Stimuli-Responsive Luminescent Bis-Tridentate Ru(II) Complexes

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Stimuli-Responsive Luminescent Bis-Tridentate Ru(II) Complexes toward the Design of Functional Materials Manoranjan Bar, Sourav Deb, Animesh Paul, and Sujoy Baitalik* Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 09/22/18. For personal use only.

S Supporting Information *

ABSTRACT: We report here the synthesis, characterization, and photophysics of two bis-tridentate Ru(II) complexes based on a heteroditopic ligand and thoroughly studied their stimuli-responsive behaviors toward the design of functional materials. Both complexes display emission at room temperature having lifetimes in the range of 0.5−70.0 ns, depending on coligand and solvent. Substantial modulations of absorption and emission spectral behaviors of the complexes were done upon interaction with anions, and anioninduced changes in the properties lead to recognition of selected anions in both organic and aqueous media. Photophysical properties of the complexes were also tuned by changing the pH of the medium, and pKa values in both ground and excited states were determined. The presence of free pyridine-imidazole motifs in the complexes leads to substantial modulation of the optical properties and switching of the emission properties upon interaction with selected cations as well as with protons. Fe2+, Co2+, Ni2+, and Cu2+ trigger emission quenching, while Zn2+ induces finite enhancement of the emission intensity in the complexes. In essence, modulation of the optical properties and switching of luminescence properties of the complexes were accomplished by a variety of the external stimuli such as anions, cations, protons, and pH, as well as solvent polarity. Importantly, the optical outputs in response to an appropriate set of stimuli were utilized to mimic the functions of two-input IMPLICATION, NOR, and XNOR logic gates.

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INTRODUCTION As many aspects of chemical synthesis are mastered to prepare complex materials, control of their properties upon interaction with different external stimuli such as light, pH, and chemical species represents a complementary step toward increasing the functionality and utility of these materials. 1,2 Stimuliresponsive materials exhibit widespread applications ranging from sensors3,4 to molecular computing, information displays, and molecular switches and machines.5−12 Transition-metalbased materials are much more advantageous over pure organic frameworks, as they can offer better tunability of the structural, optical, electrochemical, and electronic properties. Among various transition metals, coordination complexes based on Ru(II) metal are considered as potential building blocks for the design of suitable functional materials, as they possess outstanding photophysical and optoelectronic properties which primarily evolve from their metal to ligand charge transfer excited states.13−15 Generally, bipyridine- and terpyridine-type chelating units are employed for the synthesis of Ru(II) complexes. Complexes derived from bipyridine units (such as [Ru(bpy)3]2+) are better than their terpyridine counterparts (such as [Ru(tpy)2]2+) as far as the emission properties are concerned, whereas those obtained from terpyridine ligands are advantageous with respect to formation of achiral octahedral complexes but inferior with respect to their emission behaviors.16−22 [Ru(tpy)2]2+ and most of the other terpyridine complexes of Ru(II) are usually non© XXXX American Chemical Society

luminescent at room temperature, as the energy difference between 3MLCT and 3MC states is very close because of their distorted-octahedral geometries.23 For effective utilization of Ru(II)-terpyridine complexes as the components of molecular device, their room-temperature emission behaviors should be improved. Several synthetic strategies, such as insertion of electron-donating or -accepting substituents24−27 and extended π-conjugated systems onto the terpyridine unit,28−32 were followed for improvement of their emission behaviors by enlarging the energy difference between 3MLCT and 3MC states. In some cases, replacement of the pyridine moiety in the tpy unit by another heterocyclic ring yields better results by generating less distorted octahedral geometries.33−37 In the present work, our aim is to design complexes whose properties could be modulated/switched by different external stimuli, as controlling of their stimuli-responsive behavior represents a complementary step toward increasing the functionality and utility of these materials. To fulfill our goal, we report here the synthesis, characterization, photophysics, and electrochemistry of two bis-tridentate Ru(II) complexes based on a heteroditopic ligand, dipy-Hbzim-tpy, wherein the terpyridine moiety is connected with pyridine-imidazole groups via a phenyl spacer (Chart 1) and thoroughly studied their stimuli-responsive behaviors. The terpyridine site of dipyReceived: June 6, 2018

