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Journal of Luminescence 114 (2005) 197–206

www.elsevier.com/locate/jlumin

Spectroscopic characterizations of non-amphiphilic2-(4-biphenylyl)-6-phenyl benzoxazole molecules at theair–water interface and in Langmuir–Blodgett films

S.A. Hussain, S. Deb, D. Bhattacharjee�

Department of Physics, Tripura University, Suryamaninagar, Agartala: 799130 Tripura, India

Received 10 July 2004

Available online 5 February 2005

Abstract

This communication reports about the successful incorporation of a well-known non-amphiphilic derivative of

oxazole chromophore 2-(4-biphenylyl)-6-phenyl benzoxazole abbreviated as PBBO, in Langmuir–Blodgett films when

mixed with stearic acid (SA) as well as also an inert polymer matrix polymethylmethacrylate (PMMA). The surface

pressure versus area per molecule isotherms of the Langmuir films of PBBO mixed with PMMA or SA at different mole

fractions reveal that the area per molecule decreases consistently with increasing mole fractions of PBBO. Area per

molecule versus mole fraction curve shows that the experimental data points coincide with the ideality curve predicted

by the additivity rule, which leads to the conclusion of either ideal mixing or complete demixing of the binary

components. The UV-vis absorption and fluorescence spectroscopic studies of mixed LB films of PBBO reveal the

nature of complete demixing of the binary components of the sample molecules (PBBO) and PMMA or SA molecules.

This complete demixing leads to the formation of clusters and aggregates of PBBO molecules in Langmuir and

Langmuir–Blodgett films. J-type aggregates of PBBO molecules in LB films have been confirmed by UV-vis absorption

spectroscopic study. Aggregation of PBBO molecules in LB films giving rise to excimeric emission has been

demonstrated by fluorescence spectroscopic study. Excitation spectroscopic study clearly confirmed the presence of

excimeric sites.

r 2005 Elsevier B.V. All rights reserved.

Keywords: Non-amphiphilic; UV-vis absorption and fluorescence spectroscopy

e front matter r 2005 Elsevier B.V. All rights reserve

min.2005.01.001

ng author. Tel.: +91381 2375317; fax:

1.

ss: tuphysic@sancharnet.in (D. Bhattacharjee).

1. Introduction

The films obtained by successive transfer [1,2]of insoluble monolayers from a liquid onto asolid surface upon application of an externalforce (constant deposition pressure and substrate

d.

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N

O

Fig. 1. Molecular structure of 2-(4-biphenylyl)-6-phenyl ben-

zoxazole (PBBO).

S.A. Hussain et al. / Journal of Luminescence 114 (2005) 197–206198

dipping speed) have been referred to as Lang-muir–Blodgett (LB) films [3,4]. Recently theseultrathin Langmuir–Blodgett (LB) films of orga-nized molecular assemblies containing specificphotofunctional groups or chromophores areconsidered to be extremely important for nano-technology-based optoelectronic and photonicdevices and have been the subject of activeresearch in recent years [5–14]. Perhaps, the mostinteresting feature associated with LB films whichmakes them attractive for these photonic devices isthe ability to control the spatial distribution andorientation of molecules assembled in these films,to bring about desired changes [15,16] in theiroptical and electronic properties. Interestingly, LBsupramolecular assemblies closely resemble natur-al biomembranes and provide a unique platformfor mimicking energy and electron transfer reac-tions in real energy-harvesting photosyntheticreaction centers [17–20]. Advantages of bothcharged and neutral lipid mixed LB films ofmicro-heterogeneous media for control of photo-physical processes, chemical reactions and tailor-ing of charged transfer complexes or p electronarrays [21–23], are their well-defined composition,flexibility, anisotropic nature and possibility ofpolarity modulation. Lipid based supramolecularassemblies may be organized as multilamellar ortubular micro-structure [24,25] as well as in non-lamellar inverted hexagonal or cubic packings. Ashost structures, lipid/water phases possess a highcapacity to encapsulate various water solublespecies among which fluorescent and chemi-luminescent dyes have been studied [26,27]. More-over, the photophysical properties of moleculesincorporated in these supramolecular assembliesshow enormous changes, which reflect the inter-action of the molecules with their microenviron-ment providing an insight into thestructure–property co-relationship in these sys-tems. Non-amphiphilic molecules are not ideallysuited for thin film deposition by this technique.However recent studies [28–31] suggest that highquality LB films of these materials can be formed,when a long chain fatty acid or some suitable inertpolymer matrix is used as a supporting medium.The spectroscopic properties of such films arequite similar to their amphiphilic counterparts,

