Efficient Photoswitch System Combining a Dimethyldihydropyrene

Article pubs.acs.org/IC

Efficient Photoswitch System Combining a Dimethyldihydropyrene Pyridinium Core and Ruthenium(II) Bis-Terpyridine Entities Margot Jacquet,† Frédéric Lafolet,*,†,‡ Saioa Cobo,*,† Frédérique Loiseau,† Assil Bakkar,† Martial Boggio-Pasqua,§ Eric Saint-Aman,† and Guy Royal† †

Université Grenoble Alpes, DCM UMR 5250, F-38000 Grenoble, France Université Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR 7086 CNRS, 15 rue Jean-Antoine de Baïf, 75205 Paris Cedex 13, France § Laboratoire de Chimie et Physique Quantiques (UMR5626), CNRS et Université de Toulouse 3, Toulouse, France ‡

S Supporting Information *

ABSTRACT: Terpyridine ruthenium complexes linked to the dimethyldihydropyrene (DHP) photochromic unit have been synthesized and fully characterized by cyclic voltammetry and absorption and emission spectroscopy. The study of the photoisomerization reaction undergone by the DHP motif under visible light irradiation is reported. In comparison to previous work, the introduction of an electron-withdrawing pyridinium spacer between the chelating terpyridine unit and the DHP skeleton has considerably tuned the photochromic properties of the free ligands and their corresponding complexes in term of time response and photoreversibility. A rapid, reversible, and complete conversion between the closed and the open forms has been clearly evidenced under visible light irradiation. Only slight perturbations have been induced by the presence of ruthenium centers. Experimental findings and their interpretation have been supported by theoretical calculations.

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less π conjugated cyclophanediene isomer, as depicted in Scheme 1.

INTRODUCTION The increasing demand for nanoscale molecular devices and their ongoing miniaturization have given rise to an intense research activity in the area of molecular electronics and information storage. In recent years, a great deal of effort has been devoted to the construction of functional molecular components able to be addressed with an external stimulus.1 However, the ability to reversibly switch between more than two distinguishable stable states is still a stimulating concern.2 An attractive approach for the development of multiaddressable molecular switches consists in covalently combining photoactive building block with electroresponsive metal complexes exhibiting at least two reversible redox states.3 Such molecular assemblies have been particularly studied in switching luminescence and constructing integrated memories or logic gates. The design of various photochromic ligands has been reported in the literature. Among the photochromic systems, fulgides,4 spiropyrans,5 diarylethenes,6 and azobenzenes7 have been intensely employed. These systems may display high photoreversibility, good fatigue resistance upon light irradiation, and thermal irreversibility. On the other hand, our research activity in the last years has focused on the transdimethyldihydropyrene (DHP)/cyclophanediene (CPD) photochromic couple.8 The π-conjugated DHP photochromic unit can be reversibly converted under visible light irradiation to its © 2017 American Chemical Society

Scheme 1. Isomers of 2,7-di-tert-Butyl-trans-10b,10cdimethyl-10b,10c-dihydropyrene (t-DHP)

The DHP moiety represents a rare example of a negative-Tphotochrome and can be regarded as a key building block that is chemically tunable and suitable for construction of multifunctional assemblies. Recently, it has been established that the substitution of the t-DHP core with electronwithdrawing pyridinium entities greatly improves the ringopening reaction efficiency and allows a fast and complete conversion between the two isomers.9 Moreover, t-DHP-based photochromic transition-metal complexes have been reported showing that the addition of one redox activity to the Received: November 30, 2016 Published: April 3, 2017 4357

DOI: 10.1021/acs.inorgchem.6b02861 Inorg. Chem. 2017, 56, 4357−4368

Article

Inorganic Chemistry photochromic architecture, through the complexation of the terpyridine, poorly alters the photochromic properties, in terms of conversion rate.3i,j Another benefit of using metal coordination complexes is the easy control of the supramolecular shape that is guided by the geometry of the coordination sphere. In particular, terpyridinebased metal complexes are attractive because of their easy and reliable formation of octahedral structures.10 Thus, the connection of the two terpyridine moieties in their 4′-positions with a functional linker can afford linear multimetallic structures. Bimetallic architectures or coordination polymers, including a photochromic bridging ditopic ligand as a lightdriven component, have been especially developed in order to control electron or energy transfer and to modulate the metal− metal intramolecular communication. In numerous cases, ruthenium complexes have been considered as active components due to their appealing electrochemical, photophysical, and photochemical properties.11 However, the photoswitching of electronic communication between redoxactive units is still facing a great challenge in the molecular electronics field.3h,8g,i,12 As a successful example, Nishihara’s group described a ferrocenene-disubstituted DHP molecule, as the first photoswitchable mixed valence system using DHP as spacer.8i,g A strong ferrocene−ferrocene interaction has been evidenced through a π-conjugated ethynyl moiety, connected at the 4- and 5-positions of the t-DHP unit. It has been confirmed that the chemical connection between the subentities is crucial in the design of such bifunctional compounds to tune the intrinsic properties of the photochromic unit and the communication between the metallic centers. Recently, Nishihara and co-workers,13 as well as our group,14 reported photochromic systems in which two Ru(bis-terpyridine) complexes were connected at the 4- and 9-positions of the 2,7-di-tert-butyl-trans-10b,10c-dimethyl-10b,10c-dihydropyrene subunit via phenyl or ethynyl linkers. Unfortunately, it appeared that the phenyl linker led to very slow photoisomerization,14 whereas the use of an ethynyl linker completely prevents the photoconversion of the t-DHP core.13 Herein, we present the synthesis and the photophysical characterizations of two photochromic terpyridine ligands and their corresponding ruthenium(II) complexes in which the terpyridine moieties have been covalently linked to the t-DHPbased component through a pyridinium spacer (Chart 1) in order to get a rapid, reversible, and complete conversion between the closed and the open forms of the metallic architectures and to envisage the photoswitching of the electronic communication between the two ruthenium centers.

Chart 1. Chemical Structures of the Synthesized Ruthenium(II) Complexes

procedure to lead to the targeted pyridinium hexafluorophosphate salts 1c+ and 2c2+, isolated as red powders. Single crystals of 2c2+ have been obtained by slow diffusion of diethyl ether into a CH3CN solution of the ligand. The molecular structure of 2c2+ has been determined by singlecrystal X-ray structural analysis (Figure 1). 2c2+ crystallized in the triclinic system (space group P1)̅ . The length of the central C−C bond is 1.608 Å and is in agreement with the usual value found for DHP derivatives.9 This distance is 0.06 Å longer than the theoretical calculated bond length of 1.548 Å. The three units of the molecule (the DHP core, the pyridinium linker, and the terpyridine ligand) are not coplanar, with the dihedral angle between the DHP core and pyridinium plane equal to 42.6° and that between the pyridinium plane and the terpyridine ligand equal to 42.5°. The respective DFT calculated values for these dihedral angles are 43.0 and 37.2°, in good agreement with experimental data considering that the calculations were performed on an isolated model system (2c2+model, see Computational Section) and that these torsion angles (loose normal modes) are probably very sensitive to the crystal packing. Ligands 1c+ and 2c2+ were then directly metalated with the precursor Ru(tpy)Cl3 (tpy = terpyridine) to afford the corresponding mono- and binuclear ruthenium complexes Ru(1c+) and Ru2(2c2+), respectively. The Ru(1c+)2 complex was obtained using RuCl3 and 2 mol equiv of 1c+. All of the synthesized complexes have been purified by silica gel column chromatography using a CH3CN/CH3OH/aqueous saturated NaCl mixture as eluent. The obtained products were subjected to an anion exchange procedure with KPF6, in order to isolate the species as their PF6− salts. The free ligands 1c+ and 2c2+ and their corresponding complexes have been investigated by highresolution mass spectrometry and 1H and 13C NMR spectroscopy in deaerated CD3CN solution to prevent endoperoxide formation.15 As example, partial 1H NMR spectra for 2c2+ and Ru2(2c2+) (and their open forms as well) are shown in Figures S1 and S2 in the Supporting Information, respectively. Peak assignment has been reported in the Supporting Information, for all of the compounds. Electronic Absorption and Emission Properties. The UV−visible absorption spectra of the ligands and their corresponding ruthenium complexes in acetonitrile solution at

