Rational Design of a Green-Light-Mediated Unimolecular Platform for

Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 550−556

pubs.acs.org/JPCL

Rational Design of a Green-Light-Mediated Unimolecular Platform for Fast Switchable Acidic Sensing Yunyun Zhou,† Qi Zou,‡ Jing Qiu,§ Linjun Wang,§ and Liangliang Zhu*,† †

State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200433, China ‡ Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power, Shanghai University of Electric Power, Shanghai 200090, China § Department of Chemistry, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: A controllable sensing ability strongly connects to complex and precise events in diagnosis and treatment. However, imposing visible light into the molecularscale mediation of sensing processes is restricted by the lack of structural relevance. To address this critical challenge, we present the rational design, synthesis, and in vitro studies of a novel cyanostyryl-modified azulene system for green-light-mediated fast switchable acidic sensing. The advantageous features of the design include a highly efficient green-light-driven Z/E-isomerization (a quantum yield up to 61.3%) for fast erasing chromatic and luminescent expressions and a superior compatibility with control of ratiometric protonation. Significantly, these merits of the design enable the development of a microfluidic system to perform a green-light-mediated reusable sensing function toward a gastric acid analyte in a miniaturized platform. The results may provide new insights for building future integrated green materials.

W

miniaturized and point-of-care detection stage like microfluidics has to be regularly relied on.44−46 Once one can utilize the harmless and facile qualities and the ability to penetrate bulk media of a visible light, the sensing process at the molecular level will be readily controlled to meet a series of practical applications. To the best of our knowledge, precisely controlling a single visible light directive into a rationally designed unimolecular platform for the mediation of sensing behaviors remains an unsolved issue. With these considerations in mind, we attempt to combine the function of a visible-light-mediated molecular switch with a probe with acidic response to achieve that the readout signals resulting from protonation could be altered by a photochemical process to realize the regulation (erasing and restoring) of acidic sensitivity. The strategy is inspired by the fact that a molecular switching process can generally adjust the π-electron delocalization and transition to cause the change of the optical absorption and emission.47,48 Therefore, a rational band gap engineering for matching a fast visible light inclusion is extremely desired. Azulene is well-known for its anti-Kasha’s rule emission and response to ratiometric proton stimuli.49−52 The regiochemistry of azulene allows its versatile excited-state regulation upon acidic interaction.53−55 The sensitivity of azulene-based species prefers working in strong acidic environment, and such a selectivity could be suitable for application in

hile a photochemical process is usually rapid and precise and can be operated remotely without generating any chemical waste, optofunctional molecules and materials have continueed to attract attention over recent decades.1−14 In particular, chemical systems that are capable of response to visible light are playing an even more significant role in a variety of new appearing fields,15−19 because a visible light source has less harmful effects on the human body as compared with ultraviolet (UV) light and can generally ensure deeper penetration into a large scale of media or biosamples. The past years have witnessed the progress in the visible range of molecular engineering, the application of which has been largely focused on molecular switching, 20−24 molecular logic,25,26 molecular assembly,27−29 molecular catalysis,30,31 and so on.32−34 However, the mediation of molecular-scale sensing through visible light has received little attention, probably because the readout of the sensing state of a functional molecule is usually insensitive to visible-light energy; thus, the structural design will meet great challenges. As compared with using a collection of materials to realize a controllable sensing function, creating a novel unimolecular platform for integrating visible light directive and sensing feature is necessary for isometrically functional processes.35,36 Thus far, some transformable cases of organic functional sensors have been carried through a couple of approaches.37−39 Specifically, Haberfield developed a light-driven Z/E isomerizable system to control a protonation−deprotonation effect.40 Because pH sensing is always one of the crucial tasks in environmental analysis and in biomedical diagnosis,41−43 a © XXXX American Chemical Society

Received: December 6, 2017 Accepted: January 16, 2018 Published: January 16, 2018 550

DOI: 10.1021/acs.jpclett.7b03233 J. Phys. Chem. Lett. 2018, 9, 550−556

Letter

The Journal of Physical Chemistry Letters

Figure 1. (a) Comparison for input and output signals among compounds 1−4 with different end group links and (b) illustration of the protonation and green-light-driven Z/E isomerization process of the optimized compound (4), accompanied by its chromatic and luminescent change.

