Photolockable Ratiometric Viscosity Sensitivity of Cyclodextrin

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Langmuir 2009, 25, 3482-3486

Photolockable Ratiometric Viscosity Sensitivity of Cyclodextrin Polypseudorotaxane with Light-Active Rotor Graft Liang-Liang Zhu,† Xin Li,†,‡ Feng-Yuan Ji,† Xiang Ma,† Qiao-Chun Wang,† and He Tian*,† Key Laboratory for AdVanced Materials and Institute of Fine Chemicals, East China UniVersity of Science & Technology, Shanghai 200237, P. R. China, and Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, S-10691 Stockholm, Sweden ReceiVed December 23, 2008. ReVised Manuscript ReceiVed January 16, 2009 A prototype, based on light-active fluorescent rotor grafted to β-cyclodextrin, shows a good solvent viscositysensitive behavior due to the environment-dependent nonradiative decay. With the reversible photoisomerization of the cyanostilbene unit, the viscosity sensitivity of the molecular rotor could be locked and activated, and the two switchable states can be distinguished by fluorescent signals. This cyclodextrin derivative was threaded to form a novel polypseudorotaxane. Such supramolecular assembly displays a lockable ratiometric fluorescent viscosity sensitivity with two emission channels: one aroused by fluorophore’s intramolecular excimer without influenced by viscosity is used to gauge the concentration of the compound, while the other corresponding to the monomer’s rotor fluorescence acts as a viscosity-sensitive signal and it can be shut off by UV irradiation.

Introduction Viscosity, a typical rheological property of a fluid, plays an important role in all biological systems from the cellular to the systemic level. Usually, changes in fluid viscosity are linked to the perturbations and diseases in organisms.1 To date, besides the methods to measure the bulk macroscopic viscosity with mechanical devices, viscosity chemsensors or indicators2 reflecting local microscopic viscosity are also demonstrated. Numerous types of these molecular probes are based on molecular-rotor architectures,3 in which the nonradiative decay of the fluorescent excited state can be influenced by the viscosity of the medium, for measurements of local viscosity using the change of fluorescence quantum yield. Hitherto, although these promising molecular-scale viscometers have been displaying an increasingly greater potential for monitoring of biofluid viscosity or clinical diagnosis, it remains a challenge to control the viscosity-sensitive behavior of the molecular rotors using photostimuli, for spatial and temporal resolution detections in nanoscale spaces or in vivo. The conception of input-responsive molecular switches has been widely accepted.4 It is common to use subunits such as azobenzene or stilbene, which can reversibly undergo photoi* Corresponding author: e-mail [email protected]; fax (+86) 21-64252288. † East China University of Science & Technology. ‡ Royal Institute of Technology. (1) (a) Luby-Phelps, K. Int. ReV. Cytol. 2000, 192, 189–221. (b) Moriarty, P. M.; Gibson, C. A. CardioVasc. ReV. Rep. 2003, 24, 321–325. (c) Stutts, M. J.; Canessa, C. M.; Olsen, J. C.; Hamrick, M.; Cohn, J. A.; Rossier, B. C.; Boucher, R. C. Science 1995, 269, 847–850. (2) (a) Kuimova, M. K.; Yahioglu, G.; Levitt, J. A.; Suhling, K. J. Am. Chem. Soc. 2008, 130, 6672–6673. (b) Haidekker, M. A.; Brady, T. P.; Lichlyter, D.; Theodorakis, E. A. Bioorg. Chem. 2005, 33, 415–425. (c) Ghiggino, K. P.; Hutchison, J. A.; Langford, S. J.; Latter, M. J.; Lee, M. A. P.; Lowenstern, P. R.; Scholes, C.; Takezaki, M.; Wilman, B. E. AdV. Funct. Mater. 2007, 17, 805–813. (3) Haidekker, M. A.; Theodorakis, E. A. Org. Biomol. Chem. 2007, 5, 1669– 1678. (4) (a) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew. Chem., Int. Ed. 2000, 39, 3348–3391. (b) Tian, H.; Wang, Q. C. Chem. Soc. ReV. 2006, 35, 361–374. (c) Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew. Chem., Int. Ed. 2007, 46, 72–191. (d) Katz, E.; Lioubashevsky, O.; Willner, I. J. Am. Chem. Soc. 2004, 126, 15520–15532. (e) Kottas, G. S.; Clarke, L. I.; Horinek, D.; Michl, J. Chem. ReV. 2005, 105, 1281–1376. (f) de Silva, A. P.; Uchiyama, S. Nat. Nanotechnol. 2007, 2, 399–410.

