Notes

A First Study of Langmuir-Blodgett Films of1-[N-(n-Hexadecyl)-4-pyridinio]-2-[5-(dicyano-

methanidyl)thien-2-yl]ethene

Riccardo Ricceri,† Alessandro Abbotto,‡ Antonio Facchetti,‡Giorgio A. Pagani,‡ and Gabriella Gabrielli*,†

Department of Chemistry, University of Florence,via G. Capponi, 9, I-50121 Firenze, Italy, and Department ofOrganic and Industrial Chemistry, University of Milan,

via Golgi 19, I-20133 Milano, Italy

IntroductionThe dye 1-[N-(n-hexadecyl)-4-pyridinio)-2-[5-(dicya-

nomethanidyl)thien-2-yl]ethene (C16H33-PDCNTE), 1, isa new zwitterionic compound belonging to a new class ofnegativelysolvatochromicpush-pullmolecules containinga negatively charged dicyanomethanide as donor groupand a positively charged N-alkylpyridinium as acceptorgroup linked via an extended π -electron bridge.1 In aprevious work, we have shown the interesting propertiesof 4-[5-(dicyanomethanidyl)thien-2-yl]-N-(n-hexadecyl)py-ridinium (C16H33-PDCNT),2 another negatively solvato-chromic betaine dye (differing fromC16H33-PDCNTEwiththe absence of the ethene bridge between the pyridiniumand thiophene moieties) which belongs to this class offunctional dyes, which seems to be very promising forsecond-order nonlinear optical effects that can be ac-complished for example by noncentrosymmetric Lang-muir-Blodgett (LB) films.3-5 LB films of C16H33-PDCNTprepared according to a proper procedure show a sharpphotobleachable UV-vis absorption band that has in-teresting applications in a field of great interest such asthe optical data storage.3-14 In this work, we show thatalsoLB films ofC16H33-PDCNTEcanbe prepared in orderto showasharpphotobleachableUV-vis absorptionband.An LB film containing one monolayer of C16H33-PDCNTandonemonolayer ofC16H33-PDCNTEwasshowntoallow

for efficient multifrequency optical data storage becausethe two sharp UV-vis absorption bands of the twosubstances could be selectively photobleached even witha peak-to-peak separation of less than 50 nm.15

Experimental Details

The compoundC16H33-PDCNTEwas synthesized according tothe procedure described elsewhere.1 Organic solvents were ofanalytical gradeandsuppliedbyFluka. Asolution in chloroform/ethanol 1:1 v/v was used for the spreading: the typical concen-tration of dye 1 in the spreading solution was 0.7 mg/mL, andfreshly sublimed 1,4-diazabicyclo[2.2.2]octane (DABCO) (sup-plied by Aldrich) was added to the spreading solution in orderto avoid the photochemical degradation of the dye.2 Water waspurified with a Millipore Milli-RO 6 and aMilli-Q water system(organex system) obtaining a specific resistance g18 MΩ‚cm.Surfacepressurearea isothermswere recordedusingaKSV3000balance with symmetrical compression of the monolayer at acompression rate of 4mm/min. Typically, 200µLof thedescribedsolution were spread on the aqueous subphase (the procedurefor the spreading is identical to the one reported for C16H33-PDCNT2). LB films were transferred at π ) 15 mN/m (T ) 291K) onto quartz slides using the KSV 3000 balance for Y-type LBfilms. Z-type LB films were obtained using a KSV 5000 balanceat the same conditions of subphase temperature and surfacepressure (theKSV5000balance isa two-compartmentsLBtroughwithasymmetrical compressionof themonolayer).2 Immediatelyafter preparation, LB films (whichwere prepared in the absenceof light2)were stored in thedark to avoidphotodegradation.UV-vis absorption spectra were recorded using a Perkin-ElmerLambda 5 spectrophotometer. Infrared spectra were obtainedin the 4000-2000 cm-1 regionbyaBio-RadFTS-40 spectrometerat a resolution of 2 cm-1: 128 interferograms were collected for30-layer LB films, transferred onto quartz slides, of the dye.

Results and Discussion

Spreading Monolayers. Figure 1 shows the π vs A

* Author to whom correspondence should be addressed.† University of Florence.‡ University of Milan.(1) Abbotto, A.; Bradamante, S.; Facchetti, A.; Pagani, G. A. J. Org.

