Bipyridyl under External Field by Surface-Enhanced Raman

ARTICLE pubs.acs.org/JPCC

Probing the Conformational Transition of 2,20 -Bipyridyl under External Field by Surface-Enhanced Raman Spectroscopy Zhixun Luo,† Boon H Loo,‡ Xinqiang Cao,† Aidong Peng,† and Jiannian Yao*,† † ‡

Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, the Chinese Academy of Sciences, Beijing, 100190, China Department of Chemistry, Towson University, Towson, Maryland 21252, United States

bS Supporting Information ABSTRACT: Investigations on conformational transition of a small organic molecule are important to understand the conformation principles in chemistry and biology. We employed a low-temperature surface-enhanced Raman spectroscopy (LT-SERS) technique to probe the conformational changes of 2,20 -bipyridyl (22BPY) on Ag nanoparticles at the presence of external fields. An electrochemical system was used to provide an electrostatic field, and a special magnet was designed to supply a magneto-static field. High-quality and distinguishable SERS spectra of 22BPY were obtained at the different environments, which show fingerprint labels for correlative conformations of the 22BPY. The conformational transition of 22BPY is implemented via its adsorption on the Ag nanoparticles by triggers of the external electric field and magnetic field.

1. INTRODUCTION Conformational transition, involving a diversity of pathways and distribution of time scales from one molecule to another, is recognized of importance to organizing reactive and functional groups. Recently, there has been considerable interest in probing the conformational changes of biological macromolecules. For example, DNA layers were reported to exhibit a persistent electric conformation switching effect,1 and SecA-protein was investigated to function through large conformational changes and even has motor function.2,3 In particular, there is a topic of major interest on human health that conformational changes of proteins can induce the formation of amyloid fibrils which are associated with diseases including bovine spongiform encephalopathy (BSE), Alzheimer’s, Parkinson’s, and CreutzfeldtJakob disease (CJD).4,5 With rising interest in probing the conformational dynamics and potentials,6,7 however, corresponding molecular details remain elusive. It is well-known that the interconvert of macromolecules with conformation multiplicity depends on the part of the structure near a certain junction, thus general knowledge on conformational transition of correlative small organic molecules is beneficial to understand the essential attributes and principles of the large biological molecules. An aromatic nitrogen-containing heterocyclic compound, 2,20 -bipyridyl (22BPY), is chosen as the target molecule in this study.8 The 22BPY has been extensively investigated with respect to its abilities as electron acceptors, electron carriers, proton sponges, and chelating agents, etc.912 In a 22BPY molecule, two planar pyridyl rings are connected via a CC bond (Scheme 1). The influence on the rotation of the pyridyl rings around the central CC bond is the novelty associated with its conformations. r 2011 American Chemical Society

Normally, 22BPY displays a trans-conformation in the crystalline state and presents a C2h symmetry, while it also exists as a cisconformation with a C2v symmetry, especially in the condition when it forms a 22BPYmetal complex.10,1317 It is interesting to probe the transition between the two conformations in external conditions and hence provide information on isomerization of organic molecules and biologic species. It has been reported that an electric field can induce conformational transition of proteins as a result of electric birefringence and the electrooptical Kerr effects;1820 on the other hand, magnetic field effect was also found to give rise to isomerization in organic reactions.21,22 Herein, both an external electric field and a specially designed magnetic field are applied to trigger the conformational transition of the small organic molecule 22BPY on Ag nanoparticles. Surface-enhanced Raman spectroscopy (SERS)23 is employed to in situ investigate this transition process. SERS can provide a wealth of information regarding vibrational modes, chemical structures, and conformations of target molecules.2327 In particular, SERS capability of trace analysis and detection is down to a single-molecule level, which enables the probing of an individual molecular conformational transition (as a molecular personality).2830 In light of this, we have attempted to improve SERS sensitivity via a uniform assembly of Ag nanoparticles as substrates;31,32 meanwhile, ultradilute solution of 22BPY was used to approach the aim of the single-molecule level, and to Received: September 5, 2011 Revised: October 19, 2011 Published: November 04, 2011 2884

