Protonation-Induced Sign Inversion of the Cotton Effects of

Article pubs.acs.org/JPCA

Protonation-Induced Sign Inversion of the Cotton Effects of Pyridinophanes. A Combined Experimental and Theoretical Study Akinori Shimizu, Yoshihisa Inoue, and Tadashi Mori* †

Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan S Supporting Information *

ABSTRACT: The circular dichroisms (CDs) of planar chiral [2.2]- and [3.3]pyridinophanes were investigated experimentally and theoretically. Strong multisignate Cotton effects, typical for cyclophane derivatives, were observed. The CD spectra of [2.2]and [3.3]-paracyclophanes closely resembled in pattern each other, despite the much greater conformational variations in the latter. Upon protonation, both of the cyclophanes suffered dramatic CD spectral changes with accompanying complete sign inversion, which was attributed to the reversal of diploe moment of pyridinium versus pyridine moiety. This chiroptical property switching driven by protonation/deprotonation was temperaturedependent and hence applicable to thermal sensing. The protonated forms of pyridinophanes served as ideal model systems for studying the cation−π interactions and their effects on chiroptical properties. Thus, the molar CD (Δε) of the charge-transfer band of protonated [2.2]pyridinophane was 10-fold larger than that of protonated [3.3]pyridinophane, which exceeds the increased interplane electronic interactions assessed from the electronic coupling element values.

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INTRODUCTION Cyclophanes, possessing unique planar chiral scaffolds, have found a number of applications in various areas of science and technology.1−3 In the cyclophane family, pyridinophane is one of the most intensively investigated and extensively exploited members. This is due to the straightforward preparation, the hydrogen-bonding and metal-coordination abilities, and the ready access to supramolecular architectures and molecular recognition systems.4 Supramolecular chirality in self-assembled systems has been widely recognized.5 Recently, the dynamic helical chirality of cyclophanes has been reported.6 Cyclophanes also serve as a model system for studying intermolecular interactions. As such, a number of donor−acceptor cyclophanes have been prepared and investigated.7−10 In this study, we examined the circular dichroisms (CDs) of [2.2]- and [3.3]-parapyridinophanes experimentally and theoretically (Scheme 1). Besides the spectroscopic significance, CD has been used as a tool for monitoring the molecular recognition behavior of cyclophane host. Therefore, the effects of protonation on the structures and spectra of pyridinophanes have been thoroughly investigated. Cationic pyridine unit incorporated in macrocyclic hosts has similarly been employed for molecular recognition.11,12 Additionally, protonated pyridinophanes are suitable for precisely analyzing the cation−π interactions between pyridinium and benzene units, possessing rigid frameworks. In this paper, we first discuss the structural features of pyridinophanes, focusing on the geometrical changes upon protonation, the origin of which is traced back to the structural parameters with an aid of theoretical calculation as well as X-ray © 2017 American Chemical Society

Scheme 1. Chiral [2.2]- and [3.3]-Parapyridinophanes Investigated in This Study

structural analysis. Then, the experimental CD spectra of pyridinophanes in their neutral and protonated forms are compared and simulated by theoretical calculations. Finally, we demonstrate that the dramatic chiroptical property switching observed upon protonation of pyridinophanes can be applied to thermal sensing. The relationship between the strength of Received: December 6, 2016 Revised: January 12, 2017 Published: January 13, 2017 977