Figure 2. Fundamental characterization of the protonation and the green-light-induced photoisomerization. 1H NMR spectra (400 MHz, CDCl3) of compound 4 (a) at initial state (Z-4, 1 × 10−2 mol/L in CDCl3), (b) after protonation by TFA (10 μL, TFA/CDCl3: 1/50 (v/v), Z-4H), then (c) after photoirradiation by a 520 nm light beam (E-4H) and followed by (d) deprotonation (Reset Z-4).

form a new skeleton with a fine-tunable end group, for the realization of the above-mentioned hypothesis from the perspective of integration of acidic and photochemical behaviors. We anticipate that the acidic sensitivity of the azulene moiety will lead to a big shift of the band gap energy from UV to visible spectral region (longer wavelength is

technical occasions such as gastric acid detection, as demonstrated below. Cyanostilbene, a typical light-active moiety, exhibits a strong push−pull effect through derivation and isomerization, accompanied by tunable emission properties.56−58 Herein, we demonstrate a unimolecular strategy by connecting cyanostyryl unit onto 6-position59,60 of azulene to 551

DOI: 10.1021/acs.jpclett.7b03233 J. Phys. Chem. Lett. 2018, 9, 550−556

Letter

The Journal of Physical Chemistry Letters

Figure 3. Optical properties along with the protonation and the green-light-induced photoisomerization. (a) Absorption spectra of 4 at different states in CHCl3 (5 × 10−5 mol/L) at RT. (b) Photographs of the solutions at different states under (i) daylight and (ii) a 365 nm UV light. Emission spectra (λex = (c) 365 nm and (d) 520 nm) of 4 at different states in CHCl3 (5 × 10−5 mol/L) at RT.

the 1H NMR spectra, whereas that of proton 4 on 7-membered ring experienced an upfield shift (see Figures 2b and S4). These results confirmed that a tropylium cation formed on the 7membered ring upon protonation, in accordance with related findings in the literature.49−52 Such a pH-responsive process is usually regarded as a dynamically covalent interaction,62,63 and a moderate equilibrium constant (K = 4.21 ± 0.29 M−1) was calculated through a modified Benesi−Hildebrand methodology (Figure S5), indicating our system can serve as a ratiometric sensor64,65 because the optical signal was attended to different degrees along with the change of acidic concentration. Such a moderate interaction ability may thus allow a possible mediation by visible light. As expected, the Z-to-E photoisomerization occurs fast when the tropylium species Z-4H is irradiated by green light at 520 nm. With the employment of the 1H−1H COSY and NOESY spectra to assist the characterizations, the proton resonances can be well-assigned in both of the Z- and E-forms (Figures S6−S8). The resultant E-isomer was confirmed by the 1H NMR spectrum where a group of downfield shifted resonances of the cyanostyryl-modified azulene skeleton was observed (Figure 2c), due to facing of the electron-withdrawing group −CN in the E-form. The maximum photoisomerization efficiency is over 50% according to the NMR integral studies. The E-isomer can be converted back to the original Z-form by deprotonation (Figure 2d), indicating that the orthogonal inputs (protonation and light) can be reset, as illustrated in Figure 1. The protonation of compound 4 made its absorption and emission spectra change profoundly. As shown in Figure 3a, a remarkable absorption band around 520 nm appeared simultaneously accompanied by the decline of the original one (∼400 nm). The newly appearing band can be attributed to the main transition of the tropylium species, followed by the production of a long-wavelength emission at a red spectral region (∼630 nm, see Figure 3c,d). These absorption bands are in accordance with HOMO−LUMO energy calculation (Table S1 and S2). This process is quite visual, and the solution color

better), followed by a subsequent structural and optoelectronic regulation by photoirradiation. To further explore the end group effect and to optimize the optical input and output signals, four cyanostyryl-modified azulene compounds were synthesized by Knoevenagel condensation from the corresponding precursors. Figure 1a outlines the chemical structures of the four compounds (1− 4) and their response ability to acidic analyte and visible light, as well as their luminescent signal expression. The preparation route and details are provided in the Supporting Information. We finally determined that the alkoxy-substituted one (compound 4) can serve as a superior green-light erasable acidic responsive prototype with a readout by both chromatic and the anti-Kasha’s rule luminescent features (Figure 1b). While microfluidic scaffolds are increasingly used in the fields of detection, analysis, and biomedical sciences,44−46 our design can achieve a visualized in vitro sensing for gastric acid analyte and guarantee green light a sufficient penetration into glass channel to conduct an effective and fast control manner in microfluidics. Owing to such an orthogonal operation, the chemical strategy presented here can give birth to a switchable performance for the benefit of complex and precise events during diagnosis or treatment courses. At the initial state, the cyanostyryl-modified azulene structure in compound 4 adopts a Z-form evidenced by well-assigned 1H NMR signals (Figure 2a). Photoisomerization was unavailable at such an initial state upon irradiation on Z-4 either by UV or visible light (see the light degradation phenomenon in the optical spectra in Figure S2a and the unchanged signals in Figures S2b and S3), proving that the azulene moiety is prone to quench the intermediate triplet formation and cause the photoisomerization to be insensitive as in earlier findings.51,61 However, the skeleton can exhibit a light-responsive behavior in acidic environments. This property will make the optofunctionality totally different in our molecule. Upon addition of trifluoroacetic acid (TFA) into Z-4 solution, the resonances of proton 5 and 1 on the cyanostilbene and 5-membered ring of azulene underwent a downfield shift in 552

