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Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

Photosubstitution Reaction of cis-[Ru(bpy)2(CH3CN)2]2+ and cis[Ru(bpy)2(NH3)2]2+ in Aqueous Solution via Monoaqua Intermediate Megumi Kayanuma,* Mitsuo Shoji, and Yasuteru Shigeta Center for Computational Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan

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S Supporting Information *

ABSTRACT: The photoinduced ligand exchange reaction of Ru(II) complexes in aqueous solution was studied using density functional theory (DFT). The optimized structures of the lowest triplet state of cis[Ru(bpy)2(CH3CN)2]2+ (bpy = bipyridine), cis-[Ru(bpy)2(NH3)2]2+, and their monoaqua complexes were analyzed. The metal-centered (3MC) structure was lower than the metal-to-ligand charge transfer (3MLCT) structure for cis-[Ru(bpy)2(CH3CN)2]2+, whereas the 3 MLCT structure was lower than the 3MC structure for cis[Ru(bpy)2(NH3)2]2+. Such a difference would correlate with the higher quantum yield of the former complex. For the monoaqua complexes, the most stable local minimum structure was the 3MC structure, in which the Ru−OH2O and Ru−Nbpy (trans to the oxygen) bonds were elongated. Therefore, the dissociation of the H2O ligand would be preferred to that of the CH3CN (or NH3) ligand from the monoaqua intermediate, which might result in the reformation of the monoaqua intermediate, and thus, the formation of the bis-aqua product would take a longer time than that of the monoaqua intermediate.

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the populating of the 3MC state. Indeed, for a given series of complexes, it was shown that the higher energy level of the 3 MLCT state resulted in a higher photosubstitution quantum yield.18 The exchange of the ligand(s) with two solvent water molecules is considered to proceed as a sequential substitution process. For the photosubstitution reaction of cis-[Ru(bpy)2(CH3CN)2]2+ (bpy = bipyridine) to form cis-[Ru(bpy)2(H2O)2]2+, the formations of a penta-coordinated intermediate (cis-[Ru(bpy)2(CH3CN)]2+) and a monoaqua intermediate (cis-[Ru(bpy)2(CH3CN)(H2O)]2+) have been experimentally shown.19 They reported the time scale of the photosubstitution process: the 3MLCT state undergoes vibrational cooling for 5 ps and relaxes back to the S0 state of the initial complex within 51 ps, whereas the penta-coordinated intermediate undergoes vibrational cooling for 18 ps and relaxes backs to the S0 state of the initial complex within 33 ps or becomes the monoaqua intermediate in 77 ps.19 The stepwise replacement mechanism has also been studied for a nicotinamide (NA) complex cis-[Ru(bpy)2(NA)2]2+ by both experimental and theoretical analyses.20 In several previous studies, quantum chemistry calculations for such Ru(II) complexes have been performed to understand the detailed properties of the excited states and the reaction mechanisms. The geometries of the ground state (S0) and the lowest triplet state (T1) and the vertical excitation energies have been reported for several complexes using density functional

INTRODUCTION The photosubstitution reaction of the Ru(II) imine complexes, which has been known since the 1970s, has recently received attention again because of the potential application in photochemotherapy (PCT)1,2 (i.e., a strategy for anticancer therapy that can control the spatial and temporal selectivities of drugs using visible light). Traditional photodynamic therapy (PDT)3,4 depends on the availability of 3O2 around the photosensitizer; therefore, PDT is not effective for hypoxic tumors.5 Use of metal complexes, which cause photoinduced ligand dissociation to covalently bind DNA in a manner similar to that of cisplatin (cis[Pt(NH3)2Cl2]),6 is a candidate as an alternative to the traditional PDT.1,2 Cisplatin undergoes thermal ligand exchange in aqueous solution to form cis-[Pt(NH3)2(H2O)2], which binds to DNA and inhibits transcription and DNA replication.7 Metal complexes that act like cisplatin only in the area where visible light is irradiated are expected be able to reduce the drawback of cisplatin (i.e., its aggressive systemic toxicity), because it cannot distinguish between healthy and cancerous cells.8 A variety of Ru(II) complexes, which undergo light activated ligand substitution in aqueous solution, have been studied, for example, the Ru(II)-piano stool complexes,9 hexa-coordinated Ru(II) complexes which undergo monodentate ligand10,11 or bidentate ligand12−17 dissociations. During the general mechanism of the photosubstitution of the Ru(II) complexes, a metalto-ligand charge transfer (MLCT) transition occurs followed by intersystem crossing to the triplet MLCT state (3MLCT), and then, internal conversion to the triplet metal-centered (3MC) occurs.1,2 In many cases, the ligand loss is considered to occur on the 3MC state, and the photoreactivity is related to the ease of © XXXX American Chemical Society