DOI: 10.1021/acs.jpca.6b12287 J. Phys. Chem. A 2017, 121, 977−985

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by the density functional theory (DFT) with and without dispersion correction, and by the MP2 and SCS-MP2 methods. The COSMO solvation model was further tested in the SCSMP2 calculations. It is known that the simple DFT method underestimates the intramolecular attractive interactions, while the MP2 method overestimates, when they are compared to more reliable CCSD(T)-level calculations.18 In brief, the dispersion-corrected DFT and SCS-MP2 methods generally provide superior results in optimizing the geometries of a variety of cyclophane systems. Although there were found some minor alterations in the parameters obtained by the theoretical method of choice, the general trends of the structural change upon protonation were quite similar. Table 1 compares such geometrical parameters for 1 and 1-H+. The structure of 1 was found practically identical to that of parent [2.2]paracyclophane (parameters reported: d = 3.10 Å, d′ = 2.78 Å, α = 12.5°, β = 11.1°, and γ = 12.8°).19 Upon protonation, the interplane distances (d and d′) and the deformation parameters (α and β) were not much altered despite the effective donor−acceptor interaction in the protonated state (vide infra). Interestingly, however, the twist angles (γ and γ′) were significantly reduced to nearly zero. Additionally, the outward displacement (l) in neutral 1 became inward in 1-H+ to optimize the overlap between the benzene ring and the pyridinium nitrogen. The outward displacement in the neutral form is rationalized by the electrostatic repulsion between the benzene’s π and the pyridine’s lone-pair electrons. These characteristic differences in structure agree with those found in benzene−pyridine and benzene−pyridinium complexes.20 Nevertheless, the overall changes were rather insignificant. Our calculation at the DFT-D3(BJ)-TPSS/def2-TZVP level21,22 on 1 and 1-H+ afforded a pair of twisted structures (skew+ and skew−, or s+ and s−, which differ in the torsional angle of the linker, Chart 2) with quite comparable energies (Table 3). Parent [2.2]paracyclophane is known to be polymorphic, and the D2h (twisted) and D2 (centrosymmetric) structures are obtainable at different temperatures divided by the phase transition.23 Generally speaking, such conformational variations should affect the spectral properties, which is indeed the case with some substituted cyclophanes.24 However, the calculated CD spectra of these conformers were found almost identical for both 1 and 1-H+ (data not shown), since such a subtle conformational change does not significantly affect the overall electronic nature. Therefore, we will consider only one of the conformers (s+) as a representative.

cation−π interactions and the corresponding chiroptical properties will also be analyzed.

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RESULTS AND DISCUSSION Structural Considerations of Pyridinophanes and Protonated Pyridinophanes. [2.2]Parapyridinophane. The [2.2]paracyclophane skeletons have been employed as a rigid scaffold for various applications.13−15 Nevertheless, structures of cyclophanes have been more carefully considered in terms of various deformation parameters.16 The essential structural parameters are defined for the facing aromatic rings in pyridinophane as shown in Chart 1. Chart 1. Structural Parameters Illustrateda for [2.2]Pyridinophane 1 (X = none) and 2-ThiatrimethyleneBridged [3.3]Pyridinophane 2 (X = S)

a

d: the center-to-center distance between the mean planes defined by the non-bridged atoms of the two facing aromatic rings; d′: the averaged distance of the two sets of the facing bridgehead atoms; α: the averaged deformation angle between the mean plane and the bridgehead atom; β: the additional deformation angle of the linker atom from the mean plane; γ: the twist angle between the two axes passing through the mean planes (red lines); γ′: the twist angle between the two axes passing through the bridgehead atoms (green lines); r: the vertical separation between the mean planes; l: the displacement distance between the projected centers of the two mean planes, which is positive/negative when the nitrogen atom is displaced outward/inward.

X-ray crystallographic structure of 1 has already been reported,17 but the structural parameters have not been derived due to the severe disorder and multiple polymorphs. In the present study, we endeavored to obtain single crystals of [2.2]pyridinophane 1 and its protonated form 1-H+ suitable for X-ray analysis but were unsuccessful. Accordingly, we decided to discuss the structural parameters for 1 and 1-H+ obtained by theoretical calculations. Thus, the geometries of 1 and 1-H+ were optimized, and the results were comparatively discussed

Table 1. Structural Parameters for Pyridinophane 1 in its Neutral and Protonated Forms structural parametersa form 1

1-H+

geometry

b

dft-d3 (tpss) dft (tpss) mp2 scsmp2-cosmo dft-d3 (tpss) dft (tpss) mp2 scsmp2-cosmo

d, Å

d′, Å

α, deg

β, deg

γ, deg

γ′, deg

r, Å

l, Å

RSMDc

3.08 3.15 3.03 3.06 3.04 3.12 2.98 3.02

2.77 2.81 2.73 2.75 2.74 2.79 2.69 2.73

12.5 13.5 12.3 12.6 12.1 13.2 11.9 12.0

10.8 10.5 11.2 11.0 10.2 10.0 10.4 10.6

5.4 4.1 7.7 7.0 0.3 0.3 0.4 0.2

6.2 4.7 8.6 7.9 0.9 1.1 0.5 0.8

3.07 3.14 3.01 3.05 3.04 3.12 2.96 3.01

0.23 0.20 0.32 0.22 −0.84 −0.10 −0.40 −0.25

0.11 0.21 0.17 0.85 0.31 0.43

a

For definition of parameters, see Chart 1. bGeometries calculated at the DFT-D3(BJ)-TPSS/def2-TZVP, DFT-B3LYP/def2-TZVP, MP2/def2TZVPP, or SCS-MP2/def2-TZVPP with COSMO solvation correction for acetonitrile. cRoot mean square errors from parameters obtained by the SCS-MP2 method with COSMO solvation correction. 978

