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The Role of Pyridinium Groups and Iodide Ions in Photoelectrochromism in Viologen Based Ion-Pair Charge Transfer Complexes: Molecular Orbital Analysis Yuuichi Orimoto, Kosuke Ishimoto, and Yuriko Aoki J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10281 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 8, 2018

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The Journal of Physical Chemistry

The Role of Pyridinium Groups and Iodide Ions in Photoelectrochromism in Viologen Based Ion-Pair Charge Transfer Complexes: Molecular Orbital Analysis Yuuichi Orimoto,† Kosuke Ishimoto,‡ and Yuriko Aoki*,†,§ †

Department of Material Sciences, Faculty of Engineering Sciences, Kyushu University, 6-1 Kasuga-Park, Fukuoka 816-8580, Japan. ‡

Department of Energy Science and Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.

§

Japan Science and Technology Agency, CREST, 4-1-8 Hon-chou, Kawaguchi, Saitama 332-0012, Japan. *Correspondence to: Y. Aoki (E-mail: [email protected]) Abstract: Quantum chemistry calculations were performed to examine the fundamentals of photoinduced electron transfer (ET) in viologen (V) based ion-pair charge transfer (IPCT) complexes, and the resulting photoelectrochromism with respect to a photoswitching application. For the purpose, the photoinduced ET from counter anions to viologen or biphenyl (BP) derivatives was modeled and its relationship with their structures was analyzed. Our results showed that the electron reduction of V2+, assuming photoinduced ET, that is V2+→V+→V0, changes the conformational preference from a twisted to a planar structure due to the lowest unoccupied molecular orbital (LUMO) in V2+ showing a planar tendency. A similar feature appears in the reduction of neutral BP, that is BP0→BP−→BP2−, leading to a twisted→planar preference change due to the LUMO in BP0. The similarity between V2+ and BP0 can be explained by their similar MO shape and their identical number of electrons. Time-dependent density functional theory (TD-DFT) was applied to predict the absorption spectra of viologen and biphenyl derivatives with iodide ions considered as counter anions. In addition, geometrical optimization using the TD-DFT method was performed for viologen derivatives to stabilize a specific excitation to simulate laser irradiation. Our simulation implies that laser absorption can cause a twisted→planar change accompanied by weak charge transfer if we ignore the timescale. Moreover, it was found that the iodide ion is necessary for near-infrared (NIR) absorption corresponding to the telecommunication wavelength. This is because NIR absorption is

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attributed to a narrow energy gap generated by the insertion of orbital levels of iodide ions into the original energy gap of viologen. Contrary to the above-mentioned similarity, biphenyl was found to show totally different features from viologen regarding (1) the magnitude of the polarization between the molecule and counter anions, (2) the laser-induced excitation behavior, and (3) the peak position of NIR absorption. From the MO analysis, the role of pyridinium groups, which cannot be replaced by phenyl groups, was theoretically explained.

I. INTRODUCTION Electrochromic materials1,2 have attracted growing interest with respect to their application in color displays3 and smart windows.4 Viologen (V) 1, that is, 1,1’-disubstituted 4,4’-bipyridinium, as shown in Figure 1a, is a popular electrochromic molecule.5 Viologen is well known as an acceptor molecule, and its redox reaction produces an electrochromic color change.1,2 Because of the variety of chromic phenomena, viologen and its derivatives, including “extended viologen”,6 have been used as building blocks in various functional materials.7–10 Viologen makes a salt with counter anions11 and leads to ion-pair charge transfer (IPCT) complex,12–15 V2+(X−)2, as shown in Figure 1a. It is known that, in such systems, photoinduced electron transfer (ET) occurs between the ion pairs.8,12–15 For example, photoinduced ET from a counter anion (X−) to V2+ results in one-electron reduction of V2+ to V•+ (see Figure 1b). Such an ET in IPCT complexes plays an important role in photoelectrochromism.8,12–15 Here naturally, the conformation of viologen can affect its ET (or charge transfer (CT)) and chromic properties. Thus, to understand photoelectrochromism, it is essential to know the relationship between the redox states and the possible conformations of viologen,16 and its influence on the spectrum change. For this purpose, researchers have attempted to determine this, especially by using quantum chemistry (QC) calculations. For example, QC calculations were applied to conduct a conformational analysis in viologen for various redox states by computing torsional angle dependence of total energy17–19 or determining optimized geometries.20,21 In the researches, it was determined that dicationic and its reduced species prefer twisted and nearly planar conformations, respectively. Similar conformational analysis was also performed for the most relevant species such as protonated 4,4’-bipyridine whose dicationic and reduced forms prefer twisted and more planar structures, respectively.22,23 In a part of the studies,18,19 the stability in planar structure by the


