A Molecular Chameleon: Reversible pH- and Cation-Induced Control

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A Molecular Chameleon: Reversible pH- and Cation-Induced Control of the Optical Properties of Phthalocyanine-Based Complexes in the Visible and Near-Infrared Spectral Ranges Evgeniya A. Safonova,† Alexander G. Martynov,*,† Sergey E. Nefedov,‡ Gayane A. Kirakosyan,†,‡ Yulia G. Gorbunova,*,†,‡ and Aslan Yu. Tsivadze†,‡ †

Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, bldg. 4, Moscow 119071, Russia ‡ Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow 119991, Russia S Supporting Information *

ABSTRACT: A series of novel nonperipherally substituted tetra-15-crown-5-dibutoxyoxanthrenocyanines (H2, Mg, Zn), acting as chameleons with the unique properties of switchable absorption and emission in the near-infrared (NIR) spectral range have been synthesized and characterized by X-ray diffraction. The attachment of 15-crown-5-α-dibutoxyoxanthreno moieties to phthalocyanine is responsible for the high solubility of the resulting molecules and the red shift of the Q band to the NIR region and offers a unique possibility for postsynthetic modification of the optical properties of the molecules. Both aggregation of phthalocyanine and its participation in an acid−base equilibrium strongly alter their optical properties. For example, the absorption of complexes can be reversibly tuned from 686 up to 1028 nm because of the cation-induced formation of supramolecular dimers or subsequent protonation of meso-N atoms orf macrocycle, in contrast to peripherally substituted tetra-15-crown-5phthalocyanines without oxanthrene moieties. The reversibility of these processes can be controlled by the addition of [2.2.2]cryptand or amines. All investigated compounds exhibit fluorescence with moderate quantum yield, which can also be switched between the ON and OFF states by the action of similar agents.

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substituted Pc complexes (free base, PV, AsV, and SbV) with the main absorption beyond 1000 nm.17,18 Apart from the very strong bathochromic shift of the Q band, these compounds also reveal an intriguing solvatochromic behavior. For example, octa-α-alkoxy-substituted ZnPc complexes have the Q band at ∼750 nm in polar coordinating solvents [like pyridine, methanol (MeOH), and tetrahydrofuran], while in nonpolar media, like chloroform or tetrachloroethane, their Q band shifts even further to the NIR region, up to 800 nm. The addition of MeOH or pyridine to such a solution resulted in the hypsochromic shift of the Q band back to ∼750 nm.19 For a long time, the origin of such behavior remained ambiguous. Initially, researchers tended to interpret the solvatochromic behavior of α-AlkO-substituted ZnPc complexes in terms of the formation of J-aggregates, via coordination of the α-O atom to the Zn ion of a neighboring molecule.19−22 Indeed, the addition of pyridine to such aggregates could cause their dissociation with the formation of monomeric species. However, some research groups have

INTRODUCTION Phthalocyanine (Pc) complexes with near-infrared (NIR) absorption have attracted special attention as promising photodynamic therapy (PDT) sensitizers, materials for organic solar cells, nonlinear optics, NIR imaging agents, etc.1−4 There are different synthetic approaches toward Pc complexes absorbing in the NIR region, such as the extension of π conjugation or expansion of a macrocycle,5−7 oligomerization of Pc derivatives,8,9 or the lowering of the molecular symmetry.10 Among the disadvantages of these approaches are the strong tendency of extended Pc derivatives toward aggregation and oxidation and difficulties in the separation of unsymmetrical molecules. Another approach consists of nonperipheral substitution (in the α position of Pc molecules), which leads to the bathochromic shift of absorption spectra in comparison with unsubstituted or peripherally (β position) substituted Pc complexes as well as to an increase in their solubility.11 Indeed, the introduction of alkoxy substituents into the α positions of Pc complexes with 3d metals results in a decrease in the HOMO−LUMO gap, affording a red shift of the Q band from 670−680 to 740−760 nm.12−16 Recently, Kobayashi’s group reported unique examples of nonperipheral S-, Se-, and Te© XXXX American Chemical Society

Received: December 9, 2015

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DOI: 10.1021/acs.inorgchem.5b02831 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