dx.doi.org/10.1021/jp208566d | J. Phys. Chem. C 2012, 116, 2884–2890

The Journal of Physical Chemistry C

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Scheme 1. Molecular Structures and Calculated HOMO Orbitals of the 2,20 -Bipyridyl (22BPY, C10H8N2) with trans- (top) and cis-Conformation (bottom)

eliminate unwanted spectral fluctuation from such low-concentration SERS systems, a low-temperature technique was utilized.33 As a result, conformational transition of the organic molecule under the external fields is unambiguously clarified.

2. EXPERIMENTAL SECTION The 22BPY sample (purity at 99.5%) was purchased from Acros. The molecular structures and calculated HOMO orbitals of the 22BPY are shown in Scheme 1. The solution of 22BPY was prepared using Milli-Q water with a resistivity of 18.2 MΩ 3 cm1, diluted stepwise, until the final concentration up to picomolar per liter (i.e., 1012 M). The colloidal Ag nanoparticles were synthesized according to Lee’s procedure.34 All the other chemicals used were analytical reagent. The substrates (i.e., cover glasses and rough Ag electrodes) were cleaned and pretreated and then dried in vacuum before coating the as-prepared Ag nanoparticles (for detail see Supporting Information). The Ag nanoparticles were gradually added with a weak ultrasonic agitation. After they were dried again, the SEM pictures (Figure 1) show that the colloidal Ag nanoparticles were uniformly assembled on the cover glass and Ag electrode. A 20 μL volume of 22BPY target solution was rapidly added onto the substrates and dried in vacuum for Raman measurements. The Raman spectra were recorded with a Raman microprobe system (RENISHAW H13325 spectrophotometer), using the excitation line of 514.5 nm from an Ar ion laser and the laser microprobe spot at 1 μm diameter. With a holographic notch filter and a CCD detector, this system has extremely high detection sensitivity. The laser power used was 25 mW (corresponding power at the sample spot is 12 mW or so). The experimental laser acquisition time was 10 s for each scan, and the spectral resolution was 4 cm1. As an accessory of the Raman system, a low-temperature setup used in this work was also provided by the Renishaw Company. We used 150 °C for the low-temperature SERS measurements on the nonaqueous systems (cover glass samples); while for the electrode SERS system, ice-cold KCl solution (0.1 M) was employed, and cold nitrogen gas from liquid nitrogen was used to blow outside the spectro-electrochemical cell to keep the temperature at 0 °C. The electric field is supplied via a three-electrode spectroelectrochemical system according to Lambert’s method.35 On the other hand, a magnet was specially designed with necessary dimensions to fit the Raman instrument. The magnet can be raised, lowered, and rotated without disturbing the inner sample

Figure 1. SEM images of the uniform assembly of Ag nanoparticles.

lifting pillar. We used a 1.5 T magnetic field in this study, as shown in Figure 2.

3. RESULTS AND DISCUSSION We have first attempted to trigger the conformational changes of 22BPY by using an electric field based on a spectral-electrochemical electrode SERS system. Figure 3a presents the SERS spectrum of 22BPY at the case of 0 V potential, measured with ice cold (0 °C) KCl solution (0.1 M) as electrolyte. It is interesting to find the spectrum being different from the SERS features in previous literature, especially those which have been assigned to cis-conformation of 22BPY.1315 On the contrary, it shows similarity to the solid Raman of 22BPY and hence can be referred to as the trans-conformation (for details see Supporting Information, Figures S1, 2). This accords with the previous reports that a planar organic molecule favors the adsorption in a flat geometry 2885

dx.doi.org/10.1021/jp208566d |J. Phys. Chem. C 2012, 116, 2884–2890


Figure 2. Sketch map of the designed magnet. The inset label shows the strength of the magnetic field.