A

DOI: 10.1021/acs.inorgchem.8b01562 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

external stimuli such as anions, cations, protons, and pH, as well as solvent polarity. In addition, as a proof of principle of our motivation for studying this class of complexes, we utilized absorption and emission spectral responses with a specific set of ionic inputs and demonstrated that the present complexes are capable of mimicking several important logic functions such as those of two-input IMPLICATION, NOR, XNOR, and OR logic gates. The possibility of information processing at the molecular level was first explored by de Silva,46 who thereafter laid the foundation for emerging research field that ultimately led to the development of molecular computing.47−53 In this context, the design of suitable molecular and supramolecular systems capable of mimicking advanced logic functions is still a great challenge in the field of information technology.

Chart 1. Drawings of the Compounds under Study

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EXPERIMENTAL SECTION

Materials. The anions in the form of their tetrabutylammonium (TBA) salts and the cations in the form of their perchlorate salts were purchased from Sigma. Solvents and other chemicals were purchased from local vendors. The Ru(II) precursors, [(tpy-PhCH3)RuCl3] and [(H2pbbzim)RuCl3] have been prepared by refluxing RuCl3·3H2O with tpy-PhCH3 and H2pbbzim, respectively in 1:1 molar ratio in ethanol. The heteroditopic ligand, dipy-Hbzim-tpy and the Ru(II) complexes have been prepared by slight modification of our previous literature reports.52,53 Synthesis of [(dipy-Hbzim-tpy)Ru(tpy-PhCH3)](ClO4)2·2H2O (1). Ru(tpy-PhCH3)Cl3 (100 mg, 0.18 mmol) and dipy-Hbzim-tpy (130 mg, 0.25 mmol) were suspended in ethylene glycol and refluxed under argon protection for 5 h. The orange solution was filtered and poured into an aqueous solution of NaClO4·H2O (1.0 g in 5 mL of water), whereupon an orange precipitate was obtained. The compound was collected by filtration and purified by passing through a silica gel column and eluting with an acetonitrile/toluene (1/1, v/v) mixture. Upon evaporation, the microcrystalline solid that deposited was collected by filtration and recrystallized from an acetonitrile/ methanol (1/2, v/v) mixture under slightly acidic conditions. Yield: 135 mg, 62%. Anal. Calcd for C56H44N10Cl2O10Ru: C, 56.57; H, 3.37; N, 11.78. Found: C, 56.48; H, 3.42; N, 11.65. 1H NMR (300 MHz, DMSO-d6, δ/ppm): 13.00 (s, 1H, NH imidazole), 9.57 (s, 2H, 2H3′), 9.47 (s, 2H, 2H3′), 9.17−9.13 (m, 6H, 4H6 + 2H9), 8.74−8.69 (m, 4H, 4H8), 8.62 (d, 2H, J = 8.5 Hz, 2H7), 8.53 (t, 2H, J = 7.9 Hz, 2H11), 8.37 (d, 2H, J = 7.4 Hz, 2H12), 8.11−8.05 (m, 4H, 4H4), 7.91 (t, 2H, J = 6.7 Hz, 2H10), 7.56 (t, 6H, J = 6.4 Hz, 2H7 + 4H3), 7.31−7.25 (m, 4H, 4H5), 2.48 (s, 3H, CH3). ESI-MS (positive,

Hbzim-tpy is coordinated to Ru(II), forming heteroleptic complexes, and leaves two pyridine-imidazole coordination motifs free which can be utilized for modulation of the optical properties and switching of the emission properties upon interaction with selected cations as well as by protons. The imidazole NH protons in their secondary coordination sphere were utilized for tuning of properties upon interaction with anionic guests. Modulation of the photophysical properties was also made possible by changing the pH of the medium, and the pKa values in both the ground and excited states of the complexes were also determined. Although we have previously published some papers reporting photophysical and sensing behaviors of related complexes,38−45 we have not published any paper so far wherein the optical properties and in particular the emission properties of bis-tridentate Ru(II) complexes have been modulated/switched simultaneously by a wide variety of

