Theoretical and Experimental Study of Organic Nanomaterial for Acetate Anion Based on 1 , 10-phenanthroline

New phenanthroline derivatives containing phenol groups have been synthesized and optimized. The nano-material involved bromine substituent was also developed which in the orthoposition of benzene ring. Their binding properties were evaluated for various biological anions (F, Cl, Br, I, AcO and H2PO4 ) by theoretical investigation, UV-vis, fluorescence, H NMR titration experiments. These compounds all showed strong binding ability for AcO without the interference of other anions tested. The anion binding ability could be regularized by electron push-pull properties of the orthoor parasubstituent on benzene. Theoretical investigation analysis revealed the effect of intramolecular hydrogen bond existed between -OH and other atoms in the structure of these compounds.


1. Materials and Instrumentations
Most of starting materials were obtained commercially and all reagents and solvents used were of analytical grade.2,9-Dicarbaldehyde-1,10-phenanthroline and all anions, in the form of tetrabutylammonium salts (such as (n-C 4 H 9 ) 4 NF, (n-C 4 H 9 ) 4 NCl, (n-C 4 H 9 ) 4 NBr, (n-C 4 H 9 ) 4 NI, (n-C 4 H 9 ) 4 NAcO, (n-C 4 H 9 ) 4 NH 2 PO 4 ), were purchased from Sigma-Aldrich Chemical Co., stored in a desiccator under vacuum containing self-indicating silica, and used without any further purification.Tetra-n-butylammonium salts were dried for 24 h in vacuum with P 2 O 5 at 333 K before using.Dimethyl sulfoxide (DMSO) was distilled in vacuo after dried with CaH 2 .Melting points (mp) were determined with a Reichert-Jung Thermo Galen Hot Stage Microscope and reported as uncorrected.C, H, N elemental analysis was made on Vanio-EL. 1 H NMR spectrum was recorded on an UNITY Plus-400 MHz Spectrometer.ESI-MS was performed with a MARINER apparatus.SEM image was obtained by Quanta TM450 FEI coating it with Au.UV-vis Spectroscopy titration was made on a Shimadzu UV2550 Spectrophotometer at 298 K. Fluorometric titration was performed on a Cary Eclipse Fluorescence Spectrophotometer at 298 K.The affinity constant (K s ) was obtained by non-linear least square calculation method for data fitting.

3. Preparation of Nano-material
The organic nano-material of compound 2 was prepared by reprecipitation method.The DMSO and the water solution of CTAB (hexadecyl trimethyl ammonium bromide) were good solvent and poor solvent, respectively.In the experiment, the good solvent containing compound 2 (0.35 mL, 4 mmol • L -1 ) was poured into the poor solvent containing CTAB (100 mL, 3 mmol • L -1 ).The mixture was placed for 48 h and centrigugated.The expected solid was washed with water and dried with vacuum.

1. SEM Images of Compound 2
The SEM images were obtained by QuantaTM450 FEI coating it with Au (Fig. 1).From Fig. 1, compound 2 could assemble into flakiness on the whole.The width of flakiness was about 500 nm according to the scale.However, the size of flakiness was not very well-distributated which may be related with the concentration of compound 2 and the selection of poor solvent.The preparation of nano-material of compound 2 with other concentration and solvent was undergoing.

