Triphenylamine-Based Receptors in Selective Recognition of

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Triphenylamine-Based Receptors in Selective Recognition of Dicarboxylic Acids Kumaresh Ghosh,*,† Goutam Masanta,† Roland Fro¨hlich,‡ Ioannis D. Petsalakis,*,§ and Giannoula Theodorakopoulos§ Department of Chemistry, UniVersity of Kalyani, Kalyani, Nadia 741235, India, Organisch-Chemisches Institut, UniVersita¨t Mu¨nster, Corrensstraβe 40, D-48149 Mu¨nster, and Theoretical and Physical Chemistry Institute, The National Hellenic Research Foundation, 48 Vassileos Constantinou AVenue, Athens 116 35 Greece ReceiVed: February 7, 2009; ReVised Manuscript ReceiVed: April 7, 2009

New triphenylamine-based chemosensors for selective recognition of dicarboxylic acids have been designed and synthesized. They are variants of previously synthesized chemosensors, offering different possibilities for selective recognition of dicarboxylic acids. Theoretical calculations by density functional theory and time dependent density functional theory have been carried out on these as well as on one of the previously reported systems and on complexes formed by the triphenylamine based chemosensors and aliphatic dicarboxylic acids of different chain-lengths. Carboxylic acid binding takes place through charge-neutral pyridyl amide receptor sites with concomitant quenching of fluorescence of the triphenylamine moiety. The theoretical results are consistent with the experimental and correlation is found between the calculated binding energies and the experimental binding constants. 1. Introduction Design and synthesis of artificial receptors for the recognition of carboxylic acids is an important aspect in molecular recognition research. Dicarboxylic acids and carboxylates are biologically relevant1,2 species and consequently there is a continuous interest in the design and synthesis of receptor molecules, which show selective binding of dicarboxylic acid.3 During the last decades considerable progress has been made for recognition of dicarboxylic acids by different designed receptors.4-9 Recently, there have been reports by present authors (K. Ghosh and G. Masanta) on the design and synthesis of chemosensors that can function as an “Off-On” or “On-Off” fluorescence switch for dicarboxylic acids,10-12 and they are either anthracenecoupled pyridine amines10,11 or triphenylamine-based pyridine amides.12 Quenching of fluorescence to different extents, downfield shifts in the NMR signal of the amide protons, and a reduction in the absorption probability is reported to accompany the addition of different dicarboxylic acids.12 Theoretical work has been reported by present authors (I.D. Petsalakis and G. Theodorakopoulos) on the binding of different dicarboxylic acids by a triphenylamine-coupled aminopyridine receptor molecule 1 in ref 12 and the resulting changes in the geometry as well as in the absorption spectra of this system.13 According to the results of the above studies, binding of the dicarboxylic acids is achieved through two-point hydrogen bonding at each binding site and, accordingly, an important factor in determining the selectivity of the sensor molecules (as manifested by the strength of the changes in the physical properties) appears to be the chainlength of the dicarboxylic acid, or how well it fits between the two binding sites of the sensor molecule with glutaric acid possessing the ideal length for discrete 1:1 complexation with 1.12 The theoretical results yield higher binding energy for the * To whom correspondence should be addressed. (K.G.) Fax: +91-3325828282. E-mail: [email protected]. (I.D.P.) Fax +30-210-7273794. E-mail address: [email protected]. † University of Kalyani. ‡ Universita¨t Mu¨nster. § The National Hellenic Research Foundation.

complex of the sensor molecule with glutaric acid but the greatest shift in the first absorption peak is calculated for 2,2dimethyl malonic acid.13 In the present work, the binding of dicarboxylic acids to receptor molecule 212 is investigated by theoretical calculations. In addition, two variants of system 1, indicated as 3 and 4, have been synthesized and their binding of dicarboxylic acids is investigated by 1H NMR, fluorescence, and UV-vis spectroscopic methods, as well as by theoretical calculations.

