Design of Multichannel Osmium-Based Metalloreceptor for Anions and

Dec 3, 2015 - A polypyridylimidazole-based bifunctional Os(II) complex of the type [(bpy)2Os(tpy-Hbzim-dipy)](ClO4)2 (1), where tpy-Hbzim-dipy = 4′-...
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Design of Multichannel Osmium-Based Metalloreceptor for Anions and Cations by Taking Profit from Metal−Ligand Interaction and Construction of Molecular Keypad Lock and Memory Device Srikanta Karmakar, Sourav Mardanya, Poulami Pal, and Sujoy Baitalik* Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India S Supporting Information *

ABSTRACT: A polypyridylimidazole-based bifunctional Os(II) complex of the type [(bpy)2Os(tpy-Hbzim-dipy)](ClO4)2 (1), where tpy-Hbzim-dipy = 4′-[4-(4,5-dipyridin-2-yl-1Himidazol-2-yl)-phenyl]-2,2′;6′,2″-terpyridine and bpy = 2,2′bipyridine, has been synthesized and structurally characterized for the construction of multifunctional logic devices. After coordination of an [Os(bpy)2]2+ unit to one of the two bidentate chelating sites, the complex offers a terpyridine motif for binding with cationic guests and an imidazole moiety for interacting with selective anionic species. Consequently, the anion- and cation-binding aspects of the metallorecptor were examined in solution and in the solid state by different spectroscopic and electrochemical methods. The complex behaves as a bifunctional sensor for F−, AcO−, CN−, Fe2+, and Cu2+ ions in acetonitrile, whereas it is a highly selective chromogenic chemosensor for only CN− and Fe2+ ions in water. Based on various output signals with a particular set of anionic and cationic inputs, the complex mimics the functions of two-input INHIBIT, OR, NOR, and XNOR logic gates, as well as threeinput NOR logic behavior. More importantly, the complicated functions of a keypad lock and memory device were also nicely demonstrated by the complex. Finally, density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations also provide a rationale for properly understanding and interpreting the experimentally observed results.

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INTRODUCTION During past two decades, a great number of activities have been devoted to the search for information-processing systems based on molecules after the first realization of computation at the molecular level by de Silva in 1993.1−5 The main reason for these efforts is to construct computers with very small sizes, low power consumption, and high computing performance that are difficult to achieve with conventional silicon-based technology. Computation at the molecular level is possible only with molecular logic gates, which are capable of integrating simple logic gates into combinational circuits.1−7 Several molecular systems mimicking different gates performing Boolean logic operations and responding to a large variety of activating input signals (e.g., light, electrical, magnetic, and chemical) have been developed in the past two decades.1−5,8−24 In contrast to mimicking the functions of basic logic gates and their simple combinations, molecular systems capable of demonstrating sequential logic operations as required in circuits, molecular keypad locks,25−28 and memory devices are relatively rare in the literature.29−33 Moreover, there is a scarcity of coordinationcomplex-based molecular systems with versatile geometries and excellent optoelectronic properties compared with their organic counterparts in exhibiting sequential advanced logic functions.34,35 To address this gap, we designed and structurally characterized a monometallic Os(II) complex, 1, derived from a © 2015 American Chemical Society

heteroditopic terpyridylimidazole ligand. In the design of the complex, low-spin d6 Os2+ metal was chosen because of its appealing optoelectronic and electrochemical behaviors, which primarily originate from metal-to-ligand charge-transfer (MLCT) transitions in the presence of pyridine-based ligands. Moreover, complexes of this type exhibit absorption throughout the entire visible region due to both spin-allowed (1MLCT) and spin-forbidden (3MLCT) transitions. In addition, the emission wavelengths of such complexes stretch to the nearinfrared region, which is of significant importance because of their potential biological applications. Thus, the present Os(II) complex also provides more biofriendly conditions for the detection of ions. A number of Ru(II) complexes derived from polypyridine ligands were previously designed as sensors for selected anions and cations, although such complexes for the construction of sequential molecular logic gates are scarce in the literature.36,37 By contrast, analogous Os(II) complexes are extremely rare and have been reported only a few times in the literature.38−41 Moreover, most of these Ru(II) and Os(II) complexes were derived from either bidentate ligands (such as bipyridine or phenanthroline) or comparatively less explored tridentate chelating ligands (such as 2,2′:6′,2″-terpyriReceived: August 28, 2015 Published: December 3, 2015 11813