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electron reduction was explained by an electron occupation to the lowest unoccupied molecular orbital (LUMO) of dicationic species. The LUMO has a π-bonding character between C-C bond connecting two pyridinium rings, and the electron occupation to the LUMO contributes to stabilize a planar structure. As important related studies, Imamura and Hoffmann conducted a pioneering work on a conformational preference of biphenyl and its related molecules with respect to their excited states by systematic MO analysis.24 Our research group analyzed a conformation of various charged forms of poly-para-phenylene model molecules from a MO viewpoint.25 These studies gave us an insight into the relationship between redox states and conformational preference in viologen to some extent. However, more systematic analysis from a MO viewpoint should be performed to understand the unique nature of viologen and the role of pyridinium groups by examining its similarity and difference from phenyl groups as a reference. Viologen derivatives are also promising in optical switch applications.13,14 Optical switching26,27 for controlling light paths is an important technology in the field of information and telecommunications. Recently, very fast pico–femtosecond-order switching was required to control the telecommunication band, which consists of near-infrared (NIR) wavelengths with low transmission loss. Nagamura et al. reported that polymeric 2,7-bis(4-pyridiniumyl)fluorene (PFV) salts 2, as shown in Figure 1a, show photoinduced ultrafast ON/OFF NIR switching.13 PFV consists of fluorene-containing extended viologen and ether functionalities, and it forms a salt with counter anion X− to form PFV2+(X−)2. Pump-probe measurements in ref. 13 showed that the ground state polymer PFV2+(X−)2 gives no NIR absorption peak, while a transient absorption in the NIR region appears after less than 100 fs after irradiating with a 50 fs 400 nm pulse laser as the pump light. The NIR absorption is composed of double peaks at around 1400 and 1800 nm. This photoelectrochromism was explained by photoinduced ET caused by the laser irradiation. That is to say, the irradiation causes ET from (X−)2 to PFV2+, and PFV2+ is reduced to PFV•+. It was proposed that the appearance of NIR absorption under laser irradiation was due to the extension of π-conjugation in association with the ET, and the extension occurred with no change in the molecular structure. The reverse reaction from PFV•+ to PFV2+ proceeds as a thermal reaction within 240 fs (see Figure 1b). The ultrafast photoelectrochromic change in NIR absorption has potential with respect to optical switching applications. However, the understanding of its microscopic mechanism remains limited with respect to the photoinduced ET and the resulting photoelectrochromic NIR absorption change in 2. As a QC approach to an absorption spectrum of viologen derivatives, time-dependent



density functional theory (TD-DFT)28 calculations were performed to predict spectra mainly for ultraviolet (UV) and/or visible region.11,15,19,21,29 Another approach was conducted by configuration interaction (CI) for estimating UV/visible absorption.20 Although a few studies calculated a part of NIR absorption of viologen derivatives,20,21,29 the effect of counter anions on the NIR absorption remains unclear. Counter anions should play a non-negligible role in IPCT complexes and photoinduced ET phenomena. However, a relationship between counter anions and NIR absorption has not yet been sufficiently studied to our best knowledge. In this study, QC calculations and MO analysis were systematically performed to explain the relationship between the reduction process and the corresponding preferred conformation in viologen. In particular, viologen was compared with biphenyl to determine the role of pyridinium groups in the system. TD-DFT calculations were then performed to predict absorption spectra with and without laser irradiation, especially with focusing on the effects of iodide ions considered as counter anions. Here, it has been reported that TD-DFT can predict experimentally observed transient absorption spectra.30,31 The role of pyridinium groups and iodide ions in viologen based IPCT complexes was examined to determine the mechanism of photoinduced ET and photoelectrochromism accompanying the NIR absorption change.

II. COMPUTATIONAL DETAILS All the calculations were performed using the Gaussian09 program package.32 Hartree-Fock (HF) and B3LYP DFT functional33–35 methods were used for the calculations. Closed and open shell systems were calculated within the frameworks of their restricted (R) and restricted open (RO) schemes, respectively. LanL2DZ36–39 was used as a basis set for the iodine (I) atom, and 6-31G(d) was used for C, H, and N atoms. The above mixed basis set was denoted as “BS1” in this study. Geometrical optimizations were performed while maintaining the N—I distance determined by model calculations, and no other restriction was applied. The excited state was calculated by TD-DFT.28 These conditions were adopted unless otherwise stated. All the molecular structures, MOs, and absorption spectra were depicted with GaussView 5.0.40

III. RESULTS AND DISCUSSION III.A. Conformational Preference Change by Electron Reduction


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In this section, the influence of electron reduction as a result of photoinduced ET was examined using model viologen (−R = −H) 1, shown in Figure 1a. In the remaining part of this study, model 1 will be denoted as viologen (V). To determine the electron transfer effects only, counter anions (X−) are omitted from the system in section III.A. Photoinduced ET was simulated by changing the system charge, that is, V2+→V+→V0 along with the reduction. Here, the radical electron depicted by “•” is omitted for clarity. Figure 2a shows optimized structures for V2+, V+, and V0 calculated at the R(O)HF/6-31G(d) level. A twisted structure with dihedral angle D = 52.5° was obtained for V2+. In contrast, a planar structure with D = ~0° was obtained for V+ and V0. It was also found that the length of the central C–C bond decreased with the electron reduction so that it became almost a double bond in V0. This indicates that electron reduction by the ET can lead to a change in conformational preference in viologen. Such a change was investigated both experimentally16 and theoretically.18–23 As long as we ignore the time scale, there is a possibility that the reduction causes a conformational change in viologen in a finite time. In general, however, an extremely short timescale, such as 100–240 fs, mentioned in ref. 13, is not sufficient to change molecular structures, and thus the observation may not capture the conformational change. It was reported that a photoinduced structural change requires at least ~1 ps.41–43 The packing effects of surroundings in a crystal44 can also make it difficult to induce such a structural change. As a reference to determine the effects of pyridinium groups in viologen, the geometries of biphenyl (BP) were optimized with respect to the electron reduction process, BP0→BP−→BP2−. Because viologen is two electrons richer than biphenyl, BP0 (58 electrons) has the same number of electrons as V2+, as the two carbons in BP0 were replaced by two nitrogens by removing two electrons. Similarly, BP− and BP2− correspond to V+ and V0, respectively, when focusing on the number of electrons. Figure 3a shows the optimized structures for BP0, BP−, and BP2−. A twisted structure with dihedral angle D = 45.5° was found to be the stable structure for BP0. In contrast, a planar structure with D = 0° was obtained for BP− and BP2−. At the same time, the central C–C length decreases with the electron reduction to increase the double bond properties. The charge—conformation relationship in biphenyl here is consistent with our previous work.25 A similar trend in conformational preference for V2+→V+→V0 and BP0→BP−→BP2− can be explained by the similarity of the MOs of viologen and biphenyl. First, we focus on BP. Figure 4b shows an orbital interaction diagram for two benzene molecules for constructing a neutral BP calculated at the RHF/6-31G(d) level. The figure shows only π-orbitals. Each π-orbital of BP can be identified by a simple notation, SS, AS, AA, or