mmol), and KI (0.435 g, 2.62 mmol) in DMF (7 mL) was stirred at 80 °C for 3 h. Then the reaction mixture was cooled to room temperature, poured dropwise into water, and extracted with ethyl acetate (4 × 15 mL). The extract was washed with water (3 × 20 mL), dried over Na2SO4, and evaporated under vacuum. The resulting crystalline substance was purified by using chromatography on silica (elution with 1:1 hexane/CHCl3), yielding 0.233 g of 5 (52%). NMR spectra are in accordance with the previously reported data.40 1H NMR (600 MHz, acetone-d6): δ 4.29 (t, J = 6.4 Hz, 2H, CH2), 1.91− 1.83 (m, 2H, CH2), 1.62−1.54 (m, 2H, CH2), 0.99 (t, J = 7.4 Hz, 3H, CH3). 13C NMR (151 MHz, acetone-d6): δ 156.27, 135.78, 113.34, 110.41, 77.04, 32.74, 19.59, 13.99. Phthalonitrile 1. A mixture of benzo-15-crown-5-quinone obtained as described earlier37 (0.66 g, 2.21 mmol) and 10% Pd/C (34 mg) in DMF (37 mL) was degassed and then stirred for 1.5 h in a stream of H2 until it became colorless, which corresponded to the formation of catechol 6. Then, the mixture was taken into a syringe through a nylon syringe filter, and the resulting colorless filtrate was transferred into a flask with a degassed mixture of 5 (0.76 g, 2.23 mmol) and K2CO3 (0.92 g, 6.67 mmol). The resulting suspension was stirred for 24 h at 70 °C. Then, it was poured into water and extracted with chloroform (4 × 50 mL); the extract was washed with water and dried over Na2SO4, and the solvent was evaporated under vacuum. The resulting oil was purified by chromatography on silica (elution with 1:1 hexane/CHCl3), yielding 0.58 g of 1 (48%). Mp: 128 °C. 1H NMR (600 MHz, CDCl3): δ 6.51 (s, 1H, HAr), 4.19 (t, J = 6.5 Hz, 2H, OCH2), 4.11−4.07 (m, 2H, α-CH2crown), 3.92−3.87 (m, 2H, β-CH2), 3.80−3.71 (m, 4H, γ,δ-CH2), 1.84−1.78 (m, 2H, CH2), 1.57−1.53 (m, 2H, CH2), 1.00 (t, J = 7.4 Hz, 3H, CH3). 13C NMR (151 MHz, CDCl3): δ 146.79, 146.58, 140.97, 133.44, 112.83, 104.49, 103.91, 75.76, 71.08, 70.53, 69.99, 69.47, 32.09, 19.05, 13.91. ESI HRMS for C30H36N2NaO9+: (experimental) m/z 591.233 ([M + Na]+); (theoretical) m/z 591.232. Magnesium Oxanthrenocyaninate (2Mg). A mixture of nitrile 1 (102 mg, 0.18 mmol), Mg(OAc)2 (13 mg, 0.09 mmol), hydroquinone (10 mg, 0.09 mmol), and DBU (30 μL, 0.2 mmol) in 5 mL of i-AmOH was refluxed for 48 h under a slow stream of argon. After cooling, the reaction mixture was added dropwise to hexane (50 mL). The resulting precipitate was filtered and washed off the filter with CHCl3. Chromatography on alumina in a CHCl3/MeOH mixture afforded green complex 2Mg, which was additionally purified by a BioBeads SX-1 column packed in CHCl3 + 5 vol % MeOH. Yield: 39 mg (38%). 1H NMR (300 MHz, CHCl3 + MeOD): δ 6.70 (s, 1H), 5.02 (t, J = 6.6 Hz, 2H, 1CH2), 4.19−4.05 (m, 2H, α-CH2crown), 4.03−3.80 (m, β-CH2crown + HDO), 3.76−3.59 (m, 4H, γ,δ-CH2), 2.12−1.78 (m, 2H, 2CH2), 1.58 (m, 2H, 3CH2), 0.88 (t, J = 7.3 Hz, 3H). UV−vis [CHCl3; λ, nm (log ε, M−1 cm−1)]: 736 (5.35), 660 (4.63), 378 (4.81), 319 (4.75). ESI HRMS for C 120 H144N 8 Na 2 O36 Mg2+: (experimental) m/z 1171.470 ([M + 2Na]2+); (theoretical) m/z 1171.466. Zinc Oxanthrenocyaninate (2Zn). A mixture of nitrile 1 (100 mg, 0.18 mmol), Zn(OAc)2 (17 mg, 0.09 mmol), hydroquinone (10 mg, 0.09 mmol), and DBU (30 μL, 0.2 mmol) in 4 mL of i-AmOH was refluxed for 48 h under a slow stream of argon. After cooling, the reaction mixture was added dropwise to hexane (50 mL). The resulting precipitate was filtered and washed off the filter with CHCl3. Chromatography on alumina in a CHCl3/MeOH mixture afforded green complex 2Zn, which was additionally purified by a Bio-Beads SX-1 column packed in CHCl3 + 5 vol % MeOH. Yield: 25 mg (24%). 1 H NMR (300 MHz, CDCl3): δ 6.79 (s, 1H, HAr), 5.11 (t, J = 6.6 Hz, 2H, OCH2), 4.25−4.16 (m, 2H, α-CH2crown), 4.02−3.91 (m, 2H, βCH2), 3.79 (s, 4H, γ,δ-CH2), 2.08−1.90 (m, 2H, CH2), 1.77−1.57 (m, 2H, CH2), 0.97 (t, J = 7.4 Hz, 3H, CH3). UV−vis [CHCl3; λ, nm (log ε, M−1 cm−1)]: 732 (5.13), 656 (4.35), 399 (4.61), 303 (4.49). ESI HRMS for C120H144N8Na2O36Zn2+: (experimental) m/z 1191.443 ([M + 2Na]2+); (theoretical) m/z 1191.438. Oxanthrenocyanine 2H2. CF3COOH (0.5 mL) was added to a solution of 2Mg (29.5 mg, 12 μmol) in CHCl3 (10 mL). Then the mixture was stirred under reflux for 7 min. After cooling to room temperature, K2CO3 (1 g) was added to the mixture. The solvent was

proposed that this process occurs because of the protonation of molecules.23−28 In particular, in chlorinated solvents, it can be caused by traces of HCl that inevitably form upon storage of CHCl3 or C2H2Cl4, especially upon exposure to light. The addition of pyridine removes the proton from the Pc molecule, restoring the spectral appearance. Notably, β-AlkO-substituted Pc complexes do not reveal such a high sensitivity to acidic impurities in solvents, so rather high concentrations of strong acids are required for their protonation. Because both the aggregation of Pc complexes and its participation in an acid−base equilibrium strongly alter their properties, it became an attractive goal to combine both of these features in one multifunctional molecule. The target molecule could be designed on the basis of crown ether Pc derivatives whose aggregation can be precisely controlled by interaction with alkali metals. Indeed, crown ether derivatives are very promising building blocks for the assembly of molecules into cofacial and brick-wall supramolecular structures,29−33 which also lead to a change in the optical and nonlinear-optical properties of molecules.34−36 Recently, we reported that the introduction of donor oxanthrene fragments into a crown ether Pc molecule leads to moderate extension of the absorbance range and also enables control of the aggregation.37 We suppose that the combination of a crown ether group with a peripherally substituted oxanthrene moiety in the Pc complex will allow us to create a molecule with tunable absorption in the visible/NIR spectral range. Herein, we report the synthesis, X-ray structures, and optical properties of the nonperipherally substituted 15-crown-5-oxanthrenocyanine 2H2, as well as their MgII and ZnII complexes 2Mg and 2Zn, showing cation- and acid−base-induced switching of the absorption between 686 and 1028 nm.