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and 2-H+ are in the less stable conformations in crystal (vide infra). This means that the (chir)optical properties observed under a given condition should be the weight-averaged values for all the populated conformers in the system. Therefore, reproducing the experimental (chir)optical spectra by theoretical calculations is more demanding, given the fact that the properties predicted for each conformer are considerably different from each other (vide infra). The crystal structure of racemic 2 was reported to be in the syn-chair conformation,17 but the location of nitrogen was disordered. We also succeeded in isolating single crystals of enantiomerically pure 2 suitable for X-ray structural analysis. The structure determined at −150 °C revealed that the location of nitrogen is disordered about the center of symmetry and that the linkers are in the anti-chair conformation (Figure 1). Note that both of the syn-chair and anti-chair conformers (found in crystals) are not the energetically most stable ones predicted by the theoretical calculations. Nevertheless, the characteristic geometrical parameters of these conformers of 2 can be wellreproduced by the DFT-D3 calculation, as shown in Table 3. The distance between the aromatic planes was consistent at ∼3.3 Å (for all the conformers), elongated by 0.2 Å if compared with [2.2]parapyridinophane 1. The distortion of the benzene and pyridine rings was much reduced with smaller angles α and β and similar distances d and d′. The outward displacement was slightly larger in 2 than in 1. The structural parameters were found quite similar to each other with minor variations among the conformers. It is worth mentioning that the D2h symmetric geometry was found in crystal structures, while the DFT-D3 calculation afforded the twisted D2 symmetry as minima with a small twist angle of γ ≈ 1°. In the X-ray structure obtained for 2, the intermolecular interactions seem negligible, as the aromatic planes of adjacent molecules are considerably separated from each other. Similar packing structures have been reported for other cyclophanes.27 The structural changes upon protonation of 2 were also insignificant (Table 3), despite the increased donor−acceptor interaction in 2-H+. As was the case with 1/1-H+, the displacement parameter l was highly sensitive to the protonation and became negative (the nitrogen moved inward) in 2-H+. Also, the twist angles γ and γ′ slightly increased in all the conformations upon protonation. We were able to obtain the X-ray crystal structure of (racemic) 2-H+ as bromide salt (Figure 1). Substantial interaction was found between the bromide anion and the NH hydrogen with a separation of 3.33 Å, and the intermolecular cation−π interaction was also noticed between the NH and the benzene ring (∼3.0 Å). Nevertheless, the overall structure of the syn-boat conformation was within the structural variations among the conformers of 2-H+ and was well-reproduced by the theory. As such, the relative population of conformers, rather than the changes in structure, may play more significant roles in determining how and to what extent the chiroptical properties are altered upon protonation to [3.3]pyridinophane 2. Experimental and Theoretical Circular Dichroisms. Enantiomer Resolution. Pyridinophanes, being capable of metal coordination and hydrogen bonding, have been widely utilized as a scaffold for advanced materials.28 Consequently, a considerable amount of effort has been devoted to the synthesis of racemic and enantiomeric pyridinophanes. In the present study, [2.2]pyridonophane 1 was prepared by photochemical desulfurization of dithia[3.3]pyridinophane 2, according to the literature procedures.17,29,30 Chiral high-performance liquid

Chart 2. Schematic Illustrations of the Conformations of Pyridinophanes 1 (top) and 2 (bottom)

2-Thiatrimethylene-Bridged [3.3]Parapyridinophane. The conformational behavior of [3.3]pyridinophane 2 is more complex than that of 1 due to the flexibility of linkers.25 The sulfur atom in the linker closer to the pyridine nitrogen can be anti or syn to the nitrogen atom, while the two sulfur atoms can be in the boat or chair conformation. Accordingly, there are four possible conformers in total (Chart 2). We optimized all the conformers of 2 and its protonated form 2-H+ at the DFTD3(BJ)-TPSS/def2-TZVP level, and the conformer distributions were estimated by Boltzmann weighing at 25 °C. The single-point energy calculations at the more reliable SCS-MP2/ def2-TZVPP level26 using COSMO solvation model for acetonitrile were subsequently performed, and the results are compared in Table 2. The anti-boat (ab) conformation was Table 2. Relative Energiesa and Conformer Distributions of Neutral and Protonated Forms of Pyridinophanes 1 and 2 phane (form) 1 1-H+ 2

