Conjugated, Carbon-Centered Radicals

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Review Synthesis, Physical Properties, and Reactivity of Stable, π-Conjugated, Carbon-Centered Radicals

Takashi Kubo Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan; [email protected]

Received: 26 January 2019; Accepted: 11 February 2019; Published: 13 February 2019

Abstract: Recently, long-lived, organic radical species have attracted much attention from chemists and material scientists because of their unique electronic properties derived from their magnetic spin and singly occupied molecular orbitals. Most stable and persistent organic radicals are heteroatom-centered radicals, whereas carbon-centered radicals are generally very reactive and therefore have had limited applications. Because the physical properties of carbon-centered radicals depend predominantly on the topology of the π-electron array, the development of new carbon-centered radicals is key to new basic molecular skeletons that promise novel and diverse applications of spin materials. This account summarizes our recent studies on the development of novel carbon-centered radicals, including phenalenyl, ﬂuorenyl, and triarylmethyl radicals.

Keywords: π-conjugated radicals; hydrocarbon radicals; persistent; anthryl; phenalenyl; ﬂuorenyl

1. Introduction Organic radical species are generally recognized as highly reactive, intermediate species. However, recently, functional materials taking advantage of the feature of open-shell electronic structure have attracted much attention from chemists and material scientists; therefore, the development of novel, long-lived, organic, radical species becomes more important [1–7]. Nitronyl nitroxides, galvinoxyl, and DPPH are well known as stable, organic, radical species, which are commercially available chemicals. These stable radical species are “heteroatom-centered radicals”, in which unpaired electrons are mainly distributed on heteroatoms. On the other hand, “carbon-centered radicals”, in which unpaired electrons exist only on carbon atoms, are highly reactive and difficult to handle and therefore have had limited applications. However, because the physical properties of carbon-centered radicals depend predominantly on the topology of the π-electron array, the development of new carbon-centered radicals is key to new basic molecular skeletons that promise novel and diverse applications of spin materials. By investigating the physical properties and reactivity of newly developed radical species, it becomes possible to find new applications, leading to the creation of new functional materials. There are two definitions of stability of radicals: thermodynamic stability and kinetic stability. Thermodynamic stability is related to the degree of generation of radical species; for instance, it determines how many monomers are formed when the radical dimer is cleaved into a monomer. On the other hand, kinetic stability is related to the lifetime of radical species. The thermodynamic stability of radical species can be evaluated by the indices such as bond dissociation energy (BDE) and radical stabilization energy (RSE) [8]. The BDE is the energy required for homolytic cleavage of R–H into R• and H•, whereas the RSE is expressed by the difference between the BDE of a radical species of interest and the BDE of the methyl radical. Either BDEs or RSEs may be used for evaluating the thermodynamic stability of radical species. However, RSE might be easier to evaluate the stability of radicals because RSEs indicate the relative stability to the methyl radical. Table1 shows RSE values of selected carbon-centered radicals calculated at the second-order, restricted, open-shell, Møller–Plesset

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(ROMP2) level of theory. Generally, the larger the delocalization of unpaired electrons, the more thermodynamically stabilized the radical species, and the RSE shows a larger negative value.

Table 1. Radical stabilization energies (RSEs) of selected carbon-centered radicals.

Radical RSE/kJ mol−1 • Methyl ( CH3) 0 (as standard) tert-Butyl −28.3 Allyl −77.5 Benzyl −50.4 Cyclopentadienyl −98.3 Cycloheptatrienyl −134.1 Triphenylmethyl (1) −103.4 Phenalenyl (2) −201.6 9-Fluorenyl (7) −90.7

Research on stable organic radicals started with the triphenylmethyl radical (1), which was discovered by Gomberg in 1900 [9]. Although 1 reacts promptly with molecular oxygen, the radical exists for a long period of time under degassed conditions or in an inert gas atmosphere. In solution, 1 is in equilibrium with a dimer and dissociates mostly into a monomeric radical in dilute solution (about 10−5 M), indicating high thermodynamic stability of 1. The RSE value of 1 is calculated to be −103.4 kJ mol−1.

2. Phenalenyl Radical

The phenalenyl radical (2) is a spin-delocalized hydrocarbon radical with D3h symmetry. Due to its relatively long lifespan, 2 has attracted attention from organic chemists in terms of its chemical reactivity of radical species [10]. On the other hand, in 1973, Haddon proposed that a one-dimensional (1D) stack of organic radicals should show metallic behavior and also showed that 2 is a good model compound for his theoretical prediction [11]. After that, many researchers became interested in the physical properties of 1 [12–16]. However, organic radicals generally dimerize to form closed-shell compounds. Therefore, a considerable effort is required for the molecular design to obtain a one-dimensional chain of radicals. This section describes our recent studies on self-association behaviors of phenalenyl radicals, aiming at constructing a one-dimensional chain as a ﬁnal goal.

2.1. Electronic Structure of Phenalenyl Radical The phenalenyl radical (2) is an odd-alternant hydrocarbon radical, and owing to its highly symmetric structure, an unpaired electron is delocalized on six equivalent carbon atoms (1, 3, 4, 6, 7, and 9-positions: the α-position), as shown in Figure1. This spin delocalization leads to the high thermodynamic stability of 2, and indeed, the RSE value of 2 is −201.6 kJ mol−1, which is almost twice that of 1. The Hückel molecular orbital (HMO) calculation shows a singly occupied molecular orbital (SOMO) with an energy of α + 0β. The SOMO of 2 is distributed only on the α-position, and the MO coefﬁcients of the other atoms are zero, consistent with the resonance form shown in Figure1. The feature of the SOMO of 2 is a hexagonal arrangement, which allows perfect orbital overlap in both eclipsed and staggered stacking motifs. The staggered stacking is more favorable to short π–π contact (face-to-face contact) than the eclipsed stacking due to smaller atom–atom repulsion. Molecules 2019, 24, x FOR PEER REVIEW 3 of 13 Molecules 2019, 24, 665 3 of 13 Molecules 2019, 24, x FOR PEER REVIEW 3 of 13

Figure 1. Resonance structure and singly occupied molecular orbital (SOMO) of phenalenyl radical (2). Figure 1. ResonanceResonance structure structure and and singly singly occupied occupied molecular molecular orbital orbital (SOMO) (SOMO) of ofphenalenyl phenalenyl radical radical (2). ( 2). 2.2. Dimerizaion Behavior of Phenalenyl Radicals 2.2. Dimerizaion Behavior of Phenalenyl Radicals 2.2. DimerizaionThe parent Behaviorphenalenyl of Phenalenyl radical 2 is Radicals a long-lived species in the absence of air and is in equilibrium The parent phenalenyl radical 2 is a long-lived species in the absence of air and is in equilibrium with Thea σ-bonded parent phenalenyl dimer (σ-dimer) radical in 2 solutionis a long-lived state (Figure species 2) in [17]. the absenceThe thermodynamically of air and is in equilibrium stabilized with a σ-bonded dimer (σ-dimer) in solution state (Figure2)[ 17]. The thermodynamically stabilized naturewith a σof-bonded 2 suggests dimer that ( σthe-dimer) structural in solution modification, state (Figure such as2) [17].the introduction The thermodynamically of substituent stabilized groups, nature of 2 suggests that the structural modiﬁcation, such as the introduction of substituent groups, maynature lead of 2to suggests another thatassociation the structural mode othermodification, than a σ -dimersuch as form.the introduction In fact, the ofintroduction substituent of groups, bulky may lead to another association mode other than a σ-dimer form. In fact, the introduction of bulky tertmay-butyl lead togroups another at 2,5,8-positionsassociation mode afforded other thana face-to-face a σ-dimer πform.-dimer In infact, the the solid introduction [18] and ofsolution bulky tert-butyl groups at 2,5,8-positions afforded a face-to-face π-dimer in the solid [18] and solution statestert-butyl [19] groups(Figure at2). 2,5,8-positionsTwo phenalenyl afforded planes area face-to-face superimposed π-dimer at the in separation the solid distance[18] and of solution 3.25 Å states [19] (Figure2). Two phenalenyl planes are superimposed at the separation distance of 3.25 Å instates the [19]staggered (Figure stacking 2). Two manner.phenalenyl The planes π-dimer are ofsu perimposedthe tert-butyl at derivative the separation (3) shows distance a deep of 3.25 blue Å in the staggered stacking manner. The π-dimer of the tert-butyl derivative (3) shows a deep blue colorin the and staggered an intense stacking absorption manner. band The is πobserved-dimer of at the612 tertnm-butyl in the derivative solid state. ( 3A) showsquantum a deep chemical blue color and an intense absorption band is observed at 612 nm in the solid state. A quantum chemical calculationcolor and an (ZINDO/S) intense absorption for the π -dimerband is predicted observed aat fully 612 nmallowed in the absorption solid state. band A quantum around chemical600 nm, calculation (ZINDO/S) for the π-dimer predicted a fully allowed absorption band around 600 nm, with withcalculation the transition (ZINDO/S) moment for the along π-dimer the direction predicted co nnectinga fully allowed the centers absorption of the phenalenylband around rings. 600 Thenm, the transition moment along the direction connecting the centers of the phenalenyl rings. The intense intensewith the band transition can be moment assigned along to an the electronic direction transition connecting from the HOMOcenters ofto theLUMO, phenalenyl which arerings. newly The band can be assigned to an electronic transition from HOMO to LUMO, which are newly formed by formedintense byband the can orbital be assigned interaction to anof aelectronic pair of SOMO transition in the from π-dimer. HOMO The to short LUMO, π–π which contact are and newly the the orbital interaction of a pair of SOMO in the π-dimer. The short π–π contact and the orbital splitting orbitalformed splittingby the orbital indicate interaction the adequate of a pair covalent of SOMO bonding in the πinteraction-dimer. The of short two πunpaired–π contact electrons and the indicate the adequate covalent bonding interaction of two unpaired electrons [20,21]. [20,21].orbital splitting indicate the adequate covalent bonding interaction of two unpaired electrons [20,21].