SA, for occupied orbitals and SS’, AS’, AA’, or SA’ for unoccupied orbitals when the system is neutral (see the notation in refs. 24, 25). The first letter “S” or “A” indicates symmetry or asymmetry, respectively, about a mirror plane along the molecular axis through the bond between the two rings. The second letter indicates another mirror plane perpendicular to the axis through the two rings. The symmetries of the V and BP molecules both belong to the D2h point group if all the atoms are in the same plane, and the symmetry indices for D2h are also written in parentheses beside the notations SS, SA, … for each MO. We discuss here the relationship between MO phases and the change in conformational preference, as shown in Figure 4, which are described only by the π-orbitals. From the orbital interaction concept involving the nearest-neighbor interaction, the SS orbital prefers a planar structure because of in-phase orbital overlap at the central C–C bond. In contrast, the SA prefers a twisted structure because of out-of-phase overlap at the same position. The AA and AS do not contribute to the structural preference because they have no orbital coefficient at the focused position. Finally, only SS and SA orbitals affect the structural change. However, as the both the SS and SA orbitals are occupied by two electrons for each in neutral BP, stabilization by the SS orbital and destabilization by the SA orbital cancel each other out due to the opposing conformational preferences, and then two hydrogen-hydrogen repulsions at the ortho-positions around the central C–C bond become a dominant factor in determining the structure. As a result, neutral BP has a twisted conformation, as shown in Figure 3a (left), due to the hydrogen-hydrogen repulsions that overcome the effects of electronic states. Next, we focus on viologen. Figure 4a shows an interaction diagram for two neutral pyridinium radical molecules for constructing a neutral V. It was found that V0 in Figure 4a shows a similar MO shape and interaction manner to BP0 in Figure 4b. An exception was that SS and SA orbitals in V0 have different orbital shapes and energy levels to BP0, because the SS and SA orbitals have a MO coefficient on the N atoms in V0, while the AA and AS orbitals have substantially no coefficient on the N atoms. The difference leads to asymmetric splitting of the SS and SA orbitals around the AS and AA orbitals in V0. Moreover, the SOMO of the neutral pyridinium radical was strongly stabilized by electron occupation. As with BP, stabilization by the SS orbital and destabilization by the SA orbital in V0 are canceled out by the opposing structural preferences. However, the SS’ type highest occupied MO (HOMO) in V0 prefers a planar structure due to in-phase overlap at the central C–C bond. As a result of a competition between the effects of electronic states and the two hydrogen-hydrogen repulsions at the


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ortho-positions around the central C–C bond, V0 prefers a planar conformation as shown in Figure 2a (right). Finally, the reduction processes from both V2+ and BP0 will be considered. Figure 3b shows the transition of orbital energy levels for BP0→BP−→BP2−, in which the number of electrons changes as 58→59→60. The LUMO in BP0 and the corresponding MO in BP− and BP2− are shown by red circles in the panel. These MOs have in-phase orbital overlap at the central C–C bond, and they contribute to the planarity of the system by electron(s) occupation. The occupation of the LUMO in BP0 can explain the twisted→planar change during the reduction process as shown in Figure 3a, even with the two hydrogen-hydrogen repulsions at the ortho-positions in the rings around the central C–C bond. For viologen, Figure 2b shows the transition of orbital energy levels for V2+→V+→V0 with the change in the number of electrons, 58→59→60. The LUMO in V2+ and the corresponding occupied MOs in V+ and V0, shown by red circles in the panel, have in-phase orbital overlap at the central C–C bond and contribute to the twisted→planar change by electron(s) occupation. The occupation of the LUMO in V2+ can explain the change in conformational preference during the V2+→V+→V0 reduction process as shown in Figure 2a. It can be concluded that the similar trends in the conformational preference changes of viologen and biphenyl are due to the following reasons: (1) both the viologen and biphenyl consist of similar π-MOs; (2) both the reduction processes, V2+→V+→V0 and BP0→BP−→BP2−, correspond to the same transition in the number of electrons, 58→59 →60; and (3) electron occupation of the LUMO in BP0 and V2+ contributes to the preference for a planar structure due to its in-phase orbital overlap at the central C–C bond in the LUMO. The importance of the LUMO with respect to the conformational preference has also been implied in previous studies by other groups.18,19 It should be noted that the conformation was expected to be affected by the surroundings in a crystal. In fact, such packing effects become dominant for biphenyl under the specific conditions described in ref. 44.