Figure 1. 1H NMR spectra of the complexes in DMSO-d6. B

DOI: 10.1021/acs.inorgchem.8b01562 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Photophysical Data for Complexes 1 and 2 luminescence compound 1

solvent DCM

2 1

DMSO

2 1

MeOH

2 1 2

H2O

1

MeCN

2 1

absorption λmax/nm (ε/M−1 cm−1)

λmax/nm

494 (33333), 327 (sh) (65833), 312 (84166), 287 (72250) 493 (21666), 354 (54166), 336 (55833), 317 (64166), 288 (sh) (47500) 502 (29916), 372 (sh) (20750), 317 (60000), 291 (50833) 500 (18333), 348 (sh) (44166), 321 (54166), 293 (45000) 493 (25000), 311 (58333), 285 (53333)

635 674 660

CH3CN): m/z 477.06 (100%) [(dipy-Hbzim-tpy)Ru(tpy-PhCH3)]2+ and m/z 318.38 (74%) [(dipy-Hbzim-tpy+H)Ru(tpy-PhCH3)]3+. Synthesis of [(dipy-Hbzim-tpy)Ru(H2pbbzim)](ClO4)2·H2O (2). Complex 2 was synthesized by adopting the same procedure as for 1 using [(H2pbbzim)RuCl3] (95 mg, 0.18 mmol) instead of [(tpyPhCH 3 )RuCl 3 ]. Yield: 120 mg, 55%. Anal. Calcd for C53H38N12Cl2O9Ru: C, 54.92; H, 3.31; N, 14.50. Found: C, 54.85; H, 3.38; N, 14.44. 1H NMR (300 MHz, DMSO-d6, δ/ppm): 15.03 (s, 2H, NH imidazole, H2pbbzim), 13.03 (s, 1H, NH imidazole), 9.66 (s, 2H, 2H3′), 9.05 (d, 4H, J = 7.6 Hz, 2H6 + 2H9), 8.79 (t, 5H, J = 9.2 Hz, 2H8 + H17 + 2H18), 8.68−8.65 (m, 4H, 2H7 + 2H12), 8.45− 8.43 (m, 2H, 2H11), 7.97 (t, 2H, J = 7.6 Hz, 2H5), 7.85−7.82 (m, 2H, 2H10), 7.66 (d, 2H, J = 7.9 Hz, 2H3), 7.5 (d, 2H, J = 5.0 Hz, 2H19), 7.27−7.25 (m, 4H, 2H5 + 2H20), 7.01 (t, 2H, J = 7.8 Hz, 2H21), 6.08 (d, 2H, J = 8.1 Hz, 2H22). ESI-MS (positive, CH3CN): m/z 314.46 (100%) [(dipy-Hbzim-tpy+H) Ru(H2pbbzim)]3+ and m/ z 471.18 (4%) [(dipy-Hbzim-tpy)Ru(H2pbbzim)]2+. Caution! The complexes as their perchlorate salts are explosive and should be handled in small amounts with care. Physical Measurements. The details of different instruments used and experimental procedures adopted for carrying out various physicochemical measurements are given in the Supporting Information.

6.9 × 10

4.5 × 10−3 6.0 × 10−3 6.6 × 10−3

knr/s−1

5.7 × 10 , 1.7 × 106 6.0 × 105, 1.8 × 105 1.0 × 106

8.2 × 108 9.9 × 108 1.3 × 108, 3.9 × 107 1.6 × 108 3.4 × 107, 1.4 × 107 1.9 × 109, 3.9 × 108 1.4 × 108, 4.5 × 107 2.8 × 108 3.3 × 107

6

650 677

9.4 × 10−3 5.1 × 10−3

658

τ1 = 2.0

1.8 × 10−3

9.2 × 105

49.9 × 107

678

τ = 12.0

14.7 × 10−3

12.2 × 105

8.2 × 107

640

15.5

0.12

7.8 × 103

5.7 × 104

671

12.5

0.22

1.7 × 104

6.2 × 104

733

2

−3

2.2 × 105, 9.4 × 104 9.8 × 106, 1.9 × 106 1.6 × 105, 5.0 × 104 2.6 × 106 1.7 × 105

642

MeOH/EtOH (77 K)

