Two-dimensional (2D) correlation spectroscopy proposed by Noda ^[12] provides a new avenue in the field of spectroscopy. Many important subtle spectral features that are difficult to be observed in the original 1D spectra can be clearly visualized via the 2D spectra. Especially,intermolecular interactions have been extensively characterized by using cross peaks in the 2D correlation spectra ^[13]. However,interfering cross peaks due to other sources,such as concentration variations of solutes,also arise even if there is no intermolecular interaction. This problemmakes it difficult to directly usethecrosspeaksinconventional2Dspectrumasareliablecriterion to judge whether intermolecular interaction actually occurs or not.

To solve the problem,we have developed orthogonal sample design scheme (OSD),asynchronous orthogonal sample design scheme (AOSD) and double orthogonal sample design scheme (DOSD).Byusingthesemethods,interference signal thathasnothing to do with intermolecular interactions can be completely removed ^{[14, 15, 16, 17, 18, 19, 20]}. Consequently,spectral portion arisen fromdeviation to the Beer-Lambert’s law caused by intermolecular interactions between two solutes can be clearly visualized as specific cross peaks in the corresponding two-dimensional correlated spectrum. Moreover, spectral variations ona characteristic peak caused by intermolecular interaction include changes in peak position,bandwidth,and absorptivity,which can be revealed via characteristic spectral patterns of cross peaks in 2D spectra generated by using the OSD, AOSD and DOSD approaches. However,peak position,bandwidth and absorptivity of characteristic peaks of two solutes under intermolecular interaction may change simultaneously in some cases. The resultant spectral patterns are too complex to shed light on what happens on the molecular system.

In our previous work,we proposed an approach called double asynchronous orthogonal sample design (DAOSD) scheme ^[21]. By using this approach,a 2D asynchronous spectrum is decomposed into a pair of complementary sub-2D asynchronous spectra. As a result,spectral variation of different molecule can be reflected in different spectral regions. Therefore,subtle variations on the peak position,bandwidth and absorptivity from the characteristic peaks of two solutes involved in intermolecular interaction can be revealed. In our previous work,we have applied the DAOSD approach to several molecular systems and satisfactory results have been obtained ^{[22, 23]}.

In this paper,we applied the DAOSD approach on the pyridine hydrochloride/Nd³⁺ system.

Pyridine,hydrochloric acid and perchloric acid (A.R. grade) were purchased from Beijing Chemical Company. Nd₂O₃ with purity of 99.99% was purchased from Shanghai Yuelong Chemical Factory. Nd(ClO₄)₃ was prepared by dissolving the Nd₂O₃ in perchloric acid.

UV-vis spectra were recorded on a Lambda35 UV-vis spectrophotometer (Perkin Elmer) and all the spectra were measured at a scanning rate of 60 nm/min. Fluorescence spectra were measured on an F-7000 fluorescence spectrometer. The excitation wavelength was 255 nm. Both the excitation and emission slits were set as 5 nm.

To enhance the spectral signal from interaction between Nd³⁺ and pyridinium,the concentration ranges of Nd³⁺ and pyridinium should be similar. However,the absorptivity of the π-π transition band of pyridinium is very large (the molar absorptivity is around 10⁶ L mol^-1 m^-1),while the peak of Nd³⁺ is a forbidden transition, whose molar absorptivity is between 10² L mol^-1 m^-1 and 10⁴ L mol^-1 m^-1. That is to say,there is a large difference on the absorptivity between the π-π transition band of pyridinium and f-f transition band of Nd³⁺. This problem makes it difficult to get UV-vis spectra with acceptable quality when the concentration of Nd³⁺ is comparable to that of pyridinium.