III.B. TD-DFT Prediction of Absorption Spectra In this section, TD-DFT calculations were applied to predict absorption spectra of methyl viologen (MV) diiodide salt (1,–R =–CH3, X− = I−) as a model system, as shown in Figure 5a. First, to determine the distance of I− from an N atom, the potential energy surfaces (PES) at the R(O)HF/BS1 level were plotted with respect to the N—I distance (d) in a pyridinium iodide salt as shown in the inset in Figure 1c. Figure 1c shows the



PES results for both singlet and triplet spin states. The most stable distance was d’ = 3.4 Å for the singlet state. Geometrical optimizations using some possible initial structures of the MV salt, MV2+(I−)2, were performed at the B3LYP/BS1 level with fixed d’. Figure 5a (right-hand) shows the most stable structure for MV2+(I−)2. In our results, two I− ions preferentially attach at opposite sides of the molecular plane. To simplify the analysis, calculations in the remaining part of this study adopt the singlet spin state, the N—I distance d’, and the position of I− ions found in the most stable structure. In Figure 5a, the dihedral angle in MV2+(I−)2 is D = 24.4°, which is smaller than D = 52.5° in V2+ without I− ions but larger than D = 0.1° in V+, as shown in Figure 2a. Figure 5a also shows the NBO net charge of I atoms and the whole of MV moiety. Each I atom has a charge of −0.76, and the whole of MV has a charge of +1.52. Therefore, D = 24.4° in MV2+(I−)2 corresponds to a structure for the intermediate charge V1.52+, and thus it is a reasonable conformation. In a classical description, the N atom in MV2+(I−)2 is described as having a positive charge and can form an electrostatic interaction with a negatively charged I− ion (see Figure 1a). In contrast, our calculations showed that the N atom in MV2+(I−)2 has a negative charge of −0.30. The positive charge of MV of +1.52 was spread over the whole MV moiety. However, this is not surprising because an N atom can have either a positive or negative charge depending on the circumstances.45 In our results, the N–I interaction cannot be explained by the classical description. In a future study, the unique N–I interaction will be analyzed using our developed through-space/bond interaction analysis method,46,47 in which the specific interaction can be quantitatively examined by controlling the exponents in the basis functions. Spectrum for Viologen Derivative. Figure 5a (left-hand) shows an absorption spectrum predicted by TD-DFT calculations at the B3LYP/BS1 level based on the optimized structure of MV2+(I−)2 in the ground state (GS), as shown in the right panel. The spectrum shows the following three characteristic absorption peaks. MV2+(b2g→b3u): an absorption peak at 273 nm in the UV region. Figure 6a shows orbital energy levels and the excitation component for MV2+(I−)2 in the GS. The MV2+(b2g →b3u) peak indicated by red arrows in the panel corresponds to an excitation within the MV moiety. That is, the peak is an excitation from occupied π-MOs (50 and 51) to the LUMO (58), both localized in the MV moiety, while the LUMO also has a coefficient of (I−)2 ions. (I−)2→MV2+(b1g, b2g): an absorption peak around 600 nm in the visible region, consisting of several components. Excitation components for this peak are omitted in Figure 6a for clarity.


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(I−)2→MV2+(b3u): an absorption peak at 1811 nm in the NIR region. The (I−)2→ MV2+(b3u) peak shown by blue arrows in Figure 6a corresponds to an excitation from the p-orbital of (I−)2 to the LUMO (58) localized in the MV moiety. The MV2+(b2g→b3u) absorption at 273 nm might be assigned to the 400 nm laser pulse used in ref. 13. The (I−)2→MV2+(b1g, b2g) and (I−)2→MV2+(b3u) absorptions are relevant to the photoinduced ET reaction and are capable of accelerating (I−)2→MV2+ charge transfer by excitation. However, these absorptions, expected to be related to the photoinduced ET, have longer wavelengths than the assumed laser pulse. Also important is the fact that MV2+(I−)2 in the GS already shows an absorption peak in the NIR region assigned to (I−)2→MV2+(b3u) excitation, although this state is assumed to be under no laser irradiation. As shown in Figure 6a, the NIR absorption was caused by the insertion of orbital levels that consist of 5p orbitals of I− ions, as depicted by a circle in the panel. The newly generated HOMO (57) is created by inserting orbital levels of (I−)2, which realizes a narrow HOMO-LUMO gap and low energy excitation in the NIR region. As a reference, TD-DFT calculations were performed for MV2+ without counter anions (I−)2 (Figure 7a). In the predicted spectrum, there was no absorption other than a sharp peak in the UV region. This is natural because the orbital levels of (I−)2 produce absorption peaks in the visible and NIR regions, as shown in Figure 5a, even for the GS system without laser irradiation according to our calculations. Moreover, Figure 6a shows that the LUMO of MV2+ without (I−)2, corresponding to the SS’ HOMO in V0 in Figure 4a, forms an interaction with an orbital of (I−)2, and the interaction produces an occupied MO (52) and a LUMO (58). The mixture of the LUMO of MV2+ with an occupied orbital of iodide ions results in CT from (I−)2 to MV2+ in the GS. The CT results in the IPCT complex with an intermediate charge distribution such as MV1.52+(I0.76−)2 in the GS in Figure 5a. Spectrum for Biphenyl Derivative. For a comparison, Figure 7b shows an absorption spectrum predicted by the TD-DFT method for 4,4ʹ-dimethylbiphenyl (DMBP) diiodide, DMBP2+(I−)2. The spectrum shows the following three characteristic peaks, which are similar to those for MV2+(I−)2 in Figure 5a. DMBP2+(b2g→b3u): the 278 nm absorption corresponds to an excitation within the DMBP moiety, identified by DMBP2+(b2g→b3u), as shown by red arrows in Figure 6b. DMBP2+(b3u→b2g): the 508 nm absorption in the visible region can be assigned to DMBP2+(b3u→b2g). (I−)2→DMBP2+(b2g): the 1212 nm absorption in the NIR region corresponds to excitation from iodide ions to DMBP2+, identified by (I−)2→DMBP2+(b2g).



