Article pubs.acs.org/JPCA
Theoretical Studies on the Redox-Stimulated Isomerization in Electrochromic Osmium Sulfoxide Complexes Huifang Li,*,† Lisheng Zhang,† Xiaolin Fan,† and Yi Zhao*,‡ †
Key Laboratory of Organo-Pharmaceutical Chemistry, Gannan Normal University, Ganzhou 341000, R. R. China State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, P.R. China
‡
S Supporting Information *
ABSTRACT: Redox-stimulated intramolecular isomerization of the DMSO ligand in [Os(bpy)2(DMSO)2]2+ (bpy = 2,2′-bipyridine; DMSO = dimethyl sulfoxide) was explored theoretically for better understanding the electrochromic properties of osmium sulfoxide complexes. It is found that the HOMO−LUMO gap is decreased because the electron transfer amount from DMSO1 ligand to Os center using Os−S1 linkage is larger than that using Os−O1 linkage, which makes the absorption of such electrochromic Os(II) sulfoxide complexes redshifted. Moreover, it is observed that Os−O linkage is preferred by the “hard” Os(III) metal and the “soft” Os(I) metal prefers Os−S linkage, compared with Os(II). Intrinsic reaction pathway calculation results demonstrate that Os−S1 → Os−O1 isomerization is favored by Os(II) oxidation, while Os−O1 → Os−S1 isomerization is much easier to be triggered by reduction of Os(III) or Os(II). In addition, DMSO2 linkage isomerization becomes much harder to proceed attributed to the increased bond-strength between DMSO2 and Os center upon Os(II)−O1 → Os(II)−S1 rearrangement, which makes only one DMSO ligand isomerized observed experimentally.
1. INTRODUCTION Because of their potential application in redox-actuated nanoscale devices, electrochromic materials have attracted a great deal of attention.1−4 Among them, transition-metalcontaining systems are highly appealing because their redox and optical properties can be controlled via judicious ligand modification, making them superior to traditional organic chrompophores.5−7 Recently, there is a growing class of electrochromic materials based on transition metal (e.g., Ru2+, Os2+, Re+, Mo0, and W0) complexes with the isomerization capability of a coordinated ligand.8−10 Recently, electrochromic polypyridine ruthenium(II)11−18 and osmium(II)19,20 sulfoxide complexes were synthesized and characterized. In these molecules, external redox-stimulated intramolecular isomerization of the coordinated sulfoxide ligand from Mn+−S to Mn+−O linkage mode can occur on an ultrafast time scale, which is accompanied by pronounced changes of the absorption spectra.21−29 On the basis of these experimental achievements, theoretical explorations have also been performed for such transitionmetal-based chromophores.30−36 It has been demonstrated that more detailed information, including electronic and photophysical characters, as well as intramolecular isomerization mechanisms, can be obtained for such d6 transition-metal complexes. Moreover, the power of density functional theory (DFT) method for describing the transition-metal-based systems has also been proved. In this work, theoretical studies on the electrochromic characters of polypyridine osmium sulfoxide complexes will be carried out with quantum chemical methods. © XXXX American Chemical Society
Electrochromic character of osmium sulfoxide complexes was initially reported for [Os(bpy)2(DMSO)2]2+ (bpy = 2,2′bipyridine; DMSO = dimethyl sulfoxide) by J. J. Rack and his co-workers.19,20 In their studies, electron transfer triggered Os− S → Os−O, as well as Os−O → Os−S reaction of one DMSO ligand being observed. Correspondingly, their absorption maximum was changed from 355 nm to a new peak at 403 nm by such molecular rearrangement. However, intrinsic reaction pathway and the role of redox on the electron transfer triggered intramolecular linkage isomerization of such polypyridine osmium sulfoxide complexes is still unclear. Our goal here is to understand the underlying electrochromic properties of osmium sulfoxide complexes with theoretical studies. Geometrical parameter, electronic, and photophysical character changes upon intramolecular isomerization of [Os(bpy)2(DMSO)2]2+ complex were explored theoretically. The chemical structure of [Os(bpy)2(DMSO)2]2+ is displayed in Figure 1. Moreover, intrinsic reaction pathways (IRPs) of the intramolecular Os−S → Os−O and Os−O → Os−S isomerization in [Os(bpy)2(DMSO)2]3+, [Os(bpy)2(DMSO)2]2+ and [Os(bpy)2(DMSO)2]+ complexes were examined for the aim to explore the role of electron transfer in the electrochromic mechanisms of osmium complexes. Received: March 4, 2015 Revised: April 9, 2015
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Figure 2. Intramolecular S,S- and O,S-linkage isomerization located at one DMSO ligand in [Os(bpy)2(DMSO)2]2+.
Figure 1. Chemical structure of [Os(bpy)2(DMSO)2]2+.
2. METHODS The [Os(bpy)2(DMSO)2]3+, [Os(bpy)2(DMSO)2]2+, and [Os(bpy)2(DMSO)2]+ complexes were optimized with density functional theory (DFT) using the Perdew, Burke, and Ernzerhof (PBE) functional.37 Considering Os−S and Os−O are weak interactions, the empirical dispersion correction introduced by Grimme38,39 was added with Becke’s and Johnson’s rational damping function,40−42 and we have dubbed this variant PBE-D3(BJ). The SDD basis set and effective core potential were used to describe the Os atom,43 while 631G(d)44,45 was used for all remaining atoms. Moreover, solvent effect was considered with CPCM (ε = 35.688).46 For the aim to confirm that each optimized stationary point was a minimum (NIMAG = 0) or a transition state (NIMAG = 1) on the potential energy surface, harmonic vibrational frequencies were also analyzed at the same level. In addition, intrinsic reaction coordinate (IRC)47,48 calculations were also performed to make sure that the optimized transition states were connected with two relevant minima. Using these optimized geometries, single point calculations were conducted at a large basis set level, which was made of SDD for Os, a correlation-consistent polarized double-ζ basis set (cc-pVDZ)49 for H atoms, and a correlation-consistent polarized triple-ζ basis set (cc-pVTZ)49 for C, N, O, and S atoms. Binding energies between the coordinated DMSO1 ligand and central Os were calculated as the energy difference between the cluster and individual moieties.
characteristics of DMSO ligand produces distinct electronic and photophysical properties. 3.1. Geometry of Isomers. Main geometrical parameters obtained from the structural optimization of the Os(II), Os(III), and Os(I) sulfoxide complexes are collected in Table 1, and the available experimental data are also included for comparison.19 The labeling scheme is reported in Figure 2. As expected, there are considerable geometrical parameter changes with intramolecular isomerization and redox. Calculated binding energy of the DMSO1 ligand and Os center in S,S-[Os(bpy)2(DMSO)2]2+ (BESDMSO1) is 1.67 eV, which is larger than that in the S,O-linked isomer (BEODMSO1, 1.34 ev). This means the Os−S linkage mode of DMSO ligand in Os(II) complex is preferred. As listed in Table 2, natural population charge distributed on DMSO1 ligand is changed from 0.574 to 0.351 e. This means the electron transfer amount from DMSO1 ligand to central Os using the Os−S1 linkage is larger than that using the Os−O1 linkage for about 0.223 e. Correspondingly, the S1−O1 bond is weakened and its bond length is increased by 0.064 Å upon S,S- to O,S-coordination in Os(II) complexes, which is attributed to the decreased πelectron transfer from O to S.56 Moreover, the variation of the S1−O1 bond can be reflected by the spn hybridization, that is, the bond is weakened and elongated along with the increase of the p-character of the σ orbital.57,58 As collected in Supporting Information (SI) Table S1, detailed NBO analysis of the (S1− O1) σ orbital demonstrated that the spn hybrid characters of S1 and O1 atoms are increased by 2.29 and 1.14, respectively, upon intramolecular isomerization process. Optimization results indicate that the Os−S1 bond of sulfinyl group in S,S-[Os(bpy)2(DMSO)2]2+ is shorter than that in the S,S-[Os(bpy)2(DMSO)2]3+ by 0.102 Å. The Os−O1 bond in S,O-[Os(bpy)2(DMSO)2]2+ is longer than that in the S,O[Os(bpy)2(DMSO)2]3+ by about 0.117 Å upon one-electron oxidation. Differently, one-electron reduction makes the Os−S1 bonds shortened by about 0.022 Å and the Os−O1 bond lengthened by about 0.028 Å. Calculated binding energy demonstrated that BESDMSO1 is decreased by about 0.36 eV and BEODMSO1 is increased by about 0.67 eV upon one-electron removing. On the contrary, one-electron reduction makes the BESDMSO1 increased by about 0.08 eV and BEODMSO1 decreased by about 0.13 eV. In other words, compared with Os(II) complexes, O-linkage is preferred by the “hard” Os(III) metal, while the “soft” Os(I) metal prefers S-linkage.56This is similar to our previous calculation results for the Ru-DMSO bond-strength changes upon redox, which shows O-linkage is preferred by the “hard” Ru center, while the “soft” Ru prefers Slinkage.34 The variation of the binding energy between DMSO1 and Os center upon redox also can be reflected by the S1−O1 bond distance changes. As shown in Table 1, the S1−O1 bond of sulfinyl group in DMSO1 ligand is strengthened by the
BE = (E DMSO1 + E[Os(bpy)2 DMSO2]2+) − E[Os(bpy)2 (DMSO)2 ]2+
Considering the bond strength between DMSO and Os center is weak, the Boys−Bernardi counterpoise correction50 was considered for the aim to minimize basis set superposition error. Time-dependent DFT (TD-DFT)51,52 calculations based on the optimized geometries were performed at the same level to get vertical transition energies. Natural transition orbital (NTO)53 analyses were carried out to examine the nature of the excited states. In addition, for the aim to obtain the electron transfer degree from surrounding ligand to central Os, charge distribution was evaluated by natural population analysis (NPA).54 All computations were carried out with the Gaussian 0955 suite of programs.