2-H

+

conformer

ΔEDFT‑D3

% distribution

ΔESCS‑MP2

% distribution

skew+ (s+) skew− (s−) s+ santi-chair (ac) anti-boat (ab) syn-chair (sc) syn-boat (sb) ac ab sc sb

≡0 +0.001 ≡0 +0.004 +0.56

50.1 49.9 50.1 49.9 25.2

≡0 +0.004 ≡0 +0.007 +0.29

50.1 49.9 50.3 49.7 24.5

≡0

65.0

≡0

39.8

+1.2

8.2

+0.2

28.4

+2.2

1.7

+1.0

7.3

+4.2 +4.7 ≡0 +0.21

0.1 0.0 58.7 41.3

+1.4 +14 ≡0 +0.28

5.7 0.0 58.2 36.2

a Relative energy (in kcal mol−1) was calculated at the DFT-D3(BJ)TPSS/def2-TZVP or SCS-MP2/def2-TZVPP level. COSMO solvation model for acetonitrile was employed for the latter calculation.

found most stable for neutral 2 by both the theories, while the syn-chair (sc) conformer became favored for 2-H+, suggestive of the NH···S hydrogen bonding. The above two theoretical calculations provided significantly different energy profiles among the conformers, but the overall conformer preference was found less pronounced in the latter method, implying that all the conformers are in equilibrium at ambient temperatures. Indeed, the X-ray crystallographic analyses revealed that both 2 979

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Figure 1. ORTEP drawings of racemic (cited from ref 3) and enantiomerically pure [3.3]pyridinophane 2 and its protonated form 2-H+ (as bromide salt). One of the possible locations for disordered nitrogen is shown in 2 for clarity; for the detail, see the cif files in the Supporting Information.

Table 3. Structural Parameters for Pyridinophane 2 in its Neutral and Protonated Forms structural parametersa form 2

confb ac

ab sc

2-H+

sb ac ab sc sb

geometryc e

X-ray dft-d3 (tpss) dft (tpss) mp2 scsmp2-cosmo dft-d3 (tpss) X-rayf dft-d3 (tpss) dft-d3 (tpss) dft-d3 (tpss) dft-d3 (tpss) dft-d3 (tpss) X-raye dft-d3 (tpss) dft (tpss) mp2 scsmp2-cosmo

d, Å

d′, Å

α, deg

β, deg

γ, deg

γ′, deg

r, Å

l, Å

RSMDd

3.33 3.30 3.48 3.24 3.28 3.32 3.32 3.31 3.31 3.33 3.27 3.34 3.23 3.34 3.49 3.27 3.30

3.21 3.21 3.35 3.14 3.18 3.23 3.20 3.21 3.22 3.23 3.18 3.24 3.15 3.24 3.35 3.10 3.20

6.3 4.3 5.5 4.1 4.8 4.1 4.9 4.3 4.3 4.1 4.0 4.4 3.9 4.3 5.6 4.4 4.5

0.5 2.7 2.7 2.4 2.3 2.6 2.0 2.7 2.1 2.8 2.2 3.2 2.6 3.2 3.1 2.7 2.4

0.0 1.1 1.0 0.5 1.5 0.2 0.0 0.5 2.2 4.5 0.5 9.5 5.0 6.4 4.8 5.3 3.6

0.0 1.1 1.0 0.4 1.4 0.2 0.0 0.4 2.2 5.2 0.5 8.3 6.1 5.4 3.8 4.4 2.7

3.30 3.19 3.48 3.06 3.16 3.29 3.29 3.19 3.24 3.16 3.22 3.11 3.21 3.10 3.31 3.00 3.12

0.42 0.85 0.07 1.08 0.89 0.42 0.42 0.86 0.65 −1.03 −0.56 −1.17 −0.41 −1.24 −1.09 −1.29 −1.08

0.61 0.14 0.37 0.36

0.52 0.47 0.23 0.29

a

For the definition of parameters, see Chart 1. bConformation ac: antichair; ab: antiboat; sc: syn-chair; sb: syn-boat. cExperimental geometry from Xray crystal structure or calculated geometries at the DFT-D3(BJ)-TPSS/def2-TZVP, DFT-B3LYP/def2-TZVP, MP2/def2-TZVPP, or SCS-MP2/ def2-TZVPP with COSMO solvation correction for acetonitrile. dRoot mean square errors from parameters obtained by the SCS-MP2 method with COSMO solvation correction. eThis work. fReference 17.