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RESULTS AND DISCUSSION Design and Synthesis of the Compounds. Chemical structures of the synthesized compounds, and the abbreviations used, are shown in Chart 1 and Scheme 2 (the index “c” indicates the closed state of the photochromic unit). The targeted terpyridine derivatives 1c+ and 2c2+ have been synthesized following the route depicted in Scheme 2. 1c+ and 2c2+ were prepared from the previously described mono- and bis-pyridine-substituted 2,7-di-tert-butyl-trans10b,10c-dimethyl-10b,10c-dihydropyrene.9 These intermediates were then reacted in acetonitrile solution with 1 equiv of 4′-(4bromomethylphenyl)-2,2′:6′,2″-terpyridine per pyridine subunit. Upon filtration of the formed precipitates, the crude reaction products were subjected to an anion exchange 4358


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Inorganic Chemistry Scheme 2. Synthetic Pathways for Preparation of DHP-Substituted Terpyridine Ligands 1c+ and 2c2+ a

a

Legend: (i, ii) 4′-(4-bromomethylphenyl)-2,2′:6′,2″-terpyridine, CH3CN, 80 °C.

Table 1. Spectroscopic Data of the Synthesized Compounds in CH3CN Solutionsa emission absorption λ (nm) (ε (104 cm−1 M−1)) t-DHP 1c+ 1o+ 2c2+

Figure 1. Structure of 2c2+ with thermal ellipsoids plotted at the 50% probability level. Hydrogen atoms, solvent molecules, and counterions are omitted for clarity.

2o2+ Ru(tpy)2 Ru(1c+)

293 K are shown in Figure 2. Absorption wavelength maxima and the corresponding molar absorption coefficients are given in Table 1.

λ (nm)

341 (10.3); 379 (3.9); 477 (1.1); 641 (0.09) 253 (3.39); 275 (2.9); 337 (3.45); 423 (1.68); 524 (0.57); 672 (0.19) 252; 275; 315; 418 252 (6.01); 276 (5.44); 322 (2.38); 427 (2.89); 537 (0.88); 686 (0.26) 253; 278; 319; 363; 426 270 (4.23); 308 (6.52); 475 (1.46) 274 (5.38); 281 (5.35); 310 (7.04); 332 (5.94); 436 (2.75); 483 (2.67); 534 (1.1); 674 (0.23)

τ (ns)

648

10−3φem 3

733

1.6

3.8

737

1.0

2.4

739

0.25 2.4

6.1

0.3 Ru(1o+) Ru(1c+)2

275; 282; 309; 328; 440; 483 284 (12.3); 313 (7.4); 336 (7.4); 424 (3.3); 490 (2.8); 536 (1.6); 668 (0.4)

Ru(1o+)2 Ru2(2c2+)

314; 331; 420; 490 274 (10.73); 281 (10.8); 309 (13.29); 330 (8.92); 430 (5.43); 483 (5.33); 541 (2.02); 689 (0.41)

Ru2(2o2+)

275; 283; 287; 309; 329; 361; 438; 483

739

2.2

0.89

0.4 739

4.2

11

0.7

“c” and “o” indexes indicate the closed and open states of the photochromic unit, respectively.

a

+

bands are attributed to the π−π* transitions involving the four singlet electronic states of the DHP moiety. These data are in agreement with the computed absorption spectrum for 2c2+model (Figure S3 and Table S3 in the Supporting Information). Importantly, the presence of the electronwithdrawing pyridinium group, introduced by the phenylterpyridine functionalization, induces a slight bathochromic shift of absorption bands of the DHP core which is reinforced up to 0.089 eV for the band of lowest energy. In addition, the lowest energy bands at 672 and 686 nm are more intense for the pyridinium derivatives.9 Upon complexation, the introduction of the metal ions leads to the appearance of several additional absorption bands, similar to those recorded for the reference compound Ru(tpy)2. By comparison with the Ru(tpy)2 absorption spectrum, the bands at 283 and 483 nm can be assigned to a

2+

Figure 2. Absorption spectra for the ligands (a) 1c and (b) 2c and their respective ruthenium complexes (c) Ru(1c+), (d) Ru(1c+)2, and (e) Ru2(2c2+) in CH3CN.

The absorption spectra of the compounds roughly correspond to the sum of the individual typical bands belonging to the different components: i.e., the DHP skeleton and the ruthenium(II) bis-terpyridine complex. In comparison with the UV−visible absorption spectrum of the simple 2,7-di-tert-butyltrans-10b,10c-dimethyl-10b-10c dihydropyrene (t-DHP), the absorption spectra of the free ligands 1c+ and 2c2+ exhibit additional absorption features due to the π−π* transitions centered on the terpyridine substituents at 276 and 252 nm (Table 1). In the 300−800 nm range, the main four absorption 4359


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Figure 3. Evolution of the emission spectra of (a) 2c2+ and (b) Ru2(2c2+) upon irradiation at λ >630 nm in degassed CH3CN.

ligand-centered transition localized on the terpyridine ligand and to a RuII to tpy charge transfer (1MLCT) transition, respectively. The electron-withdrawing metal center causes a lowering of the LUMO on the photochromic entity, occasioning a modification of the corresponding absorption bands to lower energies in parallel with the free ligands. In addition, the molar absorption coefficient values are in accordance with the respective ruthenium/DHP ratio for the three complexes. In the visible region, a comparison between the spectra of the monometallic complex Ru(1c+) and the bimetallic one Ru2(2c2+) does not show any appreciable shift of the absorption bands, except for the two lowest-energy absorption bands, 534 and 674 nm vs 541 and 689 nm for Ru(1c+) and Ru2(2c2+), respectively. This confirms that the electronic coupling between the metallic centers is weak and that the two lowest-energy absorption bands involve orbitals close to the chelating part of the ligands. This is corroborated by the comparison of the computed absorption spectra for 2c2+model and Ru2(2c2+)model (Figures S3 and S4 in the Supporting Information). The positions of the main four absorption bands attributed to the π−π* transitions involving the four singlet electronic states of the DHP moiety are unaffected by the coordination to the two ruthenium centers. Note that the MLCT band is substantially blue shifted (by 0.57 eV) in comparison to experiment with CAM-B3LYP, as already observed in other studies,16 and that B3LYP provides a much better agreement (within 0.13 eV). Emission spectra and excited state lifetimes in acetonitrile solution for the closed-ring ligands, their corresponding complexes, and the reference compound Ru(tpy)2 have been recorded under an inert atmosphere at room temperature. Emission spectra of the compounds 2c2+ and Ru2(2c2+) are presented in Figure 3. All of the spectroscopic data related to emission properties are reported in Table 1. With regard to the free ligands, the fluorescence profiles are similar to those of pyridinium-DHP derivatives.9 A broad emission band appears at 737 nm for 2c2+, with an ∼50 nm Stokes shift (∼0.13 eV). These data are in good agreement with the vertical emission energy computed at 722 nm for isolated 2c2+model, with a Stokes shift of 0.22 eV. The substantial Stokes shift reflects the difference in geometries in the ground state and the first excited state, resulting in a loss of energy by vibrational relaxation before emission of a photon. This substantial change of the main geometrical parameters for the S1 excited state of pyridinium DHP derivatives has been previously corroborated by theoretical calculations and is further discussed in Photoisomerization Process (see Table 2). The increased electronic

