DOI: 10.1002/chem.200900968

Multi-Input/Multi-Output Molecular Response System Based on theDynamic Redox Behavior of 3,3,4,4-Tetraaryldihydro[5]helicene Derivatives:Reversible Formation/Destruction of Chiral Fluorophore and Modulation of

Chiroptical Properties by Solvent Polarity

Takanori Suzuki,*[a] Yusuke Ishigaki,[a] Tomohiro Iwai,[a] Hidetoshi Kawai,[a, b]

Kenshu Fujiwara,[a] Hiroshi Ikeda,[c, d] Yusuke Kano,[c] and Kazuhiko Mizuno[c, d]

Dedicated to Professor Takashi Tsuji on the occasion of his 70th birthday

Introduction

While multi-input/multi-output response systems[1] are rare,they have attracted considerable attention from the view-point of potential applications in molecular sensing andswitches.[2] Since multi-input systems are prototypes of mo-lecular-level logic operators,[3] the addition of a multi-outputresponse would endow such systems with the ability to actas parallel operating logic elements (“molecular CPU” [4]).In this study, we examined novel electrochromic systemsthat incorporate a chiral fluorophore, which were designedaccording to the following concept to realize a three-way-output response system.

Dihydro[5]helicene (dihydrodibenzo ACHTUNGTRENNUNG[c,g]phenanthrene) isa rigid p framework that exhibits strong fluorescence.[5]

When this skeleton can be formed/destroyed by the applica-tion of an external stimulus, this system can serve as a fluo-rescence ON/OFF switch.[6] By adopting a “dynamic-redox”protocol[7] based on reversible C�C bond formation/cleav-age upon electron transfer, the title electron donors 1 withthis fluorophore could be produced from 1,1’-binaphthyl-2,2’-diyl(diarylcarbenium)s (22+), which shows strong ab-sorption in the visible region that is characteristic of triaryl-

Abstract: 3,3,4,4-Tetaaryldihydro[5]he-licenes (1) and 1,1’-binaphthyl-2,2’-diyl-bis(diarylcarbenium)s (22+) can be re-versibly interconverted upon electrontransfer, which is accompanied by avivid color change (electrochromism)as well as by the formation/cleavage ofa C�C bond (“dynamic redox behav-ior”). Because only the neutral donor 1exhibits strong fluorescence, electro-chemical input can further modify the

fluorescent properties of the pair. Dueto the configurational stability of thehelicity in 1 and axial chirality in 22+ ,the redox reaction of optically purematerial proceeds stereospecifically,which induces a chiroptical changesuch as circular dichroism (CD) as an

additional output. The CD spectra ofdications 22+ exhibit solvent dependen-cy (chiro-solvatochromism), which isaccompanied by solvatochromic behav-ior based on the p–p interaction of thetwo cationic chromophores as well ascoordinative interaction of the Lewisbasic solvent to the Lewis acidic triar-ylcarbenium moieties. Thus, the presentsystem is endowed with multi-inputfunctionality for modifying multipleoutput signals.

Keywords: chirality · fluorescence ·molecular response systems · redoxchemistry · solvatochromism

[a] Prof. T. Suzuki, Y. Ishigaki, T. Iwai, Dr. H. Kawai, Prof. K. FujiwaraDepartment of Chemistry, Faculty of ScienceHokkaido UniversityKita 10, Nishi 8, Kita-ku, Sapporo, 060-0810 (Japan)Fax: (+81) 11-706-2714E-mail : tak@sci.hokudai.ac.jp

[b] Dr. H. KawaiPRESTO, Science and Technology Agency (Japan)

[c] Prof. H. Ikeda, Y. Kano, Prof. K. MizunoDepartment of Applied ChemistryGraduate School of EngineeringOsaka Prefecture UniversitySakai, Osaka, 599-8531 (Japan)

[d] Prof. H. Ikeda, Prof. K. MizunoThe Research Institute for Molecular Electronic Device (RIMED)Osaka Prefecture UniversitySakai, Osaka, 599-8531 (Japan)

Supporting information (experimental details of new-compound prep-aration; cyclic voltammograms of 1 a and 2 a ACHTUNGTRENNUNG[(BF4)2]; solvatochrom-ism of (R)-2 a2+ and (R)-2 b2+ ; scattering plots of UV/Vis data: lmax

(2c2+) versus several solvent parameters; electrochromic behavior of1a, 1c, 3 b, and 3 c ; reaction scheme under photoinduced electron-transfer conditions) for this article is available on the WWW underhttp://dx.doi.org/10.1002/chem.200900968. CCDC 722229 and 722230contain the supplementary crystallographic data for this paper. Thesedata can be obtained free of charge from The Cambridge Crystallo-graphic Data Centre by logging on to www.ccdc.cam.ac.uk/data_request/cif.