especially with regard to their spectroscopic andaggregating properties. These similarities justifythe study of non-amphiphilic compounds in mixedLB films since they may be readily put to largescale applications that are cost effective, comparedto their amphiphilic counterparts.The current study is focused on a well-known

oxazole derivative 2-(4-biphenylyl)-6-phenyl ben-zoxazole abbreviated as PBBO. Molecular struc-ture of PBBO is shown in Fig. 1. The dimension ofthis molecule, while considering this as a linearchain is about 1 nm. However, in the actualstructure as shown in the Aldrich catalog there isslight bending of the molecule and hence thedimension of the actual molecular structure shouldbe less than 1 nm. Generally the five-member ringoxazole chromophore does not emit fluorescencehowever when oxazole chromophore is attached toone or more phenyl rings, strong fluorescenceemission occurs [32]. Oxazole derivatives arecharacterized by their intense fluorescence andare used in light emitting diodes, solar energyconcentrators and in thin film electro-luminescentdevices [33,34]. They are also used as ultra fastoptical amplitude limit material and UV-laser dyedue to their non-linear transmission property [35].Despite such interesting properties, photophysicalcharacterization of non-amphiphilic PBBO mole-cules mixed with SA and PMMA have never beenstudied in Langmuir–Blodgett films.Here we have studied the monolayer character-

istics of PBBO mixed with stearic acid (SA) as wellas with isotactic polymethylmethacrylate (PMMA)at the air–water interface. Properties of mixed LBfilms have been discussed in the light of UV-visabsorption and fluorescence spectroscopy and

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S.A. Hussain et al. / Journal of Luminescence 114 (2005) 197–206 199

compared with the results in solution and micro-crystalline phase. The two different supportingmatrices, namely fatty acid and polymer, are usedbecause they possess different dielectric constantgiving various polar environments for the subjectmolecules. Another reason for using two differentmatrices is that these are predicted to havedifferent inner spacing, which may change theconformation of the chromophore.

2. Experimental

PBBO purchased from Aldrich chemical com-pany, USA and vacuumed sublimed followed byrepeated recrystalization before use. SA (pur-ity499%) from Sigma, USA and isotactic PMMAfrom Polyscience were used as received. Thesolvents chloroform (SRL, India) and ethanol (E.Merck, Germany) are of spectral grade and theirfluorescence spectra were checked before use. Acommercially available Langmuir–Blodgett filmdeposition instrument (Apex, 2000C, India) wasused for isotherm measurements and for multi-layer film deposition. The subphase used was tripledistilled deionised water. The pH of the sub-phasewas 6.5 and the temperature was 24 1C. Solutionsof PBBO, PMMA, SA as well as PBBO–PMMAand PBBO–SA mixtures at different mole fractionsare prepared in chloroform solvent and werespread on the sub-phase. After a delay of 15minto allow the solvent to evaporate the film wasslowly compressed for the measurement of iso-therm at ambient temperature. The barrier com-pression rate was 2� 10�3 nm2mole�1 s�1. Surfacepressure measurement was recorded using Wilhel-my plate arrangement as described elsewhere [28].All isotherms were run several times with freshlyprepared solutions. Deposition of multilayerswas achieved by allowing the substrate to dipwith a speed of 5mm/min with a drying time of15min after each lift. Fluorescence grade quartzslides were used for spectroscopic measurement.For each mole fraction of PBBO, 10 bi-layer(both sides of quartz plate) LB films weredeposited. We have chosen 15mN/m as thestandard surface pressure for lifting the LB filmsfor both matrices.

The transfer ratio was found to be 0.9870.02.Fluorescence spectra and UV-vis absorption spec-tra were recorded by a Perkin Elmer LS 55spectrophotometer and Perkin Elmer Lambda 25spectrophotometer, respectively. All the measure-ments were performed at room temperature (24 1C).

