Bipyridine Ligands - ACS Publications - American Chemical Society

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Mononuclear Ru(II) Complexes of an Arene and Asymmetrically Substituted 2,2′-Bipyridine Ligands: Photophysics, Computation, and NLO Properties Ramakrishna Bodapati,† Chakradhar Sahoo,‡ Mahesh Gudem,§ and Samar K. Das*,† †

School of Chemistry, University of Hyderabad, Central University P.O., Hyderabad 500 046, Telangana, India School of Physics, University of Hyderabad, Central University P.O., Hyderabad 500046, Telangana, India § Department of Chemistry, Indian Institute of Science Education and Research Pune, Dr. Homi Bhabha Road, Pune 411008, Maharashtra, India

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‡

S Supporting Information *

ABSTRACT: By using monosubstituted 2,2′-bipyridine asymmetric ancillary ligands with different electron donor moieties and an arene ligand (p-cymene), we successfully designed and synthesized six Ru(II) compounds (RuBPY1−6) that belong to a piano-stool-type system. The NLO properties of the synthesized complexes have been studied in both solution and the solid state. The electronic spectra of these compounds show a broad feature with two absorption bands in the visible window (350− 650 nm). RuBPY1−6 complexes exhibit NIR emission spectra in the solution state (at >720 nm), the maxima of which are bathochromically shifted in comparison to those of the concerned ligands. Interestingly, compounds RuBPY1−6 show NIR emission in their solid state too. Title compounds RuBPY1−6 have lifetimes in the range of 0.2 to 0.9 ns. An important feature of this work is the π-association of the p-cymene ligand to Ru(II) in the synthesized complexes; the π complex is formed by breaking the symmetry of p-cymene, found in the starting precursor (Ru2 dimer). This has been established by NMR spectral studies along with DFT calculations on the 1H NMR spectra. We could derive the molecular structure of the cationic part of this system by density functional theory (DFT), associated with 1H NMR spectral studies. The minimum energy structures for RuBPY1 and RuBPY2 have been optimized at DFT/B3LYP along with the LANL2DZ basis set for ruthenium atoms. These optimized structures are further considered to calculate the excited state properties using the TDDFT method. The electrochemical studies of the complexes, investigated in acetonitrile solution, show that this system is associated with a welldefined Ru(III)/Ru(II) reversible couple, rarely observed for a Ru(II) piano-stool-type compound, along with a feature of irreversible ligand oxidation. The absorption cross-section values, obtained from the two-photon absorption studies of title compounds RuBPY1−6, are worth reporting and lie in the range of 3−28 GM (in the femtosecond case).

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section (σ2) values.5 Transition metal coordination complexes with nitrogen-donor ligands, such as bipyridines and oligopyridines, are identified as important NLO materials. These metal complexes are commonly associated with intense lowenergy charge transfer transitions including metal to ligand and intraligand charge transfer transitions.6 The N-donor containing bipyridyl molecules are generally effective ligands for the synthesis of octupolar coordination compounds, e.g., [M(bpy)3]n+ of D3 symmetry and [M(bpy)2]n+ of D2d symmetry

INTRODUCTION In recent years, there has been widespread attention toward nonlinear optical (NLO)1 effects, especially two photon absorption (2PA), mostly because of their potentiality to be used in exciting technological applications, for example, optical power limiting devices, microfabrication, up-conversion lasing, bioimaging, three-dimensional data storage systems, fluorescent probes, and photodynamic therapy. Consequently, an extensive effort has been made to design and synthesize new compounds exhibiting high 2PA activity.2−4 Donor (D)− acceptor (A) associated π-conjugated systems are known to exhibit efficient two photon absorptions having larger cross© XXXX American Chemical Society

Received: April 26, 2019

A

DOI: 10.1021/acs.inorgchem.9b01235 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthesis Route for the Formation of Compounds RuBPY1−6

the relevant photoexcited state.22,23 Thus, bipyridine- and terpyridine-containing compounds have extensively been used as organic ligands in metal complexes or in organometallic compounds24−27 over the past few decades in the area of molecular materials, one of the reasons being that they easily bind with different metals with various oxidation states in addition to the ligands, which can lead to tunable linear optical properties. The Ru complexes with red emission, centered around 620 nm, also form an interesting class of electroluminescent materials, which are easily synthesized, displaying high external quantum efficiency28−30 at low voltages with good processability.31 From our laboratory, we reported a series of symmetrically substituted-2,2′-bipyridine derivatives (MS 1−8),32 and we synthesized the corresponding iridium(III) cyclometalated complexes33 that show NLO properties. Later, we described asymmetrically substituted ones by simple modification of the π-skeleton, including their photophysical properties.34 In the present work, we have used these asymmetrically substituted N-donor molecules (π-conjugated 2,2′-bipyridines)34 to form coordination complexes with Ru(II) along with a π-complexing arene ligand resulting in RuBPY1−6 (see Scheme 1), because aryl units, attached to a metal center, are known to extend the triplet lifetime of the concerned metal complex.35,36 We have described here the photophysics, electrochemistry, and NLO properties of this series of mononuclear ruthenium(II) compounds RuBPY1−6 of an arene and asymmetrically substituted 2,2′-bipyridine ligands as shown in Scheme 1. Even though several Ru(II) coordination complexes exhibit two photon absorption NLO activities, we have not come across any NLO active Ru(II)piano-stool complexes. However, Mendes and co-workers recently reported a theoretical approach (through DFT calculations) on a piano-stool type Ru(II) complex to demonstrate second order NLO properties.37 This computational report37 and our recent findings of donor−acceptor bipyridine ligated metal coordination complexes exhibiting NLO properties33 prompted us to investigate two photon absorption NLO properties of the title Ru(II) piano-stool complexes RuBPY1−6.