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methylium dye.[8] Due to the configurational stabilities inboth the neutral (helicity) and dicationic (axial chirality)states, interconversion of the redox pairs would proceed ste-reospecifically ((P)-1/(S)-22+ ; (M)-1/(R)-22+ ; Scheme 1).

Thus, chiroptical changes such as circular dichroism (CD)would also be available as an output.[9] The asymmetric ele-ments of helicity and axial chirality in a biaryl-related skele-ton are suitable for obtaining huge CD signals[10] through anexciton-coupling mechanism.[11] In this way, the opticallyactive pair should be able to exhibit three kinds of spectralchanges in response to an electrochemical input (Scheme 2).

The methoxyphenyl group is selected as an aryl group inanticipation of strong coloration and sufficient stability forthe cationic moiety in 2a2+ , as exemplified by bis(4-methoxy-phenyl)phenylcarbenium (labs =514 nm (loge=4.87); pKR+ =

�1.24[12]). At the same time, the cationic center wouldbehave as a Lewis acid, so that coordinative interaction witha Lewis base might affect the spectral properties of 22+ .[13]

Thus, the dications should exhibit solvatochromic behavior

if we use a range of solvents with the different coordinatingproperties. On the other hand, the solvent-dependent associ-ation of triarylmethyliums is a well-known process.[14] Whenp–p interaction between two chromophores in 22+ is modi-fied by the solvent polarity, this may also change the UV/Vis and CD spectral properties. To attain high solubility inless-polar solvents such as benzene or cyclohexane, dications2 b2+ and 2 c2+ with long alkoxyl chains were also includedin this study.

In this report, we first describe the preparation and X-raystructures of chiral pairs of 1/22+ . Configurationally unstablebiphenyl derivatives 3/42+ were also included as referencecompounds. The redox behavior as well as the detailedmechanism of the electrochemical interconversion are thendescribed, as determined by laser flash photolysis of 3 to ob-serve the transient absorption of cation radical intermediate4+ C. Next, the electrochemical response is studied in detailby using three kinds of spectroscopy. Finally, the solvato-chromic behavior as well as the solvent dependency of theCD spectrum are also demonstrated.

Results and Discussion

Preparation and X-ray structure : (R)-/(S)-1,1’-Binaphtholswith configurationally stable axial chirality were selected asthe starting material for 3,3,4,4-tetrakis(4-methoxyphenyl)-3,4-dihydro[5]helicenes ((M)-/(P)-1 a). Thus, both enantio-mers of binaphthol were converted, respectively, to opticallypure methyl esters (R)-/(S)-5 by means of bis ACHTUNGTRENNUNG(triflate)through Pd-catalyzed CO insertion[15] under atmosphericpressure.[16] Diol (R)-6 a in 73 % yield was obtained from(R)-5 by the four-time addition of 4-methoxyphenyllithium.Upon treatment with trimethylsilyl perchlorate (TMSClO4)in (CF3)2CHOH (Ichikawa�s method),[17] the chiral diol wasconverted into dication (R)-2 a2+ , which was directly re-duced with Zn powder to give optically pure (M)-1 a ([a]24

D =

�2318 (c=0.47 in CHCl3)) as colorless crystals in 98 % yieldover two steps (Scheme 3a). When ester (R)-5 was reactedwith 4-octyloxyphenyllithium or 4-hexadecyloxyphenyllithi-um,[18] (M)-1 b,c with four long alkyl chains were obtained inrespective yields of 79 and 68 %. They also exhibit a hugeoptical rotatory power ([a]23

D =�1538 (c=0.35 in CHCl3) for1 b ; [a]26

D =�1098 (c=0.76 in CHCl3) for 1 c), which is char-acteristic of helicene derivatives.[19] When the antipode ofthe ester (S)-5 was used as a starting material, helicaldonors with P helicity (P)-1 a–c were prepared in a similarmanner by means of diols (S)-6 a–c in 79–85 % yields overthree steps.

