquinone-based chromophore is covalently attached to an N-alkylpicolinium ester. Photolysis
of this linked system results in the clean release of a corresponding carboxylic acid. Through product analysis, laser flash photolysis
and steady-state UV/vis spectroscopy it is demonstrated that carboxylate ion release is effected by sequential absorption of two
photons. The initial photochemical step results in reduction of an anthraquinone chromophore to the corresponding hydroquinone.
The latter either reacts with O2 to regenerate the starting material, or absorption of a second photon causes an electron transfer to
the
picolinium
group
triggering
C–O
bond
scission
and
release
of
the
carboxylate.
Scheme 1. A stepwise, two-photon process.
INTRODUCTION
Photochemical reactions have long been valued for their abil-
ity to provide both temporal and spatial control of molecular
processes. Recently attention has turned to photochemical
processes that are initiated by multiple photons, as these reac-
tions have the promise of providing more precise 3-
hν2
hν1
1/τ
C
B
A
PRPGs are molecular groups that can be covalently attached to
a substrate molecule, masking its normal reactivity.36–45 Pho-
tolysis of the PRPG unit releases the original substrate. While
most PRPGs are activated by single-photon absorption, some
have shown useful behavior under non-resonant two-photon
excitation.23,46 However, other than an interesting example
described by Pirrung and coworkers using a modified o-
nitrobenzyl group PRPG for alcohols, PRPGs activated by
stepwise two-photon excitation remain underexplored.47 Here-
in we report the efficient photorelease of a carboxylate anion
that is triggered by visible light, sequential, two-photon pho-
toinduced electron transfer (PET) from an anthraquinone
chromophore to a covalently attached N-alkyl picolinium
(NAP) group by exploiting the photochemical reduction of
anthraquinone to hydroquinone in the presence of alcohols.
dimensional spatial control. One approach is non-resonant
two-photon absorption.1–8 In this case, the targeted excited
state is generated using two photons that, individually, would
not possess sufficient energy to populate an excited state, but
when absorbed simultaneously, can create one. Simultaneous
absorption requires high photon densities and, in practice, is
typically realized using focused, femtosecond laser pulses. An
attractive feature of such systems is the ability to activate pho-
tochemical or photophysical responses in media that might
absorb the lower wavelengths required for resonant excitation
and/or with 3-dimensional control, as the desired two-photon
processes only occur at the focal point of the laser beam. Ap-
plications of this phenomenon have been demonstrated in high
resolution optical imaging,9–13 photolithography,10,14–19 3-
dimensional optical storage,20,21 and more.22–25
The present study is part of a program aimed at developing
molecular systems that are activated through stepwise two-
photon excitation.26–30 Scheme 1 illustrates the general design
principles. Initial excitation of substrate A creates a short lived
excited state or reversibly formed reactive intermediate B.
The excited state of interest is created when B absorbs a se-
cond photon. In contrast to the high peak power requirements
for non-resonant multiphoton absorption, stepwise, two-
photon absorption can be achieved using low power, continu-
ous wave (CW), light sources. Some examples of this ap-
proach have been successfully applied to fluorescence imag-
ing, photochemical switches, photoacid generation, and syn-
thesis.31–35 The experiments described below focus on the
development of photoreleasable protecting groups (PRPGs)
based on stepwise, two-photon excitation.
A previous report described single photon release of carbox-
ylate anions (X-) using a N-alkylpicolinium (NAP) ion that
was covalently attached to a light–absorbing mediator (M) via
photoinduced electron transfer mechanism. The current report
demonstrates that by tuning the redox potential of M it is pos-
sible to drive a similar release process through sequential re-
lease of two photons (Scheme 2).
Scheme 2. Sequential, two-photon release of a compound,
X, using two electron transfer steps.
D
D
hν1
hν2
NAP
NAP
NAP
M
M
X
X
kBET
kBET
kFRAG
M
+
NAP
X
M
X
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Scheme 5. Synthesis of a linked AQ-NAP complex.
RESULTS AND DISCUSSION
O
O
O
H
N
4
Cl
(a) Design and Synthesis
NH2
Cl
1.4 eq.
Cl
5
6
7
8
Previous work demonstrated that the NAP (Scheme 3) group
can be used as a conventional, one-photon PRPG for carbox-
ylic acids and amines.48–52 One-electron reduction of the NAP
group triggers a rapid and efficient C–O bond scission releas-
ing a carboxylate or carbamate ion. The reduction step can
occur either directly from an excited-state electron donor (e.g.
