Bimolecular photoactvarion of NBD fluorescence

Article abstract of DOI:10.1039/c4nj01983k

The concatenation of a photochemical transformation with a chemical reaction allows the activation of nitrobenzoxadiazole (NBD) fluorescence under optical control. Specifically, the coupling of photoinduced deprotection with nucleophilic substitution converts a nonemissive NBD chromophore into a fluorescent product. These operating principles can evolve into a general mechanism to implement fluorescent switches based on the attractive photophysical properties of NBDs.

Full text of DOI:10.1039/c4nj01983k

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Bimolecular photoactivation of NBD fluorescence†
Cite this: DOI: 10.1039/#
Sherif Shaban Ragab,ab Subramani Swaminathan,a Jaume Garcia‐Amorós,a Burjor
Captaina and Françisco M. Raymo*a
Received #,
Accepted #
DOI: 10.1039/#
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The concatenation of a photochemical transformation with a offer the opportunity to probe dynamic processes and structural
chemical reaction allows the activation of nitrobenzoxadiazole features that would, otherwise, be inaccessible with the sole aid of
(NBD) fluorescence under optical control. Specifically, the their conventional counterparts.
coupling of a photoinduced deprotection with a nucleophilic
The potential applications of photoactivatable fluorophores are
substitution converts a nonemissive NBD chromophore into a stimulating fundamental studies aimed at the identification of viable
fluorescent product. These operating principles can evolve into a structural designs to switch the emission of members of the main
general mechanism to implement fluorescent switches based on families of organic dyes under optical control.
the attractive photophysical properties of NBDs. photoactivatable borondipyrromethenes, coumarins, dihydrofurans,
fluoresceins and rhodamines have already been reported in the
Conventional fluorophores emit electromagnetic radiations upon literature.2–7
Instead, only one NBD with photoactivatable
Several
illumination at an appropriate excitation wavelength (Ex).1 The fluorescence has been developed so far,10 in spite of the attractive
absorption of incoming photons at Ex excites them from the ground combination of structural and photophysical properties that this
to one of the accessible excited electronic states. The subsequent particular class of organic fluorophores has to offer. Indeed, the
relaxation and radiative deactivation of the excited species back to synthetic accessibility of NBD derivatives, the possibility to excite
their ground state results in fluorescence.
fluorophores, instead, emit only after irradiation at a given activation are encouraging their use in a diversity of analytical applications.11–14
wavelength (Ac) first and then illumination at Ex.2–7 The absorption
Literature data demonstrate that the nature of the substituents on
Photoactivatable them with visible photons and their high fluorescence quantum yields
of incoming photons at Ac initiates a photochemical transformation the NBD platform has a dramatic influence on the fluorescence
that either enables subsequent absorption at Ex or prevents quenching quantum yield of this particular chromophore.15 Derivatives with a
of the excited state responsible for emission. Both mechanisms ensure pair of electron withdrawing groups in positions 4 and 7 are generally
significant fluorescence only after activation and excitation events. not emissive. The transformation of one of the two substituents into
Under these conditions, the interplay of two light sources operating at an electron donating group, however, enables the radiative
Ac and Ex can be exploited to switch fluorescence on within a deactivation of the excited state. In fact, this general mechanism can
defined region of space at a given interval of time. Such be exploited to transduce the presence of a given reactant into a
spatiotemporal control permits the monitoring of the translocation of fluorescence signal and implement valuable sensing schemes.16–24
activated fluorophores across a sample of interest in real time with the For example, compound 1 has electron withdrawing nitro and
sequential acquisition of fluorescence images.8 Similarly, it allows sulfonamide substituents on its NBD skeleton and is not fluorescent.22
the sequential localization of closely-spaced fluorophores activated at Reaction with an appropriate nucleophile converts the sulfonamide
different intervals of time and the reconstruction of images with substituent into an electron-donating amine group to form 2 and
subdiffraction resolution.9 As a result, photoactivatable fluorophores switch fluorescence on. These observations suggest that the
photoinduced generation of a nucleophile can be exploited to convert
a Laboratory for Molecular Photonics, Department of Chemistry, University of Miami, 1301
1 into 2 and activate emission under optical control. The resulting
concatenation of photochemical and chemical steps is conceptually
analogous to schemes developed in our and other laboratories to
activate the fluorescence of coumarin25 and rhodamine26 derivatives
with photoacid generators. In turn, such bimolecular strategies for
fluorescence photoactivation permit the monitoring of the diffusion of
Memorial Drive, Coral Gables, Florida, 33146‐0431, USA
b
Department of Photochemistry, Chemical Industries Research Division, National
Research Centre (NRC), El‐Tahrir Street, Dokki, Giza, 12622, Egypt
† Electronic Supplementary Information (ESI) available: Experimental procedures
and X‐Ray Diffraction Data for 6. See DOI: 10.1039/#
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Fig. 2 Synthesis of 6 and ORTEP representation of its geometry in single
crystals (50% thermal ellipsoid probability).
