Theoretical Design of a New Allosteric Switch and Fluorescence

Article pubs.acs.org/JPCC

Theoretical Design of a New Allosteric Switch and Fluorescence Chemosensor Double Functional Devices of Aza-Crown Ether Weiwei Li,† Xinliang Yu,‡ and Xueye Wang*,† †

Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan, Hunan 411105, P. R. China ‡ College of Chemistry and Chemical Engineering, Hunan Institute of Engineering, Xiangtan, Hunan 411104, P. R. China S Supporting Information *

ABSTRACT: A novel molecular device (trans-azobenzene embedded N-(11-pyrenyl methyl)aza-21-crown-7) with double functional devices was designed on the basis of theoretical calculations. Pyrenyl methyl covalently bonded to aza-21crown-7 at the nitrogen position interacting with a series of alkaline-earth metal cations (Mg2+, Ca2+, Sr2+, and Ba2+) was investigated. The fully optimized geometries and real frequency calculations were investigated using a computational strategy based on density functional theory at B3LYP/631G(d) level. Free ligand (L) and their metal cation complexes (L/M2+) were studied using mixed basis set (6-31G(d) for the atoms C, H, O, and N and LANL2DZ for alkaline-earth metal cations Mg2+, Ca2+, Sr2+, and Ba2+. The natural bond orbital analysis that is based on optimized geometric structures was used to explore the interaction of L/M2+ molecules. The absorption spectra of L and L/M2+, excitation energies, and absorption wavelength for their excited states were studied by time-dependent density functional theory with 6-31G(d) and LANL2DZ. A new type of molecular device is found, which has the selectivity to Ca2+ and the emission fluorescence of L/Ca2+ under the condition of illumination. This molecular device would serve as an allosteric switch and a fluorescence chemosensor.

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INTRODUCTION Supramolecular chemistry is a cross science in the field of chemistry. It is closely related to some areas including material science, information science, and life science in the forefront of modern chemistry. Because researchers proposed the concept of supramolecular host−guest,1−3 supramolecular chemistry has become a hotspot gradually and attracted great attention of scientists. The supramolecular compounds not only retain the inherent molecular chemical properties but also have some specific features and properties (photochemical properties, selective catalytic, sensor, etc.).4 Therefore, it has laid a good foundation for the design of new functional molecular devices. For example, 2,2′-bipyridine crown ether is one kind of representative supramolecular compounds, which selectively bonded to alkali metal cations and transition-metal ions, with a conformational change. It was found that the crown ether rings have allosteric switch function and could be applied in the field of supramolecular chemistry and biological processes, such as in the environmental monitoring and biological disease detection.5 Moreover, the macrocycle has been widely studied and applied in the field of supramolecular chemistry and biological processes.6−9 Experimentally, Weller devised a smart fluorescence sensor based on the supramolecular compounds.10 PET-fluorescence sensor usually comprises three parts: receptor unit, linking group, and fluorescent group. The receptor unit is formed by a © 2017 American Chemical Society

recognition of cavity structure, such as polyamines, boric acid, crown ether, and calixarenes. Linking groups belong to alkyl groups. Fluorescence groups belong to aromatic derivatives (naphthalene, anthracene, pyrene,11−15 etc.). When the ligand does not selectively accept the object molecules, the lone electrons of linking groups in the visible light are induced and shift to fluorescence emitting group, resulting in fluorescence quenching. When the ligand selectively accepts the object molecules, the electron-transfer process is blocked by fluorescence chemical sensors. This is the interpretation of the “on−off” phenomenon in the PET chemical fluorescence sensor. Supramolecular chemistry has become a mature discipline, and the design of molecular sensors and switches has been rapidly developed in organic chemistry, analytical chemistry, and biological chemistry.16−22 The single function molecule cannot satisfy people’s pursuits for greater convenience, efficiency, and economy. Azobenzene is a kind of common optical switching element; trans−cis isomerization will occur under the conditions of light and heat.23 Aza-crown ether can present convertible switch property coordinated to alkali metal ions, trans isomers do not appear any coordination ability, and cis isomers that have crown ether ring cavity can Received: October 13, 2016 Revised: January 5, 2017 Published: January 5, 2017 1436