DOI: 10.1021/acs.jpclett.7b03233 J. Phys. Chem. Lett. 2018, 9, 550−556

Letter

The Journal of Physical Chemistry Letters

Figure 4. Green-light-mediated fast and high-efficiency acidic switchable sensing. (a) Time-dependent absorption spectra of 4 in CHCl3 (5 × 10−5 mol/L) at RT upon photoirradiation at 520 nm. (b) Normalized absorption changes at 520 nm during an ongoing protonation process with greenlight irradiation for several cycles. In each cycle, the solution of 4 in CHCl3 (1 × 10−3 mol/L) is treated with TFA [50 μL, TFA/CHCl3 (1/40 (v/v)) for the first cycle and then 20 μL, TFA/CHCl3 (1/100 (v/v)) for each cycle thereafter) followed by irradiation at 520 nm visible light for 2 min.

applied. The quantum yield is outstanding among the reported visible-light-responsive molecules driven with such a long wavelength. Some models triggered by visible light in the blue spectral region can reveal a quantum yield up to 50%, but those triggered by visible light in the green spectral region generally exhibit a maximum quantum yield of ∼30%.15,18,69 Only one case isomerized by green light has a quantum yield of 50%, but the conversion rate is very low.70 More interestingly, the acidic sensitivity of our system can reveal a green-light-induced erasable and reusable process. Even if a part of Z-4 was protonated, the appearing chromatic and luminescent characteristics can be readily erased by green light, and the remaining initial molecules can go on working for acidic response. Such a conversion process of unprotonated Z-4H by treating TFA upon photoisomerization can also be clearly monitored by NMR (Figure S13). With the alternation of introducing greenlight irradiation and TFA, the optical output signal was erased and restored, whereby the repeatability can be conducted for several cycles (Figure 4b). The reusability will even be strengthened to be practical for device operation. The above-mentioned acidic sensitivity, the green-light response, and the chromatic and luminescent expression differ among compounds with different attached end groups. The cyanostyryl-modified azulene compounds with nitryl (compound 1), amino (compound 2), and hydroxyl (compound 3) links were also investigated by NMR and optical spectroscopy (Figures S14−S16). It was found that these compounds cannot simultaneously exhibit these input and output signals as compound 4 did. Therefore, we can conclude that the push− pull effect and the salification of the end groups can significantly affect the establishment of a green-light-mediated photoswitchable sensing function (see the detailed explanation in the Supporting Information). The behavior of compound 4 (with an alkoxy end group) was an optimized outcome, and it certainly became a chromatic and luminescent platform for erasable acidic sensitivity, relying on the protonation and greenlight driven photoisomerization process. Next, we turn to develop a microfluidic device with the employment of this unique material to exemplify the practical applications of the switchable sensing function. Gastric acid detection is one of common patterns in clinical diagnosis, while an abnormal secretion of gastric acid is usually linked to a metabolism perturbation or diseases in some organisms.71−73 Currently, many in vitro detection procedures

and the solution luminescent color turned red notably upon protonation (Figure 3b). The time-resolved emission study indicates a fluorescence mechanism with a short lifetime for both of the emission peaks (Figure S9). The protonation is completely reversible when the neutralization by triethylamine (TEA) was applied, as indicated by those well-recovered absorption signals (Figure S10). Similar to other photoisomerizable species,66,67 the Z-to-E photoisomerization of 4 diminishes the main absorption band (∼520 nm) of the molecule (see Figure 3a). Meanwhile, the emission spectra also revealed a reduction feature (Figure 3c and 3d). The change in emission intensity is basically related to the shrinkage of the π-conjugation in the molecular structure along with the Z/E-photoisomerization process. Computational studies at the B3LYP/6-31G (d, p) level signified a change of the dihedral angle in cyanostilbene moiety from 177.9° to 16.5° along with the conformational variation (see Figure 1b). The self-quenching effect with the tropylium close to the phenoxy group might be another factor weakening the luminescence. The apparent color fading and the emission quenching can also be well-distinguished by the naked eye (Figure 3b). The above NMR studies and the isoabsorptive point suggested that such a phenomenum was definitely caused by photoisomerization rather than photobleaching or other degradation factors. In this way, a chromatic and luminescent platform with an integration of protonation and green-light response was established. The efficiency of most photoresponsive functional molecules generally decreases with increasing the excitation wavelength;1−7 however, our system can exhibit a fast response toward visible light in the green spectral region. Under a mild condition of green light irradiation (∼1 mW light power upon the sample with a 1−2 cm distance between), it reached a photostationary state within only minutes (Figure 4a). In contrast, the photoisomerization efficiency is very low upon ambient light or UV irradiation at 365 nm (Figure S11), suggesting a perfect selectivity to the irradiation wavelength.68 The green-light-driven photoisomerization rate constant (k = 0.0174 ± 0.000747 s−1, first-order exponential decay fitting) was determined in terms of the change of the main absorption band (Figure S12). In addition, the isomerization quantum yield (up to 61.3% at 517 nm irradiation) was also determined (see detailed calculation procedures in the Supporting Information). These results indicate that such a prototype is relatively practical once an optimized light source could be 553

