Article pubs.acs.org/Organometallics
Bis[alkynylplatinum(II)] Terpyridine Molecular Tweezer/Guest Recognition Enhanced by Intermolecular Hydrogen Bonds: Phototriggered Complexation via the “Caging” Strategy Tengfei Fu, Yifei Han, Lei Ao, and Feng Wang* CAS Key Laboratory of Soft Matter Chemistry, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China S Supporting Information *
ABSTRACT: The binding affinity between alkynylplatinum(II) terpyridine molecular tweezer and naphthol-derived guests can be significantly enhanced by embedding an intermolecular O−H---N hydrogen bond, facilitating the achievement of a responsive host−guest recognition system via the photolabile “caging” strategy.
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Scheme 1. Schematic Representation of the HydrogenBonding-Enhanced Complexation between Molecular Tweezer 1 and the Complementary Guests
or the bottom-up fabrication of self-assembled nanostructured materials, the fundamental molecular recognition behaviors exert significant impacts on their structural and functional complexities.1 Over the past decade, a variety of host−guest recognition systems have been exploited, which primarily consist of crown ether-, cyclodextrin-, calixarene-, and cucurbituril-type macrocyclic receptors.2−5 Beyond this scope, acyclic molecular tweezers, representing the preorganization of two π-aromatic pincers with designated orientation and distance, are capable of encapsulating the complementary guests into their cavities.6 It is highly desirable to develop a molecular tweezer/guest recognition motif with high binding affinity, which would benefit its potential applications in the fields of separation, sensing, and optoelectronics. To attain this objective, one possible approach is to expand the π-surfaces of the pincer/guest units, which is advantageous for strengthening electron donor−acceptor interactions.7 However, some severe issues, such as tedious synthesis and limited solubility, are commonly encountered for the resulting noncovalent recognition system, which limit its experimental feasibility. In this context, introduction of intermolecular hydrogen bonds represents an alternative strategy to improve noncovalent molecular tweezer/guest binding strength, due to its highly directional and stimuli-responsive character.8 We have previously demonstrated that, for the alkynylplatinum(II) terpyridine molecular tweezer receptor 1, guest 2 displays a remarkable 80 times enhancement of the binding affinity with respect to that of unsubstituted pyrene (Ka = (1.80 ± 0.11) × 105 M−1 for 1/2 versus (2.27 ± 0.05) × 103 M−1 for 1/pyrene) (Scheme 1).9 This is primarily ascribed to the formation of an © XXXX American Chemical Society
intermolecular N−H---N hydrogen bond between the amide unit on 2 and the pyridine moiety on 1. Herein we sought to expand the scope of guest toolboxes by screening a variety of naphthalene- and benzene-derived guests featuring hydrogen Received: May 28, 2016
A
DOI: 10.1021/acs.organomet.6b00429 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
UV−vis spectrum, gradually decrease in intensity upon progressive addition of 2-naphthol (Figure 1b, inset). Nonlinear curve-fitting of the collected absorbance data at 460 nm (eq S1) provides a Ka value of (1.20 ± 0.04) × 104 M−1 for 1/2naphthol (Figure 1b), which is highly consistent with the above ITC experiments. In the meantime, a similar value (Ka = (1.50 ± 0.10) × 104 M−1) can be achieved from the fluorescent titration measurements (eq S2) by monitoring the progressively decreasing MLCT/LLCT emission band of 1 centered at 600 nm (Figure 1c). In contrast, when unsubstituted naphthalene serves as the guest, both MLCT/LLCT absorption (Figure 1d) and emission (Figure S3) bands display very slight changes for 1. Such results denote very weak complexation strength between 1 and naphthalene, which prevents us from obtaining an accurate Ka value. On the basis of the above experimental results, it is evident that the presence of a hydroxyl group on the naphthalene guest facilitates an enhancement in noncovalent binding affinity to a large extent. We then resorted to using DFT theoretical calculations to elucidate the binding mechanism. For the optimized geometries of 1/2-naphthol, it is capable of forming an intermolecular O−H---N hydrogen bond between the hydroxyl group on 2-naphthol and the pyridine unit on 1, as manifested by the short H---N distance of approximately 1.6 Å, together with the O−H---N angle of 173° (Figure 2).