somerization upon UV or visible irradiation, to construct different switches with versatile performances.5 In this work, we try to combine this kind of light-active unit with the viscosity-sensitive molecular rotor to expect that the change of the fluorophore’s nonradiative decay induced by environmental viscosity could be influenced by the photoisomerization process and to realize the control (locking and activating) of microscopic viscosity response at the molecular level. In this way, a prototype of lightactive fluorescent rotor with cyanostilbene and tetramethyljulolidine unit is designed. Cyclodextrins (CDs) and their assemblies6 continue to be attractive candidates for artificial biomolecular devices due to their high hydrophilicity, low toxicity, and selective recognition toward many model substrates. To expand the applications in biological or pharmic researches, this novel switchable fluorophore was grafted onto β-cyclodextrin (β-CD), followed by transformation of the monomer into a new cyclodextrin-based polypseudorotaxane, which reveals a good ratiometric fluorescent viscosity sensitivity and can be locked by UV irradiation.

Experimental Section Instruments. 1H NMR, 13C NMR, and 2D-ROESY NMR spectra were measured on a Bru¨ker AV-400, or AV-500 spectrometer with tetramethylsilane (TMS) as internal standard. The high-resolution mass spectrum (ESI) was tested on a HP5989 mass spectrometer. (5) (a) Qu, D. H.; Wang, Q. C.; Ren, J.; Tian, H. Org. Lett. 2004, 6, 2085– 2088. (b) Browne, W. R.; Pollard, M. M.; de Lange, B.; Meetsma, A.; Feringa, B. L. J. Am. Chem. Soc. 2006, 128, 12412–12413. (c) Ma, N.; Wang, Y.; Wang, Z.; Zhang, X. Langmuir 2006, 22, 3906–3909. (d) Wang, Q. C.; Qu, D. H.; Ren, J.; Chen, K.-C.; Tian, H. Angew. Chem., Int. Ed. 2004, 43, 2661–2665. (e) Wang, Y.; Ma, N.; Wang, Z.; Zhang, X. Angew. Chem., Int. Ed. 2007, 46, 2823–2826. (f) Orihara, Y.; Matsumura, A.; Saito, Y.; Ogawa, N.; Saji, T.; Yamaguchi, A.; Sakai, H.; Abe, M. Langmuir 2001, 17, 6072–6076. (6) (a) Wenz, G.; Han, B. H.; Mu¨ller, A. Chem. ReV. 2006, 106, 782–817. (b) Klotz, E. J. F.; Claridge, T. D. W.; Anderson, H. L. J. Am. Chem. Soc. 2006, 128, 15374–15375. (c) Zhu, L. L.; Ma, X.; Ji, F. Y.; Wang, Q. C.; Tian, H. Chem.sEur. J. 2007, 13, 9216–9222. (d) Liu, Y.; Ke, C.-F.; Zhang, H.-Y.; Wu, W.-J.; Shi, J. J. Org. Chem. 2007, 72, 280–283. (e) Murakami, H.; Kawabuchi, A.; Matsumoto, R.; Ido, T.; Nakashima, N. J. Am. Chem. Soc. 2005, 127, 15891–15899. (f) Ma, X.; Wang, Q. C.; Qu, D. H.; Xu, Y.; Ji, F. Y.; Tian, H. AdV. Funct. Mater. 2007, 17, 829–837. (g) Liu, Y.; Zhao, Y.-L.; Zhang, H.-Y. Langmuir 2006, 22, 3434– 3438. (h) Choi, S.-H.; Jung, H.-H.; Kim, J.-I.; Furusho, H.; Geckeler, K. E. Macromol. Rapid Commun. 2008, 29, 1279–1286.