2. Experimental Section Synthesis. The synthesis of 1 and 2 were accomplished according to our previously reported method.11 Compounds 3 and 4 were obtained according to Scheme 1. Reduction of the amide linkage in 1 using LiAlH4 afforded compound 3 in 37% yield. For compound 4, the reaction in Scheme 1b was followed. Triphenylamine was initially formylated by using POCl3/DMF

10.1021/jp901151w CCC: $40.75  2009 American Chemical Society Published on Web 05/12/2009

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SCHEME 1: Syntheses of Receptors 3 and 4a

a Reagents and conditions: i. LiAlH4, dry THF, reflux, 9 h; ii. NBS, dry CHCl3, reflux 8 h; iii. KMnO4, acetone-water, reflux 8 h; iv. oxalyl chloride, DMF, dry CH2Cl2, rt, 9 h; v. 2-amino-6-methylpyridine, Et3N, dry THF, rt, 18 h.

to yield mono-, di- and triformylated products.12 The desired 4,4′-diformyltriphenylamine 5,14 isolated in 57% yield, on bromination followed by oxidation of the aldehydic groups gave compound 7. Subsequent reaction of 7 with oxalyl chloride in dry CH2Cl2 yielded the diacid chloride 8, which on coupling with 2-amino-6-methylpyridine gave the receptor 4 system in 80% yield as a pure yellowish product. Compounds 3 and 4 were characterized by NMR, FTIR, mass and elemental analysis. The steady state absorption and emission spectra were recorded using a PerkinElmer Lambda 25 UV-vis absorption spectrophotometer and a PerkinElmer LS55 spectrofluorimeter. N2-4-[4-[(6-Methyl-2-pyridyl)amino]methyl(phenyl)anilino]benzyl-6-methyl-2-pyridinamine (3). To a suspension of LiAlH4 (0.045 g, 1.18 mmol) in dry THF (15 mL), THF solution (15 mL) of 1 (0.2 g, 0.39 mmol) at 0 °C under nitrogen atmosphere was added dropwise. After addition, the reaction mixture was stirred under refluxing condition for 9 h. The reaction mixture was quenched by dropwise addition of 20% NaOH solution and the mixture was stirred for another 5 h at room temperature. The precipitate was filtered off and the filtrate was concentrated and extracted with CH2Cl2 (3 × 20 mL). The organic layer was separated and dried over anhydrous Na2SO4 and removed on a rotary evaporator. The crude product was purified by column chromatography using 35% ethyl acetate in petroleum ether to give foamlike light yellow solid 2 (0.07 g, yield, 37%). mp 56-58 °C. 1H NMR (400 MHz, CDCl3): δ 7.32 (t, 2H, J ) 8 Hz), 7.21 (d, 6H, J ) 8 Hz), 7.05-6.98 (m, 7H), 6.46 (d, 2H, J ) 8 Hz), 6.19 (d, 2H, J ) 8 Hz), 4.81 (brt, 2H, -NH-), 4.37 (d, 4H, J ) 4 Hz), 2.37 (s, 6H). 13C NMR (75 MHz, CDCl3) δ 158.3, 156.9, 147.6, 146.8, 137.8, 133.4, 129.2, 128.3, 124.1, 123.9, 122.6, 112.4, 102.8, 46.1, 24.3. FTIR (KBr, cm-1) νmax: 3416, 3260, 3031, 2920, 2850, 1594, 1505, 1464. UV (c ) 1 × 10-5 M) 308. Mass (LCMS) 486.6 (M + H)+, 378.4, 271.2. 4-[4-Bromo(4-formylphenyl)anilino]benzaldehyde (6). To a stirred solution of 4,4′-diformyltriphenylamine 5 (1 g, 3.32 mmol) in dry CHCl3 (50 mL) was added N-bromosuccinimide (0.65 g, 3.65 mmol). The reaction mixture was stirred under refluxing condition for 8 h. The precipitate formed was filtered off and washed with CHCl3, and the filtrate was concentrated