τ1 = 1.2, τ2 = 4.0 τ1 = 7.5, τ2 = 25.0 τ = 6.0

kr/s−1

Φ

τ1 = 29.0, τ2 = 70.0 τ1 = 0.5, τ2 = 2.5 τ1 = 7.0, τ2 = 22.0 τ = 3.5 τ = 30.0

688

508 (32500), 350 (sh) (80000), 317 (101666), 285 (84166) 492 (20000), 313 (45750), 283 (39166) 505 (19916), 349 (sh) (60833), 317 (70000), 288 (57500), 244 (65000) 492 (40320), 351 (br) (48950), 330 (74610), 311 (88770), 282 (72840) 490 (16230), 347 (53780), 333 (51360), 315 (49900), 282 (sh) (36570)

τ/ns

4.9 × 10−3 1.1 × 10−3

singlet at 2.48 ppm (not displayed in Figure 1) accounting for three protons is assignable to the −CH3 group of the tpyPhCH3 unit in 1. The upfield doublet at 6.08 ppm for 2 is assignable to two H22 protons of the H2pbbzim unit because of the anisotropic ring current of the adjacent pyridine rings. The downfield singlet at ∼13.02 ppm in both 1 and 2 is assignable to the NH proton in tpy-Hbzim-dipy, while the singlet at 15.03 ppm in 2 is assignable to two NH protons in H2pbbzim. The singlet at 9.66 ppm corresponds to the H3′ proton of dipy-Hbzim-tpy in 2, while two closely situated singlets in 1 at 9.57 and 9.47 ppm correspond to the H3′ proton of the tpy unit of tpy-PhCH3 and dipy-Hbzim-tpy, respectively. Assignments of other protons are also provided in Figure 1. Mass Spectra. ESI mass spectra of 1 and 2 acquired in acetonitrile are presented in Figures S3 and S4 (Supporting Information), respectively. The isotopic distribution patterns of experimentally observed peaks correlate well with their simulated patterns in both cases. 1 exhibits two peaks with (m/ z 318.38 and 477.06) corresponding to tripositive ([(dipyHbzim-tpy+H)Ru(tpy-PhCH3)]3+) and dipositive ([(dipyHbzim-tpy)Ru(tpy-PhCH3)]2+) species, respectively. In the case of 2, the peak at m/z 314.46 corresponds to the tripositive ion [(dipy-Hbzim-tpy+H)Ru(H2pbbzim)]3+, whereas the peak at m/z 471.18 corresponds to the dipositive species [(dipyHbzim-tpy)Ru(H2pbbzim)]2+. Thus, both complexes become protonated in the matrix of mass spectra. Electronic Absorption and Emission Spectra. The photophysical properties of both 1 and 2 have been studied in a few solvents, and relevant data are given in Table 1, while selected UV−vis absorption and emission spectra are presented in Figure 2 and Figure S5 (Supporting Information). Overall, the spectral features are similar with small variations in peak position and intensity. By comparison with the data of the structurally related complexes, the band lying between 491 and 508 nm in the UV−vis spectra is assignable to Ru(dπ) → tpy/ H2pzbzim MLCT and the band located between 311 and 354 nm is assignable to intraligand charge transfer (ILCT), while the very intense band within 230−293 nm is assignable to

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RESULTS AND DISCUSSION Synthesis and Characterization. Both complexes were obtained in a straightforward way by reacting dipy-Hbzim-tpy with the respective ruthenium(II) precursor in ethylene glycol at elevated temperature and purified by column chromatography and recrystallization under weakly acidic conditions to retain the imidazole NH protons intact within the complexes. Characterization of the complexes was done through elemental (C, H and N) analyses and ESI mass and NMR spectroscopic measurements, and the results are provided in the Experimental Section. NMR Spectra. 1H NMR spectra of 1 and 2 acquired in DMSO-d6 display a fairly large number of peaks with some overlapping features (Figure 1). Tentative assignment of the proton resonances was done with the help of their {1H−1H} COSY NMR spectra (Figures S1 and S2 in the Supporting Information) and the spectra of related complexes.38−45 The C