10.1021/la8042457 CCC: $40.75  2009 American Chemical Society Published on Web 02/09/2009

Cyclodextrin Polypseudorotaxane Absorption spectra were done on a Varian Cary 500 UV/vis spectrophotometer (1 cm quartz cell used). Fluorescent spectra were recorded on a Varian Cary Eclipse fluorescence spectrophotometer. All the fluorescent spectra recorded in this work were on an excitation at 370 nm. The ICD spectra were recorded on a Jasco J-815 CD spectrophotometer in a 1 cm quartz cell. The photoirradiation was carried on a CHF-XM 500-W high-pressure mercury lamp with a filter for 254 nm in a sealed Ar-saturated 1 cm quartz cell. The distance between the lamp and the sample cell was 20 cm. Melting points were determined by using an X-6 micromelting point apparatus. The transmission electric microscopy (TEM) was tested on a JEOLJEM2100F electron microscope. Solvent viscosities were carried on a NDJ-79 rotatory viscometer. Materials. 2-(4-Hydroxyphenyl)acetonitrile, β-cyclodextrin (βCD), hydroquinone, phosphorus oxychloride, and p-toluenesulfonyl chloride were commercially available and used without further purification. 3-Methyl-2-buten-1-ol, 1,8-diazabicyclo[5.4.0]undec7-ene (DBU), and R,ω-diaminopoly(propylene glycol)s (PPG, average molecular weight of 2000) were purchased from Alfa Aesar and used as received. Dextran (70a average MW) was purchased from Tokyo Chemical Industry Corp. Pyridine was dried over potassium hydroxide while DMF and dichloromethane were dried over calcium hydride and then distilled under reduced pressure. THF was refluxed over sodium particles and distilled before use. Preparation of RCD. To a solution of DMF (10 mL) containing mono-(6-O-p-toluenesulfonyl)-β-cyclodextrin D (0.5 g, 0.338 mmol) and potassium carbonate (0.1 g, 0.725 mmol) was added fluorescent dye A3 (0.175 g, 0.470 mmol). The resultant mixture was stirred at 90 °C for 4 days under argon. The solution was poured into 60 mL of THF, and the precipitate was collected by filtration to give a yellow powder. The crude product was washed with cold deionized water (20 mL) and neutralized with dilute HCl solution and then applied to silica gel chromatography (n-butanol:ethanol:water ) 5:4:3) to give a pure sample (0.171 g, 29.3%); mp >250 °C. 1H NMR (400 MHz, DMSO-d6, 25 °C, TMS): δ ) 7.78 (d, J ) 8.8 Hz, 2H), 7.61 (s, 1H), 7.54 (d, J ) 8.8 Hz, 2H), 7.01 (d, J ) 8.8 Hz, 2H), 6.76 (d, J ) 8.6 Hz, 2H), 5.71-5.84 (m, 14H), 4.80-4.90 (m, 7H), 4.44-4.56 (m, 5H), 4.20 (m, 3H), 3.97 (m, 2H), 3.15-3.75 (m), 1.69 (t, J ) 6.4 Hz, 4H), 1.24 (s, 12H). HRMS (ESI): m/z: 1511.5637 [RCD + Na]+. Preparation of Polypseudorotaxane PRCD. In a solution of RCD (80 mg of monomer dissolved in 8 mL of DMF was added dropwise into 40 mL of water), PPG 2000 (25 mg) was added at room temperature. The mixture was put in a supersonic bath for 1 h and then stirred for another 48 h. The formed precipitate was collected by centrifugation, washed with THF and water, and then dried in vacuo to give PRCD. 1H NMR (400 MHz, 0.2% NaOD-D2O, 25 °C, TMS): δ ) 6.61-7.74 (m), 2.69-4.15 (m), 1.20-1.51 (m), 0.85-1.15 (m). Elemental analysis calcd (the number of RCD used for calculation was estimated by 1H NMR) for (C67H96N2O35)7 (C102H208N2O33) (H2O)50: C 51.50, H 7.36, N 1.68. Found: C 51.53, H 7.20, N 1.62.

