Mixed valence radical cations and intermolecular complexes derived from indenofluorene-extended tetrathiafulvalenes

Journal ofMaterials Chemistry C

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Mixed valence ra

aDepartment of Chemistry, University of Cop

Copenhagen Ø, Denmark. E-mail: o.hammerbDepartment of Microtechnology and Nanosc

Kemivagen 9, S-41296 Goteborg, SwedencDepartment of Chemistry & Biochemistr

University of Oregon, Eugene, Oregon 97403dInstitute of Physical Chemistry and Chemic

Technology, Slovak University of Technology

Republic

† Electronic supplementary information (Echaracterization data (UV-Vis-NIR-Ecrystallographic), computational data, a1010185–1010188. For ESI and crystallogformat see DOI: 10.1039/c4tc02178a

Cite this: J. Mater. Chem. C, 2014, 2,10428

Received 26th September 2014Accepted 15th October 2014

DOI: 10.1039/c4tc02178a

www.rsc.org/MaterialsC

10428 | J. Mater. Chem. C, 2014, 2, 10

dical cations and intermolecularcomplexes derived from indenofluorene-extendedtetrathiafulvalenes†

Mikkel A. Christensen,a Christian R. Parker,a Thomas Just Sørensen,a Sebastian deGraaf,b Thorbjørn J. Morsing,a Theis Brock-Nannestad,a Jesper Bendix,a

Michael M. Haley,c Peter Rapta,d Andrey Danilov,b Sergey Kubatkin,b

Ole Hammerich*a and Mogens Brøndsted Nielsen*a

Engineering of mixed-valence (MV) radical cations and intermolecular complexes based on p-extended

tetrathiafulvalenes (TTFs) is central for the development of organic conductors. On another front, redox-

controlled dimerization of radical cations has recently been recognized as an important tool in

supramolecular chemistry. Here we show that p-extended TTFs based on the indenofluorene core,

prepared by Horner–Wadsworth–Emmons reactions, undergo reversible and stepwise one-electron

oxidations and that the detectable, intermediate radical cation forms remarkably strong intermolecular

MV ([neutral$cation]) and p-dimer ([cation$cation]) complexes with near-infrared radical cation

absorptions. The radical cation itself seems to be a so-called Class III MV species in the Robin–Day

classification. The formation of MV dimers was corroborated by ESR spectroelectrochemical studies,

revealing two slightly different ESR signals upon oxidation, one assigned to the MV dimer and the other

to the cation monomer. Crystals of the radical cation with different anions (PF6�, BF4

�, and TaF6�) were

grown by electrocrystallization. Conductance studies revealed that the salts behave as semiconductors

with the hexafluorotantalate salt exhibiting the highest conductance. Using a custom-built ESR

spectrometer with sub-femtomole sensitivity, the magnetic properties of one crystal were investigated.

While the spin-to-spin interaction between radical cations was negligible, a high cooperativity coupling

to the microwave field was observed – as a result of an exceptionally narrow spin line width and high

spin density. This could have great potential for applications in quantum computation where crystalline

spin ensembles are exploited for their long coherence times.

Introduction

Intermolecular mixed-valence (MV) salts between neutral andcationic tetrathiafulvalene (TTF) species containing anunpaired electron have attracted immense interest in materialsscience owing to their conducting properties.1 The degree of

enhagen, Universitetsparken 5, DK-2100,

[email protected]; [email protected]

ience, Chalmers University of Technology,

y and the Materials Science Institute,

-1253, USA

al Physics, Faculty of Chemical and Food

, Radlinskeho 9, 81237 Bratislava, Slovak

SI) available: Experimental procedures,SR, electrochemical, and X-raynd NMR spectra. CCDC 962757 andraphic data in CIF or other electronic

428–10438

intra- and intermolecular electronic coupling between redoxcenters is central to the properties of such materials. A morerecent exploitation of radical species relates to the eld ofquantum computing, based on the ability to make a strongcoupling between the spins of a material to an external micro-wave circuit.2 Control of spin interactions is thus crucial to thiseld.

TTF is a redox-active molecule that undergoes two sequentialone-electron oxidations (Fig. 1), in the second step generatingtwo aromatic 1,3-dithiolium rings, which are rotated out of co-planarity3 and characterized by negative nucleus independentchemical shi values.4 The radical cation generated in the rststep is a planar and fully delocalized p-system, which can beassigned as a Class III species in the Robin–Day classication,5

and which exhibits characteristic UV-Vis absorptions accordingto both solution and gas phase studies.6 When separating thetwo dithiole rings of TTF by a p-conjugated spacer,7 newchemical, electrochemical, and optical properties emerge.Thus, the bridging unit may render the two dithiole ringsindependent redox centers with two overlapping (or close to)

This journal is © The Royal Society of Chemistry 2014

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Fig. 1 Tetrathiafulvalene (TTF) and its extended derivatives.

Fig. 2 Indeno[1,2-b]fluorene-extended TTF (0/1+/2+).

