E L S E V I E R Thir~ Solid Films 312 (1~)~)8)295-2t, ~t)

Electric field induced molecular reorientation in stilbazolium salt Langmuir-Blodgett monolayers

Jianhua Xu *, Xingze Lu, Guangpeng Zhou, Zhiming Zhang • ~ltll¢ K¢)" ,lniot I.¢thnralnt3",for Materials Mod~/h'aligut !0' lxtser, hm ttt#d Eh,t'lrotl B¢¢ott.s, l)¢parlmettt t!t Phy.~ic,~. F t . h t . Unirer.~itv.

Sh<mg/tai 20ft4.¢3, ('hi~ttt

Received 21 April 19t)7.' a¢¢cpled 18 June 1997

Abstract

Elcclric field induced ,nolecuhu" reon'ient;~lion in snilbazoliuln salt Lal~gmuir-Blodgett monolayers have been investigated by usin~ i'olation-al~gie second harmonic generaliOl~ I¢~hnique. The stilbuzolium salt molecules were preferentially oriented along the dipping directi~m ;rod this anisotl-Ol'~ic fea,ure ,,va; enhanced by h~creasin~ the dipping speed. An external DC electric field (6.7 x IO s V/nO forced tile molecules to i'eorient ah,ng life field direction, and caused dissociation of .'=o,,ro,",','~.=._:.,..~ species, us demtm~trated by the stea~y slate and time resolved thmrescence mcasuremenl.s. ,i') 1998 Elsevier Sciet~ce S.A.

Ke.v~vord,~: I.:m~tllUil'-Bhu.l~¢ll morlol~tycl.'..: ,~t2u~ol|d hal't'~w~ni¢ ~eller~tlioll: Aufisotropy: Tiille-rcst)l~ed Iltl~re.,,Cellee

1. lnlrf~duc,'i,m

It was I'ound that a variety of organic molecules with donor-~.lcceptor :rod ~-electron systems have second order molecular polarizability /3 iw¢~ or three orders of magni- tude grealer tlmn Ihat of mosl inorganic materials in practi- cal use [lJ. Amphiphi l ic molecules can be well organized using the l.ullgmuir-t~h:,dgett (LB) lecimique to obtain ordered monolaycrs and tnultil:lyers with noncentmsym- Illctri¢ Stl'UCttircs so ~.s to ~tchieve substantial macroscopic second order i~onlinear susceplihilily. LB films have been received c(msiderahte altellliOli due to the scientific and technological significance [2-4]. In 1988. Lupo el al., [5] reported synthesis of a stilbazolium salt which exhibited very I'ligh quadratic nonlinearity ( ~ = "7.4 × I0 .~s ¢1113V 2), Stable rnultilayers of stilbazoliull~ s~flls have been successl'ully fabricated by mixing or alternatin~ (with inerl, material arachidic acid) melhod, displaying potenlial =~pplications for realistic SFIG devices [6].

For a given type of molecule wilh specific ftmctiOl~S, the macroscopic physical and chemical properties of I.B films arc determined by molecular assembly. Thus. molec- ttl;~r orientation, alignmenl and their ¢onlrol Jrt I.B films become significant h'l fl~eir applications in microelectronics and phottmies.

Molecular alignn=cnt along tile dipping dil'CCtion of

('orrcsp~mdi,lg i | t l lhor.

(XH41-fI()gII/"~),",I/$19.(~[t ,L ~' Iqth",; l!l',,c,.ier Sciem.'c S,A, ,,\It right', rc,,cr~,ctL. PII S0()40-f~Oq'(|( 97 )0t)3 f,~7- 2

LanTrnuir filnls at the air-water interface during deposi- tion by the conventional vertical dipping melhod has found wide appiicmions in liquid crystal aligning agent [7] and waveguidc frequency doubler [8]. which require hig.h de- gree of azimuthal alignmenl and effective control o1" molecular reorieniatiol~ based on better understanding of the mechanisms inveh'ed. One could control the in-plane orielltation of I.B films by selection of deposition parame- tern such as dipping speed [9], application of shearin~ by a rotuting disk [10], or illuminaiiort by polarized laser beams

In this paper, flle effect of dippin~ speed and e×ternal electric field poling on the in-plane molecular orientation in LB monolayer o f a stilbazofium salt was investigated by ol'jlical linear (steady stale m~d tinw-ru.:olved fluorescence) and nonlinear (rotatJOll-anb!e second harmonic ~eneration) techniques. II is reported tllal exlemal DC eleciric fields ;.llOIlg tile surface normal in Ihe order of I0 ~ V/m call reorient the m~,leu'ules towards the dil~'¢ljon of the polin~ field, which was explained by using a classicul nonlinear osci Ilator tuoc~l,

2. Experiment

2, I, S a m p h ~ I,-(7~ara/h,~

The .slilt'mzoliunl .,;all use,.! ill our experiment was syn- Ihcsized with tIle , ' , ;a l l le method as that of Lupo el al.. [5].

