Spectroscopic and Theoretical Studies of AcidâBase Behaviors of N

Article pubs.acs.org/JPCA

Spectroscopic and Theoretical Studies of Acid−Base Behaviors of N‑Confused Porphyrins: Effects of meso-Aryl Substituents Ryuichi Sakashita,† Masatoshi Ishida,‡ and Hiroyuki Furuta*,† †

Department of Chemistry and Biochemistry, Graduate School of Engineering, and ‡Education Center for Global Leaders in Molecular Systems for Devices, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *

ABSTRACT: The acid−base properties of a series of meso-arylsubstituted N-confused porphyrins (NCPs) were examined in aqueous sodium dodecyl sulfate (SDS) micellar solutions by both spectrophotometric methods and theoretical calculations. Reflecting the unsymmetrical structure of NCP having an outwardpointing pyrrolic nitrogen atom, the first and second protonations were distinguishable in the absorption and 1H NMR spectra, unlike for porphyrins, and the pK3 and pK4 values were determined discretely. The individual basicities of the NCPs were directly related to the inductive effect of para substituents on the meso-phenyl groups: A linear relationship between the pK3 (pK4) and Hammett σpara parameters was revealed. In the case of deprotonation, the structure of monoanionic NCP species was similarly characterized by the absorption and 1H NMR spectra. For the second deprotonation, the pK1 value was determined to be 11.39 for the NCP derivative with pentafluorophenyl groups. DFT calculations support the changes in electronic structures and aromaticity of the cationic and anionic species. It is demonstrated that NCPs are easily protonated and deprotonated compared to the corresponding regular congeners.

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Chart 1. Structures of NCPs 1 and Regular Porphyrin Congeners 2

INTRODUCTION Protonation and deprotonation are ubiquitous phenomena for nitrogen-containing heterocyclic compounds, and acid dissociation constants (pKa values) are used to describe the acidities of the molecules quantitatively.1 For the determination of pKa values,1,2 the absorption (and fluorescence) spectrophotometry-based technique is widely used because of its simplicity, low cost, and easy handling during experiments.3 This brings a great advantage, in particular, for porphyrins, which display characteristic absorption profiles in the visible range to monitor changes in various pH environments. In the regular porphyrin scaffolds (2-H2, Chart 1), two imino nitrogen (N) and two amino nitrogen (NH) sites are present symmetrically inside the macrocycle that allow mono- and diprotonation with constants of pK3 and pK4, respectively, as well as mono- and dideprotonation with constants of pK2 and pK1, respectively (Scheme 1b).4,5 The changes in the electronic structures of porphyrins with different charge states should give rise to pH-dependent photophysical and redox responses. Therefore, extensive investigations of the acid−base behaviors of porphyrins and related compounds have been performed.4−14 In an effort to gain structural insights into the charged species of porphyrins, we have focused on an N-confused porphyrin (NCP) known as a structural isomer of porphyrin in which one of the pyrrole rings is connected at the α and β′ positions with the surrounding meso-carbon atoms (1-H2, Chart 1).15−31 Owing to the unusual (i.e., conf used) linkage, the © XXXX American Chemical Society

NCP has an unsymmetrical structure that realizes unique NH tautomerism. Two kinds of tautomers, denoted as the 3H and 2H forms, exist at equilibrium.18,19 The 3H form, 1-H2 (22,24H) [where the configuration of the protons is described Received: December 9, 2014 Revised: January 14, 2015

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DOI: 10.1021/jp512229k J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Table 1. Summary of pKa Values of meso-Tetraaryl NCPs (1a−1h)

