Vinylruthenium-triarylamine conjugates as

DOI 10.1515/irm-2012-0005

BioInorg React Mech 2012; 8(3-4): 85–105

Walther Polit, Thomas Exnera, Evelyn Wuttke and Rainer F. Winter*

Vinylruthenium-triarylamine conjugates as electroswitchable polyelectrochromic NIR dyes Abstract: We here report on triarylamine-derived styryl ruthenium complexes 1–3 where one (1), two (2) or three (3) vinyl ruthenium moieties are appended to a triphenylamine core. The near equivalency of the styryl ruthenium and the triarylamine redox systems leads to strong interactions between these moieties and strong mixing of the respective frontier orbitals. This results, inter alia, in the observation of two to four consecutive, reversible one-electron redox couples with potential splittings of 185–435 mV. The associated radical cations and higher oxidized forms show strong absorptions whose positions vary from deep in the near-infrared (NIR) to the border region between the Vis and NIR regimes as a function of the oxidation state. Extinction coefficients and oscillator strengths reach rather impressive values of up to 90 000 l mol-1 cm-1 and ≥ 1.0. Complexes 1–3 thus constitute polyelectrochromic dyes with two to three addressable and distinguishable states that can be reversibly interconverted by application of an appropriate potential. The electronic transitions underlying the intense low energy absorptions are assigned with the aid of time dependentdensity functional theory (TD-DFT) and involve strongly delocalized molecular orbitals (MOs). Charge and spin delocalization in the (radical) cations are probed by electron paramagnetic resonance spectroscopy (EPR) and infrared (IR) spectroelectrochemistry. Keywords: density functional theory (DFT) calculations; electrochromic compounds; near-infrared (NIR) dyes; (spectro)electrochemistry; triarylamines; vinyl ruthenium complexes.

a

Present adress: Eberhard-Karls Universität Tübingen, Pharmazeutisches Institut, Auf der Morgenstelle 8, D-72076 Tübingen, Deutschland *Corresponding author: Rainer F. Winter: Universität Konstanz, Fachbereich Chemie, Universitätsstraße 10, D-78457 Konstanz, Deutschland, e-mail: [email protected] Walther Polit: Universität Konstanz, Fachbereich Chemie, Universitätsstraße 10, D-78457 Konstanz, Deutschland Thomas Exner: Universität Konstanz, Fachbereich Chemie, Universitätsstraße 10, D-78457 Konstanz, Deutschland Evelyn Wuttke: Universität Konstanz, Fachbereich Chemie, Universitätsstraße 10, D-78457 Konstanz, Deutschland

Introduction Dyes with strong absorption in the near-infrared (NIR) region of electromagnetic radiation (ca. 750 –3000 nm, 14 000 –3300 cm-1 according to ISO 20473) constitute a sought-after class of compounds. One important application of such dyes centers around photovoltaic lightto-energy conversion schemes. Because more than 50% of the solar radiation reaching the Earth’s surface lies within the NIR, considerable effort is presently devoted to developing sensitizers with superior absorption in that energy window (Burke et al., 2007; Yum et al., 2007; Silvestri et al., 2008; Choi et al., 2010; Mai et al., 2010; Clifford et al., 2011). The good penetration of NIR radiation into biological tissue makes NIR dyes absorbing in the 800 –1100 nm regime attractive for bioimaging and therapeutic purposes as it is exemplified by photodynamic therapy (Detty et al., 2004; Mihaljevic et al., 2004; Stefflova et al., 2007; Escobedo et al., 2010; Hilderbrand and Weissleder, 2010; Batat et al., 2011). Another field of interest is optoelectronics with laser recording, laser thermal imaging or printing and thermal photography as examples (Fabian et al., 1992; Ward, 2005; Qian and Wang, 2010). The ability to reversibly alter NIR absorption/transmission by outer stimuli is relevant to signal attenuation in the telecommunications window of 1300 – 1350 and 1500 –1600 nm (Mortimer, 2004; Ward, 2005). Utilization of electrical voltage as stimulus constitutes a non-invasive method that usually offers fast response times and avoids the accumulation of waste materials involved in the chemical triggering of the switching process. Materials, whose absorbance profiles can reversibly be electroswitched between different states, are termed electrochromic, or, if several differently charged states with distinguishable absorption properties can be realized, polyelectrochromic (Mortimer, 1997; 1999, 2004; Mortimer et al., 2006). NIR absorption generally requires extended chromophores with low energy gaps between the donor and acceptor levels of the relevant electronic transition(s). Compounds with such properties can be found in different substance classes. Instructive reviews of this topic are available in the literature (Fabian et al., 1992; Ward, 2005;

Bereitgestellt von | Universitaet Konstanz Angemeldet | 134.34.112.45 Heruntergeladen am | 20.02.14 08:39

86

W. Polit et al.: Vinylruthenium-triarylamine conjugates as electroswitchable polyelectrochromic NIR dyes

Qian and Wang, 2010; Kaim, 2011). Among them are highly extended, rigidified π systems, organic paramagnetic open shell systems with a high-density of states below (radical cations) or above (radical anions) the singly occupied molecular orbital (SOMO), metal complexes bearing such radicals as ligands, or mixed valent (MV) compounds, where the intervalence charge transfer (IVCT) transition involving the transfer of charge from the formally reduced to the formally oxidized redox site is usually found in the NIR. According to the nature of the redox sites, MV compounds can be purely organic or inorganic in nature (cf. Prussian Blue) or can be transition metal(-organic) coordination compounds, where the metal-based redox sites ‘communicate’ across an intervening bridge, usually an organic ligand (Robin and Day, 1967). Paradigmatic examples of such MV compounds are the pyrazine-bridged bis(pentaammine ruthenium)(5+) cation [(Ru(NH3)5(μ-pz) (NH3)5Ru)]5+ [the so-called Creutz-Taube ion (Creutz and Taube, 1969, 1973)] and its relatives (Sutton and Taube, 1981; Creutz, 1983; Richardson and Taube, 1984; Kaim et al., 2000). Less common, but not without precedent, is the converse situation where two paramagnetic ligands interact across the metal ion to which they are coordinated (Dogan et al., 2004; Jones et al., 2004; Ye et al., 2005; Lu et al., 2008). One should note here, however, that some paramagnetic ligand-bridged dinuclear complexes defy a clear-cut distinction between metal- and bridging ligandbased redox chemistry, sometimes even to the point that charge and spin density distributions vary as a consequence of relatively minor conformational changes (Fox et al., 2011). Triarylamines constitute a particularly interesting class of electrochromic dyes with intriguing NIR absorption of their radical cations. They favorably combine a straightforward modular synthesis through efficient Ullman condensation (Beletskaya and Cheprakov, 2004; Monnier and Taillefer, 2009) or Buchwald-Hartwig crosscoupling methodology (Hartwig, 1998a,b; Wolfe et al., 1998; Hartwig, 2008), reversible electrochemistry at wellaccessible potentials that can be varied by the proper choice of substituents and a good stability of their radical cations once the para positions are blocked to avoid benzidine-type rearrangements (Walter, 1966; Dapperheld et al., 1991; Bender et al., 2001; Amthor et al., 2005). Radical cations of triarylamines strongly absorb in the low energy regime of the visible and/or the NIR at wavelengths that can again be tuned by the aryl substituents (Amthor et al., 2005). Owing to its strong visible absorption (λmax = 700 nm), the tris(4-bromophenyl)aminium radical cation, N(C6H5-4Br)+3, a popular, commercially available one-electron oxidant (Connelly and Geiger, 1996),