296 ,I. Xtt el ~1. / 77fin Solid Fihns 312 f t998) 295-299

. . . . . / ' " ~ \ i ' i ~ l C 1 6 ~ , t / \ . f i F ~ ' N *. - - C H 3

• " , - - \ Of-" ~s~cj.6 / . ~

Special attention] was paid to ensure that the slm]e area was measured when tl~e sample was rotated. The results were obtained by averaging over several points on the samples.

P Donor "- ~ Acceptor

hyd.rophobie hydrophilic

E (6.7xI0 ~ V/m)

Fig. I, Molecular slructurc of the stilhazolit, m .~all and the electric field poling ,~eomet ry.

Its molecular structure is shown in Fig. I. The sti lbazolium sail was spread from 10 -~ tool I-~ chloroform solutions onto an aqueous subphase in a KSVS000 Langmuir trough made in Finhmd. The subphase was deionized, doubly distilled water at 20°C with CdCI~ of 3 X 10 4 tool ! and a pH value of 5.8. The surface pressure-area (~-A) isotherms were recorded at a compression rate or 3 nllH rain -~. and the width o1" the trough is 10.5 cm. The monolayer samples were deposited on hydrophillic quartz plates ot" a dimension 30 ram:< 18 m i n X 2 mm at a constant surface pressure of 30 mN m ~ and dilTerent dipping speeds of I r am/ra in (sample A). 5 n ln l /n l in (sample B). The transfer ratio dining deposition of all samples could be kept I + 0.05. The LB monolayer:; on one side of all samples were removed prior to optical characterizations.

2.2. Ro la t inn- (mg le SHG measurement

The experimental setup used tbr rotation angle second harmonic generation (SHG) measurements was shown in our previous paper [9]. The incident beam of 50 ps pulse width, 10 Hz repetition rate. IrnJ pulse ~ energy at 1.064 # m from a mode-locked Nd:YAG laser polarized parallel (p) to the plane of incidence by using a Ghm pri~,m and A/2 plate was directed and focused onto the samples with a spot ol" diameter I mm and a f ixed incident angle fl = 45 ° through a long-pass filter. 1"he p-pol,'lrized SHG signal at 532 nm was detected in transmission by a photomult ipl ier tube (PMT) cooled to - 2 0 ° C and a boxcar averager, then displayed on ,'m x - v recorder. The SHG intensities from the sarnples were ralioed to those from a z-cut quartz plate reference to eliminate the rneasurement errors caused by laser power fluctuations, An inl'rared blocking fi l ter and a 532 nm interference f i l ler were inserted to ensure that only the second harmonic radiation was detected. The depen- dence of the SHG intensity on the azimuthal angle (I~ between the dipping direction on the sarnple and the v-axis (it] the incident plane) were measured by rotatiiig the samples around the stnrface normal (z-axis). iLnd (I~ = ()o corresponds to a geometry with the dipping direction in the phlne of incidence (see the inset in figure 2 of ReI'. [9]).

2,3, Poling by (m ¢.~l¢l'n¢ll e lec t r i c . f iHd

The poling of the samples was achieved at u'oom tem- perature by applying a DC voltage ot" 2.0 kV across the sample plate. The diameter of the anode and cathode were 10 mm and 30 mrn respectively. Both electrodes were made of stainless stccl and the distance between them was about 3 ram. The sample was put in the center or the cathode so that the monolayer was close to the anodc lind a field of 6.7 x 105 V / m perpendicular to the substrate pointing from the hydrophobie d o n o r - N ( C , , H 33)2 to the hydropMIl ic a c c e p t o r - N ' (CH 3) was applied to the stil- bazolium salt monolayer. After poling for !0 rain. the DC vollage was renioved from the samples. Special attention was paid to ensure that the same poled area was measured when Ilic samples was rotated during the SHG and fluores- cence experiments.

