Gated photochromism in azobenzene-appended rhodamine cassette: Through-bond energy transfer-a universal strategy towards "lock and Unlock" system

Article abstract of DOI:10.1039/c8tc04170a

A dyad 1 comprising fluorescent rhodamine and photochromic azobenzene covalently linked through a phenylene bridge, which still retains their individual electronic properties, was synthesized. Gating between the fluorescent and photochromic properties of the dyad in the presence of external stimuli such as light or cations/anions or both paved the way for the development of new stimuli-responsive molecular materials. Among the several metal ions, Fe³⁺ ion was found to be selective in inducing the spirocyclic ring opening reaction of rhodamine, which can be witnessed through turn-on fluorescence. Fe³⁺-bound dyad 1 precluded the traditional trans → cis isomerization of remotely connected azobenzene and catalysed the spirocyclic ring opening reaction, which led to fluorescence enhancement by a factor of 640. The fluorescence enhancement originated from through-bond energy transfer upon excitation of azobenzene. Indeed, the turn-on fluorescence and optical changes upon UV irradiation indicated that this material can be used as a probe to monitor harmful UV radiation. The distinctly different responses of 1 concerning physical stimuli (light) and chemical stimuli (Fe³⁺ and F^- ions) make this molecule a multi-addressable functional material.

Full text of DOI:10.1039/c8tc04170a

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Fine-tuning of gold nanorod dimensions and plasmonic properties using
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Gated Photochromism in Azobenzene Appended Rhodamine
Cassette: Through Bond Energy Transfer - A Universal Strategy
Towards “Lock and Unlock” system
Received 00th January 20xx,
Accepted 00th January 20xx
Arunachalam Raman,[a,b] George Augustine,[c] Niraikulam Ayyadurai*[c] and Shanmugam
Easwaramoorthi*[a,b]
DOI: 10.1039/x0xx00000x
www.rsc.org/
The dyad, 1 comprises of fluorescent, rhodamine and photochromic, azobenzene covalently linked through phenylene
bridge which still retains their individual electronic properties was synthesized. Gating between the fluorescent and
photochromic properties of the dyad in the presence of external stimuli such as light, cations/anions or both would pave
the way for the development of new stimuli-responsive molecular materials. Among the several metal ions, Fe3+ ion was
found to be selective in inducing the spirocyclic ring opening reaction of rhodamine, which can be witnessed through turn-
on fluorescence. Fe3+ bound dyad 1 precludes the traditional trans→cis isomerization of remotely connected azobenzene
and catalyses the spirocyclic ring opening reaction that leads to fluorescence enhancement by the factor of 640. The
fluorescence enhancement originated from the through bond energy transfer upon excitation of azobenzene. Indeed, the
turn-on fluorescence and optical changes upon UV irradiation underscore that this can be used as a probe to monitor the
harmful UV radiation. The distinctly different response of 1 concerning physical stimuli (light) and chemical stimuli (Fe3+
and F− ions) make this molecule as multi-addressable functional material.
