Synthesis and spectral characterization of photoswitchable oligo(p-phenylenevinylene)–spiropyran dyad

Abstract

In view of designing a new class of photoswitchable fluorescence probes and operating them in solution as well as on solid substrates, we have envisioned the possibility of attaching photochromic spiropyran (SP) to the highly efficient fluorophore oligo(p-phenylenevinylene) (OPV). A new dyad, SP–OPV–SP (10), was synthesized and characterised both in solution as well as in a film on a solid substrate, where two SP units as photochromic acceptors are attached to the two ends of OPV, a fluorescent donor. External stimulations (ultraviolet light, visible light and acid) generate reversible changes in the structure, resulting in changes in the absorption spectrum and fluorescence emission spectrum of dyad 10 due to the presence of the two spiropyran units. Photoinduced (ultraviolet light) isomerization of the spiropyran causes a 60% decrease in the emission intensity of OPV in the photostationary state in a solution of 60 μM concentration. In the solid state, ultraviolet irradiation causes a ∼98% reduction in the fluorescence intensity of OPV. The photogenerated isomer is somewhat more stable in solid state than in solution. The fluorescence intensity of dyad 10 is modulated by reversible conversion among the three states of the photochromic spiropyran units and the fluorescence resonance energy transfer (FRET) between the MC form of SP and the OPV unit. In any case, these investigations demonstrate that the design of dyad 10 is viable for the realization of photoswitchable molecular assemblies and can evolve as efficient fluorescent probes for potential applications in molecular device design, such as an integrated logic gate with multiple inputs and a single output.

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Introduction

Materials with properties that can be modulated by external inputs, such as optical excitation, thermal excitation and chemical stimulation, are of great interest in a broad range of potential applications in advanced molecular optical devices.¹ Fluorescence spectroscopy has become a very sensitive diagnostic tool, both in bulk and at the single molecule level.² It is rapidly growing as an important methodology in many biological diagnosis,³ imaging,⁴ and detection applications⁵ and in different molecular device applications, which include fluorescent switches,⁶ fluorescent sensors⁷ and other photonic devices,⁸ primarily because of its ease of use. Furthermore, the modulation of the fluorescence emission properties of a fluorescent dye molecule is being explored in biological studies to selectively highlight cells, organelles or proteins.⁹ Hence, the control or reversible modulation of the fluorescence properties of dye molecules is a fascinating field of study. In this context, photochemical isomerisation, a light-driven transformation of a photochromic compound, between two isomeric forms could lead to a process where the two forms may have their own spectroscopic identity with different absorption and emission spectra. Because of the difference in the electron distribution in two isomers, their physical and chemical properties may differ in many ways, such as the refractive index, dielectric constant, redox potential, chelation potential, absorption spectrum, fluorescence properties, and so on, and make them suitable for different practical applications. When such photochromic compounds are attached to a fluorophore, their properties can be tuned by selective photoirradiation.^10,11 Among them, 1,3-dihydro-1,3,3-trimethyl-spiro[2H-1-benzopyran-2,2-(2H)-indole] which is popularly known as spiropyran (SP) has been extensively investigated for different fluorophores, exploiting the light driven reversible interconversion between two states, the closed ring SP and open ring merocyanine (MC).¹¹ In addition to this, the protonated form of MC can provide another state (MCH) which has different characteristic absorption properties to MC and is reversibly interconverted between MCH and SP by alternate acid–base titration. To achieve photoreversible fluorescence modulation of any fluorophore, one needs to allow photoinduced electron transfer or fluorescence resonance energy transfer (FRET)¹² to occur to quench the excited states of the fluorophore. The basic requirement of FRET is the specific fluorophore must be covalently linked to the spiropyran molecule (or linked to it through a spacer) and there must be certain degree of spectral overlap between the emission spectrum of the fluorophore (donor) and the absorption spectrum of the acceptor. The three states of spiropyran SP, MC, and MCH¹³ show quite different absorption spectra, and thus it is possible to regulate the fluorescence intensity of a suitable fluorophore by irradiation of the solution containing both spiropyran and fluorescent molecule. Indeed, Raymo and his colleagues¹³ studied the “signal communication” between pyrene (naphthalene, anthracene, and tetracene) and spiropyran, and proposed the corresponding integrated logic gates and communication network. By attaching spiropyran covalently to porphyrin, Moore et al.¹⁴ studied the quenching process of the porphyrin excited states upon irradiation by ultraviolet light.