3. RESULTS AND DISCUSSION Two different isomers of [Os(bpy)2(DMSO)2]2+ with S,S- and S,O- linkage mode are displayed in Figure 2. Triggered by formal oxidation or reduction, intramolecular Os−S1 → Os−O1 and Os−O1 → Os−S1 isomerization of one bound DMSO ligand can be observed between these two isomers.19,20 Calculated results demonstrated that differential bonding B
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Table 1. Computational Results of the Main Bond Lengths (Angstroms) and Bond Angles (Degrees) for Os(III), Os(II), and Os(I) Complexes, As Determined at the PBE0-D3(BJ)/[SDD-ECP/6-31G(d,p)] Level of Theorya S,S-bonded bond
Os(II)
Os−S1 Os−O1 S1−O1 Os−S2 Os−O2 S2−O2 Os−Nb1 Os−Nb1′ Os−Nb2 Os−Nb2′ α(O1−S1−Os)/α(S1−O1−Os) α(S1−Os−Nb1)/α(O1−Os−Nb1) α(O2−S2−Os)/α(S2−O2−Os) a
exptl
b
O,S-bonded
Os(III)
Os(I)
O,O-bonded
Os(II)
Os(III)
1.511 2.294
2.143 1.571 2.278
2.026 1.609 2.374
2.171 1.565 2.254
1.510 2.100 2.093 2.084 2.092 117.2 99.2 117.8
1.511 2.069 2.047 2.087 2.099 118.8 90.8 118.3
1.499 2.094 2.070 2.084 2.084 121.7 91.7 116.7
1.516 2.071 2.046 2.077 2.092 116.8 90.3 120.0
2.301
2.276
2.403
2.279
1.507 2.301
1.471 2.268
1.498 2.403
1.507 2.101 2.097 2.097 2.101 116.0 98.1 116.0
1.477 2.100 2.091 2.097 2.094 115.8 97.8 116.0
1.498 2.093 2.085 2.085 2.093 112.4 98.8 112.4
Os(I)
Os(II)
Os(III)
Os(I)
2.133 1.569
2.051 1.593
2.155 1.564
2.142 1.566 2.055 2.024 2.016 2.053 118.4 92.9 120.3
2.050 1.593 2.073 2.054 2.053 2.074 123.9 89.9 123.9
2.173 1.563 2.064 2.019 1.994 2.048 116.6 92.6 116.6
Optimizations were conducted in acetonitrile by using the CPCM model. bRef 19.
Table 2. Natural Population Charges Distributed on the Os Complexes (Q)a Obtained at PBE0-D3(BJ)/[SDD-ECP/631G(d,p)] Level of Theory S,S-bonded Os DMSO1 DMSO2 BPY1 BPY2 a
O,S-bonded
Os(II)
Os(III)
Os(I)
Os(II)
Os(III)
Os(I)
−0.486 0.574 0.574 0.669 0.669
−0.502 0.542 0.516 0.432 0.013
0.045 0.651 0.651 0.826 0.826
−0.109 0.351 0.524 0.631 0.602
−0.124 0.318 0.480 −0.027 0.354
0.309 0.492 0.622 0.791 0.786
Q(ligand) indicates the charge on all the atoms in ligand.
decreased ion−DMSO interaction, while it is weakened by the increased ion−DMSO interaction. This is in good agreement with previous studies for the variability of the S1−O1 distance upon Ru−DMSO interaction changes.34 In addition, attributed to the decreased electron transfer amount from DMSO1 ligand to central Os, natural population charge distributed on central Os is decreased by about 0.377, 0.378, and 0.264 e, respectively, in Os(II), Os(III), and Os(I) complexes upon Os−S1 to Os−O1 rearrangement. This means the central Os ion becomes much “harder” by DMSO1 linkage isomerization.56 The bond-strength between DMSO2 and Os center is increased correspondingly. As a result, the Os−S2 bond distances between Os center and DMSO2 ligand in these S,S-linked Os(II), Os(III), and Os(I) complexes are longer than those in the S,O-linked isomers by about 0.023, 0.029, and 0.025 Å, respectively, as shown in Table 1. 3.2. Electronic and Photophysical Properties. Calculated ionization potentials (IP) and electron affinities (EA) of Os(II) complexes are collected in SI Table S2. It is found that ionization potential is decreased by about 0.9 eV and electron affinity is increased by about 0.4 eV upon Os(II)−S1 → Os(II)−O1 isomerization. Spin density isosurfaces of the Os(I) and Os(III) complexes are shown in Figure 3. It is observed that radical ion is mainly distributed on the Os center and bpy ligands for one-electron oxidized Os(II) complexes. In [Os(bpy)2(DMSO)2]+, electron is mainly added on the bpy ligands, in good agreement with NPA charge distributions as collected in Table 2. It is known that the thermodynamic ability to gain or lose an electron is dominated by the frontier orbitals energies of the
Figure 3. Spin density isosurface (in blue) associated with the radical ion states of [Os(bpy)2(DMSO)2]2+ in acetonitrile solvent.
complex, including the lowest unoccupied (LUMO) and the highest occupied (HOMO) molecular orbital.32 The features of the frontier orbitals in these Os sulfoxide complexes are examined. Contour plots of the HOMO and LUMO for the Os(II) complexes are displayed in Figure 4. The descriptions of frontier orbital composition are summarized in Table 3. As expected, the HOMO and LUMO distributions are similar to the spin density isosurface in radical ion complexes. The HOMO electron density is mainly distributed on Os center (75%) and the LUMO is mainly distributed on bpy ligands (90%) in S,S-[Os(bpy)2(DMSO)2]2+. The HOMO electron density distributed on Os center is decreased, and that distributed on DMSO is increased by intramolecular isomerization. This can be understood from the electron rearrangement upon Os(II)−S1 → Os(II)−O1 isomerization. As mentioned above, the electron transfer degree from DMSO1 to Os center is decreased upon Os(II)−S1 → Os(II)−O1 linkage isomerization, which makes the electron density distributed on central Os atom decreased by about 0.377 e. Moreover, attributed to the increased d → π* back-bonding from Os center to DMSO ligand by Os(II)−S1 → Os(II)−O1 linkage isomerization,35 the electron transfer degree from bpy ligands to Os center is also decreased. As a consequence, the C
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transitions to/from low-lying excited states. Calculated results are collected in Table 4, and available experimental results are Table 4. Vertical Absorption and Emission Energies (in eV), Dominated Orbital Excitations, Oscillator Strength ( f), and Squares of the Transition Dipole Moments μ2ge (D) Obtained from TD-DFT Calculationsa
S,Sbonded
O,Sbonded
excitation
λabs (nm)
S0→S1
393
0.004
S0→S2
391
0.001
S0→S3
387
0.001
S0→S4
386
0.007
S0→S5
359
S0→S6
344
0.041
S0→S1
487
0.005
S0→S2
466
0.002
S0→S3
455
0.018
S0→S4
412
S0→S5
392
0.034
S0→S6
377
0.029
S,S-bonded O,S-bonded
exptl (λmax)b
355
f
0.088
403
0.074
dominated orbitals HOMO−1→LUMO (92%) HOMO→LUMO (65%) HOMO−1→LUMO +1(65%) HOMO→LUMO+1 (88%) HOMO−2→LUMO (94%) HOMO−2→LUMO +1 (90%) HOMO→LUMO (84%) HOMO→LUMO+1 (83%) HOMO−1→LUMO (77%) HOMO−1→LUMO +1 (78%) HOMO−2→LUMO (69%) HOMO−2→LUMO +1 (59%)
λem (nm) 482 651
emission T1→S0 T1→S0
a The absorption (emission) energies are based on the S0(T1) state equilibrium geometry. bRefs 19 and 20.