Figure 2. Dimerization mode of phenalenyl radicals. Figure 2. Dimerization mode of phenalenyl radicals. Because a π-dimer-dimer isis thethe smallestsmallest unitunit ofof aa 1D1D stack,stack, wewe expectedexpected thatthat aa chemicalchemical modificationmodification such as the introduction of substituent groups could lead to an equidistantly stacked 1D chain [22]. such Becauseas the introduction a π-dimer isof the substi smallesttuent unitgroups of acould 1D stack, lead towe an expected equidistantly that a stackedchemical 1D modification chain [22]. As a first trial, we decided to introduce a pentafluorophenyl (C F ) group at 2,5,8-positions [23]. Highly Assuch a asfirst the trial, introduction we decided of substi to introducetuent groups a pentafluorophenyl could lead to 6an5 (Cequidistantly6F5) group atstacked 2,5,8-positions 1D chain [23].[22]. polarizedHighly polarized C6F5 group C6F5 would group afford would aggregates afford aggregates exceeding exceeding a π-dimer a due π-dimer to the strongdue to electrostatic the strong As a first trial, we decided to introduce a pentafluorophenyl (C6F5) group at 2,5,8-positions [23]. interactionelectrostatic betweeninteraction the between C6F5 groups. the C6F Indeed,5 groups. C 6Indeed,F5 groups C6F serve5 groups as aserve supramolecular as a supramolecular synthon, Highly polarized C6F5 group would afford aggregates exceeding a π-dimer due to the strong givingsynthon, rise giving to a rise 1D to stack a 1D of stack molecules of molecules [24,25 [24,25].]. The The key key precursor precursor of of the the target target radical radical ((44)) was electrostatic interaction between the C6F5 groups. Indeed, C6F5 groups serve as a supramolecular synthesized from 2,7-dibromonaphthalene in nine steps. The dehydrogenation of the precursor synthesizedsynthon, giving from rise 2,7-dibromonaphthalene to a 1D stack of molecules in nine [24,25]. steps. The The key dehydrogenation precursor of the oftarget the precursorradical (4 )with was with dichlorodicyano-p-benzoquinone (DDQ) afforded pale yellow crystals, which is in sharp contrast dichlorodicyano-synthesized fromp 2,7-dibromonaphthalene-benzoquinone (DDQ) afforded in nine palesteps. yellow The dehydrogenation crystals, which is of in the sharp precursor contrast with to to the blue color of the π-dimer of 3. The X-ray crystallographic analysis of the yellow crystal showed thedichlorodicyano- blue color of thep-benzoquinone π-dimer of 3 .(DDQ) The X-ray afforded crystallographic pale yellow analysis crystals, of which the yellow is in sharp crystal contrast showed to the blue color of the π-dimer of 3. The X-ray crystallographic analysis of the yellow crystal showed

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1 1 thatthat 44adopts adopts a aσ -dimerσ-dimer form form (4 2()4 in2) thein the solid solid state state (Figure (Figure3a). The 3a). TheH NMR H NMR spectrum spectrum of the paleof the yellow pale crystalsyellow incrystals CDCl 3inshowed CDCl3 six showed sharp singletsix sharp signals singlet in the signals temperature in the rangetemperature 233–298 range K, consistent 233–298 with K, theconsistentσ-dimer with form. the Thus, σ-dimer4 prefers form. aThus,σ-dimer 4 prefers form overa σ-dimer a π-dimer form formover ina π the-dimer solution form and in the solid solution states. Theand σsolid-dimer states. features The σ a-dimer long σ -bondfeatures of a 1.636(7) long σ-bond Å connecting of 1.636(7) the Å two connecting phenalenyl the rings, two phenalenyl indicating therings, weakness indicating of the theσ -bond.weakness Indeed, of the a tolueneσ-bond. solution Indeed, ofa thetolueneσ-dimer solution at room of the temperature σ-dimer at showed room atemperature well-resolved showed multiline a well-resolved ESR spectrum multilin originatinge ESR fromspectrum monomeric originating4. The from enthalpy monomeric and entropy 4. The changesenthalpy forand the entropyσ-dimerization changes for of 4thewere σ-dimerization determined of to 4 be were−64.0 determined kJ mol−1 toand be− −13564.0 JkJ K −mol1 mol−1 and−1, respectively,−135 J K−1 mol from−1, respectively, the temperature from dependencythe temperature of the depe ESRndency signal of intensity. the ESR signal intensity.

Figure 3. Dimerization mode of (a)) 4 and ((b)) 5..

We alsoalso preparedprepared aa phenylphenyl derivativederivative ((55)) ofof 22[ 26[26].]. TheThe radicalradical precursorprecursor waswas preparedprepared from 2,7-diphenylnaphthalene in a similarsimilar mannermanner asas 44.. The dehydrogenation of the precursorprecursor withwith p-chloranil-chloranil affordedafforded deepdeep blueblue crystals,crystals, contrarycontrary toto ourour expectation.expectation. TheThe X-rayX-ray crystallographiccrystallographic π α α analysis of the blue crystal showed that 5 formsforms aa discreetdiscreet face-to-faceface-to-face π-dimer-dimer ((5522) with a C(α)–C(α) separation of 3.067 ÅÅ atat 300300 KK (Figure(Figure3 3b),b), much much shorter shorter than than that that of of 3322. To elucidateelucidate thethe structurestructure ofof 1 thethe dimericdimeric species species in in the the solution solution state, state, the variable-temperature the variable-temperatureH NMR 1H spectra NMR ofspectra5 was measuredof 5 was inmeasured CD2Cl2. in No CD signal2Cl2. wasNo signal observed was at observed 273 K for at the 273 solution K for the of 5 ,solution whereas of at 5 203, whereas K, two at broad 203 signalsK, two werebroad observed signals were at δ 7.5 observed ppm (phenyl at δ 7.5 protons) ppm (phenyl and 6.8 ppmprotons) (phenalenyl and 6.8 protons).ppm (phenalenyl Considering protons). that a similarConsidering behavior that wasa similar observed behavior for the wasπ-dimerization observed for ofthe3 ,π the-dimerization phenyl derivative of 3, the5 phenylprefers derivative a π-dimer σ form5 prefers over a a π-dimer formform inover the a solution σ-dimer state. form The in dissolutionthe solution of state.52 in The toluene dissolution showed of a well-resolved 52 in toluene multilineshowed a ESR well-resolved spectrum corresponding multiline ESR to monomericspectrum 5correspondingat room temperature. to monomeric The intensity 5 at ofroom the ESRtemperature. signals of The5 decreased intensity withof the decreasing ESR signals temperature of 5 decreased and almost with disappeareddecreasing temperature at 170 K due and to thealmost formation disappeared of a diamagnetic at 170 K dueπ-dimer. to the Theformation enthalpy of a and diamagnetic entropy changes π-dimer. for The the enthalpy dimerization and −1 −1 −1 wereentropy determined changes tofor be the−40 dimerization kJ mol and were−75 Jdetermined K mol ,to respectively, be −40 kJ bymol the−1 variable-temperatureand −75 J K−1 mol−1, ESRrespectively, measurements. by the variable-temperature ESR measurements. The stericsteric sizesize ofof thethe phenylphenyl groupgroup isis smallersmaller thanthan thatthat ofof thethe CC6F6F55 group.group. The preference of π-dimerization-dimerization forfor thethe phenalenylphenalenyl radicalradical withwith smallersmaller substituentssubstituents indicatesindicates thatthat thethe dimerizationdimerization mode of phenalenylphenalenyl radicals is notnot simplysimply determineddetermined by aa stericsteric factor.factor. Therefore, we decideddecided to prepareprepare aa phenalenylphenalenyl radicalradical derivativederivative withwith smallsmall substituentsubstituent groups,groups, 2,5,8-trimethyl2,5,8-trimethyl phenalenylphenalenyl radicalradical (6), inin orderorder toto investigateinvestigate the intrinsic dimerization nature of the phenalenyl radical [26,27]. [26,27]. TheThe precursorprecursor ofof 66 waswas preparedprepared fromfrom 2,7-dibromonaphthalene2,7-dibromonaphthalene inin tenten steps.steps. TheThe dehydrogenationdehydrogenation reactionreaction ofof thethe precursorprecursor with withp p-chloranil-chloranil produced produced a a pink-violet pink-violet solution, solution, indicating indicating the the generation generation of aofπ a-dimer. π-dimer. However, However, the concentrationthe concentration of the of violet the violet solution solution yielded yielded colorless colorless platelet platelet crystals, crystals, which turnedwhich turned out to be out a σto-dimer be a σ form-dimer by form the X-ray by the crystallographic X-ray crystallographic analysis (Figureanalysis4a). (Figure When 4a). the When colorless the platescolorless were plates heated were at heated 573 K at in 573 a sealed, K in a degassedsealed, degassed tube, melting tube, melting occurred, occurred, accompanied accompanied by a color by a changecolor change from colorlessfrom colorless to purple. to purple. Subsequent Subsequent cooling cooling of the purpleof the liquidpurple afforded liquid afforded purple plateletpurple crystalsplatelet togethercrystals withtogether colorless with plates.colorless The plates. X-ray crystallographicThe X-ray crystallographic analysis of the analysis purple of plate the at purple 100 K showedplate at that100 K the showed purple, that crystalline the purple, state crystalline adopts a π state-dimer adopts form a (Figure π-dimer4b) form with (Figure a short contact4b) with of a αshort-carbons contact (av. of 3.054 α-carbons Å). Thus, (av. 3.0546 can Å). exist Thus, in both 6 canσ- exist and inπ-dimer both σ forms- and π in-dimer the solid forms state. in the The solid 1D 1 state.H NMR The spectrum 1D 1H NMR of 6 recorded spectrum in degassedof 6 recorded THF- din8 atdegassed 173 K showed THF-d a8 relativelyat 173 K complicatedshowed a relatively spectral pattern.complicated The spectrumspectral pattern. consisted The of three spectrum sets of consiste signals derivedd of three from sets three of dimericsignals isomersderived (6from2-σ-chiral, three 6dimeric2-σ-meso, isomers and 6 2(-6π2-,σ shown-chiral, in 62 Figure-σ-meso,4c), and which 62-π were, shown unambiguously in Figure 4c), characterized which were byunambiguously the 2D-NMR characterized by the 2D-NMR analyses. From the integration ratio of the 1D 1H NMR signals, the