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EXPERIMENTAL SECTION

2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), Na2SO4, hydroquinone, triphenylphosphineoxide (Ph3PO), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1-bromobutane, KI, K2CO3, NaHSO3, [2.2.2]cryptand, Zn(OAc)2, Mg(OAc)2, KBPh4, and the solventsisoamyl alcohol (i-AmOH), N,N-dimethylformamide (DMF), CF3COOH, methanol (MeOH), acetonitrile (CH3CN), and toluenewere available from commercial suppliers (Acros, Merck, Aldrich, and Sigma). i-AmOH was distilled over sodium under argon. DBU was dried, distilled over CaH2 under reduced pressure, and stored under argon. Neutral alumina (Merck) and silica (Macherey Nagel, Kieselgel 60) were used for column chromatography. Chloroform (CHIMMED, stabilized with 0.6−1% ethanol) was dried over CaCl2 and distilled over CaH2. Toluene was distilled over CaH2. 4,5-Dichloro-3,6dihydroxyphthalonitrile (4) was synthesized by a previously reported method.38 NMR spectra were recorded on Bruker Avance 600 and Bruker Avance 300 spectrometers. NMR spectra were referenced to the residual solvent signal.39 The 2D 1H NOESY NMR spectra were acquired by using pulse sequences supplied by the Bruker standard pulse program library. UV−vis spectra were measured with a Thermo Evolution 210 spectrometer in quartz cells with a 1 cm optical path. Spectrophotometric titrations were performed in 1 cm quartz cells with a Teflon stopper. Dosage of the titrant was performed with an LA-100 syringe pump (Landgraf HLL). Matrix-assisted laser desorption ionization time-of-flight mass spectra were measured on a Bruker Daltonics Ultraflex spectrometer with 2,5-dihydroxybenzoic acid as the matrix. High-resolution mass spectrometry (HRMS) spectra were recorded on an Orbitrap electrospray ionization time-offlight (ESI-TOF) mass spectrometer. The photoluminescence spectra (λex = 400 nm) at 20 °C were recorded on a Horiba Scientific Flourolog spectrofluorimeter. 3,6-Dibutoxy-4,5-dichlorophthalonitrile (5). A mixture of 4 (0.3 g, 1.31 mmol), BuBr (0.42 mL, 3.93 mmol), K2CO3 (1.08 g, 7.86 B


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Inorganic Chemistry evaporated under vacuum, and the precipitate was dissolved in a mixture of CHCl3 and CH3OH and purified by chromatography on alumina in a CHCl3/MeOH mixture. The resulting oxanthrenocyanine was additionally purified by a Bio-Beads SX-1 column packed in CHCl3 + 5 vol % MeOH. Yield: 13.5 mg (46%). 1H NMR (600 MHz, CDCl3): δ 6.83 (s, 1H), 5.07 (t, J = 6.6 Hz, 2H), 4.25 (s, 2H), 4.01 (s, 2H), 3.83 (m, 4H), 2.06 (m, 2H), 1.72 (m, 2H), 1.03 (t, J = 7.4 Hz, 3H). UV−vis [CHCl3; λ, nm (log ε, M−1 cm−1)]: 759 (5.05), 736 (4.96), 695 (4.51), 321 (4.79). ESI HRMS: m/z: (experimental) m/z 1160.487 ([M + 2Na]2+); (theoretical) m/z 1160.481. X-ray Diffraction Studies. Single-crystal X-ray diffraction experiments were carried out on a Bruker SMART APEX II diffractometer with a CCD area detector (graphite monochromator, Mo Kα radiation, λ = 0.71073 Å, ω scans). Indexing was performed using APEX2.41 The semiempirical method SADABS42 was applied for absorption correction. The structures were solved by direct methods and refined on F2 by full-matrix least-squares techniques with anisotropic displacement parameters for all non-H atoms. All of the H atoms in the complexes were placed geometrically and included in the structure factor calculation in the riding motion approximation. All of the data reduction and further calculations were performed using the SAINT43 and SHELXTL44 program packages. In the structure of complexes 2Zn, the crown ethers and OBun moieties were totally disordered over nonseparated positions. For 2Mg, fragments of the crown ether substituents C44−C45−O16−C46−C47 and C104− C105−O34−C106−C107−O35 and fragments of the OBu substituents C35−C36−C37, C65−C66−C67, C95−C96−C97, and C116−C117−C118 have a multiplicity of 0.5. The structural models for solvent molecules (0.5 water molecule disordered over two positions, and 0.5 CHCl3 disordered over two positions) in 2Zn and for the CHCl3 molecule (the molecule was disordered over two positions, and all Cl atoms were disordered over nonseparated positions) in 2Mg were totally unsatisfactory, even when an isotropic approximation was used and secondary atomic positions were taken into account. Thereby, the contribution of the solvent molecules in 2Zn and 2Mg was removed from the overall scattering by using the PLATON/SQUEEZE program.45 Note that the summary and moiety formulas, atomic weight, and corresponding values including F(000) and μ(r) are given for the unit cell without all solvent molecules for 2Zn and without CHCl3 molecules for 2Mg. CCDC reference numbers are 1429458 (1), 1429459 (2Zn), and 1429460 (2Mg). The data can be obtained free of charge from the Cambridge Crystallographic Data Centre at www.ccdc.cam.ac.uk/ data_request/cif.