N-methylcarbazole), or though mediated electron transfer. In
the latter case, the mediator (M) is an excited state oxidant
(e.g benzophenone) that abstracts an electron (or H atom) from
a sacrificial donor (Scheme 3) and subsequently reduces the
NAP group in a spontaneous, ground state process. Adapting
this reaction to a stepwise two-photon scheme requires that the
first photon generate an intermediate that will not spontane-
ously reduce the NAP group, but will only do so after absorb-
ing a second photon.
O
CH2Cl2 18 hours
O
O
AQ (X=NH2)
O
9
Ph
O
O
Ph
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O
O
O
N
H
N
4 eq.
N
Cl
O
THF reflux 18 hours
PAQ-1
2-Aminoanthraquinone was covalently liked to a picolyl ester
according to the route described in Scheme 5. In brief, 2-
aminoanthraquinone and 2-chloroacetylchloride were com-
bined in dichloromethane at 0 o C for one hour followed by
stirring at room temperature overnight yielding 2-chloro-N-
(9,10-anthraquinone-2-)acetaniline. The final linked chromo-
phore-acceptor, PAQ-1, was prepared by heating 2-chloro-N-
(9,10-anthraquinone-2-)acetaniline and picolyl phenyl acetate
in THF at reflux for 18 hours.
Scheme 3. Photorelease via electron transfer to the NAP
group.
D+•
D
M*
X
H
X
hν
X
M-•
Amines
Carboxylates
M
X =
N
N
N
(b) Photochemical deprotection
Photolysis of PAQ-1 in isopropyl alcohol or methanol, either
with high intensity visible light (447 nm, ca. 1 W) or with
sequential application of 350 nm (broadband) light followed
by 447 nm light causes clean release of phenylacetic acid.
These solvents were chosen because they are expected to serve
as H atom donors to the triplet state of AQ and thus produce
the intermediates that could be subsequently photolyzed to
initiate release of from NAP. The AQ chromophore has an
absorbance maximum at 337 nm, as well as a long wavelength
tail that extends out beyond 450 nm. Thus, both light sources
are expected to excite AQ and the subsequently formed inter-
mediates (see below). A specific example of the photolysis
illustrated in Figure 1. In this case, a 0.24 mM solution of
PAQ-1 in isopropyl alcohol was irradiated for one hour at 447
nm. After removal of the solvent only phenyl acetic acid and
PAQ-0 were detected by 1H NMR (nuclear magnetic reso-
nance) and ESI+ (electro-spray ionization) mass spectrometry.
Following the irradiation, 1H NMR shows clear change as
PAQ-1 is converted into PAQ-0 and phenyl acetic acid
(Figure 2). The formation of PAQ-0 was further confirmed
by comparing photolyzed samples to that of an inde-
pendently synthesized standard. Similar photolyses carried
out on air-equilibrated samples also produced some phe-
nylacetic acid, but with a more complex mixture of by-
products.
Anthraquinone (AQ, Scheme 4) derivatives were chosen as
mediators for sequential two photon excitation purposes. With
appropriate substitution they absorb in the visible region of the
spectrum and, in most cases, excitation results in efficient
formation of excited triplet states, AQ3*. Under reducing con-
ditions (i.e. in the presence of electron donors, or in solvents
that donate H atoms), AQ3* is known to generate three poten-
tial intermediates that could serve as B in Scheme 1.53–59 One
electron reduction forms the anion radical (AQ
•
–) and/or its
conjugate acid, the semiquinone radical (AQH•). The latter
radicals can either undergo a disproportionation reaction, or a
sequential H atom abstraction, to form hydroquinones
(AQH2). The reduction potential of the anthraquinone chro-
mophore (-0.62 V vs. SCE), is less negative than that of the
NAP group (-1.1 vs SCE), therefore rapid and spontaneous
–
electron transfer from AQ
•
to the NAP group should be dis-
favored. Likewise, the protonated AQH• and the hydroqui-
none, AQH2, are expected to be even less labile toward the
subsequent electron transfer to NAP. Therefore any of these
reduced intermediates could serve as the initially prepared
state in the two-photon scheme.
Scheme 4. Photochemical reduction of AQ
3*
O
DH
D•
O
X
(1) hν
X
(2) isc
O
O
AQ3*
AQ
O2
O2
OH
OH
X
X
kd[AQH•]
X=H, NH2, NHAc
OH
O
AQH•
AQH2
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This observation, as well as experiments on unlinked model
Page 18 of 22
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*
*
*
*
O
*
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systems described below, lead us to conclude that the triplet
AQ either abstracts a H atom from the solvent, or relaxes to
ground state. Any direct interaction with the NAP group
O
N
*
*
O
O
N
*
H
*
Cl
*
*
Cl
H
N
O
N
h!, 447 nm
Isopropyl alcohol
*
*
*
+
O
H
O
O
O
PAQ-1
*
PAQ-0
would be negligible. Following the decay triplet, a new, long-
er lived, species with an apparent lmax at 410 nm is seen.