Molecules 2019, 24, 665x FOR PEER REVIEW 55 of of 13 Molecules 2019, 24, x FOR PEER REVIEW 5 of 13 most stable isomer was found to be the σ-dimer chiral form (62-σ-chiral), and the other two analyses. From the integration ratio of the 1D 1H NMR signals, the most stable isomer was found (62-mostσ-meso stable and isomer 62-π) werewas metastablefound to be forms. the σ However,-dimer chiral the formenergy (6 2differences,-σ-chiral), and estimated the other from two the σ σ σ π ratioto be(62 - theσof-meso the-dimer NMRand 6 chiral 2-signalπ) were form intensities metastable (62- -chiral), and forms. Boltzm and However, theann other distribution, the two energy (62- -meso differences,between and 6the 2estimated- )metastable were metastablefrom formsthe forms. However, the energy differences, estimated from the ratio−1 of the NMR signal intensities−1 and relativeratio toof thethe mostNMR stable signal 6 2-intensitiesσ-chiral were and veryBoltzm small:ann 0.33distribution, kJ mol forbetween 62-σ-meso the metastableand 2.8 kJ molforms for Boltzmann distribution, between2 the metastable forms relative to the−1 most2 stable 6 -σ-chiral were−1 very 62-πrelative. to the most stable 6 -σ-chiral were very small: 0.33 kJ mol for 6 -σ-meso and2 2.8 kJ mol for −1 −1 small:62-π. 0.33 kJ mol for 62-σ-meso and 2.8 kJ mol for 62-π.

FigureFigure 4. Various4. Various dimer dimer forms forms ofof of 66 .6. . ORTEPORTEP ORTEP (Oak (Oak(Oak RidgeRidgeRidge Thermal-EllipsoidThermal-Ellipsoid Plot Plot Program) Program) drawing drawing of of (a)( 6a2)- -6σ2-chiral-chiral-σ-chiral and and and ( (b ())b 6) 26--π2π-π..( (. cc()c) )Three ThreeThree structural structuralstructural isomersisomers of of thethe the dimerdimer dimer of of of 6 6.6 ..

FromFrom the the results results of of the the di dimerization dimerizationmerization behaviorbehavior of 6,, wewe concludedconcluded that that that the the the σ -dimerσ-dimer and and and π -dimerπ-dimer of ofthe the phenalenyl phenalenyl radical radical radical are areare close closeclose in in energy, energy, andand and that that its its dimerizationdimerization dimerization mode mode mode can can can be be changed be changed changed with with with a a slighta slightslight perturbation perturbationperturbation of of substituents substituents eithereither either byby by aa a steric steric or or electronicelectronic electronic factor.factor. For For For instance, instance, in in4 2,4 2the,, the attractiveattractive interactioninteraction interaction between between between the the Cthe6 F C5Cgroup6F6F55 groupgroup and naphthaleneandand naphthalenenaphthalene ring makes ringring makes themakesσ-dimer the the σ form-dimerσ-dimer preferable form form preferablein energypreferable due in inenergy to energy the electrostaticdue due to to the the electrostatic electrostatic interaction (perﬂuoroarene–areneinteinteractionraction (perfluoroarene–aren interaction),e einteraction), interaction), as shown inas as Figureshown shown5 a. inOn inFigure the Figure other 5a. 5a. hand,On On the 5the adoptsother other hand, thehand,π-dimer 5 5 adopts adopts form thethe due ππ-dimer-dimer to a cumulative formform duedue CH– toto a a πcumulative cumulativeinteraction CH– amongCH–π πinteraction interaction the phenyl amongsubstituentsamong the the phenyl (Figure phenyl substituents5 b).substituents The M05-2X/6-31G** (Figure (Figure 5b).5b). TheThe calculation M05-2X/6-31G** showed that calculationcalculation the total showed showed stabilization that that the energiesthe total total −1 −1 stabilization energies by the substituents in 4−2 1and 52 are 35 kJ −mol1 − 1and 43 kJ mol ,− 1respectively. stabilizationby the substituents energies in by42 andthe substituents52 are 35 kJ molin 42 andand 52 43 are kJ 35 mol kJ mol, respectively. and 43 kJ mol , respectively.

Figure 5. Attractive interactions between radical units of (a) 42 and (b) 52. Figure 5.5. Attractive interactions between radicalradical unitsunits ofof ((aa)) 4422 and (b) 52.. 2.3. One-Dimensional Stack of Phenalenyl Radicals 2.3. One-Dimensional Stack of Phenalenyl Radicals 2.3. One-Dimensional Stack of Phenalenyl Radicals As mentioned above, phenalenyl radicals can adopt σ-dimer and π-dimer forms. Therefore, we As mentioned above, phenalenyl radicals can adopt σ-dimer and π-dimer forms. Therefore, attemptedAs mentioned to transform above, the phenalenyl σ-dimer formradicals of 4can into adopt a π-dimer σ-dimer form and by π external-dimer forms. stimulus Therefore, [23]. The we we attempted to transform the σ-dimer form of 4 into a π-dimer form by external stimulus [23]. attemptedwhite powder to transform of 42 was the placed σ-dimer in aform sealed, of 4degassed into a π tube-dimer and form heated by externalat 573 K, stimulus resulting [23].in the The The white powder of 42 was placed in a sealed, degassed tube and heated at 573 K, resulting in the whitemelting powder of the of power 42 was and placed a color in change a sealed, from degassed white to tubepurple. and Subsequent heated at cooling 573 K, of resulting the resulting in the melting of the power and a color change from white to purple. Subsequent cooling of the resulting meltingpurple of liquid the power afforded and darka color purple change needles from whitesuitable to purple.for the Subsequent X-ray crystallographic cooling of the analysis. resulting purple liquid afforded dark purple needles suitable for the X-ray crystallographic analysis. Surprisingly, purpleSurprisingly, liquid afforded4 was found dark to bepurple equidistantly needles stackedsuitable with for thethe interplanar X-ray crystallographic distance of 3.503 analysis. Å to 4 was found to be equidistantly stacked with the interplanar distance of 3.503 Å to form a 1D chain Surprisingly,form a 1D 4chain was (Figurefound to6a,b). be equidistantlyNo Peierls transition stacked (thatwith is,the dimerization interplanar indistance the 1D ofstack) 3.503 was Å to (Figureobserved6a,b). even No at Peierls 10 K. Each transition 1D chain (that was is, surrounded dimerization by in six the adjacent 1D stack) chains, was and observed the 1D chains even at stick 10 K. form a 1D chain (Figure 6a,b). No Peierls transition (that is, dimerization in the 1D stack) was Eachtogether 1D chain by a was weak surrounded C–H•••F–C by sixhydrogen-bonding adjacent chains, interaction. and the 1D Considering chains stick togetherthat an unpaired by a weak observed even at 10 K. Each 1D chain was surrounded by six adjacent chains, and the 1D chains stick C–Helectron•••F–C resides hydrogen-bonding only on the interaction.phenalenyl Consideringmoiety, the thatC6F5 angroups unpaired serve electron as spacers resides to onlyseparate on the together by a weak C–H•••F–C hydrogen-bonding interaction. Considering that an unpaired phenalenylneighboring moiety, radical the moieties; C6F5 groups thus, serve an ideal as spacers 1D chain to separateof spin-1/2 neighboring was achieved. radical The moieties; reason for thus, the an electron resides only on the phenalenyl moiety, the C6F5 groups serve as spacers to separate idealregular 1D chain stack ofof spin-1/24 is the close was stacking achieved. of polarized The reason C6F for5 groups; the regular the interplanar stack of 4 distanceis the close between stacking the of neighboring radical moieties; thus, an ideal 1D chain of spin-1/2 was achieved. The reason for the C6F5 groups is 3.252 Å on average. Thus, the C6F5 substituent groups dominantly control the polarized C6F5 groups; the interplanar distance between the C6F5 groups is 3.252 Å on average. Thus, regular stack of 4 is the close stacking of polarized C6F5 groups; the interplanar distance between the C6F5 groups is 3.252 Å on average. Thus, the C6F5 substituent groups dominantly control the