DOI: 10.1021/acs.jpclett.7b03233 J. Phys. Chem. Lett. 2018, 9, 550−556

Letter

The Journal of Physical Chemistry Letters

sensor can work only in a nonpolar solvent, we can establish a calibration relationship in terms of the acidic distribution and conversion to quantify each operation (Figure S21). The initial gastric acid analyte with a pH < 0.4 was preferred in our experiments, corresponding to the scale of an abnormal secretion case. As compared to the relatively colorless circulation loop with a dark luminescence (Figure 5b, images i and ii), it obviously exhibited a red and luminescent color after the introduction of a gastric acid analyte (Figure 5b, image iii). These optical characteristics were erased upon the irradiation with a 520 nm light beam (Figure 5b, images iv and v). Next, when another batch of gastric acid analyte was brought in, the apparent and luminescent color turned red again (Figure 5b, image vi), suggesting the sensing ability is reusable in this way. On the basis of the device design of a microfluidic chip, the recycling can be better achieved as compared with a regular solution study. Overall, the working mechanism of the greenlight-controlled erasable and reusable microfluidic sensing process can be outlined as shown in Figure S22, essentially relying on the orthogonal operation of the protonation and photoisomerization, which results in significant input and output of the optical signals. In conclusion, we have employed visible light as an orthogonal input to control the acidic sensitivity in a welldesigned unimolecular system with chromatic and luminescent output. The switching effect of green-light-induced Z/Eisomerization with fast reactivity played a key role to regulate the optical characteristics. Four factors of the strategic design are responsible for the realization of such a unique photocontrollable sensing function at the molecular level: (1) a ratiometric acidic sensing moiety, (2) a light-active molecular switch unit, (3) a moderate responsive state that can easily be co-operated with an orthogonal directive, and (4) a matched band gap energy allowing a fast and effective visible-light interaction. Our system can also be delivered into the microfluidic detection and thereby exhibit an photoerasable and reusable sensing function toward a gastric acid analyte. We believe that the chromatic and luminescent platform demonstrated herein, which can show fast response to visible light, could be valuable for interdisciplinary development of environmentally friendly techniques for creating integrated applied optofunctional materials.

for diagnosis and treatment have been carried out through microfluidic devices, because the sample demand and the damage to living tissue can be minimized. It is easy to understand that the glass channel on a routine microfluidic chip will readily hamper the cooperation of a UV source with a low power density. However, this situation can benefit from usage of the penetration from a visible light, and that is also one of the advantages considered for the design of our probe structure. In our work, the microfluidic experiments were performed on a lab-modified microfluidic platform based on a commercially available glass chip with three ports: two for the introduction of the analyte and the sensor and one for the waste outlet (see Figures 5a and S17), ensuring a recycle task. After several

Figure 5. Green-light-controlled erasable and reusable microfluidic sensing for simulated gastric acid. (a) Schematic diagram of the microfluidic chip. (b) Photographs under daylight (and/or under a 365 nm UV light) of the chip (i) without, (ii) with 4 inlet, and (iii) mixed with a gastric acid analyte (iv and v) followed by irradiation at 520 nm for minutes, and then (vi) remixed with a gastric acid analyte. This detection process is mainly manifested by the photoerasable and the restorable color and luminescent color.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b03233. Synthesis, characterization, and spectroscopic properties, including Figures S1−S30 (PDF)

cycles, the circulation loop could be refilled with new fluids to replace the old ones, and new operations could be initiated to improve the cycling. Similar to those optical property changes upon protonation and green-light irradiation in solution, 4 is also sensitive to these stimuli in a microfluidic chip. We can clearly find that the circulation loop turned red and revealed a red luminescence after a blending of 4 and acid, and these optical characteristics are continuously obliterated upon a direct green-light irradiation on the chip, followed by the return of these signals while acid was readded (Figures S18 and S19). The detailed intensity information regarding the luminescence in the fluidic channel can also be well-distinguished using confocal microscopy (Figure S20). With these properties in mind, we turn to the detection of simulated gastric acid in this microfluidic system. As our acidic

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Liangliang Zhu: 0000-0001-6268-3351 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the NSFC/China (21644005, 21406137), National Program for Thousand Young Talents of 554

