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Synthesis 2025; 57(01): 189-195
DOI: 10.1055/a-2383-0905
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Special Topic Dedicated to Prof. H. Ila

A Multi-Stimuli Responsive Spiropyran–Tetraphenylethene Conjugate for the Control of Near-Infrared Emission

Sajal Kar
,
Sheelbhadra Chatterjee
,
Subhajit Bandyopadhyay  ∗

S.B. thanks SERB India for a research grant (SERB/CRG/2022/006776).
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Dedicated to Professor H. Ila on the occasion of her 80th birthday.

Abstract

The synthesis of a dinitrotetraphenylethene linked to two photochromic spiropyran moieties was achieved. The compound displays photochromic and acidochromic behavior. In the aggregated merocyanine photoisomeric state, the molecule is prone to form aggregates of the zwitterionic form. The merocyanine form displays near-infrared intense fluorescence (λmax_em. at 665 nm) extending into the near-infrared region up to 900 nm. On the other hand, the spiropyran form displays fluorescence in the 450 nm region. In DMF/water mixture, the near-infrared emission is quenched whereas the spiropyran form displays fluorescence with a λmax_em. at 563 nm.


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Key words

photochromism - spiropyran - acidochromism - aggregation - fluorescence

Stimuli responsive ‘smart materials’ have been the focus of recent development because of their potential applications in the fields of information,[1] electronics,[2] [3] and emerging fields such as photopharmacology[4,5] and optogenetics.[6] Central to these organic materials are light-responsive photochromic molecules that can be reversibly isomerized to two or more isomeric forms by means of light.[7] [8] Additionally, for certain class of photochromic systems, other external stimuli such as heat,[9] acids/bases[10] [11] or metal ions[12] [13] can induce isomeric transformations. These kinds of photochromic molecules can exist in two or more forms, out of which one is thermodynamically more stable over the other one. Popular photochromic systems include azobenzenes,[14] [15] dimethyldihydropyrenes (DHP),[16] [17] thioindigos,[18] hemithioindigos,[19] dithienylethenes (DTE),[20] spiropyrans,[21] and donor-acceptor Stenhouse adducts (DASA)[22] switches among others.

In the photochromic family, spiropyrans possess unique features. The closed form, known as the spiropyran form (SP), undergoes an electrocyclic ring opening followed by cis-to-trans photoisomerization, converting into the open merocyanine form (MC) (Scheme [1]). The SP to MC ring opening isomerization takes place with UV light or in the presence of an acid. The MC to SP reverse isomerization occurs upon exposure to visible light or in the presence of a base. The SP form is less polar than the zwitterionic MC form. During isomerization, a change in polarity occurs, allowing the photochromic system to reversibly alter the light-induced polarity of a system, consequently a change in color is observed as the less intensely colored SP form converts to the more intensely colored MC form, which possesses extended π-conjugation.[23] Additionally, the emission properties of the system changes upon isomerization. Liu’s group have recently published a study detailing the integration of a tetraphenylethene (TPE) system with a spiropyran moiety appended to one of its ends.[24] Their research explores the isomerization behavior under visible light/dark conditions and response to acid/base stimuli in solution. Notably, they observed the formation of aggregates by both ring-closed and ring-opened photoisomers, with distinct emission maxima for each isomer. In a separate study, Tian’s group have synthesized two distinct TPE derivatives covalently linked to a single spiropyran moiety via a C=C bond, devoid of any substituents.[25] Their investigation focused on exploring the photochromic and acidochromic properties of these compounds, as well as the modulation of emission color through treatment with acid/base in the solid state. Additionally, Yin’s group developed TPE molecules integrated with two spiropyran moieties linked by an ester linkage.[26] Their research explores the photochromism, acidochromism, and photocontrolled wettability of these compounds in the solid state.