Table 2. Main Geometrical Parameters Characterizing the Minima on the S0, S1(CT-Z), and S2(CT-LE) Potential Energy Surfaces of 2c2+model and Ru2(2c2+)model 2c2+model S0 min S1 min S2 min

Ru2(2c2+)model

q (Å)a

ϕ (deg)b

q (Å)a

ϕ (deg)b

1.548 1.568 1.560

7.5 14.2 11.0

1.548 1.567 1.553

8.4 14.8 10.5

a

Transannular bond length. bAverage deviation from planarity of the DHP core.

delocalization in 2c2+ with respect to 1c+ is reflected by a red shift of the emission maximum. The fluorescence quantum yields are accordingly rather low, i.e. 3.8 × 10−3 and 2.4 × 10−3 for 1c+ and 2c2+, respectively, with the corresponding fluorescence lifetimes being 1.6 and 1.0 ns. From fluorescence measurements and laser flash photolysis experiments, Mitchell et al. have shown that, for the simple tDHP species, the singlet excited states decay by competitive pathways involving fluorescence, internal conversion back to the DHP isomer, formation of the open CPD isomer, and intersystem crossing to the triplet state.17 The emission spectra of all ruthenium complexes exhibit a maximum at 739 nm at room temperature. This emission is assigned to the deactivation of a singlet excited state centered on the DHP core, as for 1c+ and 2c2+, respectively. Therefore, the coordination of a metal center only induces a slight emission red shift in comparison to the free ligands. At low temperature (281 K) a shoulder in the emission spectra of the complexes Ru(1c+) and Ru(1c+)2 is observed at ∼670 nm and is attributed to the Ru(tpy)2-like core. Indeed, the presence of a weak emission originating from the metal core is evidenced by lifetime measurements. For all complexes the luminescence decay appears to be biexponential, with a component of around 2 ns and a shorter value of ∼0.5 ns (Table 1). When the detector wavelength is changed from 630 to 730 nm, the respective contributions of the two lifetimes on the global decay vary. At 630 nm, the shorter component of the decay has a more important contribution than that at 730 nm. Therefore, the 2 and 0.5 ns lifetimes are assigned to the DHP emitter and the ruthenium complex emitter, respectively. This result is in accordance with the photophysical properties of the Ru(tpy)2 moiety (λem 628 nm; ϕ < 10−5; τ = 0.25 ns).18 The luminescence quantum yield values vary from 6.1 × 10−3 to 11 × 10−3 for Ru(1c+) and Ru2(2c2+), respectively, and the value is lower (0.89 × 10−3) for Ru(1c+)2. 4360


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Figure 4. 1H NMR spectra in deaerated CD3CN of the ruthenium complex Ru2(2c2+) in its (bottom) closed and (top) open forms upon irradiation at λ >630 nm in degassed CH3CN.

Figure 5. Changes in the absorption spectra of 10−5 M solutions of (a) 2c2+, (b) 1c+, (c) Ru2(2c2+), and (d) Ru(1c+) in CH3CN during irradiation of the sample at λ >630 nm.

Photoisomerization Process. The photoisomerization of the DHP moiety transforms the 14-π-electron aromatic system into a set of two isolated 6-π-electron benzenoid cores. This photoreaction can be simply observed by 1H NMR following the modifications in the spectra of a deuterated solution of the free ligands and of the corresponding complexes submitted to visible light irradiation. The NMR tubes containing initially the

closed isomers (2 mg/mL, total volume 0.7 mL) in deaerated CD3CN solution at 273 K have been irradiated. The 1H NMR spectrum of the resulting solutions shows the typical signature of the open form (CPD) with the signal of the internal −CH3 protons at +1.5 ppm instead of the signal near −3.5 ppm for the closed form. A quantitative conversion has been obtained for 1c+ and 2c2+ CD3CN solutions after 30 min of irradiation at 4361


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Inorganic Chemistry λ >630 nm. It is worth noting that under these specific experimental conditions (high concentration, lack of stirring, NMR tube geometry, ...) the complete transformation of the closed to the open forms requires a longer irradiation time in comparison to those used in quartz cuvettes. Similar changes have also been observed in the NMR spectra of ruthenium complexes upon irradiation (see for example Ru2(2c2+) in Figure 4). In all cases, after 30 min of irradiation, the signals of the initial spectra have fully disappeared at the expense of new signals, indicating a quantitative photoconversion between isomers and confirming that the functionalization of the pyridinium-DHP derivative by a free terpyridine or on complexation with ruthenium does not inhibit its photochromic properties. The photo-opening processes of the free ligands 1c+ and 2c2+ were also investigated during irradiation by monitoring the modifications in the UV−visible spectra of the compounds in CH3CN solutions at low temperature (T = 281 K, C ≈ 10−5 M). On irradiation in the visible region, the absorption bands representative of the closed isomers progressively disappeared at the appearance of new bands in the UV region corresponding to the open forms (Figure 5a,b). The kinetics of the photoinduced ring-opening process for the free ligands and the complexes are presented in Figures S5 and S6 in the Supporting Information. For the two ligands the final spectrum corresponding to the full disappearance of the initial visible bands is reached after less than 1 min of irradiation. Moreover, the stack of UV−visible spectra displays a well-defined isosbestic point at 310 nm for 1c+, suggesting a simple isomerization process. For the free ligand, the absorption bands of ruthenium complexes in the visible region gradually disappear except for that corresponding to the 1MLCT transition. The photoconversion rate and the kinetics in these terpyridine-pyridinium-DHP-bridged ruthenium complexes are quite similar to those of the ligands. Moreover, the three complexes present a good fatigue resistance upon 10 cycles of UV−vis light irradiation (Figure S7 in the Supporting Information). When the emission profiles are monitored during the DHPring-opening process, a decrease in the luminescence of the closed-ring free ligands 1c+ and 2c2+, centered at 733 and 737 nm, respectively, is observed (Figure 3) as expected, since it is known that the open form is nonemissive. In comparison to previously studied similar compounds,12 the CPD → DHP isomerization back-closing process is much more efficient. Indeed, the residual emission at the end of the opening process is more important than has been previously observed. It is due to a fast reclosing isomerization and, therefore, to a more important proportion of closed form. For the open forms of all ruthenium complexes, the emission band ascribed to the DHP motif decreased as in the case of the isolated ligands. Theoretical calculations were performed on 2c2+model and Ru2(2c2+)model compounds in order to compare their initial photoisomerization pathways. Theoretical calculations have proven useful to rationalize the photoswitching mechanisms of DHP compounds.9,16,19 In particular, a detailed ab initio study of the unsubstituted DHP back in 2007 provided useful insights into its low DHP → CPD photoisomerization efficiency.19 For the ring-opening reaction to take place, the system has to reach a funnel (conical intersection) between a biradical excited state and the ground state. This biradical excited state may be accessed via the population of a CPD precursor (denoted CPD*) on the S2 zwitterionic (Z) electronic state. CPD* is