DOI: 10.1021/acs.inorgchem.5b02300 Inorg. Chem. 2015, 54, 11813−11825

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Inorganic Chemistry dine).42−51 In the design of the present complexes, we utilized a combination of 2,2′-bipyridine- (bpy-) and 2,2′;6′,2″-terpyridine- (tpy-) type bridging ligands, which is extremely rare in the literature52−55 and provides additional advantageous features in terms of interacting with incoming cationic guests. In the present complex, the remote terpyridine and the adjacent bipyridine chelating units are suitably oriented to bind with incoming cationic guests. The NH proton in the imidazole ring in the resulting complex is sufficiently acidic to interact with an incoming anion through either hydrogen-bonding interactions or complete proton transfer.55−64 In reality, the present Os(II) complex acts as an efficient multichannel sensor for F−, CN−, AcO−, and H2PO4− in acetonitrile, whereas it acts as a highly selective chromogenic and fluorogenic receptor for CN− in pure aqueous medium. It is needless to mention that development of highly selective and efficient sensors capable of detecting cyanide in aqueous media is still a great challenge because, among the various anions, cyanide is extremely toxic to living organisms and harmful to the environment and human health.65−68 Despite its extreme toxicity, a few cyanide compounds are still widely used in several industries, which leads to the accidental leakage of cyanide ion into the aquatic environment.69−71 Thus, improved methods for cyanide detection, particularly in aqueous media, are highly necessary.72−78 Apart from sensing selective anionic species, the present system has the ability to selectively recognize and sense Cu2+ and Fe2+ ions over other 3d transition metals. The bifunctional Os(II) complex demonstrates multiple logic operations by utilizing its optical and electrochemical responses toward a particular set of ionic inputs. Of particular interest, we have used our system for the construction of a security keypad lock by proper sequencing of the addition of Fe2+ and Cu2+ as inputs and monitoring of the absorbance at a specific wavelength as the output signal. A molecular keypad lock capable of creating a secret password provides a way of protecting information at the molecular level. In addition to the construction of security keypad lock, based on the reversible and reproducible colorimetric switching of the complex, we were able to successfully design a sequential logic circuit showing “write−read−erase−read” behavior in the form of binary logic for information processing at the molecular level.29−33 Computational work using the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods was also performed for the proper assignment of the experimentally observed absorption and emission spectral bands.

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yield 60%). Anal. Calcd for C54H41N11Cl2O9Os: C, 51.92; H, 3.31; N, 12.33. Found: C, 51.90; H, 3.32; N, 12.30. 1H NMR (400 MHz, DMSO-d6, δ/ppm; see Figure S1 of the Supporting Information for proton numbering): 14.35 (s, 1H, NH imidazole), 9.28 (d, 1H, J = 8.0 Hz, H3″), 9.19−9.15 (m, 2H, 1H6 + 1H3′), 8.99 (d, 2H, J = 8.0 Hz, 2H3″), 8.93 (d, 2H, J = 4.8 Hz, 1H9 + 1H6), 8.89 (d, 1H, J = 4.8 Hz, H3″), 8.77 (d, 1H, J = 8.4 Hz, H8), 8.74 (s, 2H, 2H3′), 8.70 (d, 1H, J = 8.4 Hz, H8), 8.62 (d, 1H, J = 8.4 Hz, H3), 8.5 (d, 1H, J = 5.2 Hz, H7), 8.42−8.38 (m, 3H, 1H7 + 1H10 + 1H11), 8.25 (d, 1H, J = 8.0 Hz, H12), 8.12 (t, 1H, J = 7.6 Hz, H4), 7.96 (t, 1H, J = 7.6 Hz, H4) 7.84 (t, 2H, J = 6.4 Hz, H16 + H4′), 7.79 (t, 1H, J = 7.6 Hz, H4′), 7. 86 (d, 2H, J = 8.4 Hz, 2H6′), 7.61−7.57 (m, 2H, H4′+H15), 7.51− 7.44 (m, 3H, 1H4′+1H5 + 1H14), 7.38 (d, 1H, J = 5.2 Hz, H6′), 7.31 (t, 1H, J = 6.5 Hz, H5), 7.26−7.23 (m, 1H, H5′), 7.21−7.16 (m, 3H, 2H5′ + H6′), 6.88 (t, 1H, J = 6.7 Hz, H5′), 6.77 (d, 1H, J = 8.4 Hz, H13). ESI-MS (positive, CH3CN): m/z 344.73 (100%) [(bpy)2Os(tpy-Hbzim-dipy-H)]3+, m/z 516.59 (88%) [(bpy)2Os(tpy-Hbzimdipy)]2+.

Physical Measurements. The physical measurements and theoretical calculation methods are provided in the experimental section of the Supporting Information.

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RESULTS AND DISCUSSION Synthesis and Characterization. Synthesis and purification of the monometallic Os(II) complex [(bpy)2Os(dipyHbzim-tpy)]2+ was described in the preceding section. The complex was recrystallized under mild acidic conditions to keep the imidazole NH proton intact. The complex was thoroughly characterized through C, H, and N analysis and ESI mass and 1 H NMR spectroscopic measurements (Figures S1 and S2, Supporting Information). In MeCN, 1 exhibits two abundant peaks at m/z 344.73 and 516.09 (Figure S2, Supporting Information). The peak at m/z 516.09 corresponds to a dipositive molecular ion of the type [(bpy)2Os(dipy-Hbzimtpy)]2+. The complex was recrystallized in HClO4, in which there is a finite possibility of protonation of the free nitrogen atom(s), which can give rise to protonated species of the type [(bpy)2Os(dipy-Hbzim-tpy-H)](ClO4)3. Thus, the peak at m/z 344.73 corresponds to the tripositive species [(bpy)2Os(dipyHbzim-tpy-H)]3+. The 1H NMR spectrum of 1 is extremely complicated, with the appearance of a large number of overlapping proton resonances in the aromatic region. Tentative assignments of the different protons were made possible with the aid of its {1H−1H} COSY spectrum and comparison with the spectra of related derivatives.55 The imidazole NH proton in 1 appears at the most downfield region, δ = 14.35 ppm. The solid-state structure of the complex was determined by single-crystal diffraction studies. ORTEP79 drawing of the cation in 1 is displayed in Figure 1, and selected bond distances and angles are listed in Table S2 (Supporting Information). The complex adopts a distorted octahedral geometry around the Os(II) center, as evidenced by its bond

