Photo-driven near-IR fluorescence switch: Synthesis and spectroscopic investigation of squarine-spiropyran dyad

Article abstract of DOI:10.1039/c5ra20252c

With the objective of developing near-infrared fluorescence switch molecules for potential applications, synthesis of new dyad with two methyl 3-(3′,3′-dimethyl-6-nitrospiro[chromene-2,2′-indolin]-1′-yl)propanoate (6), spiropyran (SP), units as the photochromic acceptors and a near-infrared fluorescence probe, (E)-4-((1-(2-hydroxyethyl)-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-2-((E)-(1-(2-hydroxyethyl)-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-en-1-olate (SQ), (3) as fluorescent donor are described. Two SP units are attached to the two indole nitrogen of squarine core with almost no conjugation with the SQ unit. The spectroscopic properties of the newly synthesized dyad, SP-SQ-SP(7), and corresponding model compounds (SP, SQ) have been studied in acetonitrile solution, poly(ethylene glycol) (PEG) polymer matrices and in nanoparticle form dispersed in aqueous medium. These dyes (3 and 7) exhibited absorption in the range 550-670 nm, with significant absorption coefficients (10^-5 M^-1 cm^-1) in the ACN. The fluorescence emission spectra of these dyes cover very broad range from 630 to 750 nm and fluorescence quantum yields are of the order of 0.2 in ACN solution. External stimulations (ultraviolet light and visible light) generate reversible changes in the structure of 7, resulting in changes in the absorption spectra due to the presence of the two spiropyran units. The absorption spectrum of the MC, a ring open form of SP, in dyad SP-SQ-SP(7) has large spectral overlap with the fluorescence spectrum of the SQ unit. Thus, the fluorescence intensity of dyad 7 is modulated by reversible conversion among the two states of the photochromic spiropyran units and the fluorescence resonance energy transfer (FRET) between the MC form and the SQ unit. High-contrast "on/off" fluorescence switching is successfully achieved with remarkably fatigue resistance in solution, in polymer film and in nanoparticle form of dyad (7). The described results indicate that this system may represent an efficient fluorescent switch molecule in potentially rewritable high-density optical data or image storage utilizing near-infrared luminescence intensity readout schemes.

Full text of DOI:10.1039/c5ra20252c

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ARTICLE
Photo-driven Near-IR Fluorescence Switch: Synthesis and
Spectroscopic Investigation of Squarine-Spiropyran Dyad
Received 00th January 20xx,
Accepted 00th January 20xx
§
§
Siva Doddi, Bheerappagari Ramakrishna, Yeduru Venkatesh , and Prakriti Ranjan Bangal*
DOI: 10.1039/x0xx00000x
With the objective of developing near-infrared fluorescence switch molecules for potential applications, synthesis of new
dyad with two methyl 3-(3',3'-dimethyl-6-nitrospiro[chromene-2,2'-indolin]-1'-yl)propanoate (6), spiropyran (SP), units as
the photochromic acceptors and a near-infrared fluorescence probe, (E)- 4-((1-(2-hydroxyethyl)-3,3-dimethyl-3H-indol-1-
www.rsc.org/
ium-2-yl)methylene)-2-((E)-(1-(2-hydroxyethyl)-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-en-1-olate
(SQ),
(3) as fluorescent donor are described. Two SP units are attached to the two indole nitrogen of squarine core with almost
no conjugation with the SQ unit. The spectroscopic properties of the newly synthesized dyad, SP-SQ-SP (7), and
corresponding model compounds (SP, SQ) have been studied in acetonitrile solution, poly(ethylene glycol) (PEG) polymer
matrices and in nanoparticle form dispersed in aqueous medium. These dyes (3 and 7) exhibited absorption in the range
-
5
-1
-1
5
50-670 nm, with significant absorption coefficients (10 M cm ) in the ACN. The fluorescence emission spectra of these
dyes cover very broad range from 630 to 750 nm and fluorescence quantum yields are of the order of 0.2 in ACN solution.
External stimulations (ultraviolet light and visible light) generate reversible changes in the structure of 7, resulting in
changes in the absorption specta due to the presence of the two spiropyran units. The absorption spectrum of the MC, a
ring open form of SP, in dyad SP-SQ-SP (7) has large spectral overlap with the fluorescence spectrum of the SQ unit. Thus,
the fluorescence intensity of dyad 7 is modulated by reversible conversion among the two states of the photochromic
spiropyran units and the fluorescence resonance energy transfer (FRET) between the MC form and the SQ unit. High-
contrast "on/off" fluorescence switching is successfully achieved with remarkably fatigue resistance in solution, in polymer
film and in nanoparticle form of dyad (7). The described results indicate that this system may represent an efficient
fluorescent switch molecule in potentially rewritable high-density optical data or image storage utilizing near-infrared
luminescence intensity readout schemes.
reversible fluorescence modulation or switching of
fluorescence spectroscopic properties of the photo-responsive
molecules are of much interest not only due to their wide
Introduction
The quest to develop smart material with properties that
can be optically modulated has attracted immense attention in
the field of material science because of high demand of such
material in many scientific areas, including biotechnology and
applications in molecule-based logic systems, such as AND, OR,
6
XOR, NAND, NOR, but also for fluorescence spectroscopy as it
plays an pivotal role in different molecular device design which
7
8
includes fluorescence switch, fluorescence sensor and other
1
advanced optics. Photo-responsive molecules, in particular
9
photonic devices, primarily because of its ease of use. For
photochromic molecules can be exploit to accomplish this
objectives, as they can be transformed reversibly by light
non-fatigue fluorescence modulation, fluorescence resonance
2
–5 energy transfer (FRET) is a desirable mechanism for
between two states with different spectroscopic identities.
fluorescence quenching because, unlike electron transfer, it
does not directly create redox-active ions as photoproduct
that could lead to photodamage or other unwanted processes.