of azobenzene, a well-studied photochromic molecule is
comparatively scarce. Gated photochromism in azobenzene
can be achieved by host-guest complex formation, wherein the
Introduction
Light-induced reversible chemical reactions of photochromic
molecules have been explored for several applications owing
to their distinctly different structural and thereby functional
changes upon isomerization. The influence of other stimuli
such as pH, metal ions, and supramolecular assemblies on
photoisomerization offers gating those processes and thereby
expands their application base in non-destructive readout
ability in high-density storage, multiple addressing and optical
memory systems.1-4 Majority of the gated photochromism in
the literature is based on diarylethene wherein the ring
opening and closing action have been controlled by the
external stimuli like acid/base,5 chemicals,6-8 oxidation or
reduction,9 and temperature.10 Nevertheless, the unique
structure of diarylethene usually requires time-consuming,
expensive, multiple-step synthesis, consequently leads to high-
cost.11-12 On the other hand, gated, reversible photochromism
strong binding interaction between azobenzene and divalent
pseudo[2]rotaxanes inhibits the isomerization.13 Another
approach involved in controlling the population of π−π
*
excited state responsible for photochromism through Hg2+
binding with tetraamide substituted azobenzene.14
Importantly, most of the studies reported to the best of our
knowledge deals only with the influence of external stimuli on
the isomerization behavior. Azobenzene derivatives are known
to be non-fluorescent or weakly fluorescent because of the
prevailing isomerization reactions.15 Thus, gating the
photoisomerization with the concomitant enhancement in
fluorescence and the interplay between these two processes
expands their application base as smart functional materials.16
Gating between the competitive photochemical and
photophysical excited state deactivation pathways of
azobenzene have not been reported so far. However, recently
our group reported the reversible gated photochromism of
azobenzene by the interplay between fluorescence and
photochromism of azobenzene-rhodamine dyad through
Förster Resonance Energy Transfer (FRET) mechanism.17
Unfortunately, FRET-based cassettes need a spectral overlap of
donor emission and acceptor absorption which makes their
utility limited.18 Recently, focus on through bond energy
transfer (TBET) process is immense as it does not require the
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spectral overlap between the donor fluorescence and acceptor in moderate yields. All the intermediates and target compound
absorption bands as in the case of FRET process.19-20 In TBET were well characterized by ESI-MS and 1H and 13C NMR
1
systems, the donor and acceptor are connected through the spectroscopy (ESI, Figure S22-32). A careful analysis of the H
conjugated spacer, however, mutual orthogonal orientation NMR spectra of compound
1 reveals that the chemical shift
should be maintained to avoid extended π-conjugation values of aromatic protons in azobenzene and rhodamine
between them.21 In this work, we have synthesized moiety remains unaltered with respect to their parent
azobenzene coupled to the rhodamine moiety covalently derivatives
6 and 9 and suggests the negligible interaction
through acetamide linkage. Gating the photochromism of between those two moieties.
azobenzene can be achieved by the metal ion binding to the In line with the NMR spectra, the UV-visible absorption spectra
remotely linked rhodamine moiety.22 The binding event turns- of cassette
1
in THF solvent shown in ESI, Figure S1 is the
on the fluorescence, characteristic to rhodamine and the simple summation of the absorption spectra of individual
process operates through the through bond energy transfer chromophores
6
and
of the intervening sp3 hybridized carbon of the spirocyclic ring
that restricts the -electron delocalization between both the
chromophores. While the spirocyclic form of rhodamine is
non-fluorescent nature, weak fluorescence with the
maximum at 465 nm observed for compound is attributed to
9. This feature is ascribed to the presence
mechanism (TBET).23,24
π
a
1
the azobenzene moiety. Azobenzenes are known to be non-
fluorescent nature because of the dominant photochemical
excited state deactivation pathway, the weak emission
observed in this case is due to the presence of N,N-
dimethylamino moiety.
Figure 1. a) The principle and b) TBET based gated photochromism of azobenzene in 1
by “lock/unlock” mechanism in presence of Fe3+/F- in THF. TBAF-tetrabutyl ammonium
fluoride
The schematic representation depicted in Figure 1a explains
the hypothesis employed in this study. Stimuli-responsive
fluorophore capable of exhibiting the downhill energy transfer
from the photoexcited azobenzene before it undergoes
Chart 1: Structure and synthesis route of target compound 1 and its intermediates 6
and 9.
Upon irradiation of cassette 1 at 365 nm using 4 Watt Hg lamp,
trans-azobenzene was converted to cis-isomer as understood
from decrease in π-π* absorption at 425 nm with
concomitantly increased n–π* absorption at 537 nm (Figure
photoisomerization,
1
was synthesised. Coordination between
gates the photoisomerization of
Fe3+ and rhodamine unit of
1
trans-azobenzene (donor) at the expense of TBET process to
rhodamine (acceptor), which occurs via phenyl unit as shown
in Figure 1. Addition of F− ion decomplexes the Fe3+ from
S2). The cis isomer can be switched back to trans-
with 100% efficiency by storing the solution under dark
condition. This reaction produces the cis-rich photostationary
state (PSStrans/cis
of 32:68.25 Notably, rhodamine moiety
1 thermally
∼
rhodamine in
azobenzene. The synthetic route of cassette
intermediates are shown in Chart 1. The precursor , (E)-4'-((4-
1
and resume the photochromic action of
)
1
and
remains intact during the irradiation process which indicates
that both azobenzene and rhodamine are independent of each
other even in the presence of light and thus the compound
behaves like a cassette.
