Proton-Coupled Redox Switching in an Annulated π-Extended Core

Sep 4, 2018 - ... Janusz Gregoliński, Alan Chien, Jiawang Zhou, Yi-Lin Wu, Youn Jue Bae, Michael R. Wasielewski, Paul M. Zimmerman, and Marcin Stępi...
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Article Cite This: J. Am. Chem. Soc. 2018, 140, 12111−12119

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Proton-Coupled Redox Switching in an Annulated π‑Extended CoreModified Octaphyrin Tridib Sarma,†,‡ Gakhyun Kim,§ Sajal Sen,‡ Won-Young Cha,§ Zhiming Duan,† Matthew D. Moore,‡ Vincent M. Lynch,‡ Zhan Zhang,*,† Dongho Kim,*,§ and Jonathan L. Sessler*,†,‡ †

Center for Supramolecular Chemistry & Catalysis, Shanghai University, Shanghai 200444, China Department of Chemistry, The University of Texas at Austin, 105 East 24th Street, Stop A5300, Austin, Texas 78712-1224, United States § Department of Chemistry and Spectroscopy Laboratory for Functional π-Electronic Systems, Yonsei University, Seoul 03722, Korea

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S Supporting Information *

ABSTRACT: Proton-coupled electron transfer (PCET) is an important chemical and biological phenomenon. It is attractive as an on−off switching mechanism for redox-active synthetic systems but has not been extensively exploited for this purpose. Here we report a core-modified planar weakly antiaromatic/nonaromatic octaphyrin, namely, a [32]octaphyrin(1.0.1.0.1.0.1.0) (1) derived from rigid naphthobipyrrole and dithienothiophene (DTT) precursors, that undergoes proton-coupled two-electron reduction to produce its aromatic congener in the presence of HCl and other hydrogen halides. Evidence for the production of a [4n + 1] πelectron intermediate radical state is seen in the presence of trifluoroacetic acid. Electrochemical analyses provide support for the notion that protonation causes a dramatic anodic shift in the reduction potentials of octaphyrin 1, thereby facilitating electron transfer from halide anions (viz. I−, Br−, and, Cl−). Electronrich molecules, such as tetrathiafulvene (TTF), phenothiazine (PTZ), and catechol, were also found to induce PCET in the case of 1. Both the oxidized and two-electron reduced forms of 1 were characterized by X-ray diffraction analyses in the solid state and in solution via spectroscopic means.

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treatment with HI produced the fully aromatic [4n + 2] πelectron form. Unfortunately, initial efforts to generalize this phenomenon to related porphyrinoids proved unsuccessful.10 Recently, several porphyrinoids with formal [4n + 1] πelectron counts have been reported in the literature.11−19 However, to the best of our knowledge none of them display PCET-like redox switching behavior. The seemingly unique proton-induced reduction seen in the case of naphthorosarin may reflect its relative rigidity. Most large porphyrin analogues, including a nonannulated version of rosarin, are characterized by their inherent flexibility.20−27 This may allow alternative processes such as conformational changes to dominate over PCET in the presence of a putative reductant or a proton source.28−35 To the extent this rationale is correct, large, highly rigid porphyrin analogues should favor PCET. To test this hypothesis, we have now prepared a new, highly rigidified expanded porphyrin, namely, an octaphyrin(1.0.1.0.1.0.1.0) analogue (1), containing both fused bipyrrole and fused bithiophene subunits.36 This system relies on the use of fused

INTRODUCTION Proton-coupled electron transfer (PCET) wherein transfer of an electron is facilitated by transfer of a proton is key to the many energy conversion processes that are important in chemistry and biology.1−3 For instance, in photosystem II oxidation of tyrosine (TyrZOH) by photoexcited chlorophyll (P680*) is accompanied by a transfer of the hydroxyl proton to the imidazole moiety of a nearby histidine, thereby producing TyrZO•. In the absence of a proton transfer this process is not thermodynamically feasible due to the high oxidation potential required to produce TyrZOH•+. PCET reactions have been seen with a number of metal complexes as well as organic molecules.4−8 An intriguing subset of these reactions are those in which protonation serves to shift dramatically the redox potential of an electron acceptor, thus effectively switching on electron transfer under conditions where it would not otherwise occur. Recently, we reported that naphthorosarin, an annulated expanded porphyrin (formally a [hexaphyrin(1.0.1.0.1.0)]), exhibits such proton-promoted redox switching behavior.9 In that case, reaction with HCl served to effect reduction of the antiaromatic [4n] π-electron species to the corresponding [4n + 1] π-electron radical form. Similar © 2018 American Chemical Society

