Langmuir-Blodgett Films of a NewPyridinium-Dicyanomethanide Dye and Their Potential

Optical Applications

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

Dipartimento di Chimica, Universita di Firenze, via G. Capponi 9, I-50121 Firenze, Italy; andDipartimento di Chimica Organica e Industriale, Universita degli Studi di Milano, via Golgi

19, I-20133 Milano, Italy

Received December 9, 1996X

We report on Langmuir-Blodgett (LB) characterization of 4-[5-(dicyanomethanidyl)thien-2-yl]-N-cetylpyridinium (C16H33-PDCNT). Surface pressure-area isotherms of monolayers were investigated atT ) 291 K. LB films of Y- and Z-type were characterized by means of UV-vis and FTIR spectroscopies.The compound is photobleachable and undergoes a change in molecular structure in monolayer and insolution. The molecule strongly absorbs in the visible region due to a charge transfer band both inhomogeneous solution andLB films. Dependingupondipping conditions, a sharp absorption bandappearsat about 422 nm that could be attributed to molecular aggregates, probably of H-type. Both the 422 nmband and the broad charge transfer band at longer wavelength are photobleachable, and the former couldhave potential applications for optical data storage. The molecule seems to possess a large second-orderhyperpolarizability, and non-centrosymmetric Z-type LB films could have promising nonlinear opticalproperties.

Introduction

Molecules which consist of a donor part linked to anacceptor part by a π-electron bridge have become thesubject of intensive studybecauseof theirnonlinearopticalapplications.1 Among them, thepush-pull solvatochromiczwitterionic molecules containing as end groups a nega-tively charged dicyanomethanide donor and an N-alky-latedaziniumcationasacceptor seemtobeverypromisingfor second-order nonlinear optical effects that can berealized by preparation of non-centrosymmetric supramo-lecular structures with the LB technique.2,3 The aim ofthiswork is to studyLB filmsof 4-[5-(dicyanomethanidyl)-thien-2-yl]-N-cetylpyridinium (C16H33-PDCNT;1), a newzwitterionic dye, in order to realize different supramo-lecular geometries and optimize bulk nonlinear opticaleffects for future applications. The absorption spectra ofthedye in solutionandLB filmsare also examined. Push-pull analogs of this compoundhavebeen shown3 topossessa sharp photobleachable absorption band that could haveinteresting applications in the field of optical datastorage.4-9

Experimental DetailsThe compound C16H33-PDCNTwas synthesized according to

the procedure described elsewhere.10 Organic solvents used forpreparing solutions forUVspectra and spreading solutionswereofanalytical gradeandsuppliedbyFluka. KBr (analyticalpurity)was provided byMerck. A solution in chloroform/ethanol 1:1 v/vwas used for the spreading: the typical concentration of the dyein the spreading solutionwas 0.7mg/mL. Freshly sublimed 1,4-diazabicyclo[2.2.2]octane (DABCO) (supplied by Aldrich) wasadded to the spreading solution in order to avoid the photo-chemical degradation of the dye.11 Water was purified with aMilliporeMilli-RO6andaMilli-Qwater system(organexsystem),obtaining a specific resistance g18 MΩ‚cm. Surface pressure-area isotherms were recorded using a KSV 3000 balance withsymmetrical compressionof themonolayer (i.e. twobarriersmoveand compress symmetrically the monolayer) at a compressionrate of 4 mm/min; the accuracies were (2 Å2/molecule for areasand(0.05 mN/m for surface pressures. Typically, 200 µL of thedescribed solution was spread on the aqueous subphase and 15min was waited each time to allow the chloroform to evaporate.(The experimental conditions were kept constant in order toobtain reproducibility and avoid variations in the π vs A curvesbecause of possiblemolecularaggregation in themonolayer.Onlyfreshly prepared solutions, kept in the dark, were used for thespreading. Also the isotherms were recorded avoiding, as muchas possible, exposure of themonolayer to visible light.) LB filmswere transferred from a pure water subphase at π ) 15 mN/m(T ) 291 K) onto quartz slides using the KSV 3000 balance forY-type LB films (i.e., films with a head-to-head and tail-to-tailpacking of the amphiphilic molecules). Y- and Z-type LB films(Z-type are LB filmswith a head-to-tail packing of themoleculeswhere the polar heads of the first molecular layer are in contactwith the solid substrate)were obtainedusingaKSV5000balanceunder the same conditions of subphase temperature and surfacepressure (theKSV5000balance is a two-compartmentLB trough

* To whom correspondence should be addressed.† Universita di Firenze.‡ Universita degli Studi di Milano.X Abstract published in Advance ACS Abstracts,May 15, 1997.(1) Non linear optical properties of organic molecules and crystals;

Chemla, D. S., Zyss, J., Eds.; Academic Press: New York, 1987; Vols.1 and 2.