The DMBP2+(b2g→b3u) excitation might be assigned to the laser pulse. As shown in Figure 6b, the excitation was found to use the LUMO+1 (58), unlike MV2+(I−)2, in which the corresponding excitation uses the LUMO, as shown in Figure 6a. There is an important difference between MV2+(I−)2 and DMBP2+(I−)2 regarding the features of the laser-induced excitation. In MV2+(I−)2, an electron is excited to the LUMO, preferentially forming a planar conformation by electron occupation (see Figure 6a, corresponding LUMO of V2+ in Figure 2b, and SS’ in Figure 4a). This is the same situation as electrochemical electron reduction of MV2+(I−)2, in which the same LUMO is occupied by electron(s). On the other hand, in DMBP2+(I−)2, an electron is excited to the LUMO+1, preferentially forming a planar conformation by electron occupation (see Figure 6b, corresponding LUMO of BP0 in Figure 3b, and SS’ in Figure 4b), whereas an electron(s) occupies the LUMO, preferentially forming a twisted conformation by electron occupation for electrochemical electron reduction of DMBP2+(I−)2. As with MV2+(I−)2, the NIR absorption for DMBP2+(I−)2, that is (I−)2→DMBP2+(b2g), was caused by the insertion of orbital levels of (I−)2 (see Figure 6b). The other feature is that the (I−)2→DMBP2+(b2g) peak in DMBP2+(I−)2 consists mainly of excitation from HOMO (56) to LUMO (57) as shown in Figure 6b, while the corresponding (I−)2→MV2+(b3u) peak for MV2+(I−)2 in Figure 6a is described by two excitations, MO (53)→LUMO (58) and MO (55)→LUMO (58). Figure 7b (bottom) shows the optimized structure of DMBP2+(I−)2 with dihedral angle D = 25.6°. The figure also shows an NBO charge of −0.28 for each I atom and +0.56 for the whole DMBP moiety. It can be guessed from Figures 3b and 4b that a positively charged BP tends to form a planar structure, because electron(s) are removed from the HOMO in BP0, which preferentially forms a twisted structure due to out-of-phase overlap at the central C–C bond; this tendency was confirmed by our previous study.25 The D = 25.6° obtained here corresponds to the intermediate charge BP0.56+ and is a reasonable conformation. Figure 6b shows that the LUMO of DMBP2+, corresponding to SA in BP0 in Figure 4b, interacts with an occupied orbital level of (I−)2. The interaction produces an occupied MO (51) and the LUMO (57). The mixture of the LUMO of DMBP2+ with the occupied orbital of (I−)2 indicates IPCT from (I−)2 to DMBP2+, and this causes a charge distribution such as DMBP0.56+(I0.28−)2 in GS. Here, we compare MV2+(I−)2 with DMBP2+(I−)2 with respect to the magnitude of polarization between MV/DMBP and I2. Figure 6a shows that the interaction between the LUMO of MV2+ and one of the orbital levels of (I−)2 provides a new LUMO (58) and MO (52) of MV2+(I−)2. The shape of MO (52) indicates a small degree of MO mixing of the LUMO of MV2+ with the focused