Herein,we develop a thickness-variable cuvette whose pathlength ranges from 10 mm to 1 cm to solve the problem. Scheme 1 shows the structure of the thickness-variable cuvette.ABaF₂ window was put at one end of each stopple. Thepathlengthof the solution can be changed by adjusting the positions of the two stopples. We measured the UV-vis spectra of the pyridine hydrochloride when pathlength of the sample is set to be 100 mm.When the f-f transition peak of Nd³⁺ is recorded,the pathlength is set at 1 cm.With the help of thickness-variable cuvette,the absorbance of both the f-f transition band of Nd³⁺ and the π-π transition band of pyridine hydrochloride are similar,while the concentrations of the two solutes are close to each other (2-20 mmol L^-1) (Scheme 1).

Scheme 1

	Download: JPG larger image
Scheme 1. Schematic diagram of the thickness-variable cuvette.

The chemical systems studied here are aqueous solutions containing neodymium perchloride and pyridine hydrochloride. Some levels of solvent-solute interactions are inevitable,since a solvating layer occurs around each solute species. These interactions may lead to a deviation from linearity between absorbance and concentration of the solutes. Such a deviation may also produce cross peaks that reflected solute-solvent interactions rather than solute-solute interactions. Therefore,some additional maneuver should be made to remove the interference caused by solvent-solute interactions.

The solute and its solvating layer can be regarded as a separate entity i.e.,solvated-solute complex. Within a suitable concentration range of the solutes,the integrity of the solvated-solute complex remains undisturbed and the solvated-solute complex also follows the Beer-Lambert law. Hence,intermolecular interactions among solute and solvent do not produce any interfering cross peaks in 2D asynchronous spectra and the interference caused by the solvent-solute interactions can also be effectively removed.

In this experiment,a series of aqueous solutions containing different amount of Nd³⁺ were prepared. Similarly,another series of aqueous solutions containing different amount of pyridine hydrochloride (pH < 3) were also prepared. UV-vis spectra of the two groups of solutions were recorded. The absorbance of the pyridine hydrochloride at 255 nm is plotted against the concentration of pyridine hydrochloride and the result is shown in Fig. 1(a). Similarly,the absorbance of the f-f transition band of Nd³⁺ at 575 nm is plotted against the concentration of neodymium chloride (Fig. 1(b)). Good linearity (R > 0.999) can be obtained between absorbance and concentration. The suitable concentration ranges of Nd³⁺ and pyridine hydrochloride are 2-20 mmol/L and 20-160 mmol/L,respectively.

Fig. 1

	Download: JPG larger image
Fig. 1. Linear fitting results on the relationship between absorbance and concentration: (a) pyridine hydrochloride (R > 0.999) and (b) neodymium perchlorate (R > 0.999).

The description of the DAOSD approach in detail can be found in our previous papers. Here we just give a brief description on the procedure to construct 2D asynchronous spectra by using the DAOSD approach.

To investigate the character of the Nd(ClO₄)₃/pyridinium/H2O system,the 2D asynchronous spectra were constructed following three processes:

Step 1: A series of aqueous solutions with different concentration of pyridinium were prepared. Then the UV-vis spectra of these solutions were recorded to research the relationship between the absorbance of pyridinium and concentrations of the solutes. The suitable concentration range was chosen to make sure the mentioned relationship was linear (Fig. 1(a)). Similarly,the suitable concentration range of Nd(ClO₄)₃ aqueous solutions was obtained based on the rule that the UV-vis absorbance of f-f transition band from Nd³⁺ ions formed a linear relationship toward the concentrations of the Nd(ClO₄)₃ (Fig. 1(b)). The suitable concentration ranges for pyridinium and Nd³⁺ used in this paper were 2-16 mmol/L and 20-140 mmol L^-1,respectively.