Paper Journal of Materials Chemistry C

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oxidation potentials (Robin–Day Class I species). An apparenttwo-electron oxidation may also result from a potential inver-sion8 as observed for anthraquinone-extended TTFs (1, Fig. 1).9

Upon oxidation, the conformation changes from a “buttery-like” shape to a planar anthracene core with perpendicularlysituated dithiolium rings. The indenoindene-extended TTF 2also exhibits a two-electron oxidation,10 although now an anti-aromatic pentalene is formally generated in the central spacer.

In between strongly (Class III) and negligibly (Class I) elec-tronically coupled redox centers in odd-electron compounds, athird intermediate class (II) is dened (weak coupling).5 Class IIspecies usually exhibit a broad near-infrared (NIR) charge-transfer (CT) absorption, while a narrow NIR absorption (no CTcharacter) is sometimes seen for Class III species. Severalspectroscopic studies have been performed on intra- andintermolecular MV systems involving at least two TTF units(TTF/TTFc+),5c,6b,11 while fewer have been performed to elucidateintra- and intermolecular couplings in cationic p-extendedTTFs. However, an important modication of anthraquinone-extended TTFs by Bryce and co-workers deserves mention.12 Bytethering the two dithiafulvene (DTF) rings of 1 by alkyl bridges(RR' ¼ (CH2)5), the anthracenediylidene ring is restricted fromconformational changes, allowing a one-electron oxidation tothe cation and measurement of a broad NIR absorption thereof.In another interesting piece of work by Frere et al.,7c,13 extendedTTF cations were structurally tuned to give dimer complexeswith characteristic NIR absorptions. To obtain sequentialoxidations and hence an intermediate MV radical cation, wereasoned that by separating the two indene units of 2 by acentral benzene ring (3, Fig. 2), direct two-electron oxidationwould not only lead to an unfavorable anti-aromatic indacenecore, but also it would occur at the expense of the benzene ring.


As shown by Salle et al.,14 simply attaching two DTF units onto acentral benzene ring (para positions) provides an extended TTFexhibiting irreversible oxidations. DTFs with a proton on thefulvene carbon oen undergo oxidative radical dimerization toform a vinyl TTF species,15 unless a bridging unit, e.g., aheterocycle, can stabilize the radical cation species.7c,13 Theindenouorene unit that captured our interest is comprised ofuorene units, and uorenes incorporating a DTF unit at thecentral ve-membered ring were previously shown to exhibit areversible rst oxidation.16 This observation renders the largerindeno[1,2-b]uorene scaffold, resembling pentacene but withtwo fewer carbon atoms, particularly attractive for separatingtwo DTF units, aiming at reversible and sequential DTF oxida-tions. Moreover, the high planarity of indenouorenes shouldmake them ideally suited for governing assembly of neutral andcationic indenouorene-extended TTFs by p–p interactions inthe quest for not only conducting materials but also self-assembled supramolecular structures. For example, TTF cationdimerization was recognized by Stoddart and co-workers11h,q asan important motif in redox-switchable catenanes and rotax-anes. It should also be noted that indenouorenes are them-selves interesting polycyclic compounds, with tunable opticaland redox properties, and suitable derivatives have attractedinterest as components for organic eld-effect transistors andphotovoltaic devices.17 In addition, related concave-shapedtruxene compounds functionalized with three DTFs have beenshown to function as supramolecular partners for C60.18 Here wepresent the synthesis of indenouorene-extended TTFs, theirredox properties, the spectroscopic properties of the neutraland cationic species, and nally conductance and ESR studiesof radical cation salts obtained by electrocrystallization.

Results and discussionSynthesis and X-ray crystal structure characterization

First, dione 4 (ref. 19) and 1,3-thiol-2-thione 5 (ref. 20) weresubjected to a phosphite-mediated coupling (Scheme 1).However, these conditions only allowed for incorporation of oneDTF and the isolation of product 6. Instead, on subjecting 4 to aHorner–Wadsworth–Emmons reaction upon treatment withphosphonate ester 7,21 deprotonated by sodium hexamethyldi-silazide (NaHMDS), the desired product 8 was obtained (withSBu substituents). X-ray crystal structure determination22

revealed that the two DTFs and the indenouorene core are co-planar (Fig. 3, le). The molecules stack in layers with an inter-layer distance of 3.476(6) A. In a similar manner, 4 and 9 wereconverted to the product 10 with SEt substituents. Crystals were

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Scheme 1 Synthesis of indeno[1,2-b]fluorene-extended TTFs.

Fig. 3 Left: ORTEP plot showing the molecular structure of 8 andcrystal packing (H-atoms omitted for clarity); crystals grown fromCH2Cl2/MeOH. Right: ORTEP plot showing the molecular structure of10 and crystal packing (H-atoms omitted for clarity); crystals grownfrom CS2/heptanes.

Fig. 4 UV-Vis absorption spectra of 4, 6, and 8 in CH2Cl2. The intenseabsorptions of 4 at lmax 278 and 289 nm have been cut-off for clarity(see ESI† for full spectrum).