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MO, leading to a small IPCT from (I−)2 to MV2+. The small CT from (I−)2 to MV2+ maintains a large polarization such as MV1.52+(I0.76−)2. In contrast, in Figure 6b, the interaction between the LUMO of DMBP2+ and an orbital level of (I−)2 generates a new LUMO (57) and MO (51) of DMBP2+(I−)2. The shape of the MO (51) shows comparably large MO mixing of the LUMO of DMBP2+ with the MO in question. The mixing results in a larger IPCT from (I−)2 to DMBP2+ compared with MV2+(I−)2. The large CT from (I−)2 to DMBP2+ leads to a small polarized system such as DMBP0.56+(I0.28−)2. Figures 5a and 7b show a large difference between the NIR peak positions of MV2+(I−)2 and DMBP2+(I−)2, that is, 1811 nm for MV2+(I−)2 and 1212 nm for DMBP2+(I−)2. From another viewpoint, the BP skeleton can be considered as a part of the fluorene structure in PFV2+ 2 in Figure 1a. Under this assumption, the 1212 nm absorption of DMBP2+(I−)2 shown in Figure 7b might be relevant to the ~1400 nm peak in the NIR double-peak absorption in the experiment on PFV2+(X−)2 in ref. 13. The 1811 nm absorption in MV2+(I−)2 in Figure 5a may be related to another peak at ~1800 nm in the NIR absorption in the experiment, while it should also be noted that in our calculation, these NIR absorption peaks were predicted even for the GS structure without considering laser irradiation. To confirm the assumption, TD-DFT calculation was performed to predict absorption spectrum for the GS structure of fluorene-based extended viologen (FV) diiodide salt, FV2+(I−)2 as shown in Figure 7c. Figure 7c (bottom) shows the optimized structure of FV2+(I−)2. As the result of geometrical optimization, dihedral angle between pyridinium ring and fluorene moiety was found to be D = 34.4°, and an NBO charge shows −0.82 for each I atom and +1.64 for the FV moiety. The spectrum shows three characteristic peaks at 381, 640, and 1225 nm. Among them, we focus on the 1225 nm absorption in the NIR region. The 1225 nm absorption corresponds to an excitation from orbital level of (I−)2 to the LUMO delocalized on the whole FV moiety as shown in the inset in Figure 7c. The position of the NIR absorption in FV2+(I−)2 was found to be similar to that in DMBP2+(I−)2 with 1212 nm (Figure 7b) rather than that in MV2+(I−)2 with 1811 nm (Figure 5a). This change in NIR absorption position can be qualitatively explained by the insertion of fluorene moiety in FV2+(I−)2 to separate viologen into two pyridinium rings. As a result, the effects of viologen in the NIR absorption weaken, and fluorene moiety plays a dominant role. This leads to the NIR absorption in FV2+(I−)2 similar to that in DMBP2+(I−)2. Absorption spectrum was also calculated for FV2+(I−)2 with keeping a planarity between fluorene and two pyridinium rings during geometrical



optimization. However, there is no significant difference on the predicted spectrum from that for the relaxed model as shown in Figure 7c. Figure 7d shows the calculated absorption spectrum for ether-containing FV derivative diiodide salt, FVE2+(I−)2, and its optimized structure (Figure 7d (bottom)). It was found from the comparison between Figures 7c and 7d that the ether part in FVE2+(I−)2 does not affect largely its absorption spectrum, optimized geometries, and NBO charge distribution. It is because σ orbitals in the ether part have little mixing into the π orbitals that play an important role in the excitations or delocalization in FV part. The 1216 nm absorption of FVE2+(I−)2 in the NIR region corresponds to an excitation from (I−)2 to the LUMO localized only on FV moiety. It can be concluded that our results using small model systems may not reproduce the double-peak NIR absorption observed in the experiment in ref. 13. As the next step to improve our approach, larger systems including a viologen-based polymer with surroundings such as neighboring polymer chains will be considered by using our developed elongation method48,49 to calculate the electronic structures of huge systems with a linear-scaling computational cost.

III.C. Spectral Change by Simulated Laser Irradiation To simulate laser irradiation at 400 nm, TD-DFT calculations were applied to geometrical optimizations of MV2+(I−)2 to stabilize MV2+(b2g→b3u) excitation, because the MV2+(b2g→b3u) excitation at 273 nm in Figure 5a has the closest wavelength to the laser light. Figure 5b (right) shows the optimized structure for the excited state (ES), denoted as “ES structure.” The ES structure exhibited a completely planar structure with D = 0.0°, in contrast to the GS structure having a twisted structure with D = 24.4°. In addition, the central C–C length decreases with the change from the GS (C–C = 1.464 Å) to the ES structure (C–C = 1.404 Å), increasing its double bond character. The predicted absorption spectrum for the ES structure, as shown in Figure 5b (left), shows that the absorption in the UV region causes a red shift from 273 nm to 318 nm compared with the spectrum for the GS structure in Figure 5a. This is expected because the optimization was conducted to stabilize the MV2+(b2g→b3u) excitation at 273 nm in the GS spectrum. Simultaneously, the (I−)2→MV2+(b1g, b2g) and (I−)2→MV2+(b3u) peaks in Figure 5b showed a blue shift and red shift, respectively, compared with the GS spectrum in Figure 5a. As a result, MV2+(b2g→b3u) and (I−)2→MV2+(b1g, b2g) peaks in the ES spectrum are closer to each other than in the GS spectrum. In particular, the red shift of the (I−)2→MV2+(b3u) peak is large compared with that of the other peaks. This