Step 2: Four solutions containing pyridinium or/and Nd(ClO₄)₃ built one group,and six groups were prepared. The concentrations of solutions in the six groups were listed in Table 1. 1D spectra of solutions of group I were used to generate Ψ_Nd. According to the requirement of DAOSD approach,the concentrations of Nd(ClO₄)₃ were invariant while the concentrations of pyridinium were variable in this group. 1D spectra of solutions of groups II and III were used to generate modified reference spectra in the construction of Ψ_Nd. The concentration series of pyridinium/ Nd(ClO₄)₃ in the solutions of groups II/III met the requirement of generation of modified reference spectra which had described in our previous papers. Similarly,1D spectra of solutions of group IV were used to generate Ψ_pyridinium. Therefore,the concentrations of pyridinium were invariant while the concentrations of Nd(ClO₄)₃ were variable in the solutions of group IV. 1D spectra of groups V/VI solutions were used to generate modified reference spectra in the construction ofΨ_pyridinium. The concentration series of pyridinium/Nd(ClO₄)₃ in the solutions of groups V/VI met the requirement of generation of modified reference spectra.

Table 1

Table 1 Initial concentrations of pyridinium and Nd(ClO4)3 in aqueous solutions of the six groups of solutions.

Table 1 Initial concentrations of pyridinium and Nd(ClO4)3 in aqueous solutions of the six groups of solutions.

It should be pointed out that the concentrations of Nd(ClO₄)₃ and pyridinium in all solutions belonged to the six groups were within the selected concentration ranges obtained in step 1. Therefore,the possibility that cross peaks in the resultant 2D asynchronous spectra were caused by solute-solvent interaction rather than solute- solute interaction could be excluded in the following calculation. Then the UV-vis spectra of the solutions from the six groups were recorded.

Step 3: According to the 1D spectra obtained in the step 2,we constructed one pair of 2D asynchronous spectra (Ψ_Nd, Ψ_pyridinium),which were based on a program written in our lab using the software of MATLAB (The Math Works Inc.).

Typical UV-vis spectra of Nd³⁺ and pyridinium are shown in Fig. 2. In this paper,we use the peak at 575 nm as the characteristic peak of Nd³⁺. The broad π-π transition bands around 255 nm in UV-vis spectra are utilized as the characteristic peak of pyridinium. It should be pointed out that pyridinium does not show any absorbance in the 400-800 nm region while Nd³⁺ has no absorbance in the 240-270 nm region. Therefore,the overlapping problem of the characteristic peaks of two solutes is avoided. Moreover,the absorption peaks of solvent do not overlap with the characteristic peaks mentioned above. Thus,the requirement of the DAOSD approach is satisfied.

Fig. 2

	Download: JPG larger image
Fig. 2. The UV-vis spectrum of pyridine hydrochloride (the pathlength is 100 mm) and neodymium perchlorate (the pathlength is 1 cm).

A pair of 2D asynchronous spectra (denoted as Ψ_Nd and Ψ_pyridine) is constructed and the resultant 2D asynchronous spectra are shown in Fig. 3(a) and (b).

Fig. 3

	Download: JPG larger image
Fig. 3. The 2D asynchronous spectra of the pyridinium/Nd3+ system generated by using the DAOSD approach. (a) ΨNd and (b) Ψpyridine.

There is no cross peak around (575 nm,575 nm) in Ψ_pyridine, suggesting that neither peak position nor bandwidth of characteristic peak of Nd³⁺ undergoes changes when Nd³⁺ ions are mixed with pyridine hydrochloride. On the other hand,a strong cross peak can be observed around (575 nm,255 nm) in Ψ_Nd,suggesting that the absorptivity of the characteristic peak of Nd³⁺ increases as Nd³⁺ are mixed with pyridine hydrochloride.