Dibutoxydichlorophthalonitrile 5 was prepared in two steps from commercially available 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The reduction of DDQ was reported earlier.46 The standard method47 of alkylation was improved by using BuBr instead of BuI and introducing KI as a catalyst into the reaction. Light-yellow single crystals of phthalonitrile 1 were obtained by the slow evaporation of its saturated solution in a mixture of CH2Cl2 with CH3OH. The compound crystallizes in the monoclinic system (space group P21/c) with eight molecules per unit cell (Figures 1 and S1−S3). In the crystal lattice of

Figure 1. X-ray structure of phthalonitrile 1 (H atoms are omitted for clarity). Only one of the two translationally nonequivalent molecules that exist in the crystal lattice is shown. A comparison of the X-ray structures of these molecules is given in Figures S1 and S2.

nitrile 1, there are two translationally nonequivalent molecules (arbitrarily called as A and B) with slightly different conformations of the alkyl chains and crown ether moieties. The structure of one of these molecules is given in Figure 1, and both structures are compared in Figure S2. Both molecules A and B have an almost flat dibenzo-p-dioxin core: the bend angles between the mean planes of the benzene rings are 2.97° and 6.38°. Molecules A form dimers with B through stacking of the electron-rich tetraoxy-substituted benzene ring of molecule A with the dicyano-substituted benzene ring of molecule B (Figure S3). The distance between the centroids of these rings is 3.488 Å. This value is close to the ring-to-ring distance in unsubstituted dibenzo-p-dioxin48 and is ∼0.4 Å smaller than that in 15-crown-5-substituted dicyanooxanthrene.37 The complementary stacking between electron-rich and electron-deficient benzene rings of molecules B and A, respectively, is not observed because of the presence of butoxy groups, which create steric hindrances to such an interaction. As a result, molecules A and B do not form head-to-tail dimers but rather form pivot−joint associates with an average dihedral angle between the long axes of the oxanthrene moieties of 43.56°. The dimers form infinite chains due to two types of interactions (Figure S3). Molecules of type A form CH···N contacts, where one of the cyano groups interacts with two CH2 groups of the crown ether ring of another A molecule (N1···C17 3.177 Å and N1···C18 3.432 Å). Molecules B are linked via very weak hydrophobic CH···H contacts formed by a terminal CH3 group and CH2 atoms of the crown ether ring of another B molecule (C37···C45 3.747 Å). Further packing of chains within the crystal lattice occurs because of weak CH···O

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RESULTS AND DISCUSSION Synthesis and X-ray Single-Crystal Structures. The precursor for target compounds, 15-crown-5-dibutoxyoxanthrenodinitrile (1), was synthesized in 48% yield through the condensation of dibutoxydichlorophthalonitrile 5 with 15crown-5-cathechol (6)37 (Scheme 1). Scheme 1. Synthesis of 1a

a

Reagents and conditions: (i) NaHSO3, water/toluene (97%); (ii) BuBr, KI, K2CO3, DMF, 70 °C (52%); (iii) 5, K2CO3, DMF, 70 °C (48%). C


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Inorganic Chemistry interactions between the O atoms of the crown ether groups and the methylene groups of the butoxy groups (C49···O1 3.463 Å and C4···O15 3.218 Å). The reaction of phthalonitrile 1 with Mg(OAc)2 in refluxing i-AmOH in the presence of DBU afforded target complex 2Mg in a modest yield of 19% (Scheme 2). It was shown that the Scheme 2. Synthesis of Oxanthrenocyanines 2Zn, 2Mg, and 2H2

Figure 2. X-ray structure of 2Zn (H atoms are omitted for clarity). Selected bond lengths: Zn1−N3 1.996(6) Å; Zn1−N7 2.016(6) Å; Zn1−O37 2.114(7) Å.

introduction of hydroquinone into the template reaction increased yields twice (38%) compared to the reaction without hydroquinone. The reaction of phthalonitrile 1 with Zn(OAc)2 under the same conditions gave complex 2Zn in 24% yield (Scheme 2). The free-base ligand 2H2 was synthesized by demetalation of 2Mg using CF3COOH in CHCl3 in 46% yield (Scheme 2). Compounds were characterized by 1H NMR, ESI HRMS, and UV−vis spectroscopy after column chromatographic purification on alumina and additionally on Bio-Beads SX-1. The ESI HRMS spectra of all compounds showed the signal of the molecular ion [M + 2Na]2+. The 1H NMR spectra also corresponded to the expected chemical structures (Figures S12−S19). The crystal and molecular structures of the oxanthrenocyaninates were determined by X-ray diffraction analyses. Darkgreen single crystals of 2Mg and 2Zn suitable for X-ray analysis were obtained by the slow diffusion of hexane into a chloroform solution (Tables S1 and S2). Compounds 2Mg and 2Zn crystallize in the monoclinic system with space group P21; selected structural parameters of complexes are presented in Table S2. It was earlier shown49 that deviation from planarity of Pc molecules has an effect on its optical properties. In the case of compounds 2Mg and 2Zn, X-ray diffraction analysis revealed that the Pc core is near-planar; the dihedral angles between the mean planes of the pyrrole rings and four isoindoline N atoms vary from 1.75° to 5.48° for 2Zn (Figure 2) and from 0.37° to 6.19° for 2Mg (Figure 3). The Zn ion in 2Zn adopts a square-pyramidal coordination polyhedron, forming four almost equivalent Zn−Niso bonds and one Zn−O bond with the axially coordinated water molecule

Figure 3. X-ray structure of 2Mg (H atoms are omitted for clarity). Selected bond lengths: Mg1−N7 1.993(8) Å; Mg1−N3 2.031(8) Å; Mg1−O1 2.170(7) Å; Mg1−O2 2.101(7) Å.