Received: July 2, 2018 Published: September 4, 2018 12111

DOI: 10.1021/jacs.8b06938 J. Am. Chem. Soc. 2018, 140, 12111−12119

Article

Journal of the American Chemical Society

The high-resolution positive-ESI mass spectrum proved consistent with the molecular formula C84H61N4S6 ([M + H]+: 1317.3231). The 1H NMR spectrum of 1 (CD2Cl2) revealed β-pyrrolic CH proton signals at 6.36 ppm, naphthalene CH proton signals at 7.39 and 7.13 ppm, and DTT CH proton signals at 6.04 ppm, as well as an absence of an NH resonance at both room temperature and −60 °C (cf. Figures 1b and S3). These resonances are slightly upfield shifted with respect to the precursors, naphthobipyrrole 2 and DTT-diol 3 (7.02, 8.15, 7.34, and 6.50 ppm, respectively). The UV−vis absorption spectrum (CH2Cl2) was characterized by a broad split band with absorption maxima at 512 nm (ε = 1.1 × 105 M−1 cm−1) and 540 nm (ε = 1.1 × 105 M−1 cm−1) and an absence of any distinctive Q-like transitions (cf. Figure 1c, pink line). An X-ray diffraction analysis of single crystals of 1 grown from a dichloroethane−ethanol mixture revealed a nearly planar system (cf. Figure 3a,b). The deviation of the core heteroatoms from the mean plane (defined by the octaphyrin skeleton excluding the meso-aryl substituents) was in the range of 0.043 to 0.065 Å. Alternating patterns of longer and shorter bond length values were observed in the crystal structure of 1 (cf. Figure S52). Nucleus-independent chemical shift (NICS(0)) calculations revealed a small positive value (+4.25 ppm) estimated at the center of the molecule. Further, an anisotropy-induced current density (ACID) plot of 1 provided evidence for a broken conjugation path through the DTT moieties (cf. Figure S47a). Excited-state optical properties, such as excited-state lifetimes and two-photon absorption (TPA) cross-section values, have emerged as accepted indicators of (anti)aromaticity across a variety of π-conjugated macrocyclic systems.40−43 In general, aromatic compounds exhibit higher TPA cross-section values, as well as longer excited-state lifetimes, compared to the corresponding antiaromatic or nonaromatic counterparts. These methods were thus used to analyze the expanded porphyrin system 1 and its derivatives. Femtosecond transient absorption (TA) spectroscopic studies of 1 revealed very short-lived excited state lifetimes,

pyrrolic precursors in its synthesis. This was expected to confer rigidity and extend the π-conjugation framework, leading to potentially unique redox features.37As detailed further below, a single-crystal X-ray diffraction analysis revealed that asprepared 1 exists in a near planar structure and based on spectroscopic data and theoretical calculations is best considered as being a [4n] π-electron weakly antiaromatic/ nonaromatic system. In contrast to what is seen for the annulated naphthorosarin, treatment of macrocycle 1 with HCl leads to complete reduction to the corresponding [4n + 2] πelectron species (H41·2Cl). Similar reductions are seen in the case of HBr and HI. Reduction to the aromatic form may also be achieved by treatment with selected organic reductants and halide anions in the presence of a proton source. In these cases, evidence of a [4n + 1] π-electron intermediate is seen. The aromatic form of 1 (H21) was fully characterized, including in the solid state by X-ray diffraction analysis, in its neutral, mono-, and diprotonated forms. This also stands in contrast to naphthorosarin, which to date has only been characterized in the form of its monoprotonated chloride anion salt.9

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RESULTS AND DISCUSSION The synthesis of 1 was achieved via the reaction of naphthobipyrrole38 2 and dithienothiophene-diol (DTTdiol)39 3 under BF3·(OEt)2-catalyzed conditions. After oxidation, involving treatment with 2,3-dichloro-5,6-dicyanop-benzoquinone (DDQ), initial purification, and exposure to MnO2, macrocycle 1 was isolated in 15% yield (Scheme 1). Scheme 1. Synthesis of Octaphyrin 1

Figure 1. Selected NMR spectral data and steady-state absorption spectra of 1. (a)−(c) Comparison of the 1H NMR spectra of 1 (b) and the salt isolated after treating 1 with HI (H31·I3) (a). Spectra were recorded in CD2Cl2 at 25 °C. *An asterisk indicates residual solvent impurities. Absorption spectra of 1 (pink line), H41·3CF3COO• (red line), H21 (blue line), and H41·2Cl (black line) recorded in CH2Cl2 (c). The spectrum of H41·3CF3COO• was obtained by adding excess trifluoroacetic acid to a CH2Cl2 solution of 1. Inset shows magnified spectra over the 700−1650 nm spectral range. 12112