2. UV-vis Spectral Titration
The binding properties of four compounds with various anions were investigated through UV-vis titration experiment.The detail method was that the standard tetrabutylammonium salt solution of anion was added to dry DMSO solution of compounds at 298 K.In the absence of anion, compound 1 (4.0 × 10 -5 mol • L -1 in DMSO) displayed one obvious absorption peak at 350 nm (Fig. 2).Significant spectral changes were observed upon the addition of AcO -to the solution of compound 1.Upon the addition of AcO -, the absorption intensity of compound 1 at about 350 nm decreased and a new peak centered at 475 nm appeared gradually.In general, the red-shift phenomenon of absorption peak was occurred with the stepwise addition of AcO -.At the same time, one clear isosbestic point appeared at 387 nm which indicated compound 1 interacted with AcO -by hydrogen bond and the stable complex was formed between host and guest. 33Noticebly, the colorless solution of compound 1 turned into orange after the AcO -anion was added (Fig. 3).The color change was attributed to the significant formation of new absorption peak at 450 nm.Interestingly, the orange solution, host-guest complexation (1-AcO -), changed back to colorless after a small quantity of protic solvent (such as H 2 O or MeOH) was added to it.The above observations suggested the solvent, hydrogen-bond donor, destroyed the binding between compound 1 and AcO -and also demonstrated the binding between host and guest was hydrogenbond interaction in essence. 34,35The addition of F -also induced the spectral change of compound 1, however the spectral change was not as sensitive as the addition of acetate ion (Fig. 4).The above results maybe suggested the binding ability of F -was weaker than that of AcO -with compound 1.The following affinity constants of compounds with various anions could prove this guess.The additions of H 2 PO 4 -, Cl -, Br -and I -virtually led to very weak spectral response of compound 1 which indicated the interactions of host-guest were very weak and could be ignored (Fig. 4).
The acidity of this kind of compounds can be tuned by changing the electron property of the substituent on the ortho-or para-position.Therefore the anion binding ability also can be tuned according to resonance structure. 36electron-withdrawing substituent were synthesized in order to investigate the effect of electron properties of substituent on the hydrogen-bond.As expected, UV-vis spectra of 2, 3 and 4 indeed exhibited changes (Fig. 2b, 2c and  2d).The color of solution also changed and so these compounds could be used to detect anions by "naked-eye" (Fig. 3).The additions of other anions including F -, H 2 PO 4 -induced similar spectral changes, however, the additions of Cl -, Br -, and I -did not induce any spectral response for 2, 3 and 4.

Fluorescence Response
The photophysical responses of four compounds toward additions of various anions tested were also investigated.Just as Fig. 5 showed, compound 1 exhibited an emission peak centered at 375 nm with a shoulder at 325 nm.Upon the addition of AcO -into the solution of compound 1, the fluorescence emission was strengthened which showed AcO -interacted with hydrogen atoms of phenol groups and hydrogen bonds formed between compound 1 and anion.8][39] Without the addition of AcO -, the hydrogen atoms of free compound 1 could form an intramolecular hydrogen bond with the nitrogen atom of Shiff base.The above structure may lead to a photoinduced electron transfer and the fluorescence was weak.However, the electron transfer from compound 1 to the fluorophore became more feasible when the acetate anion was added to the host solution.Then the binding sites resulted in the PET inhibition and the emission intensity was enhanced.Similar spectral changes were observed upon the addition of F -or H 2 PO 4 -.On the other hand, no significant spectral changes were observed upon the titration of 1 with Cl -, Br -, I -, signifying compound 1 showed insignificant binding affinity toward these anions which could be ignored.
The intensity of emission spectra all increased after acetate anion was added to compound 2, 3 and 4, respectively (Fig. 5).For compound 2, the emission spectra centered at 425 nm was shifted to the long wave direction (437 nm, Δλ=12 nm).Two emission peaks (337 nm and 412 nm) incorporated one peak (380 nm) for compound 3.However, two broad emission peaks changed to sharp (410 nm and 438 nm) for compound 4. The above observation may be related with the different structures of four compounds.

4. Affinity Constant
Compound 1 interacted with anions as the binding ratio of 1:1 according to Job-plot curve.1][42] The binding ability trend of four compounds to anions followed the order of: AcO -> F -> H 2 PO 4 ->> Cl -∼ Br -∼ I -.It was apparent that the binding ability for specific anions can be rationalized on the basis of the anion's basicity and the host-guest interaction.However, multiple hydrogen-bond interactions were also necessary in high-affinity anion binding sites.As expected from their basicity, AcO -, F - and H 2 PO 4 -will bind more strongly than the other anions studied; in addition, the triangle configuration of AcO -ion may well match four compounds in terms of shape and  could form multiple hydrogen bonds.Consequently, AcO - ion can be strongly bound based on affinity constant.For the same anion, the binding ability followed the order of: 4 > 3 > 2 > 1.This order was consistent with the ability of electron-withdrawing, NO 2 > o, p-Br > o-Br > H.The stronger electron-withdrawing ability was, the easier the electron transferred from OH to electron-withdrawing group.So, the anion binding ability of compound 4 was strongest among four compounds, and compound 1 without electron-withdrawing was weakest.In addition, the intramolecular hydrogen bond could improve the acidity of receptor and so the anion binding ability could be strengthened (proved by theoretical investigation).Moreover, the affinity constants obtained by UV-vis data were in the same order of magnitude with fluorescence data.The above results indicated the affinity constants obtained by UV-vis and fluorescence data were proved each other.