(Figure 2). The Mg ion in 2Mg adopts a square-bipyramidal coordination polyhedron, forming four almost equivalent Mg− Niso bonds; in contrast to complex 2Zn, two water molecules are axially coordinated trans to each other with the respect to the Pc macrocycle, forming Mg−O bonds (Figure 3). Because D


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Inorganic Chemistry

Figure 4. Fragment of the crystal packing of 2Zn molecules. Crown ether rings not involved in hydrogen bonds with water molecules, as well as some oxanthrene moieties and butoxy groups, are omitted for clarity. Hydrogen bond lengths: O37···O12 2.885 Å; O37···O13 3.038 Å; O37···O14 2.712 Å; O37···O16 2.929 Å.

Figure 5. Fragment of the crystal packing of 2Mg molecules. Crown ether rings not involved in hydrogen bonding with water molecules, as well as the butoxy groups, are omitted for clarity. Hydrogen bond lengths: O1···O23 2.767 Å; O1···O25 2.908 Å; O1···O27 3.012 Å; O2···O5 2.879 Å; O2··· O7 2.789 Å; O2···O9 2.912 Å.

neighboring Pc planes of 27.93° (Figure 4). No more intermolecular interactions are observed within the chains. In the case of complex 2Mg, formation of the crystal lattice is also driven by bifurcated hydrogen bonds formed by two axially coordinated water molecules and two opposite crown ether moieties of neighboring molecules (Figure 5). Such a type of binding results in the formation of ruffled 2D networks. The distances between Mg ions within these networks are 15.629, 15.973, and 16.244 Å, and the tilt angle is 25.57°. The cavities of these networks are filled with the crown ether rings not involved in hydrogen bonding, as well as with the butoxy groups. Further packing of the chains and networks formed by 2Zn and 2Mg within the crystal lattice occurs via weak CH···O and CH··π contacts involving crown ether rings not participating in bonding with the axial water molecules, as well as eight butoxy groups. A similar type of hydrogen bonding, which involves the axially coordinated water molecule and a peripheral crown ether substituent, was observed in the structure of the MgII complex with tetrakis(dithia-15-crown-5)porphyrazine reported by Van Nostrum et al.50 However, in that case, the Mg complex contained only one water molecule; therefore, the general motif of crystal packing resembled the one observed in the present work for the 2Zn complex.

of the presence of the axial ligand, the Zn ion in 2Zn is displaced from the Niso plane by 0.235 Å, while the Mg ion in 2Mg is displaced by only 0.047 Å. The oxanthrene units retain their essential planarity similar to phthalonitrile 1. It should be noted that a structural feature of the obtained complexes is that all N atoms of the Pc ring lie almost in the same plane with the O atoms of the butoxy substituents (dihedral angle plane N8/plane OBuNmesoOBu = 7.2−13.8° for 2Zn and 6.4−15.3° for 2Mg) with quite short Nmeso···OBu distances of 3.090−3.126 Å for 2Zn and 3.051−3.155 Å for 2Mg. These cavities are organized in such a manner that they could be potential centers able to accept protons. The butoxy chains are arranged above and below the Pc plane, resulting in significant steric crowding. This arrangement prevents stacking interactions between complex molecules, which typically govern the crystal packing of Pc molecules. In the present case, the main driving force of crystal packing is intermolecular formation of bifurcated hydrogen bonds between the axially coordinated water molecule and O atoms of the crown ether rings of adjacent Pc molecules. However, because of the difference in the number of axially coordinated water molecules, complexes 2Mg and 2Zn form different crystal packings. The molecules of complex 2Zn form 1D zigzag chains with a Zn···Zn distance of 16.207 Å and a tilt angle of E


Article

Inorganic Chemistry Investigation of the Switchable Optical Properties of Oxanthrenocyanines. The UV−vis spectra of investigated compounds 2M in a CHCl3 solution show characteristic absorption for nonperipheral Pc complexes:23,26,49 a Q band at 730−760 nm, which is shifted by about 50 nm compared to that of tetra-15-crown-5-phthalocyanines, and a Soret band at 375−400 nm (Figure S6 and Table S3). The UV−vis spectrum of 2H2 contains a Q band at 759 nm with a shoulder at 736 nm and a Soret band at 321 nm (Figure S6). An unconventional band at 800 nm arises in the UV−vis spectra of 2M solutions in CHCl3 during storage. This phenomenon can be explained by meso-N-protonation23−25,51 or J-aggregation19,20 of molecules. The absence of the band at 800 nm in spectra of 2M in toluene (Figure S7) proves that no J-aggregation takes place. Indeed, the direct titration of chloroform or toluene solutions of 2M with CF3COOH (2 equiv) resulted in an analogous growth of the band at about 800 nm, which proves the protonation process (Figure 6). Notably, this phenomenon was not

Scheme 3. Proposed Scheme of the Protonation of 2M

system so that the downfield signals became unobservable and appeared again at lower temperatures (below 273 K). As is distinct from the 1H NMR spectra of the system in toluene, the spectra of a solution of the “salt” 2Zn·CF3COOH [obtained by evaporation of the solvent from the solution of 2Zn + CF3COOH (3 equiv) in toluene] in CDCl3 showed lowfield signals at 14.83 and 14.73 ppm (2.4:1), four separate 1CH2 signals, and resolved signals of the crown ether CH2 protons even at room temperature (Figure 7). As the

Figure 6. Spectrophotometric titration of 2Zn in CHCl3 with CF3COOH (0−1.5 equiv). The arrows indicate changes in the spectra upon the first step of protonation.