(M = Ru, Fe, Zn, etc.). With careful selection of substituents on the bipyridine ligands, these metal complexes exhibit first order hyperpolarizability (β) values that are quite comparable to the β values, shown by other dipolar systems.7 The conjugated π-bond-containing molecules exhibit high 2PA coefficient (β) values. The insertion of electron donors and electron acceptors in a molecule leads to an asymmetric molecular distribution, which may further increase the β value as well as the NLO properties. In this field, metal complexes have more advantages compared to organic ligands, because of their 3MLCT excited state having a long luminescence lifetime. The access via 2PA to the 3MLCT state could therefore generate two-photon excited fluorescence (TPEF) and thereby would have potential fluorescence-based applications, such as biological imaging and optical power limiting. Even though ruthenium bipyridine based piano stool complexes are used in multidisciplinary applications, e.g., organic light-emitting diodes (OLEDs),8,9 light-emitting electrochemical cells (LECs),10 photodynamic therapy, and multiphoton fluorescence microscopy, until now, there have only been a few reports on MLCT transitions by 2PA for ruthenium(II)11−13 piano stool analogues. Coe et al. reported the large molecular hyperpolarizabilities from ruthenium(II) donor−acceptor coordination compounds. They described a series of transruthenium(II) compounds having pyridyl N-donors as ligands that exhibit solvatochromic MLCT transitions {(dπ(Ru) → π*(acceptor)} in the visible region.14,15 Ruthenium complexes have widely been employed and intensively considered as dyes for DSSCs since very the early years and are considered the most efficient class of sensitizers up to now.16−18 By using the bipyridine based ruthenium dyes, the highest photovoltaic conversion efficiency for DSSCs has been realized. Organoruthenium(II) arene complexes are known for their successful utilization as anticancer agents and antibacterial drugs, and some of the complexes also exhibited antiproliferative activity in vitro19−21 and/or in vivo. In addition, bipyridine based π-conjugated chromophores have been employed as photoluminescent materials, e.g., nonlinear optics (NLO),1 organic light-emitting diodes (OLEDs),8,9 etc. These classes of organic π systems absorb electromagnetic radiation through intramolecular transfer (ICT) and exhibit emission involving B


Article

Inorganic Chemistry

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RESULTS AND DISCUSSION Synthesis and Characterization. The synthesis route to prepare the title arene-ruthenium bipyridine compounds RuBPY1−6 in this work is presented in Scheme 1. Asymmetrically substituted π-conjugated ligands 1−6 (Scheme 1) were synthesized according to our previous report,34 to synthesize the targeted ruthenium complexes. The reaction between the bipyridine ligands34 1−6 and η6-p-cymene ruthenium(II) chloride dimer in methanol solvent under a nitrogen atmosphere leads to the formation of a new bond between nitrogen atoms of bipyridne and ruthenium, while the ruthenium dimer breakage takes place. The title complexes were then subjected to column chromatography (for their purification). The synthesized ruthenium complexes RuBPY1−6 were characterized through NMR, mass spectrometry (HRMS), and IR spectral studies including elemental analysis. The compounds were isolated as deep red solids in good yields. Complexes RuBPY1−6 are freely soluble in common organic solvents. NMR Spectroscopy. The 1H NMR spectroscopy on the title compounds shows that the splitting of each of the CH resonances by neighboring protons gives the 3JHH coupling constant of ca. 16 Hz for the vinylic HCCH (of bipyridines); this is evidence that the synthesized materials have an E configuration. The bipyridine protons H6,6′ are largely deshielded, leading to a downfield shift from 8.5 to 9.6 ppm of their two singlets in the relevant 1H NMR spectrum. In the case of H3,3′ protons, a little upfield shift from 8.2 to 8.0 ppm is observed. The 1H NMR spectrum of the starting precursor, the {Ru2} dimer (Scheme 1), shows a doublet at around 5 ppm for the two equivalent pairs of protons of pcymene (see Figure S27, Supporting Information). But upon its π-association with Ru(II) (see Scheme 1), the story becomes different. The 1H NMR spectra of the synthesized ruthenium bipyridine complexes RuBPY1−6, in contrast to the 1H NMR spectrum of the metal source, dichloro(pcymene)ruthenium(II) dimer, clearly show four different signals (Figure S27, Supporting Information) for the η6-pcymene ligand. This means that the mirror symmetry of the η6p-cymene of the starting precursor (Ru2 dimer) is lost along the axis, resulting in four different responses for four different protons of the p-cymene and two different doublet responses for the methyl groups of the isopropyl group that are merged as a triplet.38 Likewise, 13C NMR spectra of the synthesized Ru(II) complexes show that the p-cymene gives six signals instead of four, and the isopropyl group exhibits three individual signals instead of two signals when compared to those of the metal source dichloro(p-cymene)ruthenium(II) dimer. A computational analysis on the 1H NMR spectra of the title compounds has been performed to investigate this symmetry breaking reaction (vide infra). All the complexes RuBPY1−6 exhibit clean isotopic mass spectrometry along with simulated patterns. The representative mass spectra for RuBPY1 and RuBPY2 are shown in Figure 1. Photophysical Properties. The photophysical behaviors of synthesized complexes RuBPY1−6 were investigated in three different polar solvents, viz., dichloromethane (DCM), methanol (MeOH), and dimethylformamide (DMF) at room temperature. In DCM solvent, the intense absorption bands are located at 300 to 380 nm, and a broad absorption is observed in the range of 410−560 nm. The relevant absorption maxima fall in the region of 465−520 nm as shown in Figure

Figure 1. Mass spectrometry: experimental data (top) and isotopic pattern simulations (bottom) of complexes RuBPY1 and RuBPY2.