also included for comparison. The excited-state evolutions from absorption to emission and the nature of the orbitals involved in the relevant excited states are depicted in Figure 5. It is found that the S0−S1 transition energy in S,S-[Os(bpy)2(DMSO)2]2+ is larger than that in the S,O-linked isomer for about 0.61 eV and the T1-S0* emission energy is also decreased from 2.57 to 1.90 eV attributed to the decreased HOMO−LUMO gap upon Os(II)−S1 → Os(II)−O1 isomerization. As shown in Figure 6, our calculation results reproduce the red shift of the maximum absorption peak from 359 to 412 nm upon Os(II)−S 1 →Os(II)−O 1 isomerization in [Os(bpy)2(DMSO)2]2+ complex, which is in good agreement with experimental results (from 355 to 403 nm). TD-DFT calculation results indicate that the maximum absorption peak of S,S-[Os(bpy)2(DMSO)2]2+ is made from S0 to S5 excitation, which is mainly dominated by HOMO−2 to LUMO transition.
Figure 4. Contour plots of frontier orbitals of the [Os(bpy)2(DMSO)2]2+ complexes in acetonitrile solvent.
HOMO is destabilized by about 0.9 eV and LUMO is also destabilized by about 0.4 eV.36 Correspondingly, the HOMO− LUMO gap is decreased. The frontier MOs, shown in Figure 4, play an important role in the photoresponsive materials. Among them, the HOMO and LUMO orbitals are most important because their locations can be used for the description of the first excited singlet transition. For the aim to get more detailed information about the changes of the photophysical properties of such electrochromic Os(II) sulfoxide complexes, TD-DFT calculations were carried out to get the vertical excitation energies for
Table 3. Main Distributions of the Frontier Molecular Orbitals in Os(II) Complexes S,S-bonded Os DMSO1 DMSO2 bpy1 bpy2
O,S-bonded
HOMO−2
HOMO−1
HOMO
LUMO
LUMO+1
LUMO+2
HOMO−2
HOMO−1
HOMO
LUMO
LUMO+1
LUMO+2
0.65 0.09 0.09 0.08 0.08
0.76 0.04 0.04 0.08 0.08
0.75 0.06 0.06 0.07 0.07
0.06 0.02 0.02 0.45 0.45
0.04 0.01 0.01 0.47 0.47
0.10 0.03 0.03 0.41 0.41
0.74 0.01 0.08 0.13 0.05
0.72 0.07 0.03 0.08 0.10
0.72 0.09 0.04 0.09 0.06
0.06 0.01 0.02 0.45 0.46
0.06 0.01 0.02 0.45 0.46
0.09 0.03 0.03 0.42 0.43
D
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Figure 7. Schematic representation of the intrinsic reaction pathways of the normal (black curve) and one-electron-oxidized (blue curve) or -reduced (red curve) [Os(bpy)2(DMSO)2]2+ complexes.
Figure 5. Absorption and emission information on [Os(bpy)2(DMSO)2]2+ isomers obtained from TD-DFT calculations. The nature of relevant excited states examined with NTO analyses is also included.
effects of redox on the Os(II)−S1 bond strength as mentioned above. Moreover, reaction energy for the Os(II)−S1→Os(II)− O1 reaction is decreased by about 0.91 eV with oxidation because O-bonding is preferred by the “hard” Os(III) metal. Thereby, it can be concluded that Os(II)−S1→Os(II)−O1 reaction is more thermodynamically favored in the Os(III) complexes. As mentioned above, for O-linked Os(II) complexes, Os(II)−O bond is strengthened in Os(III) complexes and is weakened in Os(I) complexes. As a result, energy barrier for the Os(II)−O1→Os(II)−S1 reaction path is increased by about 0.02 eV upon one-electron oxidation, while it is lowered by about 0.29 eV upon one-electron reduction. Then, it can be predicted that Os−O1→Os−S1 isomerization follows Os(III) or Os(II) reduction. These results are similar to those of our previous studies for the ruthenium sulfoxide complexes, which demonstrated that Ru−S→Ru−O isomerization is preferred by Ru(II) oxidation and Ru−O→Ru−S isomerization is much easier to be triggered by Ru(III) or Ru(II) reduction.34 For the aim to get more information about the DMSO2 linkage isomerization, IRPs for Os−S2→Os−O2 rearrangement from S,O-[Os(bpy)2(DMSO)2]2+ to O,O-[Os(bpy)2(DMSO)2]2+ are also examined. Calculated results are displayed in Figure 8. Similar effects of redox on the Os(II)− S2→Os(II)−O2 isomerization can be observed. This means Os−S2→Os−O2 isomerization gets easier by oxidation, while Os−O2→Os−S2 isomerization is favored by Os(III) or Os(II) reduction. However, by comparison of the IRPs shown in Figures 7 and 8, it is found that calculated energy barriers for DMSO2 linkage isomerization from Os−S2 to Os−O2 linkage mode are larger than those observed for DMSO1 linkage isomerization for about 0.07, 0.58, and 0.05 eV, respectively in Os(II), Os(III), and Os(I) complexes. Moreover, reaction energies for the DMSO2 linkage isomerization are also larger than those for the DMSO1 linkage isomerization. This means the DMSO2 linkage isomerization becomes much harder to proceed, which is attributed to the increased bond strength between DMSO2 and Os center upon Os(II)−O1→Os(II)−S1 rearrangement as mentioned above. That is why only one DMSO ligand isomerization was observed experimentally.19,20
Figure 6. Theoretical absorption spectra for the S,S- and O,S-linked [Os(bpy)2(DMSO)2]2+. The full widths at half-maximum value were set to be 4600 cm−1.
For S,O-linked isomer, S0 to S4 excitation, dominated by HOMO−1 to LUMO+1 transition, is the maximum absorption peak. 3.3. Intrinsic Reaction Pathways for Isomerization. Calculated energy results demonstrated that S,S-[Os(bpy)2(DMSO)2]2+ is more stable than its O,S-isomer, S,O[Os(bpy)2(DMSO)2]2+, for about 0.15 eV because the Os−S linkage mode of DMSO in Os(II) complex is preferred as mentioned above. One DMSO ligand isomerization from Os−S to Os−O linkage can be stimulated by oxidation, which is accompanied by pronounced changes of the absorption spectra. Here, intrinsic reaction pathways (IRPs) of the intramolecular Os−S→Os−O and Os−O→Os−S reactions of the sulfoxide group in [Os(bpy)2(DMSO)2]3+, [Os(bpy)2(DMSO)2]2+, and [Os(bpy)2(DMSO)2]+ complexes were examined. In these μS,O-linked transition states, DMSO1 ligand is chelated to central Os with both S and O atoms. IRC analyses, shown in Figure 7 and SI Figures S1−S3, were performed to confirm that the transition states were connected with two relevant minima. IRC results also demonstrate that electron transfer triggered intramolecular isomerization in osmium sulfoxide complexes is an elementary reaction. Calculated IRPs results are shown in Figure 7. It is found that activation energy for the Os(II)−S1→Os(II)−O1 reaction is decreased by 0.89 eV with oxidation and is increased by about 0.6 eV with reduction. This can be understood with different
4. CONCLUSION In this work, electronic and photophysical properties of [Os(bpy)2(DMSO)2]2+, as well as IRSs, of the intramolecular E
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21403037, 51478123, 21463003), the Natural Science Foundation of Jiangxi Province (20142BAB213014), Jiangxi Provincial “Ganpo Talents 555 Projects”, and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, PR China.