under vacuum. The organic portion was washed with 20% aqueous NaHCO3 solution and extracted with CHCl3 (3 × 40 mL). The organic layer was separated and dried over anhydrous Na2SO4 and removed on rotary evaporator. Crude product was purified by column chromatography using 15% ethyl acetate in petroleum ether to give 6 (1.07 g, yield, 85%) as yellow solid. mp, 158-160 °C. 1H NMR (500 MHz, CDCl3): δ 9.91 (s, 2H), 7.77 (d, 4H, J ) 10 Hz), 7.50 (d, 2H, J ) 10 Hz), 7.18 (d, 4H, J ) 10 Hz), 7.05 (d, 2H, J ) 10 Hz). FTIR (KBr, cm-1)νmax: 2801, 2726, 1694, 1588, 1574, 1488, 1273, 1166. 4-[4-Bromo(4-carboxyphenyl)anilino]benzoic acid (7). To a stirred solution of 6 (1 g, 2.63 mmol) in 50 mL acetone-water (4:1 v/v), KMnO4 (2.1 g, 13.29 mmol) was added portion wise at 60 °C. The reaction mixture was stirred under refluxing condition for 8 h. The solvent was removed under vacuum and water (25 mL) was added. The black precipitate was filtered off and the filtrate was acidified with conc. HCl to give a white precipitate. The precipitate was filtered off and washed with water for several times and dried under vacuum to give almost pure product 7 (0.87 g, yield, 81%). 1H NMR (500 MHz, CDCl3 + two drops d6-DMSO): δ 7.94 (d, 4H, J ) 10 Hz), 7.43 (d, 2H, J ) 10 Hz), 7.07 (d, 4H, J ) 10 Hz), 7.01 (d, 2H, J ) 10 Hz), two protons for carboxylic acids were not found. FTIR (KBr, cm-1)νmax: 3401, 2656, 2536, 1683, 1594, 1488, 1316, 1276. N1-(6-Methyl-2-pyridyl)-4-[4-bromo(4-[(6-methyl-2 pyridyl)amino]carbonylphenyl)anilino]benzamide (4). Compound 7 (0.4 g, 0.97 mmol) was next dissolved in dry CH2Cl2 (10 mL) and oxalyl chloride (1 mL, 13.44 mmol, excess) was added followed by addition of one drop dry DMF. The reaction mixture was stirred at room temperature for 9 h under nitrogen atmosphere. The solvent was removed. The diacid chloride 8 (0.4 g, 92%) was dried under vacuum and an amount of 0.2 g (0.45 mmol) of the diacidchloride was dissolved in dry THF. To this solution, 2-amino-6-methylpyridine (0.097 g, 0.898 mmol) containing triethylamine (0.3 mL, 2.17 mmol) in dry THF (10 mL) was added dropwise under nitrogen atmosphere. The reaction mixture was stirred at room temperature for 18 h. The solvent was removed under vacuum and the mixture was extracted with CH2Cl2. The organic layer was washed with 20%