Again, comparison with previously reported spectra allows the
peak at 410 nm to be assigned to the semiquinone.55 The latter
is observed to decay but is too persistent (t1/2 ca. 300 us, See
SI Figures S-15). for accurate kinetic analysis given the limits
of the LFP set up The long lifetime of the AQH• argues
against a direct, exergonic H atom or electron transfer to the
NAP group.
*
*
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Figure 1. H NMR spectra from the photolysis of PAQ-1 in
isopropyl alcohol. A stock solution of PAQ-1 (0.24 mM in
isopropyl alcohol) was prepared and filtered through a 0.45
µm cellulose acetate syringe filter prior to photolysis. To a 1
cm quartz cuvette with a stirbar was added 1.5 mL of PAQ-1
stock. The cuvette was purged with N2 for 10 minutes in the
solution and an additional 5 minutes in the headspace. The
solution was photolyzed at 447 nm for the desired time before
the solvent was removed under reduced pressure and CD3OD
was added.
Samples were photolyzed for 0, 10 and 50 min and then ana-
lyzed by mass spectrometry (ESI+, see SI Figures S4-S6).
PAQ-0 appears after 10 minutes of photolysis, and after 50
minutes, it becomes the only major product. Free phenylacetic
acid was also identified by 1H NMR in the photolysis solution
by the addition of standard phenyl acetic acid after the solvent
was removed and CD3OD was added. By monitoring changes
in their relative peak intensities it was seen that the ester peak
at 7.4 ppm is replaced by a new signal at 7.2 ppm upon irra-
diation corresponds with formation of free phenylacetic acid.
However, free phenyl acetic acid could not be explicitly quan-
tified due to the loss of product during the removal of solvent.
The formation of phenylacetic acid, along with the presence of
PAQ-0 as the only major photoproduct, suggests a mechanism
involving PET to the attached NAP group and release of sub-
strate while NAP group remains covalently linked to the
chromophore.
Figure 2. Transient absorption spectrum of PAQ-1 in 90:10
isopropyl alcohol/methanol.
(b) Mechanism of photochemical deprotection
Laser flash photolysis (LFP) with UV/vis detection was used
to identify and characterize the relevant short-lived intermedi-
ates in the photorelease process. The transient absorption spec-
trum from pulsed photolysis of PAQ-1 (354.7 nm, 10 mJ, 5-7
ns) is shown in Figure 2. Immediately following excitation
there appears a sharp absorption feature at 490 nm which is
assigned to the triplet state localized on the anthraquinone.
This assignment is made on the basis of triplet state spectra for
structurally similar anthraquinone derivatives and oxygen
quenching experiments (See SI Figure S-14).55 Its lifetime of
833 ns in an H-atom donating solvent (isopropyl alcohol) is
largely consistent with similar AQ derivatives measured in
previous work.55 Donor substitution at the 2-position attenu-
ates, but does not prevent H atom transfer. The detection of
the triplet on a ns timescale also suggests that the triplet does
not directly transfer energy or electrons to the NAP group. In a
linked system such as PAQ-1, exergonic electron and energy
transfer process generally occur on a ps or shorter timescale.
Figure 3. Steady state photolysis of PAQ-1 at 447 nm in iso-
propyl alcohol.
To characterize the intermediates present over longer time-
scales, steady state photolysis of PAQ-1 with a 447 nm, 1W
CW diode laser in isopropyl alcohol was carried out and the
long–lived products were detected by UV/vis spectroscopy.
Figure 3 shows that this photolysis generates a new absorption
band at 390 nm, which is confidently attributed to the hydro-
quinone.55,59 The latter, which is formed with a quantum yield
of 0.2 (See SI Figures S-12 and S-13), persists indefinitely (>1
day) and upon bubbling with molecular oxygen, reverts back
to its original spectrum of the starting quinone. However, it
should be noted that the hydroquinones derived from PAQ-1
and PAQ-0 are not readily distinguished by UV/vis absorp-
tion. Thus, from this experiment alone, it is not clear if the
observed AQH2 species is an intermediate on the pathway to
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release, or if it merely accumulates from subsequent photore- Two experiments show that photolysis of AQH2 results in
1
2
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duction of PAQ-0.