5. Interference Experiment
According to the affinity constants, the binding ability of four compounds with AcO -ion was the strongest  a All anions were added in the form of tetra-n-butylammonium (TBA) salts.b The affinity constant could not be determined.
among the anions tested.The above results derived from the condition that only AcO -ion existed.We determined the interference experiment whether the binding ability of AcO -was influenced by the addition of other anions tested (Fig. 6).According to UV-vis experimental data, the spectral responses of compound 2 were different when various anions (F -, AcO -and H 2 PO 4 -, 8.0 × 10 -5 mol • L -1 ) were added separately.As shown in Fig. 6, the spectral response of 2 upon the addition of mixed anions was almost as the same as the addition of only AcO -which suggested the binding ability of AcO -with compound 2 was not interfered by the existence of other anions.Similarly, the same result also existed in the interaction of other compounds with AcO -.

7. Theoretical Investigation
The geometries of four compounds were optimized (Fig. 8) using HF (Hartree-Fock) method with basis sets 3-21G.The calculation was performed with Gaussian03 program. 43From Fig. 8, the distances of hydrogen atom in one interacted site (-OH) with near nitrogen atom were 1.046 Å, 1.594 Å, 1.593 Å, 1.551 Å in compound 1, 2, 3 and 4, respectively.The corresponding minimum energies were -1549.3,-11793.8,-11794.1 and -8761.4hf.Clearly, minimum energy of compound was related with hydrogen bonds length.The shorter the hydrogen bond length was, the higher the minimum energy was.This phenomenon showed intramolecular hydrogen bond existed in four compounds and stable six-member cycle formed.In addition, the hydrogen atom in other interacted site (-OH) also could form intramolecular hydrogen bonds with near bromide or oxygen atom in compound 2, 3, 4.

6. 1 H NMR Titration
In order to look into the anion binding property of four compounds, 1 H NMR titration of compound 2 with AcO -was determined as an example.Free compound 2 displayed two peaks at 11.21 and 11.11 ppm in the downfield region which were attributed to -OH groups (Fig. 7).Upon the addition of AcO -stepwise, the signal of -OH weakened, shifted to the downfield gradually and disappeared thoroughly when the concentration of AcO -was 1 equiv. of compound 2. While slight upfield of CH protons (7.55-7.49,6.97-6.92ppm) in the aromatic was also observed.The above observation indicated the interacted sites of host-gust complex were -OH groups and the hydrogen bonds formed.After H-bonding formed, shield effect existed in the region of -OH and AcO -.Then, the proton peak of -OH shifted to the downfield direction.While, for non-interacted sites, aromatic cycle, deshield effect enhanced and their protons shifted to the upfield direction.The formation of hydrogen bonds changed ICT property of compound 1 and ultimately resulted in the observed color and spectral changes.
Shang et al.: Theoretical and Experimental Study ...These results showed four compounds could bind anion through multiple hydrogen bonds.Therefore, the binding ability of oxyanion, AcO -, with compounds was the strongest among anions tested.
In addition, selected frontier orbitals for compound 3 were shown in Fig. 9. Molecular frontier orbital was introduced in order to explain red-shift phenomenon existing in absorption spectra induced by electron transition of frontier orbital.The highest occupied orbital (HOMO) density in compound 3 was mainly localized on one phenyl cycle moiety, while the lowest unoccupied orbital (LUMO) density was localized on the whole molecular, which suggested the red-shift phenomenon in UV-vis spectra of hostguest was caused by the electron transition of HOMO.

Conclusion
In conclusion, four colorimetric anion sensors involving phenanthroline and phenol groups were successfully synthesized and optimized.They interacted with anions by hydrogen bond and showed the strong binding ability for AcO -without the interference of other anions studied.The anion binding ability was related to the electron properties of ortho-or para-substituents on benzene.The correlation between the electron properties of substituents and the binding ability will be a very useful clue to design stronger receptors to bind a certain anion.What's more, the organic nano-material of compound 2 was also prepared.The flakiness appearance of nano-material was not good which might be related to the concentration of compound 2 and the selection of poor solvent.The appearance of compound 2 will turn to better by changing the concentration of compound 2 and poor solvent.The combination of anion receptor and nano-material will be an efficient way to construct new responsive materials.The innovative aspect of this manuscript is that phenanthroline nano-material is applied to anion recognition and experiment results are accordance with theoretical investigation.This study has convincingly shown the potential of phenanthroline derivatives for future applications in the design of anion responsive materials in supramolecular chemistry.

Table 1 .
Affinity constants of compounds with various anions