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grown from CS2/heptanes and as it is evident from Fig. 3(right),22 these molecules pack quite differently from those of 8.Where in 8, there is very little p–p contact between the layersbecause of intervening butyl side chains, in 10 there issubstantial p–p stacking at a distance of 3.4718(7) A, eventhough the neighboring molecules are shied a bit. Also, thereare two orientations of stacks or molecular planes, at an angle of37.20(4)�, in the unit cell of 10, as opposed to only one orien-tation for 8. Compound 8 was more soluble than 10, whichmade it better suitable for UV-Vis-NIR/ESR spectroelec-trochemical studies, while 10 was subjected to electro-crystallization, providing crystalline materials for conductivityand ESR studies.

UV-Vis absorption spectroscopy and HOMO–LUMOcalculations

The optical properties of 4, 6, 8, and 10were examined by UV-Visabsorption spectroscopy. Spectra recorded in CH2Cl2 are shownin Fig. 4 and data are collected in Table 1. The properties of 4have been previously studied,17a but are included here for

10430 | J. Mater. Chem. C, 2014, 2, 10428–10438

comparison. The major features of 4 are the intense bands at278 and 289 nm and a weak band at 483 nm. Upon substitutionof one or two C]O by DTF groups, strong absorptions emergeat 483 nm (6) and 475 nm (8). Compound 6 has the most red-shied longest-wavelength absorption, which continues untilabout 560 nm. This same trend was previously seen in benzo-fused thiopyrane molecules where either one or two carbonylswere replaced by DTFs.23 We assign the longest-wavelengthabsorption bands of 6 and 8 to intramolecular CT absorptions,supported by solvatochromic behavior (ESI, Fig. S15†). For 8,the assignment is also supported by the HOMO and LUMO plots(Fig. 5). The HOMO is mainly located at the DTFs and thecentral part of indenouorene, while the LUMO is mainly alongthe entire indenouorene and only partly on the DTFs. Thespectra of 8 and 10 are very similar (ESI, Fig. S11†).

Electrochemistry

The redox properties of the compounds were investigated bycyclic voltammetry, and the results are summarized in Table 2and Fig. 6 (see also ESI†). In accordance with previous stud-ies,17a 4 shows two one-electron (1e) reductions located at�1.33and �1.69 V vs. Fc/Fc+. In addition, this compound exhibits anirreversible oxidation near the solvent front at Epa ¼ +1.46 V.Compound 6 only has one reversible one-electron (1e) reductionat a higher potential (�1.67 V) compared to 4 and it is followedby an irreversible reduction at Epc ¼ �2.17 V. The rst 1eoxidation associated with DTF occurs at +0.53 V, being not quitefully reversible (ia/ic ¼ 0.9; see ESI, Fig. S4†). A second irre-versible oxidation is observed at Epa ¼ +0.93 V, causing coatingof the electrode. Replacement of both C]O's with DTFs (8)gives two 1e oxidations (+0.24 and +0.38 V). A third irreversibleoxidation at +1.09 V and two irreversible reductions at �2.31and �2.47 V are also observed. The rst cyclic voltammetryoxidation wave for a 1 mM solution of 8 (Fig. 7, black line) wasvery broad, reecting that the rst electron transfer might bequasi-reversible. This, however, is not the case. The calculatedinner reorganization energies, li, for both the rst, 28.6 kJmol�1, and the second electron transfer, 34.3 kJ mol�1, are



Table 1 UV-Vis absorption data of compounds 4, 6, 8, and 10 in CH2Cl2 (sh ¼ shoulder)

Compd lmax [nm] (3 [104 M�1cm�1])

4 255 (3.1), 264 (5.4), 278 (14.4), 289 (28.8), 320 (sh, 1.6), 334 (1.3), 483 (0.08)6 252 (Sh 2.2), 260 (2.4), 283 (sh, 4.4), 293 (6.6), 325 (sh 1.6), 351 (1.2), 482 (2.3)8 269 (6.6), 294 (4.2), 304 (sh, 3.7), 331 (sh, 1.8), 344 (2.4), 380 (1.6), 423 (sh, 2.0), 449 (5.0), 475 (8.1)10 268 (6.2), 294 (4.2), 303 (sh, 3.8), 329 (sh, 1.8), 344 (2.4), 379 (1.6), 423 (sh, 2.0), 448 (5.0), 473 (8.0)

Fig. 5 Frontier orbitals (left: HOMO; right: LUMO) of 8 resulting fromB3LYP/cc-pVDZ calculations.

Table 2 Electrochemical data of compounds 4, 6, 8, and 10 in volts vs.Fc/Fc+a

Compd E1ox [V] E2ox [V] E3ox [V] E1red [V] E2red [V]

4 +1.46a �1.33 (1e) �1.69 (1e)6 (1 mM) +0.53p (1e) +0.93i �1.67 (1e) �2.17i

8 (1 mM) +0.24b (1e) +0.38 (1e) +1.09i �2.31i �2.47i

10 (0.2 mM) +0.31 (1e) +0.43 (1e) +1.13i �2.31i

a i ¼ irreversible (either Epc or Epa potential recorded for reduction oroxidation, respectively), p ¼ partially reversible (ia/ic ¼ 0.9), and b ¼broad oxidation with Ea � Ec ¼ ca. 130 mV.