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NaHCO3 solution, separated and dried over anhydrous Na2SO4, and finally removed under vacuum. The crude product was purified by column chromatography using 25% ethyl acetate in petroleum ether to give 4 (0.21 g, 80%) as yellow solid. mp 176 °C. 1H NMR (400 MHz, CDCl3) δ: 8.49 (s, 2H, -NHCO-), 8.17 (d, 2H, J ) 8 Hz), 7.84 (d, 4H, J ) 8 Hz), 7.64 (t, 2H, J ) 8 Hz), 7.46 (d, 2H, J ) 8 Hz), 7.14 (d, 4H, J ) 8 Hz), 7.03 (d, 2H, J ) 8 Hz), 6.92 (d, 2H, J ) 8 Hz), 2.44 (s, 6H). FTIR (KBr, cm-1) νmax: 3334, 1673, 1454. UV (c ) 1 × 10-5 M) 358, 324, 293. Mass (ESI) 592 (M)+, 594 (M+2)+. Compounds 1 and 2 were explored in details in dicarboxylic acid recognition. Both receptors showed good selectivity and sensitivity toward dicarboxylic acids of particular chain lengths.12 Receptor 1 showed selective binding of glutaric acid and the macrocyclic analogue 2 was found to exhibit selectivity toward 2,2-dimethylmalonic acid, a short chain analogue. Both receptors indicated the recognition events by exhibiting quenching of fluorescence of the triphenylamine moiety. The quenching of emission of the triphenylamine moiety was attributed to photoinduced electron transfer (PET) process occurring between the pyridine amide binding site and the triphenylamine moiety. For further exploration of pyridine-coupled triphenylaminebased receptor molecules in dicarboxylic acid recognition, receptors 3 and 4, which are the structural modifications of 1, were synthesized. Compound 3 is principally the same as 1, where only the amide linkage has been replaced by aminomethylene (-NHCH2-) group. Compound 4 is also a variant of 1 with a halogen atom in the triphenylamine motif. To understand the effect of the substituent on the steady state absorption and emission spectra of the triphenylamine core as well as the sensing and recognition properties of 3 and 4 with aliphatic dicarboxylic acids of different chain lengths 1H NMR, single crystal X-ray, fluorescence, and UV-vis studies were systematically carried out. General Procedure for Fluorescence Titration. Stock solutions of the receptors were prepared in UV grade CHCl3 and 2.5 mL of each receptor solution was taken in the cuvette. The solution was irradiated at the selected excitation wavelength. Upon addition of guest acids (dissolved in CHCl3 containing 0.66% DMSO), the change in fluorescence emission of the receptor was observed. The corresponding emission values during titration were recorded and used for the determination of binding constant values. The change in emission in the presence of different amounts of guest acids was used to have the Stern-Volmer plot. General Procedure for UV-vis Titration. Stock solutions of the receptors were prepared in UV grade CHCl3 and 2.5 mL of each receptor solution was taken in the cuvette. Dicarboxylic acid guest (dissolved in CHCl3 containing 0.66% DMSO) was added in different amounts to the receptor solution. The corresponding absorbance values during titration were noted. 3. Results and Discussion Prior to the interaction study of 3 and 4 with the aliphatic dicarboxylic acids of different chain lengths, the ground- and excited-state properties of the molecules were investigated in CHCl3. Both the absorption and emission spectra of molecules 1-4 were superimposed on the spectrum of triphenylamine. As can be seen from Figure 1, the absorption of triphenylamine unit is modified as the different substituents are put on the phenyl rings of the triphenylamine unit. The emission intensities of compounds 1-4 are also altered with respect to the triphenylamine unit (Figure 2). Quantum yields were calculated in CHCl315 by the relative comparison procedure using anthracene

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Figure 1. Absorption spectra of compounds 1-4 together with triphenylamine (c ) 1.0 × 10-5 M).

Figure 2. Emission spectra of compounds 1-4 together with triphenylamine (c ) 1.0 × 10-5 M) maintaining excitation and emission slit widths 12 and 3, respectively.

as standard (φant ) 0.27 in ethanol). The general equation used in the determination of relative quantum yields is shown as follows

Qu )

Qs × Fu × As × λexs × η2u Fs × Au × λexu × ηs2

where Q is the quantum yields, F is the integrated area under the corrected emission spectrum, A is the absorbance at the excitation wavelength, λex is the excitation wavelength, η is the refractive index of the solution, and the subscripts u and s refer to the unknown and the standard, respectively. With the equation mentioned above, quantum yield for triphenylamine in CHCl3 on excitation at 302 nm was determined by taking anthracene (λex ) 356 nm) in ethanol as standard. Similarly the quantum yields of the receptors 1, 2, 3, and 4 were ascertained in CHCl3 on excitation at 363, 322, 309, and 358 nm, respectively using anthracene (φant ) 0.27 in ethanol) as standard. The fluorescence quantum yields (Φ) are low and the values are 0.0036, 0.5504, 0.5974, 0.0073, 0.3563 for triphenylamine, 1, 2, 3, and 4, respectively. The fluorescence emission spectra of 3 consist of bands at 378 and 454 nm, respectively, when excited at 308 nm. Upon addition of dicarboxylic acids of different chain lengths (dissolved in CHCl3 containing 0.66% DMSO), the emission at 378 nm was increased followed by a simultaneous decrease in emission at 454 nm. Such characteristic ratiometric fluorescence was not observed in case of 1. Figure 3 demonstrates the change in fluorescence of 3 in the presence of aliphatic dicarboxylic acids of different chain lengths. In each case, the characteristic isosbestic point was noted. This indicated the formation of new species in the solution. Figure 4 shows the fluorescence titration