Figure 8. Schematic representation of the IRPs of DMSO2 linkage isomerization from S,O-[Os(bpy) 2 (DMSO) 2 ] 2+ to O,O-[Os(bpy)2(DMSO)2]2+ of the normal (black curve), one-electron-oxidized (blue curve) or -reduced (red curve) [Os(bpy) 2 (DMSO) 2 ] 2+ complexes.
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(1) Lomoth, R. Redox-Stimulated Motion and Bistability in Metal Complexes and Organometallic Compounds. Antioxid. Redox. Signaling 2013, 19, 1803−1814. (2) Roberts, M. N.; Carling, C.-J.; Nagle, J. K.; Branda, N. R.; Wolf, M. O. Successful Bifunctional Photoswitching and Electronic Communication of Two Platinum(II) Acetylide Bridged Dithienylethenes. J. Am. Chem. Soc. 2009, 131, 16644−16645. (3) Tietze, L. F.; Düfert, M. A.; Hungerland, T.; Oum, K.; Lenzer, T. Synthesis and Photochemical Investigations of Tetrasubstituted Alkenes as Molecular SwitchesThe Effect of Substituents. Chem.Eur. J. 2011, 17, 8452−8461. (4) Li, G.; Ray, L.; Glass, E. N.; Kovnir, K.; Khoroshutin, A.; Gorelsky, S. I.; Shatruk, M. Synthesis of Panchromatic Ru(II) ThienylDipyrrin Complexes and Evaluation of Their Light-Harvesting Capacity. Inorg. Chem. 2012, 51, 1614−1624. (5) Ishizuka, T.; Sawaki, T.; Miyazaki, S.; Kawano, M.; Shiota, Y.; Yoshizawa, K.; Fukuzumi, S.; Kojima, T. Mechanistic Insights into Photochromic Behavior of a Ruthenium(II)−Pterin Complex. Chem.Eur. J. 2011, 17, 6652−6662. (6) Marin, V.; Holder, E.; Hoogenboom, R.; Schubert, U. S. Functional Ruthenium(II)- and Iridium(III)-containing Polymers for Potential Electro-Optical Applications. Chem. Soc. Rev. 2007, 36, 618− 635. (7) Bonnet, S.; Collin, J.-P. Ruthenium-based Light-driven Molecular Machine Prototypes: Synthesis and Properties. Chem. Soc. Rev. 2008, 37, 1207−1217. (8) Coppens, P.; Novozhilova, I.; Kovalevsky, A. Photoinduced Linkage Isomers of Transition-Metal Nitrosyl Compounds and Related Complexes. Chem. Rev. 2002, 102, 861−884. (9) Heilweil, E. J.; Johnson, J. O.; Mosley, K. L.; Lubet, P. P.; Webster, C. E.; Burkey, T. J. Engineering Femtosecond Organometallic Chemistry: Photochemistry and Dynamics of Ultrafast Chelation of Cyclopentadienylmanganese Tricarbonyl Derivatives with Pendant Benzenecarbonyl and Pyridinecarbonyl Groups. Organometallics 2011, 30, 5611−5619. (10) Phillips, A. E.; Cole, J. M.; d’Almeida, T.; Low, K. S. Ru−OSO Coordination Photogenerated at 100 K in Tetraammineaqua(sulfur dioxide)ruthenium(II) (±)-Camphorsulfonate. Inorg. Chem. 2012, 51, 1204−1206. (11) Rack, J. J.; Mockus, N. V. Room-Temperature Photochromism in cis- and trans-[Ru(bpy)2(dmso)2]2+. Inorg. Chem. 2003, 42, 5792− 5794. (12) Rack, J. J.; Rachford, A. A.; Shelker, A. M. Turning Off Phototriggered Linkage Isomerizations in Ruthenium Dimethyl Sulfoxide Complexes. Inorg. Chem. 2003, 42, 7357−7359. (13) Rachford, A. A.; Petersen, J. L.; Rack, J. J. Designing Molecular Bistability in Ruthenium Dimethyl Sulfoxide Complexes. Inorg. Chem. 2005, 44, 8065−8075.
isomerization in [Os(bpy)2(DMSO)2]3+, [Os(bpy)2(DMSO)2]2+, and [Os(bpy)2(DMSO)2]+ complexes were examined for the aim to understand the underlying electrochromic mechanisms of osmium sulfoxide complex based electrochromic materials. Calculated binding energy of the DMSO1 ligand in S,Slinked and O,S-linked [Os(bpy)2(DMSO)2]2+ demonstrated that Os−S1 linkage mode of DMSO in Os(II) complex is preferred because the electron transfer degree from DMSO1 to Os center using the Os−S1 linkage is larger than that using the Os−O1 linkage. Then, the HOMO mainly distributed on Os center is destabilized and the LUMO mainly distributed on bpy ligands is also destabilized. Because the Os(II)−S1→Os(II)− O1 isomerization destabilizes the HOMO to a larger extent than LUMO, the HOMO−LUMO gap is decreased. Thereby, differential-bonding characteristics of DMSO ligand produce distinct electronic and photophysical properties. As a result, red-shifted absorption can be observed experimentally. Moreover, BESDMSO1 is decreased and BEODMSO1 is increased upon oxidation. On the contrary, BESDMSO1 is increased and BEODMSO1 is decreased by reduction. This means, compared with Os(II), O-bonding is preferred by the “hard” Os(III) metal, while the “soft” Os(I) metal prefers S-bonding. In addition, IRP results demonstrate Os−S→Os−O isomerization is preferred by oxidation of Os(II), and Os−O→Os−S isomerization is favored by reduction of Os(III) or Os(II). In addition, DMSO2 linkage isomerization becomes much harder to proceed due to the Os−O1→Os−S1 rearrangement attributed to the increased bond strength between DMSO2 and Os center upon DMSO1 linkage isomerization.