Fig. 6 Cyclic voltammograms of 4 (<1 mM), 6 (ca. 1 mM), and 8 (ca. 1mM) in CH2Cl2 + 0.1 M Bu4NPF6. Glassy-carbon working electrode,silver reference and platinum counter electrodes, scan speed 100 mVs�1. The traces are offset from one another for clarity; the arrowsindicate the initial scan direction.

Fig. 7 Cyclic voltammograms of 8 (concentration normalized currentvs. E) in CH2Cl2 + 0.1 M Bu4NPF6 at five different concentrations: 1 mM(black), 0.5 mM (red), 0.25 mM (green), 0.125 mM (blue), and 0.0625mM (purple), using a glassy carbon working electrode at a scan rate of0.1 V s�1.

Fig. 8 Simulated (red and blue curves) and experimental (black curve,1 mM) cyclic voltammograms of 8 (potentials uncorrected; i.e., not vs.Fc/Fc+).


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similar to that reported for the rst electron transfer of parentTTF (6.8 kcal mol�1 ¼ 28.5 kJ mol�1).11f Instead, we assign thebroad wave to an association process, which is in line with the


fact that the appearance of the wave is strongly concentration(Fig. 7, colored lines) and temperature dependent (ESI,Fig. S6†). Thus, at lower concentrations the wave is much nar-rower – approaching that for a simple reversible electrontransfer reaction at a concentration of 0.0625 mM. In contrast,when the temperature is lowered, the wave broadens. Resultsfrom digital simulations, including the presence of threedifferent associates, [8$8], [8$8c+] and [8c+$8c+], supported thisinterpretation (Fig. 8, red curve). Dissociation of such dimers isslow on the CV time scale, presumably due to strong attractiveinteractions between the large molecules. The cyclic

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voltammogram of a saturated solution of 10 (only ca. 0.2 mMdue to its poor solubility) resembled that of 8 recorded atsimilar concentrations (0.125 mM/0.25 mM) with two sharpwaves for the rst two oxidations, slightly anodically shied.Association between neutral molecules of 8 was supported by1H-NMR spectroscopy as the signals of the central aromaticprotons shied by up to 0.08 ppm when diluting a saturatedsample (>2 mM) in CDCl3 to 0.005 mM; yet, the estimatedassociation constant of dimerization in CDCl3 (16 � 6 M�1) wassignicantly smaller than that from the CV simulation (6.0 �103 M�1 in CH2Cl2 + Bu4NPF6). We did not observe UV-Visspectral changes at different concentrations, but a potentiallow-energy absorption is also absent in the solid-state absorp-tion spectrum (ESI, Fig. S14†). Thus, the association constant K[8$8] for two neutral molecules of 8 is probably overestimated bythe CV tting procedure. For that reason a new t wasattempted with K[8$8] ¼ 16 M�1, the value obtained by NMRspectroscopy. The resulting t, although not of the same qualityas that observed for the free running tting procedure, was stillsatisfactory as seen in Fig. 8 (dotted blue curve). The corre-sponding association constants K[8$8c+] and K[8c+$8c+] were 1 �104 M�1 and 4 � 103 M�1, respectively, both being much largerthan those obtained by Rosokha and Kochi11f for the relatedspecies derived from TTF: [TTF$TTFc+] (6.0 M�1) and[TTFc+$TTFc+] (0.6 M�1).24

Fig. 9 Spectroelectrochemistry (UV-Vis-NIR) of 8 (0.5 mM) in CH2Cl2+ 0.1 M Bu4NPF6. Top: spectral evolution during oxidation; the insetshows low-energy absorptions at three different concentrations.Middle: deconvolution of low-energy absorption bands of 8/8c+ after10% conversion in the electrolysis (3 Gaussian functions needed).Bottom: after 90% conversion (2 Gaussian functions needed).

UV-Vis – spectroelectrochemistry

To shed light on the nature of the radical cation and theintermolecular complexes, we turned to spectroscopic studies.We initially tried to chemically oxidize compound 8 withnitrosyl tetrauoroborate and tris(4-bromophenyl)ammo-niumyl hexachloroantimonate (“magic blue”) but it producedunstable species which degraded into unknown products, evenunder inert conditions. Fortunately, the cationic species werestable under spectroelectrochemical conditions, which allowedus to study their spectroscopic properties at various concen-trations. Upon oxidation of a 0.5 mM solution of 8, a new broadband is initially observed at 1940 nm (Fig. 9). As the oxidationproceeds, a second broad band appears around 1435 nm, whichultimately becomes stronger than the rst when the oxidationto the monocation is complete (there are also three bandsbetween 600 and 700 nm which uctuate in intensity as theoxidation progresses). Upon further oxidation, these absorp-tions disappear and a new absorption for the dication appearswith lmax ¼ 905 nm.