3. Results and discussions

3.1. Behaviour of Langmuir monolayer at the

air– water interface

When dilute solution of PBBO in chloroformwas spread onto a pure water surface andcompressed very slowly, the surface pressure didnot rise above 20mN/m and no distinct steepregion was observed in the isotherm curve.Addition of large amount of solution resulted inthe formation of crystalline domains at the air-water interface, which were visible to the nakedeye. Moreover, the islets once formed as a result ofbarrier compression did not degenerate at themolecular level but remained as smaller islets uponrelaxation of surface pressure. Repeated attemptto transfer the floating layer onto solid substratewas failed. However, when PBBO was mixed witha supporting matrix of either SA or PMMA, thefloating layer was found to be highly stable andcould be transferred onto quartz substrate.The surface pressure versus area per molecule

(p–A) isotherm characteristics of pure PBBO,PMMA or SA as well as of PBBO–PMMA andPBBO–SA mixed monolayers at the air–waterinterface are shown in Fig. 2(a) and (b), respec-tively. The area per molecule of pure PBBO isfound to be negligibly small in comparison to theactual planar area for the molecule as predicted bythe space-filling-model. This indicates that purePBBO molecules do not form monolayer at theair–water interface but form multilayers or aggre-gates of pure PBBO.However on mixing PBBO with the supporting

matrix PMMA or SA, in different mole fractions,a stable, floating compressible monolayer isobtained.In pure PMMA isotherm, there exists certain

distinct parts [36]. The ‘transition’ observed at

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10

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BO 0.8 0.6 0.4 0.2 PM

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BO 0.8 0.6 0.40.2 SA

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a pe

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2 )

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(a)

(b)

S.A. Hussain et al. / Journal of Luminescence 114 (2005) 197–206200

about 8mN/m is characteristic for the isotherm ofPMMA. The isotherm of PMMA also shows aninflection point at about 20mN/m, but above thissurface pressure, the monolayer remains no longerstable. From the isotherms of the mixed mono-layer, it is also observed that with increasing molefraction of PBBO, the inflection portion graduallyloss their distinction and at 0.6 and above molefractions of PBBO, the mixed isotherms showsteep rising upto 30mN/m surface pressure with-out any transition point.It is also interesting to note that at higher

surface pressure, in PMMA matrix, the isothermsof all molefractions of PBBO almost coincide. Ithappens because at higher surface pressure themonolayer, which consists of PMMA matrix, nolonger remain stable [36] and actually collapsing ofmonolayer occurs and multilayer and three-dimen-sional crystalline aggregates are formed at theair–water interface. Thus at higher surface pres-sure all the isotherms of mixed monolayers inPMMA matrix lose their distinguishability andoverlap on each other.The isotherm characteristics of pure SA as well

as the mixed SA–PBBO films are shown inFig. 2(b). The areas per molecule are 0.23 and0.21 nm2 for pure SA at the surface pressure 15and 25mN/m, respectively. With increasing molefractions of PBBO, the area per molecule of themixed isotherms of SA–PBBO decrease system-atically and even at higher mole fraction, they donot coincide.It may be mentioned in this context that the

thermodynamic nature of the mixing of various

Fig. 2. (a) Surface pressure ðpÞ versus area per molecule (A)

isotherms of PBBO in PMMA matrix at different mole

fractions of PBBO. The numbers denote corresponding mole

fractions of PBBO in PMMA matrix. PM and BO are the pure

PMMA and PBBO isotherms, respectively. The inset shows

area per molecule versus mole fraction plot at surface pressure

15mN/m (solid line indicates ideality: data taken from isotherm

value). (b) Surface pressure ðpÞ versus area per molecule (A)

isotherms of PBBO in SA matrix at different mole fractions of

PBBO. The numbers denote corresponding mole fractions of

PBBO in SA matrix. SA and BO are the pure SA and PBBO

isotherms, respectively. The inset shows area per molecule

versus mole fraction plot at surface pressure 15mN/m (solid

line indicates ideality: data taken from isotherm value).