Oligo(p-phenylenevinylene)s (OPVs) are the most widely used fluorophores due to their high extinction coefficient, high fluorescence quantum yield, and the fact that their excitation wavelength lies in the visible-wavelength range. OPVs are known to be efficient energy donors to different acceptors.^15–23 The research groups of Meijer, Janssen and Würthner have extensively studied the energy transfer processes in quadruple H-bonded OPV self-assemblies^24,25 and OPV–perylene bisimide coassemblies.^26,27 The properties of self-assembled OPVs make them ideal energy donor scaffolds to suitable acceptors that facilitate FRET processes.²⁸ Organogels based on OPVs with functional groups have been the target of increasing attention in recent years because of their various potential applications as soft materials.²⁹ Although some SP-functionalized macromolecular gels have also been reported,³⁰ to the best of our knowledge, OPVs with the SP moiety still remain rare.

OPV shows emission in the range of 420–650 nm and MC, a photo-driven transformed state of SP, shows an additional absorption band in the range of 500–650 nm. Hence, the overlap between the fluorescence spectrum of OPV and the absorption spectrum of MC is significantly large. By contrast, there is almost no overlap between the fluorescence spectrum of OPV and the absorption spectra of SP and MCH. As a result, for the OPV–spiropyran system the corresponding fluorescence “on/off” ratio can be well enhanced. Furthermore, OPV shows strong absorption above 400 nm, at which SP, MC, and MCH have very weak absorption. Thus, excitation of OPV at 415 nm will not perturb any of the three states of spiropyran (SP, MC, and MCH). Keeping this view in mind, we have attempted to explore the photoswitching behaviour of SP units in the presence of the OPV unit.

Here we report the synthesis and spectral studies of the as-prepared dyad (10) in solution as well as in cast film. The ease of synthesis allows us to covalently attach two SP units to both sides of an OPV unit, making the dyad SP–OPV–SP (10) (see Schemes 1–3). The demonstrated results confirm that the fluorescence intensity of the OPV unit can be regulated by alternate application of ultraviolet light, visible light, and acid–base titration. Hence, this SP–OPV–SP dyad can potentially be used in molecular device design at the single molecule level in view of processing and communicating information. For comparison, the intermolecular communication behaviour of a mixed solution of an OPV derivative (compound 7, see Scheme 1) and a SP molecule (reference compound 11, see Scheme 2) in a molar ratio of 2 : 1 was also investigated.

Scheme 1 Synthetic route of OPV: (a) bromododecane, K₂CO₃, acetonitrile, reflux, 36 h, 85%; (b) paraformaldehyde, HBr, 60 °C, 2 h, 86%; (c) KOAc, Bu₄NBr, acetonitrile, reflux, overnight, 100%; (d) LiAlH₄, THF, room temperature, 2 h, 99%; (e) PCC, CH₂Cl₂, room temperature, 2 h, 87%; (f) PPh₃, toluene, reflux, 3 h; (g) (i) 5, LiOEt, CH₂Cl₂, room temperature, 10 min; (ii) I₂, CH₂Cl₂, room temperature, overnight, 84%; (h) NaBH₄, CH₃OH, CH₂Cl₂, room temperature, 45 min.

Scheme 2 Synthetic route of the carboxyl-containing spiropyran (SPCOOH) and reference spiropyran compound.

Experimental

Materials and instrumentation

The solvents and the reagents were purified and dried using usual methods prior to use. Dodecyl bromide, triphenyl phosphine, LiAlH₄ and NaBH₄ were purchased from Sigma-Aldrich. Paraformaldehyde, HBr in acetic acid, NaH, NaOH, potassium acetate, hydroquinone and PCC were used as received from commercial suppliers. ¹H NMR spectra were recorded on a 500 MHz spectrometer (Bruker ARX500) and ¹³C NMR spectra were recorded on a Bruker 300 MHz spectrometer at room temperature in CDCl₃. The chemical shifts are reported in ppm (d) with tetramethylsilane (TMS) as the internal standard and coupling constants (J) are expressed in Hz. FT-IR spectra were recorded on a Shimadzu IRPrestige-21 Fourier Transform Infrared Spectrophotometer. MALDI-TOF mass spectrometry was conducted on a Perspective Biosystems Voyager-DE PRO mass spectrometer, using α-cyano-4-hydroxy cinnamic acid (CHCA) as the matrix, and ESI-MS on a PerkinElmer Sciex API 3000 mass spectrometer. Reactions were monitored using thin-layer chromatography (TLC) with 0.20–0.25 mm silica gel plates. Column chromatography was performed with silica gel (60–120 and 100–200 mesh). All UV/vis spectra were recorded using a Hitachi U-2910 spectrophotometer. All steady state fluorescence spectra were recorded at room temperature using a Fluorolog-3 spectrofluorimeter of Horiba Jobin Yvon, USA.