(2) 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.

(3) Ashwell, G. J. Thin Solid Films 1990, 186, 155-165.(4) Feringa, B. L.; Jager, W. F.; Delange, B. Tetrahedron 1993, 49

(37), 8267-8310.(5) Emmelius, M.; Pawlowski, G.; Vollmann, H. W. Angew Chem.,

Int. Ed. Engl. 1989, 28 (11), 1445-1471.(6) Ishimoto, C.; Tomimoro, H.; Seto, J. Appl. Phys. Lett. 1986, 49,

1677.(7) Imazeki, S.; Takeda, M.; Tomioka, Y.; Kakuta, A.; Mukoh, A.;

Narahara, T. Thin Solid Films 1985, 134, 27.(8) Clark, M. G. Chem. Ind. 1985, 258-268.(9) Ando, E.; Hibino, J.; Hashida,T.; Morimoto, K. Thin Solid Films

1988, 160, 279-286.

(10) Abbotto, A.; Bradamante, S.; Facchetti, A.; Pagani, G. A. To besubmitted.

(11) Abbotto, A.; Facchetti, A.; Pagani, G. A. To be submitted.

3434 Langmuir 1997, 13, 3434-3437

S0743-7463(96)02091-4 CCC: $14.00 © 1997 American Chemical Society

with asymmetrical compression of the monolayer; i.e., only onebarriermovesandcompresses themonolayer). Thedipping ratesfor the upstroke and the downstroke ranged from 8 to 20 mm/min, obtaining differently structured LB films depending on thespecific conditions that were chosen (see Results andDiscussionand Table 1). Immediately after preparation, LB films werestored in thedark toavoidphotodegradation. UV-visabsorptionspectra were recorded using a Perkin Elmer Lambda 5 spec-trophotometer. Infrared spectra were obtained by a BIO-RADFTS-40 spectrometer at a resolution of 2 cm-1 on KBr pellets ofthe dye and on 30-layer LB films transferred on quartz slides.

Results and Discussion

Monomolecular Layers. Figure 1 shows the π vs Aisotherm of the monolayers of the pure dye on the watersubphase (pH ) 5.6) at 291 K. The collapse pressure isabout 30 mN/m, and it is revealed by a slight inflectionof the curve, while the surface pressure still increasesafter the collapse; this could indicate an ordered stackingof themolecules during the collapse. The compressibilitymodulus (see Figure 1) shows the existence of a notstrongly condensed monolayer (the Cs

-1 value at π ) 15mN/m, the surface pressure that was chosen for LBdeposition, is 75 mN/m) and confirms the collapse of thefilm at about 30 mN/m. The limiting area per moleculeis about 21 Å2: this could indicate a vertical dispositionof the molecule at the air/water interface. In fact, theend-on Van derWaals area of C(CN)2 is 24 Å2;3 with a notvertical orientation, the polar heads should stay belowthe water surface, and this seems less likely because ofthe steric hindrance of the groups themselves at suchsurface areas in the presence of a monolayer structure.In any case, the shape of the π vs A isotherm can provideno clear proof in favor of this hypothesis.Addition of DABCO to the spreading solution does not

influence in any case the isotherm of the dye, because ofcomplete dissolution of the amine in the aqueous phase.Formation of LB Films. Conditions for LB transfer

are reported in Table 1. Y-type with symmetrical com-pression of the monolayer and Z-type LB films (asym-metrical compression of the monolayer) can be easilyobtained at 291 K with good transfer ratios (1.0 ( 0.1).Y-typeLB films obtainedwith asymmetrical compressionof themonolayer provide transferred layerswith transfer