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Figure 3. Change in emission of 3 (c ) 2.474 × 10-5 M) in the presence of (a) 2,2-dimethyl malonic (b) glutaric, (c) adipic, and (d) suberic acids [λexc ) 308 nm, slitexc width ) 6, and slitem width ) 6.]

Figure 4. Titration curves for 3 upon addition of dicarboxylic acids (measured at 454 nm).

Figure 5. Stern-Volmer plots for 3 measured at 454 nm.

curves for 3. The break of the titration curves at [G]/[H] ) 1, in each case, indicated the 1:1 stoichiometries of the complexes. The quenching of emission at 454 nm in 3 during complexation is explained from Stern-Volmer plot (Figure 5). 2,2-Dimethylmalonic acid quenches the emission of 3 at 454 nm more strongly than glutaric and any other diacids studied. This is presumably due to the stronger acidity of 2,2dimethylmalonic acid than the other acids studied as well as

best fit into the open cleft via hydrogen bonding interactions. However, receptor 3 also exhibited change in absorption in the presence of the diacids (Figure 6). In the presence of 2,2dimethylmalonic acid the absorption peak of 3 at 308 nm underwent a red shift (∆λ ) 6 nm). This further supports that 2,2-dimethylmalonic acid strongly interacts with the pyridine amine sites of 3 in the ground state according to the suggested mode in Figure 7. The binding constant values for 3 with the diacids were determined based on the fluorescence titration.16 We did not use the UV-vis titration data for determination of binding constant values, as the change in absorption of 3 was comparatively smaller than change in emission. As can be seen from Table 1, the higher binding constant value for receptor 3 is for 2,2-dimethylmalonic acid and the values for the other diacids are found to decrease with increasing chain length of the diacids. In order to determine the selectivity in binding, the interaction properties of 3 toward monocarboxylic acids such as benzoic and propanoic acids were investigated through fluorescence (see Supporting Information). Upon gradual addition of monocarboxylic acids to the CHCl3 solution of 3, minor changes in emissions at 378 and 454 nm suggested a poor interaction of 3 with the monocarboxylic acids. Even the fluorometric titrations of 3 were also carried out in CHCl3 upon addition of tetrabutylammonium salts of 2,2-dimethylmalonic, glutaric, adipic, suberic, benzoic, and propanoic acids. Importantly, negligible change in emission of 3 was noticed compared to the case of 3 with dicarboxylic acids. Figure 8 in this regard shows the comparison of change in emission of 3 in the presence of 10 equiv amounts of a particular guest. These observations thus demonstrate that the simultaneous hydrogen bonding interaction of dicarboxylic acids at the two pyridine amine sites of the receptor 3 has the marked effect on its photophysical behavior for which selective recognition of guest diacid is possible. Furthermore, the reversibility of the recognition process was established by adding tetrabutylammonium acetate to the chloroform solution of receptor 3

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Figure 6. Change in absorption of 3 (c ) 3.71 × 10-5 M) in the presence of (a) 2,2-dimethyl malonic, (b) glutaric, (c) adipic, and (d) suberic acids.

Figure 7. Hydrogen bonded complex of 3 with dicarboxylic acids.