DOI: 10.1021/jacs.8b06938 J. Am. Chem. Soc. 2018, 140, 12111−12119

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Journal of the American Chemical Society

namely, NaBH4, could be used to convert 1 into its corresponding two-electron reduced 34 π-electron aromatic form (H21). Treatment of 1 with stoichiometric excess of NaBH4 allowed H21 to be isolated as a dark blue solid after recrystallization from CH2Cl2/MeOH. Preliminary characterization via high-resolution mass spectrometry revealed a molecular weight that is two mass units greater than that recorded for 1; this is as would be expected for a two-electron reduced product (cf. Figure S51). The UV−vis−NIR absorption spectrum of H21 revealed a Soret-like band at 632 nm (ε = 2.6 × 105 M−1 cm−1) along with several Q-like transitions at 892, 1072, 1252, and 1307 nm (cf. Figure 1c, blue line). In addition, an 1H NMR spectroscopic analysis of H21 revealed downfield chemical shifts in the peripheral CH proton signals (e.g., a set of β-pyrrolic proton signals at 11.12 ppm, DTT CH− proton signals at 10.20 ppm, and two sets of naphthalene CH− resonances at 9.06 and 7.84 ppm; cf. Figure S5). Such findings are consistent with a high degree of aromatic character in H21. The two-electron reduced free-base system, H21, could be converted to its protonated form, H412+, by treatment with a proton source. Protonation of H21 leads to a more rigid structure as inferred from the associated spectral changes. For instance, the absorption spectrum of H412+ recorded in the presence of HClO4 is characterized by an intense Soret-type band at 650 nm (more than 2-fold increase in intensity compared to H21), along with three well-defined Q-like bands at 853, 957, and 1184 nm, respectively (Figure S14). Likewise, the 1H NMR spectrum of H21 recorded in the presence of trifluoroacetic acid (TFA) showed further downfield chemical shifts for the peripheral CH proton signals as compared to the free-base H21 (Figure S6). The molecular structure of H21 was further confirmed via an X-ray diffraction analysis of crystals grown via the vapor diffusion of diethyl ether into a dichloroethane solution of H21. The resulting structure revealed a relatively nonplanar geometry with respect to that of 1 (Figure 3c−d). Deviations of the core heteroatoms from the mean plane (defined by the octaphyrin skeleton excluding the meso-aryl substituents) in the range of 0.085−0.291 Å were seen. In contrast, corresponding values of 0.043−0.065 Å were seen in the in the case of 1. The C−C bond length values deduced from the crystal structure are consistent with bond delocalization in H21 (cf. Figure S53). Furthermore, NICS(0) and ACID calculations revealed large negative value (−13.36 ppm, at the center of the molecule) and a fully conjugated framework with a clockwise ring current, respectively (cf. Figure S47b). The excited-state dynamics of H21 were studied by femtosecond TA spectroscopy. According to the kinetic profile of H21, two time constants (τs: 9 and 51 ps) were obtained in the ground-state bleaching domain (λprobe = 660 nm) (cf. Figure S57b). The fast decay component is ascribed to a structural relaxation process, followed by ground-state recovery. The observed longer S1 state lifetime of 51 ps seen for H21 compared to 1 is consistent with its proposed aromatic character. Furthermore, a characteristic NIR PL peak (∼1324 nm) was observed upon photoexcitation at 630 nm (cf. Figure S56a). Taken in concert, these findings support the contention that H21 in its free-base form should be regarded as an aromatic [4n + 2] π-electron system with n = 8. The analogy between naphthorosarin and compound 1 in terms of both rigidity and their lack of aromatic character led us to consider that 1 would be amenable to PCET.

(τs = 0.22 and 4.4 ps). Deactivation is attributed to relaxation to a NIR dark state, after which internal conversion results in the ground-state recovery. A calculated vertical transition at 1119 nm (cf. Figure S57a and Table S5) provides support for this suggestion. No appreciable NIR photoluminescence (PL) was observed because the lowest transition is optically forbidden. Taken in concert, these findings are consistent with 1 being best characterized as being a weakly antiaromatic or nonaromatic [4n] π-electron system where n = 8. TPA cross-section values of 1 and its reduced congeners were probed using the open-aperture Z-scan method. Photoexcitation was carried out over the 1500−1800 nm spectral region where one-photon absorption contributions are negligible (cf. Figures S58 and S59). The maximum TPA cross-section value (σ(2)max) of 1 was determined to be 280 GM, while its reduced forms H21, H41·2Cl and H31·I3 (vide infra) exhibited largely increased σ(2)max values of 890, 850, and 750 GM, respectively. The relatively low TPA value seen for 1 is in accord with its proposed weakly antiaromatic/nonaromatic nature. We were interested in probing whether the [4n] π-electron form of 1 could be converted to a corresponding aromatic form. As prepared, 1 lacks hydrogen atoms on the core nitrogen atoms. We thus infer that it exists in its highest reasonably accessible oxidation state. Conversion to an aromatic form would thus require a two-electron reduction coupled with nitrogen atom protonation. In order to obtain insights into the redox features of 1, electrochemical analyses were carried out in CH2Cl2 in the presence of n-tetrabutylammonium hexafluorophosphate (TBAPF6) as a supporting electrolyte. On the basis of cyclic voltammetric (CV) and differential pulse voltammetric (DPV) studies, it was concluded that 1 is relatively difficult to reduce (two poorly reversible reduction events are observed at −0.88 and −0.99 V in the DPV) in its free-base form. Highly reversible one-electron oxidations at 0.45 and 0.65 V are also seen (cf. Figures 2a and 2b, Table 1). These observations led us to test whether a relatively strong chemical reducing agent,