This unique trait can then be judicially exercised to modulate
or switch various functions at the molecular as well as
supramolecular level by using light as a trigger. Photo-
Photochromic Spiropyran
unimolecular transformation between two states, spirocyclic
SP) and ring opened merocyanine (MC) forms, having
different absorption spectra. Owing to this intriguing light-
driven molecular transformation fluorophore-coupled
(SP
)
shows reversible
a.
Inorganic and Physical Chemistry Division,CSIR-Indian Institute of Chemical
Technology, Uppal Road, Tarnaka, Hyderabad, India-500607.
(
Fax: (+91)40-27160921); Tel :( +91)40-27191431), E-mail: prakriti@iict.res.in
§
b.
Academy of Scientific and Innovative Research (AcSIR), New DelhiAddress here.
spiropyran derivatives have gained substantial attention for
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
designing smart materials with fluorescence "on/off" switching
supplementary information available should be included here]. See behaviours. Fluorescence emission from the fluorophore-
DOI: 10.1039/x0xx00000x
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coupled photochromic derivatives is generally observed when internal standard and coupling constant (J) are expressed in
the photochromic compound is in the SP form, while emission Hz. All Uv/vis spectra were recorded using Hitachi U-2910
is rigorously quenched when the photochromic compound is in spectrophotometer. All steady state fluorescence spectra were
the MC form due to fluorescence resonance energy transfer recorded at room temperature as well as low temperature by
(FRET) from the excited state of the fluorophore to the MC Fluorolog-3 spectrofluorimeter of Horiba Jobin Yvon, USA.
form of the photochromic compound. Irradiation with visible Reactions were monitored by thin-layer chromatography (TLC)
light at the peak of the MC absorption results in using 0.20-0.25 mm silica gel plates. Column chromatography
transformation of the MC to the SP form of the compound and was performed with silica gel (60-120 and 100-200 mesh).
complete recovery of the fluorescence of fluorophore. By High resolution Mass spectra were recorded by electrospray
using this approach,
a
variety of spiropyran-derived ionization mass spectrometry (ESI-MS) on QSTAR XL hybrid
fluorophore-coupled photochromic compounds have been ms/ ms system (Applied Bio systems/MDS sciex, foster city,
7
-10
designed and probed in molecular switching applications.
USA). FTꢀIR spectra were recorded using a Thermo Nicolet
Recently, we have shown that Photoswitchable Oligo(p- Nexusꢀ670 Fourier Transform Infrared Spectrometer with
phenylvenyl)s-Spiropyran (SP-OPV-SP) dyad takes part in reference to KBr. Size of the nanoparticles was characterized
fluorescence switching behaiour both in solution and as well as using scanning electron microscope (Hitachi, Sꢀ3000N) and
1
1
in solid state forming gel through self-assembly.
dynamic light scattering (DLS) measurement with zetasizer
0
For effective functioning of fluorescence-coupled nanoꢀZS(Malvern Instrument, UK) using 173 scattering angle
a
spiropyran photoswitch, we have extend our quest with and laser of 633nm wavelength.
squarine dye which has strong absorption at 520-650nm and
very high fluorescent quantum yield with emission at 600-750
nm range of electrometric spectrum in acetonitrile solvent.
Hence, the red edge of absorption spectra of MC form of SP
clearly overlaps to the blue edge of emission spectrum of SQ
.
As a consequence, the spiropyran–SQ system is proficient for
incorporation into a FRET based photoreversible switching
system. Here we report the synthesis and spectral studies of as
prepared dyad in solution, in polymer matrices as well as in
nanoparticle form of SP-SQ-SP (7), (see Scheme 5). The ease of
synthesis allows us to covalently attached two SP units to the
both side of an SQ unit (SP-SQ-SP). Demonstrated results
confirm that the fluorescence intensity of the SQ unit can be
regulated by alternate application of ultraviolet light and
visible light or thermal stimulation. Hence, this SP-SQ-SP dyad
can potentially be used in molecular device design at the single
molecular level in view of processing and communicating
information. In order to compare the intermolecular
communicating behaviour of the mixture solution of an SQ
Scheme 1: Synthetic route of the Squarine SQ (3) dye.
1
-(2-Hydroxyethyl)-2,3,3-Trimethyl-3H-Indolium Iodide (2):
A mixture of 2,3,3-trimethyl-3H-indole (1), ( 5.0 g, 31.4 mmol),
and 2-iodoethanol (8.2 g, 47.7 m mol) was dissolved in 30-ml
acetonitrile and the solution refluxed for 6 h. The reaction
mixture was then cooled to room temperature and 20 ml of
diethyl ether was added. The crude product precipitated and
was collected by filtration and washed with 2–3 small portions
of ether to give Compound
2
(ref:12) as brown crystals (8.3 g,
0%) 1H-NMR (500 MHz, d6-DMSO) δ 1.57 (6H, s, 2 × CH3),
.85 (3H, s, CH3), 3.90 (2H, t, J 5.0 Hz, CH2), 4.62 (2H, t, J 5.0
8
2
derivative (compound
3
, see Scheme 1) and a SP molecule
Hz, CH2), 5.01 (1H, s, –OH), 7.64–7.62 (2H, m, ArH), 7.88–7.86
1H, m, ArH), 7.99–7.89 (1H, m, ArH); 13C-NMR (125 MHz, d6-
DMSO) δ 14.5, 21.9, 50.3, 54.2, 57.7, 115.5, 123.5, 128.7,
29.2, 141.0, 141.7.
(reference compound
6; see Scheme 2) (in a molar ratio of 1:2)
(
were also investigated.