ratios close to 1 for the upstroke but only 0.5( 0.1 for thedownstroke. The different characteristics of LB filmsobtained with different transfer conditions are shownbelow in the part regarding UV-vis and FTIR charac-terization. In any case, it must be pointed out that onlya structural analysis of LB films, e.g. by means of X-raydiffraction, can confirm the formation of Y- andZ-typeLBfilmswith their characteristic geometry of packing (head-to-head and head-to-tail, respectively).UV-vis Characterization. The UV-vis spectra of

the dye show a high-intensity absorption band in thevisible region (ε ) (6.7 ( 0.3) × 104 M-1 cm-1) in solution(see Table 2). This band is solvatochromic10 and isbathochromically shifted with decreasing solvent polar-ity: in methanol, for example, it appears at 540 nm withrespect to the 561 nmband in acetone (see Figure 2). Thisbehavior is consistentwitha transition fromazwitterionicground state to a less dipolar excited state,12 and the bandcan be attributed to a charge transfer from the negativelychargeddicyanomethanidegroup to thepositively chargedpyridinium ring, as reported for other push-pull zwitter-ionic analogs.2,3 The zwitterion studied here is readilybleached in acetone when irradiated with visible light intheair (seeFigure2). Solutionsof thedye inprotic solventslike alcohols are less readily photobleached by simpleexposure to visible light and are stable for many daysbeforea slowdegradationbegins. ThepresenceofDABCOinhibits photodegradation of the dye in solution. Resultsof our investigation on the photobleaching of the dye andon the ways to circumvent it will be reported elsewhere.11

(12) Nagy, O. B.; Nagy, J. B.; Bruylants, A. J. Chem. Soc., PerkinTrans. 2 1972, 968-971.

Table 1. Transfer Conditions of LB Films from a Pure Water Subphase (p ) 15 mN/m)

type ofLB film T(K)

v(upstroke)(mm/min) v(downstroke) (mm/min)

Y 291 9 5 (asymmetrical compression of the monolayer)Y 291 16 8 (symmetrical compression of the monolayer)Z 291 8 (asymmetrical compression of the monolayer)

Figure 1. Surface pressure-area of the C16H33-PDCNTspreading isotherm at 291 K on a pure water subphase (solidline) and compressibilitymodulus vs area of the same isotherm(dashed line) (theordinate scaleaxis is thesame forbothcurves).

Table 2. Uv-vis Data for the Dye Solution and LB Film

λmax(nm)

HWHM(nm) form

561 29 acetone solution550 28 DMF solution540 31 methanol solution530 53 Y-type LB films (T ) 291 K, symmetrical422 8 compression of the monolayer)524 54 Z-type LB film (T ) 291 K, asymmetrical

compression of the monolayer)422 about 10 Y-type LB film (T ) 291 K, asymmetrical

compression of the monolayer)

Figure 2. UV-vis absorption of C16H33-PDCNT in acetonesolution before (s) and after (‚‚‚) switching.

LB Films of a New Pyridinium-Dicyanomethanide Dye Langmuir, Vol. 13, No. 13, 1997 3435

TheUV-vis spectrumofY-typeLB films obtainedwithsymmetrical compression of the monolayer at 291 K isshown in Figure 3A. Two weak bands at 339 and 533 nmare shown, which can be attributed in the former case tothe transitions of the aromatic rings and in the latter caseto the charge transfer band corresponding to the one foundfor thehomogeneous solutions. Apeak of strong intensityat ca. 422 nm is present (Table 2). This peak is blue-shifted with respect to the charge transfer band foundboth for solutions and LB films and could be attributedto the formation of aggregates; the FTIR results shownbelow confirm somehow this hypothesis. In any case,micrographs of these LB films give clear proof of thepresence of aggregates.13 The spectral change could beattributed to the interaction of the electronic transitionmoments of the chromophore in themultilayer. Accordingto the simplified molecular exciton model, an electronictransition band shifts to the shorter wavelength, whenchromophoresarearrangedwith their transitionmomentsparallel and ordered perpendicular to the stacking direc-tion (H-aggregate structure).14 The symmetrical com-pression of the monolayer, together with the choice ofproper transfer conditions, seems to favor the appearanceof the absorption band at 422 nm. In fact, this band isstill present when the monolayer was asymmetricallycompressed (seeFigure3B),but its intensity ismuch lower.