EXPERIMENTAL SECTION

Synthesis of the Ligand 4′-[4-(4,5-Dipyridin-2-yl-1H-imidazol-2-yl)-phenyl]-2,2′;6′,2″-terpyridine (tpy-Hbzim-dipy). The ligand was synthesized by the condensation of pyridyl and 4′-(pformylphenyl)-2,2′:6′,2″-terpyridine (tpy-PhCHO) using a previously reported procedure.55 Synthesis of [(bpy)2Os(dipy-Hbzim-tpy)](ClO4)2·H2O (1). cisOs(bpy)2Cl2 (285 mg, 0.5 mmol) and dipy-Hbzim-tpy (350 mg, 0.6 mmol) were suspended in 10 mL of ethylene glycol and heated under reflux for 12 h under argon protection. The resulting solution was cooled, filtered, and finally poured into an aqueous solution (5 mL) of NaClO4 (1.0 g), at which point a dark brown compound precipitated. The precipitated crude product was filtered and purified by column chromatography (silica gel) using acetonitrile as the eluting solvent. Finally, recrystallization from a 1:1 acetonitrile/methanol mixture under slightly acidic conditions afforded the desired compound. The final product appeared as a shining black crystalline solid (380 mg, 11814

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biexponential decay behavior in the solid state with a shorter lifetime (7.1 ns). Cyclic and square-wave voltammograms recorded in MeCN indicate a reversible oxidation couple at E1/2 = 0.79 V due to the OsII/OsIII oxidation process (Figure S3, Supporting Information) and two quasireversible peaks at −1.35 and −1.67 V due to the reductions of coordinated bpy ligands. Anion-Sensing Studies of 1. Taking advantage of the NH proton of bpy-Hbzim-tpy that became appreciablely acidic upon coordination to the [Os(bpy)2]2+ unit, we performed anion recognition and sensing studies of the metalloreceptor by utilizing the secondary interaction with the anions. We investigated the sensing properties of 1 in acetonitrile, water, and solid media. Sensing Studies in Acetonitrile. Sensing experiments were performed using tetrabutylammonium (TBA) salts of F−, Cl−, Br−, I−, CN−, AcO−, OH−, and H2PO4− with the help of various spectroscopic methods and voltammetric techniques. The spectral changes are displayed in Figure 2. Among the anions, F−, AcO−, CN−, and H2PO4− led to red shifts of the MLCT bands, albeit to different extents, as well as significant quenching of the emission at 774 nm along with a red shift (Table 1). The differences in the spectral responses with different anions are due to the variations in the size and basicity of the anions. Second, among the studied anions, F−, CN−, and AcO− have higher basicities, thereby inducing comparable spectral changes of the metalloreceptor (pKa values in MeCN: HF = 25.2, HCN = 23.1, AcOH = 23.51, HCl = 10.30, HBr = 5.50, HI = 2.8). Systematic absorption and emission titration experiments were carried out with incremental addition of the anions and are presented in Figure 3 and Figures S4 and S5 (Supporting Information). With H2PO4−, two-step changes were observed in both absorption and emission spectra compared with the one-step changes with F−, CN−, and AcO− ions. For F−, CN−, and AcO− ions, spectral saturation occurred in each case with 1 equiv of the anions, whereas for H2PO4−, 5 equiv is required. Several isosbestic points and red shifts of the MLCT bands were observed in the titration profiles (Figure 3 and Figures S4 and S5, Supporting Information). The emission spectral profiles demonstrated gradual quenching of the emission upon the addition of up to 1 equiv of each F−, CN−, and AcO− ions (Figure 3b and Figure S4c,d, Supporting Information). In the case with H2PO4−, addition of 1 equiv led to emission augmentation, whereas addition beyond 1 equiv resulted in emission quenching (Figure S5, Supporting Information). The changes in the luminescence lifetimes with added anions were also found to follow the same trend as their steady-state behaviors (Figure 3c). Titration data allowed for the evaluation of the equilibrium constants (Ks) for the interaction processes, and the obtained values ranged over 6 orders of magnitude. The metalloreceptor offers very low values of detection of selective anions, on the order of 10−9 M calculated by emission titration data (Figures S6 and S7, Supporting Information). To obtain direct proof regarding the mode of the interaction between the metalloreceptor and the anions, 1H NMR titration experiments were performed with selected anions. Addition of each of the three ions F−, CN−, and AcO− first led to small downfield shifts of the NH signal from 14.35 to 14.50 ppm in DMSO-d6 (Figure S8, Supporting Information). Continued addition of the anions induced broadening and, subsequently, complete removal of the peak at 14.50 ppm. In the second step, the integrated area of the NH peak at 14.50 ppm decreased

Figure 1. ORTEP representation of 1 showing thermal ellipsoids at the 30% probability level. Hydrogen atoms except the imidazole NH proton are omitted for clarity.