DOI: 10.1021/acs.inorgchem.8b01562 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

solvent polarity as well as increased hydrogen bonding efficacy (MeOH and DMSO). Temperature-dependent emission studies were carried out for an understanding of the excited state deactivation dynamics of the complexes, and the outcomes are presented in Figure 3. It is observed that with an increase in temperature the lifetime, emission intensity, and quantum yield gradually decrease in both cases. Nonlinear least-squares fitting of lifetime vs temperature data to eq 1 yields different kinetic and thermodynamic parameters associated with the excited-state decay process.13,54 (τ(T ))−1 = (k1 + k 2 exp[−ΔE /RT ])

Figure 2. Overlay of absorption and emission spectra (λex 490 nm) of the complexes in MeCN at 298 K and in EtOH/MeOH (4/1, v/v) at 77 K.

/(1 + exp[−ΔE /RT ])

(1)

where k1 corresponds to the temperature-independent rate constant which is the summation of radiative (kr) and nonradiative (knr) rate constants from the 3MLCT state at 77 K, K2 corresponds to temperature-dependent rate constant that indicates the rate of population of the 3MC state from the 3 MLCT state, and ΔE corresponds to the activation energy involved in the process. The outcome of nonlinear fitting gives the values of k1, k2, and ΔE (Figure 3c,d). The calculated ΔE value is 3402 ± 40 cm−1 for 1, while for 2, the value is substantially higher, 4440 ± 70 cm−1. The value of k1 obtained from 77 K emission data, 6.4 × 104 s−1 for 1 and 8.0 × 104 s−1 for 2, was used for fitting purposes. The estimated k2 value is 2.9 × 1013 s−1 for 1 and 8.7 × 1013 s−1 for 2. First of all, an enormous increase in ΔE is observed in both complexes with respect to the parent [Ru(tpy)2]2+ (ΔE = 1500 cm−1).13 The increase is probably due to excited state delocalization induced by 2-(2-phenyl-5(pyridin-2-yl)-1H-imidazol-4-yl)pyridine onto the tpy moiety of the dipy-Hbzim-tpy ligand. Second, the energy difference

π−π* transitions. The MLCT maxima were found to be slightly red shifted upon an increase in solvent polarity and hydrogen bond formation capability. Excitation at the MLCT band gives rise to a broad emission band at room temperature with a maximum varying between 635 (DCM) and 660 nm (DMSO) for 1, while for 2, the range varies between 674 (DCM) and 688 nm (DMSO). The spectra at 77 K in ethanol/methanol (4/1 v/v) glass exhibit a small blue shift along with an increase in quantum yield (Figure 2). Emission maxima at 77 K were utilized to calculate the zero−zero excitation energy (E00) of the 3MLCT excited state, and the estimated value was found to vary between 1.94 and 1.84 eV. Excited state lifetimes vary between 0.5 and 70.0 ns at room temperature, depending upon the solvent, while they vary between 12.5 and 15.5 μs at 77 K (Figure S6 in the Supporting Information). Similarly to the UV−vis absorption, the luminescence maximum is also red-shifted with increased

Figure 3. Decay profiles (λex 450 nm) for 1 (a) and 2 (b) as a function of temperature in MeCN. Temperature-dependent lifetime data along with the values of different parameters and the corresponding nonlinear fits are shown in (c) and (d) for 1 and 2, respectively. D

DOI: 10.1021/acs.inorgchem.8b01562 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Absorption (a, b) and emission (λex 490 nm) (c, d) spectral changes of 2 as a function of pH. The insets of (a) and (b) show the change in absorbance, while the insets of (c) and (d) indicate the change in lifetime with pH.