Itmust be noted that theπ vsA isothermsperformedwithasymmetrical compression, not reported here, are practi-cally identical to the ones obtained with symmetricalcompressionof themonolayer. Thereforeweconclude thatalso the movement of the barriers during the depositioncan significantly affect the supramolecular arrangementof the film, in particular the packing within the chro-mophore groups region. This could account for the lowertransfer ratios for the downstroke of Y-type LB filmsobtainedwithasymmetrical compressionof themonolayer.The half width at half maximum (hwhm) of the peak is8 nm. The hwhm values for the charge transfer band forY-andZ-typeLBfilmsare larger than50nm, thusshowinga significant broadeningwith respect to the correspondingabsorption band obtained in solution (for example, hwhm) 29 nm in acetone: see Table 2). These results suggestthat chromophores pack in a less rigid structure in theLBfilms which do not show the 422 nm band as the mainpeak. The sharpbandat 422nm, that is present inY-typeLB films obtained with symmetrical compression of themonolayer (in which it represents the main absorptionband), could be a very important feature for opticalapplications ofLB films like optical data storage4,5 becauseof thepossibility of selectivephotobleachingby irradiationat the right frequency.The absorption spectra of the Y- and Z-type LB films

obtained at 291 K with asymmetrical compression of themonolayer are reported inFigures 3B and 4, respectively.These spectra differ significantly from those obtained forfilms transferred from symmetrically compressed mono-layers. The most intense bands are now those corre-sponding to the aromatic ring transitions (centered at ca.335 nm) and to the charge transfer process, equivalent totheone found insolutionat ca. 520nm(Table2). Asharperpeak at 422 nm, likely due to a highly ordered supramo-lecular structure, is still observable for Y-type LB filmsobtainedwithasymmetrical compressionof themonolayer,but its intensity is much lower with respect to the onefound for Y-type films obtained with symmetrical com-pression of the monolayer. In contrast, the higher

(13) Ricceri, R.; Grando, D., Abbotto, A.; Facchetti, A.; Pagani, A. G.;Gabrielli, G. To be submitted.

(14) Kawamura, S.; Tsutsui, T.; Saito, S.; Murao, Y.; Kina, K. J. Am.Chem. Soc. 1988, 110, 509-511. 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. Fukuda, K.; Nakahara, H. J. Colloid Interface Sci.1984, 98, 555. Shimomura, M.; Kunitake, H. Ber. Bunsen-Ges. Phys.Chem. 1983, 87, 1134.

Figure 3. (A, top) UV-vis absorption spectra of Y-type LBfilms (30 layers) of C16H33-PDCNT before (s) and after (‚‚‚)switching (symmetrical compression ofmonolayer,T) 291K).(B, bottom) UV-vis absorption spectra of Y-type LB films (30layers) of C16H33-PDCNT before (s) and after (‚‚‚) switching(asymmetrical compression of monolayer, T ) 291 K).

Table 3. Summary of Band Position (cm-1) andAssignment for the Dye

wavenumber (cm-1) assignment

3126 and 3057 CsH aromatic stretching2956 CH3 antisymmetric stretching2918 CH2 antisymmetric stretching2851 CH2 symmetric stretching2184 and 2149 CtN stretching (only KBr pellet and

LB films transferred at T ) 291 Kbefore photobleaching)

2285 CtN stretching (only photobleachedLB films)

Figure 4. UV-vis absorption spectra of Z-type LB films (30layers) of C16H33-PDCNT (asymmetrical compression of mono-layer, T ) 291 K).

3436 Langmuir, Vol. 13, No. 13, 1997 Ricceri et al.

intensity of the422nmband in theZ-type filmwith respectto that in theY-type film (bothobtainedwithasymmetricalcompression of themonolayer) is consistentwith the lowertransfer ratio recorded for the latter.All these films are stable in the dark for long periods

but are readily bleachedwhen irradiatedwithvisible lightin the presence of the air (see Figures 3 and 4).FTIR Measurements. Figure 5 shows the FTIR

spectra in the 3200-2800 and 2350-2100 cm-1 regions,respectively, of theKBrpellet of thedyeandof thedifferentkinds of LB films (the FTIR spectra of Y- and Z-type LBfilms that were obtained with asymmetrical compressionof the spreading monolayer are practically identical, andthus only one of them is reported in Figure 5). Peakassignment is summarized in Table 3.The C-H stretching region is very similar for the bulk

dye and LB films (before and after photobleaching); nosubstantial differences are shown for all types of LB films.More interesting information can be provided by the

analysis of the CtN stretching: two peaks at 2184 and2149cm-1 are found. Thepositionsof thesepeaks comparefavorably with the values found for the radical anion ofTCNQ (7,7,8,8-tetracyano-p-quinodimethane), thus con-firming a zwitterionic ground state.15-18