Synthesis of OPV

1,4-Bis(dodecyloxy)benzene (1). A suspension of 1,4-hydroquinone (5 g, 45 mmol), 1-bromododecane (32.4 mL, 135 mmol), and K₂CO₃ (18.7 g, 135 mmol) in acetonitrile (200 mL) was heated at reflux for two days before being poured into water (400 mL). The precipitate was first collected by filtration and then dissolved in a minimum of hot hexane. Subsequently, the resulting hot solution was poured into methanol (200 mL) to precipitate the product. The precipitate was filtered off and dissolved again in hot hexane (100 mL). Reprecipitation of the resulting solution in methanol gave 17.0 g of pure product 1 as a white solid, after being filtered and dried under vacuum (85% yield). ¹H NMR (CDCl₃) δ (ppm) 0.87 (m, 6H, CH₃), 1.24–1.85 (m, 40H, CH₂), 3.85 (t, J = 6.40 Hz, 4H, OCH₂), 6.82 (s, 4H, aromatic).

2,5-Bis(bromomethyl)-1,4-bis(dodecyloxy)benzene (2). To a suspension of 1 (2.28 g, 5.2 mmol) and paraformaldehyde (0.33 g, 11.0 mmol) in acetic acid (25 mL) was added HBr (2.2 mL, 31 wt% in acetic acid), all at once. This mixture was then heated to 60–70 °C with stirring for 2 h. As the reaction proceeded, the suspension changed first to a clear solution and then became a thick suspension. After cooling to room temperature, this suspension was poured into water (150 mL). The precipitate was filtered and dissolved in hot chloroform. Reprecipitation of the resulting solution in methanol gave 2 (2.83 g, 86.1% yield) as a white, loose solid, after being filtered and dried under vacuum. ¹H NMR (CDCl₃) δ (ppm) 0.87 (m, 6H, CH₃), 1.24–1.85 (m, 40H, CH₂), 3.98 (t, J = 6.42 Hz, 4H, OCH₂), 4.52 (s, 4H, CH₂Br), 6.85 (s, 2H, aromatic).

2,5-Bis(acetyl methyl)-1,4-bis(dodecyloxy)benzene (3). A solution of 2 (2.2 g, 3.4 mmol), potassium acetate (1.03 g, 10.4 mmol), and tetra n-butyl ammonium bromide (0.17 g) in a mixture of acetonitrile (50 mL) and chloroform (25 mL) was heated at reflux overnight. The resulting mixture was poured into water (50 mL) and extracted with chloroform (3 × 50 mL). The extracts were washed with water (2 × 50 mL). Solvent was removed from the resultant organic solution on a rotary evaporator after drying over anhydrous sodium sulphate. This furnished product 3 (2.0 g, 100% yield). ¹H NMR (CDCl₃) δ (ppm) 0.87 (m, 6H, CH₃), 1.24–2.06 (m, 46H, CH₂), 3.94 (t, J = 6.43 Hz, 4H, OCH₂), 5.14 (s, 4H, CH₂OAc), 6.88 (s, 2H, aromatic).

2,5-Bis(hydroxymethyl)-1,4-bis(dodecyloxy)benzene (4). To a suspension of LiAlH₄ (0.45 g, 11.2 mmol) in dry THF was added a solution of 3 (1.65 g, 2.8 mmol) in dry THF (50 mL), drop-wise. The mixture was stirred at room temperature for 2 h. The excess of LiAlH₄ was quenched by addition of ethyl acetate at 0 °C. The resulting suspension was poured into water, followed by extraction with chloroform (75 mL). The extracts were combined and washed with water (100 mL). Removal of solvent under reduced pressure on a rotary evaporator furnished a white solid. After drying under vacuum, 1.4 g (99.1% yield) of 4 was obtained. ¹H NMR (CDCl₃ + CD₃OD) δ (ppm) 0.87 (m, 6H, CH₃), 1.24–1.85 (m, 40H, CH₂), 3.97 (t, J = 6.31 Hz, 4H, OCH₂), 4.67 (s, 4H, CH₂OH), 6.92 (s, 2H, aromatic).

2,5-Bis(dodecyloxy)benzene-1,4-dialdehyde (5). A suspension of 4 (1.4 g, 2.65 mmol) and pyridinium chlorochromate (PCC) (2.3 g, 10.6 mmol) in methylene chloride (100 mL) was stirred at room temperature for 2 h. The reaction mixture was then directly transferred onto the top of a short silica gel column. The highly fluorescent product 5 was then washed off the column with chloroform. Thus, compound 5 was obtained in 85.8% yield (1.14 g). ¹H NMR (CDCl₃) δ (ppm) 0.87 (m, 6H, CH₃), 1.24–1.85 (m, 40H, CH₂), 4.08 (t, J = 6.60 Hz, 4H, OCH₂), 7.43 (s, 2H, aromatic), 10.52 (s, 2H, CHO).