From measurements at other concentrations (0.9 and 0.1mM), we nd that the intensity ratio between the 1940 and 1435nm absorptions is concentration dependent, with the latterbeing more intense at higher concentrations. Altogether, theobservations support the presence of intermolecular complexesof the radical cations. The low-energy absorptions can be ttedby three Gaussian functions at the beginning of electrolysis andby two functions at the end, providing maxima around 1450,1650, and 1996 nm. Resolution of the more complex absorptionbands resulting from the three primary transitions of theradical cation species was also possible. Based on the evolution

10432 | J. Mater. Chem. C, 2014, 2, 10428–10438

of the absorption spectrum as the electrolysis proceeds, weassigned the two bands at 1450 and 1650 nm to complexes[8c+$8c+] and [8$8c+] and the one at 1996 nm to the 8c+monomer.The absorption maximum of [8c+$8c+] corresponds to anexpected Davydov blueshi of a radical cation in a face-to-facestacked p-dimer.11f,m,25 The emergence and concomitant disap-pearance of the 1650 nm absorption as the electrolysis proceedssignal that this absorption originates from complexes withneutral 8 ([8$8c+]), which is present at the initial stage of theelectrolysis. The presence of isosbestic points allows the formalconcentration of 8, 8c+, and 82+ to be determined from theabsorption spectra. Yet, as the molecular units in the dimersshow a signicant degree of exciton coupling, the actualconcentrations [8c+], [8$8c+], and [8c+$8c+] cannot be reliablydetermined. Thus the absorption spectra only allow us to followthe speciation of 8c+, but not determination of association




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constants; here we refer instead to the values obtained by ttingof the CV (vide supra). Finally, we note that any redshiedintermolecular intervalence charge-transfer (IVCT) absorptionsare difficult to detect in this case as the intrinsic radical cationabsorptions are already at very low energies.

The full width at half maximum (FWHM) of the IV absorp-tion of 8c+ is 970 � 60 cm�1 (cf., deconvoluted spectrum) whichis signicantly smaller than the value 3400 cm�1 obtained fromthe expression (2310 � nmax)

1/2 derived for Class II complexes(nmax [cm�1] is the absorption maximum).5c This seems toindicate that the two DTFs are in fact strongly coupled corre-sponding to a Class III MV system (despite a small difference inthe rst and second oxidation potentials caused by the extendedp-system of 8).

Fig. 10 In situ ESR/UV-Vis-NIR spectroelectrochemistry for 8 (1 mM)in CH2Cl2 + 0.2 M Bu4NPF6 (scan rate 5 mV s�1). (a) In situ cyclicvoltammogram (I ¼ current) with selected points (numbered) wherethe corresponding (b) ESR (I ¼ intensity) and (c) optical spectra weretaken during the in situ voltammetric scan.

ESR/UV-Vis-NIR – spectroelectrochemistry

Further support for the presence of MV complexes came from insitu ESR/UV-Vis-NIR spectroelectrochemical studies, using a set-up described previously.26 In situ cyclic voltammograms ofsample 8 in CH2Cl2 (+0.2 M Bu4NPF6) in the spectroelec-trochemical ESR at cell at a low potential scan rate of 5 mV s�1

using a platinum mesh working electrode exhibit very similarcharacteristics to those observed in standard cyclic voltammetrystudies discussed above. Two well-dened reversible peaks wereobserved in the anodic region and the rst voltammetric peak ismore broadened by increasing the concentration of 8 in solu-tion (ESI, Fig. S34a and S35a†). For a 1 mM sample of 8 inCH2Cl2/Bu4NPF6 we observed at the foot of the rst anodic peakthe appearance of a new ESR singlet line at a g-value of 2.0062with a line width of DHpp¼ 0.235 mT and simultaneously, a newoptical transition in the visible region at 701 nm appears(Fig. 10). By increasing the potential still in the region of therst anodic peak, the ESR singlet line is shied to the slightlyhigher g-value of 2.0064 having a line width of DHpp ¼ 0.247 mT(Fig. 10b). Simultaneously, in the optical spectra a new band at623 nm starts to dominate (Fig. 10b). The shapes of both ESRspectra indicate delocalized spin and g-values that are typicalfor TTF cation radicals. At the second electron transfer, 8 showsa dominating absorption band at 905 nm with simultaneousdecrease of the ESR intensity (ESI, Fig. S34b and c†). Thepresence of the residual ESR signal also in the region of thesecond electron transfer is due to the symproportionationreaction in the bulk solution according to the reaction 8 + 82+ ¼8c+ + 8c+ and due to diffusion of the stable cations into the bulk.Applying an ESR/UV-Vis-NIR spectroelectrochemical cell withnearly thin layer conditions and using smaller concentrations of8, the decrease of ESR intensity at the second voltammetric peakis more visible (ESI, Fig. S35c†). This unambiguously conrmsformation of the diamagnetic dication 82+ having the domi-nating absorption band at 905 nm. Simultaneously, in the NIRregion the band at 1400 nm starts to dominate (ESI, Fig. S36†)as discussed above.