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Scheme 1 Schematic representation of isomerization scheme of TPE-SP1

The presence of an electron-withdrawing group in the spiropyran enhances its photoswitching capabilities. We hypothesized that incorporating two electron-withdrawing nitro groups at the ends of a large aromatic system might induce cross-conjugation with the photochromic spiropyran systems. This modification could result in an enhanced red-shifted emission profile in one of the photoisomeric forms, thereby providing a means for light-induced control of fluorescence. Thus, in this study, we have designed and synthesized a spiropyran conjugated tetraphenylethylene (TPE), namely TPE-SP1, where the para-position of the TPE is functionalized with an electron-withdrawing nitro group to extend conjugation and investigate its influence on the photoswitching process. The synthesized molecule TPE-SP1 demonstrates the ability to undergo isomerization from the SP to the MC form under exposure to UV light. The same transformation is also achievable under acidic conditions. The reverse ring-closing isomerization of the MC form to the SP form is achievable under green light (525 nm) and also under basic conditions. Steady-state fluorescence studies of the SP and the MC forms displayed a significant difference in their emission profiles. The SP form in pure DMF displayed an emission maximum at ~450 nm, whereas the photoiomerized MC form displayed an intense emission at 665 nm tailing to the infrared region up to 900 nm; in DMF/H2O (6:4, v/v), it showed a red emission with an emission maximum at 563 nm. However, the MC form under the same conditions displayed a very weak emission.

The synthesized molecule, TPE-SP1, has been comprehensively characterized by various analytical techniques including 1H NMR, 13C NMR, mass spectrometry, and infrared spectroscopy. Isomerization events were monitored and recorded using UV-visible and photoluminescence spectroscopy.

Zoom Image
Scheme 2 Synthetic scheme for TPE-SP1

Synthetic Procedures
The TPE-linked-bis-spiropyran (TPE-SP1) was synthesized (Scheme [2]) following a five-step synthetic sequence involving a mixed acid nitration of the TPE unit followed by dibromination using bromine, which, on a subsequent Suzuki coupling reaction with a pinacol ester of salicylaldehyde, afforded the key intermediate 4. The salicylaldehyde intermediate 4 on coupling with 1,2,3,3-tetramethyl-3H-indol-1-ium salt in pyridine at 120 °C yielded the target TPE-SP1 in good (70%) yield. The intermediates and the final compound were thoroughly characterized by NMR and high-resolution mass spectrometry. The existence of [TPE-SP1 + H]+ was confirmed by HRMS with molecular ion peak at m/z 973.3927 for [TPE-SP1 + H]+ , which is consistent (Figure S7) with the calculated value of 973.3960.

UV-Vis Spectroscopy
The UV-vis spectra of TPE-SP1 displayed (Figure [1a]) absorption bands at 285nm and 383 nm, corresponding to the pale-yellow solutions in CHCl3. Upon addition of trifluoroacetic acid (TFA), the pale-yellow solution (10 μM in CHCl3, purged with N2) turned red along with the generation of new absorption bands (time for saturation = 100 min) centered around 484 nm for TPE-MCH1 (Figure S11). On further addition of a base (7.2 M of Et3N), the absorption bands corresponding to TPE-SP1 were regenerated with the simultaneous changes in the color of the solution from red to pale-yellow, demonstrating a reversibility of the system. The same TPE-MCH1 to TPE-SP1 transformation was accomplished with 525 nm light irradiation (Figure [1b]). The reversible transformation of TPE-SP1 to TPE-MC1 was also achieved with 254 nm light (Figure [1a]).

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Figure 1 a) Isomerization of TPE-SP1 to TPE-MC1 under irradiation of 254 nm (10 μM in CHCl3, degassed with N2); b) TPE-MCH1 to TPE-SP1 reverse isomerization under 525 nm light in CHCl3.

Photophysical Studies
Initially the photoisomerization behavior of the synthesized derivative TPE-SP1 was studied through UV-vis spectroscopy. The isomerization was performed in chloroform under UV light irradiation (254 nm). The pale-yellow solution (10 μM in CHCl3) turned red in color when exposed to 254 nm light. The absorption spectra of the TPE-SP1 showed two absorption band at 383 nm and 285 nm. A new absorption band appeared at 484 nm under exposure to 254 nm light due to the formation of the open merocyanine form (TPE-MC1) accompanied by the simultaneous decrease in the absorption band at 285 nm in presence of an isosbestic point at 311 nm (Figure [1a]). Although the reversal of merocyanine to spiropyran is well established under visible light irradiation, TPE-MC1 form did not show in reversal when exposed to visible light. Interestingly the individual spiropyran unit attached to the TPE in TPE-SP1 displayed prominent reversibility when exposed to visible light (Figure S10). The photoisomerization was also evident from the ring opening and closing and they were also monitored by the 1H NMR studies. The NCH3 protons of TPE-SP1, denoted as ‘a’ (see Figure S8), appeared at δ = 2.72. Upon isomerization to TPE-MCH1 the nitrogen atom acquires a positive charge. Thus, the NCH3 protons become more deshielded and shifted to δ = 3.42.