Molecules 2019, 24, 665 6 of 13 Molecules 2019, 24, x FOR PEER REVIEW 6 of 13 Molecules 2019, 24, x FOR PEER REVIEW 6 of 13 thestructure C6F5 substituent of the 1D groups chain. dominantlyBecause of the control large theinterplanar structure distance of the1D (3.503 chain. Å) Becauseof the phenalenyl of the large structure of the 1D chain. Because of the large interplanar distance (3.503 Å) of the phenalenyl interplanarradical site, distance the overlapping (3.503 Å) of of the the wave phenalenyl function radicals between site, the the radicals overlapping was small. of the The wave band functions width radical site, the overlapping of the wave functions between the radicals was small. The band width betweencalculated the by radicals the extended was small. HMO The method band was width as small calculated as about by 0.29 the eV. extended As a result, HMO the method1D chain was of 4 as calculated by the extended HMO method was as small as about 0.29 eV. As a result, the 1D chain of 4 smallwas as an about insulator. 0.29 eV. As a result, the 1D chain of 4 was an insulator. was an insulator.

FigureFigure 6. 6.One-dimensional One-dimensional stackstack ofof 44,(, (aa)) TopTop viewview and ( (bb)) side side view. view. (c ()c )Side Side view view of of alternating alternating Figure 6. One-dimensional stack of 4, (a) Top view and (b) side view. (c) Side view of alternating one-dimensionalone-dimensional stack stack of of4 4and and5 5. . one-dimensional stack of 4 and 5. PentaﬂuorophenylPentafluorophenyl andand phenylphenyl groupsgroups alternately stack stack in in the the solid solid state state and and are are used used as as Pentafluorophenyl and phenyl groups alternately stack in the solid state and are used as supramolecularsupramolecular synthons synthons[ 24[24,25].,25]. WeWe heatedheated the mixed mixed powder powder of of 424 2andand 525 at2 at573 573 K Kin ina sealed, a sealed, supramolecular synthons [24,25]. We heated the mixed powder of 42 and 52 at 573 K in a sealed, degasseddegassed tube, tube, and and subsequent subsequent cooling cooling gave gave rise torise brown to brown needles. needles. The X-ray The crystallographicX-ray crystallographic analysis degassed tube, and subsequent cooling gave rise to brown needles. The X-ray crystallographic ofanalysis the brown of the needle brown at needle 200 K at showed 200 K showed that 4 thatand 45 andstack 5 stack alternately alternately to formto form a 1Da 1D chain chain with with an analysis of the brown needle at 200 K showed that 4 and 5 stack alternately to form a 1D chain with interplanaran interplanar distance distance of 3.69 of Å3.69 by virtueÅ by virt of theue of attractive the attractive interaction interaction between between C F andC6F5 phenyland phenyl groups an interplanar distance of 3.69 Å by virtue of the attractive interaction between6 5 C6F5 and phenyl groups (Figure 6c). (Figuregroups6c). (Figure 6c). 3. Fluorenyl Radical 3. Fluorenyl3. Fluorenyl Radical Radical Fluorenyl radical (7), which possesses the same sp22 carbon number as the phenalenyl radical (2), FluorenylFluorenyl radical radical (7 (),7), which which possesses possesses thethe same sp 2 carboncarbon number number as as the the phenalenyl phenalenyl radical radical (2), ( 2), is also a spin-delocalized hydrocarbon radical. However, unlike 2, about half of the spin density is alsois also a spin-delocalized a spin-delocalized hydrocarbon hydrocarbon radical. radical. However, However, unlike unlike2, about 2, about half half of the of spinthe densityspin density resides onresides the 9-position on the (Figure9-position7). Therefore,(Figure 7). 7Therefore,dimerizes 7 promptly, dimerizes and promptly, its σ-dimer and does its σ not-dimer dissociate does not into a resides on the 9-position (Figure 7). Therefore, 7 dimerizes promptly, and its σ-dimer does−1 not dissociate into a monomeric radical even in solution [28,29]. Because the RSE of− 71 is −93 kJ mol , the monomericdissociate radicalinto a monomeric even in solution radical [28 even,29]. in Because solution the [28,29]. RSE ofBecause7 is − 93the kJ RSE mol of 7 ,is the −93 thermodynamic kJ mol−1, the thermodynamic stability of 7 is much lower than that of 2. stabilitythermodynamic of 7 is much stability lower of than 7 is much that of lower2. than that of 2. (a) (b) +0.55 (c) (a)9 (b) +0.55 (c) 9

7 (BLYP/6-31G**) 7 (BLYP/6-31G**) 89 89 10 10 Figure 7. (a) Fluorenyl radical (7). (b) Spin density of 7 calculated by a BLYP/6-31G** method. (c) FigureFigure 7. 7.( a(a)) FluorenylFluorenyl radicalradical ( (77).). (b ()b )Spin Spin density density of of7 calculated7 calculated by a by BLYP/6-31G** a BLYP/6-31G** method. method. (c) Anthryl-protected dibenzo-fluorenyl radicals (8, 9, 10). (c)Anthryl-protected Anthryl-protected dibenzo-fluorenyl dibenzo-fluorenyl radicals radicals (8, (98,, 109,).10 ). 3.1. π-Extended Fluorenyl Radicals 3.1.3.1.π -Extendedπ-Extended Fluorenyl Fluorenyl Radicals Radicals To stabilize 7, which shows remarkable reactivity, a considerable device must be made in the ToTo stabilize stabilize7 ,7 which, which shows shows remarkable remarkable reactivity,reactivity, a a considerable considerable device device must must be be made made in inthe the molecular design. It would be possible to prolong the lifetime of 7 by sterically protecting only the molecularmolecular design. design. ItIt would be be possible possible to to prolong prolong the the lifetime lifetime of 7 ofby7 stericallyby sterically protecting protecting only the only spin-localized part by taking advantage of the situation where an unpaired electron is mainly thespin-localized spin-localized part part by by taking taking advantage advantage of ofthe the situation situation where where an anunpaired unpaired electron electron is ismainly mainly localized on the carbon atom at the 9-position. We decided to introduce an anthryl group at the localizedlocalized on on the the carbon carbon atom atom atat thethe 9-position.9-position. We We decided decided to to introduce introduce an an anthryl anthryl group group at atthe the 9-position, and to expand the π-skeleton to ensure extraordinary stability in air. In addition, three 9-position,9-position, and and to to expand expand the theπ π-skeleton-skeleton toto ensureensure extraordinary stability stability in in air. air. In In addition, addition, three three types of dibenzo-fluorenyl radicals (8, 9, 10), which possess an anthryl substituent group at the types of dibenzo-fluorenyl radicals8 (98,10 9, 10), which possess an anthryl substituent group at the typescentral of dibenzo-fluorenyl five-membered ring, radicals were (prepared, , ), whichby short possess step syntheses an anthryl [30]. substituent Interestingly, group the at difference the central central five-membered ring, were prepared by short step syntheses [30]. Interestingly, the difference five-memberedin the ring-condensation ring, were preparedposition bycaused short a stepbig synthesesdifference [in30 ].lifespan, Interestingly, dimerization the difference mode, and in the in the ring-condensation position caused a big difference in lifespan, dimerization mode, and ring-condensationelectronic properties position of the causedradicals. a big difference in lifespan, dimerization mode, and electronic propertieselectronic of properties the radicals. of the radicals.