The analysis of the CtN stretching for the LB filmstransferred at 291 K (before photobleaching) shows abroadening and nearly a disappearance of the 2149 cm-1

peak, with respect to the case for the bulk dye. The peakat 2184 cm-1 appears slightly broader (hwhm ) 8 cm-1)

than the one in the bulk dye (hwhm) 7 cm-1) for LB filmsofY- andZ-type obtainedwith asymmetrical compressionof the monolayer but not in the case of the Y-type LB filmobtainedwith symmetrical compression of themonolayer,which presents the sharp absorption peak at 422 nm ofvery high intensity in comparison with the band at 520-530 nm: in this case the peak is much sharper (hwhm )4 cm-1) than in all the other cases here examined. Thiscould be due to the ordered packing of the chromophoresin the films. The shape and position of the peak couldthusbedeterminedby the electronic structure of themole-cule: thepresence of highly orderedmolecular aggregates(as suggested above in the section regarding UV-vischaracterization) can account for the sharpness of thepeak, while its broadening could be due to a not veryordered packing of the chromophoreswithin the other LBstructures.TheFTIRspectra of photobleachedLB films transferred

at T ) 291 K show a single peak in the CtN stretchingregion, thus showing thata similarmolecularmodificationcould have been induced by light irradiation in thepresence of air: a single broadbandat 2285 cm-1 is shown(the FTIR spectra of all different kinds of LB films arepractically identical after photobleaching; thus, only oneof them is reported in Figure 5). The position of this bandcouldbeascribed toadisappearanceof thenegative chargedensity in proximity of the CN group, taking into accountthat the CtN stretching for neutral TCNQ is ap-proximately at 2234 cm-1.15-18

ConclusionsHighly orderedLB films of thenewdyeC16H33-PDCNT

have been obtained in this work. Their structure can bemodulated on the basis of the proper choice of themethodof compression of the spreading monolayer (symmetricalandasymmetrical), thedipping ratesduringLBdepositiononto the solid substrate, and the type of deposition (Y andZ).The dye C16H33-PDCNT forms stable monolayers at

thewater/air interfaceat291Kin thedark,while exposureto light causes a photochemical degradation of the chro-mophore. Y-type LB films that have been preparedaccording to a proper procedure show a sharp absorptionband at 422 nm which is photobleachable (LB films pre-pared according to other conditions present also a photo-bleachable band around 520-530 nm butmuch broader).The band at 422 nm has a half width at half maximumof 8 nm. A film presenting sharp bands like this could beexploited in order to accomplish three-dimensional opticaldata storage by switching the layers at different wave-lengths;moleculeswithswitchablenon-overlappingbandsare required to store several bits per pixel.9The solvatochromic response of C16H33-PDCNT seems

to indicate a very promising value of the second-orderhyperpolarizability â; non-centrosymmetrical Z-type LBfilms, as obtained in this work, are good candidates forinteresting bulk nonlinear optical properties.

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

LA962091C

(15) Chappell, J.S.;Bloch,A.N.;Bryden,W.A.;Maxfield,M.;Poehler,T. O.; Cowan, D. O. J. Am. Chem. Soc. 1981, 103, 2442-2443.

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

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

(18) Bozio,R.;Zanon, I.;Girlando,A.;Pecile,C.J.Chem.Soc.,FaradayTrans. 2, 1978, 74, 235-248.

Figure 5. FTIR spectra of C16H33-PDCNT: (a) KBr pellet; (b)Y-typeLBfilm (30 layers) transferredatT)291K(symmetricalcompressionofmonolayer); (c)Y- andZ-typeLB films (30 layers)transferred at T ) 291 K (asymmetrical compression); (d)photobleached LB films (30 layers) transferred at T ) 291 K;(A, top) 3200-2800 cm-1 region; (B, bottom) 2350-2100 cm-1

region.

LB Films of a New Pyridinium-Dicyanomethanide Dye Langmuir, Vol. 13, No. 13, 1997 3437