spectrum of 1 was composed of three domains corresponding to the 1 L b , 1 L a , and 1 B transitions (after the Scott nomenclature)32 from low to high excitation energies. Detailed investigations to assign the transitions in pyridinophanes and to elucidate their origins have been already accomplished,33 and the electronic interactions between the cyclophane planes have been recently investigated with an aid of sophisticated ab initio calculations.34 The UV−vis spectrum of 1-H+ exhibited a new broad absorption band at ∼330 nm in addition to the 1Lb, 1La, and 1B transitions that appeared near the original positions with slight modifications in intensity and excitation energy (Figure S2 in the Supporting Information). The CD spectral extrema of 1 and 1-H+ in the 1Lb, 1La, and 1 B transition regions did not exactly agree with the UV−vis spectral peak positions (Figure 2). Rather, each of the observed Cotton effects (CEs) was coupled (became bisignate) to afford a complicated positive-negative-negative-positive pattern for 1 and a quasi-mirror-imaged negative-positive-positive-negative pattern for 1-H+, where the CE extrema are labeled as A, B, D, and E in Figure 2; note that label C, not clearly observed for 1 and 1-H+, is reserved for the comparison with 2 and 2-H+.35 The CE intensities were fairly strong, typical for planar chiral cyclophanes.36 Anisotropy factor (g = Δε/ε) has been frequently used as a quantitatively more reliable chirality

chromatography (HPLC) with cellulose tris(3,5-dimethylphenylcarbamate)-based chiral stationary phases has been demonstrated to be effective for the enantiomer separation of a variety of cyclophane derivatives.31 We also succeeded to optically resolve the enantiomers of pyridinophane 1 using Chiralcel-OD column (see Figure S1 in the Supporting Information). On the contrary, only a limited number of studies have been reported for the enantiomer separation of dithia[3.3]cyclophanes, probably due to the poor solubilities of these substrates in conventional solvents for HPLC analysis. Fortunately, dithia[3.3]pyridonophane 2 showed enough solubility for the chiral HPLC separation owing to the pyridine moiety, which enabled us to resolve the enantiomers of 2. The separation (α) and resolution (Rs) factors31 for pyridinophanes 1 and 2 were determined to be α = 1.2, Rs = 1.6 and α = 1.3, Rs = 2.7, respectively, on a Chiralcel-OD column eluted with a hexane-2propanol mixture (9:1) at 25 °C. Experimental Circular Dichroisms of 1 and 1-H+. Experimental UV−vis and CD spectra of neutral [2.2]pyridinophane 1 were recorded in acetonitrile at 25 °C, while those of the protonated form (1-H+) were obtained by adding an excess amount of aqueous HBF4 to the same solution. For clarity, the first elutes from the chiral HPLC column mentioned above are discussed in the following section. The UV−vis 980

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Figure 2. Comparison of the experimental (solid lines) and theoretical (dashed lines) CD spectra of [2.2]pyridinophane 1 (blue) and its protonated form 1-H+ (red). The experimental spectra were obtained in acetonitrile at 25 °C, while the theoretical spectra were obtained at the RI-CC2/def2-TZVPP level. The calculated excitation energies were 0.2 eV red-shifted for 1 and 0.2 eV blue-shifted for 1-H+.

Figure 3. Comparison of experimental (solid lines) and theoretical (dashed lines) CD spectra of [3.3]pyridinophane 2 (blue) and its protonated form 2-H+ (red). The experimental spectra were obtained in acetonitrile at 25 °C, while the theoretical spectra were obtained by weight-averaging the spectra of four conformers calculated at the RICC2/def2-TZVPP level. The calculated excitation energies of 2 and 2H+ were red-shifted by 0.4 and 0.2 eV, respectively.

measure. The g factor of 1 at the 1Lb band was found as large as 2 × 10−2, which is much larger than the typical values (1 × 10−4 to ∼1 × 10−5)37 for allowed transitions (Figure S2 in the Supporting Information). This may be explained by a mixing of the forbidden transition of the nitrogen lone pair. The broad positive CE at ∼300−360 nm in 1-H+ is ascribable to the charge-transfer (CT) excitation from the benzene to the pyridinium unit, further details of which will be discussed below. It has been already reported that the CD spectra of pyridinophanes were substantially altered in different degrees upon coordination to various metal ions.29 Nevertheless, all the CEs (A, B, D, and E) suffered a total sign-inversion upon protonation. Because the structural change upon protonation was rather marginal (vide supra), this inversion can be understood by a reversal of the direction of electronic transition moment, which is caused by the inversion of dipole moment in the pyridine/pyridinium unit (Scheme 2). It has been