2a. This absorption band is found to be shifted bathochromically with respect to the absorption of the concerned free ligand. This absorption is due to an intramolecular LMCT (ligand to metal charge transfer) transition (vide inf ra, computation studies). Various π → π* electronic transitions within the bipyridine ligand moiety are responsible for the higher-energy absorption bands in the electronic absorption spectra.20,21 As already mentioned, the title compounds show a bathochromic shift of around 60−113 nm in comparison to that shown by the respective free ligand. Compound RuBPY2, among all, exhibits the highest bathochromic shift of ∼113 nm compared to the corresponding ligand 2.34 The electronic spectra of RuBPY1−6 in polar solvents, e.g., in MeOH and DMF, exhibit a red shift with respect to the free ligand, but those exhibit a blue shift in DCM solvent as shown in Figure 2b. This suggests that the exited state of the metal complexes gets destabilized in polar solvents. Compounds RuBPY2, RuBPY4, and RuBPY6 having cyclic pyrrolidine donors exhibit electronic absorption maxima that are bathochromically shifted by almost 18−28 nm as compared to those shown by RuBPY1, RuBPY3, and RuBPY5, having acyclic dibutylamino donor ligands, a trend similar to that observed in the case of C


Article

Inorganic Chemistry

Figure 2. Normalized UV−vis absorption and emission spectra: (a) RuBPY1−6 in DCM, (b) solvatochromic effect of RuBPY1, (c) solid state RuBPY1−6 as a thin film, (d) ligand 1 vs complex RuBPY1 in DCM.

Table 1. Photophysical Properties of the Synthesized RuBPY1−6 in DCM Solvent compound

solvent

absorption λmax (nm)

emission λmax(nm)

ε (M−1 cm−1)

RuBPY1

DCM MeOH DMF DCM MeOH DMF DCM MeOH DMF DCM MeOH DMF DCM MeOH DMF DCM MeOH DMF

497 468 470 518 490 490 465 443 453 492 450 464 502 474 481 520 473 478

727 703 635 714 707 637 690 678 637 640 637 635 751 703 637 703 680 637

48000

RuBPY2

RuBPY3

RuBPY4

RuBPY5

RuBPY6

46000

84000

38000

54000

73000

stock shiftΔv̅ (cm−1) 6365 7143 5528 5300 6264 4710 7013 7824 6377 4700 6524 5803 6605 6873 5092 5006 6436 5222

Φem

τ (ns)

0.20

0.27

0.16

0.30

0.19

0.28

0.15

0.95

0.21

0.28

0.15

0.38

various donating moieties, and by the utilization of π-extended ancillary ligands, we could demonstrate the emission toward the near IR region. The emission spectral studies of the present compounds RuBPY1−6 show a broad emission with a wide range (640−745 nm) in solution (Figure 2a) and in the solid state (Figure 2c). The emission maxima of RuBPY1−6 are red-shifted by ∼150 to 200 nm compared to those of corresponding ligands 1−6.34 Figure 2d shows this comparison for compound RuBPY1 and the relevant ligand 1 as a representative example. The emission intensity for the Ru(II) complexes was found less intensive compared to that of the concerned free ligands. Quantum yields of the synthesized complexes were measured using rhodamine 6g as a reference; the concerned values fall in the range of 0.15−0.21. The fluorescence life times of the synthesized complexes were investigated, and the experimental values fall in the range of 0.2 to 0.9 ns as shown in Table 1.

complexes versus free ligands. The emission spectra of the synthesized Ru(II) complexes are examined in both solution (Figure 2a) and the solid state (thin film, Figure 2c); the emission spectral data are given in Table 1. As already mentioned, the synthesized Ru(II) complexes exhibit electronic transitions in the visible region (465−520 nm) that are assigned to LMCT transitions, as obtained from timedependent DFT calculations (vide infra). This is in contrast to the usual MLCT transitions, generally found with Ru(II)bipyridine compounds. The molar extinction coefficients (ε) for the synthesized complexes (RuBPY1−6) have been calculated (Table 1), and the values were found in the range of 38 000−84 000 M−1 cm−1. To date, various investigations have been performed on the electroluminescence of mononuclear ruthenium complexes; a greater part of them exhibit an orange-red emission in the range of 600−650 nm.39 In the present study, we have tuned the ligands by the substitution of D


Article

Inorganic Chemistry

Figure 3. Minimum energy structure for the RuBPY1 and RuBPY2 optimized at DFT/B3LYP along with the LANL2DZ basis set for ruthenium atoms and 6-31G(d,p) for the remaining atoms in DCM solvent. Both side and top views are given for clarity, and relevant protons are denoted as H1, H2, H3, and H4.

Computational Analysis on NMR Study. It has already been mentioned that the experimentally obtained 1H NMR spectra of the synthesized ruthenium bipyridine complexes RuBPY1−6 show four signals for the η6−p-cymene protons instead of a doublet, shown by the starting precursor (dichloro(p-cymene)ruthenium(II) dimer) indicating that mirror symmetry of the p-cymene η6 ligand along the axis is broken, which leads to four different signals corresponding to the four protons of p-cymene aromatic ring as shown in Figure S27 (Supporting Information) and two different doublet signals for the methyl groups of the isopropyl group which are merged as a triplet. In order to comprehend this intriguing observation, we have performed electronic structure calculations on the cationic molecular moiety of the synthesized bipyridine complexes (see Scheme 1 for structural drawing of the cationic moiety). We determined the minimum energy structure for the RuBPY1 and RuBPY2 at DFT/B3LYP along with LANL2DZ basis set for ruthenium atoms. We performed the calculations using DFT with Becke’s three-parameter hybrid method and the Lee−Yang−Parr exchange-correlation functional theory (B3LYP)40−43 along with the LANL2DZ basis set44 for ruthenium atom and the standard 6-31G(d,p) basis set for the remaining atoms. The solvent (dichloromethane) effects have been included using PCM model. Effective core potential correction for ruthenium has also been included. Gaussian 09 quantum package45 has been used for all the calculations. We optimized the RuBPY1 and RuBPY2 complexes, that are given in Figure 3. At minimum energy structure, the p-cymene group is significantly tilted in comparison to the plane, formed by the remaining part of the complex. In other words, the p-cymene group is not oriented symmetrically with respect to the bypyridine moiety. This can be clearly seen from the side view of the complexes given in Figure 3. Due to the loss of the symmetry, four hydrogen atoms on the aromatic ring of the pcymene group interacts differently with the molecular environment, thereby resulting in different nucleus shielding for these four hydrogen atoms. Consequently, the 1H NMR spectra show four different signals for these protons. This was additionally confirmed by the theoretically calculated 1H NMR spectra as shown in Figure S28 (Supporting Information),