DOI: 10.1021/acs.jpcc.6b10228 J. Phys. Chem. C 2017, 121, 1436−1441

Article

The Journal of Physical Chemistry C bond to alkali metal ions very well. This phenomenon is interesting; however, the detailed mechanism of this sensor is still not clear. Density functional theory (DFT) provides a useful tool for the prediction of properties of supramolecular compounds.24−29 A new type of double functional molecular device, pyrenyl methyl combined with aza-crown ether, has been designed with DFT and time-dependent functional theory (TD-DFT). Figure 1 is the designed sketch of new molecular

Figure 1. Design sketch of new molecular devices.

devices. Here all of the optimization and frequency calculations are performed by Gaussian 03 at B3LYP/6-31G(d)30 and with LANL2DZ basis set for the heavy atoms (Sr2+ and Ba2+).31 Frequency calculation, the lowest energy, and the thermodynamic parameters for each configuration (binding energy, binding enthalpy, and Gibbs free energy) have been calculated. Moreover, natural bond orbital (NBO) calculations for all stable configurations of complexes L/M2+ are also performed.32,33 In addition, the excitation energy of the coordination compounds was calculated by TD-DFT/ B3LYP.34−36

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RESULTS AND DISCUSSION Optimization of the Structure of Ligand L. The optimized geometries of the free ligands in both cis-L and trans-L configurations are shown in Figure 2; more details of the structural information are shown in the Supporting Information. The (−CH2−O−CH2−)n chain of trans-azobenzene-embedded N-(10-pyrenyl methyl)aza-21-crown-7 almost becomes a straight line, forming a strip crown ring. The cisazobenzene embedded N-(10-pyrenyl methyl)aza-21-crown-7 forms a hole that is similar to the crown ring hole. According to the supramolecular predictive rules, trans aza-crown ether molecules do not coordinate with the metal ions, but cisconfiguration shows well. The calculated results show that the thermodynamic values of cis aza-crown ether are higher than those of the corresponding trans isomer, which are consistent with the experimental facts. Most trans isomer azobenzene compounds could transform into the cis isomer under UV irradiation. As can be seen from the azobenzene bond angle and bond length data of ligand L, the values change little. The azobenzene remains a monomer configuration into a crown ether ring, but alkaline earth metal ions bond free ligand, and azobenzene dihedral angles ∠CNNC and ∠NNCC and the 4,4 dC−C value greatly change. Optimization of the Structures of L/M2+ Complexes. The full optimized geometric structures of L/M2+ complexes are also shown in Figure 2. The bond length between the alkaline earth metal cations (M2+) and the oxygen or nitrogen atoms of the ligand L is listed in Table 1. Compared with the

Figure 2. Optimized geometry structures of the metal-free ligands L and L/M2+calculated with the B3LYP/6-31G(d) method.

ligand L, the conformation of the crown ether ring occurs at different degrees of distortion; in particular, azobenzene significantly changes in the ring, and the position of two benzene rings changes little for L/Mg2+, but for the L/(Ca2+, Sr2+, Ba2+), two benzene rings are located in the face-to-face manner. Because of the size mismatching, the change ranges of the bond lengths of O−M2+ and N−M2+ are relatively large for complexes L/Mg2+. The cavity of aza-crown ether ring is large, 1437


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The Journal of Physical Chemistry C

Table 3. Values of EHOMO−1(eV), EHOMO(eV), ELUMO(eV), and ΔE(eV) of L and Their Alkali−Earth Metal Cation Complexes L/M2+

Table 1. Geometrical Parameters of Complex L/M2+ Obtained Using the B3LYP/6-31G(d) method rO/N‑M2+(Å)a atoms