characterized by a stretched transannular bond and a loss of planarity of the DHP core relative to the initial ground-state structure. In the unsubstituted DHP, the population of CPD* is quenched by an efficient S2 → S1 internal conversion leading to the lowest singlet locally excited (LE) state responsible for the observed fluorescence with no Stokes shift. We showed recently that,9 in pyridinium-substituted DHPs, a substantial charge transfer (CT) character between the DHP core and the pyridinium electron-withdrawing group is observed and the electronic state correlating with the CPD precursor, CPD*, lies on the lowest excited state S1 potential energy surface (PES). This lowest excited state is denoted S1(CT-Z) because it involves transitions similar to those for the S2(Z) state of the unsubstituted DHP, with the subtlety that CT character is nonnegligible. As a result, direct excitation to S1 leads to the electronic excited state that potentially forms CPD*, and its formation is not quenched by the LE state, accounting for the increased ring-opening quantum yield in pyridinium-substituted DHPs. The analysis of the electronic nature of the S1 and S2 excited states in 2c2+model confirms that these states are S1(CT-Z) and S2(CT-LE). The main electronic transition involved in S1 is HOMO → LUMO, where the electron density on the pyridinium moiety is evident in the LUMO (Table S3 in the Supporting Information). The S2 state is constituted essentially of HOMO-1 → LUMO and HOMO → LUMO+1 transitions, with also a significant contribution from HOMO → LUMO. The relaxed geometries on the respective PESs show that CPD* lies on the lowest S1 PES, as expected. Table 2 indicates that the largest changes in the geometrical structure occur on the S1 PES, consistent with the formation of CPD*. It also accounts for the substantial Stokes shift observed in the emission spectrum of this compound. This behavior has already been observed in DHPs containing two pyridinium units.9 A similar analysis has been carried out with the bimetallic complex Ru2(2c2+)model. It is shown that the first two singlet excited states have the same nature as that for 2c2+model and that the corresponding vertical transitions are found at nearly the same wavelengths (Table S3 in the Supporting Information). In addition, the relaxed structures on the S1 and S2 PESs show distortions very similar to those observed in 2c2+model, with the CPD precursor characterized by a longer transannular bond and a loss of planarity of the DHP core located on the S1 PES (see Table 2). Thus, the coordination of 2c2+model to two ruthenium centers does not modify the initial part of the singlet photoisomerization pathway. This is consistent with the experimental observation that complexation of the terpyridine-pyridinium-DHP derivative with ruthenium does not inhibit its photochromic properties. Electrochemical Properties. The electrochemical behavior of millimolar solutions of all compounds has been investigated by cyclic voltammetry (CV) in CH3CN containing tetra-n-butylammonium perchlorate (TBAP) as supporting electrolyte (0.1 M) under an inert atmosphere. The CV curves, recorded at a scan rate of 100 mV s−1, for the free ligand 2c2+ and the corresponding complex Ru2(2c2+) are depicted in Figure 6. Table 3 summarizes the redox potentials of the reported compounds. In a first approach, the CV curve recorded can be described as resulting from the superimposition of electron-transfer signatures centered on the corresponding ligands and on the metallic complex. Upon scanning toward the positive potentials, the CV response of the free closed-ring ligand 4362


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interaction between the two metal centers (Figure 6). In the negative region, two additional reversible cathodic peaks are observed due to the reduction of the complexed terpyridine ligands as observed for the Ru(tpy)2 parent complex. The electrochemical response of the CPD isomer 2o2+ has been recorded after irradiation of the electrochemical cell under an inert atmosphere (Figure 6). The appearance of a new anodic system is detected at Ea = +0.69 V and is endorsed to the irreversible oxidation of the CPD isomer. A second irreversible system is also noticed and appears at the same potential than the second oxidation peak of the DHP isomer, suggesting an electrochemically driven ring closure reaction, as has been demonstrated for pyridinium DHP derivatives.9,10g The signature of the open form of complexes has been also recorded. The CV waves attributed to the complex subunit accounting for electron transfer centered on the terpyridine subunit or on the ruthenium center are not altered by the opening process. Photochromic Quantum Yield. The efficiency of the photoisomerization process can be estimated from quantum yields (ϕc/o), representing the number of t-CPD-derivative molecules produced per photon absorbed (Table 4). The ϕc/o values are summarized in Table 4. As already observed for pyridinium DHP derivatives,9 the quantum yields of the photoisomerization reaction have been found to be more than 1 order of magnitude higher than those obtained with the parent t-DHP. As a result, the photoisomerization reaction occurs much faster. Moreover, the presence of a metallic ruthenium center reasonably decreases the ring-opening efficiency in comparison to the free ligands. The theoretical calculations providing only a qualitative description of the initial part of the singlet photoisomerization pathway cannot account for this decrease. It is likely that photophysical processes such as efficient intersystem crossing in the ruthenium complexes will contribute to a decrease in the photoisomerization efficiency upon complexation. However, these systems represent the prototypes of rapid and reversible molecular photoswitches based on ruthenium terpyridine complexes. Study of the Thermal CPD−DHP Conversion. The CPD → DHP back-conversion can be photochemically or thermally induced. However, in order to determine the kinetics parameters associated with the CPD → DHP reaction, the thermal closure of the open-ring isomer has been used for this series of compounds. The rate constant and the activation energy associated with the CPD to DHP conversion as well as the half-life of the open-ring forms have been calculated for all of the investigated compounds assuming that first-order reaction kinetics are involved (Table 5), following the method described previously.9

Figure 6. Cyclic voltammograms of millimolar (CH3CN + 0.1 M TBAP) solutions of (a) 2c2+ and (b) Ru2(2c2+) before (black line) and after (gray line) 30 min of irradiation (λ >630 nm, 500 W; v = 100 mV s−1; WE: glassy carbon).