which shows four different peaks for the corresponding four protons on the aromatic ring of p-cymene group. In order to comprehend the photophysical behaviors of the title compounds, excited state calculations have been performed on RuBPY1 and RuBPY2 using time-dependent DFT method. Based on the computed vertical energies and the oscillator strengths at the Frack-Condon (FC) geometry, S1 state has been found to be the bright state for both the molecules and this state can be characterized as the ligand to metal charge transfer (LMCT) state. We also have obtained the minimum energy structure on the lowest singlet excited state. Figure 4 schematically displays the absorption and emission data along with the state characterizing molecular orbitals for RuBPY1 (blue) and RuBPY2 (green). The

Figure 4. Schematic representation of the absorption and emission properties of RuBPY1 (blue) and RuBPY2 (green) computed at the TDDFT/B3LYP/6-31G(d,p) level of theory along with the LANL2DZ basis set for the ruthenium atom. Corresponding experimental values (in electron volts) are given in parentheses. Molecular orbitals involved in the electronic transition are also shown. E


Article

Inorganic Chemistry

density toward ruthenium center. The current height of the Fc+/Fc couple is comparable to that of the Ru3+/Ru2+ response indicating one electron transfer for the ruthenium couple.48 Two-Photon Absorption Spectroscopy: NLO Properties. Synthesis of multiphoton absorbing materials has been an advanced topic of inorganic and organic synthesis. These compounds are widely used in many industrial and engineering applications, such as optical limiting materials,49 photoswitching materials, etc.50 Of late, there has been significant interest in ruthenium bipyridine complexes with nonlinear optical properties, as they are used in solar cells and OLEDs. In recent times, we have reasonably understood some key structural features that could increase optical nonlinearity, for instance, a system with a large number of delocalized πelectrons.51 Irrespective of the high nonlinearity that the material has, it is quite impossible to observe them under normal electric fields, as these nonlinearities show up only in the presence of strong electric field intensities, which are characteristic of the femtosecond pulse. Here, we use a femtosecond laser, to create a strong electric field in the medium and monitor the transmittance of the pulse to infer optical nonlinearities. Out of several methods, like, DFWM, ZScan, etc.,52 we have used the Z-Scan method to evaluate the optical nonlinearities, as it gives both the sign and magnitude of the nonlinear refractive index as well as multiphoton absorption coefficient. The Z-scan technique depends on the spatial distortion of the beam profile (here, a focused Gaussian beam). The detailed implementation of this experiment can be found elsewhere.53 The absorption spectra of the series of synthesized compounds indicate that the compounds absorb in the wavelength range of 465−520 nm. Ti:sapphire laser has been used that generates 1000 pulses/sec, 100 fs pulse width, 800 nm wavelength to study the two-photon absorption coefficient (β) and nonlinear refractive index (n2). The Z-scan experiments (with open aperture and closed aperture) were performed for the measurement of β and n2 respectively. A lens of 12 cm focal length focuses a single laser beam with a fluence of 150 GW/cm2. The prepared solution sample of 0.5 mM is filled in a 1 mm cuvette and is scanned across the focal volume for the measurement of intensity dependent transmittance. The experimental data of the synthesized ruthenium complexes suggest that the title system has nonlinearity as described in Table 2. The investigated ruthenium compounds exhibit emission at room temperature; we have studied the two-photon absorption properties of the synthesized ruthenium(II) compounds in DCM solvent by using open and closed aperture Z-scan technique (∼100 fs, 1 kHz pulses). The open-aperture Z-scan data provide evidence that the present system (compounds RuBPY1−6) shows reasonably

computed values are in concurrence with the experimental values. The experimentally observed red shift in the emission band in comparison to the absorption band can be ascribed to the geometry relaxation on the S1 state. It is notable that the tilt of the p-cymene group with respect the plane of the remaining complex still remains at the excited state optimized structure. The present system is unique in the sense that we are dealing with LMCT transitions (form above-mentioned TDDFT calculations) as far as electronic absorption spectroscopy is concerned, which is in contrast to MLCT transitions, usually found in the case of Ru(II) bipyridine compounds. Electrochemistry. The electrochemical behavior of RuBPY1−6 was studied by performing cyclic voltammetry. The relevant cyclic voltammograms of 10−3 M dry acetonitrile solutions of complexes RuBPY1−6 were recorded with glassy carbon working electrode, platinum wire auxiliary electrode and Ag/AgCl as reference electrode. The supporting electrolyte was tetrabutylammoniumtetrafluoroborate. Under identical conditions, ferrocenium/ferrocene (Fc+/Fc) couple was observed at E1/2 = 0.18 V (ΔEp = 110 mV). The cyclic voltammograms of complexes display three redox responses in an oxidative scan. The representative cyclic voltammogram of compound RuBPY1 is shown in Figure 5. As shown in Figure

Figure 5. Electrochemical properties of complex RuBPY1, the relevant ligand 1, and ferrocene in acetonitrile solvent in a nitrogen atmosphere. The supporting electrolyte used was tetrabutylammoniumtetrafluoroborate; the scan rate was 50 mV S−1. Peak potentials have been measured versus Ag/AgCl at 298 K.