1
Experimental
Materials and Instrumentation
The solvents and the reagents were purified and dried by
usual methods prior to use. 3, 4-Dihydroxycyclobut-3-ene-1,2-
dione, Ethanol, 4-Bromophenylhydrazine hydrochloride, 3-
Methyl-2-butanone and poly(ethylene glycol) were purchased
from Sigma-Aldrich, USA. 2-iodoethanol, Acetic acid, NaOH,
potassium carbonate, 2-Hydroxy-5-nitrobenzaldehyde and
Acetonitrile were used as received from commercial suppliers.
1
H NMR was recorded on 500 MHz (Bruker ARX500) and
1
3
C NMR spectra were recorded on Bruker 300 MHz
spectrometer at room temperature in CDCl
shifts are reported in ppm (d) tetramethylsilane (TMS) as
. The chemical Scheme2: Synthetic route of the Spiropyran (SP) (6)
3
2
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4
-((1-(2-hydroxyethyl)-3,3-dimethyl-3H-indol-1-ium-2-
6H), δ 2.85 (s, 3H), δ 2.96-3.0 (t, 2H), δ 4.63-4.67 (t, 2H), δ
7.61-7.64 (m, 2H), δ 7.82-7.84 (dd, 1H), δ7.97- 8.0(dd, 1H).
yl)methylene)-2-((1-(2-hydroxyethyl)-3,3-dimethylindolin-2-
ylidene)methyl)-3-oxocyclobut-1-en-1-olate, SQ (3):A mixture
of 2 (2.8 g, 8.76 mmol) and 3,4-dihydroxycyclobut-3-ene-1,2-
l-(β-carboxyethyl)-3’,3’-dimethyl-6 -nitrospiro (indoline-2’,2
dione (0.5 g, 4.38 mmol) was taken in a 100 ml round bottom [2H-1] benzopyran) (5): The product 4 (2.52 g, 7 mmol), 5-
flask and it was dissolved in toluene (15 ml) after that added n- nitrosalicylaldehyde (1.16 g, 7 mmol), and piperidine (0.76 mL,
butanol and 1 or 1.5 ml of pyridine added, under inert 0.7 mmol) were dissolved in anhydrous ethanol (50 mL). The
condition reflux 18 hours. The reaction mixture was then mixture was refluxed for 5 h. The resultant dark purple mixture
cooled to room temperature and it was poured into water and was cooled in an ice bath and filtered, and the filter cake was
extracted with dichloromethane. The organic layer was washed with cold ethanol. The precipitate was recrystallized
concentrated to give the corresponding purified by column from ethanol and dried in vacuum to yield
5
(ref:14, 15) (1.71
, 400 MHz): δ 1.07 (s, 3H), δ 1.19 (s,
ref:13 ) as thick blue solid (217 mg, ~70%). H-NMR (500 MHz, 3H), δ 2.45-2.57 (t, 2H), δ 3.34-3.40 (t, 2H), δ 5.98-6.01 (d, 1H),
1
2 2 2 6
chromatography [SiO : Hexane/CH Cl (1:9 v/v) to afford 3 g, 65 %). H NMR (DMSO-d
1
(
1
CDCl ) δ: H NMR (500 MHz, CDCl ) δ 7.33 (t, J = 7.9 Hz, 4H), δ 6.65-6.67 (d, 1H), δ 6.78-6.82 (t, 1H), δ 6.85-6.88 (d, 1H), δ
3
3
7.17 (t, J = 7.3 Hz, 2H), 7.10 (d, J = 7.8 Hz, 2H), 5.89 (s, 2H), 4.36 7.11-7.14 (t, 1H), δ 7.19-7.22 (d, 1H), δ7.98- 8.21 (dd, 1H), δ
(s, 2H), 4.02 (t, J = 6.1 Hz, 4H), 1.76 (t, J = 49.8 Hz, 16H). 13C- 8.21 (s, 1H). FTꢀIR (KBr) νmax = 749, 803, 946, 1026, 1087,
NMR (125 MHz, CDCl ) δ: 195, 187,173.1, 172.8, 152, 145, 1162, 1269, 1330, 1480, 1510, 1575, 1607, 1707, 2681, 2764,
3
ꢀ
1
1
6
7
1
41.4, 141.2, 128, 125, 120.8, 120.1, 116.8, 111, 108.6, 94, 2935, 2968 cm .
3.4, 59.8, 55.6, 51.1, 49.5, 26.8, 24.7. FTꢀIR (KBr) νmax = 679,
74, 834, 935, 1053, 1095, 1165, 1269, 1354, 1450, 1486, Methyl 3-(3',3'-dimethyl-6nitrospiro[chromene-2,2'-indolin]-
ꢀ1
577, 2922, 3362 cm .
1'-yl) propanoate (6):
Compound DCC (97 mg, 0.47 mmol) was added to a solution of
4
(180 mg, 0.47 mmol), methanol (1 mL, 25 mmol) and DMAP
(
3.8 mg, 0.03 mmol) in dry CH Cl (100 mL) and temperature
2
2
was initially maintained at 0°C under Ar. Then, the mixture was
allowed to warm up to ambient temperature over 12 h and
stirred for a further 12 h. Solvent was evaporated and the
residue dissolved in chloroform. It was then precipitated by
the addition of methanol and filtered. The crude mixture was
then purified by column chromatography [SiO2: Hexane/CHCl3
1
(ref:11,14,15) (130 mg, 70%). H NMR (500
(
1:1 v/v) to afford
6
MHz, CDCl3): δ 1.07 (s, 3H), δ 1.19 (s, 3H), δ 2.45-2.57 (t, 2H), δ
.34-3.40 (t, 2H), δ 3.7 (s, 3H), δ 5.98-6.01 (d, 1H), δ 6.65-6.67
d, 1H), δ 6.78-6.82 (t, 1H), δ 6.85-6.88 (d, 1H), δ 7.11-7.14 (t,
H), δ 7.19-7.22 (d, 1H), δ7.98- 8.21 (dd, 1H), δ 8.21 (s, 1H).