2c2+ is similar to that of the simple di-tBu-DHP,14 characterized by a first reversible one-electron signal (Figure S8 in the Supporting Information) followed by a second irreversible signal centered on the DHP core. On the reverse scan, the irreversible anodic signal is coupled with an irreversible cathodic signal seen at ca. 0 V. The anodic waves observed at E1/2 = 0.59 V and at Epa = 1.05 V for 2c2+ in comparison to the simple t-DHP molecule are shifted by ∼+0.2 V toward more positive potentials due to the electron-withdrawing character of the pyridinium linker. In the domain of accessible negative potentials, the voltammetric response of 2c2+ displays a first reversible signal at E1/2 = −1.09 V corresponding to a oneelectron transfer centered on the pyridinium group. In comparison to the monosubstituted pyridinium terpyridine ligand 1c+,3i,j this potential is shifted by +0.32 V, because of the presence of a second electron-withdrawing pyridinium group (Table 3). Complexation with ruthenium weakly affects the characteristic potentials of the DHP-centered electron transfers. This indicates a weak electronic interaction between the components. All ruthenium complexes exhibit an additional reversible metal-centered oxidation process at around 0.95 V. In the case of Ru2(2c2+), there is no splitting of the ruthenium-centered oxidation waves, suggesting that there is no electronic

Table 3. Redox Potentialsa of the Investigated Compounds in Their Closed Forms tpy/tpy•− t-DHP 1c+ 2c2+ Ru(1c+) Ru(1c+)2 Ru2(2c2+) Ru(tpy)2 a

tpy/tpy•−

py+/py•

DHP•+/DHP 0.26

−1.41 (200) −1.09 (80) −1.80 (90) −1.80b −1.75 (80) −1.83 (70)

0.47 (230) 0.59 (70) −1.56 (50) −1.55b −1.54 (60) −1.57 (60)

1.17b 1.05b −1.39 (45) −1.37 (60) −1.10 (30)

b

0.41 (65) 0.42 (60) 0.54 (60)

DHP2+‑/DHP•+

RuIII/RuII

b

0.84 0.79b 0.69b 0.90b 0.90b 0.93b

CPD•+/CPD nd

0.94 0.94 0.95 0.97

(60) (40) (60) (80)

0.54b 0.60b 0.79b

All potentials (E1/2 (ΔEp, mV) or Ep) are given in V vs Ag+ (10−2 M)/Ag reference electrode at 100 mV s−1. bFully irreversible system Ep 4363


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Inorganic Chemistry Table 4. Experimental and Calculated Values for the Direct Forward Quantum Yields of the Compounds λirr (nm)

ε (M−1 cm−1)

I° (10−6 einstein s−1 dm−3)

Φc/o

478 672 688 674 668 689

10800 1900 2600 2300 4000 4100

7.40 3.02 2.91 3.12 3.29 2.82

0.002 0.11 0.16 0.087 0.027 0.094

t-DHP 1c+ 2c2+ Ru(1c+) Ru(1c+)2 Ru2(2c2+)

■

Table 5. Rate Constant (k), Half-Life (t1/2), and Activation Energy (Ea) for the Thermal Relaxation of the Ligands 1o+ and 2o2+ and the Ruthenium Complexes Ru(1o+), Ru(1o+)2, and Ru2(2o2+) in CH3CN T (K) 1o

+

2o2+

Ru(1o+)

Ru(1o+)2

Ru2(2o2+)

a

298 308 318 298 308 318 298 308 318 298 308 318 298 308 318

k (10−5 s−1)

t1/2 (h)

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

17.3 5.4 1 7 2 0.7 11 3 1 19 4.8 1.8 6.8 1.9 1

1.1 3.55 20.5 2.9 9.39 28.0 1.71 6.22 17.1 1.020 4.03 10.5 2.83 1.01 18.1

0.4 0.03 0.5 0.2 0.01 0.1 0.01 0.04 0.3 0.002 0.05 0.1 0.01 0.05 0.2

METHODS AND EXPERIMENTAL PROCEDURES

General Considerations. NMR spectra were performed on a Bruker Avance 500 or 400 MHz spectrometer. Mass spectrometry analyses (ESI positive mode) were performed at the DCM mass spectrometry facility with an Esquirre 3000 Plus instrument (Bruker Daltonics). Electrochemistry. All cyclic voltammetry measurements were carried out under an inert atmosphere in CH3CN containing TBAP (0.1 M) as supporting electrolyte with a standard one-compartment, three-electrode cell using a Biologic SP300 potentiostat. CHinstrument vitreous carbon working electrodes (active surface area of 0.0314 cm2) were polished with 1 mm diamond paste (Mecaprex Presi). A CH instrument Ag|Ag+ (10−2 M) electrode was used as a reference. The auxiliary electrode was a platinum wire. Synthesis. All chemicals and solvents were used as received. 4′-(4(Bromomethyl)phenyl)-2,2′:6′,2″-terpyridine, Ru(tpy)Cl3; 2,7-di-tertbutyl-trans-10b,10c-dimethyl-10b,10c-dihydropyrene, 4-bromo-2,7-ditert-butyl-trans-10b,10c-dimethyl-10b,10c-dihydropyrene, 4,9-dibromo-2,7-di-tert-butyl-trans-10b,10c-dimethyl-10b-10c-dihydropyrene, 2,7-di-tert-butyl-4-(4-pyridyl)-trans-10b,10c-dimethyl-10b,10c-dihydropyrene, and 2,7-di-tert-butyl-4,9-bis(4-pyridyl)-trans-10b,10c-dimethyl10b,10c-dihydropyrene were synthesized as previously described.9 2,7-Di-tert-butyl-4-(N-(4′-(4-methylphenyl)-2,2′:6′,2″terpyridinyl)pyridin-4-yl)-trans-10b,10c-dimethyl-10b,10c-dihydropyrene Hexafluorophosphate (1c+). A degassed solution of 4′-(4-(bromomethyl)phenyl)-2,2′:6′,2″-terpyridine (138.3 mg, 0.34 mmol) dissolved in 5 mL of CH2Cl2 was added to a stirred solution of 2,7-di-tert-butyl-4-(4-pyridyl)-trans-10b,10c-dimethyl-10b,10c-dihydropyrene (121 mg, 0.29 mmol) in 100 mL of degassed acetonitrile. The reddish mixture was then refluxed for 15 h under an argon atmosphere. Then, the solution was concentrated under reduced pressure and the residue was precipitated with diethyl ether. The product was then dissolved in methanol and precipitated in an aqueous solution of KPF6 to afford the ion exchange. The red powder was then filtered, washed with water, and dried (62%). 1 H NMR (500 MHz, 298 K, CD3CN): δ (ppm) 8.91 (2H, d, J = 5 Hz); 8.80−8.60 (13H, m); 8.51 (2H, d, J = 5 Hz); 8.09 (2H, d, J = 10 Hz); 7.99 (2H, t, J = 5 Hz); 7.80 (2H, d, J = 5 Hz); 7.47 (2H, dd, J = 5 Hz); 5.91 (2H, s); 1.55 (9H, s); 1.53 (9H, s);-3.88 (3H, s); −3.91 (3H, s). Exact mass (m/z): calcd 743.4300 [M − PF6], found 743.4308. 1 H NMR [CPD form] (500 MHz, 298 K, CD3CN): δ (ppm) 8.76 (2H, s); 8.71 (4H, m); 8.62 (2H, d, J = 6.5 Hz); 8.09 (2H, d, J = 6.5 Hz); 8.02 (2H, d, J = 8.0 Hz); 7.97 (2H, td, 1J = 5 Hz); 7.67 (2H, d, J = 8.0 Hz); 7.46 (2H, dd, 1J = 5 Hz); 7.34 (1H, s); 6.95 (1H, s); 6.94 (1H, s); 6.90 (1H, s); 6.61 (1H, s); 6.55−6.37 (2H, m); 5.75 (2H, s); 1.48 (3H, s); 1.39 (3H, s); 1.24 (9H, s); 1.21 (9H, s). 2,7-Di-tert-butyl-4,9-bis(N-(4′-(4-methylphenyl)-2,2′:6′,2″terpyridinyl)pyridin-4-yl)-trans-10b,10c-dimethyl-10b,10c-dihydropyrene Hexafluorophosphate (2c2+). The same procedure as for 1c+ was used starting with 161.4 mg (0.40 mmol) of 4′-(4(bromomethyl)phenyl)-2,2′:6′,2″-terpyridine and 100 mg (0.20 mmol) of 2,7-di-tert-butyl-4,9-bis(4-pyridyl)-trans-10b,10c-dimethyl10b,10c-dihydropyrene in 100 mL of degassed acetonitrile. The yield of the reaction was 52%. 1 H NMR (500 MHz, 298 K, CD3CN): δ (ppm) 8.96 (4H, d, J = 6.4 Hz); 8.90 (2H, s); 8.86−8.77 (6H, m); 8.77−8.69 (10H, m); 8.51(4H, d, J = 4.5 Hz); 8.09 (4H, d, J = 6.2 Hz); 7.99 (4H, td, J = 7.7, 1.8 Hz); 7.82 (4H, d, J = 6.8 Hz); 7.47 (4H, dd, J = 6.2 Hz); 5.95 (4H, s, H);

Ea (kcal mol−1)a 19 ± 6

21 ± 0.1 (19.2)

21 ± 1

22 ± 2

18 ± 3 (18.2)

DFT values in parentheses.