in terms of the principles of lowest energy36,37 and also can be ascertained through observation of the differences in the intensity enhancements between the in-plane (i.p.) and out-of-plane (o.o.p.) molecular motions (Table 1) which will be further discussed in the following.8,3846 Similar to Figure 3a, the curves of 3b∼h refer to the spectra under different potential conditions from 0 to 1.4 V. (We did not collect the spectra under more negative potentials because the very negative potentials lead to desorption and even hydrogen evolution.)47 At first sight, all these spectra in Figure 3 can be simply separated into three groups: group I referring to Figure 3a∼c with low negative potentials (0 ∼ 0.4 V); group II for Figure 3d and 3e (0.6 V, 0.8 V); then group III for Figure 3f∼h with relatively higher negative potentials (1.0 to 1.4 V). In group I, the SERS intensities take on a minor increasing tendency with the negative potentials. This is because the negative potentials will increase the Fermi energy level of the system, and the charge transfer (between 22BPY and Ag) contributes to the SERS effect, as well as the enhanced local electric field itself (μ = αE).23 Except the small difference on the absolute intensity, the peak values and relative intensity ratio of the main modes, such as the 651/764 cm1 band (in-plane ring deformation) and 1485/1563/1595 cm1 band (ring stretching and CH in-plane deformation), show almost no changes among the three spectra in group I (Figure 3a∼c), which suggests that there is no conformational transition for the 22BPY at the case of small negative potentials (0 ∼ 0.4 V). This also accords with the previous results based on electrochemistry and second harmonic generation (SHG)17 where the 22BPY favored a flat-adsorption behavior at a negative voltage less than 0.4 V. Actually, the appearance of the strong and sharp peak at the 1027 cm1 band (ring breathing) has also been suggested as a correlation of flat adsorption of 22BPY on the metal surface.16,48 However, dramatic changes were observed with the negative potentials being further increased, shown as the two spectra in group II where the relative intensities of the main modes change gradually, and the 1027 cm1 band (also 1313/1315 cm1) is

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Figure 3. Potential dependent SERS spectra of 22BPY molecules on a Ag-coated Ag electrode (20 μL volume of 22BPY target solution was rapidly added onto the electrode surface and dried completely before adding the ice-cold KCl solution, 0.1 M, 0 °C). Curves a to h refer to the spectra collected respectively at 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, and 1.4 V. All spectra were measured at the same spot. Insets at the bottom show the three conformations of 22BPY.

weakened and turns to three small peaks (Figure 3e). Completely different spectra are displayed at group III, in particular for the relative intensity of the three high-frequency modes. As one can see, it is I1489 ≈ I1563 < I1595 in Figure 3a to Figure 3c, I1485 ≈ I1559 ≈ I1595 in Figure 3d, then I1485 < I1558 > I1595 in Figure 3e, while it turns to I1485 > I1557 > I1600 in Figure 3f∼h. Besides, the 1027 cm1 mode becomes weak and blunt, and the side peak around it disappears completely. Because all these spectra are stable and repeatable at each potential condition (see Supporting Information, Figure S3), the dramatic changes exclude random spectral fluctuation, instead resulting from conformational transition of the 22BPY. Since SERS features as group III have been identified as cis-conformation of 22BPY,10,1315 the conformational transition process with the increase of negative potentials is inferred to take place from trans-conformation to cisconformation. In principle, a 22BPY molecule prefers flat-lying adsorption and parallel orientation with respect to the Ag surface at minor negative potentials (ca. 0 ∼ 0.4 V); then with the potentials changing more negatively, the delocalized π-electrons near the two pyridyl rings will receive an unneglectable Coulomb repulsion, and hence the two pyridyl rings are forced to be tilted with two legs sticking (N-atom bonding) on the Ag surface. Further increasing the negative potentials, the two pyridyl rings will tend toward stand-up adsorption, which implies a complete conformation transition. It is undoubted that, between the two extremes of stand-up and flat-lying, there could be intermediate orientations corresponding to lower symmetry conformations of 22BPY with torsion angles.16 For instance, assuming SERS experiments had been performed in relative higher concentration of solution by mixing with Ag-colloid, a competitive adsorption of molecules might result in a tilt orientation or even a stand-up 2886