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REFERENCES
ASSOCIATED CONTENT
S Supporting Information *
Table S1 listing the NBO analysis results for the S−O bonds; frontier MOs energies listed in Table S2; Cartesian coordinates and energies of all calculated structures collected in Table S3− S17; Figures S1−S3 showing the IRC analysis results. This material is available free of charge via the Internet at http:// pubs.acs.org. F
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(34) Li, H. F.; Zhang, L. S.; Lin, H.; Fan, X. L. A DFT-D Study on the Electrochromic Mechanism of Ruthenium Sulfoxide Complexes. RSC Adv. 2014, 4, 45635−45640. (35) Li, H. F.; Winget, P.; Risko, C.; Sears, J. S.; Brédas, J. L. Tuning the Electronic and Photophysical Properties of Heteroleptic Iridium(III) Phosphorescent Emitters through Ancillary Ligand Substitution: A Theoretical Perspective. Phys. Chem. Chem. Phys. 2013, 15, 6293− 6302. (36) Li, H. F.; Zhang, L. S.; Lin, H.; Fan, X. L. A DFT-D Study on the Electronic and Photophysical Properties of Ruthenium (II) Complex with a Chelating Sulfoxide Group. Chem. Phys. Lett. 2014, 604, 10−14. (37) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. (38) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010, 132, 154104. (39) Goerigk, L.; Grimme, S. A Thorough Benchmark of Density Functional Methods for General Main Group Thermochemistry, Kinetics, and Noncovalent Interactions. Phys. Chem. Chem. Phys. 2011, 13, 6670−6688. (40) Becke, A. D.; Johnson, E. R. A Density-Functional Model of the Dispersion Interaction. J. Chem. Phys. 2005, 123, 154101. (41) Johnson, E. R.; Becke, A. D. A Post-Hartree−Fock Model of Intermolecular Interactions. J. Chem. Phys. 2005, 123, 024101. (42) Johnson, E. R.; Becke, A. D. A Post-Hartree-Fock Model of Intermolecular Interactions: Inclusion of Higher-order Corrections. J. Chem. Phys. 2006, 124, 174104. (43) Andrae, D.; Häussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Energy-adjusted ab Initio Pseudopotentials for the Second Row and Third Row Transition Elements. Theor. Chim. Acta 1990, 77, 123− 141. (44) Hehre, W. J.; Ditchfield, R.; Pople, J. A. Self-Consistent Molecular Orbital Methods. XII. Further Extensions of Gaussian-Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules. J. Chem. Phys. 1972, 56, 2257−2261. (45) Michelle, M. F.; William, J. P.; Warren, J. H.; Binkley, J. S.; Mark, S. G.; Douglas, J. D.; John, A. P. Self-consistent Molecular Orbital Methods. XXIII. A Polarization-type Basis Set for Second-row Elements. J. Chem. Phys. 1982, 77, 3654−3665. (46) Barone, V.; Cossi, M. Quantum Calculation of Molecular Energies and Energy Gradients in Solution by a Conductor Solvent Model. J. Phys. Chem. A 1998, 102, 1995−2001. (47) Fukui, K. The Path of Chemical Reactions - the IRC Approach. Acc. Chem. Res. 1981, 14, 363−368. (48) Hratchian, H. P.; Schlegel, H. B. Accurate Reaction Paths using a Hessian based Predictor−Corrector Integrator. J. Chem. Phys. 2004, 120, 9918−9924. (49) Dunning, T. H., Jr. Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007−1023. (50) Boys, S. F.; Bernardi, F. The Calculation of Small Molecular Interactions by the Differences of Separate Total Energies. Some Procedures with Reduced Errors. Mol. Phys. 1970, 19, 553−566. (51) Fantacci, S.; Angelis, F. D.; Selloni, A. Absorption Spectrum and Solvatochromism of the [Ru(4,4′-COOH-2,2′-bpy)2(NCS)2] Molecular Dye by Time Dependent Density Functional Theory. J. Am. Chem. Soc. 2003, 125, 4381−4387. (52) Censo, D. D.; Fantacci, S.; Angelis, F. D.; Klein, C.; Evans, N.; Kalyanasundaram, K.; Bolink, H. J.; Graetzel, M.; Nazeeruddin, M. K. Synthesis, Characterization, and DFT/TD-DFT Calculations of Highly Phosphorescent Blue Light-Emitting Anionic Iridium Complexes. Inorg. Chem. 2008, 47, 980−989. (53) Martin, R. L. Natural Transition Orbitals. J. Chem. Phys. 2003, 118, 4775−4777. (54) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Intermolecular Interactions from a Natural Bond Orbital, Donor-Acceptor Viewpoint. Chem. Rev. 1988, 88, 899−926. G
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Theoretical Studies on the Redox-Stimulated Isomerization in Electrochromic Osmium Sulfoxide Complexes Huifang Li,*,† Lisheng Zhang,† Xiaolin Fan,† and Yi Zhao*,‡ †
Key Laboratory of Organo-Pharmaceutical Chemistry, Gannan Normal University, Ganzhou 341000, R. R. China State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, P.R. China
‡
S Supporting Information *
ABSTRACT: Redox-stimulated intramolecular isomerization of the DMSO ligand in [Os(bpy)2(DMSO)2]2+ (bpy = 2,2′-bipyridine; DMSO = dimethyl sulfoxide) was explored theoretically for better understanding the electrochromic properties of osmium sulfoxide complexes. It is found that the HOMO−LUMO gap is decreased because the electron transfer amount from DMSO1 ligand to Os center using Os−S1 linkage is larger than that using Os−O1 linkage, which makes the absorption of such electrochromic Os(II) sulfoxide complexes redshifted. Moreover, it is observed that Os−O linkage is preferred by the “hard” Os(III) metal and the “soft” Os(I) metal prefers Os−S linkage, compared with Os(II). Intrinsic reaction pathway calculation results demonstrate that Os−S1 → Os−O1 isomerization is favored by Os(II) oxidation, while Os−O1 → Os−S1 isomerization is much easier to be triggered by reduction of Os(III) or Os(II). In addition, DMSO2 linkage isomerization becomes much harder to proceed attributed to the increased bond-strength between DMSO2 and Os center upon Os(II)−O1 → Os(II)−S1 rearrangement, which makes only one DMSO ligand isomerized observed experimentally.
1. INTRODUCTION Because of their potential application in redox-actuated nanoscale devices, electrochromic materials have attracted a great deal of attention.1−4 Among them, transition-metalcontaining systems are highly appealing because their redox and optical properties can be controlled via judicious ligand modification, making them superior to traditional organic chrompophores.5−7 Recently, there is a growing class of electrochromic materials based on transition metal (e.g., Ru2+, Os2+, Re+, Mo0, and W0) complexes with the isomerization capability of a coordinated ligand.8−10 Recently, electrochromic polypyridine ruthenium(II)11−18 and osmium(II)19,20 sulfoxide complexes were synthesized and characterized. In these molecules, external redox-stimulated intramolecular isomerization of the coordinated sulfoxide ligand from Mn+−S to Mn+−O linkage mode can occur on an ultrafast time scale, which is accompanied by pronounced changes of the absorption spectra.21−29 On the basis of these experimental achievements, theoretical explorations have also been performed for such transitionmetal-based chromophores.30−36 It has been demonstrated that more detailed information, including electronic and photophysical characters, as well as intramolecular isomerization mechanisms, can be obtained for such d6 transition-metal complexes. Moreover, the power of density functional theory (DFT) method for describing the transition-metal-based systems has also been proved. In this work, theoretical studies on the electrochromic characters of polypyridine osmium sulfoxide complexes will be carried out with quantum chemical methods. © XXXX American Chemical Society
Electrochromic character of osmium sulfoxide complexes was initially reported for [Os(bpy)2(DMSO)2]2+ (bpy = 2,2′bipyridine; DMSO = dimethyl sulfoxide) by J. J. Rack and his co-workers.19,20 In their studies, electron transfer triggered Os− S → Os−O, as well as Os−O → Os−S reaction of one DMSO ligand being observed. Correspondingly, their absorption maximum was changed from 355 nm to a new peak at 403 nm by such molecular rearrangement. However, intrinsic reaction pathway and the role of redox on the electron transfer triggered intramolecular linkage isomerization of such polypyridine osmium sulfoxide complexes is still unclear. Our goal here is to understand the underlying electrochromic properties of osmium sulfoxide complexes with theoretical studies. Geometrical parameter, electronic, and photophysical character changes upon intramolecular isomerization of [Os(bpy)2(DMSO)2]2+ complex were explored theoretically. The chemical structure of [Os(bpy)2(DMSO)2]2+ is displayed in Figure 1. Moreover, intrinsic reaction pathways (IRPs) of the intramolecular Os−S → Os−O and Os−O → Os−S isomerization in [Os(bpy)2(DMSO)2]3+, [Os(bpy)2(DMSO)2]2+ and [Os(bpy)2(DMSO)2]+ complexes were examined for the aim to explore the role of electron transfer in the electrochromic mechanisms of osmium complexes. Received: March 4, 2015 Revised: April 9, 2015
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Figure 2. Intramolecular S,S- and O,S-linkage isomerization located at one DMSO ligand in [Os(bpy)2(DMSO)2]2+.
Figure 1. Chemical structure of [Os(bpy)2(DMSO)2]2+.
2. METHODS The [Os(bpy)2(DMSO)2]3+, [Os(bpy)2(DMSO)2]2+, and [Os(bpy)2(DMSO)2]+ complexes were optimized with density functional theory (DFT) using the Perdew, Burke, and Ernzerhof (PBE) functional.37 Considering Os−S and Os−O are weak interactions, the empirical dispersion correction introduced by Grimme38,39 was added with Becke’s and Johnson’s rational damping function,40−42 and we have dubbed this variant PBE-D3(BJ). The SDD basis set and effective core potential were used to describe the Os atom,43 while 631G(d)44,45 was used for all remaining atoms. Moreover, solvent effect was considered with CPCM (ε = 35.688).46 For the aim to confirm that each optimized stationary point was a minimum (NIMAG = 0) or a transition state (NIMAG = 1) on the potential energy surface, harmonic vibrational frequencies were also analyzed at the same level. In addition, intrinsic reaction coordinate (IRC)47,48 calculations were also performed to make sure that the optimized transition states were connected with two relevant minima. Using these optimized geometries, single point calculations were conducted at a large basis set level, which was made of SDD for Os, a correlation-consistent polarized double-ζ basis set (cc-pVDZ)49 for H atoms, and a correlation-consistent polarized triple-ζ basis set (cc-pVTZ)49 for C, N, O, and S atoms. Binding energies between the coordinated DMSO1 ligand and central Os were calculated as the energy difference between the cluster and individual moieties.