Figure 7. 1H NMR spectra of complex 2Zn in CDCl3 (top) in the absence and (bottom) in the presence of CF3COOH. The fragments of the spectra in the ranges 0−3.5 and 8−14 ppm are omitted for clarity. The asterisk indicates a residual CHCl3 resonance signal.

observed for regular tetra-15-crown-5-phthalocyanine complexes; high concentrations of CF3COOH are required for their protonation, with the Q band being shifted from 686 to 735 nm (Figure S8). We can explain this fact from the X-ray diffraction data for 2Zn and 2Mg, which reveal the presence of OBu−Nmeso−OBu cavities preorganized for the formation of hydrogen bonds. Similar observations were described by Jiang and co-workers14,15 for interaction of nonperipherally octa(butoxy)-substituted Pc derivatives with a Na+ cation. The formation of Na−Nmeso and Na−OBu coordination bonds was revealed from the X-ray diffraction data. Taking all of this into account, we suggested that protonation occurs, namely, at an Nmeso atom of Pc complexes (Scheme 3). To prove this, we have studied the effect of protonation (caused by the introduction of CF3COOH) on the 1H NMR spectra. The introduction of CF3COOH (1 equiv) into a solution of 2Zn in toluene-d8 led to the splitting of the 1-CH2 signal, the narrowing of the crown ether signals, and the appearance of signals at 14.97 and 14.78 ppm, presumably corresponding to the proton transferred from CF3COOH to the 2Zn complex, which results in a lowering of the molecular symmetry and the nonequivalence of 1-CH2 groups. A further addition of the acid (2−3 equiv) intensified the exchange in the

temperature decreased from 298 to 233 K, the signals at 14.83 and 14.73 ppm shifted downfield by 0.1−0.13 ppm. The chemical shifts of these signals are consistent with the chemical shift (14.7 ppm) reported for the meso-NH protons involved in intramolecular hydrogen bonding with the O atoms of neighboring butoxy moieties in protonated Zn(OBu)8Pc16 and the signal (14.37 ppm) observed in the 1H NMR spectrum of a 2:1 mixture of 15N-3,5-dimethylimidazole and CF3COOH in freon at 120 K, which was assigned to the proton linked to one of the N heteroatoms and the trifluoroacetate ion, i.e., to the N−H···OC(O)CF3− moiety.52 On the basis of these data, we can suggest that protonation of the 2Zn complex occurs at a meso-N atom. The preferential location of the H atom at the meso-N atom is also supported by the 2D NOESY NMR spectrum of the solution of the 2Zn· CF3COOH salt in CDCl3 at 253 K (Figure 8). The spectrum showed a noticeable NOE correlation between the 1-CH2 protons (4.93 ppm) and the proton giving rise to the signal at 14.92 ppm. In addition, weaker correlations are observed between the signals at 14.92 and 14.80 ppm and the signals of the 2- and 3-CH2 groups of the butoxy substituents. Additional F


Article

Inorganic Chemistry

(2 equiv) leads to the formation of cofacial dimers with UV−vis spectra similar to those reported for the dimers of tetra-15crown-5-phthalocyaninates 31,37 (Figures 10 and 11). A hypsochromic shift of the Q band is observed: it moves to 686 and 695 nm for 2Zn and 2H2, respectively.

Figure 8. 1H NOESY (300 MHz) NMR spectrum of 2Zn in the presence of CF3COOH in CDCl3 at 253 K.

cross peaks to the other protons of the 2Zn complex were absent. The addition of bases (amines and K2CO3) to the resulting solution leads to the deprotonation and recovery of the spectrum of the starting complex. The further reaction with trifluoroacetic acid in CHCl3 gives a full set of four protonated forms for the 2Zn complexes, which is accompanied by a gradual shift of the Q band into the NIR region up to 1028 nm (Figure 9). It should be noted that all changes in the spectra are reversible; the addition of a base causes removal of the mesoprotons.

Figure 10. UV−vis titration of 2Zn in CHCl3 (6.4 × 10−6 M) with KBPh4 (4.8 × 10−4 M) in CH3CN. The arrows indicate the changes in the UV−vis spectra upon the addition of KBPh4.

Figure 11. Schematic representation of the supramolecular dimer that forms upon the interaction of 2Zn with K ions.

The formation of dimers was also proven by NMR titration in CDCl3 with a solution of KBPh4 in CD3CN (Figure 12). The resonance signal (singlet) of aromatic protons (6.80 ppm) splits into two signals (6.78 and 6.63 ppm) because the addition of KBPh4 (1 equiv) leads to equilibrium between the monomer and dimer forms of 2H2. The addition of 2 equiv of KBPh4 leads again to one resonance signal of aromatic protons shifted upfield (6.62 ppm). Similar changes are observed for the signal of the 1-CH2 protons, which moves from 5.04 to 4.94 ppm, while the α-CH2 signal of crown ether protons gradually splits into two signals of exo- and endo-protons, which is consistent with the formation of a supramolecular dimer.37 The interaction of 2Mg with KBPh4 strongly depends on the solvent. The changes in the UV−vis spectra of 2Mg + KBPh4 in CHCl3 in the presence of MeCN or MeOH are similar to those in the spectra of 2Zn and 2H2 caused by the formation of a cofacial dimer. The reaction in pure CHCl3 or toluene is

Figure 9. UV−vis spectra of 2Zn and all of its protonated forms in CHCl3 (6.4 × 10−6 M) with different amounts of CF3COOH (0, 1.5, and 16.5 equiv and 6.7% and 90%).