has been dubbed as ‘magic blue’. Advanced synthetic methodology has allowed to access a vast number of bis(triarylamine)s with bridges of differing nature and lengths. Detailed experimental and quantum chemical studies of their associated radical cations have provided valuable information as to how the inner and outer reorganization energies λin and λo, the effective charge transfer distances RAB and the electronic coupling matrix elements HAB depend on the nature of the solvent and the bridge (Lambert and Nöll, 1999; Coropceanu et al., 2002, 2004; Lambert et al., 2002, 2004, 2005; Szeghalmi et al., 2004; Barlow et al., 2005; Amthor and Lambert, 2006; Heckmann et al., 2006; Heckmann and Lambert, 2007; Zhou et al., 2007; Risko et al., 2008; Seibt et al., 2008; Kattnig et al., 2009; Lancaster et al., 2009; Kaupp et al., 2011). Most importantly, the MV radical cations of bis(triarylamine)s usually display intense NIR absorption arising from IVCT or bridge to NAr 3+ charge transfer in addition to the π→π* transitions localized at the NAr +3 core. Further oxidation to dications with two NAr +3 sites bleaches the IVCT band, whereas bridge-to-NAr +3 and π→π* transitions further intensify, sometimes with appreciable shifts. These features qualify bis(triarylamine)s as polyelectrochromic Vis/NIR dyes. Styryl ruthenium complexes (ArCH苷 CH-)RuCl(CO) L(PR3)2 (Ar = aryl, L = neutral two-electron donor ligand or free coordination site) offer similar properties to the popular triarylamines. Showing one usually reversible one-electron oxidation per vinyl ruthenium subunit at rather low half-wave potentials, they combine a rich electrochemistry, strong substituent dependence of the half-wave potentials and highly delocalized redox orbitals with a large, if not dominant, contribution of the vinyl ligand (Liu et al., 2003; Maurer et al., 2004, 2005, 2006, 2007, 2008; Xia et al., 2005; Linseis et al., 2008; Pichlmaier et al., 2009; Wu et al., 2009a,b; Záliš et al., 2010; Li et al., 2011; Man et al., 2011; Mücke et al., 2011a; Linseis et al., 2012). The latter is a consequence of the high covalency of the Ru(4d)-CH苷CHR bond and the ability of the -CH苷CH-RuCl(CO)L(PR3)2 entity to efficiently conjugate with organic π systems. MV radical cations of bridged bis- or tetrakis(vinylruthenium) complexes also display structured and strong to highly intense absorption bands in the NIR that shift to higher energies upon further oxidation while retaining their overall absorptivities (Linseis et al., 2008). Furthermore, the vinyl ruthenium redox sites offer different spectroscopic tags for addressing questions pertaining to charge and spin delocalization between the metal and the vinyl ligand or between different vinyl ruthenium subunits in a multinuclear complex. The blue shift of the Ru(CO) stretches



upon (stepwise) oxidation thus offer the often resolved P and 99/101Ru hyperfine splittings in their EPR spectra as indicators of the ‘spin density’, whereas the Ru(CO) band shift in infrared (IR) provides a sensitive measure of the ‘charge’ lost from the [RuCl(CO)L(PR3)2} site (Záliš et al., 2010; Mücke et al., 2011a). The baffling similarity between styryl ruthenium complexes and triarylamines has very recently been demonstrated through the comparison of vinyl ruthenium- and NAr3-substituted squaraines (Chen and Winter, 2012). The close relationship between these two types of systems and their great utility for the construction of potent NIR dyes prompted us to blend these two motifs into single compounds. We herein disclose that mono-, di- and trinuclear styryl ruthenium complexes with triarylamine-derived (bridging) ligands constitute potent polyelectrochromic NIR dyes, where the number of strongly NIR absorbing states scales with the number of vinyl ruthenium subunits. We note that the electrochromic properties of closely related ruthenium alkynyl complexes trans-(FcC⬅C)(dppe)2RuC⬅C-C6H4-NPhn+ and [(FcC⬅C)(dppe)2Ru-C⬅C-C6H4}3Nn+ 2 (Fc = ferrocenyl), (η5-C5H5)Fe(η5-C5H4) have recently been reported (Grelaud et al., 2012). 31

Results and discussion Synthesis and electrochemical properties The styryl ruthenium-triarylamine conjugates 1–3 of Chart 1 were synthesized by the regio- and stereoselective insertion of the hydride complex RuClH(CO)(PiPr3)2 into the terminal C⬅CH function of the corresponding amine, a process usually dubbed as hydroruthenation (Hill, 1995; Marchenko et al., 2001a,b). The required ethynylated triarylamines including the new anisyl(bis-4-ethynylphenyl) amine were prepared from their corresponding bromo precursors via Br/I exchange followed by Pd-catalyzed Sonogashira coupling with trimethylsilylacetylene and desilylation according to the literature procedures. The identity of the amines and their vinyl ruthenium complexes follows from the correct number and integral ratios of the AA′BB′ signals of the para-disubstituted phenylene rings, the methoxy substituents and the signals of the terminal ethynyl function(s) at around 3 ppm or those of the RuCH苷CH function at 8.6–8.3 ppm (RuCH), 6.3–5.9 ppm (RuCH苷CH) and those at 2.7 ppm and 1.2 ppm for the PiPr3

i

Pr3P

Cl Ru

H

OMe

H

i Cl P Pr3 H

Ru

H CO PiPr3

i Cl P Pr3 H

N

Ru

OMe

CO

N

H CO

PiPr3

1

i

2 Cl

Pr3P

OMe

Ru

H

CO

PiPr3

H

i Cl P Pr3 H

Ru

N

H CO

PiPr3

3 H H i

Pr3P

CO Ru

87

PiPr3

Cl

Chart 1 The vinyl ruthenium complexes of the present study.