characteristics of DMSO ligand produces distinct electronic and photophysical properties. 3.1. Geometry of Isomers. Main geometrical parameters obtained from the structural optimization of the Os(II), Os(III), and Os(I) sulfoxide complexes are collected in Table 1, and the available experimental data are also included for comparison.19 The labeling scheme is reported in Figure 2. As expected, there are considerable geometrical parameter changes with intramolecular isomerization and redox. Calculated binding energy of the DMSO1 ligand and Os center in S,S-[Os(bpy)2(DMSO)2]2+ (BESDMSO1) is 1.67 eV, which is larger than that in the S,O-linked isomer (BEODMSO1, 1.34 ev). This means the Os−S linkage mode of DMSO ligand in Os(II) complex is preferred. As listed in Table 2, natural population charge distributed on DMSO1 ligand is changed from 0.574 to 0.351 e. This means the electron transfer amount from DMSO1 ligand to central Os using the Os−S1 linkage is larger than that using the Os−O1 linkage for about 0.223 e. Correspondingly, the S1−O1 bond is weakened and its bond length is increased by 0.064 Å upon S,S- to O,S-coordination in Os(II) complexes, which is attributed to the decreased πelectron transfer from O to S.56 Moreover, the variation of the S1−O1 bond can be reflected by the spn hybridization, that is, the bond is weakened and elongated along with the increase of the p-character of the σ orbital.57,58 As collected in Supporting Information (SI) Table S1, detailed NBO analysis of the (S1− O1) σ orbital demonstrated that the spn hybrid characters of S1 and O1 atoms are increased by 2.29 and 1.14, respectively, upon intramolecular isomerization process. Optimization results indicate that the Os−S1 bond of sulfinyl group in S,S-[Os(bpy)2(DMSO)2]2+ is shorter than that in the S,S-[Os(bpy)2(DMSO)2]3+ by 0.102 Å. The Os−O1 bond in S,O-[Os(bpy)2(DMSO)2]2+ is longer than that in the S,O[Os(bpy)2(DMSO)2]3+ by about 0.117 Å upon one-electron oxidation. Differently, one-electron reduction makes the Os−S1 bonds shortened by about 0.022 Å and the Os−O1 bond lengthened by about 0.028 Å. Calculated binding energy demonstrated that BESDMSO1 is decreased by about 0.36 eV and BEODMSO1 is increased by about 0.67 eV upon one-electron removing. On the contrary, one-electron reduction makes the BESDMSO1 increased by about 0.08 eV and BEODMSO1 decreased by about 0.13 eV. In other words, compared with Os(II) complexes, O-linkage is preferred by the “hard” Os(III) metal, while the “soft” Os(I) metal prefers S-linkage.56This is similar to our previous calculation results for the Ru-DMSO bond-strength changes upon redox, which shows O-linkage is preferred by the “hard” Ru center, while the “soft” Ru prefers Slinkage.34 The variation of the binding energy between DMSO1 and Os center upon redox also can be reflected by the S1−O1 bond distance changes. As shown in Table 1, the S1−O1 bond of sulfinyl group in DMSO1 ligand is strengthened by the
BE = (E DMSO1 + E[Os(bpy)2 DMSO2]2+) − E[Os(bpy)2 (DMSO)2 ]2+
Considering the bond strength between DMSO and Os center is weak, the Boys−Bernardi counterpoise correction50 was considered for the aim to minimize basis set superposition error. Time-dependent DFT (TD-DFT)51,52 calculations based on the optimized geometries were performed at the same level to get vertical transition energies. Natural transition orbital (NTO)53 analyses were carried out to examine the nature of the excited states. In addition, for the aim to obtain the electron transfer degree from surrounding ligand to central Os, charge distribution was evaluated by natural population analysis (NPA).54 All computations were carried out with the Gaussian 0955 suite of programs.
3. RESULTS AND DISCUSSION Two different isomers of [Os(bpy)2(DMSO)2]2+ with S,S- and S,O- linkage mode are displayed in Figure 2. Triggered by formal oxidation or reduction, intramolecular Os−S1 → Os−O1 and Os−O1 → Os−S1 isomerization of one bound DMSO ligand can be observed between these two isomers.19,20 Calculated results demonstrated that differential bonding B
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Table 1. Computational Results of the Main Bond Lengths (Angstroms) and Bond Angles (Degrees) for Os(III), Os(II), and Os(I) Complexes, As Determined at the PBE0-D3(BJ)/[SDD-ECP/6-31G(d,p)] Level of Theorya S,S-bonded bond
Os(II)
Os−S1 Os−O1 S1−O1 Os−S2 Os−O2 S2−O2 Os−Nb1 Os−Nb1′ Os−Nb2 Os−Nb2′ α(O1−S1−Os)/α(S1−O1−Os) α(S1−Os−Nb1)/α(O1−Os−Nb1) α(O2−S2−Os)/α(S2−O2−Os) a
exptl
b
O,S-bonded
Os(III)
Os(I)
O,O-bonded
Os(II)
Os(III)
1.511 2.294
2.143 1.571 2.278
2.026 1.609 2.374
2.171 1.565 2.254
1.510 2.100 2.093 2.084 2.092 117.2 99.2 117.8
1.511 2.069 2.047 2.087 2.099 118.8 90.8 118.3
1.499 2.094 2.070 2.084 2.084 121.7 91.7 116.7
1.516 2.071 2.046 2.077 2.092 116.8 90.3 120.0
2.301
2.276
2.403
2.279
1.507 2.301
1.471 2.268
1.498 2.403
1.507 2.101 2.097 2.097 2.101 116.0 98.1 116.0
1.477 2.100 2.091 2.097 2.094 115.8 97.8 116.0
1.498 2.093 2.085 2.085 2.093 112.4 98.8 112.4
Os(I)
Os(II)
Os(III)
Os(I)
2.133 1.569
2.051 1.593
2.155 1.564
2.142 1.566 2.055 2.024 2.016 2.053 118.4 92.9 120.3
2.050 1.593 2.073 2.054 2.053 2.074 123.9 89.9 123.9
2.173 1.563 2.064 2.019 1.994 2.048 116.6 92.6 116.6
Optimizations were conducted in acetonitrile by using the CPCM model. bRef 19.
Table 2. Natural Population Charges Distributed on the Os Complexes (Q)a Obtained at PBE0-D3(BJ)/[SDD-ECP/631G(d,p)] Level of Theory S,S-bonded Os DMSO1 DMSO2 BPY1 BPY2 a
O,S-bonded
Os(II)
Os(III)
Os(I)
Os(II)
Os(III)
Os(I)
−0.486 0.574 0.574 0.669 0.669
−0.502 0.542 0.516 0.432 0.013
0.045 0.651 0.651 0.826 0.826
−0.109 0.351 0.524 0.631 0.602
−0.124 0.318 0.480 −0.027 0.354
0.309 0.492 0.622 0.791 0.786
Q(ligand) indicates the charge on all the atoms in ligand.
decreased ion−DMSO interaction, while it is weakened by the increased ion−DMSO interaction. This is in good agreement with previous studies for the variability of the S1−O1 distance upon Ru−DMSO interaction changes.34 In addition, attributed to the decreased electron transfer amount from DMSO1 ligand to central Os, natural population charge distributed on central Os is decreased by about 0.377, 0.378, and 0.264 e, respectively, in Os(II), Os(III), and Os(I) complexes upon Os−S1 to Os−O1 rearrangement. This means the central Os ion becomes much “harder” by DMSO1 linkage isomerization.56 The bond-strength between DMSO2 and Os center is increased correspondingly. As a result, the Os−S2 bond distances between Os center and DMSO2 ligand in these S,S-linked Os(II), Os(III), and Os(I) complexes are longer than those in the S,O-linked isomers by about 0.023, 0.029, and 0.025 Å, respectively, as shown in Table 1. 3.2. Electronic and Photophysical Properties. Calculated ionization potentials (IP) and electron affinities (EA) of Os(II) complexes are collected in SI Table S2. It is found that ionization potential is decreased by about 0.9 eV and electron affinity is increased by about 0.4 eV upon Os(II)−S1 → Os(II)−O1 isomerization. Spin density isosurfaces of the Os(I) and Os(III) complexes are shown in Figure 3. It is observed that radical ion is mainly distributed on the Os center and bpy ligands for one-electron oxidized Os(II) complexes. In [Os(bpy)2(DMSO)2]+, electron is mainly added on the bpy ligands, in good agreement with NPA charge distributions as collected in Table 2. It is known that the thermodynamic ability to gain or lose an electron is dominated by the frontier orbitals energies of the
Figure 3. Spin density isosurface (in blue) associated with the radical ion states of [Os(bpy)2(DMSO)2]2+ in acetonitrile solvent.