Figure 2. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) analyses of 1 showing the protonation-induced reduction potential shifts. CV (a) and DPV (b) traces of 1 (1.24 mM). CV (c) and DPV (d) traces of 1 (1.24 mM) recorded in the presence of HClO4 (5 mM). Scan rate: 100 mV. Measurements were carried out in CH2Cl2 containing 0.1 M TBAPF6 at a scan rate of 100 mV. 12113

DOI: 10.1021/jacs.8b06938 J. Am. Chem. Soc. 2018, 140, 12111−12119

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Journal of the American Chemical Society Table 1. Comparative Redox Potential of 1 and Its Protonated and Reduced Congeners (in V vs Fc/Fc+) compound

πa (N)

1 H414+ H21 H31·I3

32 32 34 34

Eox (V)b,c 0.45, 1.26, 0.20, 0.20,

0.65 1.41 0.69, 0.85, 1.06, 1.30 0.39,0.72

Ered (V)b,c −0.88, 0.56, −1.22, −0.51,

−0.99, −1.45, −1.80 0.25, −0.74, −1.84 −1.44, −2.16 −1.68

Formal number of conjugated π-electrons. bDetermined by differential pulse voltammetry. cMeasurements were carried out in CH2Cl2 in the presence of n-tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte. a

In order to obtain experimental insight into the above questions, compound 1 was tested first using HCl as a potential PCET promoter. Exposure to this hydrohalic acid (aqueous) resulted in marked changes in the UV−vis−NIR absorption spectrum of 1 recorded in CH2Cl2. For instance, emergence of an intense Soret-like band at 653 nm, along with three well-defined Q-like bands at 857, 961, and 1204 nm, was seen (cf. Figure S19). Such changes are consistent with formation of a fully aromatic [4n + 2] π-electron species. In fact, the final spectrum mirrors that of the protonated reduced form, H21. On this basis, we conclude that, unlike protonated naphthorosarin, compound H414+ undergoes a two-electron reduction when exposed to a Cl− anion source. The underlying conversion and its facility is ascribed to its relatively favorable reduction potential.44 Intermediate [4n + 1] π-electron radical species were observed upon addition of acids that are less redox active than HCl to solutions of 1. For instance, treatment of 1 with excess trifluoroacetic acid (TFA) produces absorption spectral changes characterized by the growth of an intense absorption feature at 617 nm (ε = 2.6 × 105 M−1 cm−1). Another broad absorption peak centered at 1332 nm is also seen in the NIR region. The absorption tail of this latter absorption extends to approximately 1500 nm (cf. Figure 1c, red line). The assignment of the presumed intermediate produced by treatment with TFA as a radical species was also supported by the disappearance of nearly all discernible 1H NMR spectral features (cf. Figure S4). Electron paramagnetic resonance (EPR) spectra recorded at 100 K and at room temperature (rt) revealed a broad peak with a g value of 2.0044, as would be expected for formation of an organic monoradical (cf. Figure S36). In principle, the putative one-electron reduced 33 π-electron intermediate radical species of 1 (i.e., H41•3+) generated with excess TFA should give the corresponding 34 π-electron aromatic form, H412+, upon further one-electron reduction. In accord with this expectation, the addition of 1 equiv of decamethylferrocene (Me10Fc) to H41•3+ produced a 34 πelectron aromatic form as inferred from the spectral congruence with the products obtained via chemical- or halide anion-mediated reduction (cf. Figure S25). Taken in concert, these observations provide evidence for the existence of a 33 πelectron radical form of 1, namely, H41•3+. The two-electron reduced product of 1 obtained with HCl (i.e., H41·2Cl) was further characterized via an X-ray diffraction analysis. Single crystals for this analysis were grown via the slow diffusion of pentane into a CHCl3 solution of 1 containing excess ethanolic HCl. The resulting structure revealed a dihydrochloride salt of the reduced macrocycle 1 (i.e., H41·2Cl) in a slightly saddle-shaped geometry. The two chloride anions are tethered to the macrocycle via (Npy−H··· Cl) hydrogen bonding interactions and are found on the same side of the macrocycle. The (Npy−H···Cl) hydrogen bonding

Figure 3. Single-crystal X-ray structure of 1 and its reduced congeners. (a)−(h) ORTEP showing front (a) and side views (b) of 1, front (c) and side views (d) of H21, front (e) and side views (f) of H41·2Cl(H2O) and, front (g) and side views (h) of H31·I3. Occupancy of the protons on the inner four nitrogen atoms of H31·I3 is set to 75% for each atom. Therefore, the actual number of inner NH protons is three. The meso-aryl substituents and the H atoms are omitted in the side views for clarity. Five CHCl3 molecules and one pentane molecule are likewise omitted from the structure of H41· 2Cl(H2O) for clarity. Thermal ellipsoids are scaled to the 50% probability level.