Chem. 1997, in press.(2) Ricceri, R.; Abbotto, A.; Facchetti, A.; Pagani, G. A.; Gabrielli, G.

Langmuir 1997, 13, 3434.(3) Ashwell, G. J.; Dawnay, E. J. C.; Kuczynski, A. P.; Szablewski,

M.; Sandy, I. M.; Bryce, M. R.; Grainger, A. M.; Hasan, M. J. Chem.Soc., Faraday Trans. 1990, 86, 1117-1121.

(4) Ashwell, G. J.; Dawnay, E. J. C.; Kuczynski, A. P. J. Chem. Soc.,Chem. Commun. 1990, 1355-1357.

(5) Ashwell, G. J. Thin Solid Films 1990, 186, 155-165.(6) Clark, M. G. Chem. Ind. 1985, 258-268.(7) Ishimoto, C.; Tomimoro, H.; Seto, J. Appl. Phys. Lett. 1986, 49,

1677-1679.(8) Imazeki, S.; Takeda, M.; Tomioka, Y.; Kakuta, A.; Mukoh, A;

Narahara, T. Thin Solid Films 1985, 134, 27-34.(9) Yamamoto, H.; Sugiyama, T.; Tanaka, M. Jpn. J. Appl. Phys.

1985, 24, L305.(10) Ando, E.;Miyazaki, J.;Morimoto, K.; Nakahara,H.; Fukuda, K.

Proceedings of the International Symposium on Future ElectronicDevices-Bioelectronic and Electronic Devices, Tokyo 1985, p 47.

(11) Ando, E.;Miyazaki, J.;Morimoto, K.; Nakahara,H.; Fukuda, K.Thin Solid Films 1985, 133, 21-28.

(12) Ando, E.; Hibino, J.; Hashida,T.;Morimoto, K.ThinSolid Films1988, 160, 279-286.

(13) Feringa, B. L.; Jager, W. F.; Delange, B. Tetrahedron 1993, 49,8267-8310.

(14) Emmelius,M.; Pawlowski, G.; Vollhmann, H.W.Angew. Chem.1989, 101, 1475-1502; Angew. Chem., Int. Ed. Engl. 1989, 28, 1445-1471.

(15) Ricceri, R.; Abbotto, A.; Facchetti, A.; Pagani, G. A.; Gabrielli,G. 1997, Adv. Mater., submitted.

Figure1. Surfacepressurearea isothermsofC16H33-PDCNTE,1, at 291 K on pure water subphase (solid line) and on CH3OH2 M aqueous subphase (dashed line).

4182 Langmuir 1997, 13, 4182-4184

S0743-7463(97)00134-0 CCC: $14.00 © 1997 American Chemical Society

isotherms of the monolayers of C16H33-PDCNTE on purewater subphase and on CH3OH 2 M aqueous solution at291 K. In the former case, the collapse pressure is about44mN/m, and the limiting molecular area is about 19 Å2,in accordance with a vertical closely packed dispositionof the molecules at the air/water interface.2 The limitingmolecularareaontheaqueousmethanolic solution isabout12 Å2: this value cannot be attributed to a formation ofa realmonolayerbecause it is lower than theareaoccupiedby a closely packed alkyl chain (about 19 Å2). Probably,a molecular stacking (due to the presence of methanol inthe subphase) gives rise to an inhomogeneous monolayer(the solubilization of the dye in the subphase is notremarkable, as the isotherms obtained with differentspreading quantitieswere almost identical, but cannot betotally excluded). The surface pressure tends to increasefor both isothermsafter the collapse, indicatinganorderedstacking of the molecules.UV-Vis Characterization. The UV-vis spectra of