Only the first and partially the second protonated species of the Mg complex were observed because of its further demetalation (Figure S9). Free base ligand 2H2 is less basic and requires much more acid for protonation: only the addition of 0.33% CF3COOH leads to the first step of protonation and 12% to the second step. The Q band moves up to 931 nm (Figure S10). The third step cannot be achieved by adding CF3COOH and requires the use of stronger acids. As mentioned above, 15-crown-5-phthalocyanine can interact with K cations with the formation of supramolecular assemblies.29 Indeed, the reaction of 2H2 or 2Zn with KBPh4 G


Article

Inorganic Chemistry

and fluorescence quantum yields (Table 1). The fluorescence quantum yield of the compounds decreases in the following order: 2Mg ≈ 2H2 ≫ 2Zn. Table 1. Photochemical Data of 2M in Toluene (Excitation Wavelength 400 nm) λF, nm Stokes shift, nm ΦF

2Mg

2Zn

2H2

756 18 0.38

748 11 0.15

777 16 0.35

Protonation of the 2M complexes causes fluorescence quenching and the appearance of a new emission band at 813 nm, which is significantly weaker than the band of the nonprotonated molecule (Figure 13a). Formation of the K+-

Figure 12. NMR titration of 2H2 in CDCl3 (1.88 × 10−3 M) with KBPh4 in CD3CN. (a) Starting solution and after the addition of (b) 1 and (c) 2 equiv of KBPh4. Red circles indicate the resonance signals of the BPh4− anion. The asterisk indicates the residual CHCl3 resonance signal.

accompanied by a decrease in the Q-band intensity with a negligible shift (3 nm). This fact can be explained by the presence of two water molecules coordinated to the Mg atom, as was demonstrated by X-ray diffraction data. Thus, we can propose that the water molecules prevent the formation of the dimer in pure CHCl3 or toluene; however, coordinating/polar solvents (MeCN or MeOH) lead to the dissociation/ substitution of the axial ligands and thus allow formation of the dimer. This suggestion was proven by the addition of Ph3PO to a solution of 2Mg in CHCl3. Indeed, the presence of Ph3PO in the solution of 2Mg in pure CHCl3 with the subsequent addition of KBPh4 (2 equiv) leads to formation of the cofacial dimer (Figure S11). The possible explanation can be as follows: one molecule of Ph3PO is substituted for one molecule of water and pulls the Mg atom out of the Pc plane, leading to rupture of the coordination bond with the second water molecule. A similar fact has been reported that the coordination of Ph3PO to MgPc leads to the displacement of the Mg atom from the N4 plane of Pc by ca. 0.52 Å.53 The dimerization, as well as protonation, is a reversible process: the addition of [2.2.2]cryptand to the solution of the dimer leads back to the monomer. Thus, it becomes possible to reversibly change the light absorbance of 2Zn complexes in a very wide range of wavelengths from 686 nm for the cofacial dimer to 1028 nm for the fourth protonated form! It was also important to study the photochemical properties of all types of compounds 2M (monomer, dimer, and protonated forms). To do this, the fluorescence measurements for monomeric 2M compounds were made in toluene to avoid protonation of the complexes. The compounds showed fluorescence at 750−780 nm with characteristic Stokes shifts

Figure 13. Quenching of the fluorescence of 2Zn by protonation (a) and by cation-induced dimerization (b).

induced cofacial dimer also quenches fluorescence (Figure 13b), but in contrast to the protonation process, new bands do not appear in the spectrum, which is consistent with the nonemissive behavior of Pc dimers.54 Thus, there are two different optical methods for control whether the dimers or protonated species are formed; in addition, a combination of these methods makes it possible to work in a wide range of concentrations of target compounds.

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CONCLUSIONS In summary, we have prepared and structurally characterized a new series of molecular chameleons: Pc complexes bearing 15crown-5-oxanthrene substituents (oxanthrenocyanines) with butoxy groups in α positions, possessing unique properties of reversible switching of their optical properties by means of both aggregation and acid−base equilibrium. According to X-ray diffraction data, the oxanthrene units in the molecules of Zn and Mg complexes retain their essential planarity. Depending on the metal nature, 1D zigzag chains or 2D coordination networks are observed in the crystal packing because of the intermolecular formation of bifurcated hydrogen bonds between the axially coordinated water molecule and O atoms of the crown ether rings. The detailed investigation of the optical properties of the synthesized compounds has demonstrated the possibility of reversible switching between the wavelengths of light absorption in the very wide range from 687 to 1028 nm. The changes can be induced by protonation of H