NAP reduction and carboxylate release. First, the unlinked
AQH2 was generated independently (i.e. in a non-
O
photochemical manner) and its photolysis in the presence of
NAP derivative 2 results in carboxylate release. As illustrated
in Figure 7, a solution of 1 and 2 in CD3OD was treated with
the reducing agent, sodium dithionite, in D2O under an inert
atmosphere. As expected, UV/vis spectroscopy showed that
this results in the formation of the corresponding AQH2 spe-
cies. When photolysis of the latter is carried out in the pres-
ence of NAP ester 2, clean release of the corresponding acid is
seen by 1H NMR. Control experiments showed that the release
requires the presence of reducing agent (to form AQH2) and
light. Omitting either leaves ester 2 unchanged.
O
H
N
O
O
N
ClO4
O
1
2
Figure 4. Model compounds used in the un-linked experiment
performed in this study. For synthesis and characterization,
please see SI.
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Unlinked model compounds 1, 2-acetylamino anthraquinone
and 2 (a NAP ester) were prepared and used to test the possi-
ble mechanisms. Figure 5 shows the transient absorption spec-
trum following excitation of 1 in isopropyl alcohol. As with
the linked PAQ system, a short-lived feature at 490 nm due to
the triplet decays to leave a long-lived signal at 410 nm, at-
tributed to the semiquinone. In this case the ratio of triplet to
semiquinone formation is somewhat larger than it is for PAQ-
1. The reasons for this were not examined in detail. In any
case, the semiquinone from 1, does not react at a significant
rate with NAP derivative 2. Quenching experiments where the
semiquinone signal was monitored at 410 nm as the concentra-
tion of 2 was increased up to its solubility limit in isopropyl
alcohol, (Figure 6) demonstrate the that lifetime of the sem-
iquinone is insensitive to the presence of 2. On this basis, we
exclude ground state electron transfer from the singly-reduced
aminoanthraquinone to the NAP group as a significant path-
way for release of carboxylic acid in the PAQ-1 photolysis.
*
O
O
H
N
*
O
+
*
HO
O
+
N
N
*
O
O
ClO4
1
2
Withsodium dithionite, 447 nm 60 min
*
*
Withsodium dithionite, dark
*
*
*
*
447 nm, 60 min
Dark
*
*
1
Figure 7. H NMR spectra of the control experiments per-
formed with compounds 1 and 2. Blue: A solution of 1 and 2
were kept in the dark for 60 minutes in a solution of 2:1
CD3OD/D2O. Red: A solution of 1 and 2 were photolyzed in a
solution of 2:1 CD3OD/D2O at 447 nm for 60 minutes. Green:
A 1 mL stock solution of 1 in CD3OD was purged with nitro-
gen for 5 minutes. After purging, 0.5 mL of 39.4 mM sodium
dithionite was added via cannula. The solution was then kept
in the dark for 60 minutes. Pink: A 1 mL stock solution of 1 in
CD3OD was purged with nitrogen for 5 minutes. After purg-
ing, 0.5 mL of 39.4 mM sodium dithionite was added via can-
nula. The solution was then photolyzed at 447 nm for 60
minutes.
It is also possible to prepare the linked hydroquinone in a
similar way. In this second experiment, PAQ-1 was reduced
using sodium dithionite in a mixture of D2O/CD3OD (Figure
8). UV/vis spectroscopy showed that, under an inert atmos-
phere, PAQ-1 was quantitatively converted into the doubly-
reduced hydroquinone (PAQ-1H2 Scheme 6). Two identical
samples were prepared, the first of which was photolyzed at
447 nm. After 60 min, clean conversion to PAQ-0 resulting
from release of the carboxylic acid was observed. The dark
control, however, showed no reaction. (see SI Figure S-10).
These observations further establish that the PAQ-1H2 is ca-
pable of acting as an excited state donor to NAP. Significantly,
the PAQ-1H2 does not trigger release of the carboxylate at a
significant rate in the absence of light.
Figure 5. Transient absorption spectrum of 1 in 90:10 isopro-
pyl alcohol/methanol.
Figure 6. Waveforms of 1 at 410 nm with increasing concen-
trations of 2.
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Scheme 6. Photolysis of Thermally Generated PAQ-1H2.
toward NAP in the along with the demonstrated PET reaction
between PAQ-1H2 and NAP.
1
2
3
4
5
6
7
8
O
O
O
O
Scheme 7. Proposed Photorelease Mechanism.