Electrochemical analyses revealed that upon protonation the first and second reduction potentials of 1 (at −0.88 and −0.99 V, respectively, versus Fc/Fc+) undergo dramatic anodic shifts (to +0.56 and +0.25 V) (cf. Figure 2c−d and Table 1). In comparison to naphthorosarin, these latter values are noticeably more positive (the first and second reduction potentials of the protonated form of naphthorosarin, likewise a [4n] πelectron system, are +0.42 and +0.04 V, respectively).9 The highly positive reduction potentials found in the case of H414+ led to the question of whether the preferential conversion to a one-electron reduced [4n + 1] π-electron radical species seen in the case of naphthorosarin upon exposure to HCl or HBr would be replicated in the case of 1. Alternatively, the relatively positive potential values produced upon protonation might favor full conversion to the two-electron reduced [4n + 2] πelectron aromatic form upon treatment with HX (X = Cl−, Br−). The formation of an intermediate mixture of reduced products was also considered as a possibility. 12114

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Journal of the American Chemical Society 1

distances are in the range of 3.092−3.133 Å. The chloride anions are further hydrogen bonded to a bridging water molecule. The distance between the two chloride anions is 5.432 Å (Figure 3e,f). The deviations of the core heteroatoms from the mean plane (defined by the octaphyrin skeleton excluding the meso-aryl substituents) were found to vary between 0.187 and 0.444 Å, whereas the two Cl− lie at a distance of 1.988 and 2.062 Å from the mean plane. 1 H NMR spectroscopic analysis of an isolated salt revealed features consistent with H41·2Cl having distinct aromatic character. For instance, the NH-proton signal appeared at −4.8 ppm, while the peripheral aromatic CH-proton resonances were observed in the downfield region (cf. Figures S8 and S9). The UV−vis−NIR absorption spectral features likewise proved fully consistent with what would be expected for an aromatic expanded porphyrin system (cf. Figure 1c, black line). Further support for this conclusion came from the large negative NICS(0) value (−14.37 ppm, at the center of the molecule) calculated for H41·2Cl and the clockwise ring current seen in the corresponding ACID plots (cf. Figure S47c). The excited-state dynamics of H41·2Cl were evaluated by means of femtosecond TA spectroscopy. A longer excited singlet state lifetime was observed (λprobe = 660 nm, τs = 91 ps) as compared to what was seen for H21, reflecting the rigid nature of the salt (cf. Figure S57c). A constant residual was observed in the case of H41·2Cl that was ascribed to the triplet state. Compared to H21, relatively sharp NIR PL peaks (∼1222 and 1490 nm) were observed upon photoexcitation at 650 nm (cf. Figure S56b). A small Stokes shift of 150 cm−1 was seen. This was taken as direct evidence that the excited state is relatively rigid. The proton-coupled chloride anion mediated reduction observed for 1 in the presence of HCl was also seen in the case of HBr (in acetic acid) (cf. Figure S20). This latter observation is in accord with our expectation considering the more negative oxidation potential of the bromide anion relative to chloride. For instance, the two-electron oxidation potentials for the redox couples Cl−/Cl2 and Br−/Br2 (+0.18 and +0.07 V versus Fc/Fc+, respectively) are more negative than the twoelectron reduction potential of H414+ (+0.25 V versus Fc/ Fc+).44 Although the redox chemistry of 1 with HCl and HBr was well-defined, the daughter product of the presumed halide anion oxidation was not directly characterized. In analogy to what was inferred in the case of naphthorosarin, we assumed it to be the corresponding trihalide anion. In order to provide support for the above contention, studies were carried out with HI. Here, evidence for I3− formation could come from the observation of characteristic I 3 − absorption bands at 290 and 364 nm, in analogy to what was done in the case of naphthorosarin. Again, the overall redox chemistry is thermodynamically feasible since the twoelectron oxidation potential of I−/I3− (−0.34 V versus Fc/Fc+) is more negative than the two-electron reduction potential of H414+ (0.25 V versus Fc/Fc+).9 In fact, treatment of 1 with HI (aqueous) in CH2Cl2 causes significant changes in the UV− vis−NIR absorption and 1H NMR spectra (cf. Figures S16, S21, S10, and S11). For instance, emergence of a Soret-like band at 656 nm (ε = 5.4 × 105 M−1cm−1), along with three well-defined Q-like bands at 857, 964, and 1197 nm, is seen in the UV−vis−NIR spectrum. A downfield chemical shift in the peripheral CH proton signals, as well as emergence of an NH resonance in the upfield region (−6.98 ppm), was seen in the