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could be caused by the twisted→planar conformational change. As shown in Figure 5b (right), the NBO charges for the ES structure (I: −0.67, MV: +1.34) showed smaller polarization between I2 and MV than for the GS structure (I: −0.76, MV: +1.52) in Figure 5a (right). That is, the simulated laser irradiation causes (1) CT from (I0.76−)2 to MV1.52+, resulting in suppression of the polarization, and (2) a structural change from the twisted (D = 24.4°) to the planar (D = 0.0°) conformation. The laser-induced CT from (I0.76−)2 to MV1.52+ corresponds to photoinduced CT acceleration as a part of photoinduced ET in IPCT complexes, although its magnitude was limited. The conformational change can be explained by the MV2+(b2g→b3u) excitation, assuming laser irradiation. The LUMO identified for MV2+(b3u) in Figure 6a has an in-phase orbital overlap at the central C–C bond; thus, it preferentially forms a planar structure by electron occupation. In addition, the MO (51) identified for MV2+(b2g) has an out-of-phase overlap at the central bond, and then a loss of electrons from the MO (51) by excitation also contributes to the planar structure. Therefore, due to both these effects, the MV2+(b2g→b3u) excitation is expected to increase the planarity of the system. Again, it should be stressed that the conformational change cannot occur within the extremely short femtosecond order timescale considered in the experiment. Figure 6a implies that the MV2+(b2g →b3u) excitation can help to extend π-conjugation through the C–C bonding inter-rings in MV moiety. This is because the initially occupied MO (51, b2g) has a disconnected π-orbital at the central C–C bonding inter-rings, while the newly occupied MO (58, b3u), via the excitation, has a connected π-orbital at this position. This might be related to the laser-induced extension of π-conjugation without structural changes discussed in ref. 13. As a reference, geometrical optimization was performed for MV2+(I−)2 to stabilize the (I−)2→MV2+(b3u) excitation corresponding to the NIR absorption. Figure 5c (right) shows the optimized structure for the ES, denoted as “ESʹ structure.” The conformation changed slightly from D = 24.4° to 23.3° with the change from the GS to the ESʹ structure, and the C–C length and NBO charge showed no significant change. Figure 5c (left) shows the absorption spectrum for the ESʹ structure. In the spectrum, the absorption peak in the NIR region showed a red shift from 1811 nm to 1871 nm as expected, because the optimization was performed to stabilize the (I−)2→MV2+(b3u) excitation. The other peaks in the spectrum for the ESʹ structure (Figure 5c) approximately retain the positions of those for the GS structure. The small conformational change in the ESʹ structure can be explained by the (I−)2→MV2+(b3u) excitation. Electron occupation of the LUMO (b3u) by excitation preferentially results in a planar structure (see Figure 6a). However, the loss of electrons from the (I−)2 orbitals



does not affect the conformational preference of MV moiety. Thus, in the (I−)2→ MV2+(b3u) excitation, only the former effect contributes to a conformational change. As a result, the preference for a planar structure via (I−)2→MV2+(b3u) excitation can be weaker than that via MV2+(b2g→b3u) excitation. In this study, TD-DFT calculations were performed to acquire fundamental knowledge on laser-induced photoelectrochromism in viologen derivatives. On the other hand, laser excitation is a highly energetic reaction and may not be adequately described by the perturbative TD-DFT treatment. In fact, the occurrence of photoinduced CT as a part of ET was considerably limited as shown by the small CT {GS (I: −0.76, MV: +1.52) → ES (I: −0.67, MV: +1.34)} in our results. To obtain a clearer description of the photoelectrochromism, further advanced treatment is needed. Despite such a limited situation, we believe that our approach is an important first step in understanding this phenomenon.

IV. CONCLUSIONS In the present study, QC calculations were applied to examine photoinduced ET and photoelectrochromism in viologen (V) based IPCT complexes to acquire knowledge for a photoswitching application. We modeled the photoinduced ET from counter anions to viologen or biphenyl (BP) derivatives, and analyzed its relationship with their structures to know the unique features of viologen. Our calculations showed that the electron reduction of dicationic viologen, V2+→V+→V0, assuming photoinduced ET, causes a twisted→planar conformational preference change. The change is attributed to the properties of the LUMO of V2+, in which in-phase orbital overlap occurs at the central C–C bond. The electron occupation of the LUMO contributes to the preference for a planar structure. As a reference, we investigated the reduction effects for BP. The reduction, BP0→BP−→BP2−, showed a similar twisted→planar preference change due to electron occupation of the LUMO in BP0, which has a similar MO shape to the LUMO of V2+. The similar changes in conformational preferences of V2+ and BP0 can be explained by the similarity of their MO shape and their identical number of electrons. TD-DFT calculations were applied to predict absorption spectra for MV diiodide salt. MV2+(I−)2 in the GS showed three characteristic absorption peaks identified as (1) MV2+(b2g→b3u) in the UV region, (2) (I−)2→MV2+(b1g, b2g) in the visible region, and (3) (I−)2→MV2+(b3u) in the NIR region. The MV2+(b2g→b3u) absorption at 273 nm might be assigned to the irradiating 400 nm laser pulse used in the experiment in ref. 13. The (I−)2 →MV2+(b1g, b2g) and (I−)2→MV2+(b3u) absorptions are relevant to the photoinduced ET


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from (I−)2 to MV2+ by excitation, while they have a longer wavelength than the assumed laser pulse. Another point is the fact that MV2+(I−)2 in the GS already shows an NIR absorption regardless of the laser irradiation because the insertion of orbital levels of (I−)2 produces a narrow HOMO-LUMO gap. For comparison, TD-DFT calculations were also conducted for 4,4ʹ-dimethylbiphenyl (DMBP) diiodide salt. This showed that DMBP has different features from MV with respect to (i) polarization between the MV/DMBP moiety and counter anions, (ii) the behavior of laser-induced excitation, and (iii) the peak position of NIR absorption. These differences mean that the role of pyridinium group in the photoelectrochromism cannot be fulfilled by phenyl group. To simulate laser irradiation at 400 nm, TD-DFT calculations were applied to geometrical optimizations of MV2+(I−)2 to stabilize MV2+(b2g→b3u) excitation. The optimized structure for the ES was found to be completely planar in contrast to the twisted conformation in the GS structure. Moreover, the NBO charge shows that polarization between I2 and MV is suppressed by the GS → ES structural change. This indicates that the simulated laser irradiation accelerates CT from (I−)2 to MV2+, leading to suppression of polarization. This CT corresponds to the photoinduced CT acceleration as a part of photoinduced ET, although its magnitude was very small. Compared with the GS structure, the absorption spectrum for the ES structure exhibits a red shift of the (I−)2→MV2+(b3u) peak. The shift may be caused by the twisted→planar conformational change. The LUMO (b3u) and MO (51, b2g) in MV2+(I−)2 have in-phase and out-of-phase orbital overlaps at the central C–C bond, respectively. The conformational change can be explained by electron occupation of the preferred planar LUMO and loss of electrons from the preferred twisted MO (51) by excitation. However, we should consider the fact that the femtosecond order timescale in the experiment does not allow such conformational change. Our calculations and analyses here were limited by using conventional methods and model molecules. However, we obtained a theoretical insight into the importance of using viologen (instead of biphenyl) for photoelectrochromism and the relationship between counter anions and NIR absorption from the viewpoint of MO theory. It should be noted that the explanation based on QC can contribute to the understanding of photoinduced ET in IPCT complexes and resulting photoelectrochromism, and it can facilitate the development and design of electrochromic materials for photoswitching. ACKNOWLEDGEMENTS Authors thank Ms. Ikuko Okawa for the data reduction and technical support to this work. The present study was supported by the Ministry of Education, Culture, Sports,