Racemic donors rac-1 a–c were obtained in 97–100 %yield over two steps from the corresponding racemic diolsrac-6 a–c, which were more conveniently obtained fromcommercially available rac-2,2-dibromo-1,1’-binaphthylthrough subsequent treatment with nBuLi followed by 4,4’-dialkoxybenzophenones[20] (Scheme 3b). As shown in thepreliminary communication,[21] a similar protocol was firstused to prepare 9,9,10,10-tetrakis(4-methoxyphenyl)dihydro-

Scheme 2. Three-way-output molecular response system.

Scheme 1. Dynamic redox pairs with a chiral fluorophore (a : n=1; b : n=

8; c : n=16).

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phenanthrene 3 a and related compounds, which readily un-dergo a ring-flip with an inversion of helicity. In this study,tetrakis(4-octyloxyphenyl)- and tetrakis(4-hexadecyloxyphe-nyl)dihydrophenanthrenes 3 b,c were also prepared in 99and 93 % yield as reference compounds from diols 7 b,c withfour long alkyl chains, which in turn were obtained by thereactions of 2,2’-dilithiobiphenyl and 4,4’-dialkoxybenzophe-nones. The high-yield formation of 1 a–c and 3 b,c from thecorresponding diols 6 a–c and 7 b,c clearly shows that reduc-tive generation of the dihydro[5]helicene/dihydrophenan-threne skeleton from the intermediary dicationic species(22+/42+) proceeds very smoothly.

At the same time, it is clear that the dications were pro-duced very efficiently from the diols upon treatment withTMSClO4. The binaphthylic dications 2 a2+–2 c2+ were notisolated as ClO4

� salts from the acidic mixture at thatpoint,[22] but rather were more conveniently isolated asSbCl6

� salts in high yield by oxidation of the dihydro[5]heli-cene donors 1 a–c with two equivalents of [(4-BrC6H4)3NC]-ACHTUNGTRENNUNG[SbCl6] (Scheme 4). Oxidative cleavage of the C�C bondproceeds while preserving the chiral sense, and thus only

(R)- and (S)-22+ were generated from (M)- and (P)-1, re-spectively. All of the dications ((R)-/(S)-/rac-2 a–2 c-ACHTUNGTRENNUNG[(SbCl6)2]) were quantitatively converted to the correspond-ing dihydrohelicenes ((M)-/(R)-/rac-1 a–c) upon treatmentwith Zn powder in THF.

The detailed structural features of tetraaryldihydro[5]heli-cene were revealed by X-ray analysis of (M)-1 a (Figure 1a).The dihedral angle of the two naphthalene units is 49.4(1)8,which is much smaller than that in the precursor dication(vide infra) due to the formation of a new Csp3�Csp3 bondupon reduction. This bond length is 1.646(5) �, which is

Scheme 4. Interconversion of 1a–c and 2a2+–2 c2+ .

Scheme 3. a) Preparation of optically pure (M)-/(P)-1a–c by means of(R)-/(S)-diol 6 a–c, and b) preparation of racemic diols rac-6 a–c.

Figure 1. ORTEP drawings of a) (M)-1a and b) (R)-2 a2+ in the (R)-2a-ACHTUNGTRENNUNG[(SbCl6)2] salt as determined by low-temperature X-ray analyses.

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T. Suzuki et al.

much greater than the standard value of 1.54 �, but is typi-cal for sterically congested hexaphenylethane derivatives.[23]

Two methoxyphenyl groups at the pseudo-equatorial posi-tions are located in proximity and face each other over thelong C�C bond (the shortest C�C contact: 3.14(1) �). Theother two at the pseudo-axial positions face outward.

Figure 1b shows the ORTEP drawing of (R)-2 a2+ in theSbCl6

� salt obtained by low-temperature X-ray analysis. Theabsolute configuration of the dication was confirmed by theanomalous dispersion of X-ray (Flack parameter: x=

0.000(17)). The dihedral angle of the two naphthalene unitsis 68.5(1)8 with a separation of 3.54(1) � between the twocarbenium centers. A much closer C�C contact (3.29(1) �)was observed between the two methoxyphenyl groups facingeach other (dihedral angle: 10.8(2)8), which reflects the ef-fective p–p overlap of two diarylcarbenium units. Thebroadness of the first band in the electronic absorption(labs = 534 nm in CH2Cl2) in 2 a2+ may be related to Davy-dov splitting through p–p interaction, although this bandmay represent several transitions.