The ESR intensity of the signal with the higher g-value rea-ches its maximum slightly aer the rst peak potential and inthe back scan in the region slightly aer the rst reoxidationevent, where the highest concentration of cation radicals at the


electrode is expected (ESI, Fig. S37†). In the initial stages of theoxidation only a small amount of cation radicals is formed andlogically MV dimer species [8$8c+] dominate. Therefore weassign the ESR signal that rst emerges to [8$8c+] and thesecond to uncomplexed 8c+ which ultimately will dominate asthe concentration of neutral species decreases during oxidationat the electrode (see also the red line in Fig. S34†). This inter-pretation reasonably assumes that the [8c+$8c+] p-dimer isdiamagnetic and hence ESR silent, as previously observed for p-dimers of TTF radical cations6c,11f,p and for p-dimers formedfrom a thiophene-extended TTF radical cation, which exhibiteda particularly strong tendency to dimerize due to additional S–Sinteractions.13 The very small but clearly observable difference

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Fig. 11 Crystals grown by electrocrystallization of 10 in PhCl withBu4NPF6 (left) and Bu4NTaF6 (right) as the electrolyte.

Fig. 12 ORTEP plots of the cation salts of 10 showing 50% probabilityellipsoids (top), the crystal packing of the molecules (middle), and thestacking of neighboring molecules (bottom); (a) PF6

� salt, (b) BF4� salt,

and (c) TaF6� salt.


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in the g-values (difference ¼ 0.0002, ESI, Fig. S38†) is actuallyconnected with the fact that this ESR signal comes from thesame radical in either complexed or uncomplexed form. Inter-estingly, also radicals, which diffused into the bulk (see the blueline in Fig. S34†) or formed in the bulk by symproportionation(see the green line in Fig. S34†), exhibit an ESR signal with thelower g-value, again conrming that the ESR signal with a g-value of 2.0062 can be clearly attributed to [8$8c+]. Thisphenomenon is more pronounced at higher concentrations asexpected and illustrates the powerful use of in situ ESR/UV-Vis-NIR spectroelectrochemistry for simultaneous monitoring ofboth paramagnetic and diamagnetic species at the electrodeand near the electrode surface.

Calculations

To elucidate the electronic structures of 8, 8c+, and 82+, weperformed DFT computations. Geometry optimization yielded astructure of 8c+ with the unpaired electron fully delocalized overboth halves of the molecule. The coupling in 82+ was quantiedby a broken-symmetry calculation, yielding a strong antiferro-magnetic coupling (J ¼ �688 cm�1 in HHDvV ¼ �2JS1S2convention) and concomitantly little diradical character assuggested by the quinoidal structure in Fig. 2 (and supported bythe calculated bond lengths, see ESI†). Absence of diradicalcharacter of the dication is in accordance with its ESR silence inthe ESR experiments described above.

Electrocrystallization

Attempts to prepare salts of 8 by electrocrystallization in anH-shaped cell (i ¼ 1–4 mA) in CH2Cl2/Bu4NPF6 (0.1 M) orPhCl/Bu4NPF6 (sat.) were unsuccessful; only solutions ofpresumed 8c+/82+ resulted. In contrast, a microcrystallinedeposit was observed at the anode when 10 was subjected toelectrochemical oxidation under the same conditions. A batchgrown from CH2Cl2 satised according to elemental analysis(EA) the composition (10$PF6)2$Bu4NPF6, suggesting the pres-ence of some electrolyte in the solid. We therefore devised amore rigorous washing technique to ensure removal of allelectrolyte on the crystal surfaces. In addition, we changed thesolvent to PhCl and could reproducibly obtain batches of crys-tals (ca. 3–5 mg from ca. 6 mg of 10). EAs on two separatebatches were in agreement with the composition 10$PF6.Changing the electrolyte to Bu4NBF4 gave crystals, whichaccording to EA seem to have the stoichiometry 10$(BF4)1.5(supported by X-ray crystallography, vide infra). With Bu4NTaF6as the electrolyte, we obtained 10$TaF6 according to X-raycrystallography. Fig. 11 shows the PF6

� and TaF6� crystals

grown at the platinum electrode. The long needles obtained of10$TaF6 indicate that this salt could be the most conducting inthe series, which was indeed found to be the case (vide infra).

Cation salts – X-ray crystallography

The crystals obtained from the electrocrystallization experi-ments of 10 in PhCl with PF6

� as the counter ion, thoughlooking like single crystals, are actually ensembles of minuteneedle-shaped single crystals, making X-ray structure analysis

10434 | J. Mater. Chem. C, 2014, 2, 10428–10438

difficult and only a partial structure of this compound could beobtained (Fig. 12a).22 The PF6

� ions occupy very large voidsbetween the cation stacks and consequently cannot bemodeled, not even roughly. This unmodelable electron densitywas removed using the solvent masking procedure employed byOlex2, which is similar to the PLATON Squeeze routine. Theextended TTF moieties p–p stack with an intermoleculardistance of 3.358(16) A and unlike neutral 10, the moleculesstack with the center of neighboring molecules directly aboveeach other. Because of the large cavities for accommodating thecounter ions, these stacks are quite isolated from each other.While the cations in the PF6

� salt are seen to stack directly ontop of neighboring molecules, this is not the case for the BF4

�

and TaF6� salts.22 In both of these, there are two orientations of

the molecules, though the molecular planes are parallel(Fig. 12b and c-top). The crystals of the BF4

� salt were very smalland, like the PF6

� salt, they were very akey, but because of thelimited disorder of the system, the resulting structure is muchbetter, conrming the stoichiometry 10$(BF4)1.5. The asym-metric unit contains two half molecules of 10 and 1.5 BF4