DOI: 10.1021/acs.jpclett.7b03233 J. Phys. Chem. Lett. 2018, 9, 550−556

Letter

The Journal of Physical Chemistry Letters

(19) Yang, C.; Fu, L. M.; Wang, Y.; Zhang, J. P.; Wong, W. T.; Ai, X. C.; Qiao, Y. F.; Zou, B. S.; Gui, L. L. A Highly Luminescent Europium Complex Showing Visible-Light-Sensitized Red Emission: Direct Observation of the Singlet Pathway. Angew. Chem. 2004, 116, 5120−5123. (20) Balzani, V.; Clemente-León, M.; Credi, A.; Ferrer, B.; Venturi, M.; Flood, A. H.; Stoddart, J. F. Autonomous artificial nanomotor powered by sunlight. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 1178− 1183. (21) Zhang, X.; Chamberlayne, C. F.; Kurimoto, A.; Frank, N. L.; Harbron, E. J. Visible light photoswitching of conjugated polymer nanoparticle fluorescence. Chem. Commun. 2016, 52, 4144−4147. (22) Brayshaw, S. K.; Schiffers, S.; Stevenson, A. J.; Teat, S. J.; Warren, M. R.; Bennett, R. D.; Sazanovich, I. V.; Buckley, A. R.; Weinstein, J. A.; Raithby, P. R. Highly Efficient Visible-Light Driven Photochromism: Developments towards a Solid-State Molecular Switch Operating through a Triplet-Sensitised Pathway. Chem. - Eur. J. 2011, 17, 4385−4395. (23) Qian, H.; Shao, B.; Aprahamian, I. Visible-light fluorescence photomodulation in azo-BF2 switches. Tetrahedron 2017, 73, 4901− 4904. (24) Zhang, J.; Fu, Y.; Han, H.-H.; Zang, Y.; Li, J.; He, X.-P.; Feringa, B. L.; Tian, H. Remote light-controlled intracellular target recognition by photochromic fluorescent glycoprobes. Nat. Commun. 2017, 8, 987. (25) Kortekaas, L.; Ivashenko, O.; van Herpt, J. T.; Browne, W. R. A Remarkable Multitasking Double Spiropyran: Bidirectional VisibleLight Switching of Polymer-Coated Surfaces with Dual Redox and Proton Gating. J. Am. Chem. Soc. 2016, 138, 1301−1312. (26) Chen, Z.; Wang, G.; Wang, X.; Zhao, Q. Moving toward optoelectronic logic circuits for visible light: a chalcogenide glass single-mode single-polarization optical waveguide switch. Appl. Opt. 2017, 56, 1405−1412. (27) Zeng, L.; Liu, T.; He, C.; Shi, D.; Zhang, F.; Duan, C. Organized Aggregation Makes Insoluble Perylene Diimide Efficient for the Reduction of Aryl Halides via Consecutive Visible Light-Induced Electron-Transfer Processes. J. Am. Chem. Soc. 2016, 138, 3958−3961. (28) Yeow, J.; Sugita, O. R.; Boyer, C. Visible Light-Mediated Polymerization-Induced Self-Assembly in the Absence of External Catalyst or Initiator. ACS Macro Lett. 2016, 5, 558−564. (29) Chen, L.; Li, Y.; Fan, J.; Bisoyi, H. K.; Weitz, D. A.; Li, Q. Photoresponsive Monodisperse Cholesteric Liquid Crystalline Microshells for Tunable Omnidirectional Lasing Enabled by a Visible LightDriven Chiral Molecular Switch. Adv. Opt. Mater. 2014, 2, 845−848. (30) Zhu, S.; Das, A.; Bui, L.; Zhou, H.; Curran, D. P.; Rueping, M. Oxygen Switch in Visible-Light Photoredox Catalysis: Radical Additions and Cyclizations and Unexpected C−C-Bond Cleavage Reactions. J. Am. Chem. Soc. 2013, 135, 1823−1829. (31) Fu, C.; Xu, J.; Boyer, C. Photoacid-mediated ring opening polymerization driven by visible light. Chem. Commun. 2016, 52, 7126−7129. (32) Kwon, G.; Panchanathan, D.; Mahmoudi, S. R.; Gondal, M. A.; McKinley, G. H.; Varanasi, K. K. Visible light guided manipulation of liquid wettability on photoresponsive surfaces. Nat. Commun. 2017, 8, 14968. (33) Poelma, S. O.; Oh, S. S.; Helmy, S.; Knight, A. S.; Burnett, G. L.; Soh, H. T.; Hawker, C. J.; Read de Alaniz, J. Controlled drug release to cancer cells from modular one-photon visible light-responsive micellar system. Chem. Commun. 2016, 52, 10525−10528. (34) Mayer, G.; Heckel, A. Biologically Active Molecules with a “Light Switch. Angew. Chem., Int. Ed. 2006, 45, 4900−4921. (35) Zhao, Y.-L.; Hu, L.; Grüner, G.; Stoddart, J. F. A Tunable Photosensor. J. Am. Chem. Soc. 2008, 130, 16996−17003. (36) Yin, L.; Wu, H.; Zhu, M.; Zou, Q.; Yan, Q.; Zhu, L. Sequential Block Copolymer Self-Assemblies Controlled by Metal−Ligand Stoichiometry. Langmuir 2016, 32, 6429−6436. (37) Zhu, L. L.; Li, X.; Ji, F. Y.; Ma, X.; Wang, Q. C.; Tian, H. Photolockable Ratiometric Viscosity Sensitivity of Cyclodextrin Polypseudorotaxane with Light-Active Rotor Graft. Langmuir 2009, 25, 3482−3486.