5, the first two responses, that are very close to each other, are irreversible and can be assigned to oxidation of the bipyridine ligand. This assignment is based on the cyclic voltammogram of the free ligand, that shows a broad oxidative feature in the same region of first two oxidative responses of the RuBPY1 complex. The third oxidative response, occurred at around 1 V versus Ag/AgCl, is reversible in nature and is assigned to Ru3+/ Ru2+ couple. A similar Ru3+/Ru2+ redox couple was observed46 for a similar piano stool compound,47 as described in the present study. Generally, it is known that the π-ligand (pcymene here) stabilizes the lower oxidation state of coordinating metal ion through additional back-bonding interactions. Thus, the present ligand system is very unique in the sense that this can stabilize Ru(III) oxidation state in compounds RuBPY1−6, as shown by the reversible Ru3+/Ru2+ couple (Figure S29, Supporting Information). The oxidative nature (Figure 5) of the asymmetrically substituted 2,2′bipyridine ligand makes this possible by donating electron

Table 2. NLO Coefficients of the Synthesized Compounds RuBPY1−6 Obtained with ∼100 fs Pulses at 800 nm

F

compound

n (cm2/ W; 10−16)

β (cm/ W; 10−13)

σ2 (GM)

Reχ(3) (esu; 10−15)

Imχ(3) (esu; 10−16)

χ(3) (esu; 10−15)

RuBPY1 RuBPY2 RuBPY3 RuBPY4 RuBPY5 RuBPY6

1.34 1.92 0.96 1.00 1.10 1.06

4.29 0.50 3.6 3.0 2.6 2.7

28.2 3.3 23.6 19.7 17.1 18.0

6.8 9.8 4.9 5.14 5.63 5.41

4.68 0.54 3.92 3.27 2.83 2.94

6.8 9.8 4.9 5.14 5.63 5.41


Article

Inorganic Chemistry

values decrease, when strong electron donors, e.g., dialkyl amino groups, are present in the ancillary ligand.55 As observed in the experimental data (Table 2), the nonlinear absorption cross-section value for RuBPY2 is significantly lower (3.3 GM) compared to those of rest of the Ru(II) complexes. We have offered the following explanation for this. In the case of the compound RuBPY2, there is low absorption strength at 400 nm. So, considering the two-photon absorption picture, the excitation wavelength was chosen at 800 nm. Therefore, the β and σ2 values for RuBPY2 are relatively smaller in comparison to those of other five compounds (Table 2). All other remaining complexes (Table 2) show very close values of two photon absorption parametersthe small difference may arise from the strength of linear absorption at 400 nm, fluctuation of the laser from sample to sample, scan to scan, etc. Even though compound RuBPY1 shows low absorption strength, interestingly it shows relatively high three photon absorption (γ = 1.1 × 10−23 cm3/ w2), the fitting of which is presented in the Supporting Information (Figure S31), when the rest of the compounds show one order less (10−24).

strong two-photon absorption (2PA) having nonlinear absorption cross section values in the range of 3−28 GM in femtosecond case. The two-photon absorption coefficient (β) and σ2 have been determined by using the following formula. h ϑβ = σ2N0 = h ϑσ2NAd0 × 10−3

where d0 (in units of mol L−1) is the concentration of the compound solution, β is the two-photon absorption coefficient, hν is the energy of the incident photon, and NA is Avogadro’s constant. Figure S30 (Supporting Information) shows the experimental setup of obtaining data from the open aperture and closed aperture Z-scan method. The relevant data were obtained using a ∼100 fs pulse at an 800 nm wavelength for the synthesized compounds. The wavelength of 800 nm was chosen because the energy output was relatively stable around this wavelength compared to other wavelengths in the visible spectral region (Figure 2a). The data, presented in Figure 6,

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CONCLUSIONS We have reported a series of piano-stool-type mononuclear Ru(II) compounds (RuBPY1−6) with an arene (p-cymene) as a common ligand and asymmetrically monosubstituted 2,2′bipyridines as ancillary ligands, on which extended arylvinyl−π systems have been incorporated. We have described their linear and nonlinear optical properties. Interestingly, the photophysical properties of the present Ru(II)−bipyridine system are found to be originated from LMCT (ligand to metal charge transfer) transitions (by TDDFT calculations), in contrast to other reported, numerous Ru(II)−bipyrine compounds that generally show MLCT (metal to ligand charge transfer) transitions. We have demonstrated that after metalation, the mirror symmetry of the p-cymene η6-ligand along the axis (Scheme 1) is lost, and this leads to four different signals for four aromatic ring protons of p-cymene. We have performed DFT calculations to establish the 1H NMR results of the experimentally observed four different signals. All the complexes, RuBPY1−6, exhibit the NIR emission peaks that are bathochromically shifted, compared to the emission bands of the respective free ligands. The electrochemical properties were studied for the synthesized ruthenium complexes, which exhibit a well-defined RuIII/RuII couple, which is rarely seen for a for a piano-stool-type π-system. The nonlinear absorption cross-section values of synthesized complexes are found to be in the range of 3−28 GM in the femtosecond case. To our knowledge, this might be the first report that describes NLO studies of a Ru(II)-piano-stool system having donor−acceptor ligand moieties.