6
(dimethylamino) phenyl)diazenyl)-[1,1'-biphenyl]-4-amine was
synthesized by diazotization of N,N’-dimethylaniline and 4-
bromoaniline followed by Suzuki coupling with
between with rhodamine acid chloride, gave the target
compound . Acetylation of affords the model compound
3. The reaction
An attempt was made to open the spirolactam ring of
rhodamine using Fe3+, Ag+, Al3+, Cu2+, Fe2+, Li+, Hg2+, Mg2+, Mn2+
6
8
1
6
7
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Pb2+, Ni2+, and Zn2+ ions as a stimulus. Among them, Fe3+ is
found to be selective and sensitive in transforming non-
fluorescent, spirolactam to fluorescent, ring-opened amide
form of rhodamine. This process has been evident from the
appearance of characteristic absorption of rhodamine at 556
nm, and the solution turned to pink, which can even be seen
by the naked eye. 1
-Fe3+ complex (Fe3+ concentration in ten
molar equivalents to ensure the sufficient complex formation)
in THF when subjected to photoirradiation at 360 nm, no
signatures corresponding to azobenzene isomerization
reaction has been noticed. Instead, absorbance at 556 nm
corresponding to rhodamine gradually increases with the
irradiation time as shown in Figure 2a and became saturated
after 7 hours. The appearance of an isosbestic point during
irradiation suggests the presence of only two species, i.e.
spirocyclic and ring opened amide form of rhodamine. A plot
of absorbance at 425 and 556 nm respectively corresponds to
azobenzene and rhodamine against time during the irradiation
of
-Fe3+ observed at 465, and 583 nm becomes intensified
1
+Fe3+ is shown in Figure 2b. Similarly, the fluorescence of
1
Figure 2. a) UV-visible absorption spectra and c) fluorescence spectra of 1 (10 μM) with
10 equivalents of Fe3+ (0.02 M) in THF measured at different time intervals during
photoirradiation at 365 nm b) Plot of irradiation time versus absorbance changes at
425 nm and 556 nm d) Plot of fluorescence intensity of 1 toward various metal ions
with and without irradiation.
gradually with irradiation as shown in Figure 2c. Indeed, the
course of the reaction can be monitored visually by the
intensification of pink colour and the yellow fluorescence
under the UV light. The absence of azobenzene isomerization
along with the fluorescence enhancement of rhodamine in 1-
The cumulative effect of the cassette was further ascertained
by doing the similar experiments under identical conditions
Fe3+ complex upon irradiation is quite intriguing and suggests
the stronger interaction between the two chromophores.
Intensification of both the absorption and emission band
reveals the gradually increased concentration of the
rhodamine-amide form upon photoirradiation. This feature is
because the azobenezene upon photoexcitation transfers the
energy to rhodamine which subsequently undergoes
spirolactam ring opening reaction to form a fluorescent
using the model compounds
azobenzene donor , has been quenched that of free acceptor,
has been enhanced by a factor of 95 upon addition of Fe3+
6 and 9. While the emission from
6
9
ions followed by irradiation (ESI, Figure S5-6). These features
suggest that the combined effect of both the chromophores by
TBET is more when compared to the individual chromophores.
The large enhancement in turn-on fluorescence of rhodamine
upon exposure to UV light makes this kind of molecules to be
rhodamine-amide
measurement of
form.