2,5-Bis(dodecyloxy)-1,4-bis[(2,5-didecoxy-4-formyl)phenylenevinylene]-benzene (6). A suspension of 2 (0.63 g, 1.0 mmol) and triphenylphosphine (0.55 g, 2.1 mmol) in toluene was heated at reflux for 3 h. The solvent was then removed from the resulting clear solution under reduced pressure. The resulting residue, along with dialdehyde 5 (1.0 g, 2.0 mmol), was dissolved in methylene chloride (50 mL). Lithium ethoxide solution (2.5 mL, 1.0 M in ethanol) was added to this solution drop-wise at room temperature via a syringe. The base should be introduced at such a rate that the transient red-purple colour produced upon the addition of base does not persist. The resulting solution was allowed to stir for 10 min more after the completion of the base addition. This solution was then poured into dilute aqueous HCl. The organic layer was separated, washed with water, and dried over anhydrous sodium sulphate. The residue, after removal of solvent, contained both E- and Z-isomers. A solution of this isomeric mixture and iodine (500 mg) in methylene chloride (50 mL) was stirred at room temperature overnight. The dark brown solution was then diluted with methylene chloride and washed consecutively with aqueous Na₂S₂O₃ solution (1.0 M, 2 × 75 mL) and water. After being concentrated on a rotary evaporator, this solution was loaded onto a silica gel column and eluted with a mixture of hexane and chloroform (1 : 1 v/v). This afforded 1.21 g (83.9%) of compound 6 as a yellow fluorescent solid. ¹H NMR (CDCl₃) δ (ppm) 0.87 (m, 18H, CH₃), 1.24–1.85 (m, 120H, CH₂), 4.02–4.12 (m, 12H, OCH₂), 7.15 (s, 2H, central phenyl-H), 7.20 (s, 2H, aromatic H3,H3′), 7.33 (s, 2H, aromatic H2,H2′), 7.49 (d, 2H, J = 16.56 Hz, vinyl-H), 7.58 (d, 2H, J = 16.53 Hz, vinyl-H),10.45 (s, 2H, CHO).

2,5-Bis(dodecyloxy)-1,4-bis[(2,5-didodecyloxy-4-hydroxymethyl)phenylenevinylene]-benzene (7). The bis-aldehyde 6 (0.29 g, 0.2 mmol) was dissolved in a mixture of methanol (10 mL) and dichloromethane (25 mL). To this, sodium borohydride (15 mg, 0.4 mmol) was added and stirred at room temperature for 45 minutes. The reaction mixture was poured into water and extracted with dichloromethane. The organic layer was concentrated to give the corresponding alcohol. This afforded 0.25 g (87%) of compound 7 (ref. 31) as a yellow fluorescent solid. ¹H NMR (CDCl₃) δ (ppm) 0.87 (m, 18H, CH₃), 1.25–1.83 (m, 120H, CH₂), 2.4 (s, 2H, OH), 3.9–4.0 (m, 12H, OCH₂), 4.67–4.69 (s, 4H, CH₂OH), 6.86 (s, 2H, aromatic), 7.12 (s, 2H, aromatic), 7.14 (s, 4H, aromatic), 7.40–7.45 (d, J = 16.45, 2H, vinylic), 7.46–7.51 (d, J = 16.39, 2H, vinylic).

Synthesis of SP

l-(β-Carboxyethyl)-2,3,3-trimethylindolenine iodide (8). A mixture of 2,3,3-trimethylindolenine (2.5 g, 15.7 mmol) and 3-iodopropanoic acid (3.14 g, 15.7 mmol) was dissolved in toluene (5 mL) and heated under nitrogen at 100 °C for 3 h. The resulting solution was evaporated; the remaining product was dissolved in water (100 mL) and washed with chloroform (50 mL) 3 times. Evaporation of water gave product 8 (4.07 g, 72%) as a red oil. ¹H NMR (DMSO-d₆, 400 MHz): δ 1.53 (s, 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), δ 7.97–8.0 (dd, 1H).

l-(β-Carboxyethyl)-3′,3′-dimethyl-6-nitrospiro-(indoline-2′,2[2H-1]benzopyran) (9). The product 8 (2.52 g, 7 mmol), 5-nitrosalicylaldehyde (1.16 g, 7 mmol), and piperidine (0.76 mL, 0.7 mmol) were dissolved in anhydrous ethanol (50 mL). The mixture was refluxed for 5 h. The resultant dark purple mixture was cooled in an ice bath and filtered, and the filter cake was washed with cold ethanol. The precipitate was recrystallized from ethanol and dried under vacuum to yield 9 (ref. 32) (1.71 g, 65%). ¹H NMR (DMSO-d₆, 400 MHz): δ 1.07 (s, 3H), δ 1.19 (s, 3H), δ 2.45–2.57 (t, 2H), δ 3.34–3.40 (t, 2H), 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, 1H), δ 7.19–7.22 (d, 1H), δ 7.98–8.21 (dd, 1H), δ 8.21 (s, 1H).