Scheme 1. Reaction Schemes for Protonation and Deprotonation of NCPs 1 and Regular Porphyrin Congeners 2

in parentheses, e.g., 1-H2 (22,24H) indicates the NCP that has NH protons on 22N and 24N; also see Table 2 for the nomenclature of the isomers], has three hydrogen atoms in the core which includes an 18 π-electron conjugated circuit with distinct aromaticity as seen in the regular porphyrins. In contrast, the 2H form, 1-H2 (2,23H), has two hydrogen atoms in the core and one hydrogen atom at the peripheral nitrogen, which does not include a complete 18 π-electron conjugated circuit, demonstrating a moderate aromatic nature. This tautomerism-coupled structural change of the NCP results in a distinctive switching of the internal hydrogen-bonding network and the characteristic photophysical properties such as nearinfrared (NIR) absorption and emission. Herein, we report the acid−base behaviors of various mesoaryl-substituted NCPs (1a−1h) for determining the pKa constants in aqueous sodium dodecyl sulfate (SDS) solutions by spectrophotometric and pH metric techniques.6 The surfactant aggregates known as micelles are responsible for solubilizing the hydrophobic substances in water.32 As it is known that micelles can affect the apparent pKa values of porphyrins through electrostatic and microenvironmental influences,6d we examined the acid−base behaviors of NCPs and regular porphyrin as a reference and quantitatively analyzed both types of molecules with comparative optical studies performed in nonaqueous solvents and NMR characterizations. Furthermore, the total energies of NCP molecules in various charged states and their gas-phase acidities were also evaluated by the density-functional-theory- (DFT-) based formalism, and the theoretical calculations were consistent with the experimental results (Scheme 1a).

Ar: meso aryl substituents. aReference 28. bReference 29.

Spectroscopic Titrations. For sample preparations, ultrapure water obtained through the ion-exchange resin Organo Puric-Z was used. The sample concentration of NCPs (1a−1f) and TPP (2c) was set to ∼10−5 M in 2.5% aqueous SDS solution.15 Nitric acid and sodium hydroxide aqueous solutions were used to adjust the pH values of the sample solutions at room temperature using a D-71 pH meter (Horiba) equipped with a 9611-10D electrode (Horiba). The ionic strength of the sample solutions was maintained at 0.1 M by adding appropriate amounts of sodium nitrate. The UV−vis−NIR absorption spectra were measured on a Shimadzu UV-3150PC spectrometer, and fluorescence spectra were recorded on a Horiba Fluorolog spectrometer with 10-mm quartz cells under ambient conditions. pKa values of each sample were obtained from the resulting absorption plots fitted by the least-squares method. NMR Spectroscopy. 1H NMR spectra were recorded in CDCl3 solution on a JEOL JNM-AL Series FT-NMR (300 MHz) spectrometer, and chemical shifts are reported relative to the residual proton of deuterated solvent, CHCl3 (δ = 7.26) or dimethyl sulfoxide (DMSO; δ = 2.50), in ppm. Concentrations of the porphyrin samples were set to 20 mM for all measurements. Computational Protocols. DFT calculations were performed with the Gaussian 09 program package33 without symmetry treatment. Initial structures were based on the X-ray structures of the related compounds. The geometries were fully optimized using Becke’s three-parameter hybrid functional combined with the Lee−Yang−Parr correlation functional, denoted as the B3LYP level of density functional theory, with the 6-31G(d,p) basis set for all calculations.34 Values of the nucleus-independent chemical shifts [NICS(0)]35 were determined at the mean position of the 24 core atoms of the optimized porphyrin structures by the GIAO/B3LYP/ 6-31G(d,p) method. Harmonic oscillator model of aromaticity (HOMA) values were calculated from the bond lengths of optimized structures.36

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METHODS Materials and Instruments. Commercially available solvents and reagents were used without further purification unless otherwise stated. Thin-layer chromatography (TLC) was carried out on aluminum sheets coated with silica gel 60 F254 (Merck). Preparative separation was performed by silica gel column chromatography (Kanto silica gel 60 N, spherical, neutral, 40−50 or 63−210 μm). The samples of NCPs (1a−1f) and tetraphenylporphyrin (TPP) (2c) used in this study were prepared according to the reported methods.16,25,26 Recrystallized samples of porphyrin derivatives were utilized for photophysical experiments. B


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Table 2. Plausible Structures of 1c in Various Charged States and Their Relative Free Energiesa (kcal/mol), NICS(0) Values (ppm), HOMA Values (Å), and mpd Values (Å) Calculated at the B3LYP/6-31G** Level

a

With respect to the most stable tautomer.