China. Y.Z. acknowledges China Postdoctoral Science Foundation funded project (2016M601491) for additional research support. L.Z. and Y.Z. thank Mr. Z.-W. Zhang and Dr. J.-J. Zhang for assistance with the quantum yield measurement and calculation.

■

REFERENCES

(1) Coskun, A.; Friedman, D. C.; Li, H.; Patel, K.; Khatib, H. A.; Stoddart, J. F. A Light-Gated STOP−GO Molecular Shuttle. J. Am. Chem. Soc. 2009, 131, 2493−2495. (2) Vicario, J.; Walko, M.; Meetsma, A.; Feringa, B. L. Fine Tuning of the Rotary Motion by Structural Modification in Light-Driven Unidirectional Molecular Motors. J. Am. Chem. Soc. 2006, 128, 5127−5135. (3) Irie, M.; Kobatake, S.; Horichi, M. Reversible Surface Morphology Changes of a Photochromic Diarylethene Single Crystal by Photoirradiation. Science 2001, 291, 1769−1772. (4) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. Thermal and Light Control of the Sol-Gel Phase Transition in Cholesterol-Based Organic Gels. Novel Helical Aggregation Modes As Detected by Circular Dichroism and Electron Microscopic Observation. J. Am. Chem. Soc. 1994, 116, 6664−6676. (5) Zhu, L.; Ma, X.; Ji, F.; Wang, Q.; Tian, H. Effective Enhancement of Fluorescence Signals in Rotaxane-Doped Reversible Hydrosol−Gel Systems. Chem. - Eur. J. 2007, 13, 9216−9222. (6) Lv, J.-a.; Liu, Y.; Wei, J.; Chen, E.; Qin, L.; Yu, Y. Photocontrol of fluid slugs in liquid crystal polymer microactuators. Nature 2016, 537, 179−184. (7) Zhu, L.; Yan, H.; Wang, X. J.; Zhao, Y. Light-controllable cucurbit[7]uril-based molecular shuttle. J. Org. Chem. 2012, 77, 10168−75. (8) Andersson, S.; Zou, D.; Zhang, R.; Sun, S.; Sun, L. Light driven formation of a supramolecular system with three CB[8]s locked between redox-active Ru(bpy)3 complexes. Org. Biomol. Chem. 2009, 7, 3605−3609. (9) Zhu, L.; Tran, H.; Beyer, F. L.; Walck, S. D.; Li, X.; Agren, H.; Killops, K. L.; Campos, L. M. Engineering topochemical polymerizations using block copolymer templates. J. Am. Chem. Soc. 2014, 136, 13381−7. (10) Raymo, F. M. Intermolecular Coupling of Motion under Photochemical Control. Angew. Chem., Int. Ed. 2006, 45, 5249−5251. (11) Kundu, P. K.; Samanta, D.; Leizrowice, R.; Margulis, B.; Zhao, H.; Börner, M.; Udayabhaskararao, T.; Manna, D.; Klajn, R. Lightcontrolled self-assembly of non-photoresponsive nanoparticles. Nat. Chem. 2015, 7, 646−652. (12) Zhang, Q.; Wang, W. Z.; Yu, J. J.; Qu, D. H.; Tian, H. Dynamic Self-Assembly Encodes A Tri-stable Au−TiO2 Photocatalyst. Adv. Mater. 2017, 29, 1604948. (13) Zhou, H.; Xue, C.; Weis, P.; Suzuki, Y.; Huang, S.; Koynov, K.; Auernhammer, G. K.; Berger, R.; Butt, H. J.; Wu, S. Photoswitching of glass transition temperatures of azobenzene-containing polymers induces reversible solid-to-liquid transitions. Nat. Chem. 2017, 9, 145−151. (14) Cox, J. R.; Simpson, J. H.; Swager, T. M. Photoalignment Layers for Liquid Crystals from the Di-π-methane Rearrangement. J. Am. Chem. Soc. 2013, 135, 640−643. (15) Beharry, A. A.; Sadovski, O.; Woolley, G. A. Azobenzene Photoswitching without Ultraviolet Light. J. Am. Chem. Soc. 2011, 133, 19684−19687. (16) Fredrich, S.; Göstl, R.; Herder, M.; Grubert, L.; Hecht, S. Switching Diarylethenes Reliably in Both Directions with Visible Light. Angew. Chem., Int. Ed. 2016, 55, 1208−1212. (17) Wegner, H. A. Azobenzenes in a New LightSwitching In Vivo. Angew. Chem., Int. Ed. 2012, 51, 4787−4788. (18) Bléger, D.; Hecht, S. Visible-Light-Activated Molecular Switches. Angew. Chem., Int. Ed. 2015, 54, 11338−11349. 555