Synthesis of SP–OPV–SP (10)

DCC (29 mg, 0.14 mmol) was added to a solution of 9 (35 mg, 0.09 mmol), 7 (202 mg, 0.14 mmol) and DMAP (2 mg, 0.01 mmol) in dry CH₂Cl₂ (100 mL) with the temperature 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 [SiO₂: hexane–CHCl₃ (1 : 1 v/v)] to afford 10 (217 mg, 70%). ¹H NMR (500 MHz, CDCl₃): δ 8.02–7.95 (m, 4H), 7.45 (s, 4H), 7.2–7.06 (m, 8H), 6.92–6.81 (m, 6H), 6.72 (d, J = 8.8 Hz, 2H), 6.62 (d, J = 7.7 Hz, 2H), 5.82 (d, J = 10.3 Hz, 2H), 5.09 (s, 4H), 3.86–4.10 (m, 12H, OCH₂), 3.48–3.72 (m, 4H, CH₂N), 2.59–2.81 (m, 4H, CH₂CO), 1.09–1.91 (m, 132H, CH₂ & CH₃), 0.79–0.91 (m, 18H, CH₃); ¹³C NMR (300 MHz, CDCl₃): δ 171.8, 159.4, 151.1, 150.4, 146.3, 141.0, 136.0, 128.3, 127.8, 127.3, 127.0, 125.8, 123.2, 122.7, 121.9, 119.8, 118.6, 115.5, 115.1, 110.6, 109.5, 106.8, 69.6, 69.4, 62.0, 52.9, 39.4, 33.7, 33.4, 31.9, 29.7, 29.5, 29.4, 25.7, 22.7, 19.7, 14.1. FT-IR (KBr) ν_max = 748, 806, 851, 963, 1022, 1089, 1123, 1206, 1269, 1339, 1383, 1421, 1485, 1509, 1579, 1610, 1737, 2852, 2924, 3057, 3447 cm⁻¹.

Reference compound SP (11). DCC (97 mg, 0.47 mmol) was added to a solution of 9 (180 mg, 0.47 mmol), methanol (1 mL, 25 mmol) and DMAP (3.8 mg, 0.03 mmol) in dry CH₂Cl₂ (100 mL) with the temperature 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 : 1 v/v)] to afford 11 (ref. 33) (130 mg, 70%). ¹H NMR (500 MHz, CDCl₃): δ 1.07 (s, 3H), δ 1.19 (s, 3H), δ 2.45–2.57 (t, 2H), δ 3.34–3.40 (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, 1H), δ 7.19–7.22 (d, 1H), δ 7.98–8.21 (dd, 1H), δ 8.21 (s, 1H).

Results and discussion

The synthesis of SP-functionalized OPV (10) is shown in Scheme 3. Compounds 7 and 9 were prepared according to the sequences shown in Schemes 1 and 2. An acid–alcohol coupling between 7 and 9 in the presence of DCC and DMAP led to the SP–OPV–SP dyad (10) in 50% yield.

Scheme 3 Synthetic route of SP–OPV–SP from SPCOOH and OPV.

Absorption spectra

The as-prepared reference spiropyran compound SP (11) shows the typical reversible interconversion between the corresponding SP, MC, and MCH states upon irradiation with ultraviolet (350 nm) and visible light (580 nm) and addition of acid (ESI, Fig. 1S†).^13a Similar phenomena were observed for the mixed solution of reference compounds 11 and 7 (in a molar ratio of 2 : 1), but no change in the absorption spectrum of OPV (7) was observed.

Fig. 1 shows the absorption spectra of OPV (7), SP (11) and the SP–OPV–SP (10) dyad in ACN. The absorption bands of 10 are essentially identical to those observed for OPV (7) and reference SP (11). Thus, the absorption spectrum of the dyad SP–OPV–SP is a superposition of the spectra of the OPV and SP chromophores. However, a red shift of around 60 nm of the OPV absorption peak at 410 nm is observed in dyad 10, which may be due to the enhanced conjugation of the OPV moiety after addition of the SP units. However, after the ACN solution of dyad 10 was irradiated with ultraviolet light at 350 nm (150 W xenon lamp) for 10 minutes, the yellow-greenish ACN solution of 10 changed to a blue-greenish solution. The characteristic absorption band of the MC form of SP with an absorption maximum at 580 nm emerged, but the main absorption bands of the OPV unit remain unchanged (Fig. 2A, curve b). In comparison to the absorption spectrum of SP (11) upon ultraviolet light irradiation (ESI, Fig. 1S†), it is confirmed that the formation of the corresponding MC form of SP in dyad 10 under the same conditions has occurred. This photogenerated isomer reverts to the original one in two ways: (i) upon 580 nm irradiation for less than 2 minutes and (ii) upon room temperature thermal isomerisation for ∼5–6 minutes. Upon addition of 2 equivalents of CF₃COOH to the solution of 10 immediately after 350 nm light irradiation, the absorption band at 580 nm disappeared (Fig. 2A, curve c) due to the complete transformation of MC to MCH (Scheme 4).