[1-H3+ (2,22,24H)] and monoanion [1-H− (23H)] were used to calculate GA and PA.

The gas-phase acidity (GA) of the acid (PH) and the proton affinity (PA) of the corresponding conjugate base (P−) are defined by the changes in Gibbs free energy (G) and enthalpy (H), respectively, in the following equilibrium

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RESULTS AND DISCUSSION Basicity of NCPs in Water. The acid dissociation constants of NCPs, pK3 and pK4, can be given under the acid−base equilibria shown in Scheme 1. The UV−vis−NIR absorption spectral changes of the NCPs (1a−1f) were analyzed in the range from pH 1 to 13 in SDS micellar solutions (Table 1; Figure S1, Supporting Information). At pH 7.0, in 2.5% SDS solution, which is higher than the critical micelle concentration (cmc),37 the phenyl derivative (1c) as a representative displayed a spectrum similar to that obtained in a nonpolar organic solvent, namely, CH2Cl2 (Figure S3, Supporting Information).

ΔH , ΔG

PH XoooooooY P− + H+ GA(PH) = ΔG

(1)

PA(P−) = ΔH

(2)

The 3H-type tautomer [1-H2 (22,24H)], which is more stable than the 2H-type tautomer [1-H2 (2,23H)], was used as a standard neutral structure. The most stable monocation C


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The Journal of Physical Chemistry A This suggests that 1c exists as a 3H-type tautomer [1-H2 (22,24H)] in this medium (Table 2). The linear relationship in the Beer’s plots indicated that NCP 1c (and also 1f) is well dispersed in the micelles without molecular aggregation (Figure S4, Supporting Information).38 Upon an increase in the acidity of the aqueous medium, the formation of monoprotonated NCP [1c-H3+ (2,22,24H)] was inferred from the red shift of the Q band at the longest wavelength from 728 to 782 nm with increasing intensity (Figure 1b).

In contrast to 1c, the reference compound 2c exhibits the nondistinctive pKa value of 3.20 in the mono- and diprotonation processes (Figure S2, Supporting Information).41 Thus, in the structural sense, the molecular basicity of 1c is significantly larger than that of the corresponding regular porphyrin, 2c. The inherently higher basicity of 1c could originate from the “bare” nitrogen atom at the periphery. Next, the second protonation was explored for the relatively electron-rich NCPs 1a−1d. The UV−vis−NIR absorption spectrum of dication 1c-H42+ (2,22,23,24H) formed in a strongly acidic solution exhibited distinct spectroscopic features, namely, further red shifts of the Soret and Q bands (Figure 1a). The structure of 1c-H42+ was likewise confirmed by 1H NMR spectroscopy in the presence of 10 equiv of trifluoroacetic acid (TFA) in CDCl3 (Figure 2c); diprotonation of both the inner and outer nitrogen sites was characterized by the NH signals at δ = 1.49, 1.77, and 1.90 ppm for the internal protons (Hb″) and 11.7 ppm for the peripheral Hc″. The pK4 values were obtained in the lower pH range from 1.56 to 3.58 depending on the meso substituents of the NCPs (Table 1; Figure S1, Supporting Information). Generally, for the protonation of regular porphyrins, the constants pK3 and pK4 cannot be distinguished, because the first protonation causes a loss of planarity and this makes the second protonation more facile. When the molecule is predistorted significantly, however, the stepwise protonation occurs as illustrated with the dodeca-substituted porphyrin: β-Octabromo-substituted TPPS (2g) has pK3 and pK4 values of 4.83 and 1.96, respectively.14a In contrast, NCPs are rather planar but have intrinsic unsymmetrical structures with discrete nitrogen-atom configuration. To gain insight into the relationship between the acidity and the meso substituents, the pKa values were plotted against the Hammett parameters (σ). Interestingly, the pK3 and pK4 values of 1a−1e were linearly correlated with σpara (Figure 3).42 From the least-squares analysis, the linear relationships pK3 = −0.754 × 4σpara + 7.577