2,4, 3"lc~dv ,~'l~tle dtld tinr'-resnh'ed lhun'e,s('('nce me~£sure- I IH ' l l l

The lluorescencc ext; : i imental setup is given in Fig. 2. Fluorescence was excilect by a laser buani of pulse width gO ps. repeli l ion rate 82 MHz. and average power 40 mW lit 532 nm generated from an actively mode-locked Nd :YAG laser. The diamut::r o!' the pump beam t'ocused on the samples was al~out I ram. The I]uorescence signals were dispersed by a mOllOChromalor, detected and ana- lyzed by a synchroscan slrcilk Cil l l leu'a system of a resolu- l ion I() ps. Both the steady slate (integrated) and lime-re- solved I luorescence spectra could be measured sinltllt:Lne- ously, Al l experiments were performed at room tempera- lure.

YAG :: 1 i ',. "~ 1 .06 .m 0.~3.,n. a2.MHz,80ps :v.n,," ~--P---~-~--~-- ~ I S H G ~, M1

L,._J Oelf~lQr [

[ irfl~ delay

j : i i i ,lnnn..hrom.llo f ! MOIlllgl Pl~lh";r ;

~$treak ' c.,:,~l,o,,~, i '!camera i ~ - - - " i

: Power suppl',g--, dispersion Compuler ; I L ~ ,,.._.l.__ . . . . . [ analyzer i[ _ _ .

Fill 2, I-,q~crinlci~Ial ~cLup for lh¢ ~leady ~tak' and linic ic~ohcd tluoic.s- cu'nu'¢ I'neasttrcnlu'rll.

,I. Xu el al. / 7hin );nlid I"ilm,~ 312 l .t99~) 29.5--299 297

3. Results and discussion

3, i, Dipifing.imlt.'ed m'ientntim~

For the fresh sample A and B, the dependence of the p-in/p-out SHG intensities on (lie rotation angle t/~ al an incident angle 0 = 4 5 ° are shown in Fig, 3(a and b, respectively) with Ihe same intensity scale. The radM length between the center and a dau.l point represents the intensity in thin direction. The data were reproducible in different measured areas. Since the SHG intensity IYom the substrate was at least one order of magnitude smaller than that of the stilbazolitml salt monolayers, those patterns directly rellecled the in-phme symmetry o1' the stil- bazol ium salt nuolecular arrangement in the samples. Both patterns in Fig. 3a and b (solid dots) displayed significant in-plane anisotropy with a long axis (SHG intensity unaxi- mun]) at & -- 0 ° and a mirror symmetry about the dipping line. We defined the nonlinear ardsolropy ratio rx~. = ! (0°)/ i (180°), and measured rm. = I, I +_ 0, I from Fig. 3a for fresh sample A, and rs t = 3.2 for fresh sample B from Fig, 3b, i.e., the in-plane anisotropy increases with increas- ing the dipping speed. This result agreed with the conehl- sion in our previous paper [9] u,~ing optical linear tech- nklue (polarized UV-visible absorption spectroscopy) in hemicyanine LB multilayers.

According to our ela,~sical theory for SHG [9]. one could treat eacil stilbazolium Malt molecule as a one-dimen- sional anharmonic (non-liilear) oscillator with its electric dipole n l o m e n t i11 the direction el' tile molecular chro- mophore axis. The average molecuhu" dipole moment of stilbazolittm sail can he written:

d = dt,(sin d,.v+ cos ~hy) + d, - ( I )

where th is the i|zJnlulhal ar, gle of d with n'espect t(1 the y-axis, d h and d, are its components in the surface plane (.v~r.') and ill the z-d i rec t ion respectively. Ill the simple treatment [13], tile dh angle ~/i of the molecular dipole 1110111ellt was aSStlllled to he a coflstant, the . .-component o f the dipole d, was the same for all stilhazolium salt molecule,,,, while the in-plane componen| oriented ran- domly and the average (net) value d h was zero. Therefore. the nonvanisl'fing d h directly reflected the degree of the in-I)lane anisotrol'~y. The detected p-in/l~-Out SHG inten- sit)' could he derived [9]