Fig. 1 Absorption spectra of (a) SP (11), (b) OPV (7) and (c) SP–OPV–SP (10) in ACN solution (∼10⁻⁵ M) at room temperature.

Fig. 2 Normalized absorption spectra of SP–OPV–SP (10), (A) in ACN solution (∼10⁻⁵ M) and (B) as a cast film on a quartz plate at room temperature. (a) Black curve, before irradiation with UV light, (b) magenta curve, after irradiation with UV light (350 nm), (c) orange curve, after irradiation with visible light (580 nm) and (d) blue curve, addition of 2 equivalents of CF₃COOH immediately after UV irradiation. The inset in (A) shows the enlarged view of the absorbance in the visible range.

Scheme 4 Three switching states of the reference SP (11) compound.

In order to check the photoinduced reversible interconversion of 10 in the solid state, we have prepared a cast film of 10 on a quartz plate from a (∼5 mM) DCM solution. The surface morphology of the cast film of 10 shows good network structure, indicating the formation of a gel (ESI, Fig. 3S†), like the reference OPV (7).³⁴ The absorption spectra of the as-prepared cast film of 10 were very similar to those observed in solution (Fig. 2B) with a little red shift in the OPV absorption peak. The colour of the film was greenish-yellow. However, as shown in Fig. 3, the colour of the film turned to dark blue after the film was exposed to 350 nm (150 xenon lamp) light for just 10 seconds. Consequently, the absorption spectrum of this film shows a prominent peak at 570 nm corresponding to the MC form of 10 (curve b, Fig. 2B). This dark blue coloured film turned back towards the original colour upon irradiation with 570 nm light for 10 minutes. It is important to mention here that, unlike in solution, the photogenerated isomer (MC form) of 10 in the film thermally reverts very slowly and it takes more than ∼120 minutes to complete 80% conversion. However, this cycle of photoinduced reversible colour change was repeated several times without degrading the sample colour or the optical density. Upon exposure to CF₃COOH vapour the colour of the dark blue film of the MC form of 10 turns to yellow (Fig. 3) and the corresponding absorption spectrum is almost identical to the absorption spectrum of 10 (curve d, Fig. 2B).

Fig. 3 Photograph of the cast film of 10 prepared from DCM solution (∼10 mM), (a) before 350 nm light irradiation, (b) after 350 nm light irradiation for 10 seconds, (c) after 570 nm light irradiation and (d) after CF₃COOH vapour exposure.

Fluorescence spectra

To ascertain if there is any self quenching of the fluorescence emission of dyad 10, we performed concentration as well as excitation wavelength dependent emission studies of 10 in ACN solution, but no self quenching was observed until 100 μM concentration (ESI, Fig. 6S–8S†). Fig. 4 shows the fluorescence spectra of the ACN solution (60 μM) of dyad 10 under different experimental conditions with an excitation wavelength of 415 nm. Before exposure to ultraviolet light, dyad 10 showed a broad emission band in the range of 425–650 nm with the maximum around 460 nm (curve a, Fig. 4A). This fluorescence spectrum is quite similar to the fluorescence emission spectrum of reference OPV (7) in terms of the fluorescence intensity as well as the fluorescence emission spectral structure (ESI, Fig. 4S†). A red shift in the emission maximum of dyad 10 is observed with respect to the emission peak of reference OPV (7) (ESI, Fig. 4S†). However, upon irradiation with ultraviolet light (350 nm) for 10 minutes, the intensity of the fluorescence band around 460 nm decreased to 60% of that of the initial solution (curve b, Fig. 4A) and a very weak new fluorescence band peaking at around 630–640 nm appeared. It is important to note that in ACN solution the MC form of SP (11) shows fluorescence emission peaking at 650 nm (ESI, Fig. 1S†). Upon irradiation with ultraviolet light, the SP moieties of dyad 10 transform into the MC conformer, resulting in the formation of MC–OPV–MC (Scheme 5) which quenches the excited state of the OPV unit through fluorescence resonance energy transfer. Energy transfer from the excited OPV state to MC produces the excited MC state, which should show a new emission band at around 650 nm. However, in the present case we do observe such a new fluorescence band relating to the fluorescence of the MC moiety of MC–OPV–MC, but the intensity of this band is too low to be resolved with high fidelity. Furthermore, the same ACN solution of MC–OPV–MC shows a very weak fluorescence emission above 600 nm upon excitation at 580 nm, the absorption peak of MC moiety in MC–OPV–MC. This weak fluorescence of the MC form of dyad 10 could be more advantageous in using this compound in fluorescence switching applications with the enhanced contrast between the fluorescence of the “on/off” modes. However, after irradiation of the MC absorption peak with 580 nm light for 2 minutes, MC–OPV–MC completely changed to the SP–OPV–SP form and the fluorescence intensity was restored to its initial value without distortion of the fluorescence band shape (curve c, Fig. 4A). These fluorescence “on/off” states are repeated by alternate application of 350 and 580 nm light for several cycles without degrading the compound (Fig. 4A). This result confirms the light-driven transformation between two states of dyad 10 is fatigue resistant in the solution phase.