(R2 = 0.9875)

(3)

pK4 = −0.542 × 4σpara + 2.644

(R2 = 0.8877)

(4)

were established. These equations allow for the systematic estimation of the acidities of modified NCPs. Importantly, the observed pKa values were much less correlated with the metapositioning parameter, σmeta, which represents almost exclusively an inductive effect (e.g., R2 = 0.8430 for pK3 and R2 = 0.3647 for pK4). Therefore, a certain extent of resonance contribution from the 4-substituents on the meso-phenyl moieties to the core macrocycle was inferred. Through the fluorometric technique, signal changes of 1c were also observed upon lowering of the pH of the SDS solution (Figure S6, Supporting Information). The fluorescence emission emerging at λmax = 743 nm in the neutral form at pH 11.0 was red-shifted to λmax = 824 nm at pH 4.28 and λmax = 842 nm at pH 1.74, which is in agreement with the red shifts of the lowest-energy absorption band. The overall intensity of the emission was decreased upon protonation, which implies the deactivation of the excited state by the protonation-induced deformation of the macrocyclic core. Acidity of NCPs in Water. Although the protonation behaviors of various regular porphyrins have been widely explored, little study has been done on the deprotonation events because of the inherently low acidity of the freebase porphyrins,

Figure 1. Absorption spectral changes of 1c in aqueous SDS micellar solution over the pH ranges (a) 4.28−11.01 and (b) 1.74−4.28.

These spectral changes are consistent with those observed in the organic solvents CH2Cl2 and dimethylformamide (DMF).31 Curvefitting analysis afforded a pK3 value of 7.49 for 1c in aqueous SDS solution. Separately, the structure of the monoprotonated species15 (1c-H3+) was characterized by the 1H NMR spectrum in CDCl3 (Figure 2). The spectrum of 1c obtained in the presence of 1 equiv of dichloroacetic acid (CHCl2COOH) confirmed that the monoprotonation occurs at the peripheral imino nitrogen of the confused pyrrole ring to give 1c-H3+ (2,22,24H); the peripheral NH (Hc′) signal appeared at δ = 17.4 ppm in the far downfield region.39 A broad signal of the inner amino protons (Hb′) at δ = −1.29 ppm together with the broad signal around 5 ppm assigned to the CH group of the dichloroacetate counteranion is characteristic, reflecting the effect of the aromatic ring current.40 D


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Figure 2. 1H NMR spectra of (a) 1c, (b) 1c with 1 equiv of CHCl2COOH, and (c) 1c with 10 equiv of CF3COOH in CDCl3 at −50 °C. [1c] = 20 mM. Asterisks indicate residual solvent and impurity peaks.

monoanionic porphyrin derivatives have been reported.9−14 The dissociation constant for the first deprotonation of 2c, for instance, was estimated only in the aprotic organic solvents DMSO (pK2 = 21.2) and DMF (pK2 = 16.4).11 In aqueous systems, the pK2 value of the water-soluble tetrasulfonatesubsituted porphyrin (TPPS, 2g) was determined to be 16.4.12 Accordingly, several porphyrins containing meso-tetrapyridinium cations (TPyP, 2i) were shown to be more acidic, with pK2 = 11.8− 13.5, because of their electron-withdrawing nature (Chart 2).13 Furthermore, the distinct acidity of TPPS and TPyP was altered by the peripheral β-substituents. β-Octabromo-substituted Chart 2. Chemical Structures of Water-Soluble Tetraarylporphyrins (2g, 2i, 2j) Reported in Refs 13 and 14

Figure 3. Plots of pK3 and pK4 vs Hammett parameters 4σpara for p-CH3O (1a), p-CH3 (1b), H (1c), p-CF3 (1d), p-CN (1e).

which makes the formation of anionic species difficult, especially in water, under typical basic experimental conditions (∼pH 14).5 In fact, very few studies on the formation of E