PiPr3

88


C

*

B

* A

1.0

0.8

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

-0.8

E in V vs. Fc0/+

Figure 1 Cyclic voltammograms of (A) complex 1, (B) complex 2 and (C) complex 3 in NBu4PF4/CH2Cl2 (0.1 m) at room temperature (r.t.) and v = 0.100 V s-1.

ligands in 1H NMR spectroscopy. 13C NMR spectra of complexes 1–3 feature the characteristic signals of the vinyl ruthenium moiety with two triplets at 147 ppm (2JPC≈11 Hz) and at 134 ppm (3JPC≈3 Hz) and of the carbonyl ligands at 203 ppm (2JPC≈13 Hz), whereas the 31P NMR spectra show a sharp singlet near 38 ppm for the PiPr3 ligands. Cyclic voltammograms of complexes 1–3 (see Figure 1) display two to four oxidation waves within the electrochemical window of the NBu4PF6/CH2Cl2 electrolyte. The total number of redox processes equals the number of styryl ruthenium subunits plus one additional wave for the likewise electroactive NAr3 core. All waves appear to be chemically and electrochemically reversible by the usual criteria. For sweep rates in the range of 0.05–2 V s-1, peak current ratios for the reverse and forward peaks are essentially unity and peak-to-peak separations and forward-peak half-widths are identical or close to those of

1 2 3 RuOMe RuNMe2 An3N DMAPh3N Ru1 Ru3

the internal Fc*/Fc*+ redox standard [Fc* = (η5C5Me5)2Fe]. Half-wave potentials are collected in Table 1 and compared with those of other donor substituted triarylamines, styryl complexes and the triarylamine-derived alkynyl ruthenium complexes Ru1 and Ru3 of Chart 2 with RuCl(dppe)2 metal termini instead of the vinyl ruthenium ones of the present study. It has recently been noted that the (CH苷CH)RuCl(CO)(PiPr3)2 moiety resembles the NMe2 substituent in terms of donor capacity, but is an inferior donor when compared with -C⬅C-Ru(dppe)2Cl (Pevny et al., 2010; Mücke et al., 2011a). This is clearly seen by the comparison of the half-wave potentials of complexes 1 and 3 to those of Ru1 and Ru3 (Onitsuka et al., 2006; Grelaud et al., 2012) and those of tris-(4-dimethylaminophenyl)amine, DMAPh3N (Pragst et al., 1979). Halfwave potential separations ΔE1/2 between individual waves of complexes 1–3 are in the range of 186–434 mV. They attest to the high thermodynamic stabilities of all redox species of each complex with respect to disproportionation as it is quantified by the comproportionation constant Kc = exp [(n‧F/R‧T)‧ΔE1/2}, where ΔE1/2 represents the difference of half-wave potentials between the nth and (n+1)th redox processes. Sufficiently large Kc values are a necessary requirement for the generation of the various oxidation products as (spectroscopically) pure samples. The splitting of all redox potentials in complexes 2 and 3 with more than one styryl ruthenium subunit on one hand and the close similarity of half-wave potentials between similarly substituted styryl ruthenium complexes and triarylamines on the other already suggests substantial charge and spin delocalization for any oxidation state between the neutral and the fully oxidized forms. We also note that the redox splittings between the first three waves of complex 3 are larger than those for the closely related triruthenium tris(ethynyl) complex Ru3, whose radical

E1/20/+ a

E1/2+/2+ a

E1/22+/3+ a

E1/23+/4+ a

-0.118 -0.168 -0.218 0.135 -0.190 +0.109 -0.260 -0.19 -0.19 -0.33 -0.32

+0.266 +0.152 +0.114 0.725 +0.236 +0.860 – 0.32 0.34 -0.07 -0.08

– +0.586 +0.444 – – – – –

– 0.384/3.1 × 106 – 0.330/3.8 × 105 +0.630 0.332/4.1 × 105 – 0.590/9.5 × 109 – 0.426/1.6 × 107 – 0.751/5.0 × 1012 – – – 0.51/4.2 × 108 0.53/9.2 × 108 +0.44 0.26/2.5 × 104 +0.44 0.24/1.1 × 104

+0.16 +0.11

ΔE1/21/K1c

ΔE1/22/K2c

ΔE1/23/K3c Reference

– – 0.434/2.2 × 107 – 0.330/3.8 × 105 0.186/1.4 × 103 – – – – – – – – – – 0.23/7.7 × 103 0.19/1.6 × 103

0.28/5.4 × 104 0.33/3.8 × 105

This work This work This work (Ott et al., 2010) (Záliš et al., 2010) (Amthor et al., 2005) (Pragst et al., 1979) (Onitsuka et al., 2006) (Grelaud et al., 2012) (Onitsuka et al., 2006) (Grelaud et al., 2012)

Table 1 Electrochemical data for complexes 1–3 and related triarylamines, styryl complexes and triarylamine-derived ethynyl ruthenium complexes. a All potentials are given in Volts relative to the ferrocene/ferrocenium scale; E1/2 Cp*2Fe = -0.545 V vs. Cp2Fe.



89

50 000 1 2 3

ε (I mol-1 cm-3)

40 000 30 000 20 000 10 000 0 300

350

400

450

500

550

600

650

700

λ (nm)

Chart 2 Reference compounds for the electrochemical properties of complexes 1–3.

cation constitutes a Class II MV system (Robin and Day, 1967) with mutually interacting, yet electronically distinct alkynyl ruthenium subunits (Onitsuka et al., 2006). Cyclic and square wave voltammograms of diruthenium complex 2 display another small wave 170 mV anodic of the 2+/2+ couple which also shows features of chemical reversibility with an E1/2 of ca. 0.320 V as is indicated by the star symbol in Figure 1. The small height of the individual peaks and the overlap with the anodic/cathodic realm of the forward/reverse peak of the 2+/2+ wave does, however, not allow for a quantitative assessment of the degree of chemical reversibility for this couple. Similar behavior is also seen in other electrolytes such as ortho-difluorobenzene/NBu4PF6, THF/NBu4PF6 or upon replacement of PF6- by the even less nucleophilic B[C6H3(CF3)2}4- counterion and thus seems not to be the result of an electron transfer-initiated detrimental chemical process following the second oxidation. Thus, the chemical reversibility of the 20/+ and 2+/2+ couples was found to be independent of whether the sweep direction was reversed before or after traversing this small wave. NMR spectra of 2 showed no impurities that may cause this additional wave. We defer a discussion of its possible origin until we turn to the aspect of electron delocalization in these unsymmetrical MV systems.

UV/Vis/NIR spectroscopy on the neutral and the various oxidized forms UV/Vis spectra of neutral complexes 1–3 are characterized by an intense structured UV band and a much weaker absorption at 525 nm, whose intensity roughly scales with the number of vinyl ruthenium moieties (see Figure 2 and Table 2). Although only weakly allowed and little intense, it is still responsible for the brilliant pur-

Figure 2 Comparison of the UV/Vis spectra of complexes 1–3.

ple-red coloration of the complexes. The structured UV band consists of two peaks with the more intense one at lower energy. In triarylamine complexes 1 and 2 the splitting of the UV band is a consequence of the symmetry lowering from C3 to C2. A lifting of the degeneracy of the lowest unoccupied molecular orbital (LUMO) in triarylamines with three equal substituents such as complex 3 is, however, also not without precedence (Amthor et al., 2005). The main peak shifts bathochromically and intensifies as the number of vinyl ruthenium subunits increases, whereas the position of the other one is much more constant across this series. To demonstrate the (poly)electrochromism of complexes 1–3, solutions in 0.2 m NBu4+ PF6- /1,2-C2H4Cl2 (DCE) were placed inside an optically transparent thin layer electrolysis (OTTLE) cell (Krejcik et al., 1991) and exhaustively electrolyzed at potentials sufficiently positive of every respective n+/(n+1)+ redox couple. Owing to the large halfwave potential separations and Kc values, every accessible oxidation state could be generated in this manner. Clean isosbestic points were observed for each step and electrolytic reduction restored the original spectra of the neutral starting materials in high optical yields (generally > 95%). The last oxidation step of complexes 2 and 3 could, however, not be pursued to full conversion to 23+ or 34+ before some degradation occurred as was indicated by deviations from the isosbestic points and irreversible spectroscopic changes. Figure 3 compares the UV/Vis/NIR spectra of the neutral, singly and doubly oxidized forms of monoruthenium complex 1. Radical cation 1+ is characterized by two groups of intense, structured absorptions. The one at the higher energy peaks at 487 nm with a distinct shoulder to the red. The other one is a broad absorption which covers the entire spectroscopic window from 750 to 1500 nm. Distinct peaks are observed at 1040 and 1115 nm with extinction coefficients of ca. 20 000 l mol-1 cm-1.