distances and angles. The Os−N bond distances (2.049−2.083 Å) lie in the expected ranges. Absorption and emission spectra of the Os(II) complex (1) were measured in MeCN and H2O. In the two solvents, the absorption spectra of 1 are of similar types, showing very intense bands in the range of 200−400 nm due to π−π* transitions of coordinated ligands and moderately strong overlapping absorption bands in the range between 400 and 500 nm due to Os(dπ) → bpy and Os(dπ)→ bpy-Hbzim-tpy MLCT transitions. In addition, 1 exhibits a broad and less intense band in the range of 600−700 nm in both solvents due to spin-forbidden 1[OsII(dπ)6] → 3[OsII(dπ)5bpy/bpy-Hbzimtpy(π*)1] MLCT transitions.52,53 Upon excitation to its MLCT absorption maximum, complex 1 displays a broad emission band at 774 nm in MeCN and at 772 nm in H2O at room temperature due to the 3MLCT (OsII(dπ) → bpy/bpy-Hbzimtpy (π*) excited state (Table 1). The complex also displays strong luminescence at 767 nm in the solid powered state in a slightly blue-shifted position compared with the solution state. Excited-state decay characteristics of 1 were also measured at room temperature. In contrast to the single-exponential decay with lifetimes of 17.5 ns in MeCN and 8.9 ns in H2O, 1 exhibits Table 1. Luminescence Maximum (λmax) and Quantum Yield (Φ) of 1 in the Presence of Different Anions and Cations at 298 K in acetonitrile compound 1 1 1 1 1 1 1 1 1 1 1 1

+ + + + + + + + + + +

F− CN− H2PO4− (1 equiv) H2PO4− (5 equiv) Mn2+ Fe2+ Co2+ Ni2+ (0.5 equiv) Ni2+ (3.0 equiv) Cu2+ (0.5 equiv) Cu2+ (2.5 equiv)

in water −4

λ (nm)

Φ (10 )

λ (nm)

Φ (10−4)

774 833 833 773 775 773 774 774 774 773 770 745

8.21 1.09 1.09 15.08 4.59 5.95 1.01 3.39 4.11 1.35 3.01 0.47

772 772 801 − 772 − 772 − − − − −

5.61 5.61 1.37 − 5.61 − 1.44 − − − − − 11815

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Figure 2. UV−vis absorption and emission spectral changes of 1 in (a,b) acetonitrile and (c,d) water upon addition of different anions as their TBA salts. The excitation wavelength was 490 nm.

Figure 3. Changes in (a) UV−vis absorption, (b) steady-state emission, (c) excited-state lifetime, and (d) square-wave voltammetry (SWV) of 1 upon incremental addition of F− in acetonitrile. The excitation wavelength for steady-state emission was 490 nm.

anion, the proton resonances again resolved properly, and a good-quality NMR spectrum was obtained. Addition of anions beyond 1.0 equiv did not produce any change of the spectrum

gradually, maintaining its position almost constant. During this process, most of the aromatic proton signals became broadened and unresolved. Finally, in the neighborhood of 1.0 equiv of 11816

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Inorganic Chemistry Scheme 1

energy of the intersecting point of the absorption and fluorescence spectra of free dipy-Hbzim-tpy (Figure S11a, Supporting Information). Similarly, the E00 value of the deprotonated dipy-bzim-tpy moiety was estimated to be 2.76 eV (Figure S11b, Supporting Information). Thus, taking into consideration the first reduction potential of 1 (−1.35 V) and the oxidation potential of the deprotonated ligand (+1.16 V), the free energy change of the photoinduced electron transfer from the deprotonated ligand to the excited Os(II) center was calculated as −0.25 eV, making the electron-transfer process thermodynamically feasible. The recognition phenomenon was also studied by the SWV technique. Upon progressive addition of F− to 1, the current height of the original SWV peak at 0.79 V due to OsII/OsIII oxidation gradually diminished, and at its expense, a new peak at 0.53 V grew in height (Figure 3d). After addition of ∼1.0 equiv of F−, the peak at 0.79 V was completely replaced by the 0.53 V couple. 1 also exhibited similar oxidation responses with AcO− and CN−. A very weak wave at ∼1.2 V in the SWV of 1 was observed that can be assigned to the oxidation of the coordinated ligand by comparison with the oxidation of the free ligand, dipy-Hbzim-tpy. During the addition of anion, it was observed that the peak at 1.2 V also became intensified with a small cathodic shift (Figure S12, Supporting Information). This indicates that anion-induced deprotonation of the NH proton leads to charge accumulation on the dipy-bzim-tpy moiety and thus favors oxidation of the deprotonated dipy-bzim-tpy.

except the emergence of a triplet at 16.0 ppm due to the formation of HF2− ion. Meanwhile, the protons signals attached to the phenyl moiety (H7 and H8), as well as H3 and H3′ of the free pyridine groups of the terpyridine moiety, moved toward the upfield region (Figure S8, Supporting Information). A downfield shift along with broadening of the imidazole NH peak indicates the initial formation of a hydrogen bond (Scheme 1). In the subsequent step, a gradual decrease of the NH peak followed by its complete removal suggests abstraction of the proton associated with the imidazole ring. Upfield shifts of the selected protons are probably because of an increase in negative charge on the aromatic backbone of 1 brought about by N−H deprotonation. Again, close resemblances of the titration profiles of 1 in the presence of F−, CN−, and AcO− ions with that of OH − are consistent with ultimate deprotonation of the imidazole NH proton (Figure S9, Supporting Information). The quenching of emission in all cases is most likely due to photoinduced intramolecular electron transfer from the negatively charged dipyridin-2-yl-1H-imidazol-2-yl ligand moiety to the excited-state Os(II) center. However, to check the thermodynamic feasibility of the electron-transfer process, we calculated the free energy change of the redox process. The excited-state energy (E00), estimated from the intersection of the absorption and emission spectrum, was found to be 1.77 eV for 1 and 1.68 eV for the deprotonated form (Figure S10, Supporting Information). The E00 value of the dipy-Hbzim-tpy moiety in 1 can be approximated to be 2.83 eV by taking the 11817