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The Journal of Physical Chemistry A TPPS and TPyP were reported as the most acidic water-soluble porphyrins, with corresponding pK2 values of 10.0 and 6.5, respectively, because the distorted saddle core planes facilitate the contact of the amino protons with surrounding bases (Chart 2).14 The NCP derivatives exhibiting 2H tautomeric forms with outward-pointing amino protons are thus deemed to show stronger acidities than the corresponding porphyrins, in response to their anion binding abilities.21 However, only a few studies [e.g., of a water-soluble NCP (1h)29 with mesotetrapyridinium substituents] on the deprotonation were addressed with moderate pK2 = 9.8. Very recently, Ziegler et al. reported the deprotonation behavior of NCP 1c in organic solvents, but quantitative investigations with apparent equilibrium constants have not yet been unveiled.31 To analyze the deprotonated forms accessible under a reasonable basic environment, two electron-deficient derivatives, namely, 4-cyanophenyl-substituted NCP (1e) and pentafluorophenyl-substituted NCP (1f), were utilized. When the basicity of the aqueous SDS medium was increased, the Soret band of neutral 1f-H2 at 435 nm was broadened and redshifted to 449 nm, which could be explained by the formation of monodeprotonated species (Figure 4a). Similar characteristics were seen for 1e. The corresponding pK2 values of 1e and 1f were determined to be 11.43 and 8.04, respectively, by spectroscopic analysis (Table 1). Judging from the smaller pK2 value of 1f, it can be concluded that meso-C6F5-substituted NCP 1f is more acidic than the cationic NCP 1h and the corresponding regular porphyrin 2f. Importantly, it was found that the second deprotonation of 1f was achieved in strong basic environments (e.g., pK1 > 11.39; Figure 4b).43 The structures of monodeprotonated 1f-H− and dideprotonated 1f2− were both characterized by 1H NMR spectroscopy in DMSO-d6 in the presence of 1 equiv of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and excess amounts of tetra-n-ethylammonium hydroxide (TEAOH) (Figure 5).44 In a polar solvent (e.g., DMSO), 1f is known to exist as a 2H tautomer [1-H2 (2,23H)], as is evident from the spectroscopic patterns (Figure 5a).45 Upon addition of DBU, the 1H NMR spectrum revealed the formation of deprotonated 1f-H− as confirmed by the disappearance of outer NH resonance and the shifts of inner NH and CH resonances to higher magnetic field (Figure 5b). The stronger base, TEAOH, led to the disappearance of two inner NH resonances; instead, a singlet signal of the inner CH (Ha″) appeared at δ = −2.83 ppm in the 1H NMR spectrum, which suggested the formation of the dianionic species 1f2− (Figure 5c).46 The stronger electron-withdrawing nature of the pentafluorophenyl groups present at the meso positions in 1f play a significant role in the enhanced acidity of NCPs. Correspondingly, the fluorescence nature of the monoanionic and dianionic species derived from 1f was certainly altered; red shifts of the emission peaks and decreased emission quantum yields were observed similarly to the cationic derivatives (Figure S7, Supporting Information). This indicates the fluorometric method can also be used to quantify the degree of deprotonation state of 1. Calculated Energetics and Aromaticity of Protonated and Deprotonated Species. The apparent acidities (pK1 and pK2) and basicities (pK3 and pK4) of a series of meso-arylsubstituted NCP derivatives 1 were described above (Table 1). Between the two tautomeric forms, 2H and 3H, the structure of 1-H2 (22,24H) is energetically more favorable [i.e., −5.29 kcal/mol vs 1-H2 (2,23H)] due to the significant resonance aromatic stabilization and the pyrrolic amine−imine hydrogenbonding interactions in the core.19 Upon monoprotonation and

Figure 4. Absorption spectral changes of 1f in aqueous SDS micellar solution over the pH ranges (a) 6.18−9.68 and (b) 9.68−13.07.