3
(
1
SP-SQ-SP (7): Compound DCC (29 mg, 0.14 m mol) was added
to a solution of 5 (35 mg, 0.09 mmol), 3 (69.5 mg, 0.14 mmol)
and DMAP (2 mg, 0.01 mmol) in dry CH Cl (100 mL) and
2
2
temperature was initially maintained at 0°C under Ar. Then,
the mixture was allowed to warm up to ambient temperature
over 12 h and stirred for a further 24 h. Solvent was
evaporated and the residue dissolved in chloroform. It was
then precipitated by the addition of methanol and filtered. The
crude mixture was then purified by column chromatography
[
SiO
2
+
: Hexane/CHCl
3
(1:1 v/v) to afford
ES ): calculated for C72 12 (M+): 1209.490; found,
209.4943. H NMR (500 MHz, CDCl ): δ 8.02-7.96 (m, 4H),
7 (74 mg, 70%). HRMS
Scheme 3: Synthetic route of SP-SQ-SP (7) from SQ (3) and SPCOOH (5).
(
68 6
H N O
1
1
7
6
3
l-(β-carboxyethyl)-2,3,3-trimethylindolenine iodide (4):
A
.35-7.32 ( d, 2H), 7.25-7.21 (t, 3H), 7.05-7.03 (d, 4H), 6.98-
.96 (d, 2H), 6.90-6.84 (dd, 4H), 6.70-6.67 (d, 2H), 6.47-6.44
mixture of 2,3,3-Trimethylindolenine (2.5 g, 15.7 mmol) and 3-
iodopropanoic acid (3.14 g, 15.7 mmol) were dissolved in
toluene (5 mL) and heated under nitrogen at 100 °C for 3 h.
The resulting solution was evaporated; the remaining product
dissolved in water (100 mL) and was washed with chloroform
(d, 2H), 5.98(s, H), 5.8-5.76 (d, 2H), 4.45-4.41 (t, 4H), 4.31-4.08
(m, 5H), 3.52-3.3 (m, 6H), 2.6-2.4 (d, 2H), 1.96-0.86 (m,
1
4H). C NMR (101 MHz, CDCl
3
2
1
3
) δ 171.4, 159.3, 146, 141, 135,
28.4, 127.8, 125.9, 124, 122.8, 122.4, 121.8, 119.8, 118.5,
(
50 mL) for 3 times. Evaporation of water gave product 4 (4.07
1
g, 72 %) as a red oil. H NMR (DMSO-d , 400 MHz): δ 1.53 (s,
115.5, 106.7, 52.9, 38.9, 33, 29.7, 29.5, 27, 25, 19.7. FTꢀIR
(
1
6
KBr) νmax = 744, 806, 951, 1069, 1185, 1282, 1337, 1457,
496, 1603, 1739, 2853, 2922, 2958, 3409 cm .
ꢀ1
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Preparation of SP-SQ-SP(7) incorporated PEG film
dependent studies of absorption spectra of SQ(3) as well as
-
7
-5
dyad SP-SQ-SP(7) in the range of 10 to 10 M concentration
In order to incorporate SP-SQ-SP(7) into polymer matrices, we
have taken 0.8 g of poly (ethylene glycol) and ~0.01 g of SP-SQ-
and found no substantial change in absorptions spectra (ESI,
SP(7) and dissolved them in 5 ml of dichloromethane. Few Fig. 3S) in the studied concentration range. Hence, all solution
drops of as prepared solution was cast onto quartz plate and phase experiments are performed with 5-6 micro molar
then drier in ambient condition over a period of 2 days. A thick concentration of the dyad molecule.
light blue colour film was obtained (Fig. 2B, Inset).
Preparation of SP-SQ-SP nanoparticles
To prepare nanoparticles we adopted ‘reprecipitation method’
1
6
and it is described elsewhere. In brief, first of all stock
solution SP-SQ-SP ) of milli Molar concentration were
(
7
prepared in tetrahydrofuran (HPLC grade, Aldrich) and stored
in the dark place at room temperature. Then few micro litters
(30-50 µl) of stock solution of SP-SQ-SP (7) was rapidly injected
Scheme 4: Two switching states of references SP (6) compound.
into large excess of deionised water at room temperature and
sonicated for few minutes. A very light blue colloid was
obtained and used for characterization. The sizes of the as
prepared nanoparticles are found to be ~ 60-100 nm (Fig. 1B,
Inset), for more see (ESI, Fig. 8S).
Figure 1A shows the absorption spectra of SP(6), SQ(3) and
SP-SQ-SP(7) dyad in ACN solution and Figure 1B shows the
absorption spectra of SP-SQ-SP (7) in polymer matrices and its
nanoparticle dispersed in water. As it is observed in solution,
the absorption bands of SP-SQ-SP (7) are the exact
superposition of absorption bands of photochromic SP (6) unit
and fluorescence probe SQ (3) unit. This clearly indicates that
in ground state, there is no interaction between SP (6) and
SQ(3) moieties. Thus, SP-SQ-SP (7) turns out to be an ideal
dyad molecule. In ACN solution, dyad
7 shows a strong
absorption peak at 634 and a shoulder band at 590 nm
corresponding to SQ (3) absorption bands and a relatively
weak absorption peak at 330 nm corresponding to the
absorption of SP (6). In polymer matrices, no change in
absorption spectra pertaining to SQ (3) moiety of dyad
7 but a
systematic red shift of 10 nm is observed for the absorption
bands corresponding to SQ (3) moiety along with the
broadening of the spectral shape. In polymer matrices the
ratio of the intensities of 643 nm band to 600 nm band
Figure1. (A) Absorption spectra of SP(6), SQ(3) and SP-SQ-SP (7) in ACN solution decreases than that of in ACN solution. This result indicates
-
6
(
~10 M) at room temperature, (B) absorption spectra of SP-SQ-SP (7) in PEG matrices
that in polymer matrices SP-SQ-SP (7) get bit aggregated. Not
surprisingly, the absorption band correspond to SQ(3) moiety
of SP-SQ-SP(7) nanoparticles is found to be very broad and
apparently composed of two almost equally intense band at
and it nanoparticles dispersed in water. Inset shows the SEM micrograph of
nanoparticles
6
00 and 643 nm. However, non-linear least square fitting
Results and Discussion
analysis to the absorption spectra yields three Gaussian peaks.