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Inorganic Chemistry Sensing Studies in Water. Although very sensitive toward F−, CN−, AcO−, and H2PO4−, 1 lacks selectivity to differentiate each ion in the presence of the others in neat MeCN. In contrast to MeCN, the metalloreceptor exhibited remarkable selectivity toward CN− among the other studied ions in pure aqueous medium (Figure 2c,d). The titration profiles of 1 recorded in pure water and in 2-[4-(2-hydroxyethyl)-1piperazinyl]ethanesulfonic acid (HEPES) buffer solution at pH 7.2 are very similar to each other and were found to vary to some extent from the profiles in neat MeCN (Figure 4). In line

limit, which is a very important parameter for practical applications, is strikingly low for the present Os complex and was calculated to be 5.75 × 10−8 M from emission titration data (Figure S13, Supporting Information). The selectivity of 1 for CN− over other anions was also studied through UV−vis absorption measurements by taking a solution of 1 (1.5 × 10−5 M) and treating it with 10 equiv of CN− in the presence of an excess of each of the other studied anions. Cyanide-ion-induced spectral and color changes were clearly observed in the competition experiments, indicating very high selectivity of the metalloreceptor toward CN− over other studied anions in aqueous media. Thus, multichannel detection, high selectivity, and a very low detection limit for CN− make the novel Os(II) complex an excellent candidate for the practical detection of CN− ion in aqueous solution. The inability of 1 to sense F− ions in pure water is probably due to its higher Gibbs energy of hydration for F− (ΔGh° = −465 kJ/mol) compared to CN− (ΔGh° = −295 kJ/mol).80 Moreover, the higher pKa value for HCN in water (pKa = 9.0) compared to HF (pKa = 3.17) strongly suggests that CN− is a stronger base than F− in aqueous medium.80 Both of these factors are responsible for the selective sensing of CN− in water. Sensing Studies in the Solid State. Sensing experiments in the solid state were carried out by mechanically grinding the complex with anions with the help of a mortar and pestle, and an instantaneous color change was observed with NaCN, as shown in Figure S14 (Supporting Information). The color change is also reflected in the absorption spectral profile. The broad emission at 767 nm of 1 is found to quench significantly. The luminescence lifetime of 1 also decreased markedly upon interaction with NaCN in the solid state (Figure S14, Supporting Information). The sensitivity of the metalloreceptors of 1 toward CN− was also checked by grinding a fixed amount of 1 with crystals of NaCN of varying dimensions. Instantaneous color changes occurred in each case, indicating the high sensitivity of 1 toward CN− in the solid state as well. All of these observations strongly suggest that the present Os(II) complex is a useful candidate for the detection of CN− in real solid samples. Cation-Sensing Studies of 1. The cation-sensing aspects of 1 can be exploited by utilizing the vacant terpyridine- and bipyridine-type coordinating sites. The spectral changes with 0.5 equiv of the cations in acetonitrile are shown in Figures 5 and 6 and Figures S15−S17 (Supporting Information). Among the cations, only Fe2+ induced a marked color change from greenish yellow to red-violet with evolution of a strong band at 572 nm due to the Fe(dπ) → tpy(π*) MLCT transition of tpyHbzim-dipy. On the other hand, ∼2.5 equiv of Cu2+ led to a colorless solution with the complete disappearance of both 1 MLCT and 3MLCT bands. Thus, 1 behaves as an efficient colorimetric sensor for Fe2+ and Cu2+ in MeCN. In contrast to the absorption spectrum, the emission spectrum shows significant quenching of luminescence in the presence of other metal ions. Thus, considering the emission spectral behavior, the present metalloreceptor is unselective toward the studied metal ions. Figure 6 displays the changes in the absorption and luminescence spectral profiles of 1 with Fe2+ and Cu2+ ions, whereas the spectral changes with other metal ions are presented in Figures S15−S17 (Supporting Information). Spectral saturation occurred in all cases (except Cu2+) upon addition of 0.5 equiv of the metal ions, indicating the formation of the trimetallic species [(bpy)2Os(dipy-Hbzimtpy)M(tpy-Hbzim-dipy)Os(bpy)2]6+ (1·M·1) (Scheme 1).

Figure 4. Changes in (a) UV−vis absorption, (b) steady-state emission, and (c) excited-state lifetime of 1 upon incremental addition of CN− in pure water. The excitation wavelength for steady-state emission was 490 nm.

with the steady-state emission spectral trends, the lifetime of the metalloreceptor in water decreased gradually with increasing CN− ion concentration. Significant lowering of the lifetime in the presence of CN− makes 1 a suitable and more reliable lifetime-based metalloreceptor for CN− in aqueous media (Figure 4c). Utilizing the titration data, the equilibrium constants (Ks) of the receptor−CN − interaction were calculated to be on the order of 105 M−1. The detection 11818

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Figure 5. (a) UV−vis absorption and (b) emission spectral changes of 1 in acetonitrile upon addition of different cations as their perchlorate salts. The excitation wavelength was 490 nm.