corresponding UV−vis spectra and anisotropy (g) factor profile can be found in Figure S2 in the Supporting Information. Despite the much larger conformational flexibilities and more complex conformer equilibria before and after protonation, the CD spectra of 2 and 2-H+ were strikingly comparable to those of 1 and 1-H+. Some additional points to note here are that the bands C and D became more apparent, the corresponding CEs were coupled (when compared with those of 1 and 1-H+), and the positive CE at the CT transition in 2-H+ became much smaller in intensity. Accordingly, the signs of all the CEs (A to E) were reversed from the positive-negative-positive-negativepositive pattern to the opposite upon protonation, which is explained in principle by the reversal of electronic transition moment and the inversion of dipole moment in the pyridine/ pyridinium unit (Scheme 2). Theoretical Investigation of CD Spectra. To gain some insights into the electronic interactions and the chiroptical behaviors upon protonation of pyridinophanes, we performed the theoretical CD calculations on all the pyridinophanes at the RI-CC2/def2-TZVPP level38 for the geometries optimized at the DFT-D3(BJ)-TPSS/def2-TZVP level, and the results were compared with the corresponding experimental spectra (Figures 2 and 3, dotted lines). The characteristic features of spectra, that is, the relative excitation energies and the CE intensities of 1 and 1-H+, were well-reproduced by this theory, but each transition was more resolved in the theoretical spectra to show apparently coupled CEs. The spectra of 2 and 2-H+ were better reproduced by the theory, but this might be rather incidental, as the theoretical spectra for 2 and 2-H+ were constructed by Boltzmann-averaging those predicted for the relevant conformers. The spectrum calculated for each conformer considerably differed from each other, implying that the locations of sulfur atoms have substantial influence on the electronic and magnetic transition moments and thus on the observed CD (Figure S3 in the Supporting Information). Nevertheless, the signs of all CEs were predicted to flip upon protonation, as observed in the experiment. The CE at the CT band was also well-reproduced in sign and intensity as well as relative excitation energy. More detailed configurational

Scheme 2. Reversal of the Dipole Moment in the Pyridine Versus Pyridinium Chromophore

established that the low-energy transitions (bands A−C) involve the excimeric excited states of predominant exciton resonance nature, while the high-energy transitions involve the excimeric excited states of dominant charge-resonance nature.33 The local excitation (within a single aromatic ring) appears at the left edge of this spectral window. As such, the reversal of dipole moment in a single plane of cyclophane can ultimately flip the signs of the whole CD spectral extrema. Experimental CD of 2 and 2-H+. The CD spectra of [3.3]pyridinophane 2 and its protonated form 2-H+ were also measured in acetonitrile at 25 °C (Figure 3). The 981

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Table 4. Comparison of Calculated Excitation Energya (ΔE) and Oscillator and Rotatory Strengths ( f and R) for the First Transitions of [2.2]Pyridinophane 1 in its Neutral and Protonated Forms 1-H+

1 theory experiment RI-CC2/def2-ZTVPP SAC−CI/B95(d) CAM-B3LYP/def2-TZVP B3-LYP/def2-TZVP M06-2X/def2-TZVP BH-LYP/def2-TZVP

ΔE 3.97 4.27 4.07 4.41 4.08 4.41 4.50

(312) (290) (305) (281) (304) (281) (276)

f

R

b 0.007 0.010 0.012 0.005 0.011 0.013

+33 +56 +55 +83 +55 +91 +81

ΔE 3.76 3.47 3.34 3.83 3.18 3.88 3.91

(330) (357) (371) (324) (390) (320) (317)

f

R

b 0.001 0.002 0.002 0.001 0.002 0.003

+15 +5.8 +21 +21 −1.3 +18 +30

a ΔE is reported in electronvolts and in nanometers (in parentheses), and f and R are reported in 1 × 10−40 cgs unit. For calculations with TD-DFT method, COSMO solvation model for acetonitrile was employed. bNot obtained due to the structureless absorption (see Figure S2 in the Supporting Information).