The synthesis of SP-functionalized dyad SP-SQ-SP (7) is An additional new peak at 660 nm is resolved along with two
shown in Scheme 3. Compounds SQ 3) and SP 5) were distinct bands at 600, 643nm. The appearance of this new
prepared according to the sequence shown in Scheme 1 and band indicates that existence of J-type aggregates in the
Scheme 2. Reaction of acid-alcohol coupling between and
formation of SP-SQ-SP nanoparticles. No changes in
in the presence of DCC, DMAP led to SP-SQ-SP dyad (7) in 50% absorption band correspond to SP (6) absorption was notices
(
(
3
5
yield.
neither in PEG polymer matrices nor in nanoparticle form of
SP-SQ-SP
After the ACN solution of dyad
.
Absorption Spectra:
7
was irradiated at room
0
As prepared reference spiropyran compound SP
(6) shows temperature (27 C) with ultraviolet light at 330 nm (150W
the typical reversible interconversion between spirocyclic (SP) Xenon lamp) for 30 seconds, the light blue colour ACN solution
and ring opened merocyanine (MC) states (Scheme 4) upon of dyad
turned to dark blue colour solution (Fig. 2A). The
irradiation of ultraviolet (330 nm) and visible light (580nm), colourless SP solution turns blue upon 330 nm photo-
7
1
0
and it is identified by absorption spectra of respective irradiation due to transformation of SP to MC form. Hence,
1
7
conformer (ESI, Fig. 1S & 2S) . Since, SQ(3) is known to form this colour change of ACN solution of dyad
H-aggregates
7
confirms the
1
8,19
we have performed concentration transformation of SP to MC in dyad SP-SQ-SP (7). However,
4
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the absorption spectra of UV (330nm) light irradiated solution MC to SP enhanced and it takes 20 minutes to restore the
shows a very weak new band at round 555 nm (ESI, Fig. 4S). original state. This cycle of photoinduced reversible change of
The characteristic absorption band of the MC form of SP which dyad
7 was repeated several times without degrading the
shows absorption maximum at 555 nm and it is superimposed molecule both in solution and in polymer film.
with the absorption bands of SQ but the main absorption
bands of the SQ unit remain unchanged (Figure 4S, Curve b). In
comparison to the absorption spectrum of SP (6) upon
ultraviolet light irradiation (ESI, Fig. 1S), it is confirmed that the
a
b
c
Before Irr
a
(
A)
2
1
1
00.0k
50.0k
00.0k
50.0k
0.0
(λ
) Irr = 330 nm, 30 Sec
ꢀ
2 min
c
b
2
40.0k
λ irr = 330 nm, 30 Sec
formation of the corresponding MC form of SP in dyad
7 under
2
00.0k
the same conditions is occurred. This photogenerated isomer
reverts to original one by thermal isomerisation within 30
seconds at room temperature in solution phase making it
difficult to collect absorption spectra of MC-SQ-MC form with
160.0k
ꢀ
2 min
1
20.0k
0
2
4
6
8
10
12
Cycle number
SP-SQ-SP(7) in ACN
at 300 K
high fidelity. However, upon decreasing the solution
0
temperature to 7 C, the rate of forward (i.e. SP to MC
)
6
25
650
675
700
725
750
transformation induced by 330 nm photo-irradiation does not
change but the rate of backward reaction decrease
dramatically and the absorption spectra of photo-irradiated
SP-SQ-SP clearly shows new band at 555nm (Figure 2A curve
b), a band which correspond to absorption spectra of MC. At
this temperature, the MC form of SP is very stable and
irradiation of 555 nm light for 20 minutes reverts the MC form
Wavelength (nm)
3
2
1
1
00.0k SP-SQ-SP(7) in ACN at 280K a - Before Irr
b - λ Irr = 330 nm, 30 sec
c - Irr = 580 nm, 2 min
λ
40.0k
80.0k
20.0k
λ
Irr = 33Onm, 30 sec
(B)
300k
200k
to SP
.
100k
λ Irr = 580 nm, 2 min
0
2
4
6
8
10
UV-Vis Cycle no.
60.0k
0.0
650
700
750
Wavelength (nm)
a - Before Irr
b - λ Irr = 330 nm, 20 sec
c - λ Irr = 580 nm, 2 min
SP-SQ-SP(7)
1
1
50.0k Cast Film in polymer
a
2
1
00k
(
C)
λ Irr = 33O nm, 30 sec
c
00.0k
00k
0
λ Irr = 580 nm, 2 min
5
0.0k
0
2
4
6
8
UV-Vis Cycle no.
b
-
6
Figure 2. Absorption spectra of SP-SQ-SP (7), (A) in ACN solution (~10 M) at 280 K and
(
B) in PEG polymer matrices onto quartz plate at room temperature, 300K. (a) Black
0
.0
curve, before irradiation of UV light, (b) Blue curve, after irradiation of UV light (330
nm) for 30 sec in ACN solution of dyad 7 and polymer film respectively. Insets are
photographic image of reversible colour change of SP-SQ-SP in solution and in polymer
matrices at 300K.