Figure 6. (a−c) UV−vis absorption and (d−f) emission spectral changes of 1 upon incremental addition of Fe2+ and Cu2+ as their perchlorate salts in acetonitrile. The excitation wavelength was 490 nm.

broad band at ∼700 nm probably due to LMCT transitions (Figure 6c and Figure S17, Supporting Information). The spectral changes in this process were also accompanied by the appearance of several isosbestic points (Figure 6b,c and Figure S17, Supporting Information). The quenching of the emission

Interestingly, Cu2+ ion induces a second change that is clearly visible in both the absorption and emission spectral profiles and to the naked eye. Further addition of Cu2+ ion induced the complete disappearance of both the 1MLCT and 3MLCT bands and, at their expense, the evolution of a weak and relatively 11819

DOI: 10.1021/acs.inorgchem.5b02300 Inorg. Chem. 2015, 54, 11813−11825

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Figure 7. Energy-level diagram depicting the dominant transitions that comprise the lowest-energy absorption bands for (a) 1 and (b) 1a in acetonitrile.

Interference Studied. The interference of 1 toward a particular anion and cation over the other ions was also investigated through absorption and emission studies, and the relevant spectral changes in terms of bar diagrams are presented in Figures S23−S30 (Supporting Information). Among the cations, visual color changes were observed only with Fe2+ and Cu2+ in the presence of the other cations, whereas among the anions, F−, CN−, and AcO− showed selectivity over other anions. Although very sensitive toward each F−, CN−, and AcO−, 1 lacks selectivity to differentiate the ions in the presence of the others in neat acetonitrile. In contrast to MeCN, the metalloreceptor exhibits high selectivity toward CN− even in the presence of an excess of each of the other studied anions in aqueous medium (Figure S30, Supporting Information). Finally, the interference of anion(s) in the cation-binding affinity and the interference of cation(s) in the anion-binding affinity of 1 were investigated. Addition of Fe2+ (up to 0.5 equiv) in the presence of 1 equiv of F− ion induced the emergence and gradual enhancement of the absorption band at 572 nm and quenching of the emission at 774 nm due to the formation of [(bpy)2Os(dipy-bzim-tpy)Fe(tpy-bzim-bipy)Os(bpy)2]4+, derived from the anion-induced deprotonated [(bpy)2Os(dipy-bzim-tpy)]+ complex (Figure S31, Supporting Information). Second, addition of F− up to 2 equiv in the presence of 0.5 equiv of Fe2+ enhanced the absorbance of the MLCT bands with concomitant red shifts of some of these bands. Further addition of F− (∼10 equiv) induced the gradual decrease and ultimate removal of the band at 572 nm (Figure S32, Supporting Information). 1·Fe·1 is weakly luminescent with its broad band at 774 nm. Addition of ∼2 equiv of F− led to quenching of the band along with a red shift. Continued addition of F− up to ∼10 equiv, on the other hand, led to a significant increase in the emission intensity of the said band. It is likely that the initial 2 equiv of F− induce the abstraction of

at 774 nm also continuously decreased in two successive steps, and complete quenching of the emission required 2.5 equiv of Cu2+ ion. In this second step, the initially formed [1·Cu·1]6+ was gradually oxidized by an additional 2.0 equiv of Cu2+ to form a [1·Cu·1]8+ complex, where the two Os(II) units were oxidized to Os(III) species.41 The second step is characterized by the complete disappearance of the color of the solution. Chemical oxidations of 1 and [1·Cu·1]6+ were also performed with Ce4+ ion through spectrophotometric methods (Figure S18, Supporting Information). The similarity of the spectral profiles confirms the oxidation of the Os(II) center in both the mono- and trimetallic complexes. Remarkably, the oxidation of the Os(II) center in 1 occurs with only Cu2+ ion. Moreover, this oxidation process is not affected by the presence of the other 3d divalent metal ions under investigation (Figure S19, Supporting Information). Although 1 interacts with all of the studied cations in aqueous medium, the visual color change was observed only with Fe2+ ion (Figures S20 and S21, Supporting Information). In acetonitrile, after oxidation of the Os(II) center, the reduced Cu(I) becomes stabilized by coordinating with acctonitriles, whereas in water, Cu(I) is not stabilized, and this why Cu2+ is unable to decolorize 1. In the structure of 1, four uncoordinated pyridyl nitrogen atoms are present, so there is a finite possibility of the protonation of one or more of these nitrogen atoms. We explored this possibility by performing spectral titration with a standard HClO4 solution (Figure S22, Supporting Information). Addition of HClO4 up to 1 equiv led to only a small increase in the intensity of the shoulder at ∼330 nm, keeping the MLCT region almost constant. Addition of acid also led to a gradual quenching of the luminescence as well as a decrease of the lifetime (Figure S22b,c, Supporting Information). The observed spectral changes are probably due to the protonation of one of three nitrogen atom on the terpyridine moiety. 11820

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Figure 8. (a) INHIBIT and OR logic gates based on 1 when monitoring the absorbances at 572 and 360 nm, respectively. (b) Corresponding truth table of the combined logic circuit. (c) Schematic representation of a combined logic circuit of INHIBIT and OR logic gates.