Results and Discussion Preparation and Characterization of β-Cyclodextrin Derivative RCD and Polypseudorotaxane PRCD. In the present work, we first prepared the monomer β-CD derivative RCD, which comprises a D-π-A fluorescent molecular rotor structure linked covalently to the 6-position of β-CD (Scheme 1). It was prepared by coupling mono-(6-O-p-toluenesulfonyl)-β-CD (D) with fluorescent dye (A3) in alkaline DMF and purified by silica gel chromatography with a mixed-solvent eluent (n-butanol: ethanol:water ) 5:4:3). RCD is soluble in water and some high polar organic solvents, and it avoids the β-CD’s intra- or intermolecular inclusion with the bulky stopper (tetramethyljulolidine unit). Thus, the single molecular compound is qualified to be further assembled to functional supramolecular or biomacromolecular materials by the encapsulation behavior of the chiral β-CD cavity to other guests. RCD was fully characterized

Langmuir, Vol. 25, No. 6, 2009 3483 Scheme 1

by 1H NMR, 13C NMR, and HRMS. In the initial state of RCD, the cyanostilbene unit exists almost entirely in the trans-form. Sufficient irradiation at 254 nm can induce a photoisomerization of the cyanostilbene unit to the cis-form. With a photoisomerization efficiency of 70%, new peaks in the range of aromatic protons arise in 1H NMR.7 The design and synthesis of pseudopolyrotaxanes with novel functions are desirable for both materials science and biochemistry. The fluorescent polypseudorotaxane (PRCD) was prepared by threading RCD onto the poly(propylene glycol) bis(2aminopropyl ether) (PPG-NH2, MW ≈ 2000) chains (Scheme 1). However, for the sparing solubility of PRCD in D2O, we use a 0.2% NaOD aqueous solution for all the NMR measurements. From the 1H NMR spectrum of PRCD, a comparison of the integral area of proton peaks indicates that the ratio between the PPG-NH2’s methyl protons together with tetramethyljulolidine unit’s methyl protons (a molecule of PPG-NH2 2000 contains ca. 34 methyl protons while a molecule of RCD contains 12 methyl protons in tetramethyljulolidine unit, δ ) 0.85-1.15) and the proton of aromatic rings in RCD (a molecule of RCD contains 7 protons in downfield region, δ ) 6.61-7.74) is 27.13:7.00.8 Therefore, we can calculate that one PPG-NH2 chain threads ca. 7 RCD monomers, which is consistent with the results of elemental analysis. Moreover, the 2D ROESY spectra of PRCD show the NOE correlations between the methyl protons of PPGNH2 and interior protons of β-CD cavity, which demonstrates the threading of β-CD cavities onto the PPG-NH2 chain.7 TEM was performed to provide further insight into the size and shape of PRCD. The nanowires, which are in the range of more or less 20 nm, should belong on the pseudopolyrotaxane molecules on the substrate observed from TEM images (see Supporting Information, Figure S6). Viscosity Response of RCD Detected by Fluorescence Readout. RCD belongs to a class of fluorescent molecules, which are derived from the group of p-(dialkylamino)benzylidenemalonitriles and form twisted intramolecular charge transfer (TICT) states upon photoexcitation.9 Their competition of fluorescence emission and nonradiative decay is environmental viscosity or (7) Further details of synthesis and characterization details of RCD and PRCD are shown in the Supporting Information. (8) Although CDs may be disassembled to some extent from the polypseudorotaxane under a high-pH condition, the peaks in NMR of different functional groups of this compound will only have a minor shift within a certain range. (9) The precursor A3 also shows a good environment-dependent viscositysensitive behavior. But for some difference of electronic structure and emission band between A3 and RCD, A3 is hard to be photoisomerized under the 254 nm irradiation. Sources of higher frequency light that might be more effective are not readily available.