Fig. 4 Fluorescence emission spectra of SP–OPV–SP (10), (A) in ACN solution (∼60 μM) and (B) as a film on quartz plate at room temperature excited at 415 nm. (a) Black curve, before irradiation with UV light, (b) magenta curve, after irradiation with UV light (350 nm) for 10 min in ACN solution and for 10 second in the film, (c) orange curve, after visible light irradiation at 580 nm for 2 min in ACN solution and 570 nm for 10 min in the film and (d) blue curve, addition of 2 equivalents of CF₃COOH immediately after UV irradiation. The inset 1 in (A) shows the enlarged view of the tail end of the emission spectra where emission from the MC form of SP was visible, and inset 2 in (A) and inset 1 in (B) show the reversible modulation of the fluorescence intensity of the OPV moiety at 470 nm of dyad SP–OPV–SP (10) in ACN solution and in cast film, respectively.

Scheme 5 Reversible fluorescence switching cycle of dyad 10 under different external stimuli: ultraviolet light (350 nm), visible light (580 nm) and acid.

In order to explore the role of the third state, the protonated form of MC, in the fluorescence modulation, we added 2 equivalents of CF₃COOH to the solution of dyad 10 immediately after UV light irradiation. After addition of the acid, the fluorescence intensity of the solution was restored to 70% to its initial value (curve d, Fig. 4A) without any deformation of the spectral shape. Hence, the efficiency of the fluorescence modulation in the case of treatment with ultraviolet light irradiation followed by addition of acid is bit lower than the treatment with alternate UV/vis light. The reason behind this reduction in fluorescence modulation could be three fold: (a) the conversion of MC to MCH is not complete and there may be equilibrium between MC and MCH, (b) MCH–OPV–MCH could show FRET and (c) MCH–OPV–MCH has higher non-radiative transition than SP–OPV–SP. Since the absorption spectrum of MCH–OPV–MCH does not show any trace of absorption in the range of 500–650 nm (Fig. 2A, curve d), the first two reasons cannot account for the reduction in fluorescence modulation by the MCH form. Hence, it can safely be attributed to the fact that MCH–OPV–MCH has a lower fluorescence yield than SP–OPV–SP.

Similar sets of experiments were performed for the mixtures of reference compounds SP (7) and OPV (11) (in a molar ratio of 2 : 1) in order to compare the difference between intra-and intermolecular effects in the fluorescence switching behaviour. Before ultraviolet light irradiation, the solution showed a broad and featureless emission band with the maximum around 420 nm. After irradiation of the solution at 350 nm for 15 min, the fluorescence intensity was reduced by about 13%, which should be due to the quenching of the excited state of OPV (11) by the corresponding MC form generated from SP (7) upon ultraviolet light irradiation. The fluorescence intensity of the solution returned to its initial value upon irradiation with visible light. Similarly, UV irradiation followed by addition of 2 equivalents CF₃COOH restored the fluorescence intensity back to its initial value. As compared to dyad SP–OPV–SP (10), the modulation of the fluorescence intensity for the mixed solution of 7 and 11 is significantly less. As per Förster theory,³⁵ the energy transfer efficiency is strongly dependent on the donor–acceptor distance. The donor and acceptor units in dyad SP–OPV–SP (10) are much closer to each other than those in the case of the mixed solution of 7 and 11 (intermolecular). Thus, the intermolecular energy transfer is not as effective as for the intramolecular case.