Indeed,
the
fluorescence
1
-Fe3+ complex was carried out by exciting
used as
a reversible sensor for detecting harmful UV
radiations. Interestingly, the addition of Fe3+ to the cis rich PSS
the azobenzene moiety and the observed emission
corresponding to rhodamine moiety indicates the occurrence
of excitation energy transfer processes. The fluorescence of
azobenzene and rhodamine components were found to
enhanced respectively by a factor of 6 and 640 as shown in ESI,
Figure S3. Since, there exists a large spectral gap between the
emission of azobenzene with the absorption of rhodamine
state immediately regenerates the absorption spectra of trans-
-Fe3+ complex (as given in ESI, Figure S7) which indicates that
1
the coordination of Fe3+ with
1 is favorable only in the trans
state.
To get more insights on light-induced structural changes,
proton NMR spectra of was measured before and after
addition of Fe3+ in THF-d8 and is given in ESI, Figure S8-9
1
even after the formation of 1
-Fe3+ complex as can be seen
.
from Figure S4 (ESI), energy transfer observed in this case
Though we are able to observe the spectral changes
corresponding to trans→cis isomerization in from UV-visible
spectra measured during irradiation, unfortunately no
characteristic NMR signals for the cis isomer during irradiation
was observed. It may be due to the fact that the cis isomers
formed upon photoirradiation is probably in smaller
concentration, which subsequently undergoes the reverse
reaction during the measurement time as shown in ESI, Figure
could be attributed to the TBET mechanism.
S8
.
However, after addition of Fe3+ ions, followed by
irradiation in
1, rhodamine protons observed at 7.93 (Ha), 7.33
(Hi,h), 7.0 (Hd), 6.6 (Hj,g) and 6.4 (Hb,c,e,f) ppm becomes
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broadened, which has been gradually downfield shifted during
photoirradiation as shown in ESI, Figure S9. NMR studies
indicate that the initial interaction between
1
and Fe3+ seems
to be a simple electrostatic interaction that would probably
reduce the electron density around it and causes downfield
shift. Light-induced spirolactam ring opening reaction further
reduces the electron density around it and subsequently
causes more downfield shifts. Remarkably, during this process,
azobenzene phenyl protons remains intact indicating that
photoisomerization has been arrested, which is in line with the
UV-visible spectral studies. Mass spectral studies also confirm
the formation of the
peaks were observed at m/z value of 741.1, 825.34, 925.28
and 1042.16 corresponds to
C44H39N6O2+Fe3+,
C43H36N6O2+Fe3++ClO4, C43H36N6O2+Fe3++2ClO4
and
C43H36N6O2+Fe3++3ClO4+H2O respectively as shown in ESI,
Figure S10-11
The gated photochromism of
1
+Fe3+ complex where the molecular ion
.
1
shows excellent selectivity
towards Fe3+ ions over various metal ion species such as Ag+,
Al3+, Cu2+, Fe2+, Li+, Hg2+, Mg2+, Mn2+ Pb2+, Ni2+ and Zn2+ before
and after irradiation except for Al3+ which shows slight
enhancement in absorption and emission from rhodamine
unit. In fact, the enhanced fluorescence intensity is nearly two
times higher for Fe3+ than the Al3+ ions as shown in Figure 2d
(ESI, Figure S12-15). The stoichiometry of 1
+Fe3+ complex is
Figure 3. Molecular logic gate [(I1 OR I2) AND (NOT I3)] based on
1 (10 μM
) using an absorption at 420 nm as output in THF).
calculated from the Job’s method to be 1:1 and the binding
constant value calculated from the Benesi-Hildebrand
equation corresponds to 4.2X105 M-1 (ESI, Figure S16-19).
Reversible nature of these processes has been tested by
adding tetrabutylammonium fluoride (TBAF) before and after
A logic gate could be constructed with the three inputs viz.