Received: November 26, 2018 Revised: February 21, 2019

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DOI: 10.1021/acs.jpca.8b11399 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

Figure 1. Optimized structures of (a) 1a, (b) 1b, (c) 1c, and (d) 2 of the S0 state

theory (DFT) methods.9,11,17 For cis-[Ru(bpy)2(py)2]2+ (py = pyridine), both of the 3MLCT and 3MC structures have been analyzed.21 With respect to the intermediates, the equilibrium structure and the vertical excitation energies of cis-[Ru(bpy) 2 (NA) 2 ] 2+ , the monoaqua intermediate (cis-[Ru(bpy)2(NA)(H2O)]2+), and the penta-coordinated intermediates (cis-[Ru(bpy)2(NA)]2+) have been analyzed using the DFT methods.20 The minimum energy crossing points between the 3 MC and 1MC states and between the 1MC and S0 states of the penta-coordinated intermediate have also been obtained using the complete active space self-consistent field (CASSCF) method.20 However, the detailed mechanism of the photosubstitution, especially for the formation of the bis-aqua product from the monoaqua intermediate, has not yet been clarified. The loss of the second ligand to form the bis-aqua product had been reported to occur on a much longer time scale than that of the first ligand for cis-[Ru(bpy)2(NA)2]2+.20 In the present study, we analyzed two Ru(II) complexes, which undergo photosubstitution reaction in aqueous solution10,19 but are stable in the dark.10,11 We examined the 3 MLCT and 3MC structures of the Ru(II) complexes and their monoaqua complexes using DFT methods to shed light on the reaction mechanism of the sequential ligand substitution induced by visible light irradiation in aqueous solution.

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database.30 The characters of the transitions in the TD-DFT calculations were attributed using natural transition orbitals (NTOs).31 The calculations were performed using the Gaussian 09 quantum chemistry software.32

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RESULTS AND DISCUSSION Absorption Spectra of 1a−c and 2. The optimized structures of the S0 state of 1a−c and 2 in water are shown in Figure 1 and Tables S1−S3. The calculated absorption spectra of these complexes are shown in Figure 2, and the characteristic

THEORETICAL METHODS

The structures of the cis-[Ru(bpy)2(CH3CN)2]2+ (1a), cis[Ru(bpy)2(CH3CN)(H2O)]2+ (1b), cis-[Ru(bpy)2(CH3CN)]2+ (1c), cis-[Ru(bpy)2(H2O)2]2+ (2), cis[Ru(bpy)2(NH3)2]2+ (3a), and cis-[Ru(bpy)2(NH3)(H2O)]2+ (3b) were optimized using DFT with the B3LYP functional22,23 and Stuttgart/Dresden ECP24 for the Ru atom and Dunning/ Huzinaga full double-ζ basis sets with polarization functions (D95(d,p))25 for the others. For several structures, we examined the effects of the dispersion correction using the D3 version of the Grimme’s dispersion26 and confirmed the effects are small (Table S1). The geometries of the S0 and T1 (both 3MLCT and 3 MC) states were analyzed. The solvent effect was considered with the polarizable continuum model (PCM)27 using the integral equation formalism variant (IEFPCM) for water (ε = 78.3553). Vibrational analyses were performed for all the optimized structures to confirm that they were energy minima or transition states. Intrinsic reaction coordinate (IRC) calculations were carried out for the transition states to verify whether they connect the expected minima. Vertical excitation energies in the water were calculated using a time-dependent density functional theory (TD-DFT) method with the cc-pVTZ-PP28 and cc-pVTZ29 basis sets for the Ru atom and other atoms, respectively, which were taken from the Basis Set Exchange