Molecules 2019, 24, x FOR PEER REVIEW 7 of 13 Molecules 2019, 24, x FOR PEER REVIEW 7 of 13 MoleculesMolecules 20192019,, 2424,, 665x FOR PEER REVIEW 77 of 1313 The radicals 8, 9, and 10 showed relatively long half-life times of 7, 3.5, and 43 days, The radicals 8, 9, and 10 showed relatively long half-life times of 7, 3.5, and 43 days, respectively, in air in the solution state. As for dimerization, 8 formed a σ-bond on the fluorenyl respectively,The radicals in air8, 9in, and the 10solutionshowed state. relatively As for long dimerization, half-life times 8 formed of 7, 3.5, a andσ-bond 43 days, on the respectively, fluorenyl ring, whereas 10 formed a σ-bond on a bent anthracene ring (Figure 8). In contrast, no dimerization inring, air whereas in the solution 10 formed state. a σ As-bond for dimerization,on a bent anthracene8 formed ring a σ(Fig-bondure on8). theIn contrast, fluorenyl no ring, dimerization whereas reaction was observed in 9. Regarding physical properties such as redox potentials and UV-Vis-NIR 10reactionformed was a σ observed-bond on in a bent9. Regarding anthracene physical ring (Figure properties8). In such contrast, as redox no dimerizationpotentials and reaction UV-Vis-NIR was absorptions, 8 and 10 showed similar properties, whereas 9 showed behavior different from those observedabsorptions, in 9 8. Regardingand 10 showed physical similar properties properties, such whereas as redox 9 potentials showed behavior and UV-Vis-NIR different absorptions, from those radicals. For instance, the redox waves of 8 and 10 in the cyclic voltammogram appeared on the 8radicals.and 10 Forshowed instance, similar the properties,redox waves whereas of 8 and9 10showed in the behaviorcyclic voltammogram different from appeared those radicals. on the higher potential side (that is, not easily oxidized and easily reduced) compared with 9. Regarding Forhigher instance, potential the side redox (that waves is, not of easily8 and oxidized10 in the and cyclic easily voltammogram reduced) compared appeared with on 9. theRegarding higher the UV-Vis-NIR absorption spectra, 8 and 10 showed weak absorptions in the NIR region, but 9 did potentialthe UV-Vis-NIR side (that absorption is, not easilyspectra, oxidized 8 and 10 and showed easily weak reduced) absorptions compared in the with NIR9 .region, Regarding but 9 thedid not absorb light beyond the visible region. These differences arise from the difference in the manner UV-Vis-NIRnot absorb light absorption beyond spectra,the visible8 and region.10 showed These di weakfferences absorptions arise from in the the NIR difference region, in but the9 didmanner not of condensation of the naphthalene ring with respect to the five-membered ring (Figure 9). It is absorbof condensation light beyond of the the naphthalene visible region. ring These with differencesrespect to the arise five-membered from the difference ring (Figure in the manner9). It is well-known that naphthalene possesses a larger double-bond character at the 1,2-bond than at the ofwell-known condensation that ofnaphthalene the naphthalene possesses ring a withlarger respect double-bond to the five-memberedcharacter at the ring1,2-bond (Figure than9). at It the is 2,3-bond. Therefore, the five-membered ring condensation to the 1,2 or 2,3 region of naphthalene well-known2,3-bond. Therefore, that naphthalene the five-membered possesses aring larger conden double-bondsation to characterthe 1,2 or at2,3 the region 1,2-bond of naphthalene than at the leads to endo- or exo-double bonds at the five-membered ring, respectively. This difference in the 2,3-bond.leads to endo- Therefore, or exo-double the five-membered bonds at the ring five-membered condensation ring, to the respectively. 1,2 or 2,3 region This difference of naphthalene in the condensation affects the manner of spin delocalization, as shown in Figure 10. Thus, 8 and 10 leadscondensation to endo- affects or exo-double the manner bonds of at spin the five-membereddelocalization, as ring, shown respectively. in Figure This 10. differenceThus, 8 and in the 10 behave like a cyclopentadienyl radical, whereas 9 behaves like a linear pentadienyl radical. The condensationbehave like a affects cyclopentadienyl the manner ofradical, spin delocalization, whereas 9 behaves as shown like in a Figure linear 10pentadienyl. Thus, 8 and radical.10 behave The higher redox potentials of 8 and 10 are attributable to the antiaromatic character of the likehigher a cyclopentadienyl redox potentials radical, of 8 whereas and 109 behavesare attributable like a linear to pentadienylthe antiaromatic radical. character The higher of redox the cyclopentadienyl cation in the cation state and the aromatic character of the cyclopentadienyl anion potentialscyclopentadienyl of 8 and cation10 are in attributable the cation state to the and antiaromatic the aromatic character character of theof the cyclopentadienyl cyclopentadienyl cation anion in in the anion state. thein the cation anion state state. and the aromatic character of the cyclopentadienyl anion in the anion state.

Figure 8. σ-Dimer structures of (a) 8 andand ((b) 10 determineddetermined byby X-rayX-ray crystallographiccrystallographic analysis.analysis. Figure 8.8. σ-Dimer-Dimer structuresstructures ofof ((aa)) 88 andand ((bb)) 1010 determineddetermined byby X-rayX-ray crystallographiccrystallographic analysis.analysis.

Figure 9. RingRing condensation condensation modes modes for 8,, 9,, andand 10.. Figure 9. Ring condensation modes for 8, 9, and 10.

cyclopentadienyl (cycliccyclopentadienyl conjugation) (cyclic conjugation)

8 10 8 10 pentadienyl (linearpentadienyl conjugation) (linear conjugation)

9 9

Figure 10. Manner of the delocalization of an unpaired electron based on the spin density calculation Figure 10. MannerManner ofof thethe delocalizationdelocalization ofof anan unpairunpaired electron based on the spin density (UBLYP/6-31G**//UB3LYP/6-31G**).Figure 10. Manner of the delocalization of an unpaired electron based on the spin density calculation (UBLYP/6-31G**//UB3LYP/6-31G**). calculation (UBLYP/6-31G**//UB3LYP/6-31G**).

Molecules 2019, 24, x 665 FOR PEER REVIEW 88 of of 13

As shown in Figure 8b, σ-dimerization of 10 occurs not on the dibenzofluorenyl moiety, where an unpairedAs shown electron in Figure predominantly8b, σ-dimerization distributes, of 10 occurs but on not the on anthryl the dibenzofluorenyl group that is moiety,introduced where as an a stericallyunpaired electronprotecting predominantly group. This distributes,σ-dimerization but onis accompanied the anthryl group by athat large is introducedstructural change as a sterically of the anthrylprotecting group group. from This the σplanar-dimerization form to isthe accompanied butterfly form. by We a large estimated structural the changeenergy profile of the anthrylfor the structuralgroup from change the planar upon form the toσ-dimerization the butterfly form.of 10We by estimatedquantum thechemical energy calculations profile for the (Figure structural 11). Althoughchange upon the themostσ-dimerization stable form of of the10 monomericby quantum 10 chemical is the twisted calculations form, 10 (Figure can adopt 11). Although a metastable the foldedmost stable form,form where of an the unpaired monomeric electron10 is resides the twisted mostly form, on the10 can10-positon adopt aof metastable the anthryl folded group. form, The structurewhere an of unpaired the transition electron state resides (10*) mostlyfor the onmost the stable 10-positon twisted of form the anthryl ⇄ the group.metastable The folded structure form of isthe close transition to the state folded (10 *)form, for theexcept most that stable the twisted anthryl form group isthe contorted. metastable The folded calculated form is enthalpy close to the of ‡ activationfolded form, (ΔH except‡) for the that twisted the anthryl form group to 10* is was contorted. 95.5 kJ mol The− calculated1 (Figure 11). enthalpy Although of activation a σ-dimer ( ∆(10H2)) −1 offor 10 the is twistedmore stable form than to 10 the* was twisted 95.5 form, kJ mol the energy(Figure difference 11). Although is just a 0.6σ-dimer kJ mol (−101. The2) of low10 is barrier more −1 heightstable thanfor the the dissociation twisted form, of 10 the2 into energy the differencemonomeric is just10 is 0.6 in kJline mol with. the The fact low that barrier the dissolution height for the of thedissociation solid of 10 of2 10in2 ainto solvent the monomeric gradually resulted10 is in line in the with equilibrium the fact that with the the dissolution monomeric of the 10 solid. The offacile102 dissociationin a solvent graduallyof 102 suggests resulted the inweakness the equilibrium of the σ with-bond the connecting monomeric two10 anthryl. The facile groups. dissociation In reality, of the102 suggestssolid of 10 the2 showed weakness mechanochromic of the σ-bond connecting behavior. twoHard anthryl grinding groups. of the In crystals reality, theof 10 solid2 led of to10 a2 drasticshowed color mechanochromic change from behavior. orange Hardto purple. grinding The of purple the crystals color ofcorresponds102 led to a drasticto the colorcolor changeof the monomericfrom orange 10 to. purple. The purple color corresponds to the color of the monomeric 10.

Figure 11. Energy diagram for twisted 10 to 102 (σ-dimer) calculated with the UB3LYP/6-31G** method. Figure 11. Energy diagram for twisted 10 to 102 (σ-dimer) calculated with the UB3LYP/6-31G** method. 3.2. Ring Expansion of Fluorenyl Radical 3.2. Ring Expansion of Fluorenyl Radical We also investigated the effect of ring size of the ﬂuorenyl radical on chemical and physical properties.We also Three investigated kinds ofthe anthracene-attached effect of ring size of tricyclic the fluorenyl radicals radical (11, 12 on, and chemical13) were and designedphysical properties. Three kinds of anthracene-attached tricyclic radicals (11, 12, and 13) were designed and

Molecules 2019, 24, x FOR PEER REVIEW 9 of 13 synthesized (Figure 12) [31]. It is well-known that five- and seven-membered rings have electron-deficient and electron-rich properties, respectively. Additionally, the size of the central polygon in the tricyclic system affects steric congestion in the fjord region. These electronic and steric factors would cause the difference in the properties among the tricyclic radicals. From the decay of the ESR signal intensities in air-saturated toluene, the half-life time of 11 at room temperature was found to be 5.6 days. Considering that the 9-phenylfluorenyl radical rapidly decomposes [32], it seems that the effect of steric protection by an anthryl group on the stability is unexpectedly large. 12 showed a longer half-life time (9 days) than 11 presumably due to the perpendicular conformation between the anthryl group and the tricyclic scaffold [33]. In contrast, Molecules 2019, 24, 665 9 of 13 13, which also adopts the perpendicular conformation, was a short-lived species with a half-life time of about 5 min and was easily oxidized. This high reactivity toward oxygen would be related toand the synthesized high SOMO (Figure level originating12)[31]. Itfrom is well-known the electron-rich that ﬁve-character and of seven-membered the seven-membered rings havering. Theelectron-deﬁcient oxidation and and reduction electron-rich potentials properties, (Eox respectively. and Ered, respectively) Additionally, determined the size of theby centralcyclic polygonvoltammetry in the are tricyclic summarized system affectsin Table steric 2. congestion13 is most in easily the fjord oxidized region. among These electronicthe threeand radicals, steric whereasfactors would 11 shows cause the the highest difference electron-accepting in the properties ability. among the tricyclic radicals.