Mg

N57 N58 N59 O60 O61 O62 O63 O64 O65 averageb

2+

2.211 6.916 7.367 6.226 6.139 2.008 2.074 5.188 2.105 4.470

Ca

2+

4.134 6.695 6.706 2.465 2.538 2.411 2.520 2.484 2.450 3.600

Sr2+

Ba2+

molecule

EHOMO−1

EHOMO

ELUMO

ΔE

4.209 6.651 6.655 2.623 2.681 2.564 2.647 2.606 2.596 3.692

4.351 6.719 6.717 2.826 2.896 2.740 2.819 2.795 2.777 3.849

L L/Mg2+ L/Ca2+ L/Sr2+ L/Ba2+

−5.4 −10.8 −10.1 −10.1 −10.0

−5.1 −10.2 −9.2 −9.2 −9.1

−1.7 −7.2 −6.6 −6.6 −6.6

3.4 3.0 2.6 2.6 2.5

some important roles in the perfect coordination between the alkaline earth metal cations and crown ether ring. From Table 1, compared with the other O and N in the ring, we can find that the bond length values of M2+ with N58 and N59 have great difference, which indicates that metal ions may not interact with N58 and N59. Natural Bond Orbital Analysis. The optimized structures of the free ligand and metal complexes are used for natural bond orbital (NBO) calculation. The second-order perturbation stabilization energies (E2) are usually applied to reflect the interaction energies of host−guest system.37 The stabilization energies E2 of complexes L/M2+ are listed in Table 2. The interaction of host and guest molecules is mainly caused by the interactions between the bonding orbitals of oxygen atoms and nitrogen atoms (provide the lone pair electrons) on the crown ether ring and the antibonding orbitals of alkaline earth metal cations. As the stabilization energy E2 increases, the interaction becomes stronger, and the structures of complexes become more stable. For alkaline earth metal ions and ligand L, the host−guest molecular interaction of the empty antiorbitals of M2+ and the lone-pair electrons of N57, N58, N59, O60, O61, O62, O63, O64, and O65 in aza-crown ether ring reflect in the stabilization energy E2. The results indicate that N58 and N59 do not participate in the bond formation at the azobenzene, which are consistent with the experimental speculation results. For L/Mg2+, a part of the N and O atoms in the crown ether ring interact with Mg2+. But for L/M2+ (Ca2+, Sr2+, Ba2+), the alkaline earth cations could coordinate with all O atoms on the crown ether ring and N57. Compared with Mg2+, Sr2+, and Ba2+, the Ca2+ has the strongest effect with aza-crown ether ring, which is the most stable complex. NBO analysis results conform to the bond length analysis results above. In the L/Mg2+ complex, Mg2+ located near the pyrene group, the π electronic of the double bond

a

rO/N‑M2+: The metal−heteroatom coordination bond lengths (Å) of each optimized complex L/M2+. bAverage: The metal−heteroatom average coordination bond lengths (Å) in the parent rings of each optimized complex L/M2+.

while the Mg2+ radius is small. They cannot cooperate very well. The Mg2+ only interacts with a part of O or N in the crown ether ring. As the radii of metal ions (Mg2+, Ca2+, Sr2+, Ba2+) increase, the change range of the bond lengths is also much different. The average bond lengths of complexes L/M2+ (Mg2+, Ca2+, Sr2+, Ba2+) are 4.470, 3.600, 3.692, and 3.849 Å, respectively. The average bond length of L/Ca2+ is shortest, so it has the strongest effect. The results indicate that the Ca2+ has the strongest interaction with L, Sr2+ and Ba2+ come secondary, and Mg2+ is the weakest one. The reason is the size match rule between the cavity size of the ligand L and the radius of the alkaline earth metal cations (M2+) during the molecular recognition process. Because the radius of Mg2+ is too small, the crown ring of L must twist significantly to close to it. The radii of Sr2+ and Ba2+ are larger than Ca2+. Above all, the cavity size of the crown ether ring and the cation diameter are two dominant factors that affect the coordination behavior of the crown ether ring and cations. It is indicated that compared with Mg2+, Sr2+, and Ba2+, the alkaline earth metal cation Ca2+ has the best coordination with the six oxygens of the crown ether ring, thus forming a stable structure. On one hand, the ion radii of Sr2+, Ba2+ are larger than Ca2+; on the other hand, pyrene is a large fluorescence substituent group that connects to N through a −CH2−. Thus the steric hindrance should play