Figure 2. Calculated absorption spectra of 1a (blue line), 1b (green line), 2 (red line), and 1c (violet dashed line).

Table 1. Transition Energies of Several Characteristic Peaks of 1a, 1b, and 2 1a

B

1b

2

character

nm

f

nm

f

nm

f

MLCT MLCT MC MLCT MLCT IL MC IL

462.4 420.5 322.5 315.8 300.5 285.1 280.0 281.5

0.010 0.131 0.001 0.088 0.071 0.262 0.001 0.822

481.3 434.4 370.5 320.9 301.5 286.2 285.4 281.8

0.006 0.143 0.002 0.128 0.047 0.200 0.126 0.790

497.2 449.7 392.3 325.7 301.4 285.4 287.7 282.9

0.009 0.153 0.005 0.146 0.023 0.271 0.004 0.861


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experimental values: 444.0 nm for 1b (434.4 nm without any explicit solvent water molecule, 458 nm by experiment19) and 468.2 nm for 2 (449.7 nm without any explicit solvent water molecules, 490 nm by experiment19). On the other hand, the explicit solvent water molecules did not affect the IL states around 282 nm as expected (i.e., 282.1 and 282.6 nm for 1b and 2, respectively). 3 MC and 3MLCT Structures of 1a and 3a. For the optimized structure of 1a on the S0 state, the T1 state was the MLCT state, of which the relative energy was 2.59 eV (Table 2).

peaks are summarized in Tables 1 and S4. All the complexes showed an intense peak around 280 nm, which corresponds to the intraligand (IL) state. A moderate peak around 400−450 nm corresponds to the MLCT state. Both peaks became longer in wavelength during the photoinduced ligand exchange (i.e., 1a < 1b < 2). The difference in the former peaks was small, i.e., they were located at 281.5, 281.8, and 282.9 nm for 1a, 1b, and 2, respectively. The latter peaks were located at 420.5, 434.4, and 449.7 nm for 1a, 1b, and 2, respectively. This tendency is in agreement with the experimental absorption spectra,19 though the calculated spectra overestimated the transition energies, especially for the mono and bis-aqua complexes. Liu et al. have reported the peak shift in the absorption spectra of 1a during photolysis in water and showed the shift in the absorption maximum (i.e., 427 nm at 0 min (corresponds to 1a), 458 nm at 2 min (assigned to 1b), and 490 nm at 10 min (corresponds to 2).19 In the previous experimental study using transient absorption spectra, absorption at >500 nm at 10 ps was considered to be the penta-coordinated structure.19 This peak corresponds to the two lowest singlet excited states of 1c, which were located at 563.3 and 510.9 nm and assigned as the MC states. A previous theoretical study showed that 2 undergoes photoisomerization to form trans-[Ru(bpy)2(H2O)2]2+ (2′) via the penta-coodinated [Ru(bpy)2(H2O)]2+ on the T1 state, in which the trans structure was lower in energy than the cis structure.33 To examine whether such isomerization would also occur for 1c, we optimized the structures of trans-[Ru(bpy)2(CH3CN)]2+ (1c′) on the S0 and T1 state (Figure S1a,b and Table S3) and 1c on the T1 state (Figure S1c and Table S3). The relative energy of 1c′ (1.19 eV) was higher than that of 1c (1.04 eV) even in the T1 state, though the energy difference was smaller than that in the S0 state, where 1c′ was higher in energy than 1c by 0.55 eV. In addition, the trans (1c′)structure of the T1 state resembled both cis (1c) and trans (1c′)structures of the S0 state (e.g., the dihedral angle of four N atoms of bpy ligands was −124.9° for the trans structure of the T1 state, whereas they were −93.7 and −152.8° for cis and trans structures of the S0 state, respectively). Therefore, isomerization would not occur in 1c, and the formation of the trans form of 1b would be negligible, in contrast to the case of trans-[Ru(bpy)2(H2O)]2+.33 However, 2′ would be formed from 2 during light irradiation;33 thus, we also calculated the geometry (Figure S1d and Table S3) and the absorption spectra (Table S4) of 2′. The MLCT peak was located at 468.7 nm, which was longer than that of 2 (449.7 nm). As already mentioned, the TD-DFT calculations overestimated the excitation energy of the peak of the MLCT transitions around 400−450 nm for the aqua complexes. Such a blue shift in the TD-DFT spectrum had also been reported for the monoaqua intermediate of the nicotinamide complex (cis[Ru(bpy)2(NA)(H2O)]2+).20 A possible reason for the difference with the experimental values would be the effect of the hydrogen-bond interactions with the solvent water molecules; thus, we examined the effect of the explicit water molecule(s) on the transition energy of this MLCT state. The optimized structures of 1b and 2 with an explicit solvent water molecule (hydrogen-bond acceptor) for each H2O ligand are shown in Figure S2. In these structures, the Ru−OH2O distances became slightly shorter (2.18 Å) compared to those without any explicit solvent water molecules (2.21 and 2.20 Å for 1b and 2, respectively). By considering the explicit water molecules, the MLCT peak was red-shifted and became closer to the