complex, including the lowest unoccupied (LUMO) and the highest occupied (HOMO) molecular orbital.32 The features of the frontier orbitals in these Os sulfoxide complexes are examined. Contour plots of the HOMO and LUMO for the Os(II) complexes are displayed in Figure 4. The descriptions of frontier orbital composition are summarized in Table 3. As expected, the HOMO and LUMO distributions are similar to the spin density isosurface in radical ion complexes. The HOMO electron density is mainly distributed on Os center (75%) and the LUMO is mainly distributed on bpy ligands (90%) in S,S-[Os(bpy)2(DMSO)2]2+. The HOMO electron density distributed on Os center is decreased, and that distributed on DMSO is increased by intramolecular isomerization. This can be understood from the electron rearrangement upon Os(II)−S1 → Os(II)−O1 isomerization. As mentioned above, the electron transfer degree from DMSO1 to Os center is decreased upon Os(II)−S1 → Os(II)−O1 linkage isomerization, which makes the electron density distributed on central Os atom decreased by about 0.377 e. Moreover, attributed to the increased d → π* back-bonding from Os center to DMSO ligand by Os(II)−S1 → Os(II)−O1 linkage isomerization,35 the electron transfer degree from bpy ligands to Os center is also decreased. As a consequence, the C
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transitions to/from low-lying excited states. Calculated results are collected in Table 4, and available experimental results are Table 4. Vertical Absorption and Emission Energies (in eV), Dominated Orbital Excitations, Oscillator Strength ( f), and Squares of the Transition Dipole Moments μ2ge (D) Obtained from TD-DFT Calculationsa
S,Sbonded
O,Sbonded
excitation
λabs (nm)
S0→S1
393
0.004
S0→S2
391
0.001
S0→S3
387
0.001
S0→S4
386
0.007
S0→S5
359
S0→S6
344
0.041
S0→S1
487
0.005
S0→S2
466
0.002
S0→S3
455
0.018
S0→S4
412
S0→S5
392
0.034
S0→S6
377
0.029
S,S-bonded O,S-bonded
exptl (λmax)b
355
f
0.088
403
0.074
dominated orbitals HOMO−1→LUMO (92%) HOMO→LUMO (65%) HOMO−1→LUMO +1(65%) HOMO→LUMO+1 (88%) HOMO−2→LUMO (94%) HOMO−2→LUMO +1 (90%) HOMO→LUMO (84%) HOMO→LUMO+1 (83%) HOMO−1→LUMO (77%) HOMO−1→LUMO +1 (78%) HOMO−2→LUMO (69%) HOMO−2→LUMO +1 (59%)
λem (nm) 482 651
emission T1→S0 T1→S0
a The absorption (emission) energies are based on the S0(T1) state equilibrium geometry. bRefs 19 and 20.
also included for comparison. The excited-state evolutions from absorption to emission and the nature of the orbitals involved in the relevant excited states are depicted in Figure 5. It is found that the S0−S1 transition energy in S,S-[Os(bpy)2(DMSO)2]2+ is larger than that in the S,O-linked isomer for about 0.61 eV and the T1-S0* emission energy is also decreased from 2.57 to 1.90 eV attributed to the decreased HOMO−LUMO gap upon Os(II)−S1 → Os(II)−O1 isomerization. As shown in Figure 6, our calculation results reproduce the red shift of the maximum absorption peak from 359 to 412 nm upon Os(II)−S 1 →Os(II)−O 1 isomerization in [Os(bpy)2(DMSO)2]2+ complex, which is in good agreement with experimental results (from 355 to 403 nm). TD-DFT calculation results indicate that the maximum absorption peak of S,S-[Os(bpy)2(DMSO)2]2+ is made from S0 to S5 excitation, which is mainly dominated by HOMO−2 to LUMO transition.
Figure 4. Contour plots of frontier orbitals of the [Os(bpy)2(DMSO)2]2+ complexes in acetonitrile solvent.
HOMO is destabilized by about 0.9 eV and LUMO is also destabilized by about 0.4 eV.36 Correspondingly, the HOMO− LUMO gap is decreased. The frontier MOs, shown in Figure 4, play an important role in the photoresponsive materials. Among them, the HOMO and LUMO orbitals are most important because their locations can be used for the description of the first excited singlet transition. For the aim to get more detailed information about the changes of the photophysical properties of such electrochromic Os(II) sulfoxide complexes, TD-DFT calculations were carried out to get the vertical excitation energies for
Table 3. Main Distributions of the Frontier Molecular Orbitals in Os(II) Complexes S,S-bonded Os DMSO1 DMSO2 bpy1 bpy2
O,S-bonded
HOMO−2
HOMO−1
HOMO
LUMO
LUMO+1
LUMO+2
HOMO−2
HOMO−1
HOMO
LUMO
LUMO+1
LUMO+2
0.65 0.09 0.09 0.08 0.08
0.76 0.04 0.04 0.08 0.08
0.75 0.06 0.06 0.07 0.07
0.06 0.02 0.02 0.45 0.45
0.04 0.01 0.01 0.47 0.47
0.10 0.03 0.03 0.41 0.41
0.74 0.01 0.08 0.13 0.05
0.72 0.07 0.03 0.08 0.10
0.72 0.09 0.04 0.09 0.06
0.06 0.01 0.02 0.45 0.46
0.06 0.01 0.02 0.45 0.46
0.09 0.03 0.03 0.42 0.43
D
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Figure 7. Schematic representation of the intrinsic reaction pathways of the normal (black curve) and one-electron-oxidized (blue curve) or -reduced (red curve) [Os(bpy)2(DMSO)2]2+ complexes.
Figure 5. Absorption and emission information on [Os(bpy)2(DMSO)2]2+ isomers obtained from TD-DFT calculations. The nature of relevant excited states examined with NTO analyses is also included.
effects of redox on the Os(II)−S1 bond strength as mentioned above. Moreover, reaction energy for the Os(II)−S1→Os(II)− O1 reaction is decreased by about 0.91 eV with oxidation because O-bonding is preferred by the “hard” Os(III) metal. Thereby, it can be concluded that Os(II)−S1→Os(II)−O1 reaction is more thermodynamically favored in the Os(III) complexes. As mentioned above, for O-linked Os(II) complexes, Os(II)−O bond is strengthened in Os(III) complexes and is weakened in Os(I) complexes. As a result, energy barrier for the Os(II)−O1→Os(II)−S1 reaction path is increased by about 0.02 eV upon one-electron oxidation, while it is lowered by about 0.29 eV upon one-electron reduction. Then, it can be predicted that Os−O1→Os−S1 isomerization follows Os(III) or Os(II) reduction. These results are similar to those of our previous studies for the ruthenium sulfoxide complexes, which demonstrated that Ru−S→Ru−O isomerization is preferred by Ru(II) oxidation and Ru−O→Ru−S isomerization is much easier to be triggered by Ru(III) or Ru(II) reduction.34 For the aim to get more information about the DMSO2 linkage isomerization, IRPs for Os−S2→Os−O2 rearrangement from S,O-[Os(bpy)2(DMSO)2]2+ to O,O-[Os(bpy)2(DMSO)2]2+ are also examined. Calculated results are displayed in Figure 8. Similar effects of redox on the Os(II)− S2→Os(II)−O2 isomerization can be observed. This means Os−S2→Os−O2 isomerization gets easier by oxidation, while Os−O2→Os−S2 isomerization is favored by Os(III) or Os(II) reduction. However, by comparison of the IRPs shown in Figures 7 and 8, it is found that calculated energy barriers for DMSO2 linkage isomerization from Os−S2 to Os−O2 linkage mode are larger than those observed for DMSO1 linkage isomerization for about 0.07, 0.58, and 0.05 eV, respectively in Os(II), Os(III), and Os(I) complexes. Moreover, reaction energies for the DMSO2 linkage isomerization are also larger than those for the DMSO1 linkage isomerization. This means the DMSO2 linkage isomerization becomes much harder to proceed, which is attributed to the increased bond strength between DMSO2 and Os center upon Os(II)−O1→Os(II)−S1 rearrangement as mentioned above. That is why only one DMSO ligand isomerization was observed experimentally.19,20
Figure 6. Theoretical absorption spectra for the S,S- and O,S-linked [Os(bpy)2(DMSO)2]2+. The full widths at half-maximum value were set to be 4600 cm−1.