Spiropyrans exhibit isomerization in the presence of acid and base.[10] The isomerization behavior of TPE-SP1 was also studied under acidic and basic conditions. Thus,TPE-SP1 (10 μM in CHCl3) in the presence of TFA (0.06 M) isomerized to the TPE-MCH1 as evident from the appearance of the characteristic absorbance bands of TPE-MCH1 (Figure [2a]). Further addition of triethylamine (0.04 M) to the same solution led to a distinct spectral change suggesting the reverse isomerization of TPE-MCH1 → TPE-SP1 (Figure [2b]). Further the reversibility of TPE-MCH1 was also checked with visible light and indeed a significant reversal from TPE-MCH1 → TPE-SP1 was obtained when exposed to 525 nm light (Figure [1b]).

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Figure 2 a) Isomerization of TPE-SP1 to TPE-MCH1 in CHCl3 acidified with TFA; b) TPE-MCH1 to TPE-SP1 reverse isomerization upon addition of base (Et3N).
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Figure 3 Changes in a) absorbance and b) emission of TPE-SP1(10 μM, λex = 370 nm, slit width 10/5 nm, 298 K) in various DMF/H2O solvent mixture; changes in the c) absorbance and d) emission of TPE-MCH1 (10 μM, λex = 370 nm, slit width 10/5 nm, 298 K) in various DMF/H2O solvent mixture.

Having an excellent AIE-gen (AIE: aggregation-induced emission), TPE, attached to the system,[27] TPE-SP1 was expected to form aggregates upon addition of an increasing percentage of water leading to a change in both the absorption and the steady-state fluorescence emission spectra. The aggregation behavior of both the isomers TPE-SP1 and TPE-MCH1 was investigated using absorption and emission spectroscopy in DMF solvent with various percentages of water. In the absorption spectra of TPE-SP1, a maximum red-shift of 25 nm was observed in 6:4 DMF/H2O mixture (v/v), which indicated the formation of J-aggregates (Figure [3a]). In pure DMF, the SP form (TPE-SP1) exhibited an emission maximum at 438 nm (Figure [3b]), while the photoisomerized MC form (TPE-MCH1) displayed an intense emission with a λmax_em. at 665 nm, extending into the near-infrared region up to 900 nm (Figure [3d]). Under the same conditions, namely DMF/H2O mixture (6:4, v/v), the TPE-SP1 form showed a red emission with a maximum intensity (λmax_em) at 563 nm. This shift in the emission band is consistent with the literature reports.[28] [29] [30] The absorption and emission studies were also carried out with the TPE-MCH1 isomer. In contrast to the TPE-SP1, no significant shifts in the absorption bands were observed for TPE-MCH1 in any of the DMF/H2O solvent system suggesting either no or a weak aggregate formation (Figure [3c]). The intense emission at 665 nm of the merocyanin form was quenched progressively upon addition of an increasing amount of water and with 100% water, it was minimum (Figure [3d]). It was envisioned that the open isomer, TPE-MCH1, being a charged species is hydrophilic. Therefore, with addition of an increasing amount of water, instead of exhibiting an induced enhancement, the species progressively dissolved in the solvent system as the percentage of water was increased. Thus, for the TPE-MCH1 isomer a gradual decrease in emission intensity, unlike the corresponding TPE-SP1 isomer, was observed as the fraction of water was increased. This shows a strong dependence of the emission of the two forms TPE-SP1 and TPE-MCH1 under various DMF/H2O ratio. This can potentially be used to map the acidity in microenvironment inside cellular systems. It is to be noted that this difference in emission of the two forms is primarily an effect of the cross-conjugation of the two spiropyran units with the electron-withdrawing nitro group through the TPE spacer. The relative fluorescence quantum efficiency (ΦF) of TPE-SP1 and TPE-MCH1 with respect to a commonly used standard, quinine sulfate (in 0.1 M H2SO4) was measured and found to be 4.2 × 10–3 and 6.4 × 10–3 for TPE-SP1 and TPE-MCH1, respectively.[31]