Table 2. Relative Energies of the T1 State for the Optimized Structures of the S0 and T1 (3MLCT and 3MC) States and the Transition State Structures of the T1 State for 1a and 3a optimized state S0 T1 T1 T1

character of T1 3

MLCT 3 MLCT 3 MC TSMC/MLCT

1a

3a

2.59 2.33 2.12 2.50

2.14 1.94 2.03 2.17

Two local minimum structures of the T1 state were obtained (Figure 3 and Table S1), which were confirmed to correspond to

Figure 3. Optimized structures of the (a) 3MLCT and (b) 3MC of 1a.

the 3MLCT and 3MC structures based on the nature of the singly occupied molecular orbitals (SOMOs) (Figure S3a,b). For the 3MLCT structure (2.33 eV, Figure 3a), a C−C single bond of a bpy ligand became shorter than that of the S0 structure by 0.06 Å because of the π-bonding nature of the Kohn−Sham orbital, to which the electron excitation occurred (Figure S3a). For the 3MC structure (2.11 eV, Figure 3b), a Ru−NCH3CN bond and a Ru−Nbpy bond, which are trans-coordinated, were elongated by 0.66 and 0.27 Å, respectively. The 3MC structure was lower in energy than the 3MLCT structure by 0.21 eV (Table 1). In the previous study of cis-[Ru(bpy)2(py)2]2+, the 3 MC structure had also been reported to be lower in energy than the 3MLCT structure by 0.46 eV (calculated using B3LYP functional with LanL2DZ and 6-31+G** basis sets).21 For comparison, we also calculated the S0, 3MLCT, and 3MC structures for 3a (Figure S4 and Table S5), because the quantum yield of the photosubstitution of 3a had been reported to be lower than that of 1a by 1 order of magnitude.10,19 In contrast to 1a, the 3MLCT structure (1.94 eV, Figure S4b) was lower in energy than that of the 3MC structure (2.03 eV, Figure S4c) by 0.09 eV for 3a (Table 2). The higher energy level of the 3MC structure for 3a would be a reason for the lower quantum yield of 3a compared to 1a. It is considered that the photoreactivity of these complexes is related to the ease of the population of the C