bond donating groups (such as 1-/2-naphthol and 1-/2naphthalenamine) (Scheme 1). It is anticipated that different types of hydrogen bonds (such as O−H---N and N−H---N) can be formed between 1 and the complementary guests, facilitating the examination of the versatility to enhance molecular tweezer/guest complexation via the assistance of hydrogen bonds. More importantly, the strength of intermolecular hydrogen bonds can be elaborately modulated, by “caging” the hydroxyl and amine units on the guests,10 which could efficiently revert to their original forms in respond to external stimuli. Hence, it is highly expected that stimuli responsiveness can be assigned to the resulting molecular tweezer/guest recognition system via the “caging” strategy. Noncovalent complexation was first examined between 1 and the complementary guests naphthalene and 2-naphthol (Figure S1). For 1 itself, severe signal broadening behaviors are observed for the aromatic proton resonances in 1H NMR spectrum, primarily ascribed to the formation of self-associated structures in CHCl3 solution. This phenomenon is still maintained upon adding an equimolar amount of naphthalene to 1. In stark contrast, for a 1:1 mixture of 1 and 2-naphthol, well-defined sharp signals emerge for the resulting 1H NMR spectrum. In particular, H5 appears in the remarkably downfield region (δ = 8.79 ppm), whereas the terpyridine protons H1−4 on 1 are located at 8.66, 7.31, 8.21, and 8.29 ppm, respectively. Such phenomena indicate distinct complexation behaviors between naphthalene and 2-naphthol guests toward the same molecular tweezer receptor. We then turned to understand the binding thermodynamics for the resulting noncovalent recognition system. For complex 1/2-naphthol, the binding stoichiometry is determined to be 1:1, as manifested by the abrupt changes in the isothermal titration calorimetry (ITC) curves (Figure 1a). Fitting the exothermic isotherm data with the one-site model provides the Ka value of (1.18 ± 0.06) × 104 M−1 for 1/2-naphthol. Additionally, the MLCT (metal-to-ligand charge transfer) and LLCT (ligand-to-ligand charge transfer) absorption bands of 1, predominately located in the region of 400−500 nm in the
Figure 2. Optimized structures of complexes 1/2-naphthol and 1/ naphthalene via DFT methods.
Moreover, the electron density predominately distributes over the electron-rich 2-naphthol moiety in the HOMO orbital, while it is fully occupied by electron-deficient alkynylplatinum(II) terpyridine motifs in LUMO and LUMO+1 orbitals. Such phenomena definitely support the involvement of sufficient electron donor−acceptor interactions between 1 and 2naphthol. In terms of 1/naphthalene (Figure 2), although the electron distribution in LUMO and LUMO+1 orbitals is analogous to that of 1/2-naphthol, in the HOMO orbital it mainly distributes on the diphenylpyridine backbone, which unambiguously indicates a rather weak charge transfer interactions between naphthalene and the electron-deficient pincers on 1. Hence, it is evident that the formation of an intermolecular hydrogen bond, together with the strengthening
Figure 1. (a) ITC data for titrating 2-naphthol (8.00 mM in CHCl3) into the CHCl3 solution of 1 (0.40 mM); (b) UV/vis absorption spectral changes of 1 (5.00 × 10−5 M) upon addition of 2-naphthol; (c) emission spectral changes of 1 (5.00 × 10−5 M) upon addition of 2-naphthol; (d) UV/vis absorption spectral changes of 1 (5.00 × 10−5 M) upon addition of naphthalene. B
DOI: 10.1021/acs.organomet.6b00429 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
reported that the hydroxyl group can be conveniently protected by the o-nitrobenzyl dimethyl ether caging unit, which is restored to its original form in response to UV irradiation.11 Hence, we have further designed compound 7 (Figure 4), with
of donor−acceptor interactions, contributes to the remarkably enhanced binding affinity for 1/2-naphthol. To obtain an in-depth understanding of hydrogen bond enhanced complexation behaviors, the substitution groups on the guest moiety were further varied (Table 1 and Figures S4− Table 1. Binding Constants (Ka) of 1 with Various Naphthalene and Benzene Derivatives in CHCl3 via UV−Vis Titrationa
Figure 3. 1H NMR spectra (300 MHz, CDCl3, room temperature) of (a) 1; (b) 7; (c) a 1:1 mixture of 1 and 7; (d) a 1:1 mixture of 1 and 7 after irradiation at 365 nm for 15 min; and (e) a 1:1 mixture of 1 and 6.
[1] = 5 × 10−5 M in CHCl3. bNo interaction can be detected on the basis of UV−vis titration experiments. a
S12). In detail, 1/1-naphthol is also prone to forming intermolecular O−H---N hydrogen bonds, resulting in a similar binding strength to that of 1/2-naphthol. The presence of ortho-substituted groups (such as naphthol-derived guests 3 and 4) hampers the formation of intermolecular hydrogen bonds, primarily due to the steric hindrance effect. In comparison, when the substitution group is located on the para-position, both guests 5 and 6 still maintain sufficient binding strength toward molecular tweezer 1. Notably, naphthalene guests bearing dihydroxyl substituents show a relatively higher binding affinity (Ka ∼105 M−1) than the monohydroxyl counterparts toward the receptor 1. Moreover, when the hydroxyl group is replaced by the amine unit, the resulting guests 1- and 2-naphthalenamine display considerably weaker binding affinity toward 1 (Table 1). Such phenomena are highly plausible, since nitrogen shows a relatively lower electronegativity (3.04) than that of oxygen (3.44). Meanwhile, 1-naphthalenemethanol also shows a relatively low binding affinity toward 1 (Table 1), ascribed to its relatively weak hydrogen bond donating capability. These results demonstrate the significant role of O−H---N hydrogen bonds for the enhanced complexation between 1 and naphtholderived guests. On the other hand, when phenol-derived compounds (such as phenol and methyl p-hydroxybenzoate) serve as the complementary guests, they show remarkably lower binding affinity than the corresponding naphthalene derivatives, primarily attributed to the weakened donor−acceptor interactions for the resulting noncovalent complexes. After validating the crucial role of the O−H---N hydrogen bond for enhanced molecular tweezer/guest complexation, we endeavored to construct a stimuli-responsive supramolecular recognition system via the “caging” strategy. It is widely
Figure 4. (a) Schematic representation of the phototriggered modulation of binding selectivity in a three-component supramolecular recognition system consisting of 1, 7, and 8. (b) Fluorescent spectral changes of the mixtures of 1, 7, and 8 upon UV irradiation at 365 nm. (c) Dependence of emission intensity at 800 nm with prolonged irradiation time.