Redox behavior and mechanism of interconversion : Theredox potentials of newly prepared electron donors 1 a–cwere measured by cyclic voltammetry in CH2Cl2 (23 8C, scanrate 100 mV s�1) and the oxidation potentials (Eox) are sum-marized in Table 1 along with those of reference compounds

3 a–c. The electrochemical oxidation of 1 occurs at approxi-mately +1.3 V versus SCE, which is slightly less positivethan Eox of 3 (ca. + 1.5 V), probably due to the electron-do-nating properties of the binaphthyl core. The oxidation pro-cess is irreversible since the corresponding reduction peakwas observed in the far cathodic region (ca. + 0.2 V), whichwas assigned to the reduction process of the bond-dissociat-ed dication, as confirmed by the independent measurementsof 22+ (and 42+) under similar conditions (Figures S1a and bin the Supporting Information). Such separation of redoxpeaks in the voltammogram is a characteristic feature of dy-namic redox systems,[7] and provides electrochemical bista-bility for the redox pairs of 1/22+ (and 3/42+).

Both peaks correspond to the two-electron transfer pro-cess, and there are no other peaks in the voltammograms.The negligible steady-state concentration of the intermedi-ary cation radicals 1+ C/2+ C (and 3+ C/4+ C) can be accounted forby assuming the reaction mechanism shown in Scheme 5.[21b]

In the oxidation process of the donors, the as-preparedcation radicals 1+ C (and 3+ C) would undergo rapid long-bondfission[24] to give more stable species 2+ C (and 4+ C), in whichthe unpaired electron and positive charge can be delocalizedover each of the triarylmethane units. The lifetimes of 2+ C

(and 4+ C) would also be very short during the oxidation pro-cess because their oxidation potentials (ca. +0.2 V) must befar less positive than those of the neutral donors 1 (and 3).In the reduction process, electron capture at the two cationicsites in 22+ (and 42+) would proceed in a stepwise mannerwith marginal separation between the two reduction poten-tials (Ered

1 , Ered2 ) due to electronic interaction of the two chro-

mophores through the face-to-face overlap.[6h, 21a,25] Due tothe facile disproportionation of 2+ C into 22C/22+ (and 4+ C into42C/42+) as well as the rapid bond formation of 22C to 1 (and42C to 3), the steady-state concentration of the intermediaryspecies would also be negligible upon reduction.

The above idea regarding the mechanism shown inScheme 5 was supported by the following experimental re-sults. For the reduction process, voltammetric analyses werecarried out at low temperature (�78 8C) with a fast scanrate (5 V s�1) in CH2Cl2, which supported the sequentialtwo-step one-electron reduction (Ered

1 =++0.09 V; Ered2 =

�0.05 V for 2 a2+) by retarding the disproportionation pro-cess (Figure S1c in the Supporting Information). In thosevoltammograms, the oxidation waves remained unchanged,suggesting that a much faster method is necessary to eluci-date the mechanism of the oxidation process. Therefore, forthis purpose, photoinduced electron-transfer (PET) reac-tions of 3 a,b were conducted with nanosecond-absorptionspectroscopy on laser flash photolysis. A cosensitizingsystem that used an N-methylquinolinium tetrafluoroborate([NMQ] ACHTUNGTRENNUNG[BF4], sensitizer, 1.0 mm)–toluene (cosensitizer,1.0 m) couple in aerated MeCN at ambient temperature wasused, since this system allows us to directly detect the inter-mediary cation radicals in a one-electron oxidation process(Scheme S1 in the Supporting Information).[26]

Scheme 5. Mechanism of interconversion.