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Table 1. Experimental Raman and SERS Bands of 22BPY and Assignment (cm1)a SERS of 22BPY Raman of 22BPY

type I

type II

type III

Raman of

(trans-conf.)

(trans-conf.)

(twisted conf.)

(cis-conf.)

Ru(22BPY)3Cl2 (ref 41)

assignment

612 (m)

651 (w)

657 (m)

658 (s)

658 (s)

i.p. ring def.

762 (m)

736/764 (w)

764 (m)

765 (w)

765 (w)

i.p. ring def.

807 (w)

811 (w)

810 (w)

993 (first)

1009/1025 (s)

1024 (s)

1022 (s)

1027 (s)

ring breathing

1041(m)

1060 (w)

1061 (w)

1061 (w)

1089 (w)

1108 (w)

1143 (w) 1234 (s)

1170 (w) 1266/1278 (w)

1169 (m) 1268 (m)

1169 (s) 1271 (s)

1171 (s) 1272 (s)

ring str. + CH i.p. def. ring str. (CC, CN) + CC inter-ring str.

1300 (s)

1313 (s)

1316 (s)

1318 (vs)

1318 (vs)

CC inter-ring str. + ring str.

1428 (w)

1479 (s)

1487 (s)

ring str. + CH i.p. def.

1110 (w)

1357 (w) 1443 (s)

CH o.p. def.


1365 (m) 1427 (w)


1485 (s)

1485 (first)

1485 (first)


1571 (vs)

1563 (s)

1558 (first)

1558 (s)

1557 (s)

ring str. (CC, CN)

1587 (first)

1595 (first)

1595 (s)

1600 (s)

1606 (s)

ring str. + i.p. def.

1685 (w) a

Note: conf., conformation; i.p., in-plane; o.p., out-of-plane; str., stretching; def., deformation; w, weak; s, strong; first, strongest.

Figure 4. SERS of 22BPY on Ag-nanoparticle coated cover glass, measured at 150 °C, in the absence (a) and presence (b) of a 1.5 T magnetic field. The 1022 cm1 mode is enlarged on the right. Both spectra were measured at the same spot. The sample was prepared by rapidly adding a 20 μL volume of 22BPY solution (pM) onto the Agcoated cover glass and drying completely.

orientation directly.49 This is the reason why previous investigations have ascertained the cis-conformation of 22BPY, but SERS features due to the trans-conformation have not been observed by far. We have also considered a strong magneto-static field to trigger the conformational transition of 22BPY. A magnet (Figure 2) was specially designed for this study. As a result, SERS spectra of 22BPY molecules with and without the magnetic field (1.5 T, i.e., 15 000 G) present a remarkable difference, as shown in Figure 4. First, magic changes on relative intensity of the Raman modes are observed again; for instance, I651 < I762 in Figure 4a, while I651 > I762 in Figure 4b. The three main modes in the high-frequency region, in sequence, come in an order of I1485 ≈ I1558 < I1600 in Figure 4a, while they turn to I1485 > I1558 >