The measured activation energies are about 20 kcal mol−1, this value being quite standard for CPD derivatives.3i,j They are in good agreement with the computed CPD → DHP potential energy barriers obtained with DFT: i.e., 19 kcal mol−1 for 2o2+model and 18 kcal mol−1 for Ru2(2o2+)model (Table 5). The half-life values found for all the studied compounds are on the order of a few hours at 308 K and are much lower than that of 54 h reported for the reference t-DHP in cyclohexane.20

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CONCLUSION We have reported the synthesis and the characterization of two dimethyldihydropyrene ligands functionalized with one or two terpyridine units connected through an electron-withdrawing pyridinium unit. The redox and photochemical properties of ligands have been investigated, showing that a rapid and complete conversion between the closed and the open forms occurs. TD-DFT calculations are in line with these results, confirming the favorable effects of the pyridinium electronwithdrawing groups on the efficiency of the photoisomerization process. Such high photoconversion and switching efficiency of this photochromic ligands opens the way for construction of supramolecular light-responsive polynuclear 1D chains or metallo-rings. Moreover, the complexation of the ligands with Ru(II) ions does not affect the efficiency of the ring-opening photoprocess and it represents the first efficient system combining a DHP unit and a ruthenium metal ion. No communication between the metallic centers has been evidenced for the bimetallic species. 4364


Article

Inorganic Chemistry 1.63 (18H, s); −3.62 (6H, s). Exact mass (m/z): calcd 571.2856 [M − 2PF6]2+, found 571.2851. 1 H NMR [CPD form] (500 MHz, 298 K, CD3CN): δ (ppm) 8.77 (4H, s); 8.75−8.69 (8H, m); 8.66 (4H, d, J = 6.6 Hz); 8.11 (4H, d, J = 6.8 Hz); 8.04 (4H, d, J = 8.1 Hz); 7.98 (4H, td, 1J = 7.7, 2J = 1.8 Hz); 7.69 (4H, d, J = 8.1 Hz); 7.47 (4H, dd, 1J = 7.7, 2J = 1.8 Hz); 7.41 (2H, s); 7.12 (2H, s); 6.75 (2H, s); 5.77 (4H, s); 1.48 (6H, s); 1.20 (18H, s). [Ru(1c+)][3PF6−]. A 30 mL portion of a methanol solution containing 15 mg (0.017 mmol) of 1c+, 15 mg (0.034 mmol) of [(tpy)RuCl3], and a few drops of N-ethylmorpholine was heated under reflux for 2 days. After it was cooled to room temperature, the suspension was filtered through Celite and washed with EtOH. The solution was concentrated, and aqueous KPF6 was added to precipitate the product. The solid was filtered, washed with diethyl ether, and purified by column chromatography on silica gel (CH3CN/CH3OH/ H2O/saturated NaCl 4/1/1/1). The reddish collected fraction was concentrated before adding aqueous KPF6 to precipitate the product. The solid was washed with diethyl ether to obtain a red powder (53%). 1 H NMR (500 MHz, 298 K, CD3CN): δ (ppm) 9.03 (2H, s); 8.99 (2H, d, J = 5 Hz); 8.84 (1 H, s); 8.82 (1H, s); 8.80 (2H, s); 8.76 (2H, d, J = 8 Hz); 8.69 (1H, s); 8.67−8.64 (4H, m); 8.59 (2H, d, J = 5 Hz); 8.50 (2H, d, J = 8 Hz); 8.43 (1H, t, J = 8 Hz); 8.36 (2H, d, 1J = 10, 2J = 5 Hz); 7.98−7.90 (6H, m); 7.42 (2H, d, J = 5 Hz); 7.36 (2H, d); 7.21- 7.17 (4H, m); 6.00 (2H, s); 1.71 (9H, s); 1.68 (9H, s); −3.80 (3H, s); −3.85 (3H, s). Exact mass (m/z): calcd 359.4722 [M]3+, found 359.4709. 1 H NMR [CPD form] (500 MHz, 298 K, CD3CN): δ (ppm) 8.99 (2H, s); 8.76 (2H, d, J= 10 Hz); 8.69 (2H, d, J = 5 Hz); 8.64 (2H, d, J = 10 Hz); 8.50 (2H, d, J = 10 Hz); 8.42 (1H, t, J = 10 Hz); 8.30 (2H, d, J = 10 Hz); 8.17 (2H, d, J = 5 Hz); 7.96−7.91 (4H, m); 7.81 (2H, d, J = 5 Hz); 7.41−7.37 (3H, m); 7.5 (2H, d, J = 5 Hz); 7.24−7.11 (4H, m); 6.97 (2H, s); 6.92 (1H, s); 6.66 (1H, s); 6.52 (1H, d, J = 10 Hz); 6.48 (1H, d, J = 10 Hz); 5.84 (2H, s); 1.51 (3H, s); 1.42 (3H, s); 1.26 (9H, s); 1.23 (9H, s). [Ru2(2c2+)][6PF6−]. The same procedure as for [Ru(1c+)][3PF6‑] was used starting with 50 mg of 2c2+ (0.035 mmol) and 46 mg (0.105 mmol) of [(tpy)RuCl3]. The yield of the reaction was 32%. 1 H NMR (400 MHz, 298 K, CD3CN): δ (ppm) 9.10−9.01 (8H, m); 8.95 (2H, s); 8.86 (2H, s); 8.78 (2H, s); 8.76 (4H, d, J = 8 Hz); 8.66 (4H, d, J = 8 Hz); 8.60 (4H, d, J = 8 Hz); 8.51 (4H, d, J = 8 Hz); 8.43 (2H, t, J = 8 Hz); 8.38 (4H, d, J = 8 Hz); 8.02−7.88 (12H, m); 7.42 (4H, d, J = 8 Hz); 7.37 (4H, d, J = 8 Hz); 7.21−7.15 (8H, m); 6.04 (4H, s); 1.69 (18H, s); −3.57 (6H, s). Mass (m/z): calcd 694.7 [M + Na+], found 694.8. 1 H NMR [CPD form] (500 MHz, 298 K, CD3CN): δ (ppm) 9.00 (4H, s); 8.76 (8H, m); 8.65 (4H, d, J = 8 Hz); 8.50 (4H, d, J = 8 Hz); 8.43 (2H, t, J = 8 Hz); 8.32 (4H, d, J = 8 Hz); 8.18 (4H, d, J = 8 Hz); 7.96−7.91 (8H, m); 7.84 (4H, d, J = 8 Hz); 7.48 (2H, s); 7.41 (4H, d, J = 5 Hz); 7.37 (4H, d, J = 5 Hz); 7.20−7.15 (10H, m); 6.81 (2H, s); 5.87 (4H, s); 1.5 (6H, s); 1.26 (18H, s). [Ru(1c+)2][4PF6−]. A 3 mL portion of an ethanol solution containing 40 mg (0.045 mmol) of 1c+, 6 mg of RuCl3 (0.022 mmol), and a few drops of N-ethylmorpholine was heated under reflux for 5 days. Then the same treatment was followed. A red powder was obtained with a 40% yield. 1 H NMR (500 MHz, 298 K, CD3CN): δ (ppm) 9.05 (4H, s); 8.99 (4H, d, J = 6.85 Hz); 8.84 (2H, s); 8.82 (2H, s); 8.80 (4H, s); 8.69− 8.62 (10H, m); 8.58 (4H, d, J = 6.85 Hz); 8.37 (4H, d, J = 8.4 Hz); 7.98−7.94 (8H, m); 7.45 (4H, d, J = 5.55 Hz); 7.21−7.18 (4H, m); 6.00 (4H, s); 1.70 (18H, s); 1.68 (18H, s); −3.80 (6H, s); −3.85 (6H, s). Mass (m/z): calcd 2023.6 [M + Na+], found: 2023.7. 1 H NMR [CPD form] (500 MHz, 298 K, CD3CN): δ (ppm) 9.01 (4H, s); 8.70 (4H, d, J = 6.75 Hz); 8.65 (4H, d, J = 8 Hz); 8.31 (4H, d, J = 8.25 Hz); 8.17 (4H, d, J = 7 Hz); 7.96 (4H, t, J = 8 Hz); 7.82 (4H, d, J = 8.3 Hz); 7.43 (4H, d, J = 5.5 Hz); 7.39 (2H, s); 7.20−7.18 (4H, m); 6.98−6.97 (4H, m); 6.92 (2H, s); 6.66 (2H, s); 6.52 (2H, d, J = 11 Hz); 6.48 (2H, d, J = 11 Hz); 5.84 (4H, s); 1.51 (6H, s); 1.42 (6H, s); 1.26 (18H, s); 1.23 (18H, s).