90


λmax (ε in l mol-1 cm-1) 1a 1+ b 12+ b 2a 2+ b 22+ b 23+ b 3a 3+ b 32+ b 33+ b

322 (sh, 26 000), 343 (30 000), 535 (300) 365 (7500), 487 (24 000), 516 (sh, 20 000), 575 (sh, 3000), 870 (sh, 10 000), 1030 (19 600), 1160 (22 000) 380 (11 300), 482 (sh, 10 500), 535 (sh, 13 000), 595 (sh, 15 500), 664 (sh, 27 000), 730 (59 500) 320 (sh, 27 000), 357 (37 000), 525 (700) 372 (13 000), 467 (sh, 20 500), 517 (29 000), 800 (sh, 7400), 913 (12 600), 1205 (sh, 15 400), 1561 (38 500) 425 (22 000), 515 (15 000), 575 (sh, 12 000), 770 (10 000), 941 (14 000), 1185 (sh, 48 000), 1355 (91 000)c 412 (23 000), 464 (30 000), 658 (35 000), 743 (44 500), 820 (45 000)c 311 (sh, 29 000), 367 (49 000), 525 (1500) 296 (17 700), 358 (25 000), 480 (sh, 18 000), 525 (26 000), 711 (2000), 805 (3000), 1280 (sh, 26 000), 1555 (52 400) 296 (17 000), 388 (19 000), 424 (20 700), 536 (8100), 696 (6200), 1340 (sh, 60 000), 1542 (69 000) 296 (18 600), 395 (sh, 19 000), 443 (26 000), 644 (9700), 737 (9000), 1352 (57 000)

Table 2 UV/Vis/NIR data for complexes 1–3 in their various oxidation states. In CH2Cl2.; bin 1,2-C2H4Cl2/0.2 m NBu4+ PF6- ; cextinction coefficients estimated based on the degree of conversion of 22+ to 23+.

a

The spectrum of 1+ differs from that of simple triarylaminium radical cations such as N(C6H4-4-OMe) 3+ (An3N+) by its enormous breadth and structuring as well as by its unusually low energy (An3N+: λmax = 728 and 641 nm) (Amthor et al., 2005). It, however, resembles those of MV radical cations of linearly bridged bis(triarylamine)s (Lambert and Nöll, 1999; Low et al., 2004; Barlow et al., 2005; Lambert et al., 2005; Risko et al., 2008; Ramírez et al., 2012). Spectral deconvolution revealed that each of the two main transitions is composed out of three individual Gaussian bands. Total oscillator strengths as calculated from the equation f = 4.6 × 10-9 × ε × Δν1/2 are 0.36 and 0.35 for the lower and the higher energy transitions. They are more than twice as large as those observed for An3N+ and similar, simple triarylaminium ions (Amthor et al., 2005). The red shift and the increase in oscillator strength of the low energy band are both tokens of electron richness and extensive ground-state delocalization of 1+. Further oxidation to 12+ shifts the prominent low energy band to higher energy such that it now resides 60 000

1 1+ 12+

ε (I mol-1 cm-1)

50 000 40 000 30 000 20 000 10 000 0 400

600

800

1000 λ (nm)

1200

1400

1600

Figure 3 Comparison of the UV/Vis/NIR spectra of complex 1 in its various oxidation states (spectra in 0.2 m NBu4+ PF6- ).

at the border of the Vis and NIR regimes. The transition dipole moment f of 0.44 as computed from the deconvoluted spectrum indicates that the overall absorptivity even increases. The overall Vis/NIR band pattern and energy of 12+ is highly reminiscent of that of homovalent bis(triarylaminium) dications (Lambert and Nöll, 1999; Heckmann et al., 2004; Low et al., 2004; Barlow et al., 2005). This underlines once more the very similar properties of triarylamine and styryl ruthenium type redox sites (Chen and Winter, 2012). Complex 1 thus constitutes a polyelectrochromic system with three distinct and interconvertible states, one of which is transparent below 600 nm (1), another one strongly absorbing in the NIR (1+) and the other one strongly absorbing at the border of the Vis and NIR regimes (12+). Qualitatively similar electron transfer-induced absorption changes have just been documented for [Fc-C⬅C-Ru(dppe)2-C⬅C-C6H4-NPh2]n+ (Fc = (η5-C5H5)Fe(η5-C5H4), ferrocenyl, n = 1, 2), a close relative to Ru1, although the authors propose a more localized description of the redox processes (0→+: Fc based, +/2+: Ru-ethynyl centered, 2+/3+ triarylamine based) and, consequently, a somewhat different assignment of the underlying transitions, e.g., a ligand-to-metal charge-transfer (LMCT) for the dication (Grelaud et al., 2012). As the total number of accessible redox states increases by one with every vinyl ruthenium moiety, diand triruthenium complexes 2 and 3 have the potential to act as four- and five-state switches. This is indeed the case as follows from the comparison of the spectra of every member of these two redox series (see Figures 4 and 5). Oxidation of 2 to its radical cation induces the growth of two structured NIR absorptions with main peaks at 913 and 1561 nm and oscillator strengths of 0.15 and 0.35. Upon further oxidation to 22+ the NIR absorption further intensifies and shifts to the blue, now peaking at 1355 nm with an impressive extinction coefficient of 91 000 and a



80 000

100 000

ε (I mol-1 cm-1)

60 000

40 000

3 3+ 32+ 33+ 34+ (45% conversion)

70 000 60 000 ε (I mol-1 cm-1)

2 2+ 22+ 23+

80 000

91

50 000 40 000 30 000 20 000 10 000

20 000

0 0 500

1000

1500

2000

0

500

combined oscillator strength of 0.86. During the third and final oxidation, the low energy absorption shifts further to the blue to again reside at the border between the Vis and NIR regimes (Figure 4). Spectroscopic changes during the stepwise conversion of 2 to 23+ are collected in Figure S1 of the supplementary material and show sets of clean isosbestic points for each individual process. The final oxidation to 23+ could, however, not be pursued to full conversion before the onset of chemical decomposition. Our estimates of the extinction coefficients and total oscillator strength of ca. 0.9 are based on the degree of conversion and may only qualitatively be correct. Triruthenium complex 3 has a total of five accessible oxidation states, at least three of which are strongly absorbing in the NIR, although with different absorption profiles, absorptivities and energies (Figure 5). Extinction coefficients are up to 69 000 l mol-1 cm-1 for 32+ and total oscillator strengths assume values of 0.52 (3+), 1.12 (32+) and 0.99 (33+). The final oxidation caused the rising of a rather strong absorption in the visible peaking at 773 nm but could only be pursued to ca. 45% conversion before the onset of decomposition. No absorptivity data of 34+ are therefore provided. Spectra recorded during the individual oxidation steps can be found in Figure S2 in the supplementary material. Data pertaining to the absorption properties of complexes 1–3 in all investigated redox states are compiled in Table 2. To gain insight into the molecular and electronic structures of complexes 1–3 and their associated oxidized forms, we performed quantum chemical calculations based on density functional theory (DFT, Gaussian 09 (Frisch et al., 2009), for details see Experimental section) on simplified PMe3 substituted model complexes which are hereafter denoted as 1Me, 2Me and 3Me. Previous

1500

2000

2500

λ (nm)

λ (nm)

Figure 4 Comparison of the electronic spectra of complexes 2 to 23+ in DCE/0.2 m NBu4+ PF 6- .