Ira, o: [( d,,cos 0)cos &-+-d,,,,in 0] 4 (2)

with a maxin~um at eh = ()'~ Major featui'es of the SilG angular pattern could be

extracted fl'O111 this simple equatioil. The first terl l l

(dhCOStrl)cosO represei~ts the oplieal arfisotropy hy the Ik~ctor cos~h. The degree of anisotropy increa.,,es with inereasin~ tl~e in-plane alignment (imlarization) i~aramcler tit,, or decreasing the meastn'ing incident an~le O. The second term (d, sin0) is ~h-independent. It plays a rote of 'bias' representing the inversion asyillmetry, The tle#ree of

(a) 90

/ / _ .o" , ' r" . - . \ '

_ T" +', \

' e _ Q" /

27O (b) gO

/ : l e o F . . . . . . . . . . . . . . . . . . . . . ',,..-lo \ :/

~ , . ../"

270 (c) 90

/" ,.;.,.., \ / , i "'~'- I

1801 . . . . . t ........ ~ o \ , , / e. 41' '\ • ' ~ " l " " e "

.\ /

270 Fig, ),. Angular pattern,, of the p-in/p-oLII S|t(; inlen.ql)' from lai sample A dcpo.,,i|cd with the dipping ~,pccd of I ram/rain: (h) tresh ~arnple B dcpt~,,,ited xxith the dipping speed of 5 ram/rain: (c) sample B aflcr electric field poling. I)axh lim.'~, are the theoretical fi! results,

the inversion asynunelry increases with i,lc,'easing the normal polarization parameter d, or the incident angle O. if d h = 0, ira. becomes isotropic, as the way most workers previously considctvd. If d, = O, Ira. becomes centrosym- metric, just like tile paUern observed by K;Liikawa et al,, [ 14.15] who conehided a C2, symmetry with the C~ axis in the surl'ace plane for LB monolayer samples of mcmcya- nine with the chromophorc axe~ lying in the surface plane ( d, = 0).

When stilbazolium salt monolayers were deposited dur- ing upsm&es, the molecules would be aligned v,.ith their Jll-pl~ltle COlllpOllellt o,f dipole 1110111ellIS di~ il l the same direction a~ Ihc dippin,~ direction, Thus, d, ~ {}, dl~ 4= O, Ihe anisotropic angtdar pattern Ira, shouM hc nor|cen- trosynln|etric, c,g,, Ira,( tb = 0 °) > / r l , ( tb -- 180 °) d~le to tile ditTcreilt geonlclry (eOllstriB.-tixc or ticslrtlu'tive in tile p-di- rection) of the extra in-plane polarization d h relative to the normal lmlari/ation d, an shown in Fi~. 4a (¢h = 0 °) and t:i~. 4b (th = IS(V). Tile SHG intensity pattern possessed only a reflection s)'mmetr) xvitll the mirror plane peq'~cn-

¢ "~ 2?5 29q 2t),R .I . \ ' t tet a t . / T h i n S~,lid F i h m . I . f I~,~,~¢1

(a) (b)

' ~f't \ I o .l~ "'l' ~ . t I%lI

" ' - - -2e) o) I

I I

p p'

-" 2to

I"ig. 4. (a} ( 'Ol l , , l r t lCl i~e {~h :=: {I ) lii' (1~] de,,lrucli~.e (d* .... I l l() I ge~l iuelry t~l lhe ¢~11':i in -p laue pol;n'J/alJon dl~ I '¢l: l l i~c I . Ihe nOl'l l l l i l pohu'i~,,! i,, l , d , p-d i i 'ec l i lm.

Jn l i l t

dieuhu" Io tile stu'l'ace plane ¢ontailling Ihe dipping line, This was exactly the chtiracler obsei'ved ill our e×perinlenl tlS shown in Pi!z. 3a and !. We define _l ~ (d,,/dl,)lanO .'.irid deduce .] = 47.7 and 7.() fronl fii of Eq. (2) io Ille in0a.~tlred SI4G d;iia ill Fig. 3ti and b rest)e¢iively. Tile inverse of .1 lef'le¢led tile rehilive degree ill" file in.-phine ani~oll'opy. When tile dipping speed was increased. _~ would deCl-011se be¢iltlSO ill" lhe ellhIlllCelllelll of Ihe iil-plailc' conlponenl tit,, and file allisOli'Ol)i¢ ralio rxl would in- crease :is shown hy a ¢llnlpal'islln of Fig, 3a alld b. The ealeulliled fii resuli~ ~ire thc'n disphiyed hy Ihe dash lines in Fig. 3a aild b and agl'Ted well v,,ilh die experinlenlal dala.