Fe3+ ions, F− ions, light and the absorption at 425 nm would be
the visible logic output for gating between the photochromism
and fluorescence. Schematic diagram and truth table for
irradiation, where
the free receptor
1
+Fe3+ complex dissociates and regenerates
compound
1 obtained based on the above parameters are
1
as revealed from the reappearance of
summarized in Figure 3. The designed logic gates consist of a
absorption and fluorescence at 425 nm and 465 respectively,
and disappearance of absorption and fluorescence at 556 nm
and 583 nm respectively. Further, the regenerated receptor
was found to retain its original photochromic and metal ion
binding properties. The results indicates that the amount of F−
NOR, a NOT and an OR input function and these inputs are
performed by
1
in THF solution. Monitoring the absorbance
when it was subjected to all the three
value at 425 nm of
1
possible input operations proved the expected output.
ion required to dissociate the
1
+Fe3+ complex after the
Taking into account of On/Off photochromic reactivity of
1 in
irradiation is 20 times higher than the one required before
irradiation of the complex. This is due to the changes in the
chemical equilibrium before and after irradiation. After
the presence of physical stimuli, light and chemical stimuli,
Fe3+, it might serve as a promising fluorescent candidate for
biological imaging. For that cells (HeLa) was incubated with
at different concentrations and subjected to MTT assay. A cell
viability assay supports a non-cytotoxic effect of
with Fe3+ up
to 40 µM with longer incubation time (12 hrs.) (ESI, Figure
S21). Though gradually increasing concentrations of
with Fe3+
1
addition of Fe3+ ion, only a few molecules of
1 might get
complexed with Fe3+ and therefore a few molecules of fluoride
1
ions (5 μl, 0.02 M) are sufficient to dissociate the complex. In
case of after irradiation, the concentration of 1
+Fe3+ complex
1
formed is relatively higher, thus it require more number
fluoride ion (100 μl, 0.02 M) to decomplex it and the results
are toxic to live cells; interestingly, it supports that around 45
to 55% of cell retain their viability even at higher
concentrations (60 and 90 µM). These observations indicates
are shown in ESI, Figure S20
.
that
1
with Fe3+ could be used as a bioimaging probe even at
higher concentration (60 to 90 µM).
Further, the intracellular uptake and distribution of 1, and 1-
Fe3+ was examined by fluorescent microscopy as shown in
Figure 4. In contrast, cell treated with
1
or Fe3+ alone is non-
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4. N. Tamaoki, S. Song, M. Moriyama and H. Matsuda, Adv.
Mater, 1999, 12, 94-97.
5. J. Massaad, J. C. Micheau, C. Coudret, R. Sanchez, G. Guirado
and S. Delbaere, Chem Eur. J, 2012, 18, 6568 - 6575.
6. C.-T. Poon, W. H. Lam and V. W.-W. Yam, J. Am. Chem. Soc,
2011, 133, 19622-19625.
fluorescent during microscopy analysis. Also, with an increased
incubation time, bright red emission was observed throughout
the cells, suggesting that a considerable amount of
exchanged into the cytoplasm and nucleus of HeLa cells. This
result reveals that the compound can be efficiently
1 was
1
7. Y. Wu, S. Chen, Y. Yang, Q. Zhang, Y. Xie, H. Tian and W. Zhu,
Chem. Comm, 2012, 48, 528-530.
8. J. Zhang, W. Tan, X. Meng and H. Tian, J. Mater. Chem, 2009,
19, 5726-5729.
transported into the cells and could be a suitable probe for
bio-imaging and biological applications.
9. X. Li, Y. Ma, B. Wang and G. Li, Org. Lett, 2008, 10, 3639-
3642.
10. D. Dulić, T. Kudernac, A. Pužys, B. L. Feringa and B. J.
vanꢀWees, Adv. Mater, 2007, 19, 2898-2902.
11. H.-B. Cheng, H.-Y. Zhang and Y. Liu, J. Am. Chem. Soc, 2013,
135, 10190-10193.
12. S. Mahvidi, S. Takeuchi, S. Kusumoto, H. Sato, T. Nakagawa
and Y. Yokoyama, Org. Lett, 2016, 18, 5042-5045.
13. M. Lohse, K. Nowosinski, N. L. Traulsen, A. J. Achazi, L. K. S.
von Krbek, B. Paulus, C. A. Schalley and S. Hecht, Chem.