1000

Figure 5 Comparison of the UV/Vis/NIR spectra of complexes 3 to 33+ in DCE/0.2 m NBu4+ PF6- .

calculations on mono- and diruthenium complexes have shown that this simplification under- (Ru-P) or overestimates (Ru-C) the ruthenium-ligand bond lengths by just a few pm and underestimates somewhat PR3 ligand contribution to the corresponding MOs but gives otherwise satisfactory results (Maurer et al., 2008; Záliš et al., 2010). Most importantly, structure optimization correctly reproduces the propeller-like arrangement of the aryl substituents around the central nitrogen atom, which is a common feature of neutral triarylamines. The most important structural parameters of 1Me, 2Me and 3Me and their monoand dioxidized forms are listed in Table 3. Although they are not strictly identical, equivalent bond lengths of like n+ substituents (i.e., the two anisyl groups of 1Me or the two n+ n+ or three vinyl ruthenium moieties of 2Me and 3Me , respectively) do not deviate by more than 0.01 Å or 0.2° such that only average values are provided. Our DFT calculations reveal that the structural changes accompanying stepwise oxidation are rather small (Table 3). Typical assets are a shortening of the RuCvinyl and the Cvinyl-CAryl bonds, an elongation of the vinylic C苷C bonds and a shortening of the N-Cipso and the opposing 苷C-Cipso bonds of the ruthenium-bonded styryl rings that bridge the individual Cl(CO)(PMe3)2Ru-CH苷CH- and the NAr3 redox sites in complexes 1–3. The overall magnitude of the structural alterations is largest for the + 2+ 1Me /1Me /1 Me redox series and decreases consistently as the number of the vinyl ruthenium moieties increases. Taken together with the fact that the structural parameters at equivalent sites are essentially identical, our calculations therefore indicate that the positive charge(s) [hole(s)] generated upon oxidation(s) equally spread over the styryl ruthenium subunits. Our computations also indicate that the styryl ring which is common to the NAr3 and the styryl


92


Ru-Cl Ru-P Ru-CCO Ru-CVi C 苷C 苷C-CAryl N-C Ar1b Ar2b Ar3b Ru-C 苷C C 苷 C-Ci-CAr1 C苷 C-Ci-CAr2 C苷 C-Ci-CAr3

1

1+

2.449 2.363 1.807 1.985 1.348 1.468 1.407, 1.415a 34.9 44.4 43.2 134.4 6.3 – –

2.432 2.379 1.820 1.932 1.372 1.434 1.378, 1.417a 26.7 45.8 44.5 133.6 3.5 – –

12+

2+

22+

3

3+

32+

2.407a 2.425a a 2.368 2.385a a 1.825 1.827a 1.947a 1.908a 1.364a 1.383a a 1.444 1.421a a a 1.396 , 1.419 1.381 , 1.416 35.3 34.4 33.1 32.7 46.6 45.8 134.1a 133.5a -0.4 0.7 4.9 3.7 – –

2.448a 2.363a 1.808a 1.984a 1.348a 1.468a 1.411a 38.8 39.2 41.7 134.3a -0.1 -0.9 8.8

2.409a 2.365a 1.822a 1.955a 1.361a 1.449a 1.403a 37.2 39.5 37.1 134.0a -1.2 1.9 2.4

2.429a 2.381a 1.823a 1.923a 1.375a 1.431a 1.392a 37.3 36.5 36.8 133.9a 0.0 1.7 1.6

2

2.411 2.421a 2.394 2.353a 1.839 1.815a 1.871 1.989a 1.409 1.347a 1.397 1.468a a a 1.355, 1.413 1.411 , 1.411 26.2 40.3 44.3 39.9 43.2 38.8 132.5 134.3a 2.5 7.4 – 4.4 – –

Table 3 Calculated structure parameters of complexes 1–3 in the neutral, radical cation and dication states (bond lengths in Å, bond angles in °). a

Average value (see text); b angle between best plane of the corresponding arene rings and the NC3 plane containing the ipso-carbon atoms; the styryl ring(s) bonded to ruthenium are denoted as Ar1 (1) or Ar1, Ar2 (2).

ruthenium redox sites rotates to become more coplanar to + the NAr3 plane. This is again most evident for 1Me/1Me and, + to a lesser extent, also for the 2Me/2Me pair of compounds, whereas no such effect is evident for 3Me. This rotation is accompanied by a shortening of the Cortho-Cortho, bonds of ca. 0.01 to 0.02 Å and an equal lengthening of the flanking Cipso-Cortho bonds. The structural effects on the ‘terminal’ anisyl rings are even smaller. These structural changes, although not large, are still indicative of an increased quinoidal distortion and a decreased electron density at the ‘central’ aryl substituent. We also note that our results for + the 1Me/1Me pair of compounds closely resemble those of quantum chemical (Malagoli and Brédas, 2000; Renz et al., 2009) and crystallographic (Low et al., 2004; Zheng et al., 2006) studies on closely related N,N′-tetraphenyl-1,1′biphenyl-4,4′-diamines. Time dependent (TD)-DFT calculations (Runge and Gross, 1984) (1,2-C2H4Cl2 solvent included by the polarizable conductor continuum model, PCM, (Cancés et al., 1997; Mennucci and Tomasi, 1997; Cossi et al., 2003; Scalamani and Frisch, 2010) based on the optimized structures reproduce the general appearances of the UV/Vis/NIR spectra of the mono- and dications rather well. Figures S3 and S4 in the supplementary material demonstrate this for compounds 1 and 2 in their mono- and dioxidized states. Calculated transitions are compiled in Table S1 in the supplementary material along with their positions and oscillator strengths. Deviations from the experimental spectra arise from the fact that individual transition energies and transition dipole moments are somewhat overestimated and the structuring of the transitions are not accounted for by the calculations. The experimentally observed band structuring is most