.'1.2. Eh'¢'tric.liehl indm'ed m<,h,ruhw rem'ienmtiml

The rolalhlll angle dopendellce of file p. i i l / /~-oUl S H G intensily i'roln sanlpie B ;il'ler heiilg poled by Ihe exlernal I)C ele¢Iric field for I0 inJn as shown in Fig. 3c gives a sniallcr degl'ee ill" ani.~otrop)' ( rxl = 2,5 -± II. I, _l ~-.-- ~.4),

= 3.2 -+ (). I con~pal'ed with that of tile fresh sanlple B ( r \ l _ . 21 = 7.{)i given in Fig. 3h, Nolice lhat file iiltensily scale llSed in Fig. 3c is 1() limes hirger ihtin Ihal used hi Fi,,4. 3a tlild t1. Ihus, file Lll.'Ittill illeiistlred tiVel'tlge SttG inlensily fronl Salllple B after poling was itbottt lille order of ilitl,,ni-e. lude hirger Ihan Ihal froln fresh Salllple B.

When the exlerilal elecirh: field E was aprdied io the nlonilhiyers as slloWll iil Fig. 1, lhe iiilera¢lJlnl belween ille tltOlecular dipole P ( l ' l 'O ln d o t l o r I l l acceplol) and lhe exlerllal DC field croaled a iOl'qUe leadillg Ill re~lrieillalJoil of lhe stilbazolium siill nlolecuhir axes tlhltl~ Ii1¢ fictd dh'¢clion agahlsl Itle Iilerlnal randoll~J/.alion I1~,1 i . iniflJ- IllJ/.C ihe inlcraclJon energy (/ : ' . P), which caused an in- crease of d~ and SHG inlcnsily, as well as a deL'le~.ise ifl' di, aild r x i . Those predh.'lions lll'e all verified tly lit lt cnh:uhiied rcsuh wilh _l = S.4 shmvn hv Ihe dash line in Fig. 3c.

Ill I,B I'ilms, since ainphirJl i t l ic lntflecules are essen- tially linked together by ~'an der ~,aals iilleraclion instead of slroltg cheiuic:il bollds, itlolecuhu ic'orienlalillll c'lltlllt be realized inore easily than in nlO.xl .~olhl fJllll~, t;or illSltinco, Ihc illolectllar Iill ant ic t'oilld Jll¢l-Ca.~c flOlll 3Y Ill 30 '> in

Ilenlicyanine I,B Iililis when tile fcnlperaltire was raised froin 3()~C io "7()°C II 7]. On lhe oilier hand. lhe aili.~olrllpic I0alUl¢S tn ihe poled .~;illlple B wa.~ found 1o rentain al least a few weeks Ill roonl lenlF, eraltire, which sllow.~ Ihiil such aligiillleill :ire slable agaillSl Iherillal rehl×alJon. Tile possi- bil i ly of polirJg of organic nloleeules by rehiiivcly weak el¢ciri¢ field as shown in Itlis s¢clion stigge.~ted il valuablc challnel for eOllii'ol or nlol¢cuhir uss0111hly and fabrJealion in pi'aclical poh.irized I,B fillllS for various purposes.

Nolice ihal file increase of ihe .~HG inlensily was also possibly euuscd by disso¢ialion of ihe l-/-aggr0galion in slJlhazliliunl sah nlonolayers, which wil l he strewn hi lhe I\~llowing seclion by usin 7 steatly slalc and lime i'¢solved fl uoresceilee lech iliq ties,