Sequentially, similar types of experiments were performed with the as-prepared cast film of dyad 10. As shown in Fig. 4B, the fluorescence emission spectrum of the film of 10 is red shifted by 60 nm compared to that in ACN solution, peaking at 520 nm (curve a, Fig. 4B). A weak fluorescence peak is also observed at 750 nm. Excluding this tail end band, the fluorescence spectrum of dyad SP–OPV–SP (10) resemblances the typical OPV fluorescence characteristics in film form³⁶ (see ESI, Fig. 4S†). Hence, the OPV moiety retains its identity in the SP–OPV–SP (10) dyad. In other words, attachment of the SP moiety to OPV does not alter the physical properties of OPV. However, after irradiation with 350 nm light for just 10 seconds, the fluorescence intensity of the OPV moiety of dyad 10 at around 520 nm was quenched by 99% of its initial value and a strong new band appeared, peaking at 750 nm (curve b, Fig. 4B). This new fluorescence emission band strongly resembles the fluorescence emission spectrum of the MC form of SP (11) in the solid state as a film (ESI, Fig. 2S†). This result confirms the efficient FRET between the OPV and MC moieties. It is also important to note here that the weak appearance of this 750 nm band, even before UV irradiation of the SP–OPV–SP film, suggests that 415 nm excitation (for collection of the fluorescence emission) converts some SP moieties of SP–OPV–SP to the MC form. To check the photo-reversibility, the film was irradiated with 570 nm light for 10 minutes and the fluorescence of the OPV moiety came back to 30% of its initial value, whereas fluorescence corresponding to the MC moiety disappeared completely (curve c, Fig. 4B). This cycle was repeated several times and no remarkable degradation of the compound was observed (Fig. 4B). At this point, it is not clear why the fluorescence of the OPV moiety did not retain its initial value after the first cycle of 570 nm irradiation, whereas the fluorescence of the MC moiety vanished totally. To resolve this issue, systematic studies with different parameters of film preparation are essential and are planned soon. However, after exposure of the film to CF₃COOH vapour just after UV irradiation, the OPV fluorescence intensity was restored to 30% of its initial value along with concomitant decrease in the MC fluorescence at around 750 nm (curve d, Fig. 4B). It is important to mention here that the film of 10 was removed from the fluoremeter and replaced back after undergoing CF₃COOH vapour exposure. In the process, the excitation was not performed exactly at the same time as before to monitor the fluorescence. Since the thickness of the cast film was not precisely uniform, quantitative analysis of the fluorescence intensity was hampered. Moreover, the top surface of the film is exposed to acid vapour while bottom surface may not be exposed to acid vapour properly. As a result, the colour of the acid vapour-exposed film is not exactly the same as that of the unexposed one (Fig. 3). However, qualitatively it can be assured that the film of dyad 10 on the quartz plate attained the three states of the SP moiety upon alternate application of UV light, visible light and acid. Furthermore, for all of the above studies, the observed results are qualitative in nature and the quantitative estimations were not performed. During the collection of the fluorescence spectra, irradiation was not carried out. 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 the SP form is very fast. Finally, since SP, MC and OPV absorb at 350 nm, irradiation at this wavelength creates only a photostationary equilibrium between two forms (SP and MC) of the spiropyran units, rather than complete conversion of the system to one form.

The above fluorescence modulation in both solution and film observed for dyad SP–OPV–SP (10) can be rationalized with the switching cycles starting and ending with SP–OPV–SP (10) as shown in Scheme 5. There should exist several states of dyad SP–OPV–SP (10) 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, we consider three ultimate states (Scheme 5) to clarify the mechanism. Upon irradiation with ultraviolet light, the SP moieties in dyad 10 are transformed to MC moieties, forming MC–OPV–MC, which quenches the excited state of the OPV moiety and the fluorescence intensity of the OPV moiety is reduced to about 50–60% of the initial value. Upon addition of 2 equiv. of CF₃COOH (in solution), the MC moieties are converted to the MCH moiety and form MCH–OPV–MCH, whose absorption spectrum is almost similar to that of SP–OPV–SP (10) and has no overlap with the fluorescence spectrum of the OPV moiety, a requirement for the FRET process to occur. Thus, no energy transfer occurs and the fluorescence intensity of the OPV moiety returns. On the other hand, upon irradiation of the solution of MC–OPV–MC with visible light, complete conversion from MC–OPV–MC to SP–OPV–SP occurs (in solution) and the fluorescence intensity returns to its initial value. Thus, alternate application of ultraviolet light, visible light, and acid regulates explicitly the fluorescence intensity of the OPV moiety. This external stimulation-dependent fluorescence spectrum of 10 can be exploited to design integrated logic circuits on the molecular level.

Conclusion

In summary, we have successfully synthesized a new dyad, SP–OPV–SP, and characterized its spectral properties both in solution and in solid state. The presence of SP units controls the fluorescence switching ‘on/off’ states of the OPV moiety by way of reversible interconversion among the three different states of the photochromic spiropyran moiety and fluorescence resonance energy transfer (FRET) between the MC and OPV moieties. These results may have implications for the invention of efficient fluorescent probes for potential applications towards molecular device design, such as an integrated logic gate with multiple inputs and a single output.

Acknowledgements

S.D. and B.R.K. thank the CSIR and UGC for providing fellowships, respectively. PRB acknowledges the support from the CSIR Network project INTELCOAT, CSC-0114.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: UV/vis spectra, fluorescence spectra of reference compounds, SEM image of cast film and relevant fluorescence spectral data along with ¹H NMR & ¹³C NMR data. See DOI: 10.1039/c5ra06628j

‡ Academy of Scientific and Innovative Research (AcSIR), New Delhi.

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