H NMR spectrum. These spectral changes are consistent with formation of an aromatic species (cf. Figure 1a). Surprisingly, and in contrast to what was proved true for naphthorosarin, in the case of 1, efforts to extract the proposed HI daughter product, I3−, into an aqueous layer after reaction with 1 proved unsuccessful. This observation led us to speculate that the I3− formed in this redox reaction might form a complex with the reduced macrocycle itself. To the extent this occurred, it would preclude its extraction into an aqueous solution. The absorption spectra of 1 were thus recorded after reaction of a fixed amount of HI (3 mM) with various concentrations of 1 (0−0.4 mM) in CH2Cl2 (5 mL). These studies revealed that the absorption corresponding to the I3− ion observed at 290 and 364 nm increases linearly as the concentration of 1 is raised (cf. Figure S22). This observation led us to study the isolated product of the iodide anionmediated reduction of 1. Here, we sought both to obtain a single crystal suitable for structural determination of the proposed complex and to study it spectroscopically such that any interference from I3− contaminated with HI could be excluded. The UV−vis−NIR spectrum of the salt of 1 isolated after treatment with HI (for details of the experimental procedure please see the SI) clearly shows characteristic absorption features ascribable to I3− at 290 and 364 nm, along with the other aromatic spectral features that would be expected for a two electron reduced product of 1 (cf. Figure S16). Crystals were then grown by slow evaporation of pentane into a solution of 1 containing a stoichiometric excess of HI. The resulting X-ray diffraction analysis proved consistent with a monoprotonated triiodide salt (H31·I3) (Figure 3g−h). The system is roughly planar with the deviation of the core heteroatoms from the mean plane (defined by the octaphyrin skeleton excluding the meso-aryl substituents) being in the range of 0.072−0.294 Å. The triiodide anion was found to lie outside the macrocyclic core. The monoprotonation inferred in the case of HI stands in contrast to what is seen upon treatment with HCl, where a diprotonated species is seen. This difference is attributed to the comparatively larger size of the counteranions in question (triiodide vs chloride). In order to garner further insights into the protonation state in solution, UV−vis−NIR titrations were performed in CH2Cl2 with isolated H21 and H41·2Cl using both acid (as a solution in acetonitrile) and base, respectively. The results revealed that H21 requires approximately twice the amount of HCl to reach the point of saturation (defined as the point where no further changes were observed in the absorption spectrum) compared to HI (7 equiv versus 3 equiv) (cf. Figures S60 and S61). Moreover, the isolated diprotonated HCl salt, H41·2Cl, requires approximately 6.5 equiv of triethylamine in order to produce corresponding free-base H21 (cf. Figure S62). These observations are consistent with the conclusion that a diprotonated species dominates in the case of HCl, whereas monoprotonation occurs in preference upon treatment with HI. Nevertheless, a symmetric peak pattern is seen in the 1H NMR spectrum of H31·I3. While we ascribe this observation to rapid tautomerization of the inner NH protons within the macrocyclic core,45 it could also reflect simple diprotonation and formation of a more symmetric chemical species. Furthermore, the absorption spectra of the species assigned as being mono- vs diprotonated do not differ appreciably. Thus, the assignment under solution-phase conditions remains 12115

DOI: 10.1021/jacs.8b06938 J. Am. Chem. Soc. 2018, 140, 12111−12119

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Journal of the American Chemical Society Scheme 2. Summary of Redox Reactions Seen for 1

The highly positive reduction potential of 1 seen upon protonation (but not in its absence) led us to explore the possibility of electron transfer from electron donors other than the three simple halide anions tested above. With this objective in mind, the protonated form of 1 (i.e., H41·4X with X = ClO4) was titrated separately with solutions of tetrathiafulvene (TTF) or phenothiazine (PTZ) in CH2Cl2. The UV−vis−NIR absorption spectral changes were monitored as a function of added TTF and PTZ. In both cases, spectral features corresponding to the formation of first the intermediate 33 π-electron radical form and then the two-electron reduced product of 1 were seen (cf. Figures 4a and S34a). For instance the addition of up to 0.3 molar equiv of TTF to an initial CH2Cl2 solution of H41·4ClO4 was found to give rise to spectral changes that were congruous with those of the 33 π-