Article

Inorganic Chemistry

(9) Makarov, S.; Litwinski, C.; Ermilov, E. A.; Suvorova, O.; Röder, B.; Wöhrle, D. Chem. - Eur. J. 2006, 12 (5), 1468−1474. (10) Mack, J.; Kobayashi, N. Chem. Rev. 2011, 111 (2), 281−321. (11) Cook, M. J.; Dunn, A. J.; Howe, S. D.; Thomson, A. J.; Harrison, K. J. J. Chem. Soc., Perkin Trans. 1 1988, No. 8, 2453−2458. (12) Soldatova, A. V.; Kim, J.; Peng, X.; Rosa, A.; Ricciardi, G.; Kenney, M. E.; Rodgers, M. A. J. Inorg. Chem. 2007, 46 (6), 2080− 2093. (13) Zhong, A.; Zhang, Y.; Bian, Y. J. Mol. Graphics Modell. 2010, 29 (3), 470−480. (14) Zhang, H.; Wang, R.; Zhu, P.; Lai, Z.; Han, J.; Choi, C.-F.; Ng, D. K. P.; Cui, X.; Ma, C.; Jiang, J. Inorg. Chem. 2004, 43 (15), 4740− 4742. (15) Gao, Y.; Chen, Y.; Li, R.; Bian, Y.; Li, X.; Jiang, J. Chem. - Eur. J. 2009, 15 (47), 13241−13252. (16) Honda, T.; Kojima, T.; Fukuzumi, S. Chem. Commun. 2011, 47, 7986−7988. (17) Kobayashi, N.; Furuyama, T.; Satoh, K. J. Am. Chem. Soc. 2011, 133 (49), 19642−19645. (18) Furuyama, T.; Satoh, K.; Kushiya, T.; Kobayashi, N. J. Am. Chem. Soc. 2014, 136 (2), 765−776. (19) Huang, X.; Zhao, F.; Li, Z.; Huang, L.; Tang, Y.; Zhang, F.; Tung, C.-H. Chem. Lett. 2007, 36 (1), 108−109. (20) Chen, Z.; Zhong, C.; Zhang, Z.; Li, Z.; Niu, L.; Bin, Y.; Zhang, F. J. Phys. Chem. B 2008, 112, 7387−7394. (21) Islam, M. R.; Sundararajan, P. R. Chem. - Eur. J. 2011, 17 (22), 6098−6108. (22) Shishkin, V. N.; Kudrik, E. V.; Shaposhnikov, G. P. Rus. J. Inorg. Chem. 2004, 49, 1023−1027. (23) Weitman, H.; Schatz, S.; Gottlieb, H. E.; Kobayashi, N.; Ehrenberg, B. Photochem. Photobiol. 2001, 73 (5), 473−481. (24) Donzello, M. P.; Ercolani, C.; Gaberkorn, A. A.; Kudrik, E. V.; Meneghetti, M.; Marcolongo, G.; Rizzoli, C.; Stuzhin, P. A. Chem. Eur. J. 2003, 9 (17), 4009−4024. (25) Makarov, D. A.; Derkacheva, V. M.; Kuznetsova, N. A.; Kaliya, O. L.; Lukyanets, E. A. Makrogeterotsikly 2013, 6 (4), 371−378. (26) Ayhan, M. M.; Altınbaş Ö zpinar, G.; Durmuş, M.; Gürek, A. G. Dalt. Trans. 2013, 42 (41), 14892−14904. (27) Kanat, Z.; Dinçer, H. Dalt. Trans. 2014, 43 (23), 8654−8663. (28) Derkacheva, V. M.; Kaliya, O. L.; Luk’yanets, E. A. J. Gen. Chem., USSR 1983, 53, 163−167. (29) Gorbunova, Y. G.; Martynov, A. G.; Tsivadze, A. Y. In Handbook of Porphyrin Science; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; World Scientific Publishing: Singapore, 2012; Vol. 24, pp 271−388. (30) Martynov, A. G.; Gorbunova, Y. G.; Tsivadze, A. Y. Russ. J. Inorg. Chem. 2014, 59 (14), 1627−1656. (31) Kobayashi, N.; Lever, A. B. P. J. Am. Chem. Soc. 1987, 109 (24), 7433−7441. (32) Ahsen, V.; Yilmazer, E.; Ertas, M.; Bekâroğlu, Ö . J. Chem. Soc., Dalton Trans. 1988, 2, 401−406. (33) Gorbunova, Y. G.; Enakieva, Y. Y.; Sakharov, S. G.; Tsivadze, A. Y. J. Porphyrins Phthalocyanines 2003, 07 (12), 795−800. (34) Gorbunova, Y. G.; Grishina, A. D.; Martynov, A. G.; Krivenko, T. V.; Isakova, A. A.; Savel’ev, V. V.; Nefedov, S. E.; Abkhalimov, E. V.; Vannikov, A. V.; Tsivadze, A. Y. J. Mater. Chem. C 2015, 3, 6692− 6700. (35) Grishina, A. D.; Gorbunova, Y. G.; Zolotarevsky, V. I.; Pereshivko, L. Y.; Enakieva, Y. Y.; Krivenko, T. V.; Savelyev, V.; Vannikov, A. V.; Tsivadze, A. Y. J. Porphyrins Phthalocyanines 2009, 13 (01), 92−98. (36) Vannikov, A. V.; Grishina, A. D.; Gorbunova, Y. G.; Krivenko, T. V.; Laryushkin, A. S.; Lapkina, L. A.; Savelyev, V.; Tsivadze, A. Y. Polym. Sci., Ser. A 2011, 53 (11), 1069−1075. (37) Safonova, E. A.; Martynov, A. G.; Zolotarevskii, V. I.; Nefedov, S. E.; Gorbunova, Y. G.; Tsivadze, A. Y. Dalt. Trans. 2015, 44 (3), 1366−1378. (38) Al-Raqa, S. Y. Dyes Pigm. 2008, 77 (2), 259−265.

molecules, leading to a significant red shift of absorption: fully reversible switching was observed for the four protonated forms of Zn complexes, which can be deprotonated by the addition of a base, in contrast to peripherally substituted tetra-15-crown-5phthalocyanines without oxanthrene moieties. Only two reversible protonated forms are observed for the free-base ligand and the Mg derivative. At the same time, the cationinduced formation of a supramolecular dimer leads to a blue shift of the Q band from the NIR region to the visible one, which can be reversed by the addition of [2.2.2]cryptand to the solution of the dimer. It has also been demonstrated that the investigated compounds exhibit fluorescence with a moderate quantum yield, which is also sensitive to both aggregation and acid−base equilibrium. Therefore, both absorption and emission control can be achieved for postsynthetic modification of the electronic and structural properties of molecules. Taking into account the results of this study, we believe that in future the investigated Pc complexes might be suitable molecular switchers for optoelectronic materials and sensors.

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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/acs.inorgchem.5b02831. X-ray crystallographic data in CIF format for 1 (CIF) X-ray crystallographic data in CIF format for 2Mg (CIF) X-ray crystallographic data in CIF format for 2Zn (CIF) Additional data on the X-ray structures of 1, 2Zn, and 2Mg, additional photophysical data for 2Zn and 2Mg, and NMR and ESI HRMS spectra for synthesized compounds (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The design and synthesis of the complexes was performed within the project supported by the Russian Science Foundation (Grant 14-13-01373). Physical−chemical studies were supported by the Russian Foundation for Basic Research (Grant 14-03-00977). The authors are grateful to A. A. Averin for fluorescence measurements.

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REFERENCES

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