two NH protons in 1·Fe·1. Excess F− ion induces sequestering of Fe2+ ion from 1·Fe·1 by forming a stable complex of the type [FeF6]4−. Thus, after binding with cations or anions, the restoration of the initial state of the metalloreceptors is possible. Moreover, as demonstrated later, the various logic functions exhibited by the metalloreceptors rely on its coordination with a particular metal such as Fe2+ or Cu2+ followed by its reversible decomplexation by some specific anions such as F−. Computational Investigations. The geometries of 1 and its deprotonated form (1a) were optimized in MeCN (Figure S33, Supporting Information), and selected geometrical parameters and frontier orbitals along with their energies and compositions are presented in Figure S34 and Tables S2 and S3 (Supporting Information). From the electrostatic potential plots, it can be clearly observed that, upon deprotonation of the imidazole NH proton in 1, the additional negative charge in the complex is mainly distributed over pyridine, imidazole, and the phenyl-tpy unit of dipy-bzim-tpy, where the red color is indicative of an electron-rich area whereas the blue color corresponds to the electropositive sites of the molecule (Figure S35, Supporting Information). TD-DFT calculations were performed on the optimized geometries of 1 and 1a in MeCN to interpret the origins of electronic transitions within the complexes, and the spectral data are reported in Table S4 (Supporting Information). The involvements of different molecular orbitals (MOs) in the lowest-energy absorption band in the complexes are shown in Figure 7. By considering the contributions of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs), the calculated bands at 466 and 414 nm can be assigned as MLCT transitions. The electron density difference maps (EDDMs) and natural transition orbital (NTO) calculations also allow the two lowerenergy transitions corresponding to the S6 and S9 states for 1 to be confidently assigned as MLCTs, whereas the lower-energy bands (S5 and S10) in the case of 1a have mixed MLCT and intraligand charge-transfer (ILCT) character (Figures S36 and S37, Supporting Information). The experimentally observed red shift of the MLCT and ILCT band upon deprotonation of the imidazole NH proton in 1a is also reproduced by the calculated results. For the sake of comparison, the experimental and theoretical spectra of the complexes are overlaid in Figure S38 (Supporting Information). In both the protonated and deprotonated forms, there is a nice correlation between the experimental and calculated results. To locate the redox-active center in complex 1, DFT calculations on the one-electron-

oxidized form of 1 were also carried out, and the spin-density plot indicates the active involvement of osmium metal in the oxidation process (Figure S39, Supporting Information). From the excited-state [lowest-energy triplet excited state (T1)] calculations, we found the calculated emission peak at 772 nm and at 854 nm for 1 and 1a, respectively (Table S5, Supporting Information). The corresponding experimental results are at 774 for 1 and 833 nm for 1a and the correlations with the calculated values are really good. The MLCT nature of the emissions in both 1 and 1a was also verified from the analysis of MOs in the excited states and from the NTO analyses of the complexes on the triplet excited states based on their calculated transition density matrices (Figures S40−S42, Table S6, Supporting Information). Molecular Logic Gates, Keypad Lock, and Memory Device. Significant modulation of the spectroscopic and electrochemical properties of the bifunctional osmium-based receptor was achieved by selected anionic and cationic and proton inputs. In this section, we explore various roles, such as those of different logic gates, molecular keypad lock, and memory device, played by the metallorecptor by utilizing these multireadout output responses. INHIBIT Logic Gate. A particular combination of the logic functions of AND and NOT gate is essential for the construction of an INHIBIT logic gate. To demonstrate the INHIBIT logical behavior of complex 1, we chose two inputs, namely, Fe2+ (input 1) and F− (input 2), and utilized the absorbance signal at 572 nm as the output. The high value of absorbance at 572 nm is assigned as 1 (ON state), and the low value is designated as 0 (OFF state). When both of the inputs (Fe2+ or F−) are absent, the output is low, indicating the OFF state of the system. In the presence of input 1, a significant enhancement of the absorbance occurs at 572 nm, implying the ON state of the system, whereas in the presence of input 2, the absorbance value becomes very low (OFF state), thereby implementing the necessity of a NOT gate. In addition, the receptor acts in parallel with the absorbance output signals, implementing the required AND function. When we take the combination of the two inputs, the output absorbance value at 572 nm is again low, in accordance with the truth table in Figure 8. Therefore, by monitoring the absorbance at 572 nm upon sequential addition of Fe2+ and F−, the function of an INHIBIT logic gate can be achieved (Figure 8). OR Logic Gate. To mimic the function of an OR logic gate, the absorbance at 360 nm is used as the output response, and the inputs are the same as for the INHIBIT gate (Fe2+and F−). The output signal of an OR gate is low in the absence of both 11821

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Figure 9. (a) XNOR logic gate based on emission at 774 nm. The horizontal dotted line represents the threshold value. (b) Truth table. (c) Schematic representation of an XNOR gate.

Figure 10. (a) Three-input NOR logic gate when monitoring the luminescence at 774 nm. The horizontal dotted line represents the threshold value. (b) Corresponding truth table of the gate. (c) Schematic representation of a NOR gate.

Figure 11. (a) Truth table of the molecular keypad lock. (b) Schematic representation of the keypad to access a secret code by monitoring absorbance at 572 nm with different input sequences. (c) Visual color changes in the presence of different inputs.

NOR Logic Gate. At the end of our investigation of the twoinput logic system, we again took the same output and inputs (1.0 equiv of F− and 5.0 equiv of H+) to check the NOR logical behavior (Figure S43, Supporting Information). The NOR logic gate, which is the integration of the NOT and OR logic gates, is a universal gate and allows the combinatorial creation of all other Boolean operations. It should be mentioned that the output signal of the metalloreceptor is high (ON state) only in the absence of each or both of the inputs. In the presence of any of these inputs, the emission intensity of the receptor is substantially decreased. The observations correlate well with the function of a two-input NOR logic gate. Three-Input NOR Gate. So far, the output signal have originated from combinations of two ionic inputs. Now, we discuss the nature of the output signal in the presence of three ionic inputs. With the help of the three cations Co2+, Ni2+, and

inputs, indicating the OFF state of the system, but it is ON when either one or both inputs are present. From the truth table in Figure 8, the OR logical behavior of the complex 1 is clearly observed. XNOR Logic Gate. In the two previous cases, the absorbance value at a particular wavelength was chosen as the output signal. To investigate the XNOR logical behavior of 1, the emission intensity at 774 nm is considered as the output signal. The two ionic inputs are F− (input 1) and H+ (input 2) (Figure 9). When input 1 (1.0 equiv of F−) or input 2 (1.0 equiv of H+) is present, the emission intensity decreases substantially, but simultaneous addition of the inputs (1.0 equiv of F− and 3.0 equiv of H+) leads to a significant enhancement of the emission intensity. Thus, the XNOR logic function of the complex is verified. 11822