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S.A. Hussain et al. / Journal of Luminescence 114 (2005) 197–206 201

two component systems was established by analyz-ing the p–A isotherm of the pure components aswell as the binary mixtures [37]. The excess areasof mixing AE which provides a measure of non-ideality, were calculated as a function of surfacepressure, mixture of composition and the mole-cular weight using the additivity rule [38–40]

AE ¼ A12 � Aav; (1)

Aav ¼ N1A1 þ N2A2; (2)

where A12 is the actual area per molecule of mixedmonolayer, Aav is the calculated average area permolecule of mixture, assuming molefraction ad-ditivity, A1 and A2 are the area per molecule (ormonomer) of each of the single component at aspecific surface pressure, N1 and N2 are theircorresponding molefractions.The insets of Fig. 2(a) and (b) show the plot of

the data of the actual area (A12) per moleculeversus molefraction of PBBO in the mixed mono-layer with PMMA and SA respectively, at theair–water interface and at a surface pressure of15mN/m. The solid lines in the figures representthe ideality curve that corresponds to the plotof average area per molecule (Aav) of themixture versus molefraction, assuming molefrac-tion additivity.The experimental data in case of both PMMA

and SA matrices almost coincide with the idealitycurve. That is from Eqs. (1) and (2) we may saythat AE ¼ 0 and

A12 ¼ Aav ¼ N1A1 þ N2A2: (3)

It should be mentioned in this context that forbinary mixture of molecules exhibiting either idealmiscibility or complete immiscibility, AE ¼ 0;which is our case. It is important to note that inbulk binary miscible mixtures if two liquids areideally miscible, it usually means that they arephysically and chemically similar, so that theintermolecular forces of these two componentsare identical and the presence of the solute doesnot affect the solvent molecules. However, com-plete immiscibility means strong attractive inter-actions among like molecules and almost nointeraction between the unlike molecules [37].The existence of strong attractive interaction

among PBBO molecules may lead to the formationof clusters and aggregates.Although from this additivity characteristics

study, we cannot say definitely whether completemixing or demixing occurs in the monolayer,however our later studies of UV-vis absorptionand fluorescence spectroscopic characteristics defi-nitely conclude that complete demixing of mole-cules occur which lead to the formation ofaggregates of PBBO molecules.

3.2. Absorption spectroscopic study

The absorption spectra of the mixed LB films ofPBBO in two different matrices PMMA and SA atdifferent mole fractions of PBBO, together withthe spectra in ethanol solution and PBBO micro-crystal are shown in Fig. 3a and b, respectively.The solution absorption spectrum in the200–400 nm region shows distinct (0–0) band at328 nm, a broad and weak hump which extendsfrom 250–290 nm and an intense, prominent highenergy band with a peak at 205 nm. The micro-crystal absorption spectrum shows an overallbroadening of the band system having a broadand weak band at around 287 nm and anotherhigh energy band at around 241 nm. The LB filmabsorption spectra at various mole fractions ofPBBO in both matrices are almost similar tomicrocrystal spectra having identical peak positionand broadened spectral profile with respect to thesolution spectrum. However it is also observedthat there is an extremely weak hump at around365 nm, which is somehow observable in case of0.5–0.7 molefractions of PBBO in PMMA mixedLB films and existence of this band has beenconfirmed by excitation spectroscopic study. Incomparison with the solution spectrum, it isevident that the 328 nm intense band in solutionspectrum is red shifted to 365 nm and becomesextremely weak, which is somehow visible incertain mole fractions of PBBO–PMMA mixedLB films. The weak and broadened hump in250–290 nm region and the high energy intenseband with peak at around 205 nm in solutionspectrum are red shifted to give intense prominentbands with peak at around 287 and 241 nm,respectively, in the LB films and microcrystal

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EtOH

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(a.

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EtOH

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S.A. Hussain et al. / Journal of Luminescence 114 (2005) 197–206202

spectra. Moreover all the absorption bands in LBfilms are overall broadened. Although only thebroadening in the spectra cannot be assignedsolely to aggregation of molecules in general, thered shift and broadening observed in the mixed LBfilms are likely to originate due to aggregate.It is relevant to mention in this context that

according to the intermediate strength excitoncoupling theory [41,42], dipole–dipole interactionresults in the raising or lowering of the excitonband to a position either energetically higher orlower than the monomer band. Such a change inenergy is given by