necessarily tentative. However, in the solid state the number of counteranions associated with H31·I3 and H41·2Cl (one vs two) allows the assignment of mono- vs diprotonated to be made with a greater degree of confidence. The aromatic nature of H31·I3 was further inferred from the large negative NICS(0) value calculated at the center of the molecule (−13.34 ppm), as well as the clockwise ring current seen in the corresponding ACID plot (cf. Figure S47d). A relatively long-lived excited state was also seen (τs = 101 ps), as would be expected for a relatively rigid aromatic species (cf. Figure S57d). According to the kinetic profile of H31·I3, the proportion of the triplet state increases in the order HCl < HI, presumably reflecting the relative conformational rigidity seen for expanded porphyrins upon protonation.46,47 Relatively sharp NIR PL peak (∼1211 nm) and small Stokes shifts (ca. 100 cm−1) were observed upon photoexcitation at 650 nm (cf. Figure S56c). A summary of the redox reactions observed in the case of 1 and its protonated forms is provided in Scheme 2. Identical spectral changes corresponding to an antiaromatic−aromatic conversion were also observed upon treating protonated 1 with the corresponding TBAX salts (X = Cl−, Br−, and I−) (cf. Figures S30−33). Spectral signatures corresponding to the intermediate radical species were observed. The neutral, mono, and diprotonated aromatic forms of 1 (H21, H31·I3, H41·2X where X = Cl, Br) can be converted to 1 via treatment with an oxidizing agent such as MnO2 (for experimental details for this reoxidation chemistry, please see the SI). We thus believe analogous chemistry is occurring in the case of all three hydrohalic acids and that the byproduct is X3−, where X = Cl, Br, or I.

Figure 4. Absorption spectral changes for a CH2Cl2 solution of H41· 4ClO4 (0.5 × 10−5 M) (red line) seen upon the addition of tetrathiafulvene (TTF) (a) and phenothiazine (PTZ) (b). The intermediate radical and final aromatic states (one- and two-electron reduced species) are represented by pink and blue lines, respectively. 12116

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Journal of the American Chemical Society

order to obtain mechanistic insight into the inferred PCET process, a solution of 1 in CH2Cl2 was titrated against CH3COOH (aqueous pKa = 4.76) as a representative weak, nonredox active acid. The corresponding UV−vis−NIR spectral changes were monitored. It was found that treatment with CH3COOH did not produce an appreciable change in the spectrum of 1. This was true even after the addition of >10 000 equiv (cf. Figure S64a). However, addition of TTF (approximately 7 equiv) to the above solution containing CH3COOH induced complete reduction of 1 over the course of 6 h (cf. Figure S64b). Similarly, titration of 1 with TTF (up to 10 equiv) in the absence of a proton source did not induce any changes in the UV−vis−NIR spectrum of 1. On the other hand, upon the addition of HClO4 (as a solution in acetonitrile) spectral changes corresponding to the reduction in 1 (i.e., conversion to the aromatic form) are seen (cf. Figure S65). Although not a proof, these observations are taken as evidence that electron transfer to 1 is facilitated by prior or simultaneous protonation. To the extent such a conclusion is correct, the use of strong, redox-active acids (e.g., HI, HBr, and HCl) would be expected to induce near synchronous, possibly concerted protonation and electron transfer without producing appreciable quantities of intermediate protonated states or radical forms as seen by experiment (cf. Figures S19−21). In contrast, the protonated state of 1 could be easily observed by means of UV−vis spectroscopy when less redox active acids, including TFA (pKa = −0.25), methanesulfonic acid, MSA (pKa = −2.60), HClO4 (pKa = −10.0), and triflic acid, TfOH (pKa = −14.0), were used (cf. Figures S24 and S26−29). Moreover, to a solution of 1 in CH2Cl2 containing HClO4 (4 equiv, acetonitrile solution), the addition of an I− source (also Br− and Cl−) leads to spectral changes consistent with the formation of a 33 π-electron radical intermediate (cf. Figures S30−34). Thus, by judicious control of the conditions it is possible to observe both intermediate protonated and radical species that are not seen in the case of the strong, redox-active hydrohalic acids, HCl, HBr, and HI.