DOI: 10.1021/acs.jpclett.7b03233 J. Phys. Chem. Lett. 2018, 9, 550−556

Letter

The Journal of Physical Chemistry Letters

(58) Hang, C.; Wu, H. W.; Zhu, L. L. π-Conjugated cyanostilbenebased optoelectric functional materials. Chin. Chem. Lett. 2016, 27, 1155−1165. (59) Lash, T. D.; Lammer, A. D.; Idate, A. S.; Colby, D. A.; White, K. Preparation of Azulene-Derived Fulvenedialdehydes and Their Application to the Synthesis of Stable adj-Dicarbaporphyrinoids. J. Org. Chem. 2012, 77, 2368−2381. (60) Muranaka, A.; Yonehara, M.; Uchiyama, M. Azulenocyanine: A New Family of Phthalocyanines with Intense Near-IR Absorption. J. Am. Chem. Soc. 2010, 132, 7844−7845. (61) Saltiel, J.; Ko, D.-H.; Fleming, S. A. Differential Medium Effects on the Trans → Cis Photoisomerization of all-trans-1,6-Diphenyl1,3,5-hexatriene. Competing Diradicaloid vs Zwitterionic Pathways. J. Am. Chem. Soc. 1994, 116, 4099−4100. (62) Murai, M.; Amir, E.; Amir, R. J.; Hawker, C. J. Azulene-based conjugated polymers: unique seven-membered ring connectivity leading to stimuli-responsiveness. Chem. Sci. 2012, 3, 2721−2725. (63) Liang, E.; Zhou, H.; Ding, X.; Zheng, Z.; Peng, Y. Fabrication of a rhythmic assembly system based on reversible formation of dynamic covalent bonds in a chemical oscillator. Chem. Commun. 2013, 49, 5384−5386. (64) Guliyev, R.; Coskun, A.; Akkaya, E. U. Design Strategies for Ratiometric Chemosensors: Modulation of Excitation Energy Transfer at the Energy Donor Site. J. Am. Chem. Soc. 2009, 131, 9007−9013. (65) Charier, S.; Ruel, O.; Baudin, J. B.; Alcor, D.; Allemand, J. F.; Meglio, A.; Jullien, L. An Efficient Fluorescent Probe for Ratiometric pH Measurements in Aqueous Solutions. Angew. Chem., Int. Ed. 2004, 43, 4785−4788. (66) Zhu, L.; Ang, C. Y.; Li, X.; Nguyen, K. T.; Tan, S. Y.; Ågren, H.; Zhao, Y. Luminescent Color Conversion on Cyanostilbene-Functionalized Quantum Dots via In-situ Photo-Tuning. Adv. Mater. 2012, 24, 4020−4024. (67) Zhu, L.; Lu, M.; Qu, D.; Wang, Q.; Tian, H. Coordinationassembly for quantitative construction of bis-branched molecular shuttles. Org. Biomol. Chem. 2011, 9, 4226−4233. (68) Turro, N. J.; Ramamurthy, V.; Cherry, W.; Farneth, W. The effect of wavelength on organic photoreactions in solution. Reactions from upper excited states. Chem. Rev. 1978, 78, 125−145. (69) Knie, C.; Utecht, M.; Zhao, F.; Kulla, H.; Kovalenko, S.; Brouwer, A. M.; Saalfrank, P.; Hecht, S.; Bléger, D. orthoFluoroazobenzenes: Visible Light Switches with Very Long-Lived Z Isomers. Chem. - Eur. J. 2014, 20, 16492−16501. (70) Kurihara, M.; Hirooka, A.; Kume, S.; Sugimoto, M.; Nishihara, H. Redox-Conjugated Reversible Isomerization of Ferrocenylazobenzene with a Single Green Light. J. Am. Chem. Soc. 2002, 124, 8800− 8801. (71) Ghosh; Lewis; Axon; Everett. Review article: Methods of measuring gastric acid secretion. Aliment. Pharmacol. Ther. 2011, 33, 768−781. (72) Fujiki, S.; Iwao, Y.; Kobayashi, M.; Miyagishima, A.; Itai, S. Stabilization mechanism of clarithromycin tablets under gastric pH conditions. Chem. Pharm. Bull. 2011, 59, 553−558. (73) Anirudhan, T. S.; Sandeep, S. Synthesis and characterization of a novel pH-controllable composite hydrogel for anticancer drug delivery. New J. Chem. 2011, 35, 2869−2876.