among 3MLCT and 3MC states is a delicate function of the electronic environment of the coligand as reflected in the lifetime of the complexes. It is quite possible that the energetic position of the 3MC state remains almost constant, while the 3 MLCT state is lowered substantially leading to a decrease in 3 MLCT → 3MC surface-crossing efficiency. Both complexes exhibit biexponential decay at room temperature. The first short-lived component probably corresponds to deactivation of the 3MLCT level, while the second long-lived component probably appears from deactivation of the equilibrated triplet state of 3MLCT and 3IL of dipy-Hbzim-tpy. Emission spectra at 77 K also exhibit vibrational features with spacings (1216 cm−1 for 1 and 1279 cm−1 for 2) that indicate quite a substantial contribution from the triplet IL emission in the emission spectra at 77 K (Figure 2). Thus, an important outcome of the present work is that a new type of luminescent heteroleptic bis-tridentate Ru(II) complex having moderately long lifetime at room temperature with dangling pyridine-imidazole motifs has been designed. Electrochemical Properties. The electrochemical behaviors of the complexes have been examined by CV and SWV in CH3CN solutions (Figure S7 in the Supporting Information). One reversible oxidation and four reversible/quasi-reversible reductions were found for both complexes. The couple with E1/2 = 1.3 V for 1 and E1/2 = 1.0 V for 2 corresponds to the RuII/RuIII oxidation process, while the couples with their E1/2 values ranging between −0.88 and −2.09 V correspond to the reductions of the pyridine moieties in the complexes.55 pH-Induced Changes in the Photophysical Properties and Ground- and Excited-State pKa Values of the Complexes. The imidazole NH protons residing on tpyHbzim-dipy and H2pbbzim groups become acidic and could be easily removed upon an increase in pH. In order to modulate the ground and excited state properties, we performed

absorption and emission spectral measurements as well as acquired lifetimes of the complexes within the pH range of 2.0−12.0 by using Robinson−Britton buffer.56 Figure 4 and Figure S8 (Supporting Information) display UV−vis absorption and luminescence spectral change with pH. Complex 1 exhibits a one-step change while 2 exhibits a twostep change in the spectral profiles. The single-step change in 1 corresponds to dissociation of the NH proton on tpy-Hbzimdipy (Figure S8, Supporting Information). For the two-step process in 2, the first step change in the pH range of 3.0−6.0 corresponds to dissociation of one of the two benzimidazole NH protons attached to the H2pbbzim moiety, while the second step change occurring within the pH range of 6.0−9.0 is due to the dissociation of the second NH proton from H2pbbzim (Figure 4). The proton associated with the tpyHbzim-dipy moiety in 2 is not dissociated within the studied pH range. A number of isosbestic points were observed in each case. Ground-state pKa values have been estimated from absorbance vs pH data utilizing eq 2.56 pH = pK − log(A − A 0)/A f − A 0

(2)

Figure S8b (Supporting Information) shows substantial quenching of luminescence at 655 nm for 1 with an increase in pH. For 2, quenching is observed in two consecutive steps (Figure 4c,d). In the first step, quenching occurs with the position of luminescence maximum remaining constant, while quenching is accompanied by a gradual red shift of the emission maximum in the second step. Excited state decay curves along with their lifetimes as a function of pH are shown in the insets of Figure S8b (Supporting Information) and Figure 4c,d. The lifetimes of both 1 and 2 decrease gradually with an increase in pH. Lifetime vs pH data were used to calculate the excited state pKa (pKa*) values.57 E

DOI: 10.1021/acs.inorgchem.8b01562 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 5. UV−vis absorption and emission (λex 490 nm) spectral changes of 1 ((a) and (c), respectively) and 2 ((b) and (d), respectively) in MeCN upon addition of different anions. The visual color changes are shown in the insets of (a) and (b).

Figure 6. UV−vis absorption and emission (λex 490 nm) spectral changes of 1 in MeCN upon addition of F− ((a) and (c), respectively) and H2PO4− ((b) and (d), respectively). The insets show the fits of the experimental absorbance and luminescence data to a 1:1 binding profile.

pK a* = pH + log τacid /τbase

Cl−, Br−, I−, AcO−,CN−, SCN−, and H2PO4− were employed in this study. The spectral changes in the presence of 10 equiv of anions are shown in Figure 5. Among the anions, only F− and CN− induce a red shift of the MLCT band in 1. For 2, the extent of the shift is much greater in comparison to that in 1, and the observed shift takes place with F−, AcO−, CN−, and H2PO4−. Interestingly, the anion-induced spectral changes are also reflected in their visual color changes (Figure 5). The emission band at 658 nm for 1 undergoes a minor change with the other anions except for F−, AcO−, CN−, and H2PO4−. With F− and CN−, the emission gets fully quenched, while the extent of quenching is much less with AcO−. In contrast, H2PO4−