probably due to vibronic coupling which was not included in our calculations. The weak Vis band observed for neutral 1 involves excitations from the d(Ru)/π-arene mixed HOMO/ HOMO-1 to the metal-based unoccupied LUMO (Figure 6), whereas the more intense UV band comprises excitations from the highly delocalized HOMO to the bridge- or the NAr2based LUMO+1 and LUMO+2, respectively. Replacement of a second and third OMe substituent of N(C6H4-4-OMe)3 (An3N) by vinyl ruthenium subunits increases the number of energetically close-lying MOs below the HOMO and above the LUMO and hence the complexity of the individual transitions as it is exemplified by diruthenium complex 2. Owing to even more extensive delocalization of all frontier molecular orbitals (FMOs) except HOMO-4, HOMO-3 and LUMO+1 (Figure 7), the charge transfer character of the individual transitions further diminishes. Triruthenium complex 3 features sets of three nearly degenerate MOs in the FMO region that are localized on a single vinyl ruthenium subunit (HOMO-6 to HOMO-4, LUMO to LUMO+2), whereas HOMO-1, HOMO-2 and LUMO+3 are delocalized over two styryl ruthenium entities with virtually no contributions from the central nitrogen atom. Only MOs HOMO and HOMO-3 are delocalized over all three arene rings and receive contributions from the nitrogen lone pair (see Figure S5 in the supplementary material). As a consequence of the further extension of the π-conjugated system, the experimental spectrum of complex 3 constitutes an overlap of several mixed transitions with differing character. According to the TD-DFT results, the main transi+ tions of 1Me can be assigned as the SOMO-1→SOMO (or, more strictly speaking, the β-HOSO→β-LUSO) and the



HOMO-1

LUMO

93

HOMO

LUMO+1

LUMO+2

Figure 6 MOs involved in the Vis and the low energy UV transitions of complex 1Me.

SOMO→SOMO+2 (α-HOSO→α-LUSO+1) transitions, whereas the weaker absorption in between these two is ascribed to excitation from the lower lying SOMO-2 to the SOMO (β-HOSO-1→β-LUSO, Table 4). This latter band may account for the low energy shoulder (λ ca. 575 nm) of the 487 nm band of 1+. From Figure 8 it follows that both orbitals involved in the NIR transition are delocalized over the entire molecule. The NIR band of 1+ is thus best described in terms of a π→π* transition within an

HOMO-4

HOMO-3

HOMO-1

HOMO

LUMO

LUMO+3

extended metal-organic radical cation (polaron) with only little charge transfer. In keeping with such an assignment, we experimentally observe just a very small positive solvatochromism for chemically oxidized 1+ (oxidation with ferrocenium hexafluorophosphate) with the main peaks shifting from 1030 and 1160 nm in DCE/NBu4+ PF6- to 1030 and 1166 nm in neat CH2Cl2, 1020 and 1140 nm in THF, and 1005 and 1110 nm in acetone (total shift 240 or 430 cm-1, respectively). The second, intense transition (calculated

HOMO-2

LUMO+1

LUMO+2

LUMO+4

Figure 7 MOs involved in the optical transitions of complex 2Me.


94


E (cm-1)

λ (nm)

f

9925 17 136 23 001

1008 584 435

0.6344 0.1391 0.5085

23 089

433

0.116

Assignment β-HOSO→β-LUSO (97%) β-HOSO-1→β-LUSO (98%) α-HOSO→α-LUSO+1 (61%), β-HOSO-6→β-LUSO (14%) α-HOSO→α-LUSO+1 (14%), β-HOSO-6→β-LUSO (66%)

+ Table 4 Calculated transitions for radical cation 1Me .

at 435 nm, experimental value: 487 nm) has some Ar2N→ruthenium charge transfer contribution but is also only slightly solvatochromic. The strong Vis/NIR band of the singlet dication 12+ corresponds to the HOMO→LUMO transition of that species. The orbitals involved in that transition have essentially the same composition as the βHOSO and β-LUSO of the radical cation. From TD-DFT calculations on 2+ (Table 5, Figure 9), we learn that the main NIR band arises from the SOMO1→SOMO (β-HOSO→β-LUSO) transition and involves an excitation from an MO which is delocalized over the two styryl ruthenium moieties to the totally delocalized SOMO. This implies a very small degree of charge transfer which is in good agreement with our experimental observation of small positive solvatochromism similar to 1+. The second, lower intensity Vis/NIR band is computed to arise from the excitation from the anisyl-centered, lower lying SOMO-2 (βHOSO-1) to the same acceptor orbital. Calculations on dication 22+ provide a rather small energy difference between the singlet and triplet states of just 4 kJ mol-1. The spectrum calculated for the singlet ground state matches the experimental one rather well, whereas that computed for the triplet state has some features that are not observed in the experimental spectrum. According to our results, the intense NIR band of 22+ in its singlet ground state is composed of the HOMO-1→LUMO and the HOMO-3→LUMO transitions with partial charge transfer from the anisyl ring or one ruthenium atom to the totally delocalized LUMO (see Table S2 and Figure S6 in the supplementary material).

β-LUSO

⇑

β-HOSO

β-LUSO

α-LUSO+1

⇑

⇑

β-HOSO-1

α-HOSO

The character of the transitions underlying the NIR absorption of 3+ is basically equivalent to that in 2+ and involves two nearly degenerate excitations (λ = 1174, 1169 nm) from SOMO-2 (β-HOSO-1) and SOMO-1 (β-HOSO) that each spread over two styryl ruthenium subunits to the completely delocalized SOMO (β-LUSO) with combined + oscillator strengths of 1.12 for 3Me (Table S3 in the supplementary material). Figure S7 in the supplementary material provides a graphical representation of these orbitals. The NIR transitions in singlet 32+ retain the character of those in 3+ (see Figure S8 and Table S4 in the supplemen2+ tary material). Calculated total oscillator strengths for 3 Me double to 2.54, which, apart from the gross overestimation, match the experimentally observed trends. The triplet state 2+ of 3 Me is calculated as being 26.5 kJ mol-1 higher in energy.

Charge and spin delocalization as probed by EPR and IR spectroscopy The close similarity between styryl ruthenium and triarylamine-based redox sites and the close resemblance of the Vis/NIR spectra of the oxidized forms of complexes 1–3 to those of bridged bis(triarylamine)s along with the results of our quantum chemical calculations suggest extensive delocalization of the hole(s) [positive charge(s)] or the unpaired spin in the associated cations. Styryl ruthenium complexes offer various spectroscopic tags to experimentally probe for such delocalization (Maurer et al., 2006; Linseis et al., 2008, 2012; Mücke et al., 2011a,b; Wuttke et al., 2012). Spin delocalization in the radical cations is conveniently probed by EPR spectroscopy. Resolved EPR spectra have been obtained for the chemically prepared (oxidation with ferrocenium hexafluorophosphate) radical cations of complexes 1–3, from which the hyperfine interaction constants to the 31 P and the 99/101Ru nuclei of the vinyl ruthenium as well as the 14N nucleus of the triarylamine moieties could be determined by digital simulation. Graphical accounts

β-LUSO

⇑

β-HOSO-6

+ Figure 8 Electronic transitions in radical cation 1 Me .