Spectroscopy. All experimental details for absorption and emission spectroscopy have been previously reported.14 Experimental uncertainties are as follows: absorption maximum, 2 nm; molar absorption, 20%; emission maximum, 5 nm; emission lifetime, 10%; emission quantum yield, 20%. Irradiation Procedures. Samples were prepared in a Jaram glovebox with carefully degassed solvents. The solutions were irradiated in UV−visible quartz cells or NMR tubes. The concentrations used for UV−visible spectroscopy and NMR experiments were between 2 × 10−5 and 3 × 10−3 M. The visible irradiation for the isomerization of the “closed” isomer to its corresponding “open” form were carried out with a Xe−Hg lamp, using a 630 nm cutoff filter, and the samples were placed in a 8 °C bath in order to limit the reverse thermal reaction. Samples were placed at a distance of 15 cm of the visible lamp. Alternatively, irradiation was performed at room temperature with a 500 W tungsten−halogen lamp equipped with a 590 nm cutoff filter. The quantum yield ϕc/o has been extracted from the fitting curve using a Levenberg−Marquardt iterative program within the Origin 6.0 software package. The irradiation power I0 was measured with a Newport photodetector (818-SL). The conversions between the different species were investigated by UV−visible and NMR experiments. Intermediate spectra were recorded at different times depending on the isomerization process rates. The ratio between the different species was determined by 1H NMR from the relative integration of the characteristic resonance peaks of the N+−CH3 groups of the different forms. Photochromic Quantum Yield Determination. Evolution of the absorbance values as a function of irradiation time under monochromatic visible irradiation light (λirr) was used to evaluate the quantum yield of the opening process. The optimal irradiation wavelength (λirr) was selected as the maximum wavelength of the lowest energy transition. Using these experimental conditions, the closed-ring form can be quantitatively converted in its open isomer. All absorption profiles were fitted with a simple photochromic model involving the two isomers by means of a numerical integration procedure using the simplified differential eq 1 and the phenomenological eq 2

dC DHP 1 − 10−A(λ irr) = − ϕc/oI0(λ irr)εDHP(λ irr)lC DHP dt A(λ irr)

(1)

A = εDHP(λ irr)lC DHP + εCPD(λ irr)lCCPD

(2)

, where CDHP and CCPD denote the concentrations of the closed-ring and the open-ring isomers, l denotes the optical path length, ϕc/o denotes the quantum yield of the photochemical reaction, I0 denotes the incident irradiation intensity, εDHP and εCPD denote the molar absorption coefficients of the closed-ring and open-ring forms, and A denotes the absorbance of the solution defined by eq 2. It is worth noting that CPD does not absorb at the selected irradiation wavelength. The quantum yield ϕc/o has been extracted from the fitting curve using a Levenberg−Marquardt iterative program. Thermal Closing Process. Key kinetic parameters corresponding to the thermal conversion process from the opened forms to their corresponding closed forms were deduced from UV−visible spectroscopy measurements conducted at three different temperatures, as previously described.9 Computational Section. Density functional theory (DFT) and time-dependent DFT (TD-DFT) have been used to perform calculations on the ground and first excited singlet states, respectively, of isolated model systems for DHP 2c2+ and Ru2(2c2+). The model systems, denoted 2c2+model and Ru2(2c2+)model in the following, were obtained by simply replacing the bulky tert-butyl groups by hydrogen atoms in order to reduce the computational cost. The ground-state geometry was optimized using the B3LYP functional,21 while excitedstate geometry optimizations and excited-state potential energy profile calculations were performed with the long-range corrected CAMB3LYP functional.22 Triple-ζ quality basis sets including polarization functions were used throughout: 6-311G(d,p) was used for H, C, and 4365


Article

Inorganic Chemistry N atoms,23 and a Stuttgart relativistic small-core effective potential along with its basis augmented by an f polarization function with an exponent of 0.96 was employed for the ruthenium atoms.24 This level of calculation provides a good description of the ground state and first two excited states of the unsubstituted DHP reference compound.16 Note that several conformers lying very close in energy (within 0.01 eV) exist for the ground-state structures of both 2c2+model and Ru2(2c2+)model. To compare with the X-ray structure of 2c2+, the geometry of 2c2+model was first optimized in the same conformational state. For the computation of the absorption spectrum of 2c2+model, the ground-state geometry used is the one of lowest energy. The relaxation in the excited states was also investigated starting from the lowest energy ground-state structure. Because CAM-B3LYP is known to produce a degradation of the absorption spectrum for polypyridine ruthenium complexes in the case of charge-transfer transitions taking place between relatively spatially close regions,25 the absorption spectrum of Ru2(2c2+)model was computed with both B3LYP and CAM-B3LYP at the TD-DFT level. All absorption spectra were computed using the polarizable continuum model (PCM) to describe the acetonitrile solvent. The thermal CPD → DHP potential energy barrier was obtained by optimizing the corresponding open-shell singlet transition state at the broken-symmetry unrestricted B3LYP level. To account for the spin contamination, spin-projected energies have been calculated with the approximate spin-correction procedure proposed by Yamaguchi and co-workers.26 All of the optimized Cartesian coordinates and energies are collected in the Supporting Information. All calculations were performed with Gaussian 09.27