Figure 12.12. Anthryl-substitutedAnthryl-substituted tricyclic tricyclic radicals radicals (11 (11(a),, 12,( band), and 13).13 Spin(c)). Spindensity density maps maps calculated calculated by UBLYP/6-31G**//Uby UBLYP/6-31G**//UωB97X-D/6-31G**ωB97X-D/6-31G** are also are shown. also shown.

From the decay of theTable ESR 2. signalRedox potentials intensities (V in vs air-saturated Fc/Fc+) of 11, 12 toluene,, and 13. the half-life time of 11 at room temperature was found to be 5.6 days. Considering that the 9-phenylﬂuorenyl radical rapidly decomposes [32], it seems that theCompounds effect of steric E protectionox/V Ered/V by an anthryl group on the stability is unexpectedly large. 12 showed a longer11 half-life+0.28 time − (91.11 days) than 11 presumably due to the perpendicular conformation between the12 anthryl group−0.10 and−1.76 the tricyclic scaffold [33]. In contrast, 13, which also adopts the perpendicular13 conformation, −0.30 was−1.78 a short-lived species with a half-life time of about 5 min and was easily oxidized. This high reactivity toward oxygen would be related to theSimilar high SOMOto 10, 11 level also originating gradually fromgave therise electron-rich to a colorless character σ-dimer ofat theroom seven-membered temperature in ring. the solution state. In contrast, 12 showed no dimerization behavior for a long period of time. The ΔH‡ of The oxidation and reduction potentials (Eox and Ered, respectively) determined by cyclic voltammetry −1 −1 12are to summarized a σ-dimer was in Table calculated2. 13 is to most be 138 easily kJ mol oxidized, which among is much the three larger radicals, than that whereas of 11 (9011 shows kJ mol the). Therefore,highest electron-accepting 12 could not exceed ability. the transition state at room temperature. As the central polygon of the tricyclic scaffold becomes a six-membered ring, the steric hindrance of the fjord region became larger than 11, leading Tableto a higher 2. Redox activation potentials (Vbarrier. vs Fc/Fc However,+) of 11, 12 13, and, where13. the fjord region was anticipated to be crowded more than 11 and 12, possessed a ΔH‡ of only 83 kJ mol−1. The reason for the unexpectedly low barrier isCompounds that in the transitiEoxon/V state, theEred seven-member/V ed ring moiety adopts a tub-like structure, whereby the tricyclic11 radical+0.28 moiety greatly−1.11 bends and the steric hindrance of 12 − − 0.10 1.76 13 −0.30 −1.78

Similar to 10, 11 also gradually gave rise to a colorless σ-dimer at room temperature in the solution state. In contrast, 12 showed no dimerization behavior for a long period of time. The ∆H‡ of 12 to a σ-dimer was calculated to be 138 kJ mol−1, which is much larger than that of 11 (90 kJ mol−1). Therefore, 12 could not exceed the transition state at room temperature. As the central polygon of the tricyclic scaffold becomes a six-membered ring, the steric hindrance of the fjord region became larger than 11, leading to a higher activation barrier. However, 13, where the fjord region was anticipated to Molecules 2019, 24, 665 10 of 13 be crowded more than 11 and 12, possessed a ∆H‡ of only 83 kJ mol−1. The reason for the unexpectedly MoleculesMolecules 2019 2019, 24, 24, x, xFOR FOR PEER PEER REVIEW REVIEW 1010 of of 13 13 low barrier is that in the transition state, the seven-membered ring moiety adopts a tub-like structure, thewherebythe fjord fjord regionthe region tricyclic is is alleviated alleviated radical (Figure moiety(Figure 13). greatly13). The The bendsflexibility flexibility and of theof the the steric seven-membered seven-membered hindrance of ring thering fjordmoiety moiety region is is also also is relatedalleviatedrelated to to (Figurethe the stability stability 13). Theof of the ﬂexibilitythe σ σ-dimer.-dimer. of theThe The seven-membered energy energy differences differences ring between between moiety isthe the also monomeric monomeric related to radical the radical stability and and −1 theofthe the σ σ-dimerσ-dimer-dimer. are are The 64 64 energykJ kJ mol mol− differences1− 1and and 109 109 kJ betweenkJ mol mol−1− 1for thefor 11 monomeric11 and and 13 13, ,respectively respectively radical and (the the (theσ σ-dimer σ-dimers-dimers are are 64are lower kJ lower mol in in −1 energyandenergy 109 than kJthan mol the the monomeric monomericfor 11 and species).13 species)., respectively Indeed, Indeed, (the13 13 afforded σafforded-dimers a a areσ σ-dimer-dimer lower quickly inquickly energy at at room thanroom thetemperature temperature monomeric in in thespecies).the solution solution Indeed, state state 13under underafforded the the degassed degassed a σ-dimer condition; condition; quickly atfu furthermore, roomrthermore, temperature no no dissociation dissociation in the solution reaction reaction state was was underobserved observed the fordegassedfor the the σ σ-dimer.-dimer. condition; furthermore, no dissociation reaction was observed for the σ-dimer.

FigureFigure 13. 13. Structures Structures of of the the transition transition state state of of ( a()a )11 11 and and (b ()b 13) 13 calculatedcalculated calculated atat at thethe the UBLYP-D3/6-31G**UBLYP-D3/6-31G** UBLYP-D3/6-31G** levellevellevel of of of theory. theory. theory.

4. Trianthrylmethyl Radical 4.4. Trianthrylmethyl Trianthrylmethyl Radical Radical 1 TheThe stability stability of of of the the triphenylmethyl triphenylmethyl radical radicalradical ( (1()1) )is is is largely largely largely dependent dependent dependent on on on the the the steric steric steric factor factor factor of of of the the phenylphenyl groups groups [34]. [ 34[34].]. We We We decided decided to to investigate investigate the the the effe effect effectct of of of larger larger aryl aryl groups groups on on the the stability stability of of of the the methylmethyl radical radical and and designed designed a anew new highly highly congestedcongeste congestedd hydrocarbon hydrocarbon radical, radical, trianthrylmethyl trianthrylmethyl radical radical (14(14))[ )[35]. 35[35].]. AsAs As shownshown shown in in Figurein Figure Figure 14 ,14 the14, ,the centralthe central central carbon carbon carbon atom atom atom of 14 of, whichof 14 14, ,which possesseswhich possesses possesses the largest the the largest spin largest density, spin spin density,isdensity, more effectivelyis is more more effectively effectively protected protected protected by three by anthrylby three three groupsanthryl anthryl than groups groups that than ofthan1 .that Wethat of expected of 1 .1 .We We expected thatexpected14 could that that 14 be 14 couldisolatedcould be be inisolated isolated the solid in in the state the solid solid with state state no substituent with with no no substituent substituent groups on groups groups the anthryl on on the the group. anthryl anthryl group. group.

FigureFigureFigure 14. 14. 14. ((a a()a) Trianthrylmethyl) Trianthrylmethyl Trianthrylmethyl radicals radicals radicals ( (14 (14,, 15, 15,, and and, and 16 16). ).Space-filling Space-ﬁlling Space-filling models models of of ( b ()b 1) 1and and ( c ()c 14) 14. .