DE ¼2M2ð1� 3 cos2 yÞð1� 1=NÞ

r3; (4)

where M is the dipole moment vector, N is thenumber of monomers in the aggregate, y is theangle between the dipole moment of the moleculeand r, the vector joining the centers of two dipoles.When 01oyo54:71; the exciton band is energeti-cally located below the monomeric band thatcauses a red shift and the corresponding aggre-gates are referred to as the J-aggregates [43], whilefor 54:71oyo901; the exciton band is locatedenergetically above the monomeric band thatcauses a blue shift and the corresponding aggre-gates are referred to as H-aggregate [43]. Corre-sponding to the magic angle y ¼ 54:71; no shift inthe absorption spectrum is observed and corre-sponding aggregates are referred to as I-aggregate.The broadening of the absorption bands accom-

panied with a red shift in the LB film of PBBO,seems to be due to the formation of J-aggregates.It may be mentioned in this context that certain

rigid nearly planar plate like molecular structure aspyrene [44] or rod like molecule as anthracene [45]form aggregate in the mixed LB films. Thesemolecules are actually sandwiched among thematrix molecules (stearic acid) to form aggregates

Fig. 3. (a) UV-vis absorption spectra of PBBO in ethanol

solution (EtOH), in microcrystal (MC) and in PBBO–PMMA

mixed LB films. The numbers denote corresponding mole

fractions of PBBO in PMMA matrix. (b) UV-vis absorption

spectra of PBBO in ethanol solution (EtOH), in microcrystal

(MC) and in PBBO–SA mixed LB films. The numbers denote

corresponding mole fractions of PBBO in SA matrix.

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Fig. 4. Schematic representation of molecular organization of

PBBO molecules in the mixed LB films.

300 350 400 450 500 5500.0

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a.u.

)

S.A. Hussain et al. / Journal of Luminescence 114 (2005) 197–206 203

in the LB films and actually partial or total binarydemixing is occurred in the LB films. In the presentwork the oxazole derivative PBBO has almostlinear rod like structure. For aggregation to beoccurred in the mixed LB films of PBBOmolecules, the most possibility is the sandwich ofPBBO molecules within SA or PMMA moleculesin the LB films. A schematic representation ofmolecular organization is shown in Fig. 4.

Wavelength (nm)

0.5

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Inte

nsity

(a.

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(a)

3.3. Fluorescence spectroscopic study

In Fig. 5a and b, we have plotted the fluorescencespectra of PBBO in PMMA and SA matrices atdifferent mole fractions of PBBO together with thefluorescence spectra in ethanol solution and ofPBBO microcrystal. The fluorescence spectrum ofPBBO in ethanol solution (1� 10�5M) shows abroad band in the 350–550nm region having intensepeak (0–0 band) at around 398nm and an over-lapping hump at around 380nm. The fluorescencespectrum of PBBO microcrystal is somewhat differ-ent from the solution spectrum. The high energyhump at 380nm is totally absent in the microcrystalspectrum having its (0–0) band at about 411nm andan overall broadened spectral profile is observed.Fig. 5a shows also the fluorescence spectra of

the mixed LB films at different mole fractions of

300 350 400 450 500 5500.0

EtOH

Wavelength (nm)(b)Fig. 5. (a) Steady state fluorescence spectra of PBBO in ethanol

solution (EtOH), in microcrystal (MC) and in PBBO–PMMA

mixed LB films. The numbers denote corresponding mole

fractions of PBBO in PMMA matrix. (b) Steady state

fluorescence spectra of PBBO in ethanol solution (EtOH), in

microcrystal (MC) and in PBBO–SA mixed LB films. The

numbers denote corresponding mole fractions of PBBO in SA

matrix.

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EtOH

0.1

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MC

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0.1

0.2

0.3

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MC

Inte

nsity

(a.

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(a)

S.A. Hussain et al. / Journal of Luminescence 114 (2005) 197–206204

PBBO (0.1–0.7M of PBBO) in PMMA matrix. Upto 0.6molefraction of PBBO in PMMA matrix,the LB films spectra have weak but prominent highenergy band at about 380 nm. The intensity of thishigh energy band is considerably reduced above0.6 molefraction of mixed LB films in PMMAmatrix and also in PBBO microcrystal spectra.Moreover the intense band in the solutionspectrum (peak at 398 nm) is well resolved intotwo vibrational peaks at about 398 and 419 nm.With increasing molefractions, the vibrationalbands are gradually diffused and at 0.7 molefraction of PBBO in mixed LB films ofPBBO–PMMA, an overall broadened band isobserved. However still at that mole fraction, thehigh energy band at 380 nm is prominent. Inmicrocrystal spectrum, no high energy band at380 nm is observed. Moreover a broad spectrumprofile is observed.Fig. 5b shows the fluorescence spectra of the

mixed LB films of PBBO–SA at different molefractions of PBBO (0.1–0.7M of PBBO).Here also it is observed that the high energy

band at 380 nm is present in all the LB films.Another interesting thing is that unlike that ofPMMA mixed LB films, here the broad bandprofile does not consist of any prominent vibra-tional structure. In this case the fluorescencespectra have distinct similarity with the micro-crystal spectrum.