O
O
N
O
O
H
N
Ph
Ph
OH
N
Cl
H
N
Cl
O
O
3*
Sodium Dithionite
D2O/CD3OD
O
O
O
N
O
N
H
N
H
N
Cl
Cl
hν1, 447 nm
1) 447 nm 60 min
OH
O
O
2) O
OH
OH
H
H2O2
2
O
O
1) Dark
2
PAQ-1
PAQ-13*
2) O
9
λmax = 490 nm
O
O2
O
O
N
10
11
12
13
14
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17
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25
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27
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30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
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56
57
58
59
60
H
N
Cl
O
O
O
Ph
+
Ph
O
O
O
O
O
N
H
N
Cl
hν2, 447 nm
PET
OH
N
OH
O
N
H
N
Cl
Cl
H
N
-H+
Disproportionation
O
O
O
O
PAQH•-1
PAQ-1
OH
OH
PAQH2-1
PAQH-1
λ
max = 390 nm
λ
max = 410 nm
O
Ph
O
Rearrange
Re-Oxidation
O
O
N
H
N
Cl
OH
O
N
O
H
N
+
O
H
O
O
PAQ-0
CONCLUSION
The experiments described herein demonstrate a linked chro-
mophore-protecting group system, PAQ-1, which can release
a model carboxylate upon sequential absorption of two visible
light photons in the presence of a hydrogen atom donor. The
mechanism involves photochemically generating hydroqui-
none in solution and eventual photolysis of the hydroquinone
triggering PET to the NAP group and release of free carbox-
ylate. Further work is currently in progress to increase the
substrate scope to include amines, and other biologically rele-
vant molecules such as amino acids and phosphates. Further-
more, it would be ideal if the absorption spectrum of ground-
state and intermediate or transient species (in this case the
groundstate anthraquinone and the groundstate hydroquinone)
were shifted relative to one another to allow for selective irra-
diation of the intermediate. To address this issue and improve
upon the current system, additional chromophores are being
investigated. The development of sequential two-photon
PRPGs will allow for continued improvement in the spatial
control over photochemical process.
Figure 8. PAQ-1 was converted to PAQ-1H2 by reduction
with sodium dithionite in CD3OD/D2O under an inert atmos-
phere. To 1.5 mL of a 0.893 mM solution of PAQ-1 in CD3OD
under N2, 0.5 mL of 0.1 M sodium dithionite in D2O was add-
ed via cannula. The UV/vis spectrum shows the chemical con-
version to hydroquinone using sodium dithionite. The dark
control showed no change by UV/vis.
Given these observations, along with the transient spectrosco-
py and photoproduct analysis demonstrated earlier, a mecha-
nism for the stepwise, two-photon release of carboxylate is
described in Scheme 7. The first photon excites the AQ chro-
mophore, forming the corresponding triplet state, PAQ-13*.
The latter abstracts a H atom from the solvent, to form the
PAQ-1H•. A relatively slow disproportionation reaction of
PAQ-1H• provides the hydroquinone PAQ-1H2. The second
photon drives a PET from PAQ-1H2 to the NAP, resulting in
release of the substrate. A relatively slow reaction of PAQ-
1H2 with O2 resets the chromophore back to its original state
(PAQ-1). In the present system, the overlap of PAQ-1H2 ab-
sorption with that of PAQ-1, along with a fast intramolecular
photoreduction of the NAP group, make accumulation of the
intermediate complex infeasible. Moreover, given the stability
of PAQ-1H2 along with its slow rate of formation via dispro-
portionation, it is not expected that the release process will
exhibit the sort of non-linear intensity dependence that is char-
acteristic of reaction driven by non-resonant two-photon abso-
prtion. However, the requirement for sequential absorption of
two photons is demonstrated through the lack of ground-state
reactivity of the key intermediates (PAQ-1H•, PAQ-1H2)
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website.
AUTHOR INFORMATION
Corresponding Author
* falvey@umd.edu
Author Contributions
All authors have given approval to the final version of the manu-
script.
Funding Sources
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The Journal of Physical Chemistry
565.
This work was supported by the National Science Foundation
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1
2
3
4
5
6
7
8
(CHE-1112018)
ABBREVIATIONS
CW, continuous wave; PRPG, photoreleasable protecting group;
PET, photoinduced electron transfer; NAP, N-alkyl picolinium;
NMR, nuclear magnetic resonance; ESI, electrospray ionization;
LFP, laser flash photolysis.
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Storage Using Photochromic Materials. Chem. Rev. 2000, 100,
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Erskine, L. L.; Heikal, A. A.; Kuebler, S. M.; Rumi, M.; Wu,
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Initiators for Three- Dimensional Optical Data Storage and
Microfabrication. Solid State Phys. 1999, 398 (March), 51–54.
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Raymo, F. M. Bimolecular Photoactivation of NBD
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Bort, G.; Gallavardin, T.; Ogden, D.; Dalko, P. I. From One-
Photon to Two-Photon Probes: “Caged” Compounds, Actuators,
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