TABLE 1: Binding Constant Values for 3 and 4 from Fluorescence Methods in CHCl3 Solvent guest

Ka in M-1 for 3

Ka in M-1 for 4

2,2-dimethyl malonic acid Glutaric acid Adipic acid Suberic acid

104 1.1 × 104 6.85 × 103 4.54 × 103

7.83 × 104 1.38 × 105 3.95 × 104 a

a Because of irregular change binding constant value was not determined.

containing dicarboxylic acid. Upon gradual addition of AcOion to the solution of 3 containing dicarboxylic acid, the emission of 3 at both 378 and 454 nm reached the initial stage (see one representative example in the Supporting Information). The binding interaction of 3 with the diacids was also realized from 1H NMR in CDCl3. In the presence of equivalent amounts of diacids, the signal for amine protons of 3 at 4.81 ppm became too broad to detect accurately. In the presence of adipic acid, the signal at 4.81 ppm underwent significant downfield shift (∆δ ) 2.08 ppm). Similarly, for receptor 4, quenching of fluorescence to different extents results from the addition of the dicarboxylic acids where for this system the effect is found to be greatest with glutaric acid (Figure 9b). It is worth noting

that the change in fluorescence of 4 is drastically smaller than the corresponding change in 1.12 This is possibly due to the presence of a bromine atom at position-4 of a phenyl ring of the triphenylamine core. The conjugation of the lone pair of the central nitrogen of the triphenylamine core with the vacant d-orbital of bromine presumably perturbs the emission. Figure 10 exhibits the change in absorption of 4 during interaction with the diacids in CHCl3. In each case the isosbestic point was noticed. This clearly indicated the formation of a new complex in the solution. The binding stoichiometries of the complexes were confirmed from the break of the fluorescence titration curves at [G]/[H] ) 1, which indicated 1:1 stoichiometries of the complexes. Binding constant values were determined by fluorescence method and they are summarized in Table 1.16 As can be seen from Table 1, receptor 4 selectively recognizes glutaric acid and shows the same trend as noticed in the case of 1.11 The downfield chemical shift of the amide protons of 4 was found in the presence of the equivalent amounts of the diacids in CDCl3. The largest change in chemical shift value was observed for glutaric acid (see Table 2). The selective binding of glutaric acid into the open cleft of 4 was further examined from single crystal structure. Crystallization of 4 was carried out in the presence of glutaric acid from a mixture solvent (dry CHCl3, MeOH, and pet ether in the 4:1:2 ratios). Figure 11 shows the hydrogen bonding structure of 4 with glutaric acid [hydrogen bonding scheme: O12-H12...N21B. 0.83 Å, 1.79 Å, 167.5°; O52-H52...N21A, 0.83 Å, 1.89 Å, 162.3°; N19A-H19A...O51, 0.75 Å, 2.11 Å, 173.6°; N19B-H19B...O11, 0.75 Å, 2.30 Å, 172.6°].17a We also attempted the crystallization of the previously reported receptor 1 in the presence of adipic acid from a mixture solvent (dry CHCl3, MeOH and pet ether in the 4:1:2 ratio). In this case, it was found that the open cleft was not dimensionally fitted with the chain length of adipic acid and thus induced a dynamic supramolecular assembly according to Figure 12 [N10B-H1A... O21a, 0.78 Å, 2.05 Å, 177.8° a, x + 1, y, z + 1; N10B-H1B...O23, 0.85 Å, 2.22 Å, 169.8°; O22-H22...N12Ab, 0.83 Å, 1.94 Å,

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Figure 8. Plot of ∆I/I0 for receptor 3 in the presence of 10 equiv amounts of a particular guest at (a) 378 nm and (b) 454 nm in CHCl3 [λex ) 308 nm, slitex width ) 6, and slitem width ) 6 ].

Figure 9. Change in fluorescence of 4 (c ) 1.22 × 10-6 M) in the presence of (a) 2,2-dimethylmalonic, (b) glutaric, (c) adipic, and (d) suberic acids [λexc ) 360 nm, slitexc width ) 12, and slitem width ) 2.5].