monodeprotonation, we could consider four tautomers for each anion and cation species of 1 (Table 2). Between 1c-H3+ and 1c-H−, 1c-H3+ (2,22,24H) and 1c-H− (23H) are suggested to be most stable, according to the DFT (B3LYP/6-31G**) calculations, which might be caused by the difference in the number of effective hydrogen-bonding stabilization inside the core.47 This result is consistent with the fact that both of 1c-H3+ (2,22,24H) and 1c-H− (23H) have the smallest mean plane deviation (mpd) among their tautomers (Table 2). Consequently, all cationic and anionic species are based on the 3H-type tautomeric structures that are realized by the contribution from the effective hydrogen-bonding network within the planar conformation. The similar energetic trend and parameters are seen in the NCP derivatives with different mesosubstituents (Tables S1 and S2, Supporting Information). For the energetic analysis of 1c, protonation- and deprotonation-induced alterations of aromaticity were evaluated in terms of the NICS(0) and HOMA values (Table 2; Table S4, Supporting Information). Observed ring current shifts in the 1H NMR spectra of 1c and 1f are also summarized in Table S7 (Supporting Information), where Δppm is defined as the difference in chemical shifts between the signals of the outer α proton (3CH) and the inner β proton (21CH) of the confused pyrrole ring. It is known that the degree of aromaticity of 1c-H2 (2,23H) is smaller than that of 1c-H2 (22,24H) judging from the NICS(0) (i.e., −6.3 vs −12.2 ppm), HOMA F


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Figure 5. 1H NMR spectra of (a) 1f, (b) 1f with 1 equiv of DBU, and (c) 1f with 10 equiv of TEAOH in DMSO-d6 at 25 °C. [1f] = 20 mM. Asterisks indicate residual solvent and reagent peaks.

(0.539 vs 0.631), and Δppm (8.97 vs 13.85 ppm) values, despite the highly planar core structure of 1c-H2 (2,23H). Upon protonation, the extent of aromaticity of the monocationic structure, 1c-H3+ (2,22,24H), becomes slightly weakened as characterized by 1H NMR spectroscopy and theoretical assessments (Table S7, Supporting Information). The Δppm and NICS(0) values of 1c-H3+ (2,22,24H) were estimated to be 12.53 and −9.028 ppm, respectively. In the case of the dication [1c-H42+ (2,22,23,24H)], the aromaticity is further weakened as confirmed by the smaller magnitude of NICS(0) = −7.63 ppm and Δppm =10.20 ppm. The steric hindrance among the inside CH and NH groups might cause the deformation of the core structure [mpd values of 0.2139 Å for 1c-H3+ (2,22,24H) and 0.3628 Å for 1c-H42+ (2,22,23,24H)] to decrease the global aromaticity. Similarly, for 1f, the aromatic characters of the monoanion [1f-H− (23H)] and the dianion (1f2−) decreased compared to that of the neutral 3H tautomer [1f-H2 (22,24H)], which is evident from the values of the characteristic parameters: Δppm =12.33 and 11.64 for 1f-H− and 1f2−, respectively, and NICS(0) = −10.71 and −10.69 ppm, respectively. However, the decrement of the aromaticity indices of the anionic species is not pronounced compared to that of the cationic systems, which could be due to the higher planarity of the dianion species (i.e., mpd = 0.0595 Å for 1f2−; Table S5, Supporting Information).