I1600 in Figure 4b. Second, there is the appearance of some additional peaks (1365, 1427, and 1685 cm1) and a small redshift (1489/1485, 1563/1558) or blue-shift (1594/1600 cm1) in Figure 4b. In addition, the 1022 cm1 mode (appearing at 993 cm1 for solid and 10021015 cm1 in solution16) takes on a minor decreased intensity and an enlarged half-width, shown as the inset in Figure 4. Similar to the result above, all these changes are associated with the conformational transition of 22BPY (from trans- to cis-) in the presence of the magnetic field.21 To further verify the 22BPY transiting from trans- to cisconformation, we employed the first-principles calculation to reproduce the experimental results. Figure 5 presents the calculated Raman activities by using density functional theory (DFT) at the B3LYP level with Lanl2dz basis sets as implemented in the Gaussian03 package.50 Optimized geometry is used for the calculations. The calculated results accord well with the experimental Raman frequencies, even with a similar spectral profile, which confirms the conclusion drawn above. In brief, SERS features as Figure 3a∼c and Figure 4a are assigned to the transconformation of 22BPY, and SERS features like Figure 3f∼h and Figure 4b aim at cis-conformation. The other SERS features (similar to Figure 3d, e) can be referred to as lower symmetry conformations existing between the two extremes, named as twisted conformation with different torsion angles.8 It is important how the electrostatic and magneto-static field have both triggered conformation transition of the 22BPY. Previous reports have shown possible mechanisms of isomerization behind the field effects.6 Some of them were based on macroscopic results of statistical nature, and others focused on the direct effect of electric field and magnetic fields on the ions, atoms, and molecules of biological systems. We quote here a dipole mechanism to explain the field effect on conformational transition of the 22BPY molecules.51 In physics, there are two sorts of dipoles: an all-known electric dipole and a magnetic dipole which is defined as a closed circulation of electric current. Although there are no known magnetic monopoles in nature, 2887



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Figure 5. (A/B/C) Calculated Raman activities of 22BPY, based on three models with different conformations on the Ag surface (a), respectively, compared with correlative experimental results (b), same as Figure 3b, e, and f. The Ag10 clusters were assumed to represent Ag nanoparticles, and the distances between Ag atoms were set to be 2.82 Å.

there are magnetic dipoles in the form of the quantum-mechanical spin associated with particles such as electrons, for it generates a magnetic field which is identical to that generated by a very small current loop.51 So it is not difficult to understand that the magnetic dipole of electrons in a 22BPY molecule will change subsequently when an external magnetic field was employed and result in the reconfiguration of the electrons and current loops. This is actually a function similar to that of an external electric field which affects the molecular electric dipole. Molecules have electric dipole moments due to nonuniform distributions of positive and negative charges on the atoms, including permanent dipoles, induced dipoles, and instantaneous dipoles which occur when electrons happen to be more concentrated at one place than another in a molecule.52 An induced dipole of polarizable charge distribution F generally originates from an electric field external to F. This resembles the potential-induced electric field near the electrode of the above SERS system, and accordingly, instantaneous dipoles of 22BPY might exist at the case of 0.6 to 0.8 V potential (Figure 3). Besides, the laser excitation might be of assistance. The photoisomerization in an external field has been proposed for cis and trans isomers of stilbene and piperylene,53 where the formation of cis-stilbene is preceded by singlettriplet conversion in a radical pair intermediate.21 Similarly, the conformational transition of 22BPY might also be associated with its ground states and excited states.54 On the other hand, a possible contribution from the laser excitation could be correlated with localized surface plasmon resonance (LSPR) of the Ag nanoparticles. It is well-known that the LSPR can be tuned at the presence of an external electric or magnetic field. The surface electromagnetic waves generated by LSPR propagates in a direction parallel to the metal/dielectric interface, where the oscillations on the boundary of the metal and the external medium are very sensitive to any changes of this boundary, involving the adsorption behavior and possible conformational transition. It is still worthy of note that the absolute Raman intensity of the spectrum in the presence of a magnetic field (Figure 4b) was enhanced, such as the modes of 658, 1169, 1271, 1318, 1485, 1558, and 1600 cm1, etc. On the basis of the quantum