the attachment of a photolabile group on guest 6. As an initial step, the photostability for 7 was examined. Although it is quite stable in the dark state, deprotection of the caging group on 7 occurs upon irradiation by UV light (365 nm). Briefly, for a 2.00 mM solution of 7 in CDCl3, the 1H NMR resonance for the benzylic proton located at 5.64 ppm, as well as the aromatic proton resonance located at 8.55 ppm, progressively decreases in intensity and totally disappears after irradiation for 15 min (Figure S16). In the meantime, the newly formed 1H NMR resonances coincide very well with those of guest 6, indicating the smooth removal of the photocaged group. The complete conversion from 7 to 6 within 15 min is further monitored via HPLC experiment (Figure S17), which is highly consistent with the above 1H NMR study, and thereby validates the efficiency of the “decaging” process. We then turned to exploit noncovalent complexation behaviors between 1 and 7. Due to the absence of a hydrogen C
DOI: 10.1021/acs.organomet.6b00429 Organometallics XXXX, XXX, XXX−XXX
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ACKNOWLEDGMENTS This work was supported by the Fundamental Research Funds for the Central Universities (WK3450000001, WK2060200012), CAS Youth Innovation Promotion Association, and the National Natural Science Foundation of China (21274139).
bond donating group, 7 does not tend to be encapsulated into the cavity of molecular tweezer 1, as reflected by the persistence of 1H NMR signals for a 1:1 mixture of 1/7 (Figure 3b,c). Upon UV irradiation, the resulting mixture shows remarkable changes for their 1H NMR signals (Figure 3d), which correlate well with those of complex 1/6 (Figure 3e). Simultaneously, depending on the fluorescent measurements, the MLCT/ LLCT emission bands of 1/7 display a significant decrease in their intensities (Figure S19). Hence, it can be concluded that photocleavage of 7 leads to the favorable noncovalent complexation between 1 and the decaged species. In light of the fact that noncovalent complexation between 1 and 7 could only occur upon exposure to UV light, we sought to probe such a transition, by taking advantage of the optical signal changes. Yam et al. and we have previously reported that, Pt2+−Pt2+ metal−metal interactions exist in complexes 1/8 (Figure 4a), leading to remarkable bathochromic shifts for the absorption (λmax = 560 nm) and emission peaks (λmax = 805 nm) (Figure S20).6c,12 Considering that the binding affinity for 1/8 (Ka = (1.25 ± 0.07) × 104 M−1) is a little lower than that of 1/6, but dramatically stronger than that of 1/7, we set out to establish a three-component supramolecular recognition system consisting of 1, 7, and 8. At the initial state, noncovalent complexation is more favored between 1 and 8, as evidenced by the presence of MMLCT absorption (Figure S20) and emission (centered at 805 nm, Figure 4b) bands. Upon UV irradiation, 1 is more biased to complex with the decaging product from 7 and thereby leads to the escape of 8 from the cavity of the tweezer receptor. As a consequence, both MMLCT absorption and emission signals exhibit drastic decreases in their intensities. A detailed examination of the kinetic process reveals that the MMLCT band levels off after 20 min of irradiation (Figure 4c), illustrating that light-triggered decaging of 7 is the rate-determining step for guest uptake/release switching within the three-component supramolecular recognition system. In conclusion, we have demonstrated that implementing an intermolecular O−H---N hydrogen bond facilitates the reinforcement of the binding affinity between alkynylplatinum(II) terpyridine molecular tweezer 1 and the naphthalenederived guests. On this basis, a phototrigged supramolecular recognition system has been successfully fabricated via caging the hydroxyl group on the guests. The distinct optical changes in the multicomponent supramolecular recognition system are promising for the development of novel types of chemo- and biosensors, which is now under way in our laboratory.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00429. Synthetic route to compounds 1 and 7, characterizations, detailed 1H NMR, UV−vis, and fluorescent measurements between 1 and the complementary guests (PDF)
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AUTHOR INFORMATION
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[email protected]. Notes
The authors declare no competing financial interest. D
DOI: 10.1021/acs.organomet.6b00429 Organometallics XXXX, XXX, XXX−XXX