C16H33-PDCNTE show a high-intensity absorption band(ε ) 7.5 ( 0.6 × 104 M-1 cm-1) in solution. This band isnegatively solvatochromic, that is hypsochromically shiftedwith increasing polarity of the solvent: in methanol, forexample, it appears at λmax ) 614 nm, with respect to λmax) 642 nm in acetone (see Figure 2). Similarly to what isshown inapreviouswork forC16H33-PDCNT,2 thebehavioris consistentwith a transition froma zwitterionic ground-state to a less dipolar excited state; the band has atransitiondipolemomentµeg of 9.8Dandcanbeattributedto an intramolecular charge transfer from the negativelychargeddicyanomethanidegroup to thepositively chargedpyridinium ring.2 These characteristics make this com-pound an interesting candidate for studies in the field ofnonlinearoptics. Thecharge transferbandof themoleculeis bleached in solution when irradiated with visible lightin the presence of air (see Figure 2): only the 370 nmband, attributable to transitions of the aromatic rings, isstill present after the bleaching. The presence ofDABCOinhibits the photodegradation, as it will be explainedextensively in a forthcoming paper.16Figure 3 reports the electronic spectra of different types

of LB films. Y-type LB films of 30 layers, transferredfrom a pure water subphase with a dipping rate of 6 mm/min for the upstroke and 3 mm/min for the downstroke(transfer ratio)1.0(0.1 forall layers) reveal thepresenceof an absorption band at λmax ) 635 nm that correspondsto the charge transfer band found in solution. The 365nm band and other peaks at shorter wavelengths are dueto transitions of thearomatic rings; a small peak ispresent

at λmax ) 432 nm: this peak, blue-shifted with respect tothe charge transferbandof themonomer, canbeattributedto the formation of molecular aggregates of H-type17-20 (adetailed treatment of such absorption characteristicswillbe given in a forthcoming paper21). The presence ofmethanol in the subphase also allows for preparation ofgood quality 30-layer LB films (transfer ratio close to 1for all layers) of Y-type, with dipping rates of 5 mm/minfor the upstroke and 2 mm/min for the downstroke. TheUV-vis absorption spectrum shows a new absorptionregion of high intensity, due to the overlapping of threedifferent bands at λmax ) 467, 451, and 432 nm (the 432nm band is revealed by a shoulder in the new absorptionpeaks) (Figure 3). These peaks, attributable to theformation of different molecular aggregates containingeach a different number of monomers,17-20 are due to thepresence of methanol in the subphase that causes varia-tions in the structure of the interfacialwater andprobablyinteracts in a specificwaybothwith the dye and thewaterbound to the dye itself (the presence of aprotic solventslike DMF or CH3CN in the subphase do not cause theappearance of the sameabsorptionpeaks but only a slightincrease in intensity of the 432 nm peak). The 30-layerLB film of Z-type is transferredwith transfer ratios equalto 1 for all layers from the aqueous methanolic solutionwith an extraction rate of 13mm/min. LB films preparedaccording to thisprocedurepresentapeakofhigh intensityat λmax ) 467 nm; the two bands at λmax ) 432 and 451 nmare revealed only by two veryweak shoulders in thismainpeak, which presents a half-width at half-maximum(HWHM) of 17 nm (Figure 3). All the LB films can bephotobleached by illumination with visible light in the

(16) Abbotto, A.; Facchetti, A.; Pagani, G. A. 1997, in preparation.

(17) Lehmann, U. Thin Solid Films 1988, 160, 257-269 andreferences therein.

(18) Fukuda, K.; Nakahara, H. J. Colloid Interface Sci. 1984, 98,555-564.

(19) Shimomura, M.; Kunitake, H. Ber. Bunsen-Ges. Phys. Chem.1983, 87, 1134-1142.

(20) Mooney,W.F.;Brown,P.E.;Russel, J.C.;Costa, S.B.; Pederson,L. G.; Whitten, D. G. J. Am. Chem. Soc. 1984, 106, 5659-5667.

(21) Ricceri, R.; Abbotto, A.; Facchetti, A.; Grando, D.; Pagani, G. A.;Gabrielli, G. 1997, in preparation.

Figure 2. UV-vis absorption spectra of C16H33-PDCNTE, 1,in acetone solution before (solid line) and after (dashed line)bleaching.

Figure 3. UV-vis absorption spectra of LB films (30 layers)of C16H33-PDCNTE, 1: (a) Y-type LB films (pure watersubphase, T) 291 K, symmetrical compression of monolayer);(b) Z-type LB films (CH3OH 2 M aqueous subphase, T ) 291K,asymmetrical compressionofmono layer); (c)Y-typeLB films(CH3OH 2 M aqueous subphase, T ) 291 K, symmetricalcompression of monolayer); (d) photobleached LB films.