Since the π-π transition band of pyridine hydrochloride is a broad peak with two shoulders locating around 250 nm and 262 nm,the complex spectral structure of the π-π transition band of pyridine hydrochloride produce complex cross peaks around (255 nm,255 nm) inΨ_Nd. However,a pair of cross peak at (250 nm, 262 nm) and (262 nm,250 nm) can still be observed. The cross peak around (262 nm,250 nm) is positive. This spectral pattern suggests that the π-π transition band of pyridine hydrochloride undergoes a red-shift as a consequence of interaction between Nd³⁺ and pyridine hydrochloride. In Ψ_pyridine,three cross peaks appear around (249 nm,575 nm),(254 nm,575 nm),(261 nm, 575 nm). The pattern of cross peak suggests that the absorptivity of the π-π transition band increases upon interaction with Nd³⁺. The extent of the variation of absorptivity is more pronounced. Thus, spectral pattern relevant to band-shift observed around (254 nm, 575 nm) in Ψ_pyridine is masked.

Thus,experimental results from a pair of 2D asynchronous spectra generating by using the DAOSD approach clearly demonstrate that intermolecular interactions occur between Nd³⁺ and pyridinium.

We would like to discuss the physicochemical nature of the interaction between Nd³⁺ and pyridinium. First,the possibility that pyridine coordinates with Nd³⁺ can be excluded. Nd³⁺ is a hard Lewis acid while the nitrogen atom of pyridine is a soft Lewis base. Thus,pyridine,as a monodentate ligand,has little chance to coordinate with Nd³⁺ in aqueous solution. Moreover,the pH value of the aqueous solution is below 3.0. As a result,more than 99.99% nitrogen atoms of pyridine are locked by proton. Furthermore, experimental results from 2D asynchronous spectrum provide additional evidence to support that no coordination occurs between Nd³⁺ and pyridinium. If pyridine molecules take part in coordination with Nd³⁺,the resultant changes on coordination sphere will bring about changes on the f-f transition peaks. However,no cross peak is observed around (575 nm,575 nm) in Ψ_pyridine,confirming that there is no change on either peak position or bandwidth of the f-f transition peak.

Therefore,observable cross peak in 2D asynchronous spectrum generated by using the DAOSD approach confirmed that interaction does occur between Nd³⁺ and pyridinium. The only reasonable explanation is that the interaction stems from interaction between π electron of pyridinium from aromatic system and f electron from Nd³⁺ ions. Such an interaction may bring about considerable influence on the spectroscopic behavior of the system. Here we provide an example.

Pyridine was dissolved in D₂O and acidified by DCl. Then fluorescent spectrum of pyridinium was recorded. When H2O and HCl were replaced by D₂O and DCl,high frequency vibrations relevant to hydrogen atoms can be suppressed so that fluorescent signal can be enhanced significantly. Afterwards,neodymium perchlorate was added into the solution and the fluorescent spectrum was collected. The results are shown in Fig. 4. The fluorescent bands of pyridinium in emission spectrum exhibit remarkable variation upon introducing neodymium perchlorate. From this fact,we suggest that interaction between p electron from aromatic system and f electron from lanthanide ions can be an important factor in the development of functional material containing lanthanides.

Fig. 4

	Download: JPG larger image
Fig. 4. The emission spectra of pyridinium-D2O solution and pyridinium/Nd3+-D2O solution (excitation wavelength = 255 nm).

We apply 2D asynchronous spectra generated by using the DAOSD approach to probe interactions between Nd³⁺ and pyridinium. Intermolecular interaction is proved to occur between Nd³⁺ and pyridinium via a series of cross peaks in the resultant 2D asynchronous spectrum. Since the possibility of coordination between Nd³⁺ and pyridine is excluded,the interaction stems from interaction between p electron from aromatic system and f electron from lanthanide ions. The interaction can produce remarkable influence on the spectroscopic behavior of the system. For example, the emission spectrum of pyridinium exhibits significant change upon introducing neodymium perchlorate into the system. The result demonstrates that 2D asynchronous spectroscopy,in conjunction with DAOSD scheme,provides us an experimental approach to characterize metal ions/organic functional groups interactions. Furthermore,the method can be applied to more extensive chemical systems with the help of the thickness-variable cuvette.

This project is financially supported by the National Natural Science Foundation of China (No. 51373003) and Beijing Natural Science Foundation (No. 2122059).