Table 2. Selected Stabilization Interaction E2 (kcal/mol) for L/M2+ (Mg2+, Ca2+, Sr2+, Ba2+) donor NBO(i) → acceptor NBO(i)

E2 (kcal/mol)

donor NBO(i) → acceptor NBO(j)

E2 (kcal/mol)

BD(1) C74-C75→LP*Mg BD(1) C74-76→LP*Mg BD(1) C76−78→LP*Mg LP N57→LP* Mg LP O62→LP* Mg LP O63→LP*Mg LP O65→LP*Mg LP N57→LP*Sr LP O60→LP*Sr LP O61→LP*Sr LP O62→LP*Sr LP O63→LP*Sr LP O64→LP*Sr LP O65→LP*Sr

1.78 2.46 1.20 8.76 9.97 9.57 7.11 0.41 5.98 4.97 5.27 5.45 5.33 5.12

LPN57→LP*Ca LPO60→LP*Ca LPO61→LP*Ca LPO62→LP*Ca LPO63→LP*Ca LPO64→LP*Ca LPO65→LP*Ca LPN57→LP*Ba LPO60→LP*Ba LPO61→LP*Ba LPO62→LP*Ba LPO63→LP*Ba LPO64→LP*Ba LPO65→LP*Ba

0.08 8.74 7.90 7.58 8.26 7.86 6.98 0.12 3.15 2.62 3.10 2.33 3.02 2.99

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Figure 3. Ultraviolet absorption spectra of L and L/M2+ (Mg2+, Ca2+, Sr2+, Ba2+).

C74C75, C74C76, and C76C78 of the pyrene group could interact with Mg2+, which can effectively stabilize the L/Mg2+. Frontier Orbitals Analysis. The frontier molecular orbital diagrams of L and L/M2+ are shown in the Supporting Information. The results indicated that the HOMO and LUMO of L are mainly distributed in the azobenzene, HOMO−1 orbital is mainly distributed in the pyrene fluorescence group,

and the N atom connects to the interval group. The HOMO of L/Mg2+ is mainly distributed in the azobenzene, HOMO−1 and LUMO mainly distributed in the pyrene fluorescence group, and N atom connects with the interval group. The HOMO−1 and HOMO of L/M2+ (Ca2+, Sr2+, and Ba2+) mainly distributed in the pyrene fluorescence group and N atom connects with the interval group; LUMO is mainly distributed 1439


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fluorescence. Compared with L/M2+(Ca2+, Sr2+, and Ba2+), the fluorescence intensity of L/Mg2+ has decreased. According to the above analysis, we know L/Ca2+ is the most stable in the four metal complexes. On the basis of stability and fluorescence, we believe that L/Ca2+ is the best choice. The results are wellconformed to the above conclusions.

Table 4. Calculated Data for the Gas-Phase Free Ligand L and Its Metal Complexes L/M2+(Mg2+, Ca2+, Sr2+, Ba2+) at the Excited States Using the TD-DFT Method at the B3LYP/6-31G(d) Level molecule L L/Mg2+

L/Ca2+ L/Sr2+ L/Ba2+

configuration (CI coefficient) 179→180 178→180 182→186 184→186 184→185 183→184 182→184 183→184 182→184 183→184 182→184

(0.675) (0.697) (−0.181) (0.670) (0.702) (0.707) (0.707) (0.707) (0.707) (0.707) (0.707)

E (eV)