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Figure 4. Optimized structures of the (a) 3MLCT′, (b) 3MLCT, (c) 3MC, and (d) 3MC′ of 1b. 3

instead of the CH3CN ligand, the penta-coordinated intermediate 1c would be regenerated, and then, the monoaqua complex 1b would be formed again. Therefore, the photosubstitution of the second CH3CN ligand (1b + H2O → 2 + CH3CN) would take a longer time than the first ligand (1a + H2O → 1b + CH3CN) because of the competition between the H2O ligand loss (on the 3MC state) and the CH3CN ligand loss (on the 3MC′ state) pathways, in which the former might be preferred. In the previous experimental study of the photosubstitution of cis-[Ru(bpy)2(NA)2]2+ in water, a much longer time scale for the second substitution had been reported.20 The low-lying 3MC state, which would cause the dissociation of the H2O ligand, in the monoaqua complex could explain the slower reaction of the formation of the bis-aqua product than that of the monoaqua intermediate during the photosubstitution reactions in aqueous solutions. For comparison, the optimized structures of the T1 state of 3b were also calculated (Figure S6 and Tables 3 and S6). Among the four local minimum structures, the lowest one was the 3MC structure, in which the Ru−OH2O bond and Ru−Nbpy bond, which are trans-coordinated, were elongated (denoted as 3MC, 1.56 eV, Figure S6d), as in the case of 1b. Another 3MC structure was the highest in energy among the four structures, in which the Ru−NNH3 bond and Ru−Nbpy bond, which are transcoordinated, were elongated (denoted as 3MC′, 2.11 eV, Figure S6e). The 3MC′ structure was slightly higher in relative energy than the 3MC structure of 3a (2.03 eV). The energy difference between the 3MC and 3MC′ structures was 0.55 eV, and the two 3 MLCT structures (2.01 and 2.02 eV) were more stable than the 3 MC′ structure. Therefore, in the case of 3b, photoinduced ligand loss might have a higher preference for the dissociation of the H2O ligand compared to 1b, which would result in a longer time to complete the formation of the bis-aqua product from the monoaqua intermediate for 3b compared to 1b. These results suggest that the efficiency of both the first ligand exchange (1a/ 3a + H2O → 1b/3b + CH3CN/NH3) and the second ligand exchange (1b/3b + H2O → 2 + CH3CN/NH3) might cause the higher quantum yield of the photosubstitution of 1a compared to 3a. The transition states between the 3MC and 3MLCT structures in the T1 state (TSMC/MLCT) were also obtained for both complexes (Figure S7 and Tables S2 and S6). The relative energies were 2.25 and 2.05 eV for 1b and 3b, respectively (Table 3). Thus, the reaction barriers from the 3MLCT structure to the 3MC structure on the T1 state were very low (i.e., 0.02 and 0.04 eV, respectively). These results indicate that the internal conversion between the 3MC and 3MLCT structures would very easily occur for both 1b and 3b.