convert 6 into 4. Consistently, new absorption bands develop at ca.
240 and 280 nm (b–e in Fig. 3) during the photolytic transformation.
These absorptions resemble those observed in the spectrum (f in Fig.
3) of 4 and confirm the photoinduced formation of this species.
The addition of 6 to a solution of 1 does not affect the absorption
band (a in Fig. 4) of the NBD chromophore and does not produce any
fluorescence (b in Fig. 4). Furthermore, the absorption and emission
spectra of the mixture remain unchanged even after storage for hours
in the dark. These observations indicate that 6 cannot react with 1 to
produce 2 and activate fluorescence. Upon illumination of the
mixture at a Ac of 350 nm, however, the 2-nitrobenzyl group of 6
cleaves to generate 4. The corresponding thiolate then disconnects the
sulfonamide group of 1 to release 2. In fact, the absorption and
emission spectra (c–l in Fig. 4), recorded during the course of the
photolytic transformation, show essentially the same changes
observed after the physical addition of 4 to a solution of 1 (Fig. 1).¶
Specifically, the emission of the product develops into an intense band
(h–l in Fig. 4), demonstrating that the fluorescence of the NBD
Fig. 1 Absorption (
[10 µM, MeCN:PBS (95:5 v/v), 25 °C, λEx = 440 nm] of
and after ( and ) the addition of (2 equiv.) and stirring for 25 min in
the dark. Absorption ( ) and emission ( ) spectra of [10 µM, MeCN:PBS
(95:5 v/v), 25 °C, λEx = 440 nm].
a
and
c
) and emission (
b
and
d
) spectra of a solution
1
before ( and b)
a
c
d
4
e
f
2
the activating reactant in real time25a as well as allow the
reconstruction of images with subdiffraction resolution.25
The absorption spectrum (a in Fig. 1) of 1 shows an intense band
at 479 nm. Illumination at a Ex positioned within this absorption does
not cause any detectable fluorescence (b in Fig. 1), in agreement with
literature data.22 After the addition of 4 under experimental conditions
that ensure the formation of the corresponding thiolate,22 noticeable
changes can be observed in the absorption spectrum (c in Fig. 1).
Furthermore, an intense band appears at 540 nm in the corresponding
emission spectrum (d in Fig. 1). This band is essentially identical to
that observed in the spectrum (f in Fig. 1) of 2. Thus, the treatment of
1 with 4 produces 2 and activates fluorescence, consistently with
literature precedents.22
The transformation of 1 into 2 is a consequence of the nucleophilic
displacement of the sulfonyl group of one species with the
concomitant formation of the other. If the nucleophile is protected
with an appropriate photocleavable group, then the conversion of 1
into 2 can be achieved under photochemical control. Specifically,
treatment of 4 with 5, in the presence of potassium carbonate,
produces 6 in a yield of 94%. The electrospray ionization mass
spectrum of the product together with its 1H and 13C nuclear magnetic
resonance spectra confirm the covalent incorporation of a 2-
nitrobenzyl group. Furthermore, single crystals of 6 could be obtained
from a dichloromethane/hexane solution of the compound, after the
slow evaporation of the solvent. The corresponding structure§ (Fig.
2) clearly reveals the presence of a 2-nitrobenzyl group within the
covalent skeleton of the molecule.
The absorption spectrum (a in Fig. 3) of 6 shows a band at 346 nm
for the 2-nitrobenzyl chromophore. In agreement with extensive
literature data on this particular photocleavable protecting group,Error!
illumination at a Ac of 350 nm cleaves the bond
between the benzylic carbon atom and the adjacent sulphur atom to
Fig. 3 Absorption spectra of a solution [20 µM, MeCN:PBS (95:5, v/v), 25
°C] of
5 ( ), 10 (
MeCN:PBS (95:5, v/v), 25 °C].