6
50
700
750
800
Wavelength (nm)
Figure 3. Fluorescence switching behaviour of SP-SQ-SP upon alternate application of
0
UV and Vis/ꢀ in ACN solution at room temperature and at 7 C as well as in PEG
matrices at room temperature. (A) in ACN at 300K, (B) in ACN at 280 K and (C) in PEG
At room temperature, photo-irradiation at 330 nm (150 polymer matrices at 300K. Insets show the reversible modulation of the fluorescence
intensity of the SQ moiety at the emission peak for the respective medium.
Xenon Lamp) light for just 30 seconds, the polymer film of
dyad 7 undergoes to change the colour of the film and it
turned into dark blue from light blue (Figure 2B, inset). Fluorescence Spectra:
Consequently, the absorption spectra of this film showed a
Although there was no noticeable change in concentration
prominent peak at 580 nm corresponding to the MC form of dependent absorption spectra of SQ(3) or dyad SP-SQ-SP(7) in
dyad
unlike in solution the photogenerated isomer (MC form) of fluorescence spectra of SQ and dyad SP-SQ-SP(7) are observed
dyad
in film is very stable at room temperature. It reverts in ACN solution. Upon increasing concentration of SQ or dyad
7
(Fig. 2B, curve b). It is important to mention here that ACN solution, a slight change in concentration dependent
7
-7
-6
thermally very slowly to SP form and takes more than 24 hours SP-SQ-SP from 10 M to 10 M in ACN solution, a
to complete the conversion. However, upon 580 nm photo bathochromic shift of fluorescence emission maxima was
irradiation (150 W, xenon lamp) the rate of conversion from observed and no further shift of emission peak was observed
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till few hundred of micro molar concentration of the SQ or SP- in the shape of fluorescence spectra of SP-SQ-SP(7) in this low
SQ-SP (ESI, Fig. 5S). Hence, all fluorescence spectroscopic temperature but bit increase in florescence intensity. Upon
studies are performed in 6-7 micromolar concentration of the irradiation with ultraviolet light (330 nm, peak position of SP)
compounds. It is also important to mention that no self for 30 seconds, the intensity of the fluorescence band around
quenching of fluorescence was observed for these compounds 656 nm decreased to 70% of that of initial solution (Figure 3B,
till few hundreds of micro molar solution. Excitation curve b) and it remains constant with time. It clearly reveals
wavelength dependent studies of dyad
7 shows good linearity that a substantial amount of conversion occurs from SP-SQ-SP
in the studied concentration range. However, very small to its ring opening conformer MC-SQ-MC and MC-SQ-MC is
0
fluorescence Stokes shift of dyad
wavelength to the blue side of the absorption spectra. Figure with 555 nm (absorption peak position of MC form of SP) light
shows the fluorescence spectra of the ACN solution (6 µM) for 20 minutes, MC–SQ–MC completely changed to the SP–
of dyad under different experimental conditions with an SQ–SP form and the fluorescence intensity was restored to its
excitation wavelength of above 600 nm. Before exposing to initial value without distortion of the fluorescence band shape
ultraviolet light, dyad showed a broad emission band in the (Figure 3B, curve c). This fluorescence “on/off” states are
range of 625-750 nm with the maximum around 656 nm repeated by alternate application of 330 and 555 nm light for
Figure 3A curve a,). This fluorescence spectrum is broadly several cycles without degrading the compound (Figure 3B).
similar to that of fluorescence emission spectra of reference This result confirms the light-driven transformation between
SQ (3) in terms of fluorescence emission spectral structure. two states of dyad is also fatigue resistant in the solution
The fluorescence quantum yield of dyad is found to be phase.
higher than that of SQ and it is estimated to be 0.7(ESI, Fig. Sequentially, similar types of experiments were performed
S). However, upon irradiation with ultraviolet light (330 nm, with dyad within PEG polymer matrices at room
peak position of SP) for 30 seconds at room temperature, the temperature. As shown in Figure 3C, the fluorescence emission
intensity of the fluorescence band around 656 nm decreased spectrum of dyad became broad and the peak position was
7 limits the excitation quite stable at 7 C in ACN solution. However, after irradiation
3
7
7
(
7
7
2
0
6
7
7
and could be managed to estimate only 28% reduction to that red shifted by 10 nm compared to that in ACN solution,
of the initial solution (Figure 3A curve b,) and no new peaking at 670 nm (Figure 3B curve a,). This change of
fluorescence peak was detected. It is important to note that in emission spectra of SP-SQ-SP in the polymer matrices is
ACN solution the MC form of SP
emission peaking at 650 nm (ESI, Fig. 1S). Upon irradiation with However, after irradiation with 330 nm light for just 30
ultraviolet light, the SP moieties of dyad transform into the seconds at room temperature, the fluorescence intensity of
MC conformer, resulting in the formation of MC–SQ–MC the SQ moiety of dyad was quenched by 85% of its initial
Scheme 5) which quenches the excited state of the SQ unit value (Figure 3B, curve b) and it did not substantially change
(6) shows fluorescence perhaps due to some aggregation effect of SQ moiety.
7
7
(
through fluorescence resonance energy transfer. Energy with time. This result not only confirms the efficient FRET
transfer from the excited SQ state to MC produces the excited between the SQ and MC moieties in polymer matrices but also
MC state, which should show a new emission band at around confirms the hindrance of thermal conversion of MC-SQ-MC to
650 nm. However, in the present case we do observe such a SP-SQ-SP at room temperature. To check the photo-
new fluorescence band relating to the fluorescence of the MC reversibility, the polymer film was irradiated with 580 nm light
moiety of MC–SQ–MC, perhaps due to strong overlap of for 2 minutes and the fluorescence of the SQ moiety came
florescence of MC to that the red edge florescence of SQ
.
back almost to 90% of its initial value (Figure 3C, curve c). This
Since SQ has very high fluorescence quantum yield with cycle was repeated several times and no remarkable
respect to fluorescence yield of MC, the fluorescence emission degradation of the compound was observed (Figure 3C, Inset).