165.2° b, x - 1, y, z - 1; O24-H24...N12B, 0.83 Å, 1.80 Å, 171.9°].17b In the crystal, the triphenylamine core of receptor 1 was found to contain methyl group at the position-4 of the phenyl ring. We suggest that this methyl group comes from MeOH under acidic condition. In both cases, carboxylic acid motif is complexed into the pyridine amide site without showing any proton transfer. We also tried several times to have the single crystals of 3 in the presence of diacids, but we failed. Therefore, it is found that receptors 3 and 4 (reduced analogue of 1) are important and useful in the selective recognition of dicarboxylic acids just as our previously reported receptors 1 and 2. Receptor 112 being dimensionally fitted selectively bound glutaric acid with concomitant change in fluorescence. In the present report, it has been further confirmed by single crystal structure of 4 with glutaric acid. More importantly, whenever the amide linkage in 1 is changed into the amine function, the dimension of the cleft is changed and prefers 2,2-dimethylmalonic acid, which is a shorter chain analogue. The macrocyclic analogue 2 with amide functionality also selectively bound 2,2dimethylmalonic acid.12 In order to substantiate the experimental findings, we carried out theoretical calculations on receptors 3 and 4 as well as on

our previously published macrocyclic receptor 2. The details are given below. 4. Calculations The present calculations involve binding of dicarboxylic acids of different chain lengths, COOH-(CR2)n-COOH, with R ) H for n ) 2, 3,4 and R ) CH3 for n ) 1, to molecules 3 and 4, and with R ) H for n ) 1, 2, 3 and also with R ) CH3 for n ) 1 to molecule 2, using density functional theory (DFT)18 calculations with the B3LYP functional19,20 and the basis set 631G**, provided by Gaussian 03.21 A similar study for molecule 1 has been published previously.13 Geometry optimization was carried out for the ground electronic state of the above systems. Binding energies are determined by the difference between the energy calculated for the complexes of the sensor molecules with the different dicarboxylic acids and the sum of the calculated energies of the separate chemosensor and dicarboxylic acid molecules in each case. Time-dependent density functional theory calculations (TDDFT)22 were carried out at the DFT optimum geometry of each complex in order to determine the vertical transition energies to excited electronic

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Figure 10. Change in absorption of 4 (c ) 1.7 × 10-5 M) in the presence of (a) 2,2-dimethylmalonic, (b) glutaric, (c) adipic, and (d) suberic acids.

TABLE 2: Change in Chemical Shift of 4 in the Presence of Equivalent Amount of Dicarboxylic Acids in CDCl3 guests

∆δ in ppm

2,2-dimethylmalonic acid glutaric acid adipic acid suberic acid

1.26 2.05 1.58 1.59

states and the oscillator strength, giving an indication of the intensity of the absorption spectra. The above calculations are relevant to the gas phase and in order to better mimic the experimental conditions, the TDDFT calculations were also carried out including CH2Cl2 solvent, using the method of the IEF-PCM model23 provided in Gaussian 03. Figure 12. Single crystal structure of the complex of 1 with adipic acid.

Figure 11. Single crystal structure of the complex of 4 with glutaric acid.

4a. Results of the Calculations on the Macrocyclic Receptor 2. The geometry optimization of a complex consisting of molecule 2 and dicarboxylic acids of different lengths presents a challenge because of the confined region available for the formation of the 2-site 2-point hydrogen bonding.12,13 Thus, it is necessary for the dicarboxylic acid to “pucker” outside the plane of the 2-point hydrogen bonds. This can be seen in Figure 13, where the geometries of the complexes of 2 with malonic, succinic, and glutaric acids are shown. In Table 3, the results of the DFT and TDDFT calculations on system 2 and its complexes with different dicarboxylic acids are summarized. The calculated optimum geometries do not have the two amidic protons exactly symmetric, hence the different proton NMR shifts. In terms of geometric distortion of 2, the complex with succinic acid has the smallest CNC angle at the triphenyl amine nitrogen atom and also has the smallest binding energy, about half of the BE of the malonic acid complex. The experimental result12 of increased selectivity of 2 for 2,2dimethyl malonic acid could be related to the highest value for

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Figure 13. Calculated optimum structures for the complexes of receptor 2 with malonic acid in (a) and (b), with succinic in (c), and with glutaric acid in (d).