6
before (
a
) and after illumination at λAc (350 nm, 2.48 mWcm–2) for
Bookmark not defined.
b
c
), 15 (d) and 20 min (e). Absorption spectrum (f) of 4 [20 µM,
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procedures.10,22 Electrospray ionization mass spectra (ESIMS) were
recorded with a Bruker micrOTO-Q II spectrometer. Nuclear
magnetic resonance (NMR) spectra were recorded with a Bruker
Avance 400 spectrometer. Absorption spectra were recorded with a
Varian Cary 100 Bio spectrometer, using quartz cells with a path
length of 1.0 cm. Emission spectra were recorded with a Varian Cary
Eclipse spectrometer in aerated solutions. Solutions were irradiated
at 350 nm (2.48 mW cm–2) with a Luzchem Research LZC-4V
photoreactor.
4 (124 mg, 1 mmol) was added dropwise to a mixture of 5 (276
mg, 1 mmol) and K2CO3 (179 mg, 1.3 mmol) in dimethylformamide
(DMF, 25 mL) maintained at ambient temperature under Ar. The
mixture was stirred for 10 hours, diluted with H2O (20 mL) and
extracted with EtOAc (40 mL). The organic phase was washed with
brine (3 × 15 mL), dried over MgSO4 and the solvent was distilled off
under reduced pressure. The resulting oil solidified upon standing in
air to give 6 (300 mg, 94%) as a yellow crystalline solid. ESIMS: m/z
= 342.0784 [M + Na]+ (m/z calcd. for C16H17NNaO4S = 342.0776); 1H
NMR (400 MHz, CDCl3): δ = 2.31 (3H, s), 3.73 (3H, s), 3.92 (3H, s),
4.39 (2H, s), 6.50 (1H, s), 7.06 (2H, d, 8 Hz), 7.20 (2H, d, 8 Hz), 7.64
(1H, s) ppm; 13C NMR (400 MHz, CDCl3): δ = 21.0, 21.2, 38.4, 56.1,
56.3, 108.4, 113.2, 128.7, 129.6, 129.8, 131.1, 132.8, 133.4, 137.7,
138.6, 140.2, 147.7, 152.5 ppm.
Notes and references
§ Crystal data for 6: C16H17NO4S, Mr = 319.37, monoclinic, space group P21/c,
a = 15.7805(9) Å, b = 5.0908(3) Å, c = 21.0600(13) Å,  = 110.851(1)°, V =
1468.32(17) Å3, Z = 4, T = 296 K, Mo K = 0.71073 Å. GOF = 1.038, No.
Parameters = 202, 2max = 56°. The final R1(F2) was 0.0380 for 2912
reflections I>2(I). CCDC No. 1033127.
Fig. 4 Absorption (
MeCN:PBS (95:5 v/v), 25 °C, λEx = 440 nm] after the addition of
equiv.). Absorption ( ) and emission ( ) spectra of the same solution
after illumination at λAc (350 nm, 2.48 mW cm–2) for 5 (
and ), 10 ( and
), 15 ( and ), 20 ( and ) and 30 min ( and ).
a
) and emission (
b
) spectra of a solution of
1
[10 µM,
6
(2
c
–
g
h–l
c
h
d
i
e
j
f
k
g
l
¶ Irradiation of 1 in the absence of 6, under otherwise identical conditions,
does not cause any change in the absorption and emission spectra (Fig. S1).
chromophore can, indeed, be activated on the basis of these operating
principles.
1
2
3
4
5
6
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Our results prove that the photoinduced generation of
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nucleophile can initiate a chemical reaction to convert a nonemissive
reactant into a fluorescent product. In particular, the established
photochemistry of the 2-nitrobenzyl group together with the
photophysical properties of the NBD chromophore translate into the
opportunity to activate the fluorescence of the latter with the
photocleavage of the former. The resulting bimolecular mechanism
for fluorescence activation can evolve into a general strategy for the
implementation of photoresponsive ensembles of molecules based on
the coupling of photochemical transformations with chemical
reactions.
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