Table 1. Redox potentials[a] of dihydro[5]helicene-donors 1a–c and bi-naphthylic dications 2 a2+–2 c2+ measured in CH2Cl2 along with those ofrelated compounds.

n Eox [V] Ered [V]

1a 1 +1.29 –1b 8 +1.32 –1c 16 +1.32 –3a 1 +1.483b 8 +1.503c 16 +1.532a2+ 1 – + 0.232b2+ 8 – + 0.212c2+ 16 – + 0.214a2+ + 0.214b2+ + 0.184c2+ + 0.17

[a] E versus SCE, scan rate 100 mV s�1, 0.1 m Bu4NBF4 as a supportingelectrolyte, Pt electrode, 298 K.

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FULL PAPERMulti-Input/Multi-Output Molecular Response System

At a delay time of 100 ns after the laser excitation of[NMQ]ACHTUNGTRENNUNG[BF4] with the methoxy derivative 3 a (10�5

m) using a100 ns pulse from the yttrium aluminum garnet (YAG) laser(335 nm), transient absorption spectra were observed, asshown in Figure 2. A characteristic band with labs at 523 nm

is assigned to the transient 4 a+ C, formed by rapid bondcleavage of 3 a+ C, since this band corresponds to the intensechromophore of bis(4-methoxyphenyl)phenylcarbenium.Absorption of the bis(4-methoxyphenyl)phenylmethyl partmust be much less than that of the cationic chromophore,[21b]

which would explain its absence in the spectrum. A similartransient spectrum with labs at 527 nm assigned to 4 b+ C wasobserved with the octyloxy derivative 3 b but appearedmuch slower than 4 a+ C (Figure 2, inset), which suggested theslower ring-opening of 3 b+ C than of 3 a+ C. This difference inreactivity between 3 a+ C and 3 b+ C may be accounted for bythe solvatophobic/fastener effects,[27] which give two octy-loxy groups on the vicinal phenyl groups of 3 b+ C with an at-tracting force.

Three-way-output response : Figure 3 shows the continuouschanges in the three kinds of spectra of (M)-1 b upon con-stant-current oxidation in CH2Cl2. The colorless solutiongradually turned deep red with a steady increase in absorp-tions at 429 and 541 nm in the UV/Vis spectrum (Figure 3a),which are assigned to dication 2 b2+ . Complete regenerationof 1 b with decoloration was attained by reverse electrolysisof the as-prepared dicationic solution. Similar electrochro-mic behavior was also observed with the use of other donors1 a,c/3 b,c (Figure S3 in the Supporting Information). Sinceonly the neutral donors 1 a–c, but not dications 2 a2+–2 c2+ ,have the fluorophore of dihydro[5]helicene (lem =407, 408,and 409 nm and y= 0.18, 0.16, and 0.18 for 1 a, 1 b, and 1 c,respectively, in CH2Cl2), electrochemical oxidation to thecorresponding binaphthylic dication induced a steady de-crease in the fluorescence intensity (Figure 3b).

Both the optically pure donors and dications ((M)-1/(R)-22+ and (P)-1/(S)-22+) exhibit strong CD signals thanks tothe exciton coupling, but in different wavelength regions.Thus, electrochemical input induced drastic changes in theCD spectrum, as shown in Figure 3c. The emerging couplet

Figure 2. Transient absorptions of cation radical species generated byphotoinduced electron-transfer reactions of 3a and 3 b by laser flash pho-tolytic conditions. Inset shows the time-course of the absorption intensity.

Figure 3. Changes (every 20 min) in a) UV/Vis, b) fluorescence, andc) CD spectra upon constant current (24 mA) electrolysis of (M)-1 b inCH2Cl2 containing 0.05 m Bu4NBF4 as a supporting electrolyte.

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T. Suzuki et al.

with a large amplitude (A>100) in the longer-wavelengthregion upon electrolysis is notable. This represents a suc-cessful demonstration of a three-way-output molecular re-sponse system based on the reversible formation/destructionof a configurationally stable chiral fluorophore of dihy-dro[5]helicene upon electron transfer.[28]