of MC hides under the envelop of the fluorescence of dyad SP-
SQ-SP even in reduced fluorescence condition. Moreover, MC-
SQ-MC form of SP-SQ-SP thermally reverts very fast to dyad
SP-SQ-SP and hinders to monitor efficient conversion of dyad
SP-SQ-SP. However, MC-SQ-MC comes back to SP-SQ-SP form
within 30 sec after photo-irradiation and fluorescence
intensity is restored to its initial value without distortion of the
fluorescence band shape (Figure 3A, curve c) and this cycle of
fluorescence “on/off” states are repeated by application of
UV
330 nm
580 nm
Vis/ ∆
330 nm light followed by thermal conversion for several cycles
without degrading the compound (Fig. 3A, Inset). This result
confirms that light and thermal-driven transformation
MC-SQ-MC
SP-SQ-SP(7)
between two states of dyad
7 is fatigue resistant in the
Scheme 5. Reversible fluorescence switching cycle of dyad 7 under different external
solution phase at room temperature. However, to control the
stimuli: ultraviolet light (330 nm) and visible light (580 nm).
rate of thermal conversion of MC-SQ-MC to SP-SQ-SP we have
0
performed fluorescence switching experiment at 7 C in ACN
Solution. As shown in Figure 3B, there is no substantial change
6
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The as prepared water dispersed nanoparticles of dyad
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7
polymer matrices at room temperature (300K) in order to
shows good fluorescence with emission peak at 675 nm. At compare the difference between intra and inter molecular
room temperature, upon UV irradiation (330 nm) for 30 sec, effect in fluorescence switching behaviour (Figure 4). Before
the florescence intensity of the nanoparticles reduced to 50 % ultraviolet light irradiation, the solution showed the exactly
of its initial value (ESI, Fig. 7S) and after two minutes it reverts similar fluorescence emission spectra to that of dyad
7 (Figure
to its initial value. However, irradiation by visible light 4A, curve a). After irradiation of the mixture solution at 330
enhances the reversion process and complete reversion is nm for 5 min, less than 5% reduction of the fluorescence
observed by irradiation of 580 nm light for 30 sec. This cycle is intensity was observed (Figure 4A, curve b) and it returns to its
repeated several times and no change of initial fluorescence original value upon irradiation of 550 nm light for 2 minutes.
intensity is observed. It is worthy to note that at room However, after irradiation of PEG polymer film incorporated
temperature in nanoparticle form, the rate of back reaction i.e. with 2:1 mixture of SP (6) and SQ (3) at 330 nm for 30 seconds
MC-SQ-MC form to SP-SQ-SP (7) form is slowed by two times reduction of the fluorescence intensity was observed only
than that in ACN solution. Like in solution phase, on lowering about 21 % to its initial value (Figure 4B, curve b) and it returns
0
the temperature to 7 C, efficiency of thermal reversion tends to its original value upon irradiation of 580 nm light for 2
to be zero and visible light induced reversion becomes minutes, (Figure 4B, curve c) similar to what is described above
predominantly active.
for dyad 7. Although the extent of fluorescence reduction is
quite low, but these unavoidable reduction of fluorescence
intensity is due to the quenching of the excited state of the SQ
unit by the corresponding MC form generated from the SP
upon ultraviolet light irradiation both in solution and in
a
(A)
8
0.0k
a - Before Irr
b -After irradiation
polymer matrices. As compared to dyad
7
, the fluorescence
and is
rster theory, the energy
transfer efficiency is strongly dependent on the donor-
acceptor distance. The donor and acceptor units in dyad are
much closer to each other than those in the case of the mixed
solution of and (intermolecular) and it strongly depends on
b
intensity reduction for the mixed solution of
3
6
2
1
(λ Irr = 330 nm, 30 sec)
significantly less. According to the F
ö
40.0k
7
3
6
SP(6) & SQ(3)
in ACN with (2:1)
rate of diffusion of the molecules. However, the efficiency of
FRET is bit enhanced in polymer matrices but it is quite low
0.0
than for dyad
7 in same polymer matrices. Thus, it is quite
6
50
700
750
clear that the intermolecular energy transfer is not as effective
as that for the intramolecular cases.
Wavelength (nm)
Furthermore, all the studies described here are qualitative
in nature and the quantitative estimations were not
performed. During the collection of fluorescence spectra
irradiation was not carried out as well as the time gap after
photo-irradiation and before collecting the spectral signature
of photoproducts whether it fluorescence or absorption has
not been considered. Hence, thermal reversion to the SP form
could occur before and during the collection of the
fluorescence spectra, especially in solution where conversion
of MC to SP form is very fast. Finally, since MC and SQ absorb
at 550 nm and irradiation at this wavelength creates only
1
50.0k
a - Before Irr
b - After Irr,
λ Irr = 330 nm, 30 sec)
c - After Irr,
λ Irr = 580 nm, 120 sec)
(B)
a
c
(
(
100.0k
b
5
0.0k
SP(6) & SQ(3)
in PEG matrices with 2:1
photostationary equilibrium between two forms (SP and MC
)
of spiropyran units, rather than complete conversion of the
system to one form. The studied fluorescence modulation in
solution at room temperature as well as low temperature,
polymer matrices and in nanoparticle form observed for dyad
SP–SQ–SP (7) can be rationalized with the switching cycles
starting and ending with SP–SQ–SP (7) as shown in Scheme 5.
There should exist several states of dyad SP–SQ–SP (7) due to
the partial conversion between SP and MC or vice versa on
exposure to ultraviolet or visible light. Since changes in the
fluorescence intensity are the sole parameter to account for
the conversion due to irradiation with UV/Vis light or thermal,
we consider two ultimate states (Scheme 5) to clarify the
mechanism. Upon irradiation with ultraviolet light, the SP
6
50
700
750
800
Wavelength (nm)
Figure 4: Fluorescence switching behaviour of the mixture of SP(6)and SQ(3) in 2:1 ratio
upon alternate application of UV and Vis/ꢀ in ACN solution and in PEG polymer
matrices at room temperature. (A) in ACN at 300K, (B) in PEG polymer matrices at
3
00K.