3,3. ,b'leadv s/ale aml lime re,~oh'ed.lhuwexcence ,Vwo'ra

('on~ider a dinler fornled fronl Iwl~ illolecule.~ with ;lleJl" li'all.~ilion dipole nlonlClli~ parallel In each iJIher. Accord- lng ht Ihe sinlpli' ltleoi'f of excilon splil l ing l i,xl. file eieclronicalJy exciied slale splhs intll Iwo levels wJlell lwl l illol¢cules approach io clio't1 oilier and re, ill a dinler thi0 Io Ihc eteclric dipole inlera¢liOliS. ]'tie sign nnd ~i/.o of Ihe ex¢ilon ~pliuing 2_11£ ii~ well as the selectilm rules i~ll" optical lransilions were e.~senlially delei'nlincd I~y the inctJ ilalion allglc f-J helwcen Ihc illllh.,ctihir Irailshhlll IllOlllenl,~ alld ihe ¢l)nlleclJn,,~. lines of Ihe two nlttlectlJcs. I:or hil'ger O)'s us in tile case ill' H-I),pc aggregation, the lower level is oplh:ally I'orbiddcn alld Ille Iiall.~ilJon belwcen file gfound ,Male and lippor level loads Io all absilrptil~n build whk'h is Iqtlc-shJllctl fronl Ihe original itt~sorl~liol~ baild ,ll' nlOilOlllel'jC tllOlectlle,~. II was found thai lhe fltlol'e.~¢ellCe ~pcctra would al,o he t4ue-.,,hiflcd ~allen illolecules furnled //-aggl'egales, whirl1 was similar Io Ihe :dlit't behavior ~lhser~,ed ill file al~.~Ol'ption spc¢li'a l i<Jl. Moreover, i f Itle i'atlJativ¢ decay nile of an i.,,ttlliletl lwn-lev¢l Sillgle nlolecttle Js y. ihe size i l l till , i , , ro, .m, inchidJn- N stlc'h lllOlectlles is inudi simlller Ihan Ihe wuveletlgth associaled wJlh tilt.' nplic':d II'tillsJlitlllS, we enn use Ih¢ dipole aptmlxinlalhtn for Ihe inlerilloleeuhlr iillera¢tiollS in Ihe ¢lllJre i.i -un'-:~lo [2(),21i. According m ihe lhcory of aggi'egale dec;ly dy-

J, Xu ('t . i . / 77fi, Solid b'ilm.~ 312 ( I~tt8~ 295-29q 2t)9

1.0

t~ 0.8

== o.6

0.4

g 0,2

8 o.o It.

..0.2

- (a)

I

_ ' ~ " . ,..,

500 550 600 65(1 700 750 800

Wavelen~lh (nm}

an characterized by the rotation-angle SHG technique. It wan demonstrated that an external DC electric field of 6.7 × i0 -~ V / m could be used to control molecular reori. entation effectively by aligning the chmmophore axes along the direction of the poling field and cause dissocia- tion of the H-aggregates in LB monolayers.

Acknuwledgements

_ i O

~ 0.8

g 0.6

0.4 r -

~ 0.2 Lt.

0.0

(b)

-300 0 300 BO,3 9001200151301800

l ~n~e ;.ps)

Fig. 5. (a) Slcatly std.lc I]uol'C~,CCllCC npeCll'a alId (h) lime remfl~cd Iltmresccnce plt~tile.s of sample B belorc (dash line) :tnd :tllcr t.solid lille) clcclric field puling.

namics [22], the radiative decay rate of an agg~vgate in ivy. N times of a single molecule radiative decay rate due to the coherent interactions of all molecules in the aggregate with the radiation field, i.e., the lifetime of aggregates is smaller than that ot" single molecules.

The s t i lbazol ium stilt rnolccules would f o rm H-aggre- gates [6] anti d issociat ion of H-agglega t i tm would cause an increase of hypcrpohu izab i l i ty /3 and S H G intensi ty in I,B

films [23]. Fig. 5a show.,, the measured steady slate fluores-

cence spectra of the sattlplc B before (dash line) and after

(solid l i n c ) b e i n g poled by an electric ficld (6.7 x l0 s V / m ) for I() n~in. The red-shift o f the f luorescence band after pol ing in attributed to dissociat ion of the H-aggrega- lion. This was also COld'il'nled by our t ime resolved iluores-

cence spectra shown in t:ig. 5b. The average decay rate of

sample B (solid line) after i-~olillg was obvious ly smal ler

than that of the fresh sample B (dash line), because some aggregates were dissociated into single molecules. Appli- cat ion of the external I)C field wotild weaken the inter- molecular cot lpl ing and ctltlSe the dissociat ion o f the ae,=re-

gated s l i lbazol ium salt rntflecules in I,B monolayers .

4. Conclusions

l)ipping-induced optical anisotropy in stilbazolium s~lt O LB monulaycrs increases with incrca , in~ the dipping spcctl,

This work was supported by the National Science Foun- dation of China, National Postdoctoral Science Foundation of China and Shanghai Postdoctoral Science Foundation.

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17ira& [1 It J,H. Xu, X.Z, i,u, K. Ilan, (;,P. Zhou. Z,M, ZlwrL,.:. I,an~muir t3

(Itlt)7) 3187.

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