Comm, 2015, 51, 9777-9780.
Figure 4. Fluorescence microscope images of HeLa cells before and after incubation
with 1, Fe3+ and 1+Fe3+ in 90 μM, respectively, at 37 °C following excitation at 530 nm.
14. Y. Ouyang, Z. Yuan and J. Wang, New J. Chem, 2018, 42
5660-5663.
,
Conclusions
15. H. M. D. Bandara and S. C. Burdette, Chem. Soc. Rev, 2012,
41, 1809-1825.
16. W. Feng, T. Wee Beng, Z. Yong, F. Xianping and W. Minquan,
Nanotech, 2006, 17, R1.
17. A. Raman, G. Augustine, N. Ayyadurai and S.
Easwaramoorthi, New J. Chem, 2018, 42, 9300-9305.
18. G.-S. Jiao, L. H. Thoresen and K. Burgess, J. Am. Chem. Soc,
2003, 125, 14668-14669.
19. A. B. Othman, J. W. Lee, J.-S. Wu, J. S. Kim, R. Abidi, P.
Thuéry, J. M. Strub, A. Van Dorsselaer and J. Vicens, J. Org.
Chem, 2007, 72, 7634-7640.
20. M. H. Lee, H. J. Kim, S. Yoon, N. Park and J. S. Kim, Org. Lett,
2008, 10, 213-216.
21. M. Kumar, N. Kumar, V. Bhalla, H. Singh, P. R. Sharma and T.
Kaur, Org. Lett, 2011, 13, 1422-1425.
22. K. Li, Y. Xiang, X. Wang, J. Li, R. Hu, A. Tong and B. Z. Tang, J.
Am. Chem. Soc, 2014, 136, 1643-1649.
23. G.-S. Jiao, L. H. Thoresen, T. G. Kim, W. C. Haaland, F. Gao, M.
R. Topp, R. M. Hochstrasser, M. L. Metzker and K. Burgess,
Chem. Eur. J, 2006, 12, 7816-7826.
In summary, we have developed azobenzene-rhodamine
cassette
1 connected covalently through the phenyl ring and
the latter moiety was susceptible to show turn-on
fluorescence in presence of Fe3+ in solution as well as
biological samples. While the azobenzene and rhodamine
moiety of
1
behave independently in the free form, Fe3+
cis
binding with the rhodamine moiety arrests the trans
→
isomerization of azobenzene upon photoexcitation at 365 nm
by transferring the excitation energy to rhodamine moiety
which later undergoes light catalysed spirocyclic ring opening
reaction. The energy transfer occurs through TBET mechanism
and is found to be reversible. An integrated molecular logic
gate was constructed using the light, Fe3+ and F− ions as
variables. The present work demonstrates the possibility of
using the cassette as a multi-addressable, photocontrollable,
turn-on fluorescent sensor for detecting Fe3+ ions and harmful
UV radiation.
24. R. Bandichhor, A. D. Petrescu, A. Vespa, A. B. Kier, F.
Schroeder and K. Burgess, J. Am. Chem. Soc, 2006, 128,
10688-10689.
25. A. Airinei, N. Fifere, M. Homocianu, C. Gaina, V. Gaina, B. C.
Simionescu, Inter. J. Mol. Sci, 2011, 12, 6176-6193.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors gratefully acknowledge the Council of Scientific
and Industrial Research (CSIR), New Delhi for funding this
research through “STRAIT” project. NA thanks Indo-Belarus
joint collaboration (Project reference number INT/BLR/P-
12/14) funded by Department of Science and Technology, New
Delhi. RA thanks DST for INSPIRE fellowship.
Notes and references
1. B. L. Feringa, Molecular switches; Wiley-VCH: Weinheim,
Germany, 2001.
M. Irie, Chem. Rev, 2000, 100, 1685-1716.
2
3. H. Tian and S. Yang, Chem. Soc. Rev, 2004, 33, 85-97.
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