analysis revealed that the bands A−C correspond to the HOMO, HOMO−1, and HOMO−2 to LUMO transitions, while the bands D and E are composed of complex mixtures of the HOMO, HOMO−1, HOMO−2 to LUMO+1, LUMO+2 transitions. At any rate, the absolute configurations (ACs) of all the first elutes in chiral HPLC were unambiguously assigned to the (Sp)-(+)-enantiomer. The AC assignment of 1 is in accord with that of the previous study.29 Recently, the time-dependend density functional theory (TD-DFT) method has been widely employed for the simulation of UV−vis and CD spectra of relatively large molecules.39 The method is clearly advantageous in efficiency and cost, but it is often compromised by deteriorated accuracy. Therefore, we also investigated the applicability and limitations of such methods for our pyridinophane systems and briefly discuss the results here. Table 4 compares the theoretical excitation energy and the oscillator and rotational strengths of the first transition of 1 and 1-H+ predicted by different theoretical ansatz. Comparison of the theories in the whole spectral range is also shown for 1 and 1-H+ in Figure S4 in the Supporting Information. Among the various functionals, we tested global hybrid functionals (B3-LYP40 and BH-LYP41), long-range corrected functional (CAM-B3LYP),42 as well as hybrid meta functional (M06-2X)43 for comparison. Most of the functionals overestimated the excitation energies as well as the rotational strengths of neutral 1. Moreover, the rotational strengths predicted for protonated 1-H+ were found quite sensitive to the choice of functionals. The overall spectral shapes were significantly less satisfactorily reproduced by all the TD-DFT methods (Figure S4 in the Supporting Information) but would be useful for the provisional assessment of absolute configuration. Therefore, it is recommended to carefully evaluate the results of the TD-DFT method, and more reliable theory, such as RI-CC2 and SAC−CI, should be employed whenever possible. The SAC−CI method44−46 was found superior in reproducing the CE at the first transition, but the RI-CC2 method achieved slightly better overall performance (to reproduce the whole spectra in the full wavelength range) at least for the pyridinophane system. To show how the individual structural parameters affect the observed CD spectra, we calculated the hypothetical CD spectra using the face-to-face model system composed of pxylene and 2,5-dimethylpyridine (or 2,5-dimethylpyridinium), the results of which are detailed in Figure S5 in the Supporting Information. Although the variation of separation distance (d) induced rather systematic CD changes, altering twist angle (γ) and displacement (l) led to unpredictable changes. Considering

possible additional effects of the linker atoms, we refrained from further analyzing the CD spectral changes by using the model systems. Switching CD Signs upon Alternating Protonation− Deprotonation. Because both pyridinophanes 1 and 2 exhibited the characteristic CD sign inversion upon protonation, we wanted to demonstrate their ability to switch the chiroptical properties upon repeated protonation−deprotonation cycle by sequential additions of acid and base. This attempt enabled us to utilize this phenomenon as a tool for sensing temperature, that is, a thermal sensor. The temperature dependence of the CD spectra of neutral pyridinophanes 1 and 2 was examined (Figure S6 in the Supporting Information). When gradually cooled, slight enhancement of CE intensity was observed for most of the transitions, due to the conformational freezing at lower temperatures.47,48 In contrast, the CD spectra of the protonated forms of 1 and 2 exhibited an unusual temperature dependence. As exemplified in Figure 4, left, the CD spectrum started to

Figure 4. Temperature dependence of CD spectra of [3.3]pyridinophane 2 (left) and the sign inversion of the CE, monitored at 298 nm, in aqueous methanol containing HBF4 upon repeated cooling−heating cycles (right). Blue, 25 °C; red: −25 °C.

change when a solution of 2 in aqueous methanol containing tetrafluoroboric acid was cooled to temperatures below 0 °C. Further investigations revealed that this change is reversible and most probably triggered by the temperature-induced acidity change of tetrafluoroboric acid.49 Figure 4, right, illustrates the CD switching behavior (monitored at 298 nm) of an aqueous methanol solution of 2-H+ upon repeated cooling−heating cycles between −25 and +25 °C. As the molecule is quite stable, this chiroptical switching behavior is practically persistent as thermal sensor. 982

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The Journal of Physical Chemistry A Donor−Acceptor Interactions in 1-H+ and 2-H+. We now discuss the donor−acceptor interactions between the benzene and pyridinium rings in cyclophanes 1-H+ and 2-H+. The cation−π interaction is rather ubiquitous in biological and supramolecular systems and has been extensively investigated for more than three decades.50 Being electrostatic in nature, the cation−π interaction is quite strong in the gas phase but becomes weaker in solution. The effect of cation−π interaction on chiroptical properties has been studied in some detail.51,52 The current pyridinophanes can be regarded as apposite model systems to analyze the cation−π interaction and its relationship with chiroptical properties. The degree of interaction can be quantitatively evaluated by the electronic coupling of frontier orbitals, which is represented by the electronic coupling element HDA.53 Table 5 summarizes