Regarding the relative stability between the NCP and the regular porphyrin, the energies of 1c in each state were compared to those of 2c. The reference 2c-H2 is an ideally planar aromatic macrocycle stabilized by the characteristic hydrogenbonding network in the core as inferred from the following parameters: mpd = 0.0734 Å, NICS(0) = −14.0478 ppm, and HOMA = 0.6303. Compared to 2c-H2, the neutral 3H tautomer 1c-H2 (22,24H) is 14.1 kcal/mol less stable because of the decreased hydrogen-bonding network and deformation of the core (Figure 6). However, the energy difference between these two species (ΔE) becomes much smaller when protonation takes place. For monocations 1c-H3+ (2,22,24H) and 2c-H3+, ΔE is 6.4 kcal/mol, and for dications 1c-H42+ (2,22,23,24H) and 2c-H42+, ΔE is only 2.6 kcal/mol (Figure 6). These results can also be explained in terms of the structural parameters (e.g., similar hydrogen-bonding networks and a smaller mpd difference; Table 2). On the other hand, in the case of deprotonation, the relative stability is reversed; that is, 1c-H− (23H) and 1c2− are more stable than 2c-H− and 2c2− by 4.4 and 13.5 kcal/mol, respectively (Figure 6). For the monoanions, the two species share similar inner 1NH-type structures with the same hydrogen-bonding networks, but the core planarity is better for 1c-H− (23H) (mpd = 0.1330 Å) than for 2c-H− (mpd = 0.1692 Å). For the dianions, two anionic lone-pair G


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of the neutral state of 1. In other words, protonation at the imino nitrogen of the confused pyrrolic ring and deprotonation of the amino-site congener of 1 take place preferentially. Moreover, the gas-phase basicity of dianionic species 1c2− was estimated to be 414.1 kcal/mol, which is 9.2 kcal/mol more favorable than that of 2c2−. The easier second deprotonation can be explained by the distinct thermodynamic stability of dianion 1c2− (13.5 kcal/mol vs 2c2−). Finally, it was found that the predicted gas-phase acidities of NCPs 1 bearing various meso substituents are well correlated with the corresponding values of pK3 and pK4 determined in this study (Figure S8, Supporting Information).48 The curvefitting treatment afforded the equations pK3 = 0.1386 × GA(1‐H3+) − 27.8360

(R2 = 0.9293) (5)

pK4 = 0.05149 × GA(1‐H4 2 +) − 20.2649 (R2 = 0.9406)

(6)

The calculated GA term explains the substituent effect of basicity for the series of 4-substituted tetraaryl NCPs (1a−1e) with reasonable accuracy. This suggests that the acid−base behaviors of NCPs can be predicted by DFT-based calculations of the relative stabilities of the neutral, anionic, and cationic species.49

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Figure 6. Gas-phase acidity diagrams of (a) 1c and (b) 2c calculated at the B3LYP/6-31G** level.

CONCLUSIONS The comprehensive acid−base properties of NCP derivatives 1a−1f were examined using spectrophotometric titrations for the determination of pKa values in aqueous SDS micellar solutions. The 1H NMR spectra of both the cationic and anionic forms of 1 revealed that the outward-pointing nitrogen site of the N-confused pyrrole ring is a key moiety for the protonation and deprotonation processes. The stronger acidities (pK2) and basicities (pK3) of a series of NCPs compared to the corresponding porphyrin derivatives 2 were elucidated. The facile protonation and deprotonation of NCPs can be critically explained in terms of the significant destabilization of neutral 1 because of its unsymmetrical structure, as well as the decreased internal hydrogen-bonding network, as confirmed by the computational assessments. Furthermore, the acid−basic character of NCP derivatives 1 could be fine-tuned by the meso-aryl substituents and predicted based on the linear correlation between the Hammett σpara parameters and the pKa values. Such controllable acid−base properties involving multiple protonation/deprotonation states should make NCPs more attractive for various optoelectronic material applications.