the binding energy calculated for this system among those considered experimentally (malonic acid was not included in the previous experimental study12). The present TDDFT calculations may be related to the absorption spectra of the complexes. As shown in the first row of Table 3 for molecule 2 without any acid, the calculated lower energy peaks (at 379, 331, and 293 nm) with appreciable oscillator strength do not fall exactly on the experimental (at 360, 323, and 291 nm) but their shifts and the lowering of the corresponding oscillator strengths, resulting from the addition of the different dicarboxylic acids, may be of use. As shown in Table 3, the present calculations indicate the highest selectivity for receptor 2 to be for malonic acid, because the corresponding complex has the largest red shifts in the absorption peaks and the largest binding energy. Malonic acid was not included in the experimental investigations but the next best candidate (in terms of binding energies and shifts in the absorption peaks, see Table 3) is dimethyl malonic acid, which is in agreement with experimental results.12 The complex of 2 with succinic acid (also not included in the experimental study12) shows large shifts in the absorption peaks and a dramatic drop in the intensity of

the two lowest-energy peaks, but it has a significantly smaller value of binding energy. It should be noted that as found previously for receptor 1,13 the calculated values of the binding energies are smaller if an estimate of the basis superposition error (BSSE) is included. For the complexes of receptor 2 the BSSE correction was calculated by the counterpoise method (available in Gaussian 03) as 40 kJ/mol. Finally, the corresponding quantities calculated in the presence of CH2Cl2 solvent (for molecule 2 without any acid the calculations with solvent did not converge) are also included in Table 3. The effect of the solvent is to generally increase the calculated oscillator strength values and to shift the absorption peaks to slightly higher or lower energies depending on the particular complex. 4b. Results of the Calculations on the Two Open Receptors 3 and 4. The optimum geometries for the complexes of receptors 3 and 4 are very similar to those obtained previously for receptor 1.13 In Figure 14, top views are given for the complexes with glutaric acid and top and side views for the complexes with adipic acid, and it can be seen that the 2site 2-point hydrogen bonds are in the same plane, unlike the situation in the complexes of receptor 2, above. A comparison of some theoretical geometrical quantities with those derived from the single crystal structure of the complex of 4 with glutaric acid shows reasonable agreement, given the inherent problems of the DFT/B3LYP approach for weak bonds. The calculated bond lengths relevant to the 2-site -2-point hydrogen bonding are for N...H-O 1.68, 1.02 and 1.70,1.02 Å (angle 172.9 and 173.4°) and for N-H...O 1.02, 1.93 Å and 1.02, 1.90 Å (angle 175.3 and 177.0°). Thus, the theoretical 2-site -2-point hydrogen bonding is more symmetrical than in the crystal structure with the N-H and O-H bonds elongated and shorter O...H and N...H distances than the corresponding values in the crystal structure (see Section 3 above). In Tables 4 and 5 the DFT and TDDFT results for the complexes with receptors 3 and 4, respectively, are summarized. As shown, generally higher binding energies are calculated for the complexes of 3 and 4 as compared to those of 2, consistent with the fact that the geometric distortions are easier to occur for the open systems 3 and 4. In both Tables 4 and 5, the highest binding energy is found for the complexes with succinic acid, indicating preferred selectivity of both 3 and 4 for succinic acid. In the absence of succinic acid in the experimental work, the preferred selectivity of 3 for 2,2-dimethyl malonic acid and of 4 for glutaric acid discussed in the Experimental Section above, may be related to the greater values for the binding energy calculated respectively for these complexes (see Tables 4 and 5). Similarly, in the previous study13 the highest binding energy was calculated for the complex of receptor 1 with glutaric acid in agreement with the experimental result that 1 has greater

TABLE 3: Results of the DFT and TDDFT Calculations: Geometrical Parameters and Binding Energies of the Complexes Formed by 2 with Different Dicarboxylic Acidsa complex (2) no acid 2,2,DMMA malonic succinic glutaric a

R(Nam-Nam) Å R(Npy-Npy) Å