�

moieties. As the BF4� with half occupancy does not lie on a

special crystallographic site, there are two possible positions ofthis counter ion 3.2 A apart. Unlike the PF6

� salt, the dithiolerings are slightly rotated out of the molecular plane (14.103(2)�

and 13.063(2)�, respectively, for the two crystallographicallydifferent molecules). This supports the presence of moredithiolium rings in this salt as these can rotate freely. For



Table 3 Selected average bond lengths in the DTF unit of the differentcrystallographically characterized species of 10

Salt S1–C2 (C2–S3) bond [A] Exocyclic fulvene C–C bond [A]

10 1.753(3) 1.363(3)10$PF6 1.729(3) 1.388(11)10$(BF4)1.5 1.714(16) 1.390(10)10$TaF6 1.731(11) 1.388(21)

Fig. 13 Solid-state absorption spectra of cation salts of 10 incomparison to the solution-state spectrum of cationic 8.

Fig. 14 a) Microwave resonator design concept (see text). (b) Opticalmicroscope image of an area indicated in panel (a) with a frame. The10$PF6 crystal (black) coupled to the two parallel superconductinglines (grey). The crystal was introduced into the resonator by amechanical micromanipulator. A static magnetic field was applied inplane along the resonator symmetry line.


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comparison, TTF2+ has a twisted gauche-like conformation witha mean torsional angle of 60� associated with the two dithio-lium rings.3 The stacking distance between the molecules of10+/2+ is 3.6902(4) A.

In the TaF6-salt, the stoichiometry is 1 : 1 and so it consistsentirely of radical cations. Like the BF4

� salt, the dithiole ringsare slightly out of plane, by 4.9(2)� and 9.3(2)�, respectively, forthe two crystallographically different molecules, comparable tothe 3.2(2)� of the PF6

� salt. The stacking distance betweenneighboring molecules is 3.325(12) A and where the PF6

� andBF4

� salts stack in a similar way (the BF4� salt being slightly off

the center), the stacking of the TaF6� salt is quite different as

seen in the bottom panel of Fig. 12. As seen from Table 3, thereis a change in bond distances in the DTF units upon oxidation,the S1–C2 bonds (dithiole numbering, S1–C2–S3) becomingshorter (especially for the more oxidized BF4

� salt) and exocyclicfulvene C–C bonds becoming longer.

Cation salts – solid-state absorptions

To shed further light on the composition of crystals grown byelectrocrystallization of 10 from PhCl, we measured the solid-state absorptions of selected batches (Fig. 13). First, character-istic cation absorptions centered around 450 and 660 nm areobserved in the solid-state spectra of the salts, located at aboutthe same positions as in the solution spectrum of cationic 8(451/474 nm and 626/686 nm), with small changes in the nestructure and energy induced by the crystal packing. Moreover,the characteristic and very strong absorption of the neutralspecies 8 at ca. 475 nm (cf., Fig. 9) does not seem to overlap thecation absorptions in this region, which indicates that the saltsonly contain cationic species. The low-energy cation absorp-tions were very dependent on the counter anion and hence thecrystal packing (in contrast to TTFc+ dyads11f). The PF6

� saltshowed a broad absorption around 1050 nm (Fig. 13), which isblue-shied relative to those present in the solution spectrumof cationic 8. The position of the broad low-energy absorption ofthe TaF6

� salt was closer to that of the radical cation in the p-dimer in solution.

Cation$PF6� salt – ESR Measurements

We characterized the magnetic properties of 10$PF6 crystalsusing a custom-built ESR spectrometer with sub-femtomolesensitivity, described in detail elsewhere.27 The spectrometer isbased on a planar superconducting resonator, which is anelectromagnetic analog of a mechanical tuning fork (Fig. 14a).The area between oscillating prongs is lled with a high-density


inter-digital capacitor. The spin sample is placed between twoparallel narrow lines of the superconductor where the magneticcomponent of the microwave eld is concentrated. The ESRspectrum recorded on a micrometer-scale crystal (�6 � 6 � 70m3 in size) of 10$PF6 reveals a single peak corresponding tog ¼ 2.0 (Fig. 15). The observed ESR resonance is exceptionallysharp, the extracted inhomogeneous spin line width being�0.086 mT (2.4 MHz). This suggests that the spin-to-spininteraction in this compound is negligible and spins areessentially independent. In consistence with this observation,ESR spectra recorded at temperatures down to 0.3 K present notraces of formation of cooperative spin states.

We measured a total of ve different crystals of 10$PF6 withdifferent orientations of the c-axis with respect to the appliedstatic magnetic eld. The sample oriented along the magneticeld axis yielded the most narrow spin linewidth of 0.032 mT(0.9 MHz) at 0.3 K. Another crystal was rotated �30 degrees andthis yielded a much wider spin resonance 0.264 mT (7.4 MHz)similar to the measurements in solution. Three other sampleswith intermediate rotation angles showed line widths of 0.150,0.159 and 0.176 mT (4.2, 4.5 and 4.9 MHz), respectively, at 0.3 K.The increase in spin polarization between 1.6 and 0.3 K is inagreement with the expected tanh(hf/2kBT)-dependence at lowtemperatures, further supporting the picture of non-interactingspins. We attribute the increased line width to the anisotropy inthe g-factor (calculated using DFT to be �0.5%) and single-crystal irregularities.