For comparative studies, similar set of experiments were
performed for the mixtures of reference compounds SP(6) and
SQ ) (in a molar ratio of 2:1) both in ACN solution and in PEG
(
3
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moieties in dyad
7
are transformed to MC moieties, forming
Chem. Int. Ed., 2007, 46, 5472; (c) X. Chen, Y. Wang, Q. Liu, Z.
Zhang, C. Fan, L. He, Angew. Chem., 2006, 118, 1791; Angew.
Chem. Int. Ed., 2006, 45, 1759; (d) D. H. Qu, Q. C. Wang, H.
Tian, Angew. Chem., 2005, 117, 5430; Angew. Chem. Int. Ed.,
MC–SQ–MC, which quenches the excited state of the SQ
moiety and the fluorescence intensity of the SQ moiety is
reduced. On the other hand, upon irradiation of the solution of
MC–SQ–MC with visible light or thermal exposure, complete
conversion from MC–SQ–MC to SP–SQ–SP(7) occurs and the
fluorescence intensity returns to its initial value. Thus,
alternate application of ultraviolet light, visible light, and
thermal exposure regulates explicitly the fluorescence
intensity of the SQ moiety. This external stimulation-
2
005, 44, 5296; (e) H. Miyaji, H. K. Kim, E. K. Sim, C. K. Lee,
W. S. Cho, J. L. Sessler, C. H. Lee, J. Am. Chem. Soc., 2005,
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1
Angew. Chem. Int. Ed., 2007, 46, 4026; (g) E. Katz, V.
Privman, Chem. Soc. Rev., 2010, 39, 1835; (h) S. Angelos, Y.
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Soc., 2009, 131, 11344.
dependent fluorescence spectra of dyad
7
can be exploited to
7
8
A. P. de Silva, D. B. Fox, T. S. Moody and S. M. Weir, Trends
Biotech., 2001, 19, 29.
(a) E. A. Lemke and C. Schultz, Nat. Chem. Biol., 2011, 7,
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80;(b) K. P. Carter, A. M. Young and A. E. Palmer, Chem.
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Conclusion
9
1
(
2
a
) Y. Li, T. Liu, H. Liu, M.-Z. Tian and Y. Li, Acc. Chem. Res.,
014, 47, 1186; (b) A. P. De Silva, H. Q. N. Gunaratne,T.
We have successfully synthesized a new dyad SP-SQ-SP(7)
and have examined their photophysical properties devoted to
understanding their light driven fluorescence switching
behaviour in solution, in PEG polymer film and in its
nanoparticle form dispersed in water. The presence of SP (6)
units controls the fluorescence switching ‘on/off’ states of the
Gunnlaugsson, A. J. M. Huxley, C. P. Mccoy, J. T. Rademacher
and T. E. Rice, Adv. Supramol. Chem., 1997, , 1.
0 (a) F.M. Raymo , S. Giorgani, J. Am. Chem. Soc., 2002, 124
4
,
,
,
2004; (b) F. M. Raymo and S. Giorgani, Org. Lett., 2001,
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3
3
3475.(d) E. Deniz, S. Sortino , F. M. Raymo, J. Phys. Chem.
Lett., 2010, , 1690.
D. Siva, B. Ramakrishna, Y. Venkatesh, P. R. Bangal, RSC Adv.,
015, , 56855
SQ(3) moiety of dyad
among the two different states of the photochromic
spiropyran moiety and fluorescence resonance energy
7
by way of reversible interconversion
1
1
1
2
5
.
1
2 (a) M. D. H. Bhuiyan , G. J. Gainsford , Y. Kutuvantavida , J. W.
Quilty , A. J. Kay , G. V. M. Williams & M. R. Waterland Mole.r
Cryst. and Liq. Crys., 2011, 548, 272; (b) S. Kuster, T. Geiger,
Dyes and Pigments, 2012, 95, 657; (c) D. B. Liu, W. W. Chen,
K. Sun, K. Deng, W. Zhang, Z. Wang, X. Y. Jiang, Angew.
Chemie Int. Ed., 2011, 50, 4103.
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Chem., 2008, 18, 264; (b) A. jayaghosh, Acc. Chem. Res.,
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Hibino, N. Tanaka, H. Miura, Y. Himori,T. Yoshida, Y. Kubota,
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4 (a) R. Klajn, Chem. Soc. Rev., 2014, 43, 148; (b) A. Nayak, H.
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Takai, T. Kanamori, Phys. Chem. Chem. Phys., 2011, 13, 7322;
transfer (FRET) between the MC and SQ moieties. At room
temperature in solution phase MC form of SP in SP-SQ-SP(7) is
found to be less stable resulting very fast thermal reversion of
MC form to SP which could be advantageous in developing
temperature sensor. Near-infrared fluorescence on/off states
of nanoparticle dispersed in aqueous media may have
potential applications in fluorescence imaging. Overall, these
results may have implication for the invention of efficient
near-infrared fluorescent probes for potential applications
towards molecular device design with multiple inputs and a
single output.
1
1
Acknowledgements
(
e) J. Whelan, D. Abdullah, J. Wojtyka, E. Buncel, J. Mat.
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TOC
Novel near-infrared fluorescent dyad molecule was
synthesized by coupling squarine derivatives (SQ) and
Spiropyran (SP) and characterized in solution, in
polymer matrices and in its nanoparticle form. Light
driven reversible transformations between ring close
and ring open isomers of SP modulate the near-infrared
fluorescence of SQ leading to fluorescence "ON/OFF"
switching behaviours.