λabs (nm)

f

2.35 3.44 2.47

527.7 360.0 502.3

0.010 0.004 0.025

2.60 2.40 3.29 2.40 3.28 2.33 3.21

476.0 515.3 377.1 516.7 377.8 531.6 385.6

0.022 0.032 0.003 0.038 0.001 0.034 0.000

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CONCLUSIONS DFT and TD-DFT methods are chosen to calculate (transazobenzene embedded N-(11-pyrenyl methyl)aza-21-crown-7) L and L/M2+. The optimized geometric structures and NBO analysis results show that under the condition of illumination, trans L changes into cis configuration, cooperates with alkaline earth metal ions Ca2+ very well, and displays the performance of the molecular switch. According to the frontier orbital analysis, absorption spectra, and excited-state calculation, L combined with Ca2+ is a classic PET fluorescence sensor. We found that the new molecular devices have double functions.

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in the azobenzene. It can be seen that in free ligand L the lonepair electrons of N are shifted. Compared with the free ligand L, metal ions (Ca2+, Sr2+, and Ba2+) cooperate with aza-crown ethers ring. The HOMO of the complexes is mainly distributed in the pyrene fluorescence group, and a part of the lone-pair electrons in N is still existing. As mentioned above, the NBO analysis can explain the results well. It shows that the metal ions prevent the lone-pair electrons of N from shifting to the fluorescent pyrene group. The EHOMO−1, EHOMO, ELUMO, and ΔE energy values are listed in Table 3. Because of the interactions between the ligand L and the alkaline earth metal cations, the EHOMO−1, EHOMO, and ELUMO values are decreased, and accordingly, the ΔE also has a little decrease. Absorption Spectra and Excited-State Calculations. The TD-DFT/B3LYP method was chosen to perform the absorption spectrum calculation of the optimized geometry of L and L/M2+. Figure 3 displays the calculated ultraviolet absorption spectra of L and L/M2+. The range of absorption wavelength of free ligand L is 220−600 nm. There are three obvious absorption peaks, with the wavelength 266.0, 329.0, and 527.8 nm, respectively. For L-cooperated metal M2+, its absorption wavelength range is 220 to 450 nm, becoming much narrower. L/(Mg2+, Ca2+, and Sr2+) has two obvious absorption peaks. L/Ba2+ only has a strong absorption peak, and the peak absorption spectra have a blue shift in comparison with ligand L. Pyrene has four ultraviolet absorption bands, respectively, at 240, 270, 290−340, and 360 nm. L/Ca2+ has two ultraviolet absorption bands, respectively, in 272 and 340 nm. The results show that Ca2+-cooperated L retains the nature of pyrene monomer. L/Ca2+ is a kind of fluorescence molecular sensor. The longwave absorption of L/M2+ absorption spectra peak disappears. This is due to the electron-donating atoms in the crown ether ring that have strong interaction with metal ions. It forms a weak charge transfer in molecular so that the longwave peaks disappear. From Table 4, it is found that the first excitation state mainly takes place by the HOMO → LUMO transition for L, which corresponds to the π → π* transitions of pyrene fluorophore. Meanwhile, the orbital occupied by the lone pair electrons of nitrogen atom to the fluorophore π* orbital n → π* transition also exists, which produced photoinduced electron transfer (PET). Therefore, the fluorescence quenching of L occurred. For L/M2+(Mg2+, Ca2+, Sr2+, and Ba2+) complexes, only π → π* transitions of fluorophore occurred; they do not have a PET process, so the excited state of L/M2+complexes can produce

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b10228. Table S1. Comparison of parameters of the calculated parameters of trans- and cis-isomer ligands optimized at the B3LYP/6-31G(d) and LANL2DZ level. Figure S1. Frontier molecule orbitals of L and its metal complexes L/M2+ (Mg2+, Ca2+, Sr2+, Ba2+). (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 0731 58292206. Fax: +86 0731 58292251. E-mail: [email protected]. ORCID

Xueye Wang: 0000-0001-5578-6504 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Scientific Research Fund of Hunan Provincial Education Department (16A047). REFERENCES

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Theoretical Design of a New Allosteric Switch and Fluorescence

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