Notes Langmuir, Vol. 13, No. 15, 1997 4183

air: after photodegradation the absorption spectra areall similar and present only a main absorption band atλmax ) 390 nm, which is likely to be due to the aromatictransitions of the molecule that has been obtained byphotobleaching (see Figure 3). The 467 nm sharp peakshown by the Z-type LB film transferred from CH3OH 2Maqueous subphase can thusbeexploited forapplicationsin the field of optical data storage.5-14

FTIR Characterization. The FTIR spectra of the LBfilms in the C-H stretching region (not shown) do notpresent remarkable differences. The CtN stretchingpeaks (see Figure 4) compare favorably in all cases withthe values found for the radical anion of TCNQ (7,7,8,8-tetracyano-1,4-quinodimethane),22-24 thus confirming thezwitterionic ground-state ofC16H33-PDCNTE. TheY-typeLB film transferred frompurewater reveals anabsorption

peak at νj ) 2173 cm-1 and a broader absorption regionat νj ) 2143 cm-1: the splitting in three peaks at νj ) 2174,2179,and2186cm-1 of the2173cm-1 absorptionmaximumshown by the LB films transferred from the methanolicsubphase could be ascribed to electron-molecular vibra-tions interactions in the stacked arrays, evidenced previ-ously by UV-vis spectroscopy23,24 (a detailed interpreta-tion of these features in progress). The CtN stretchingregion of the photobleached LB films presents a weak,broad absorption at about νj ) 2270 cm-1; the position ofthe band could be ascribed to the disappearance of thenegative charge density in proximity of the -CN groups(the CtN stretching for neutral TCNQ is at νj ) 2234cm-1).

Conclusions

Y- and Z-type LB films of the new dye C16H33-PDCNTE(1) have been studied by means of UV-vis and FTIRspectroscopies. Thestructureof theLBfilmscanbevariedby the proper choice of various parameters like thesubphase, the symmetrical or asymmetrical compressionof themonolayers, type of deposition (Y or Z), and dippingrates. The presence of methanol 2 M in the subphasefavors the formation of molecular aggregates, which giverise to new absorption peaks that are blue-shifted withrespect to themonomer absorption band. Z-typeLB filmsprepared on an CH3OH 2 M aqueous subphase present asharp photobleachable absorption band at λmax ) 467 nm:this band has a HWHM of 17 nm and can be exploited forefficient multifrequency optical data storage.15 Thesharpness of the absorptions bands is very useful in orderto increase resolution: the synthesis of novel derivativesof C16H33-PDCNTE and C16H33-PDCNT can provide fur-ther compounds with narrow UV-vis absorption peaksuseful for optical data storage devices that could storemany bits per pixel. C16H33-PDCNTE (1) is negativelysolvatochromic and possesses a high dipole transitionmoment: these characteristics make this compound apromising candidate for nonlinear optical applications.

Acknowledgment. Financial support fromM.U.R.S.T.,C.N.R., and C.S.G.I. is gratefully acknowledged.

LA970134Z

(22) Terzis, A.; Kamitsos, E. I.; Psycharis, V.; Zambounis, J. S.;Swiatek, J.; Papavassilou, J. Synth. Met. 1987, 19, 481-486.

(23) Meneghetti, M., Girlando, A.; Pecile, C. J. Chem. Phys. 1985,83, 3134-3145.

(24) Bozio,R.;Zanon, I.;Girlando,A.;Pecile,C.J.Chem.Soc.,FaradayTrans. II 1978, 74, 235-248.

Figure 4. FTIR spectra of C16H33-PDCNTE, 1, in the 2370-2100 cm-1 region: (a) Y-type LB films (pure water subphase,T ) 291 K, symmetrical compression of monolayer); (b) Z-typeLB films (CH3OH 2 M aqueous subphase, T ) 291 K,asymmetrical compression of monolayer); (c) Y-type LB films(CH3OH 2 M aqueous subphase, T ) 291 K, symmetricalcompression of monolayer); (d) photobleached LB films.

4184 Langmuir, Vol. 13, No. 15, 1997 Notes