14 15 However,However, the the radical radical that that could could be bebe isolated isolated in inin the thethe solid solid state statestate was waswas not notnot 14 14 but but 15 15,, ,where where where the the the carboncarbon atom atomatom at atat the the 10-position 10-position of of ofall all all anthryl anthryl anthryl groups groups groups was was was sterically sterically sterically protected protected protected by by a by amesityl mesityl a mesityl group. group. group. The The 15 stabilityThestability stability ofof 1515 of waswaswas extraordinarilyextraordinarily extraordinarily highhigh high andand and showedshowed showed nono no decompositiondecomposition decomposition eveneven even inin air airair at atat roomroom temperature.temperature.temperature. The TheThe absence absence of of of attack attack attack of of ofmolecular molecular molecular ox ox oxygenygenygen against against against the the the central central central carbon carbon carbon indicates indicates indicates that that thatthe the 16 stericthesteric steric protection protection protection by by the bythe theanthryl anthryl anthryl groups groups groups works works works very very very effectively. effectively. effectively. However, However, However, it it itwas was found foundfound that thatthat 16 16,, , whichwhich removed removed one one of of the the three three mesityl mesityl groups, groups, immediatelyim immediatelymediately undergoes undergoes a adimerization dimerization reaction reaction at at ω thethethe 10-position 10-position 10-position of of the the non-substituted non-substituted anthrylanth anthrylryl group group (Figure(Figure (Figure 1515). 15).). AccordingAccording According toto to aa UaU UωωB97X-D/6-31G**B97X-D/6-31G**B97X-D/6-31G** ‡ calculation,calculation,calculation, thethe the ﬂippingflipping flipping motionmotion motion of of theof the the anthryl anthryl anthryl blade blade blade in in14 in 14occurs 14 occurs occurs very very very rapidly rapidly rapidly because because because of the of of smallthe the small small∆H −1 Δ(43ΔHH‡ kJ ‡(43 (43 mol kJ kJ mol ),mol as−1−), shown1), as as shown shown in Figure in in Figure Figure16. Near 16. 16. theNear Near transition the the transition transition state, an state, state, unpaired an an unpaired unpaired electron electron is electron mostly is localizedis mostly mostly localizedlocalized at at the the 10-position 10-position of of the the anthryl anthryl group, group, and and this this localized localized radical radical species species shows shows high high reactivityreactivity with with the the σ σ-dimer.-dimer.

Molecules 2019, 24, 665 11 of 13 at the 10-position of the anthryl group, and this localized radical species shows high reactivity with Molecules 2019, 24, x FOR PEER REVIEW 11 of 13 Moleculesthe σ-dimer. 2019, 24, x FOR PEER REVIEW 11 of 13

Mes Mes

H H 16 16 Mes Mes Mes Mes H H

Mes Mes

Figure 15. Dimerization reaction of 16. Mes = mesityl. Figure 15. Dimerization reaction of 16.. Mes = mesityl. mesityl.

Figure 16. Relative energies for propeller flippingflipping motionmotion ofof 1414 calculatedcalculated atat thetheU UωωB97X-D/6-31G**B97X-D/6-31G** Figure 16. Relative energies for propeller flipping motion of 14 calculated at the UωB97X-D/6-31G** level of theory. level of theory. 5. Conclusions 5. Conclusions 5. Conclusions Our recent studies on stable, π-conjugated, carbon-centered, radical species were described. Our recent studies on stable, π-conjugated, carbon-centered, radical species were described. The πOur-conjugated recent studies radical on species stable, with π-conjugated, high thermodynamic carbon-centered, stability radical were foundspecies to were be able described. to show The π-conjugated radical species with high thermodynamic stability were found to be able to show variousThe π-conjugated association radical modes species due to with steric high and thermodynamic electronic effects stability of substituent were found groups. to be Onable the to show other various association modes due to steric and electronic effects of substituent groups. On the other varioushand, it association was found thatmodes the due anthryl to steric group and used electr foronic kinetic effects stabilization of substitu hasent not groups. only a simple,On the steric,other hand, it was found that the anthryl group used for kinetic stabilization has not only a simple, steric, protectivehand, it was effect found but that also the a role anthryl as a reaction group used site accompanied for kinetic stabilization by a large structural has not only change. a simple, By utilizing steric, protective effect but also a role as a reaction site accompanied by a large structural change. By protectiveelectronic andeffect steric but effects also a of role substituent as a reaction groups, site organic accompanied radicals couldby a offerlarge astructural further possibility change. forBy utilizing electronic and steric effects of substituent groups, organic radicals could offer a further utilizingdeveloping electronic new reactions and steric [36] effects and new of functionalitiessubstituent groups, for optoelectronics organic radicals [37 ],could electronics offer a [38 further], and possibility for developing new reactions [36] and new functionalities for optoelectronics [37], possibilityspintronics for [13 ,16developing] devices. new reactions [36] and new functionalities for optoelectronics [37], electronics [38], and spintronics [13,16] devices. electronics [38], and spintronics [13,16] devices. Acknowledgments: The author gratefully acknowledge all the collaborators for their extensive work, Acknowledgments: The author gratefully acknowledge all the collaborators for their extensive work, particularlyAcknowledgments: Yasukazu The Hirao author (Osaka gratefully University), acknowledge Tomohiko Nishiuchi all the (Osakacollaborators University), for andtheir Masayoshi extensive Nakano work, (Osakaparticularly University). Yasukazu Hirao (Osaka University), Tomohiko Nishiuchi (Osaka University), and Masayoshi particularly Yasukazu Hirao (Osaka University), Tomohiko Nishiuchi (Osaka University), and Masayoshi Nakano (Osaka University). NakanoConflicts (Osaka of Interest: University).The author declare no conflict of interest. Conflicts of Interest: The author declare no conflict of interest. Conflicts of Interest: The author declare no conflict of interest. References References References1. Miller, J.S.; Glatzhofer, D.T.; Vazquez, C.; McLean, R.S.; Calabrese, J.C.; Marshall, W.J.; Raebiger, J.W. II 1. Miller,Electron-Transfer J.S.; Glatzhofer, Salts ofD.T.; 1,2,3,4,5-Pentamethylferrocene, Vazquez, C.; McLean, R.S.; Calabrese, Fe (C5Me 5J.C.;)(C 5Marshall,H5). Structure W.J.; Raebiger, and Magnetic J.W. 1. Miller, J.S.; Glatzhofer, D.T.; Vazquez, C.; McLean, R.S.; Calabrese, J.C.; Marshall, W.J.; Raebiger, J.W. Electron-TransferProperties of Two Salts 1:1 andof 1,2, Two3,4,5-Pentamethylferrocene, 2:3 Fe(C5Me5)(C5H5) Electron-Transfer FeII(C5Me5)(C5H Salts5). Structure of Tetracyanoethylene. and Magnetic Electron-Transfer Salts of 1,2,3,4,5-Pentamethylferrocene, FeII(C5Me5)(C5H5). Structure and Magnetic PropertiesInorg. Chem. of 2001Two ,1:140 ,and 2058–2064. Two 2:3 [Fe(CCrossRef5Me5)(C][PubMed5H5) Electron-Transfer] Salts of Tetracyanoethylene. Inorg. Chem. Properties of Two 1:1 and Two 2:3 Fe(C5Me5)(C5H5) Electron-Transfer Salts of Tetracyanoethylene. Inorg. Chem. 2. 2001Kothe,, 40 G.;, 2058–2064. Denkel, K.-H.; Sümmermann, W. Schlenk’s Biradical.—A Molecule in the Triplet Ground State. 2001, 40, 2058–2064. 2. Kothe,Angew. G.; Chem. Denkel, Int. Ed.K.-H.; Engl. Sümmermann,1970, 9, 906–907. W. Schlenk’s [CrossRef ]Biradical.—A Molecule in the Triplet Ground State. 2. Kothe, G.; Denkel, K.-H.; Sümmermann, W. Schlenk’s Biradical.—A Molecule in the Triplet Ground State. Angew. Chem. Int. Ed. Engl. 1970, 9, 906–907. Angew. Chem. Int. Ed. Engl. 1970, 9, 906–907. 3. Hicks, R.G. What’s new in stable radical chemistry? Org. Biomol. Chem. 2007, 5, 1321–1338. 3. Hicks, R.G. What’s new in stable radical chemistry? Org. Biomol. Chem. 2007, 5, 1321–1338. 4. Rajca, A. Organic Diradicals and Polyradicals: From Spin Coupling to Magnetism? Chem. Rev. 1994, 94, 4. Rajca, A. Organic Diradicals and Polyradicals: From Spin Coupling to Magnetism? Chem. Rev. 1994, 94, 871–893. 871–893.