Solvatochromic and chiral-solvatochromic behavior: Triaryl-methylium dyes often form intermolecularly associated dyador higher oligomeric species, depending on the nature of thesolvent, such as its polarity, which causes changes in theirspectral properties. As shown by the X-ray structure of the(R)-2 a2+ salt, the binaphthylic dications adopt a conforma-tion in which two dye chromophores can electronically inter-act with each other through p–p overlap. Thus, the dications22+ would be associated with different degrees of intramo-lecular association of dye chromophores in various solventsto exhibit solvatochromic behavior. This is indeed the case,and the electronic spectra of dications 22+ exhibit a batho-chromic shift of the first band when the solvent is changedfrom MeCN to CH2Cl2, benzene, and cyclohexane (Fig-ure 4a), with a color change from red to purple. The strongabsorption band of hexadecylated dication 2 c2+ at 532 nmin MeCN is shifted to 543 nm in cyclohexane, and is accom-panied by new shoulder absorptions at 500 and 592 nm. In

other solvents with intermediate polarity between MeCNand cyclohexane, the first band of 2 c2+ appeared at the po-sitions expected from their dielectric constants (Table 2, Fig-ures S2a and c in the Supporting Information).

Such behavior can be accounted for by considering thelower stabilization of the cationic chromophores in 22+ bysolvation in the nonpolar solvents, where p–p interactionthrough the effective overlap of two chromophores is impor-tant (Scheme 6). On the other hand, the coordinative inter-

action with a Lewis basic solvent would be a dominantfactor in a polar solvent, and this would reduce the contribu-tion of the p–p interaction. Accordingly, the degree of p–p

overlap would be greater in nonpolar solvents than in polarsolvents, which would induce greater Davydov splitting andmore low-energy absorption in the former. The coordinativeinteraction at the cationic center raises the energy level ofthe LUMO, which might also cause the blueshift of the ab-sorption in Lewis basic solvents.

The CD spectra of optically pure 22+ are also changed bysolvent polarity (Table 3, Figure 4b; Figures S2b and d in the

Figure 4. a) UV/Vis and b) CD spectra of the (R)-2c ACHTUNGTRENNUNG[(SbCl6)2] salt mea-sured in various solvents.

Table 2. UV/Vis spectral data[a] of 2a–2c ACHTUNGTRENNUNG[(SbCl6)2] in various solvents.

Dielectric constant 2a2+ 2 b2+ 2c2+

MeCN 37.5 524 ACHTUNGTRENNUNG(4.71) 532 ACHTUNGTRENNUNG(4.82) 532 ACHTUNGTRENNUNG(4.87)CH2Cl2 9.1 534 ACHTUNGTRENNUNG(4.83) 541 ACHTUNGTRENNUNG(4.86) 542 ACHTUNGTRENNUNG(4.88)CHCl3 4.9 – 543 ACHTUNGTRENNUNG(4.84)

590sh ACHTUNGTRENNUNG(4.50)543 ACHTUNGTRENNUNG(4.81)590sh ACHTUNGTRENNUNG(4.47)

benzene 2.28 – 543 ACHTUNGTRENNUNG(4.82)590sh ACHTUNGTRENNUNG(4.49)

543 ACHTUNGTRENNUNG(4.77)590sh ACHTUNGTRENNUNG(4.44)

cyclohexane 2.05 – – 543 ACHTUNGTRENNUNG(4.78)592sh ACHTUNGTRENNUNG(4.50)

[a] Data of absorption maxima (labs and e) and shoulders for the firstband.

Scheme 6. Plausible mechanism of solvatochromism.

Table 3. CD spectral data[a] of 2 a–2c ACHTUNGTRENNUNG[(SbCl6)2] in various solvents.

Dielectric constant 2a2+ 2 b2+ 2 c2+

MeCN 37.5 569 ACHTUNGTRENNUNG(+123)488 ACHTUNGTRENNUNG(�24.9)

574 ACHTUNGTRENNUNG(+164)525 ACHTUNGTRENNUNG(�23.4)

573 ACHTUNGTRENNUNG(+182)525 ACHTUNGTRENNUNG(�26.8)

CH2Cl2 9.1 577 ACHTUNGTRENNUNG(+147)496 ACHTUNGTRENNUNG(�28.9)

584 ACHTUNGTRENNUNG(+174)533 ACHTUNGTRENNUNG(�22.2)

583 ACHTUNGTRENNUNG(+183)531 ACHTUNGTRENNUNG(�22.3)

CHCl3 4.9 – 592 ACHTUNGTRENNUNG(+215)544 ACHTUNGTRENNUNG(�60.2)

593 ACHTUNGTRENNUNG(+199)544 ACHTUNGTRENNUNG(�55.3)

benzene 2.28 – 594 ACHTUNGTRENNUNG(+231)546 ACHTUNGTRENNUNG(�68.4)

595 ACHTUNGTRENNUNG(+201)545 ACHTUNGTRENNUNG(�59.2)

cyclohexane 2.05 – – 595 ACHTUNGTRENNUNG(+242)549 ACHTUNGTRENNUNG(�112)

[a] Data of extrema (lext and De) of Cotton effects for the first band.