3.4. Excitation spectra

The excitation spectrum of PBBO in ethanolsolution, monitored at its emission maximum(380 nm) gives rise to broad spectrum (Figs. 6and 7) and is in good agreement with theabsorption spectrum. Microcrystal excitation spec-tra monitoring at two different wavelengths at 380and 415 nm, show almost the same spectral profile,however these spectra are somewhat different fromtheir absorption counterpart. The existence of

225 250 275 300 325 350 375 400

EtOH

Wavelength (nm)(b)Fig. 6. Excitation spectra of PBBO in ethanol (EtOH) solution,

in microcrystal (MC) and in PBBO–PMMA mixed LB films.

The numbers denote corresponding mole fractions of PBBO in

PMMA matrix. The monitoring wavelength is 380 nm for Fig.

6a and 415nm for Fig. 6b.

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225 250 275 300 325 350 375 400

EtOH0.1

0.2

0.3

0.40.5

0.6

0.7

MC

Inte

nsity

(a.

u.)

Wavelength (nm)

225 250 275 300 325 350 375 400

EtOH

0.1

0.2

0.30.4

0.5

0.6

0.7

MC

Inte

nsity

(a.

u.)

Wavelength (nm)

(a)

(b)

Fig. 7. Excitation spectra of PBBO in ethanol (EtOH) solution,

in microcrystal (MC) and in PBBO–SA mixed LB films. The

numbers denote corresponding mole fractions of PBBO in SA

matrix. The monitoring wavelength is 380 nm for Fig. 7a and

415 nm for Fig. 7b.

S.A. Hussain et al. / Journal of Luminescence 114 (2005) 197–206 205

longer wavelength band at around 365 nm hasbeen confirmed in the microcrystal excitationspectra. The other bands are somewhat different.The excitation spectra of the mixed LB films of

PBBO–PMMA in different mole fractions, andmonitoring at two different wavelengths of 380and 415 nm, are shown in Fig. 6a and b,respectively. The excitation spectra are almostsimilar irrespective of the monitoring wavelength,indicating that both bands originate from the samekind of species of PBBO molecules. In these cases,the 365 nm band is prominent unlike that ofabsorption spectra. Moreover the excitation spec-tra give broadened band profile and there may besome difference in the relative intensity distribu-tion among various bands. However the excitationspectra support the various band positions of theabsorption spectra. For all these spectra, weobserve broadening and change in intensitypattern in comparison to dilute solution spectrumthat might be due to the closer association of themolecules. The excitation spectra resemble thesame while monitoring at the high energy bandand longer wavelength position. This certainlygives the conclusion that the broadband spectralprofile of the emission spectrum originates due tothe formation of excimer.It may be mentioned in this context that

excimeric emission from short oxazole derivativeshave also been observed when they are incorpo-rated into constrained media such as cyclodex-trines [46]. Well-known oxadiazole derivative bu-PBD was also observed to emit excimeric emissionwhen organized in LB films [47]. LB films of someother molecules as pyrene [44] anthracene [45]chromphores and their derivatives [48,29] etc. alsogive rise to excimeric emission in the longerwavelength region and is an indication of aggrega-tion for such molecules in the LB films.

4. Conclusion

In summary, non-amphiphilic PBBO mixed withPMMA or SA form excellent monolayers at theair–water interface. Isotherm studies as well asarea per molecule versus mole fraction study ofPBBO indicate either ideal mixing or complete

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demixing of the binary components. The UV-visabsorption study of the mixed LB films of PBBOreveal the nature of complete demixing of thebinary components of the sample molecules PBBOand PMMA or SA which leads to the formation ofclusters or aggregates. Fluorescence spectroscopicstudies show that excimer emission occurs fromthe microcrystalline aggregates of PBBO. Excita-tion spectroscopic study confirms the formation ofonly excimeric sites.

Acknowledgement

The authors are grateful to DST, Govt. of Indiafor providing financial assistance through SERCDST project No. SR/FTP/PS-05/2001.

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