We also note that the reversal of TPE-SP1 was not very effective under neutral pH or under 525 nm light. This is not surprising as it is well known that several photochromic systems such as azobenzenes do not isomerize well under aggregated states.[32]

Fluorescence Response of TPE-SP1 and TPE-MCH1 to p-Nitrophenol
Nitroarenes, that are explosive and hazarders for human health, are known to interact with aromatic fluorophores displaying a change in their emission profile, and thus serving as a method to detect them.[33]

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Figure 4 a) Quenching of emission intensity of TPE-SP1 with increasing concentration of PNP; b) of emission of TPE-MCH1 with increasing concentration of PNP; c) and d) quenching of fluorescence of TPE-SP1 and TPE-MCH1, respectively, with gradual increase in PNP concentration.

In this work, we have also explored how both TPE-SP1 and TPE-MCH1 responded to a nitroalkene. Systematic emission study showed a turn-off response with both the fluorophores in the presence of p-nitrophenol (PNP). In both cases, a gradual decrease of fluorescence intensity with the increasing concentration of the PNP was observed (Figure [4]).

In conclusion, our study demonstrates the successful design and synthesis of a spiropyran-conjugated tetraphenylethylene (TPE) molecule, functionalized with electron-withdrawing nitro groups to extend conjugation and enhance photoswitching capabilities. The incorporation of these nitro groups effectively induces cross-conjugation within the photochromic spiropyran system. The synthesized molecule exhibits the ability to undergo isomerization from the SP form to the MC form upon exposure to UV light, as well as under acidic conditions. Conversely, the reverse isomerization from the MC form to the SP form is achievable with green light (525 nm) and under basic conditions resulting in a blue-shifted fluorescence. Steady-state fluorescence studies also reveal significant differences in the emission profiles of the SP and MC forms: the SP form displays a pronounced red emission with a maximum at 450 nm, whereas the MC form shows a strong NIR-emission under the same conditions. This study highlights the potential of the TPE-spiropyran hybrid with cross-conjugated nitro groups as a versatile material for applications requiring precise light-induced modulation of fluorescence.


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4,4′-(2,2-Diphenylethene-1,1-diyl)bis(nitrobenzene) (1)

To a mixture of concd HNO3 (15.7 M, 20 mL) and glacial AcOH (50 mL) was added tetraphenylethylene (3 g, 9.03 mmol) dissolved in CH2Cl2 (75 mL). After 2 h of stirring at RT (25 °C), a deep green color appeared. The resulting mixture was extracted with CH2Cl2 (4 × 100 mL) and the combined organic layers were washed with H2O, dried (anhyd Na2SO4), and concentrated in vacuo. The desired product 1 (yellow solid, 3.2 g, yield 85%) was further purified by column chromatography on silica gel using CH2Cl2/hexane (1:10) as the eluent.

1H NMR (400 MHz, CDCl3): δ = 8.03–7.95 (m, 4 H), 7.73 (dd, J = 5.7, 3.3 Hz, 1 H), 7.53 (dd, J = 5.7, 3.3 Hz, 1 H), 7.19–7.13 (m, 8 H), 7.04–6.95 (m, 4 H).

The NMR data was consistent with the literature report.[34]

HRMS (ESI+): m/z [M]+ calcd for C26H18N2O4: 422.1267; found: 422.1270.


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4,4′-(2,2-Bis(4-bromophenyl)ethene-1,1-diyl)bis(nitrobenzene) (2)

To a mixture of compound 1 (2 g, 4.74 mmol) and FeCl3 (0.76 g, 4.74 mmol) suspended in CCl4 (50 mL) was added Br2 (1 mL, 19 mmol) at RT. After 24 h of stirring at RT, a yellowish green color appeared. The solution was quenched with sat. aq Na2S2O3 and diluted with H2O. The resulting mixture was extracted with CH2Cl2 (4 × 100 mL) and the combined organic extracts were washed with H2O, dried (anhyd Na2SO4­) and concentrated in vacuo. The crude was purified by column chromatography on silica gel with CH2Cl2/hexane (1:10) as an eluent to furnish the compound 2 as a yellow solid product; yield: 2.5 g (90%); mp 90–91 °C.