(3)

where the pH is at the inflection point of the emission intensity vs pH curve. τacid corresponds to the lifetime of the protonated form, while τ base corresponds to the lifetime of the deprotonated form. pKa* values are found to be somewhat greater with respect to their pKa values, and greater pKa* values suggest that the MLCT state is primarily located on the dipy-Hbzim-tpy moiety in the complexes.58−60 Anion-Induced Modulation of the Photophysical Properties of the Complexes. Both acetonitrile and aqueous solutions of the complexes and TBA salts of F−, F

DOI: 10.1021/acs.inorgchem.8b01562 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 7. UV−vis absorption ((a) and (b)) and emission (λex 490 nm) ((c) and (d)) spectral changes of 2 in MeCN upon addition of H2PO4−. The insets show the fit of the experimental absorbance and luminescence data to a 1:1 binding profile.

Table 2. Equilibrium/Binding Constantsa,b (K) for 1 and 2 toward Various Anions in MeCN and Water at 298 K

induces substantial emission enhancement in 1. On the other hand, complete emission quenching occurs in 2 with F−, AcO−, CN−, and H2PO4−. To obtain quantitative information, we carried out titration measurements with selected anions, and the spectral profiles are presented in Figures 6 and 7 and Figure S9 (Supporting Information). The MLCT band intensity at ∼500 nm for 1 gradually diminishes and finally saturates with 1 equiv of the anions in a single step. In contrast, diminution and evolution of the MLCT band in 2 takes place in two consecutive steps with a red shift of the band in each step and final saturation occurs with 10 equiv of the anions. The differences in spectral behaviors arise due to variation in both the number and extent of the acidic character of the NH protons in 1 and 2. From the pH study, it is evident that benzimidazole NH protons of the H2pbbzim moiety in 2 are more acidic and one of these NH protons ia removed from the complex initially, while the second NH proton is dissociated in the second step. The absorption spectral profile in each step is accompanied by several isosbestic points. The equilibrium constants of the complex−anion interaction process have been estimated with the help of their titration data, and the calculated values lie on the order of 106 M−1 (Table 2). On the emission side, gradual emission quenching takes place in 1 on addition of both F− and CN− and quenching stops upon reaching 1 equiv of the anions. In contrast to the case for both F− and CN−, emission enhancement occurs in the presence of H2PO4− and the enhancement continued up to addition of 1 equiv of the anion. The observed differences in spectral responses among the anions arise because of their differences in charge density, size, and basicity. For 2, emission quenching occurs in two consecutive steps and the quenching processes were complete upon addition of ∼10 equiv of the anions. Equilibrium/binding constants were also calculated from emission titration profiles, and fairly good correlation was observed between the values calculated by the two methods (Table 2). The shift of the MLCT band to longer wavelength is probably because of second-sphere hydrogen-bonding inter-

2 anion/cation F− H2PO4− Fe2+ Ni2+ Cu2+ Zn2+ H+ CAN CN− SCN− F− H2PO4− Fe2+ Ni2+ Cu2+ Zn2+ H+ CAN CN− SCN−

1K

K1

From Absorption Spectra (MeCN) 1.63 × 106 2.03 × 106 6 1.30 × 10 2.08 × 106 1.84 × 106 1.55 × 106 1.68 × 106 1.96 × 106 6 2.80 × 10 3.76 × 106 6 1.10 × 10 1.13 × 106 2.12 × 106 2.22 × 106 2.28 × 106 From Absorption Spectra (Water) 3.40 × 104 2.28 4.48 × 104 3.71 From Emission Spectra (MeCN) 2.50 × 106 2.16 × 106 6 1.18 × 10 2.02 × 106 2.70 × 106 1.62 × 106 6 1.10 × 10 1.57 × 106 6 1.61 × 10 1.06 × 106 1.27 × 106 1.24 × 106 2.23 × 106 2.00 × 106 2.07 × 106 From Emission Spectra (Water) 1.09 × 104 1.03 2.51 × 104 3.98

K2 1.22 × 106 1.36 × 106

× 104 × 104 1.78 × 106 1.41 × 106

× 104 × 104

a

tert-Butyl salts of the anions and perchlorate salts of the cations were used for the studies. bEstimated errors were