E (cm-1)

λ (nm)

f

8019 12 333 21 425 21 537 21 659

1247 811 467 464 462

0.696 0.242 0.1309 0.2882 0.1121

95

Assignment β-HOSO→β-LUSO β-HOSO-1→β-LUSO β-HOSO-9→β-LUSO (52%), α-HOSO→α-LUSO+2 (14%), β-HOSO-7→β-LUSO (14%) α-HOSO→α-LUSO+2 (34%), β-HOSO-9→β-LUSO (13%), β-HOSO→β-LUSO+1 (11%) α-HOSO→α-LUSO (11%), α-HOSO→α-LUSO+2 (15%), β-HOSO-1→β-LUSO+1 (11%), β-HOSO→β-LUSO+1 (38%)

+ Table 5 Calculated transitions for radical cation 2Me .

β-LUSO

β-HOSO

β-HOSO-1

+ Figure 9 MOs involved in the NIR transitions of radical cation 2Me .

of the experimental results along with those of spectral simulations are displayed in Figure 10, whereas pertinent data along with those for related compounds (see Chart 3) are collected in Table 6. From these data the following information can be gathered: (i) the 14N hyperfine coupling constant in 1+ is approximately half of that in the tris-4-anisylaminium or the phenyl(bis-4-anisyl)aminium radical cations An3N+ and PhAn2N+ and resembles that in the radical cations of E-stilbenyl- or tolanyl-bridged bis(dianisylamine)s. By contrast, 31P hyperfine splitting is approximately half of those in comparable mononuclear styryl ruthenium radical cations and resembles that in the oxidized 1,4-divinylphenylene-bridged diruthenium complex Ru2DVP+. This signals that the unpaired spin is approximately equally shared between the styryl ruthenium and triarylamine redox sites. (ii) All hyperfine splittings including that to the 14N nucleus tend to decrease as the total number of redox sites and hence spin-bearing centers increases. This again points to complete delocalization of the unpaired spin in these radical cations. (iii) The spectra of radical cations 2+ and 3+ featuring two or three equivalent 1+

styryl ruthenium-type redox sites display only one set of P and 99/101Ru hyperfine coupling constants. More specifically, they show that all three styryl ruthenium subunits are equivalent in the radical cation state. On the EPR time scale, complexes 2+ and 3+ thus constitute inherently delocalized MV systems of Class III according to the Robin and Day classification scheme (Robin and Day, 1967). It should be mentioned here that the g-tensor remains isotropic in frozen solution (T = -170°C, CH2Cl2, see Figure S9 in the supplementary material) and that the g-value is only slightly larger than the free electron value ge of 2.0023. Similar behavior has already been observed for arylene-bridged bis(vinyl) ruthenium complexes and is a token of the large contribution of the organic ligand to the corresponding SOMO (Maurer et al., 2006; Pevny et al., 2010; Mücke et al., 2011a; Linseis et al., 2012; Wuttke et al., 2012). The intense, charge-sensitive Ru(CO) labels provide a convenient means for probing charge delocalization between the styryl ruthenium and the triarylamine redox sites in complexes 1–3 and between the individual styryl ruthenium subunits in complexes 2 and 3 by IR spectroscopy. Under the conditions of in situ IR 31

2+

3300

3320

3340

B (G)

3360

3+

3300

3320

3340

3360

3300

B (G)

3320

3340

3360

B (G)

Figure 10 EPR spectra (X-band) of radical cations 1+ to 3+ in CH2Cl2 at r.t.; black (bottom): experimental; blue (top): simulated.


96


↑

↑

1 → 1+ ↓

2000

1900

↓

Chart 3 Reference compounds for EPR spectroscopy on complex cations 1+ to 3+.

spectroelectrochemistry, the oxidation of complex 1 to first 1+ and then 12+ proceeds in two well-defined steps with clean isosbestic points (Figure 11). Most importantly, the Ru(CO) band experiences a blue shift from 1910 cm-1 to 1944 cm-1 during the first and then to 1985 cm-1 during the second oxidation. These shifts compare favorably with those observed for the 0/+/2+ redox series of the closely related 1,4-divinylphenylene bridged diruthenium complex Ru2DVP [Chart 3; 1910 cm-1 → 1932 cm-1 (delocalized form) → 1991 cm-1]. From this we conclude that the amount of charge lost from the styryl ruthenium site of 1 during the first and second oxidation is similar to that in Ru2DVP. This finding is in complete agreement with our EPR data (see above) and establishes that complex 1, despite its dislike redox-active subunits, constitutes a strongly (if not completely) delocalized MV system. Other spectroscopic changes (see Table 7) include the growth of

+

1 2+ 3+ RuSt+ Ru2DVP+ An3N+ PhAn2N+ [An2N}2Tol+ [An2N}2St+

↑

1800 1700 ν (cm-1)

1500 ↓

1+ → 12+ ↓↑↑

↑

2000

1600

1900

1800 1700 ν (cm-1)

1600

1500

Figure 11 IR spectroscopic changes during the first (top) and second (bottom) oxidation of complex 1 in NBu4PF6/0.2 m 1,2-C2H4Cl2.

intense C苷C-type vibrations in the vinylic and aromatic region as it is typical of the styryl ruthenium-type redox systems. We also note that the low energy NIR band was also observed to first grow (1 → 1+) and then disappear (1+ → 12+) as oxidation proceeds, although its maximum is outside the detector range. The first oxidation of diruthenium complex 2 gives rise to a pattern of two overlapping CO bands whose peak

giso/g103a

A (31P)

A (99/101Ru)

A (14N)

Reference

2.015/2.016 2.020/2.016 2.013/2.017 2.045 2.028 n.p.b n.p.b n.p.b n.p.b

10.4 G (2 P) 6.6 G (4 P) 7.3 G (6 P) 21.5 G 9.0 G – – – –

5.1 G (1 Ru) 4.5 G (2 Ru) 4.6 G (3 Ru) n.p.b 6.0 G – – – –

5.1 G 5.9 G 2.9 G – – 9.05 G (1 N) 9.21 G (1 N) 4.1 Gc (2 N) 3.7 Gc (2 N)

This work This work This work (Maurer et al., 2008) (Maurer et al., 2006) (Pearson et al., 1978) (Kattnig et al., 2009) (Lancaster et al., 2009) (Lancaster et al., 2009)

Table 6 EPR spectra of radical cations 1+ to 3+ and comparison to related styryl ruthenium complexes and triarylamines. giso = isotropic g-value at r.t.; g103 g-value in frozen CH2Cl2 solvent at T = -170°C; bnot provided; cin the limit of static delocalization over both amine sites. a



1 1+ 12+ 2 2+ 22+ 23+ b 3 3+ 32+ 33+

97

ν(CO)

ν(C 苷C), ν(CC)

1910(s) 1944(vs) 1985(s) 1910(s) 1919(m), 1936(s) 1947(m), 1953(s) 1984(m) 1912(m) 1919(s), 1924(m) 1939(s), 1963(m) 1953(s), 1970(m)

1503(s) 1608(s), 1503(vs), 1251(vs), 1197(m), 1183(m), 1163(s) 1605 (m), 1591(m), 1503(w), 1164(m, sh), 1154(vs) 1575(w), 1545(w), 1502(m) 1606(w), 1578(w), 1517(sh), 1502(vs), 1252(s), 1138(s), 1091(m), 1061(m) 1600(m), 1501(vs), 1153(vs), 1090(m), 1060(m) 1601(w), 1502(m), 1155(m) 1502(s), 1138(vs), 1090(m) 1500(s), 1146(vs), 1090(s) 1502(m), 1146(m)