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FULL PAPERMulti-Input/Multi-Output Molecular Response System

Supporting Information).[29] Along with the above explana-tion for the absorption-band shift, a more distinct and largercouplet is observed for (M)-2 c2+ in cyclohexane (lext (De)=

595 (+242), 549 nm (�112 mol�1 dm3 cm�1)), whereas Cottoneffects in MeCN are much weaker. Thus, chiroptical proper-ties are also modified by solvent polarity (chiro-solvatochro-mism),[1b,c,30, 31] and this provides the multi-input (electric po-tential and solvent polarity) and multi-output (UV/Vis, fluo-rescence, CD) functionalities in the present molecular sys-tems.

Conclusion

The present results demonstrate that the reversible forma-tion/destruction of a chiral fluorophore in redox reactions isthe key to realizing novel three-way-output response sys-tems based on the interconvertible colorless dihydro[5]heli-cenes 1 and the corresponding binaphthylic dications 22+ .Since we adopted a “dynamic redox” protocol, the steady-state concentration of open-shell species is negligible, whichallows high reversibility[7,32] in their mutual transformation.The solvent dependency of the UV/Vis and CD spectra for22+ endows the system with an additional function, al-though, unlike the electrical potential, solvent polarity (as inthe case of pH and heat) cannot be applied at molecular res-olution as an input signal. Still, a sensitivity to several stimu-li is an essential feature for the further development of mo-lecular response systems into “parallel-operating molecularlogic gates”.

Acknowledgements

The authors express sincere thanks to Prof. Takashi Ooi at the Depart-ment of Applied Chemistry, Graduate School of Engineering, NagoyaUniversity for his valuable and helpful suggestions. The financial supportfrom the Global COE Program by MEXT “Catalysis as the Basis for In-novation in Materials Science,” Hokkaido University is gratefully ac-knowledged. We thank Prof. Tamotsu Inabe (Department of Chemistry,Faculty of Science, Hokkaido University) for the use of facilities to ana-lyze the X-ray structures. Elemental analyses were done at the Center forInstrumental Analysis of Hokkaido University. Mass spectra were mea-sured by Mr. Kenji Watanabe and Dr. Eri Fukushi at the GC-MS &NMR Laboratory (Faculty of Agriculture, Hokkaido University).

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[16] The CO-insertion reaction proceeds not only under pressurized con-ditions but also under atmospheric pressure. A typical procedure isas follows: MeOH (50 equiv) and iPr2EtN (4.4 equiv) were added toa mixture of (S)-bis ACHTUNGTRENNUNG(triflate) (16.5 g), Pd ACHTUNGTRENNUNG(OAc)2 (25 mol %), anddppp (25 mol %) in DMSO, and the mixture was stirred at 70 8C for3 d under a CO atmosphere (balloon). The resulting suspension was

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T. Suzuki et al.

cooled to 23 8C and diluted with EtOAc. The whole mixture waswashed with 1n HCl, sat. NaHCO3 (aq.), and brine, and dried overanhydrous MgSO4. After filtration, the solvent was concentratedunder reduced pressure. The residue was purified by column chro-matography on silica gel (CH2Cl2/hexane 1:1) to give (S)-5 (9.39 g,85%).

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[28] Dihydro[5]helicenes with two electron-donating spiro ACHTUNGTRENNUNG(thio)xantheneunits at 3,4-positions were previously studied as a multi-output mo-lecular response system (ref. [25]). Because those helicenes are non-fluorescent, they only afforded UV/Vis and CD signals as outputs.In another system forming the dihydro[5]helicene unit attached withcationic chromophores upon oxidation, its fluorescence is complete-ly quenched by intramolecular charge shift (ref. [10f]).

[29] Spectra were measured by using the (SbCl6�)2 salts. Similar results

were obtained when the (BF4�)2 salts were used.

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