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Zerbetto, F. Synthetic Molecular Motors and Mechanical Machines. Angew. Chem., Int. Ed. 2007, 46, 72−191. (d) Irie, M. Diarylethenes for Memories and Switches. Chem. Rev. 2000, 100, 1685−1716. (e) Saha, S.; Stoddart, J. F. Photo-driven molecular devices. Chem. Soc. Rev. 2007, 36, 77−92. (f) Kawata, S.; Kawata, Y. Three-Dimensional Optical Data Storage Using Photochromic Materials. Chem. Rev. 2000, 100, 1777−1788. (2) (a) Bléger, D.; Hetch, S. Visible-Light-Activated Molecular Switches. Angew. Chem., Int. Ed. 2015, 54, 11338−11349. (b) Fihey, A.; Perrier, A.; Browne, W. R.; Jacquemin, D. Multiphotochromic molecular systems. Chem. Soc. Rev. 2015, 44, 3719−3759. (3) (a) Akita, M. Photochromic Organometallics, A StimuliResponsive System: An Approach to Smart Chemical Systems. Organometallics 2011, 30, 43−51. (b) Kume, S.; Nishihara, H. Photochrome-coupled metal complexes: molecular processing of photon stimuli. Dalton Trans. 2008, 3260−3271. (c) Aubert, V.; Guerchais, V.; Ishow, E.; Hoang-Thi, K.; Ledoux, I.; Nakatani, K.; Le Bozec, H. Efficient Photoswitching of the Nonlinear Optical Properties of Dipolar Photochromic Zinc(II) Complexes. Angew. Chem., Int. Ed. 2008, 47, 577−580. (d) Sud, D.; McDonald, R.; Branda, N. R. Synthesis and Coordination Chemistry of a Photoswitchable Bis(phosphine) Ligand. Inorg. Chem. 2005, 44, 5960−5962. (e) Lee, J. K.W.; Ko, C.-C.; Wong, K. M.-C.; Zhu, N.; Yam, V. W.-W. A Photochromic Platinum(II) Bis(alkynyl) Complex Containing a Versatile 5,6-Dithienyl-1,10-phenanthroline. Organometallics 2007, 26, 12−15. (f) Nishihara, H. Multi-Mode Molecular Switching Properties and Functions of Azo-Conjugated Metal Complexes. Bull. Chem. Soc. Jpn. 2004, 77, 407−428. (g) Jukes, R. T. F; Adamo, V.; Hartl, F.; Belser, P.; De Cola, L. Electronic energy transfer in a dinuclear Ru/Os complex containing a photoresponsive dithienylethene derivative as bridging ligand. Coord. Chem. Rev. 2005, 249, 1327−1335. (h) Tanaka, Y.; Inagaki, A.; Akita, M. A photoswitchable molecular wire with the dithienylethene (DTE) linker, (dppe)(η5C5Me5)Fe−C[triple bond, length as m-dash]C−DTE−C[triple bond, length as m-dash]C−Fe(η5-C5Me5) (dppe). Chem. Commun. 2007, 1169−1171. (i) Bakkar, A.; Cobo, S.; Lafolet, F.; Roldan, D.; SaintAman, E.; Royal, G. A redox- and photo-responsive quadri-state switch based on dimethyldihydropyrene-appended cobalt complexes. J. Mater. Chem. C 2016, 4, 1139−1143. (j) Bakkar, A.; Cobo, S.; Lafolet, F.; Roldan, D.; Jacquet, M.; Bucher, C.; Royal, G.; Saint-Aman, E. Dimethyldihydropyrene−cyclophanediene photochromic couple functionalized with terpyridyl metal complexes as multi-addressable redoxand photo-switches. Dalton Trans. 2016, 45, 13700−13708. (4) Yokoyama, Y. Fulgides for Memories and Switches. Chem. Rev. 2000, 100, 1717−1740. (5) Berkovic, G.; Krongauz, V.; Weiss, V. Spiropyrans and Spirooxazines for Memories and Switches. Chem. Rev. 2000, 100, 1741−1754. (6) (a) Terao, F.; Morimoto, M.; Irie, M. Light-Driven MolecularCrystal Actuators: Rapid and Reversible Bending of Rodlike Mixed Crystals of Diarylethene Derivatives. Angew. Chem., Int. Ed. 2012, 51, 901−904. (b) Fukaminato, T.; Doi, T.; Tamaoki, N.; Okuno, K.; Ishibashi, Y.; Miyasaka, H.; Irie, M. Single-Molecule Fluorescence Photoswitching of a Diarylethene−Perylenebisimide Dyad: Nondestructive Fluorescence Readout. J. Am. Chem. Soc. 2011, 133, 4984−4990. (c) Mori, K.; Ishibashi, Y.; Matsuda, H.; Ito, S.; Nagasawa, Y.; Nakagawa, H.; Uchida, K.; Yokojima, S.; Nakamura, S.-I.; Irie, M.; Miyasaka, H. One-Color Reversible Control of Photochromic Reactions in a Diarylethene Derivative: Three-Photon Cyclization and Two-Photon Cycloreversion by a Near-Infrared Femtosecond Laser Pulse at 1.28 μm. J. Am. Chem. Soc. 2011, 133, 2621−2625. (d) Morimoto, M.; Irie, M. Photochemical control of dielectric properties based on intermolecular proton transfer in a hydrogenbonded diarylethene crystal. Chem. Commun. 2011, 47, 4186−4188. (e) Morimoto, M.; Irie, M. A Diarylethene Cocrystal that Converts Light into Mechanical Work. J. Am. Chem. Soc. 2010, 132, 14172− 14178. (f) Meng, F.; Hervault, Y.-M.; Shao, Q.; Hu, B.; Norel, L.; Rigaut, S.; Chen, X. Orthogonally modulated molecular transport junctions for resettable electronic logic gates. Nat. Commun. 2014, 5,

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02861. Crystal data, additional NMR spectra, Cartesian coordinates and energies, computational absorption spectra and TD-DFT results, kinetic data, additional electrochemical data, and HR-MS (PDF) X-ray crystallographic data for 2c2+ (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for F.L.: [email protected]. *E-mail for S.C.: [email protected]. ORCID

Saioa Cobo: 0000-0002-9353-3417 Martial Boggio-Pasqua: 0000-0001-6684-5223 Funding

The present work was supported by the Labex Arcane (ANR11-LABX-0003-01) and by the ANR program (MULTICOM project, ANR-13-JS07-0012). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the ICMG Chemistry Nanobio Platform for technical support and Dr. Christian Philouze for X-ray measurements.

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REFERENCES

(1) (a) Feringa, B. L. Molecular Switches; Wiley-VCH: Weinheim, Germany, 2001. (b) Balzani, V.; Venturi, M.; Credi, A. Molecular Devices and Machines: Concepts and Perspectives for the Nanoworld; Wiley-VCH: Weinheim, Germany, 2008. (c) Kay, E. R.; Leigh, D. A.; 4366


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Inorganic Chemistry

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Efficient Photoswitch System Combining a Dimethyldihydropyrene

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