J. Mater. Chem. C, 2014, 2, 10428–10438 | 10435


Fig. 15 Spin induced loss rate of a microwave cavity loaded with amicrometer-scale crystalline sample; a single dissipation peak corre-sponding to g ¼ 2.0. The measured FWHM translates to DHpp ¼ 0.086mT. Data were taken at 1.6 K.

Fig. 16 (Left) Measured transmission (in color code) of the coupledspin–resonator system vs. magnetic field and microwave frequency,measured at 0.3 K. (Right) Theoretical fit.


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Fig. 16 (le) presents the transmission spectroscopy28 data ofa coupled spin–resonator systemmeasured at T¼ 0.3 K. Aroundthe magnetic eld corresponding to g ¼ 2.0 one can clearly seetwo distinct eigen modes for spins oscillating in-phase and out-of-phase with the microwave eld. This is a characteristicsignature of a strong cooperativity between the spin ensembleand the resonator, when the energy exchange rate between theresonator and spin system becomes comparable to or greaterthan the dissipation to the environment. The t to experimentaldata (Fig. 16, right) returns eigen mode splitting D � 1.5 MHzand the spin resonance width 0.9 MHz < D. A high cooperativitycoupling is a crucial pre-requisite for implementing quantumcomputing schemes.2 So far this was only demonstrated formacroscopic samples of spins diluted in a crystalline matrix,29

indicating that there is potential for implementing indeno-uorene-extended TTF cations into quantum circuits, especiallywith the good crystalline properties obtained with the PF6

� saltsthat allow for dense spin systems without spin–spin interac-tions, important for many high density circuit applications.

Table 4 Electrical resistivities (r) and conductivities (s)

Sample

298 K 77 K

r [U m] s [S m�1] r [U m] s [S m�1]

10$PF6 0.326 3.07 2809 0.0003510$(BF4)1.5 7.72 0.13 556 000 0.000001810$TaF6 0.188 5.32 14173 0.000071

10436 | J. Mater. Chem. C, 2014, 2, 10428–10438

Cation salts – conductance measurements

Electrical conductivities were measured on compressedpowdered samples of the cation salts at ambient temperatureand at 77 K using a dual electrode conguration and results arelisted in Table 4. Firstly, the samples behaved as semi-conductors. Secondly, by comparing the salts 10$PF6 and10$TaF6, we see a dependency on the anion – the room-temperature conductance of the hexauorotantalate salt ishigher by a factor of ca. 1.7. Finally, it is noted that the salt10$(BF4)1.5 exhibited the lowest conductance, but it is notdirectly comparable to the other two salts as both the cati-on$anion stoichiometry and nature of the anion are differenthere.

Conclusions

The indenouorene core is a particularly useful spacer forseparating dithiafulvene rings when aiming at extended TTFradical cations with interesting optical and material propertiesand with a strong ability to associate. Thus, indenouorene-extended TTFs undergo sequential and reversible one-electronoxidations. This behavior allowed detailed spectroscopic char-acterization of the radical cation, which revealed it to exhibit arather narrow NIR IV absorption (based on curve deconvolu-tion), placing it as a Class III MV species in the Robin–Dayclassication. The cation has a remarkable tendency to formdimer complexes, which was uncovered by a very broad rstoxidation wave in the cyclic voltammogram at high concentra-tions or at low temperatures and by blueshied radical cationabsorptions. The formation of MV dimer species upon oxida-tion was supported by ESR studies in solution, which showedthe presence of two paramagnetic species with slightly differentg-values, assigned to radical cation monomers and MV dimers.The strong MV and p-dimer associations render indeno-uorene-extended TTFs particularly interesting as redox-controlled building blocks for supramolecular chemistry.

Electrocrystallization of an indenouorene-extended TTFwith peripheral SEt groups generated radical cation salts. Themagnetic properties of the PF6

� salt were studied using acustom built state-of-the-art ESR spectrometer withsub-femtomole sensitivity. Spin-to-spin interactions in thecrystal were found to be negligible and the spins were thusessentially independent (down to 0.3 K). Instead, a strongcoupling between the spins of the material and the externalsource was observed, which is a prerequisite for the futureexploitation of such materials for quantum computing. Thesalts behaved as semiconductors with conductivities dependingon the counter anion; yet, more detailed conductancemeasurements will be the subject of future work, and we alsohope to achieve MV crystals of the neutral and cationic species.

Acknowledgements

The Danish Council for Independent Research | NaturalSciences (#10-082088), the European Union 7th FrameworkProgramme (FP7/2007–2013) under the grant agreement no.




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270369 (“ELFOS”), the Carlsberg Foundation, the US NationalScience Foundation (CHE-1301485), and the University ofCopenhagen are acknowledged for support. We thank Prof.K. Bechgaard (University of Copenhagen) for helpful discussions.

Notes and references

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Mixed valence radical cations and intermolecular complexes derived from indenofluorene-extended tetrathiafulvalenes