Figure 6. SEM image of the as-prepared 22BPY microfibers via reprecipitation method by injecting saturated 22BPY solution into icecold Ag colloid (a), microscopy bright-field image of a single fiber (b), and correlative micro-Raman spectrum measured from this microfiber of 22BPY (c).

mechanical treatment of Raman scattering, the interaction Hamiltonian H is the sum of several terms including the magnetic dipole operator,55 H = Helectric + Hmagnetic + Hquadrupole + ..., where Helectric is the electric dipole operator, Hmagnetic the magnetic dipole operator, and Hquadrupole the electric quadruple operator. Accordingly, a strong magnetic dipole may have a possible enhancement effect; however, there was no enhancement observed for bulk 22BPY powder samples or the usual colloidal SERS samples where the corresponding spectra present a feature as group III (Supporting Information, Figure S4). So a conclusive origin for the enhancement on the intensity, rather than minor contributions from a magnetic dipole, is just due to the conformational transition (on Ag) which has been triggered by the magnetic field. Because of the bonding of 22BPY on Ag with N-atoms after conformation changes, the in-plane stretching modes are strengthened. Apparently, there is no chance for the 22BPY molecules to have a conformation transition in the bulk sample due to the molecular stacking, as well as in a colloidal system where the conformational transition of 22BPY has resulted in a [22BPYAgn] “complex” (Figure 5C). 2888


The Journal of Physical Chemistry C It is worth mentioning that all the changes in the presence of magnetic field (and also electric field) were not found to be reversible. This agrees with the previous theory that, in the initial steps of kinetics of isomerization, the back reaction is weak and can be neglected.21 Actually, once the conformational transition of a 22BPY molecule is triggered from trans to cis, the strong adsorption of the two N-atoms on Ag will not allow further conformational change from an opposite direction.10 Such principles enable the preparation of microcosmic architectures of the 22BPY with alternative conformations. In addition to the usual microcrystals with trans-conformation, we have gained microfibers of 22BPY with the cis-conformation by injecting saturated 22BPY solutions into ice-cold electronegative Ag colloid, as shown in Figure 6 (for more details, see Supporting Information, Figure S5 and S6). It is supposed that a number of 22BPY molecules chemically adsorb (with N-atom bonding) onto the electriferous Ag nanoparticles;10 simultaneously, they assemble into microfibers in rows15 but keep the cis-conformation without changes (Supporting Information, Figure S8). This is in accordance with the previous reports on cis-conformation of 22BPY in Ag hydrosols due to the solution-phase interactions and the strong adsorption of 22BPY on colloidal Ag nanoparticles.8 The high-quality spectrum in Figure 6b also suggests that microfibers of 22BPY combined with Ag nanoparticles could be a sort of SERS-active system with assembly of the probe molecules.56,57

4. CONCLUSIONS LT-SERS spectroscopy is introduced to probe the conformational transition of a small organic molecule. With the electric field and magnetic field as triggers, we investigated the conformation dynamics of 22BPY on Ag nanoparticles. Conformation labels of 22BPY were clarified upon the joint experimental and theoretical results. The external-field triggers and LT-SERS verification enable the control and trace of molecular conformations and indicate potential applications to control molecule structures in synthesis. ’ ASSOCIATED CONTENT

bS

Supporting Information. Samples and methods and experimental/calculation details. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 20733006, 50720145202) and the National Research Fund for Fundamental Key Project 973 (2006CB806200). ’ REFERENCES (1) Kaiser, W.; Rant, U. J. Am. Chem. Soc. 2010, 132, 7935. (2) Economou, A.; Wickner, W. Cell 1994, 78, 835. (3) Singhal, K.; Kalkan, A. K. J. Am. Chem. Soc. 2010, 132, 431. (4) Budi, A.; Legge, F. S.; Treutlein, H.; Yarovsky, I. J. Phys. Chem. B 2005, 109, 22641.

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Bipyridyl under External Field by Surface-Enhanced Raman

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