Molecules 2019, 24, 665 12 of 13

3. Hicks, R.G. What’s new in stable radical chemistry? Org. Biomol. Chem. 2007, 5, 1321–1338. [CrossRef] [PubMed] 4. Rajca, A. Organic Diradicals and Polyradicals: From Spin Coupling to Magnetism? Chem. Rev. 1994, 94, 871–893. [CrossRef] 5. Hicks, R.G. Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds; Wiley-Blackwell: Hoboken, NJ, USA, 2010; ISBN 9780470666975. 6. Gaudenzi, R.; Burzurí, E.; Reta, D.; Moreira, I.d.P.R.; Bromley, S.T.; Rovira, C.; Veciana, J.; van der Zant, H.S.J. Exchange Coupling Inversion in a High-Spin Organic Triradical Molecule. Nano Lett. 2016, 16, 2066–2071. [CrossRef][PubMed] 7. Pasini, D.; Takeuchi, D. Cyclopolymerizations: Synthetic Tools for the Precision Synthesis of Macromolecular Architectures. Chem. Rev. 2018, 118, 8983–9057. [CrossRef][PubMed] 8. Hioe, J.; Zipse, H. Radical stability and its role in synthesis and catalysis. Org. Biomol. Chem. 2010, 8, 3609–3617. [CrossRef][PubMed] 9. Gomberg, M. AN INSTANCE OF TRIVALENT CARBON: TRIPHENYLMETHYL. J. Am. Chem. Soc. 1900, 22, 757–771. [CrossRef] 10. Reid, D.H. The chemistry of the phenalenes. Q. Rev. Chem. Soc. 1965, 19, 274–302. [CrossRef] 11. Haddon, R.C. Design of organic metals and superconductors. Nature 1975, 256, 394–396. [CrossRef] 12. Morita, Y.; Nishida, S. Phenalenyls, Cyclopentadienyls, and Other Carbon-Centered Radicals. In Stable Radicals; Hicks, R.G., Ed.; John Wiley & Sons, Ltd.: Chichester, UK, 2010; pp. 81–145. 13. Raman, K.V.; Kamerbeek, A.M.; Mukherjee, A.; Atodiresei, N.; Sen, T.K.; Lazić,P.; Caciuc, V.; Michel, R.; Stalke, D.; Mandal, S.K.; et al. Interface-engineered templates for molecular spin memory devices. Nature 2013, 493, 509–513. [CrossRef][PubMed] 14. Yoneda, K.; Nakano, M.; Fukuda, K.; Matsui, H.; Takamuku, S.; Hirosaki, Y.; Kubo, T.; Kamada, K.; Champagne, B. Third-Order Nonlinear Optical Properties of One-Dimensional Open-Shell Molecular Aggregates Composed of Phenalenyl Radicals. Chem. A Eur. J. 2014, 20, 11129–11136. [CrossRef][PubMed] 15. Salustro, S.; Maschio, L.; Kirtman, B.; Rérat, M.; Dovesi, R. Third-Order Electric Field Response of Infinite Linear Chains Composed of Phenalenyl Radicals. J. Phys. Chem. C 2016, 120, 6756–6761. [CrossRef] 16. Mukherjee, A.; Sau, S.C.; Mandal, S.K. Exploring Closed-Shell Cationic Phenalenyl: From Catalysis to Spin Electronics. Acc. Chem. Res. 2017, 50, 1679–1691. [CrossRef][PubMed] 17. Reid, D.H. Stable π-electron systems and new aromatic structures. Tetrahedron 1958, 3, 339–352. [CrossRef] 18. Goto, K.; Kubo, T.; Yamamoto, K.; Nakasuji, K.; Sato, K.; Shiomi, D.; Takui, T.; Kubota, M.; Kobayashi, T.; Yakusi, K.; et al. A Stable Neutral Hydrocarbon Radical: Synthesis, Crystal Structure, and Physical Properties of 2,5,8-Tri-tert-butyl-phenalenyl. J. Am. Chem. Soc. 1999, 121, 1619–1620. [CrossRef] 19. Suzuki, S.; Morita, Y.; Fukui, K.; Sato, K.; Shiomi, D.; Takui, T.; Nakasuji, K. Aromaticity on the pancake-bonded dimer of neutral phenalenyl radical as studied by MS and NMR spectroscopies and NICS analysis. J. Am. Chem. Soc. 2006, 128, 2530–2531. [CrossRef] 20. Small, D.; Zaitsev, V.; Jung, Y.; Rosokha, S.V.; Head-Gordon, M.; Kochi, J.K. Intermolecular pi-to-pi bonding between stacked aromatic dyads. Experimental and theoretical binding energies and near-IR optical transitions for phenalenyl radical/radical versus radical/cation dimerizations. J. Am. Chem. Soc. 2004, 126, 13850–13858. [CrossRef] 21. Kolb, B.; Kertesz, M.; Thonhauser, T. Binding Interactions in Dimers of Phenalenyl and Closed-Shell Analogues. J. Phys. Chem. A 2013, 117, 3642–3649. [CrossRef] 22. Koutentis, P.A.; Chen, Y.; Cao, Y.; Best, T.P.; Itkis, M.E.; Beer, L.; Oakley, R.T.; Cordes, A.W.; Brock, C.P.; Haddon, R.C. Perchlorophenalenyl radical. J. Am. Chem. Soc. 2001, 123, 3864–3871. [CrossRef] 23. Uchida, K.; Hirao, Y.; Kurata, H.; Kubo, T.; Hatano, S.; Inoue, K. Dual association modes of the 2,5,8-tris(pentafluorophenyl)phenalenyl radical. Chem. Asian J. 2014, 9, 1823–1829. [CrossRef] 24. Blanchard, M.D.; Hughes, R.P.; Concolino, T.E.; Rheingold, A.L. π-Stacking between Pentafluorophenyl and Phenyl Groups as a Controlling Feature of Intra- and Intermolecular Crystal Structure Motifs in Substituted Ferrocenes. Observation of Unexpected Face-to-Face Stacking between Pentafluorophenyl Rings. Chem. Mater. 2000, 12, 1604–1610. [CrossRef] 25. Smith, C.E.; Smith, P.S.; Thomas, R.L.; Robins, E.G.; Collings, J.C.; Dai, C.; Scott, A.J.; Borwick, S.; Batsanov, A.S.; Watt, S.W.; et al. Arene-perfluoroarene interactions in crystal engineering: Structural preferences in polyfluorinated tolans. J. Mater. Chem. 2004, 413–420. [CrossRef] Molecules 2019, 24, 665 13 of 13

26. Mou, Z.; Uchida, K.; Kubo, T.; Kertesz, M. Evidence of σ- and π-Dimerization in a Series of Phenalenyls. J. Am. Chem. Soc. 2014, 136, 18009–18022. [CrossRef] 27. Uchida, K.; Mou, Z.; Kertesz, M.; Kubo, T. Fluxional σ-Bonds of the 2,5,8-Trimethylphenalenyl Dimer: Direct Observation of the Sixfold σ-Bond Shift via a π-Dimer. J. Am. Chem. Soc. 2016, 138, 4665–4672. [CrossRef] [PubMed] 28. Rakus, K.; Verevkin, S.P.; Schätzer, J.; Beckhaus, H.-D.; Rüchardt, C. Thermolabile Hydrocarbons, 33. Thermochemistry and Thermal Decomposition of 9,90-Bifluorenyl and 9,90-Dimethyl-9,90-bifluorenyl—The Stabilization Energy of 9-Fluorenyl Radicals. Berichte der Deutschen Chemischen Gesellschaft 1994, 127, 1095–1103. [CrossRef] 29. Font-Sanchis, E.; Aliaga, C.; Focsaneanu, K.-S.; Scaiano, J.C. Greatly attenuated reactivity of nitrile-derived carbon-centered radicals toward oxygen. Chem. Commun. 2002, 2, 1576–1577. [CrossRef] 30. Tian, Y.; Uchida, K.; Kurata, H.; Hirao, Y.; Nishiuchi, T.; Kubo, T. Design and Synthesis of New Stable Fluorenyl-Based Radicals. J. Am. Chem. Soc. 2014, 136, 12784–12793. [CrossRef] 31. Nishiuchi, T.; Ito, R.; Takada, A.; Yasuda, Y.; Nagata, T.; Stratmann, E.; Kubo, T. Anthracene-Attached Persistent Tricyclic Aromatic Hydrocarbon Radicals. Chem. Asian J. 2019.[CrossRef] 32. Frenette, M.; Aliaga, C.; Font-Sanchis, E.; Scaiano, J.C. Bond Dissociation Energies for Radical Dimers Derived from Highly Stabilized Carbon-Centered Radicals. Org. Lett. 2004, 6, 2579–2582. [CrossRef] 33. Zeng, Z.; Sung, Y.M.; Bao, N.; Tan, D.; Lee, R.; Zafra, J.L.; Lee, B.S.; Ishida, M.; Ding, J.; López Navarrete, J.T.; et al. Stable tetrabenzo-Chichibabin’s hydrocarbons: Tunable ground state and unusual transition between their closed-shell and open-shell resonance forms. J. Am. Chem. Soc. 2012, 134, 14513–14525. [CrossRef] [PubMed] 34. Sabacky, M.J.; Johnson, C.S.; Smith, R.G.; Gutowsky, H.S.; Martin, J.C. Triarylmethyl Radicals. Synthesis and Electron Spin Resonance Studies of Sesquixanthydryl Dimer and Related Compounds. J. Am. Chem. Soc. 1967, 89, 2054–2058. [CrossRef] 35. Nishiuchi, T.; Aibara, S.; Kubo, T. Synthesis and Properties of a Highly Congested Tri(9-anthryl)methyl Radical. Angew. Chem. Int. Ed. 2018, 57, 16516–16519. [CrossRef] 36. Vijaykumar, G.; Pariyar, A.; Ahmed, J.; Shaw, B.K.; Adhikari, D.; Mandal, S.K. Tuning the redox non-innocence of a phenalenyl ligand toward efficient nickel-assisted catalytic hydrosilylation. Chem. Sci. 2018, 9, 2817–2825. [CrossRef][PubMed] 37. Ai, X.; Evans, E.W.; Dong, S.; Gillett, A.J.; Guo, H.; Chen, Y.; Hele, T.J.H.; Friend, R.H.; Li, F. Efficient radical-based light-emitting diodes with doublet emission. Nature 2018, 563, 536–540. [CrossRef] 38. Mas-Torrent, M.; Crivillers, N.; Rovira, C.; Veciana, J. Attaching Persistent Organic Free Radicals to Surfaces: How and Why. Chem. Rev. 2012, 112, 2506–2527. [CrossRef]

Sample Availability: Samples of the compounds are not available from the authors.

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