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Figure 12. (a) Truth table of the logic circuit. (b) Schematic representation of the reversible logic operation for the memory element with “writing− reading−erasing−reading” behavior. (c) Visual color changes in the presence of different inputs.

Cu2+ as input 1, input 2, and input 3, respectively, and using the luminescence intensity of the band at 774 nm as the output, a combinational serial NOR logic operation can be constructed (Figure 10). Molecular Keypad Lock. A molecular keypad lock that creates a secret password provides a method of protecting information at the molecular level. To create the keypad lock, the absorbance of 1 at 572 nm is monitored as a function of two ionic inputs, Fe2+ and Cu2+. Moreover, proper sequencing of the additions of Fe2+ and Cu2+ must be strictly maintained for the construction of a security keypad lock. For the generation of the input sequence as a password entry in a keypad lock, the Fe2+ and Cu2+ inputs are designated as “A” and “T” (Figure 11). In the absence of any ionic inputs, 1 exhibits no prominent absorbance band at 572 nm, indicating the OFF state of the system. The first input sequence A followed by T gives rise to a high absorbance value at 572 nm, implying the ON state of the system, and it creates the secret code ATM (M represents the ON state). On the other hand, when the input sequence is reversed, that is, the first input is T and the second input is A, the absorbance value is below the threshold limit, indicating the OFF state of the system. Thus, this sequence fails to open the keypad lock because of the incorrect entry TAX (X represented the OFF state). Therefore, the sequence-dependent inputs are essential for the construction of the molecular keypad lock, namely, ATM only in the present system. This observation indicates that only the authorized user who knows the exact password ATM can open the lock, which is a new approach for protecting information at the molecular scale. When the numerical digits (0−9) are used as personal identification numbers (PINs) in a two-digit password, a total of 90 different combinations are allowed, whereas the choice increases to 650 different combinations with individual letters (A−Z), each signifying a specific ionic input, are used as PINs. Thus, the cracking of the keypad lock becomes more difficult and, therefore, markedly improves the security of the molecular devices. Moreover, it is important to mention that the present molecular-level security device has advantages in the fact that it is easy to detect Fe2+ and Cu2+ by the naked eye from visual color changes. Molecular Memory Device. For the construction of memory devices that are capable of storing information, sequential logic circuits are essential. These operate through a feedback loop in which one of the outputs of the memory device serves as the input and is memorized as a “memory element”. For the

construction of a useful mimic of a memory element, we took F− and Cu2+ for the set (S) and reset (R) processes, respectively, and the absorbance at 490 nm as the output signal (Figure 12). When the set input is high (S = 1), the system writes and memorizes binary state 1, whereas the state is erased under a high value of the reset input (R = 1), which results in writing and memorization of the binary state 0. Sequential logic circuits having behaviors similar to those of traditional logic devices constructed using semiconducting materials would be beneficial to the development of molecular microprocessors of integrated logic circuits for memory elements in the future. It is important to note that the write−erase cycles could be repeated many times with the same solution of the complex without any appreciable reduction in intensity in the absorption spectra.

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CONCLUSIONS

In this work, we designed a new Os(II) complex based on a heteroditopic polypyridylimidazole ligand, whereby a free terpyridine moiety is available for the secondary interaction of transition-metal cations and the imidazole NH proton for interaction with selected anions. Taking advantage of these receptor units, the present system is capable of the selective recognition of Fe2+ and Cu2+ ions among other 3d divalent metal ions and selective anions such as cyanide in water through different channels. The reasons for selecting the Os(II) complex are that its absorption and emission spectra stretch to the near-infrared region and that its oxidation potential is low. One of the most interesting aspects of this study is that the complex shows very high selectivity toward cyanide ion in aqueous medium and also exhibits a very low detection limit of 10−8 M. In addition, we have successfully demonstrated the first lifetime-based sensor for cyanide ion using the Os(II) complex in aqueous solution as well as in the solid state. Another important aspect of the study is the mimicking of the functions of a molecular keypad lock and a memory device based on the optical and electrochemical responses of the complex toward selected anionic and cationic guests. To the best of our knowledge, this is the first example of a molecular keypad lock with an Os-based polypyridine complex with Cu2+ and Fe2+ as ionic inputs. 11823

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02300. UV−vis absorption, steady-state, and time-resolved luminescence spectra; electrochemical data; and molecular orbital pictures and spectra related to DFT and TDDFT calculations (PDF) CIF data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support received from the Council of Scientific and Industrial Research, New Delhi, India, through Grant 01(2766)/13/EMR-II. Thanks are due to the DST for providing the time-resolved nanosecond spectrofluorimeter in the PURSE program at the Department of Chemistry of Jadavpur University. S.K. is grateful to CSIR, and S.M. and P.P. are grateful to UGC for their fellowships.

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REFERENCES

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