Figure 6. Open aperture Z-scan data shown by compounds RuBPY1−6 in DCM solvent; the relevant data are obtained with ∼100 fs pulses at 800 nm. Black curves represent experimental data, and red curves represent theoretical fits. The plots, corresponding to the closed aperture Z-scan, are shown in “insets.”

evidence that the present system (synthesized compounds) exhibits two-photon absorpton behavior, because theoretically fitted data are consistent with the experimentally obtained observations of femto-second data. The insets, shown in Figure 6 correspond to closed aperture Z-scan data. All the complexes exhibit positive nonlinearity; the summary of the NLO experimental data is presented in Table 2. The experimentally obtained n2 values fall in the range of (0.9−1.9) × 10−16 cm2/ W, demonstrating clean nonlinearity. As per previous reports, the design principles used for those organic chromophores can also be applied to inorganic materials in order to get large two photon absorption σ2 values, which, upon further increase in conjugation and/or increase in donor/acceptor strength, can be increased.54 Ancillary ligands, which are poor electron donors, e.g., alkyl-oxy or alkyl-thio groups, exhibit increased two photon absorption cross-section values upon metal coordination. But, the two photon absorption cross-section

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EXPERIMENTAL SECTION

Materials and Methods. The reactions, carried out in this study, were done under a pure nitrogen atomosphere. The deuterated NMR solvents were obtained from Sigma-Aldich. Column chromatography was carried out using neutral alumina (SRL, India) and silica gel (100−200 mesh). The solvents, which are used for chromatographic purification purposes, were distilled before their use. The NMR spectra were taken from a Bruker AV-400 400 MHz spectrometer, using tetramethylsilane (TMS) as an internal standard. Signal multiplicities are described by the following notations: s = singlet, d = doublet, t = triplet, q = quartet, pent = pentet, m = multiplet, and br = broad. Elemental analyses were performed using a FLASH EA series G


Article

Inorganic Chemistry

124.3, 123.7, 122.1, 120.1, 115.1, 110.8, 104.3, 103.8, 99.3, 87.3, 87.1, 84.1, 84.0, 57.1, 56.4, 50.5 (2C), 31.1, 25.2 (2C), 22.2, 22.2, 21.4, 19.1. IR spectrum (ν/cm−1): 2960.5, 2926.4, 2852.6, 1728.1, 1621, 1597, 1467, 1356, 1287, 1209, 1041. HRMS (m/z) calcd: 774.2400 (RuBPY4−Cl). Found: 774.2400. Anal. Calcd for C43H47Cl2N3O2Ru: C, 63.77; H, 5.85; N, 5.19. Found: C, 63.86; H, 5.79; N, 5.23. RuBPY5. Yield: 89%. 1H NMR (400 MHz, CDCl3): δ 9.56 (d, J = 5 Hz, 1H), 9.45 (d, J = 6 Hz, 1H), 8.08 (s, 1H), 7.99 (s, 1H), 7.84 (d, J = 16 Hz, 1H), 7.70 (d, J = 5 Hz, 1H), 7.56 (d, J = 5 Hz, 1H), 7.50 (d, J = 16 Hz, 1H), 7.34 (d, J = 16 Hz, 1H), 7.17 (d, J = 16 Hz, 2H), 7.11 (d, J = 8 Hz, 2H), 6.50 (s, 1H), 6.21 (d, J = 6 Hz, 1H), 6.15 (d, J = 6 Hz, 1H), 6.04 (d, J = 6 Hz, 1H), 6.02 (d, J = 6 Hz, 1H), 4.12 (t, J = 6 Hz, 2H), 4.05 (t, J = 6 Hz, 2H), 3.86 (s, 6H), 3.16 (t, J = 7 Hz, 4H), 2.671 (t, J = 6 Hz, 1H), 2.58 (s, 3H), 2.29 (s, 3H), 1.86 (sextet, J = 7 Hz, 4H), 1.59 (pent, J = 7 Hz, 4H), 1.49 (pent, J = 7 Hz, 4H), 1.29 (quart, J = 7 Hz, 4H), 1.07 (unresolved, 6 H), 1.05−1.00 (m, 6H), 0.91 (t, J = 7 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ 155.8, 155.8, 154.3, 154.2, 152.2, 151.9, 151.7, 150.7, 149.0, 147.3, 141.5, 132.6, 130.9, 128.9, 124.9, 124.1, 123.6, 123.3, 122.9, 120.6, 119.5, 119.0, 112.04, 112.0, 110.5, 109.2, 105.0, 104.0, 103.96, 87.2, 87.1, 84.2, 84.0, 69.3, 69.0, 56.4, 56.2, 52.2 (2C), 31.5, 31.4, 31.0, 29.4 (2C), 22.1 (2C), 21.4, 20.5 (2C), 19.48, 19.45, 19.0, 14.0 (4C). IR spectrum (ν/cm−1): 2962, 2918, 2850, 1726, 1603, 1461, 1282, 1121, 1069, 772, 719. HRMS (m/z) calcd: 976.4333. (RuBPY5−Cl). Found: 976.4333. Anal. Calcd for C55H73Cl2N3O4Ru: C, 65.27; H, 7.27; N, 4.15. Found: C, 65.16; H, 7.31; N, 4.23. RuBPY6. Yield: 92%. 1H NMR (400 MHz, CDCl3): δ 9.56 (d, J = 4 Hz, 1H), 9.46 (d, J = 5 Hz, 1H), 8.07 (s, 1H), 7.97 (s, 1H), 7.81 (d, J = 16 Hz, 1H), 7.67 (d, J = 5 Hz, 1H), 7.52 (br, 1H), 7.49 (d, J = 16 Hz, 1H), 7.27 (d, J = 15 Hz, 1H), 7.15 (d, J = 15 Hz, 2H), 7.10 (s, 2H), 6.27 (s, 1H), 6.18 (d, J = 5 Hz, 1H), 6.14 (d, J = 5 Hz, 1H), 6.01 (t, J = 6 Hz, 2H), 4.10 (t, J = 6 Hz, 2H), 4.03 (t, J = 6 Hz, 2H), 3.86 (s, 3H), 3.82 (s, 3H), 3.40 (br, 4H), 2.67 (pent, J = 6 Hz, 1H), 2.55 (s, 3H), 2.26 (s, 3H), 1.94 (br, 4H), 1.85 (pent, J = 7 Hz, 4H), 1.58 (pent, J = 7 Hz, 4H), 1.04−0.99 (unresolved, 12H). 13C NMR (100 MHz, CDCl3): δ 155.8, 154.3, 154.2, 152.7, 152.3, 151.6, 150.6, 149.1, 144.0, 141.2, 132.8, 131.3, 128.9, 125.1, 124.0, 123.6, 122.9, 122.6, 119.5, 119.0, 115.9, 112.1, 110.9, 109.6, 104.1, 103.9, 99.4, 87.2, 87.1, 84.3, 84.2, 69.3, 69.0, 56.8, 56.4, 50.4, 31.6, 31.4, 31.1, 25.1 (2C), 22.2 (2C), 21.4, 19.54, 19.51, 19.03, 14.06, 14.04. IR spectrum (ν/cm−1): 2962, 2925, 2863, 1683,1603, 1467, 1362, 1208, 1164, 1035, 979, 750. HRMS (m/z) calcd: 918.3551. (RuBPY6−Cl). Found: 918.3550. Anal. Calcd for C51H63Cl2N3O4Ru: C, 64.21; H, 6.66; N, 4.40. Found: C, 64.38; H, 6.57; N, 4.48.