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Figure 1. Emission spectra of initial trans-RCD (6.0 × 10-6 M, 298 K) in (a) mixtures of ethylene glycol and glycerol with different viscosity and in (b) aqueous solutions of dextran at different concentrations. The insets show a double-logarithmic plot between fluorescent intensity at 500 nm of trans-RCD and the corresponding viscosity.

Figure 2. Emission spectra of photoisomerized cis-RCD (6.0 × 10-6 M, 298 K) in (a) mixtures of ethylene glycol and glycerol with different viscosity and in (b) aqueous solutions of dextran at different concentrations after irradiation at 254 nm to the photostationary state.

fluid flow dependent. It is believed that the intramolecular rotation of the phenyl group (here is the tetramethyljulolidine group) is relatively free when the fluid outside is not sheared, whereas it is readily inhibited by high viscosity of the microenvironment. Hence, the balance of relaxation will shift with the variation of environmental viscosity and the fluorescence intensity increases with increased viscosity of the solvent.3,10 The viscosity sensitivity of RCD was tested by measuring its fluorescent intensity11 in mixtures of ethylene glycol and glycerol. Increased glycerol content is known to increase viscosity with only minimal changes of solvent polarity. Glycerol contents of 0, 20, 40, and 60% resulted in viscosities of 19, 32, 72, and 155 mPa · s, respectively. As shown in Figure 1a, in the initial state (i.e., trans-RCD), the emission intensity of the β-CD derivative boosts dramatically with the increase of the solvent’s viscosity. What is more, the logarithm of its fluorescence has a good linear relationship with the logarithm of viscosity according to the Fo¨rster-Hoffmann equation.12 RCD could also be dispersed in water for the rotor linked to the hydrophilic β-CD unit. The dextran was added to make the following weight/volume aqueous solutions, 2.5, 5.0, 7.5, and 10.0%, leading to viscosities of 1.6, (10) (a) Loutfy, R. O.; Arnold, B. A. J. Phys. Chem. 1982, 86, 4205–4211. (b) Iwaki, T.; Torigoe, C.; Noji, M.; Nakanishi, M. Biochemistry 1993, 32, 7589– 7592. (11) If only the absorbed light intensity Iab is known, the emission intensity Iem and the quantum yield are directly and proportionally related through Iem ∝ IabΦ. Thus, we could directly make use of fluorescent intensity readout in this measurement. (12) Fo¨rster, Th.; Hoffmann, G. Z. Phys. Chem. (Munich) 1971, 75, 63–76.

2.6, 4.2, and 6.5 mPa · s, respectively. The emission intensity of trans-RCD is enhanced with increasing concentration of dextran (Figure 1b) and shows a similar linear relationship to the one in the glycol/glycerol system. Lockable Viscosity Sensitivity of RCD with Photoisomerization. The cyanostilbene unit of RCD will undergo trans-to-cis photoisomerization by irradiation at 254 nm in both glycol/ glycerol and aqueous solutions. The maximum absorption of RCD will be weakened by the photoisomerization, with a stronger blue-shifted emisson peak generated. An interesting phenomenon found as shown in Figure 2 is that, when reaching the photostationary state, the new emission intensity of RCD almost no longer elevates with the increase of the solvent’s viscosity not only in glycol/glycerol system but also in aqueous solution, different from the fluorescence behavior of the initial state of RCD. Exactly, only the emission band around 500 nm, which belongs to the fluorescence of some residual unisomerized tranform RCD, slightly ascends with increase of viscosity in highviscosity range. But the new emission peak at 450 nm, which should arise from cis-form RCD, keeps unchanged in variational viscosity environment. To explain the peculiar spectral change, both the conformational forms (trans- and cis-form) of RCD’s fluorophore were optimized by density functional theory with hybrid B3LYP functional and 6-31G(d,p) basis set as implemented in the Gaussian 03 program.13 The dihedral of the cyanostilbene (the unit in the frame shown in Figure 3a) in the trans-form is 177.6°, which is nearly coplanar so that the two phenyl groups can rotate freely to each other

Cyclodextrin Polypseudorotaxane

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Figure 3. Optimized conformational (a) trans- and (b) cis-form of RCD’s fluorophore by B3LYP/6-31G(d,p).