IR: 1511 cm–1 (N–O).

1H NMR (400 MHz, CDCl3): δ = 7.95 (d, J = 8.8 Hz, 4 H), 7.22 (s, 4 H), 7.12 (d, J = 8.8 Hz, 5 H), 6.83 (d, J = 8.4 Hz, 4 H).

13C NMR (100 MHz, CDCl3): δ = 148.92, 146.73, 144.07, 140.32, 140.24, 137.79, 132.80, 132.08, 131.93, 131.75, 123.74, 122.62.

HRMS (ESI+): m/z [M]+ calcd for C26H16Br2N2O4: 577.9431; found: 577.9425.


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2-Hydroxy-5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzaldehyde (3)

To a mixture of 5-bromo-2-hydroxybenzaldehyde (1 g, 5mmol), bis(pinacolato)diborane (1.4 g, 5.5 mmol), and KOAc (0.98 g,10 mmol) in a 20 mL pressure tube was added 1,4 dioxane. The reaction mixture was degassed under N2 atmosphere. Then Pd(dppf)Cl2 (180 mg, 5 mol%) was added under N2 atmosphere. The mixture was kept stirring for 12 h at 110 °C. The mixture was allowed to cool to RT and solvents were removed under reduced pressure. The residue was extracted with EtOAc (4 × 100 mL) and the combined organic extracts were washed with H2O, dried (anhyd Na2SO4) and concentrated in vacuo. The organic residue was further purified by column chromatography on silica gel with EtOAc/hexane (1:20) as an eluent to furnish 3 as a white solid product; yield: 0.8 g (64%).

1H NMR (400 MHz, CDCl3): δ = 11.21 (s, 1 H), 9.91 (s, 1 H), 8.03 (d, J = 1.7 Hz, 1 H), 7.93 (dd, J = 8.4, 1.7 Hz, 1 H), 6.96 (dd, J = 8.4, 0.6 Hz, 1 H), 1.34 (s, 12 H).

The NMR data was consistent with the literature report.[35]

HRMS (ESI+): m/z [M + H]+ calcd for C13H18BO4: 249.1293; found: 249.1301.


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4′,4′′′-(2,2-Bis(4-nitrophenyl)ethene-1,1-diyl)bis(4-hydroxy-[1,1′-biphenyl]-3-carbaldehyde) (4)

To a mixture of 2 (464 mg, 0.8 mmol), 3 (600 mg, 2.4 mmol), and K2CO3­ (221 mg, 1.6 mmol) in a pressure tube was added a mixture of THF (3 mL)/H2O (1 mL) (3:1) and reaction vessel was placed under an atmosphere of N2. Further, Pd(PPh3)4 (90 mg, 0.08 mmol, 10 mol%) was added to the reaction mixture under N2 atmosphere. The reaction mixture was stirred for 24 h at 85 °C. The mixture allowed to cool to RT and the volatiles were removed under reduced pressure. The residue was extracted with EtOAc (4 × 100 mL) and the combined organic layers were washed with H2O, dried (anhyd Na2SO4) and concentrated in vacuo. The crude was further purified by column chromatography on silica gel with EtOAc/hexane (1:10) as eluent to furnish 4 as a yellow solid; yield: 397 mg (60%); mp 95–97 °C.

IR: 1511 (N–O), 1648 (C=O), 2913 cm–1 (O–H).

1H NMR (500 MHz, CDCl3): δ = 11.02 (s, 2 H), 9.97 (s, 2 H), 8.05–8.03 (d, J = 8.7 Hz, 4 H), 7.75–7.74 (m, 4 H), 7.40–7.38 (d, J = 8.3 Hz, 4 H), 7.24–7.22 (d, J = 8.8 Hz, 4 H), 7.13–7.11 (d, J = 8.3 Hz, 4 H), 7.08–7.05 (d, J = 9.3 Hz, 2 H).

13C NMR (125 MHz, CDCl3): δ = 196.53, 161.30, 149.46, 146.57, 140.72, 138.95, 135.49, 132.10, 132.07, 131.93, 131.79, 126.33, 123.60, 120.75, 118.34, 77.28, 77.02, 76.77.