Science and Technology of Japan (MEXT) / Japan Society for the Promotion of Science (JSPS) (JSPS KAKENHI Grant Number JP: 23245005, 16KT0059, 25810103, 15KT0146, and 16K08321), and the Japan Science and Technology Agency (JST), CREST. The computations in this work were conducted by Linux OS system in our laboratory and computer facilities at the Research Institute for Information Technology, Kyushu University.

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Figure 1. (a) 1,1’-disubstituted 4,4’-bipyridinium salt (viologen (V)) 1 and polymeric 2,7-bis(4-pyridiniumyl)fluorene (PFV) salts 2,13 where X− is a counter anion. (b) Photoinduced electron transfer in viologen based ion-pair charge transfer (IPCT) complex.14 (c) Potential energy surface for N—I distance in pyridinium iodide salt, as shown in the inset.


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Figure 2. (a) Optimized structures for V2+, V+, and V0. “D” indicates the dihedral angle between two six-membered rings. The central C–C bond length is also given in parentheses. (b) Orbital energy levels for V2+, V+, and V0. Relevant MOs with the same shape are connected by supporting lines. The number near each level shows the orbital energy in Hartree units. For clarity, occupying electron(s) are shown by arrow(s) only for the highest occupied MO. Red circles indicate the LUMO of V2+ and the corresponding MOs in V+ and V0.



Figure 3. (a) Optimized structures for BP0, BP−, and BP2−. “D” indicates the dihedral angle between two six-membered rings. The central C–C bond length is also given in parentheses. (b) Orbital energy levels for BP0, BP−, and BP2−. Relevant MOs with the same shape are connected by supporting lines. The number near each level shows the orbital energy in Hartree units. For clarity, occupying electron(s) are shown by arrow(s) only for the highest occupied MO. Red circles indicate the LUMO of BP0 and the corresponding MOs in BP− and BP2−.


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Figure 4. (a) Orbital interaction diagram for two neutral pyridinium radical molecules for constructing neutral viologen (V0). The number near each level indicates the orbital energy in Hartree units. (b) Orbital interaction diagram for two benzene molecules for constructing neutral biphenyl (BP0). Notations SS, AS, AA, and SA indicate π-orbitals of b3u, b1g, au, and b2g, respectively. The first letter indicates symmetry (S) or asymmetry (A) about a mirror plane along the molecular axis through the central C–C bond between two rings, while the second letter indicates another mirror plane perpendicular to the axis through the two rings.



Figure 5. Absorption spectra of dicationic methyl viologen (MV2+) with iodide ions (X− = I−) predicted by TD-DFT calculations for (a) ground state (GS) structure, and two excited states (ES) structures obtained by stabilizing the energy for (b) MV2+(b2g→b3u) and (c) (I−)2→MV2+(b3u) excitations. The optimized structures and NBO charges for each condition are given at the right-hand side. Dihedral angles (D) between two rings and central C–C bond lengths are shown near the structures. It should be noted that the symmetry indices assume a D2h point group when all the atoms are located in the same molecular plane without iodide ions.


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Figure 6. Excitation coefficients calculated by TD-DFT method for ground state (GS) structure of (a) dicationic methyl viologen (MV2+) and (b) dicationic dimethylbiphenyl (DMBP2+) with iodide ions (X− = I−). The number near each model indicates MO numbering, and the numbers near the orbital levels show the orbital energy in Hartree units. The numbers near the arrows show the transition coefficient in each excitation.



Figure 7. Absorption spectra predicted by TD-DFT method for ground state (GS) structure of (a) dicationic methyl viologen (MV2+) without counter anions, (b) dimethylbiphenyl (DMBP) diiodide salt, DMBP2+(I−)2, (c) fluorene-based extended viologen (FV) diiodide salt, FV2+(I−)2, and (d) its ether-containing derivative, FVE2+(I−)2. Optimized structures are depicted beneath each graph. The NBO charge is given except for (a). Dihedral angles (D) and C–C bond lengths between two adjacent rings are also shown near the structures. It should be noted that the symmetry indices for (a) and (b) assume a D2h point group when all the atoms are located in the same molecular plane without iodide ions.


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TOC Graphic


research 1..11 - American Chemical Society

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