For S,O-linked isomer, S0 to S4 excitation, dominated by HOMO−1 to LUMO+1 transition, is the maximum absorption peak. 3.3. Intrinsic Reaction Pathways for Isomerization. Calculated energy results demonstrated that S,S-[Os(bpy)2(DMSO)2]2+ is more stable than its O,S-isomer, S,O[Os(bpy)2(DMSO)2]2+, for about 0.15 eV because the Os−S linkage mode of DMSO in Os(II) complex is preferred as mentioned above. One DMSO ligand isomerization from Os−S to Os−O linkage can be stimulated by oxidation, which is accompanied by pronounced changes of the absorption spectra. Here, intrinsic reaction pathways (IRPs) of the intramolecular Os−S→Os−O and Os−O→Os−S reactions of the sulfoxide group in [Os(bpy)2(DMSO)2]3+, [Os(bpy)2(DMSO)2]2+, and [Os(bpy)2(DMSO)2]+ complexes were examined. In these μS,O-linked transition states, DMSO1 ligand is chelated to central Os with both S and O atoms. IRC analyses, shown in Figure 7 and SI Figures S1−S3, were performed to confirm that the transition states were connected with two relevant minima. IRC results also demonstrate that electron transfer triggered intramolecular isomerization in osmium sulfoxide complexes is an elementary reaction. Calculated IRPs results are shown in Figure 7. It is found that activation energy for the Os(II)−S1→Os(II)−O1 reaction is decreased by 0.89 eV with oxidation and is increased by about 0.6 eV with reduction. This can be understood with different
4. CONCLUSION In this work, electronic and photophysical properties of [Os(bpy)2(DMSO)2]2+, as well as IRSs, of the intramolecular E
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21403037, 51478123, 21463003), the Natural Science Foundation of Jiangxi Province (20142BAB213014), Jiangxi Provincial “Ganpo Talents 555 Projects”, and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, PR China.
Figure 8. Schematic representation of the IRPs of DMSO2 linkage isomerization from S,O-[Os(bpy) 2 (DMSO) 2 ] 2+ to O,O-[Os(bpy)2(DMSO)2]2+ of the normal (black curve), one-electron-oxidized (blue curve) or -reduced (red curve) [Os(bpy) 2 (DMSO) 2 ] 2+ complexes.
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(1) Lomoth, R. Redox-Stimulated Motion and Bistability in Metal Complexes and Organometallic Compounds. Antioxid. Redox. Signaling 2013, 19, 1803−1814. (2) Roberts, M. N.; Carling, C.-J.; Nagle, J. K.; Branda, N. R.; Wolf, M. O. Successful Bifunctional Photoswitching and Electronic Communication of Two Platinum(II) Acetylide Bridged Dithienylethenes. J. Am. Chem. Soc. 2009, 131, 16644−16645. (3) Tietze, L. F.; Düfert, M. A.; Hungerland, T.; Oum, K.; Lenzer, T. Synthesis and Photochemical Investigations of Tetrasubstituted Alkenes as Molecular SwitchesThe Effect of Substituents. Chem.Eur. J. 2011, 17, 8452−8461. (4) Li, G.; Ray, L.; Glass, E. N.; Kovnir, K.; Khoroshutin, A.; Gorelsky, S. I.; Shatruk, M. Synthesis of Panchromatic Ru(II) ThienylDipyrrin Complexes and Evaluation of Their Light-Harvesting Capacity. Inorg. Chem. 2012, 51, 1614−1624. (5) Ishizuka, T.; Sawaki, T.; Miyazaki, S.; Kawano, M.; Shiota, Y.; Yoshizawa, K.; Fukuzumi, S.; Kojima, T. Mechanistic Insights into Photochromic Behavior of a Ruthenium(II)−Pterin Complex. Chem.Eur. J. 2011, 17, 6652−6662. (6) Marin, V.; Holder, E.; Hoogenboom, R.; Schubert, U. S. Functional Ruthenium(II)- and Iridium(III)-containing Polymers for Potential Electro-Optical Applications. Chem. Soc. Rev. 2007, 36, 618− 635. (7) Bonnet, S.; Collin, J.-P. Ruthenium-based Light-driven Molecular Machine Prototypes: Synthesis and Properties. Chem. Soc. Rev. 2008, 37, 1207−1217. (8) Coppens, P.; Novozhilova, I.; Kovalevsky, A. Photoinduced Linkage Isomers of Transition-Metal Nitrosyl Compounds and Related Complexes. Chem. Rev. 2002, 102, 861−884. (9) Heilweil, E. J.; Johnson, J. O.; Mosley, K. L.; Lubet, P. P.; Webster, C. E.; Burkey, T. J. Engineering Femtosecond Organometallic Chemistry: Photochemistry and Dynamics of Ultrafast Chelation of Cyclopentadienylmanganese Tricarbonyl Derivatives with Pendant Benzenecarbonyl and Pyridinecarbonyl Groups. Organometallics 2011, 30, 5611−5619. (10) Phillips, A. E.; Cole, J. M.; d’Almeida, T.; Low, K. S. Ru−OSO Coordination Photogenerated at 100 K in Tetraammineaqua(sulfur dioxide)ruthenium(II) (±)-Camphorsulfonate. Inorg. Chem. 2012, 51, 1204−1206. (11) Rack, J. J.; Mockus, N. V. Room-Temperature Photochromism in cis- and trans-[Ru(bpy)2(dmso)2]2+. Inorg. Chem. 2003, 42, 5792− 5794. (12) Rack, J. J.; Rachford, A. A.; Shelker, A. M. Turning Off Phototriggered Linkage Isomerizations in Ruthenium Dimethyl Sulfoxide Complexes. Inorg. Chem. 2003, 42, 7357−7359. (13) Rachford, A. A.; Petersen, J. L.; Rack, J. J. Designing Molecular Bistability in Ruthenium Dimethyl Sulfoxide Complexes. Inorg. Chem. 2005, 44, 8065−8075.
isomerization in [Os(bpy)2(DMSO)2]3+, [Os(bpy)2(DMSO)2]2+, and [Os(bpy)2(DMSO)2]+ complexes were examined for the aim to understand the underlying electrochromic mechanisms of osmium sulfoxide complex based electrochromic materials. Calculated binding energy of the DMSO1 ligand in S,Slinked and O,S-linked [Os(bpy)2(DMSO)2]2+ demonstrated that Os−S1 linkage mode of DMSO in Os(II) complex is preferred because the electron transfer degree from DMSO1 to Os center using the Os−S1 linkage is larger than that using the Os−O1 linkage. Then, the HOMO mainly distributed on Os center is destabilized and the LUMO mainly distributed on bpy ligands is also destabilized. Because the Os(II)−S1→Os(II)− O1 isomerization destabilizes the HOMO to a larger extent than LUMO, the HOMO−LUMO gap is decreased. Thereby, differential-bonding characteristics of DMSO ligand produce distinct electronic and photophysical properties. As a result, red-shifted absorption can be observed experimentally. Moreover, BESDMSO1 is decreased and BEODMSO1 is increased upon oxidation. On the contrary, BESDMSO1 is increased and BEODMSO1 is decreased by reduction. This means, compared with Os(II), O-bonding is preferred by the “hard” Os(III) metal, while the “soft” Os(I) metal prefers S-bonding. In addition, IRP results demonstrate Os−S→Os−O isomerization is preferred by oxidation of Os(II), and Os−O→Os−S isomerization is favored by reduction of Os(III) or Os(II). In addition, DMSO2 linkage isomerization becomes much harder to proceed due to the Os−O1→Os−S1 rearrangement attributed to the increased bond strength between DMSO2 and Os center upon DMSO1 linkage isomerization.
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REFERENCES
ASSOCIATED CONTENT
S Supporting Information *
Table S1 listing the NBO analysis results for the S−O bonds; frontier MOs energies listed in Table S2; Cartesian coordinates and energies of all calculated structures collected in Table S3− S17; Figures S1−S3 showing the IRC analysis results. This material is available free of charge via the Internet at http:// pubs.acs.org. F
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