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Compound TPE-SP1

A mixture of 4 (60 mg, 0.09 mmol) and 1,2,3,3-tetramethyl-3H-indol-1-ium (56 mg, 0.18 mmol) were dissolved in pyridine (2 mL). The resulting mixture was sealed in a vessel and placed under an atmosphere of N2. The reaction mixture was kept stirring for 10 min at 120 °C. The mixture was allowed to cool to RT and extracted with EtOAc (4 × 100 mL). The combined organic extracts were washed with H2O, dried (anhyd Na2SO4) and concentrated in vacuo. Further purification was done by column chromatography on silica gel with EtOAc/hexane (1:10) as eluent to furnish TPE-SP1 as a yellow solid; yield: 61 mg (70%); mp 180–183 °C.

IR: 1511 cm–1 (N–O).

1H NMR (400 MHz, CD3CN): δ = 7.97 (d, J = 8.8 Hz, 4 H), 7.38 (d, J = 8.4 Hz, 4 H), 7.34 (d, J = 2.3 Hz, 2 H), 7.30 (dd, J = 8.4, 2.4 Hz, 2 H), 7.27–7.23 (m, 4 H), 7.11 (td, J = 7.7, 1.3 Hz, 2 H), 7.09–7.05 (m, 6 H), 6.95 (dd, J = 10.4, 0.8 Hz, 2 H), 6.78 (td, J = 7.4, 1.0 Hz, 2 H), 6.62 (d, J = 8.4 Hz, 2 H), 6.53 (dd, J = 7.8, 0.8 Hz, 2 H), 5.77 (d, J = 10.3 Hz, 2 H), 2.67 (s, 6 H), 1.23 (s, 6 H), 1.10 (s, 6 H).

13C {1H}NMR (125 MHz, CDCl3): δ = 154.52, 149.87, 148.16, 146.40, 146.28, 140.26, 140.00, 136.70, 136.31, 132.15, 131.90, 131.77, 129.31, 128.25, 127.64, 126.14, 125.09, 123.51, 121.53, 120.05, 119.23, 119.10, 115.48, 106.86, 104.59, 77.29, 77.04, 76.79, 51.84, 28.94, 20.17.

HRMS(ESI+): m/z [M + H]+ calcd for C64H53N4O6: 973.3960; found: 973.3927.


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Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

S.K. is supported by a fellowship from UGC, India, and S.C. is supported by a fellowship from DST-INSPIRE, India.

Supporting Information

    Supporting information for this article is available online at https://doi-org.accesdistant.sorbonne-universite.fr/10.1055/a-2383-0905.
  • Supporting Information

Corresponding Author

Subhajit Bandyopadhyay
Indian Institute of Science Education and Research (IISER) Kolkata
N304 RC, Mohanpur, WB 741246
India   

Publication History

Received: 29 May 2024

Accepted after revision: 08 August 2024

Accepted Manuscript online:
08 August 2024

Article published online:
04 September 2024

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Zoom Image
Scheme 1 Schematic representation of isomerization scheme of TPE-SP1
Zoom Image
Scheme 2 Synthetic scheme for TPE-SP1
Zoom Image
Figure 1 a) Isomerization of TPE-SP1 to TPE-MC1 under irradiation of 254 nm (10 μM in CHCl3, degassed with N2); b) TPE-MCH1 to TPE-SP1 reverse isomerization under 525 nm light in CHCl3.
Zoom Image
Figure 2 a) Isomerization of TPE-SP1 to TPE-MCH1 in CHCl3 acidified with TFA; b) TPE-MCH1 to TPE-SP1 reverse isomerization upon addition of base (Et3N).
Zoom Image
Figure 3 Changes in a) absorbance and b) emission of TPE-SP1(10 μM, λex = 370 nm, slit width 10/5 nm, 298 K) in various DMF/H2O solvent mixture; changes in the c) absorbance and d) emission of TPE-MCH1 (10 μM, λex = 370 nm, slit width 10/5 nm, 298 K) in various DMF/H2O solvent mixture.
Zoom Image
Figure 4 a) Quenching of emission intensity of TPE-SP1 with increasing concentration of PNP; b) of emission of TPE-MCH1 with increasing concentration of PNP; c) and d) quenching of fluorescence of TPE-SP1 and TPE-MCH1, respectively, with gradual increase in PNP concentration.