electron radical form obtained by treating with TFA as noted above. Specifically, the characteristic sharp band at 619 nm and the broad band centered at 1325 nm with an absorption up to 1500 nm were seen at this point in the titration with TTF. Further addition of TTF gives rise to a Soret-like band at 648 nm accompanied by three well-defined Q-like bands at 856, 960, and 1183 nm. This latter spectral pattern corresponds directly to that observed for the 34 π-electron aromatic form of 1 generated via halide anion mediated reduction in the presence of a proton source. Similar spectral changes were seen when PTZ was used as the reductant (cf. Figures 4b and S34c). The driving force for these reductions came from the favorable redox potentials of TTF48 and PTZ,49 respectively. For instance, the one-electron oxidation potentials for the TTF/TTF•+ and PTZ/PTZ•+ redox couples (−0.0450 and +0.22 V versus Fc/Fc+, respectively) are more negative than the two-electron reduction potential of H414+ (+0.25 V versus Fc/Fc+). The plausible side products of these reductions are presumed to be the corresponding radical cations (e.g., TTF•+ and PTZ•+). However, unlike I3−, monitoring these radical cations via UV−vis−NIR absorption spectroscopy is experimentally more challenging due to the relatively low absorption spectral intensity of the putative oxidized organic donors compared to 1 and its two-electron reduced form.51 Nevertheless, efforts were made to identify the proposed radical cation byproduct in the case of PTZ for which characteristic absorption features at 434 and 519 nm are seen in the case of its corresponding radical cation (PTZ•+).52 A careful inspection of the spectrum of 1 obtained after treatment with PTZ reveals the presence of these bands (cf. Figure S63a). Notably, these two characteristic bands are absent in the two-electron reduced product of 1 obtained via reduction with a hydrohalic acid or the other reductants used in the present study. In order to gain further insights into the redox chemistry promoted by treatment with PTZ, the absorption spectra of 1 were recorded after various concentrations of 1 (0−0.4 mM) in CH2Cl2 (5 mL) in the presence of 4 equiv of HClO4 (as a solution in acetonitrile) were treated with a fixed amount of PTZ (3 mM). A plot of the intensity of the bands corresponding to the PTZ•+ absorption feature versus the concentration of 1 revealed a direct proportionality. This is as expected if the formation of PTZ•+ is driven by the reduction of 1 in the presence of a proton source (cf. Figure S63b). We also tested catechol as a potential reductant for 1. This particular putative reaction partner was chosen because it contains mildly acidic protons. It was thus envisioned that reduction of 1 by catechol might be promoted by the hydroxyl protons of catechol itself. This would preclude the need for an external proton source. In fact, titrating a solution of 1 with a solution of catechol in CH2Cl2 gave rise to spectral changes consistent with the formation of the two-electron reduced aromatic form of 1. Specifically, the emergence of a Soret-like band at 653 nm, along with comparatively red-shifted Q-like bands at 1263, 1206, 1078, and 954 nm, was observed as the result of this addition (cf. Figure S35). In this case, no appreciable quantities of the intermediate one-electron reduced species were formed as inferred from the associated spectral changes. That reduction of 1 is induced by catechol despite the latter being a relatively weak acid (pKa = 9.48) is attributed to its ability to undergo oxidation along with proton transfer. In

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CONCLUSION In conclusion we have synthesized a core-modified rigid octaphyrin derivative 1 and characterized it as a 32 π-electron weakly antiaromatic/nonaromatic species on the basis of various spectroscopic measurements, as well as by means of a single-crystal X-ray diffraction analysis and theoretical studies. A PCET-type reduction of 1 was observed in the presence of hydrogen halides (HX, X = Cl, Br, and I) yielding the 34 πelectron aromatic congener of 1 (i.e., H412+). Evidence of reduction by halide anions came from spectroscopic studies, as well as from single-crystal X-ray diffraction analysis of the product obtained in the case of HI. The resulting structure revealed formation of a monoprotonated triiodide salt (H31· I3). An intermediate radical species characterized by 33 πelectron peripheries were also identified and characterized spectroscopically including via EPR spectral analyses. In addition to halide anions, PCET was observed with TTF and PTZ in the presence of a proton source and catechol without any external proton source. The latter species is a relatively weak proton donor and proved less effective than other reductants tested in the course of this study. The proton coupled electron transfer chemistry seen in the case of the present system makes it an interesting environmentally sensitive probe; it could have a potential role to play where strongly absorbing species with stimulus-based switchable 12117

DOI: 10.1021/jacs.8b06938 J. Am. Chem. Soc. 2018, 140, 12111−12119

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features are needed, such as organic photovoltaics and smart molecular electronic devices. Studies of these applicationsrelated possibilities are in progress.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b06938. Experimental procedures and characterization data, NMR, EPR, and UV−vis−NIR absorption and NIR emission spectra, CV, DPV, excited-state absorption spectra, two-photon absorption spectra, and DFT calculation detail (PDF) X-ray diffraction structures of 1 (CCDC 1846899) (CIF) X-ray diffraction structures of H21 (CCDC 1846954) (CIF) X-ray diffraction structures of H 4 1·2Cl (CCDC 1846898) (CIF) X-ray diffraction structures of H31·I3 (CCDC 1846900) (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] *[email protected] ORCID

Zhiming Duan: 0000-0002-8332-6131 Matthew D. Moore: 0000-0001-6401-6667 Dongho Kim: 0000-0001-8668-2644 Jonathan L. Sessler: 0000-0002-9576-1325 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Shanghai University and the National Natural Science Foundation of China (grant 21672141 to Z.Z.) for financial support. The work in Austin was supported by the U.S. National Science Foundation (grant CHE-142004 to J.L.S.) and the Robert A. Welch Foundation (F-0018). The work at Yonsei was supported by the Global Research Laboratory (GRL) Program funded by the Ministry of Science, ICT & Future, Korea (2013K1A1A2A02050183). We thank the staff of Beamline BL17B at the National Facility for Protein Science Shanghai and Shanghai Synchrotron Radiation Facility for assistance during the single-crystal Xray data collection for compounds 1, H2·1, and H21·I3.

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REFERENCES

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