Table 7 IR data for complexes 1–3 in their various oxidation statesa. a In 1,2-C2H2Cl2/0.2 m NBu4+ PF6-; bonly partial conversion could be achieved.

positions were estimated by spectral deconvolution as 1919 cm-1 and 1936 cm-1 (Figure 12). This result contradicts that from EPR spectroscopy insofar as it indicates that the two styryl ruthenium moieties are electronically inequivalent, whereas the observation of just one set of 31P and 99/101 Ru hyperfine interactions in EPR spectroscopy points to the opposite. This seeming paradox can be resolved when taking into account the different time scales inherent in these methods: 10-8 to 10-9 s for EPR and 10-11 to 10-12 s for IR spectroscopy. It thus seems that radical cation 2+ is a strongly coupled, yet intrinsically localized MV system of Class II with an unequal distribution of the hole over the two styryl ruthenium subunits. By contrast, intramolecular electron transfer, which renders these two sites equivalent, occurs at faster time scale than that of the EPR experiment. Several examples of such time-dependent valence trapping (or detrapping) have already been reported in the literature (Atwood et al., 1993; Atwood and Geiger, 2000), amongst them Ru2DVP+ and other bis(vinyl ruthenium) complexes (Maurer et al., 2006; Mücke et al., 2011a,b; Linseis et al., 2012). Quantum chemical calculations based on the optimized structures grossly underestimate the experimental splitting of the CO bands. An almost quantitative agreement is, however, achieved, when one of the styryl ruthenium moieties is forced to a coplanar arrangement with the NC3 plane such as to maximize overlap between that unit and the nitrogen lone pair (see Table S5 in the supplementary material). We note that there is ample literature precedence for a close link between the torsional motions of aryl rings of the bridge and the efficacy of intramolecular electron transfer, notably in oligoethynylphenylenes and bis(ethynyl) phenylene bridged dimetal complexes (Levitus et al., 2001; Schmieder et al., 2002; Berlin et al., 2003; Fox et al., 2011). Curiously, the simulation of the experimental spectrum of dication 22+ requires the inclusion of three CO bands to

achieve a satisfactory fit. A strong one at 1953 cm-1 and two weaker, broader ones at 1948 and 1969 cm-1 (Figure S10 in the supplementary material). A possible explanation is the presence of several conformers or rotamers that differ with respect to the mutual orientations of the individual styryl ruthenium moieties and the NC3 plane and those of the -HC苷 CH-RuCl(CO)(PiPr3)2 π systems to those of the phenyl bridges. Fox and coworkers have made a convincing case of how such torsions can influence the hole distribution between terminal redox sites and the bridge in electron-rich, π-conjugated organometallics and give rise to a dichotomy between MV and bridge-localized radical cations (Fox et al., 2011). In our system this may give rise to two different valence tautomers, one of which features two electronically equivalent styryl ruthenium moieties, whereas in the other one the hole is mainly distributed between one styryl ruthenium subunit and the triarylamine redox system. This one probably gives rise to the higher and lower energy CO bands. We here revert to our observation of a small, additional voltammetric wave 170 mV positive of the apparent 2+/2+ couple which also hints to the presence of two different species at the 22+ oxidation state. A similar situation has recently been documented in the 1,4-dithynylphenylene bridged mixedmetal complex [Cp*(dppe)Fe(μ-C⬅C-C6H4C⬅C)Mo(dppe) (η7-C7H7)]+ (Cp* = η5-C5Me5, dppe = 1,2-Ph2PC2H4PPh2), where two different valence tautomers with the unpaired spin either associated with the iron or the molybdenum center exist (Fitzgerald et al., 2011). Additional work is, however, necessary before any firm conclusions on that particular issue can be reached for 22+. As in UV/Vis/NIR spectroelectrochemistry, the final oxidation of complex 2 to the trication could not be pursued to completion before the onset of degradation. Still, the new CO band of that species could be observed at 1984 cm-1 and thus at the same value as in 12+.


98


↑

↑ ↑

↓

2 → 2+ ↑

2000

1900 ↑

1800 1700 ν (cm-1)

1600

1500

Conclusions

2+ → 22+

↓ ↓ ↑

2000

1900 ↓

1800

1700

1600

1500

-1

ν (cm ) 22+ → 23+

↓

↑ ↓

2000

1900

1800 1700 ν (cm-1)

to obtain reasonable fits, but even more may be involved. It thus seems that charge fluctuations in these MV cations occur on a time scale faster than 10-8 s but slower than 10-12 s. Whether the presence of electronically inequivalent styryl ruthenium sites is just a consequence of slow intramolecular electron transfer (with respect to the IR time scale) between the individual styryl ruthenium subunits or also involves different valence tautomers remains to be explored.

1600

1500

Figure 12 Spectroscopic changes during the first (top), second (middle) and incomplete third oxidation (bottom) of complex 2 in NBu4PF6/0.2 m 1,2-C2H4Cl2.

A lower signal-to-noise ratio of the experimental spectra and extensive broadening of the CO band in the 3+, 33+ and, particularly, the 32+ states renders the determination of the ν(CO) values for the individual components difficult and considerably less accurate than for complexes 1n+ and 2n+. We can therefore only state that results are qualitatively similar to those for complex 2 with a gradual shift of the composite CO band to higher energies with increasing oxidation level. The distinctly asymmetric shape of the CO bands requires at least two separate peaks

In conclusion, complexes 1, 2 and 3 constitute polyelectrochromic systems with three, four and five discernible states that differ with respect to their absorption profiles. For complex 1, each of these states absorbs in a different energy window with the main peak shifting from the UV (1) deep into the NIR (1+) and finally in the low energy regime at the border of the Vis and the NIR (12+). For complexes 2 and 3, oxidation profiles of the mono- and dications or mono- to trications resemble each other more closely with their main absorption peaks in the 1340–1560 nm range but display consecutive blue shifts and intensity gains with increasing oxidation level. The higher oxidized forms reach truly remarkable extinction coefficients of up to 90 000 l mol-1 cm-1 with oscillator strengths of 0.9–1.1. These complexes clearly outscore more conventional MV systems with metal-based redox sites (Ward, 2005; Kaim, 2011). Triarylamine-derived vinyl ruthenium complexes 1–3 rank among the best polyelectrochromic systems reported in the literature. Underlying the intense NIR absorptions are transitions between MOs which are extensively delocalized between the styryl ruthenium and triarylamine redox sites with hardly any charge transfer involved. Spin and charge delocalization in the various oxidized forms was experimentally probed by EPR and IR spectroscopies. Radical cation 1+ appears to be fully delocalized on the EPR and IR time scale as inferred from the analysis of the 14N, 31P and 99/101Ru and hyperfine splittings (EPR) and the position of the Ru(CO) band (IR). For radical cation 2+, the pattern of two Ru(CO) bands indicating only partial charge delocalization contrasts to that of complete spin delocalization. Charge equilibration thus probably occurs on a time scale in between these two techniques, i.e., 108 s-1