(38) Chang, K. C.; Su, I. H.; Senthilvelan, A.; Chung, W. S. TriazoleModified Calix[4]crown as a Novel Fluorescent On−Off Switchable Chemosensor. Org. Lett. 2007, 9, 3363−3366. (39) Haberfield, P. Phototropic molecules. 2. A light-powered hydrogen-ion pump. J. Am. Chem. Soc. 1987, 109, 6178−6179. (40) Tian, H. Data Processing on a Unimolecular Platform. Angew. Chem., Int. Ed. 2010, 49, 4710−4712. (41) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Signaling Recognition Events with Fluorescent Sensors and Switches. Chem. Rev. 1997, 97, 1515−1566. (42) Shiraishi, Y.; Tokitoh, Y.; Nishimura, G.; Hirai, T. SolventDriven Multiply Configurable On/Off Fluorescent Indicator of the pH Window: A Diethylenetriamine Bearing Two End Pyrene Fragments. J. Phys. Chem. B 2007, 111, 5090−5100. (43) Cui, D.; Qian, X.; Liu, F.; Zhang, R. Novel Fluorescent pH Sensors Based on Intramolecular Hydrogen Bonding Ability of Naphthalimide. Org. Lett. 2004, 6, 2757−2760. (44) Lin, C. F.; Lee, G. B.; Wang, C. H.; Lee, H. H.; Liao, W. Y.; Chou, T. C. Microfluidic pH-sensing chips integrated with pneumatic fluid-control devices. Biosens. Bioelectron. 2006, 21, 1468−1475. (45) Kou, S.; Lee, H. N.; van Noort, D.; Swamy, K. M. K.; Kim, S. H.; Soh, J. H.; Lee, K. M.; Nam, S. W.; Yoon, J.; Park, S. Fluorescent Molecular Logic Gates Using Microfluidic Devices. Angew. Chem., Int. Ed. 2008, 47, 872−876. (46) Macounová, K.; Cabrera, C. R.; Holl, M. R.; Yager, P. Generation of Natural pH Gradients in Microfluidic Channels for Use in Isoelectric Focusing. Anal. Chem. 2000, 72, 3745−3751. (47) Wu, H.; Zhou, Y.; Yin, L.; Hang, C.; Li, X.; Agren, H.; Yi, T.; Zhang, Q.; Zhu, L. Helical Self-Assembly-Induced Singlet-Triplet Emissive Switching in a Mechanically Sensitive System. J. Am. Chem. Soc. 2017, 139, 785−791. (48) Zhu, L.; Li, X.; Zhang, Q.; Ma, X.; Li, M.; Zhang, H.; Luo, Z.; Agren, H.; Zhao, Y. Unimolecular photoconversion of multicolor luminescence on hierarchical self-assemblies. J. Am. Chem. Soc. 2013, 135, 5175−5182. (49) Amir, E.; Amir, R. J.; Campos, L. M.; Hawker, C. J. StimuliResponsive Azulene-Based Conjugated Oligomers with Polyanilinelike Properties. J. Am. Chem. Soc. 2011, 133, 10046−10049. (50) Tang, T.; Lin, T.; Wang, F.; He, C. Origin of Near-Infrared Absorption for Azulene-Containing Conjugated Polymers upon Protonation or Oxidation. J. Phys. Chem. B 2015, 119, 8176−8183. (51) Zhou, Y.; Zhuang, Y.; Li, X.; Agren, H.; Yu, L.; Ding, J.; Zhu, L. Selective Dual-Channel Imaging on Cyanostyryl-Modified Azulene Systems with Unimolecularly Tunable Visible-Near Infrared Luminescence. Chem. - Eur. J. 2017, 23, 7642−7647. (52) Xin, H.; Ge, C.; Yang, X.; Gao, H.; Yang, X.; Gao, X. Biazulene diimides: a new building block for organic electronic materials. Chem. Sci. 2016, 7, 6701−6705. (53) Xia, J.; Capozzi, B.; Wei, S.; Strange, M.; Batra, A.; Moreno, J. R.; Amir, R. J.; Amir, E.; Solomon, G. C.; Venkataraman, L.; Campos, L. M. Breakdown of Interference Rules in Azulene, a Nonalternant Hydrocarbon. Nano Lett. 2014, 14, 2941−2945. (54) Yamaguchi, Y.; Ogawa, K.; Nakayama, K.-i.; Ohba, Y.; Katagiri, H. Terazulene: A High-Performance n-Type Organic Field-Effect Transistor Based on Molecular Orbital Distribution Control. J. Am. Chem. Soc. 2013, 135, 19095−19098. (55) Amir, E.; Murai, M.; Amir, R. J.; Cowart, J. S.; Chabinyc, M. L.; Hawker, C. J. Conjugated oligomers incorporating azulene building blocks - seven- vs. five-membered ring connectivity. Chem. Sci. 2014, 5, 4483−4489. (56) An, B.-K.; Gierschner, J.; Park, S. Y. π-Conjugated Cyanostilbene Derivatives: A Unique Self-Assembly Motif for Molecular Nanostructures with Enhanced Emission and Transport. Acc. Chem. Res. 2012, 45, 544−554. (57) Zhu, L.; Zhao, Y. Cyanostilbene-based intelligent organic optoelectronic materials. J. Mater. Chem. C 2013, 1, 1059−1065. 556

DOI: 10.1021/acs.jpclett.7b03233 J. Phys. Chem. Lett. 2018, 9, 550−556