Figure 4. Emission spectra of PRCD (6.0 × 10-6 M, 298 K) in alkaline aqueous solutions (pH ) 11) of dextran at different concentrations (a) before and (b) after irradiation at 254 nm to photostationary state. The inset in (a) shows double-logarithmic plot between fluorescent intensity ratio of initial PRCD and the corresponding viscosity.

when exposed in low-viscosity solvent, leading to a drastic nonradiative decay of the excited state and consequently a low fluorescent emission. This rotary motion, however, will be restricted in a high-viscosity medium, and a strong fluorescent emission will emerge. There is something different about the cis-form. As shown in Figure 3b, the dihedral of the cyanostilbene in the cis-form is 9.21° due to the two phenyl groups’ steric effect. The hydrogen atoms (the ones in the ellipse shown in Figure 3b) in the two aromatic rings could not be closer to each other, and this steric factor would sharply hinder the relative intramolecular rotation.14 Therefore, this conformational form would hardly feel the environmental viscosity change, resulting in a lockable or shielded viscosity-sensitive behavior. The cis-form of RCD can return back to trans-form by visible irradiation. This reversible photoisomerization can be operated (13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; MontgomeryJ. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 03, reVision C.01; Gaussian, Inc.: Pittsburgh, PA, 2004. (14) (a) Kee, H. L.; Kirmaier, C.; Yu, L. H.; Thamyongkit, P.; Youngblood, W. J.; Calder, M. E.; Ramos, L.; Noll, B. C.; Bocian, D. F.; Scheidt, W. R.; Birge, R. R.; Lindsey, J. S.; Holten, D. J. Phys. Chem. B 2005, 109, 20433–20443. (b) Yamada, K.; Toyota, T.; Takakura, K.; Ishimaru, M.; Sugawara, T. New J. Chem. 2001, 25, 667–669.

for several cycles (see Supporting Information, Figure S9), indicating that the function of a molecular switch has been successfully combined to a molecular viscosity sensor. Photolockable Ratiometric Viscosity Sensitivity of Polypseudorotaxane PRCD. To a fluorescence intensity-based measurement, the emssion intensity may alter inevitably with the fluctuation of fluorophore’s concentration, environmental temperature, and fluid optical properties. A ratiometric approach,15 using probes that incorporate two independent emssion peaks, when one is not influenced by viscosity and is used to gauge the concentration while the other acts as a viscosity sensitive signal, has been suggested to overcome the problem. There comes out a obvious long wavelength emission band at around 600 nm wavelength in the fluorescent spectra of PRCD (Figure 4), different from that of monomer RCD. This characteristic emission peak cannot originate from the PRCD’s intermolecular association because it will not completely disappear with the decrease of the concentration of the polypseudorotaxane (see Supporting Information, Figure S11). On the contrary, it is inferred that an intramolecular excimer has formed in PRCD by the interaction of two neighboring monomer’s fluorophores on the PPG backbone. It is reported that excimers (15) (a) Haidekker, M. A.; Brady, T. P.; Lichlyter, D.; Theodorakis, E. A. J. Am. Chem. Soc. 2006, 128, 398–399. (b) Luby-Phelps, K.; Mujumdar, S.; Mujumdar, R. B.; Ernst, L. A.; Galbraith, W.; Waggoner, A. S. Biophys. J. 1993, 65, 236–242.