MC state, which is involved in the photoinduced ligand loss; therefore, the easier access to the 3MC state in 1a compared to 3a would increase the quantum yield of the former complex. The transition states between the 3MC and 3MLCT structures in the T1 state (TSMC/MLCT) were also obtained for both complexes (Figure S5a,b and Tables S1 and S5). The relative energies were 2.50 and 2.17 eV for 1a and 3a, respectively (Table 2). Thus, the reaction barriers from the 3MLCT structure to the 3MC structure, in other words, the reaction barriers for the photodissociation of a ligand, in the T1 state were 0.17 and 0.23 eV, respectively. These results indicated that internal conversion between the 3MC and 3MLCT structures would occur more easily for 1a compared to 3a, which would also be related to the higher quantum yield of 1a. In addition, another local minimum structure of the T1 state was obtained for both 1a and 3a, in which two trans-Ru−Nbpy bonds were elongated (3MC″, Figure S5c,d and Tables S1 and S5). The relative energies of the 3MC″ structures were 2.35 and 2.14 eV, respectively, which are higher than those of the 3MC and 3MLCT structures. 3 MC and 3MLCT Structures of the Monoaqua Complex 1b and 3b. To understand the second step of the photosubstitution reaction, that is, the formation of the bis-aqua product from the monoaqua intermediate, we analyzed the local minimum structures of the T1 state of 1b. Four structures were obtained (Figure 4 and Tables 3 and S2), which were confirmed to correspond to the two 3MLCT and two 3MC structures based on the nature of the SOMOs (Figure S3c−f). In the lower 3 MLCT structure, the C−C single bond of the bpy ligand, which lies in the same plane of the CH3CN ligand, was shorted (denoted as 3MLCT′, 2.18 eV, Figure 4a). In the higher 3MLCT structure, the C−C single bond of another bpy ligand was shorted (denoted as 3MLCT, 2.23 eV, Figure 4b). The energy difference between the two 3MLCT structures was small (0.05 eV). Both of them were slightly lower in relative energy than the 3 MC structure of 1a (2.33 eV). The lower 3MC structure, in which the Ru−OH2O bond and its trans-Ru−Nbpy bond were elongated (denoted as 3MC, 1.69 eV, Figure 4c), was the most stable among the four local minimum structures of the T1 state. Another 3MC structure, in which the Ru−NCH3CN bond and Ru−Nbpy bond, which are trans-coordinated, were elongated (denoted as 3MC′, 2.05 eV, Figure 4d), was higher in energy than that of the lower 3MC structure by 0.36 eV. The 3MC′ structure was slightly lower in relative energy than that of 1a (2.12 eV). These results suggested that when the monoaqua intermediate 1b absorbs a photon, the release of the H2O ligand would be more favorable than that of the CH3CN ligand because of the lower energy of the 3MC structure than that of the 3MC′ structure. When the ligand loss occurs at the 3MC structure, the H2O ligand would be released D



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Table 3. Relative Energies of the T1 State for the Optimized Structures of the S0 and T1 (3MLCT′, 3MLCT, 3MC, and 3 MC′) States and the Transition State Structures of the T1 State for 1b and 3b optimized state

character of T1

S0 T1 T1 T1 T1 T1

3

MLCT MLCT′ 3 MLCT 3 MC 3 MC′ TSMC/MLCT 3

1b

3b

2.44 2.18 2.23 1.69 2.05 2.25

2.23 2.02 2.01 1.56 2.11 2.05

CONCLUSION We analyzed the photoinduced ligand substitution reaction of two Ru complexes, 1a and 3a, in aqueous solution using DFT methods. Comparing the two optimized structures of the T1 state of 1a and 3a, the lower one was the 3MC state for 1a and the 3MLCT state for 3a. In addition, the reaction barrier from the 3MLCT structure to the 3MC structure in the T1 state was lower for 1a than for 3a. Such differences might be the reasons for the higher efficiency of the photosubstitution of 1a compared to that of 3a. The T1 structures for the monoaqua complexes (1b and 3b) were also analyzed. The lowest one was the 3MC structure, in which the Ru−OH2 bond was elongated, for both complexes, and thus, the dissociation of the H2O ligand would be preferred to that of the CH3CN or NH3 ligand. In addition, the reaction barriers from the 3MLCT structure to the 3MC structure in the T1 state were low for both complexes. Therefore, the formation of the bis-aqua product would take a longer time than that of the monoaqua intermediate because of the competition between the H2O ligand loss and CH3CN/NH3 ligand loss pathways. ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.8b11399. Optimized structures, SOMOs of the optimized structures of the T1 state, and transition energies (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel +81-29-853-6578; E-mail: [email protected]. ORCID

Megumi Kayanuma: 0000-0003-1149-5857 Mitsuo Shoji: 0000-0001-7465-6326 Yasuteru Shigeta: 0000-0002-3219-6007 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research of the Innovative Areas “Photosynergetics” (No. JP26107004) from MEXT, Japan. The computations were performed using (1) the Research Center for Computational Science, Okazaki, Japan, and (2) COMA, provided by the Multidisciplinary Cooperative Research Program in Center for Computational Sciences, University of Tsukuba. E


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