1112 CHNS analyzer. HRMS spectra were taken from a Bruker Maxis spectrophotometer (ESI positive mode). The electronic absorption spectra were recorded by using a Cary 100 Bio UV−visible spectrophotometer. The electrochemical experiments were performed by using a CH-Instruments Model 620A electrochemical analyzer. Synthesis and Characterization. A mixture of the dimer precursor [Ru(p-cymene)Cl2]2 (0.091 g, 0.15 mmol) and the corresponding bipyridine ligands (1−6, 0.30 mmol) was put in 30 mL of MeOH, and the resulting reaction mixture was stirred for 7 h at room temperature in a nitrogen atmosphere. It was then evaporated to dryness. This results in a dark red-colored solid that was purified by column chromatography over neutral alumina taking methanol/ CH2Cl2, 2:95 v/v, as an eluent. RuBPY1. Yield: 92%. 1H NMR (400 MHz, CDCl3): δ 9.55 (d, J = 5 Hz, 1H), 9.44 (d, J = 6 Hz, 1H), 8.01 (s, 1H), 7.98 (s, 1H), 7.55 (d, J = 16 Hz, 1H), 7.65 (d, J = 6 Hz, 1H), 7.49 (d, J = 5 Hz, 1H), 7.04 s,1H), 6.95 (d, J = 16 Hz, 1H), 6.38 (s, 2H), 6.16 (S, 1H), 6.14 (s, 1H), 6.00 (d, J = 6 Hz, 2H), 3.84 (s, 3H), 3.81 (s, 3H), 3.32 (t, J = 7 Hz, 4H), 2.66 (pent, J = 7 Hz, 1H), 2.50 (s, 3H), 2.23 (s, 1H), 1.52− 1.46 (m, 4H), 2.7 (sextet, J = 7 Hz, 4H), 0.97 (qurt, J1 = 6 Hz, J2 = 2 Hz, J3 = 6 Hz, 6H), 0.88 (t, J = 7 Hz, 6H). 13C NMR (100 MHz, CDCl3): 155.8, 155.7, 154.3, 154.2, 153.6, 151.6, 149.6, 146.4, 143.9, 132.7, 128.8, 123.6, 123.5, 120.0, 119.4, 115.3, 111.8, 104.0, 103.9, 103.0, 87.3, 87.0, 84.3, 84.1, 56.6, 56.1, 52.1 (2C), 31.1, 29.6 (2C), 22.2, 22.2, 21.5, 20.4 (2C), 19.1, 14.0 (2C). IR spectrum (ν/cm−1): 2962, 2925, 2884, 1720, 1593, 1464, 1278, 1208, 1034. HRMS (m/z) calcd.: 730.2713 (RuBPY1−Cl). Found: 730.2714. Anal. Calcd for C39H51Cl2N3O2Ru: C, 61.17; H, 6.71; N, 5.49. Found: C, 61.26; H, 6.78; N, 5.42. RuBPY2. Yield: 87%. 1H NMR (400 MHz, CDCl3): δ 9.55 (d, J = 5 Hz, 1H), 9.33 (d, J = 6 Hz, 1H), 7.95 (d, J = 8 Hz, 2H), 7.77 (d, J = 16 Hz, 1H), 7.61 (d, J = 5 Hz, 1H), 7.51 (d, J = 5 Hz, 1H), 7.05 (s, 1H), 6.87 (d, J = 16 Hz, 1H), 7.61 (d, J = 5 Hz, 1H), 6.16 (s, 2H), 6.10 (d, J = 6 Hz, 1H), 6.02 (d, J = 6 Hz, 1H), 5.98 (d, J = 6 Hz, 1H), 5.3 (DCM), 3.88 (s, 3H), 3.81 (s, 3H), 3.49 (br, 4H), 2.66 (pent, J = 7 Hz, 1H), 2.54 (s, 3H), 2.27 (s, 1H), 1.94 (br, 4H), 1.03 (t, J = 6 Hz, 6H). 13C NMR (100 MHz, CDCl3): 155.9, 155.4, 154.5, 154.3, 154.0, 151.5, 149.7, 143.4, 143.1, 132.7, 128.8, 123.5, 123.2, 119.1, 118.2, 112.5, 112.1, 104.0, 103.6, 97.9, 87.3, 87.1, 84.3, 84.0, 57.2, 56.0, 50.5 (2C), 31.0, 25.4 (2C), 22.2 (2C), 21.5, 19.1. IR spectrum (ν/cm−1): 2962, 2925, 2950, 1743, 1591, 1518, 1458, 1366, 1210, 1039, 863, 767. HRMS (m/z) calcd: 672.1931 (RuBPY2−Cl). Found: 672.1934. Anal. Calcd for C35H41Cl2N3O2Ru: C, 59.40; H, 5.84; N, 5.94. Found: C, 59.32; H, 5.87; N, 5.98. RuBPY3. Yield: 90%. 1H NMR (400 MHz, CDCl3): δ 9.5 (s, 1H), 9.44 (s, 1H), 8.3 (s, 1H), 8.26 (s, 1H), 7.58 (d, J = 14 Hz, 2H), 7.43 (d, J = 8 Hz, 2H)), 7.40−734 (m, 4H), 7.10 (s, 1H), 6.95 (d, J = 16 Hz, 1H), 6.88 (d, J = 16 Hz, 1H), 6.44 (s, 1H), 6.08 (d, J = 17 Hz, 2H), 5.96 (s, 2H), 3.81 (s, 6H), 3.12 (t, J = 7 Hz, 4H), 2.54 (br, 1H), 2.37 (s, 3H), 2.21 (s, 3H), 1.45 (t, J = 7 Hz, 4H), 1.25 (q, J = 7 Hz, 4H), 0.91 (br, 6H), 0.85 (t, J = 7 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ 155.9, 155.3, 154.4, 154.3, 152.0, 148.4, 147.3, 141.7, 139.9, 137.6, 133.8, 128.7, 128.3 (2C), 126.5 (2C), 125.2, 124.5, 124.4, 122.4, 120.1, 118.2, 110.6, 104.8, 104.3, 103.7, 87.3, 87.1, 84.1, 84.0, 56.5, 56.4, 52.2 (2C), 31.0, 29.4 (2C), 22.1 (2C), 21.4 (2C), 20.5, 19.1, 14.0 (4C). IR spectrum (ν/cm−1): 2962, 2918, 2851, 1730, 1597, 1462, 1283, 1207, 1117, 1137, 841, 744. HRMS (m/z) calcd: 832.3183 (RuBPY3−Cl). Found: 832.3191. Anal. Calcd for C47H57Cl2N3O2Ru: C, 65.04; H, 6.62; N, 4.84. Found: C, 64.87; H, 6.68; N, 4.79. RuBPY4. Yield: 93%. 1H NMR (400 MHz, CDCl3): δ 9.46 (d, J = 5 Hz, 1H), 9.40 (d, J = 6 Hz, 1H), 8.23 (d, J = 16 Hz, 2H), 7.57 (d, J = 5 Hz, 1H), 7.54 (d, J = 16 Hz, 1H), 7.43 (t, J1 = 7 Hz, J2 = 10 Hz, 3H), 7.37 (d, J = 7 Hz, 3H), 7.02 (s, 1H), 6.94 (d, J = 16 Hz, 1H), 6.82 (d, J = 16 Hz, 1H), 6.23 (s, 1H), 6.09 (d, J = 6 Hz, 2H), 6.04 (d, J = 6 Hz, 1H), 5.95 (t, J = 6 Hz, 2H), 3.83 (s, 3H), 3.80 (s, 3H), 3.38 (t, J = 6 Hz, 4H), 2.56 (pent, J = 7 Hz, 1H), 2.40 (s, 3H), 2.24 (s, 3H), 1.92 (t, J = 6 Hz, 4H), 0.94 (qurt, J = 3 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ 155.7, 155.1, 154.4, 154.3, 152.7, 151.1, 148.5, 144.0, 141.3, 140.3, 137.6, 133.4, 128.7, 128.3, 126.4, 124.7, 124.5,