the cation−π interaction. Accordingly, the NH+ group was positioned just above the benzene ring in the protonated forms, while the nitrogen was sledded out in the neutral forms. The positive CE was observed for the (Sp)-(+)-isomer at the CT band, and the intensity was found ca. 10-fold larger for [2.2]than for [3.3]pyridinophane. Remarkably, most of the characteristic CEs observed for neutral pyridinophanes (at the 1 Lb, 1La, and 1B bands) were completely inverted in sign, which was comprehensively explained by the theoretical calculations. The reversal of CD sign was accomplished not by the marginal structural changes upon protonation but by the reversal of dipole moment in the pyridine versus pyridinium ring as a consequence of the excitonic nature of the electronic transitions. This unique switching behavior was demonstrated for an application as a thermal sensor. Our study revealed for the first time that the strong CEs observed for planar chiral rigid cyclophanes can switch their signs by simply alternating the direction of dipole moment on one of the planes in cyclophanes. This is because most of the characteristic transitions in cyclophanes are composed of excimeric excited states of either charge or exciton resonance nature. Such consideration could be beneficial in designing the advanced sensing systems based on planar chirality, and pyridine moiety, although not limited to, is certainly one of the most expedient groups to inverse the plane’s dipole. Further studies on (chir)optical properties of related pyridinophanes are now underway.

Table 5. Characteristic Propertiesa of the Charge-Transfer Transition of Protonated Pyridinophanes 1-H+ and 2-H+ phane +

1-H 2-H+

rDA,a Å

λCT, nm

3.04 3.32

369 352

ΔεCT, M−1 cm−1 εCT, M−1 cm−1 +8.2 +0.8

290 40

HDA,b cm−1 840 330

a

DFT-D3-TPSS/def2-TZVP calculated mean plane distance d was used as donor−acceptor separation. For 2-H+, averaged d for four conformers was used. bElectronic coupling element calculated by the ̂ equation: HDA = 0.0206 × (νmax × Δν1/2 × εCT)(1/2)/r DA. See refs 55−57.

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the spectroscopic data for CT transition and the corresponding CD spectral features, together with the HDA values for 1-H+ and 2-H+. As expected, the HDA value for [2.2]paracyclophane 1-H+ was markedly larger (by a factor of 2.5) than that for [3.3]cyclophane 2-H+, as a consequence of the smaller separation between the planes. The difference in transannular interaction between [2.2]- and [3.3]paracyclophanes has been recently investigated by comparing their electron transfer processes.54 Considerably greater differences were found in CE intensity (10-fold in Δε) and also in anisotropy (g) factor (4.7fold, 0.014 vs 0.003) for the CT band of 1-H+ and 2-H+. This may be better explained by the conformational diversity of dithia[3.3]cyclophane and the mutual cancellation of CEs, rather than the intrinsic reduction of CEs for [3.3]cyclophane, as judged from the calculation results for models with varying separations (Figure S5 in the Supporting Information). Although qualitative, the CE at the CT band can be wellreproduced by the model system, where the donor (p-xylene) and acceptor (2,5-dimethylpyridinium) in 1-H+ and 2-H+ are separated by 3.1 and 3.3 Å, respectively (Figure S7 in the Supporting Information).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b12287. Experimental and theoretical details and extended spectral data (PDF) Crystallographic data (CIF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +81-6-6879-7923. ORCID

Tadashi Mori: 0000-0003-3918-0873 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial supports by Grant-in-Aids for Scientific Research, Challenging Exploratory Research, and on Innovative Areas “Photosynergetics” (Grant Nos. JP15H03779, JP15K13642, and JP15H01087) from JSPS, the Matching Planner Program from JST (Grant No. MP27215667549), and Cosmetology Research Foundation are greatly acknowledged.

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CONCLUSIONS The CDs of planar chiral pyridinophanes exhibited fairly strong, mostly coupled CEs at the 1Lb, 1La, and 1B bands. The (Sp)(+)-isomers, the first elutes under the chiral HPLC conditions employed, showed a positive CE at the lowest-energy band, which was followed by negative, positive, and negative CEs. Upon protonation, strong cation−π interaction occurred between the pyridinium and benzene ring, as revealed by the large electronic coupling element (HDA) values, to induce the characteristic CT transition in the lower energy region. Despite this strong interaction, the changes in molecular geometry were quite minor, due to the rigid cyclophane framework. The displacement of nitrogen was, nevertheless, evident to maximize

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