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could be effectively formed in some certain cyanostilbene-based system,16 and a red shift of the maximum emission peak of RCD is observed indeed in a concentrated solution (see Supporting Information, Figure S12). In a PRCD molecule, the intramolecular excimers are more likely to be formed while the monomer’s fluorophores are fixed to be closer to each other on the backbone. Ionized CDs will be easily disassembled from a polypseudorotaxane under strong-base condition,17 so we introduced an excessive amount of NaOH into PRCD alkali aqueous solution and found that the long wavelength emission peak gradually declined with the dissociation of monomer RCD from PRCD (see Supporting Information, Figure S13). This process also proved that the intramolecular interaction of the RCD’s fluorophore on PRCD produces the long wavelength emission, while it is no finding of appearance of special emission even if upon addition of an excessive amount of NaOH into the monomer RCD aqueous solution (Figure S13). Interestingly, the emission band at around 500 nm, corresponding to the monomer’s rotor fluorescence, is enhanced with increasing viscosity of dextran aqueous solutions, but the excimer’s emission at 600 nm did not change (Figure 4a). The two emissions that origined from different mechanisms behave differently in viscosity-variable media. In this way, we recorded the relative fluorescent intensity (the emission signal at 500 nm, F500, divided by the one at 600 nm, F600) and plotted in a doublelogarithmic scale with respect to viscosity. This ratiometric fluorescent viscosity sensitivity fluctuates within a very tight range upon the change of the compound’s concentration and environmental temperature. However, the long wavelength emission will be eliminated after irradiation at 254 nm to photostationary state because the trans-to-cis photoisomerization may lengthen the distance between the nearby fluorophores on the polypseudorotaxane, so as to preclude the formation of intramolecular excimer. There is a significant difference on the value of fluorescent intensity ratio between the initial and the photostationary state of PRCD seen from Figure 4b. Negative logarithmic intensity ratios of the initial state of PRCD are exhibited because F500 is less than F600, while positive values of logarithmic intensity ratios of the photostationary state of PRCD are obtained as F500 is larger than F600. The two entirely different states can be clearly distinguished even by the naked eye (Figure 5), displaying an “activated” state of initial PRCD compared with a “locked” state after UV (16) (a) An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410–14415. (b) Nam, H.; Granier, M.; Boury, B.; Park, S. Y. Langmuir 2006, 22, 7132–7134. (17) Huh, K. M.; Tomita, H.; Ooya, T.; Lee, W. K.; Sasaki, S.; Yui, N. Macromolecules 2002, 35, 3775–3777.

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Figure 5. Fluorescent photograph of PRCD (6.0 × 10-6 M, 298 K) in alkaline aqueous solutions (pH ) 11) before and after irradiation at 254 nm.

irradiation, which are corresponding to the turn-on and turn-off of the ratiometric fluorescent viscosity sensor function.

Conclusion In summary, a β-CD derivative whose fluorophore has a photolockable viscosity-sensitive behavior has been synthesized. The environmental viscosity-dependent nonradiative decay of its fluorescent rotor can be modulated by the cyanostilbene’s photoisomerization. This β-CD derivative has been further transformed into a polypseudorotaxane which shows a lockable ratiometric fluorescent viscosity sensitivity due to the assistance of a fluorophore’s intramolecular excimer emission. This type of pseudopolyrotaxanes, with novel functions of sensor and switch, is an ideal candidate for biomaterials or biological detection systems, although it can be only used in a certain pH aqueous solution now. Next, we would try to attach this kind of switchable viscosity probe to a surface or living cells to further demonstrate its practical applications in a broad viscosity range. Acknowledgment. This work was supported by NSFC/China (50673025, 20603009), National Basic Research 973 Program (2006CB806200), Key Project of Chinese Ministry of Education (107044), and Scientific Committee of Shanghai. Supporting Information Available: Synthetic procedures, 1H NMR, 13C NMR, and HRMS (ESI) spectra of RCD; elementary analysis, 1 H NMR, 2D ROESY NMR, and TEM image of PRCD; absorption spectra of RCD and its precursor; repetitive cycles of RCD and PRCD described in fluorescence signal at 450 nm; confirmation of the intramolecular excimer of PRCD by fluorescent spectra. This material is available free of charge via the Internet at http://pubs.acs.org. LA8042457