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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.9b01235. Figures S1−S24 (1H and 13C NMR spectra, HRMS spectra, and CHN analysis results), Figure S25 (mass spectrometry: experimental data and isotopic pattern simulations of complexes), Figure S26 (IR spectra), Figures S27 and S28 (experimental and theoretical NMR studies), Figure S29 (electrochemistry), Figures S30 and S31 (Z-scan experiment setup and NLO study), and computational outputs (Cartesian coordinates of the optimized geometries of of complexes RuBPY1−6) (PDF)

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*Phone: +91 40 2313 4853. Fax: +91 40 2301 2460. E-mail: [email protected], [email protected]. H


Article

Inorganic Chemistry

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ORCID

Samar K. Das: 0000-0002-9536-6579 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank SERB, DST, Government of India (Project No. EMR/2017/002971) for financial support. We acknowledge M. Sethupathy and D.N. Rao (School of Physics, University of Hyderabad) for the NLO characterization. We thank Mrs. Asia Perwez and Mr. G. M. Subrahamanyam for helping us to record the isotopic pattern simulations of HRMS spectra of complexes RuBPY1−6. The 400 MHz NMR facility at University of Hyderabad by DST, Government of India, is gratefully acknowledged. We thank Mr. Satish Malkapuri and Dr. Sathish Kumar Kurapati for helping in electrochemical analysis; we would like to thank Dr. Monima Sarma for insightful discussions. We acknowledge UGC-CAS, DST Purse, and UPE-II. R.B. thanks UGC, New Delhi for a fellowship. Special thanks are due to Mr. Arunkumar Kanakati and Prof. S. Mahapatra, School of Chemistry, University of Hyderabad, for helping us to perform DFT calculations.

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Bipyridine Ligands - ACS Publications - American Chemical Society

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