electrons facing each other largely avoid the planarity, but the degree of distortion of 1c2− (mpd = 0.2233 Å) is smaller than that of 2c2− (mpd = 0.3580 Å), probably because of the presence of a positively polarized CH hydrogen in the core. The protonation- and deprotonation-induced alterations of the aromaticity of the NCPs are indeed reflected in the molecular orbital (MO) interactions and, hence, in the optical transitions. The smaller energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of 1c-H3+ (2,23,24H) (ΔEHL = 1.92 eV) is in good agreement with the emergence of an NIR absorption band at λ = 782 nm (Figure 1a and Figure S14, Supporting Information). The symmetry-broken degeneracy of the LUMO and LUMO + 1 allow the electronic transition from the HOMO to the LUMO. In the case of 1f-H− (23H), the ΔEHL value (2.39) is slightly smaller than that of the neutral species (ΔEHL = 2.51), so a small red shift of the Q band was observed (Figure 4a and Figure S14, Supporting Information). Gas-Phase Acidity of NCPs. From the theoretical energy diagrams (Figure 6), the gas-phase acidity of 1c-H3+ (2,22,24H) was estimated to be 256.0 kcal/mol, whereas that of TPP (2c-H3+) was estimated to be 248.3 kcal/mol (Table S3a, Supporting Information). The difference of 7.7 kcal/mol arises from the larger destabilization of neutral 1c-H2 (22,24H) (14.1 kcal/mol vs 2c) compared to cationic 1c-H3+ (2,22,24H) (6.4 kcal/mol vs 2c-H3+). This supports the experimental results that NCP 1c is apparently more basic than the corresponding porphyrin (2c). In this context, the gas-phase basicity of 1c-H− (23H) was also estimated. The smaller value of 329.2 kcal/mol compared to that of 2c-H− (347.7 kcal/mol) with respect to the corresponding neutral species suggests the stronger acidic nature of the NCP than the porphyrin (Table S3, Supporting Information). Such characteristic stronger acidity and basicity of the NCP can be explained by the significant destabilization

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ASSOCIATED CONTENT

* Supporting Information S

Figures of absorption spectra and pH titration spectral changes and tables of structures, total energies, calculated transitions, gas-phase acidities, and Cartesian coordinates for optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. H


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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (No. 25248039 to H.F.) and Young Scientists (No. 26810024 to M.I.) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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The Journal of Physical Chemistry A (37) Domínguez, A.; Fernández, A.; González, N.; Iglesias, E.; Montenegro, L. J. Chem. Educ. 1997, 74, 1227−1231. (38) (a) Brown, S. B.; Shillcock, M.; Jones, P. Biochem. J. 1976, 153, 279−285. (b) Pasternack, R. F.; Schaefer, K. F.; Hambright, P. Inorg. Chem. 1994, 33, 2062−2065. (39) Protonation at the outside of the macrocycle was reported for meso-aryl and β-alkyl NCPs; see refs 16 and 27. (40) The original chemical shift of the CH proton of CHCl2COOH in CDCl3 is δ = 5.99 ppm; see ref 8. (41) The monoprotonation (pK3) and diprotonation (pK4) of symmetrical porphyrin derivatives are usually not distinguishable; see refs 4 and 5. The pKa values of 2c in nitrobenzene were reported to be pK3 = 4.4 and pK4 = 3.9, which are consistent with our experimental results; see ref 7. (42) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165−195. (43) In the case of regular porphyrins, the pK1 value was reported to be 10.2 in the highly electron-deficient and distorted derivative 2j; see ref 14. (44) When the NMR spectroscopic titration of 1f with DBU in CDCl3 was conducted, precipitates were seen in the sample tube. Therefore, the polar solvent DMSO-d6 was used for this study. (45) The 1H NMR spectrum of 1c in DMSO-d6 indicated relatively weaker aromaticity (δ = −3.99 ppm for Ha′) than for the neutral 3Htype tautomer [δ = −5.11 ppm for Ha of 1c-H2 (22, 24H) in CDCl3]. (46) Although the absorption spectra of the monoanion 1f-H− and dianion 1f2− obtained in aqueous SDS solution are rather broadened, the spectral characteristics are consistent with those in DMSO solution (Figure S3, Supporting Information). (47) Furuta, H.; Maeda, H.; Osuka, A. J. Org. Chem. 2001, 66, 8563− 8572. (48) In the case of 1f, the strong acidic nature is relatively underestimated (Table S3, Supporting Information) because of the potential factors, such as solvation energy. A detailed analysis is now in progress. (49) When the calculations were performed in water using the PCM method, stabilization of the ionic species was observed for both 1c and 2c. However, the result that the NCP is more acidic and basic than the corresponding regular porphyrin did not change even in an aqueous environment (Figure S15, Supporting Information).

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