Optical Properties of Crystalline Pseudoisocyanine (PIC)

Optical Properties of Crystalline Pseudoisocyanine (PIC)

Hans von Berlepsch,*,† Sven Mo1 ller,‡ and Lars Da1hne*,†,§

Institut fur Chemie-Physikalische und Theoretische Chemie, Freie UniVersitat Berlin, Takustrasse 3,D-14195 Berlin, Germany, Fachbereich Physik der Philipps-UniVersitat Marburg, Renthof 5,D-35032 Marburg, Germany, and Max-Planck-Institut fu¨r Kolloid und Grenzfla¨chenforschung,D-14424 Potsdam, Germany

ReceiVed: December 31, 2000; In Final Form: March 23, 2001

Normal incidence reflectance spectra of different crystal faces of 1,1′-diethyl-2,2′-cyanine (pseudoisocyanine)bromide (PIC-Br) and chloride (PIC-Cl) single crystals have been analyzed by a polariton model. The spectraare anisotropic with wide bands of quasimetallic reflectance for two orthogonal polarizations of light due todirectional dispersion of polariton resonances. The measured spectra were analyzed by Kramers-Kronigtransformation and compared with theoretical spectra obtained by solving the wave equation. Because of theherringbone-like packing of PIC molecules in single crystal, theπ f π* transition is split into two Davydovcomponents with energy separation of about 30 meV. Most of the oscillator strength is concentrated in thefundamental exciton peaks, while a fine structure related to molecular vibrations is also present. Structuralisomorphism of PIC-Br and PIC-Cl single crystals is reflected in identical reflection spectra. Glassy PIC-Brfilms of thicknesses between 5 and 150 nm were prepared by spin-coating. The films are built up by a networkof randomly distributedJ-aggregates. By treatment with humid air they become highly ordered, showingthree differently colored crystalline domains in polarized light. The absorption spectra of the domains coincidewith that of the (100), (-101), and (101) faces of single crystal. Surface force microscopy (SFM) revealsheterogeneous layers composed of aligned aggregates of about 0.5µm width.

I. Introduction

Cyanine dyes are known to form in concentrated aqueoussolution tightly bound molecular assemblies, exhibiting a strongspectral shift of their absorption band toward longer wavelengthswith respect to the monomer absorption. The assemblies havebeen named Scheibe or Jelly (J-) aggregates in honor of thetwo researchers who first discovered this phenomenon1,2 inaqueous solution of the dye 1,1′-diethyl-2,2′-cyanine chloride(pseudoisocyanine chloride, PIC-Cl):

Scheibe attributed the peculiar spectroscopic behavior to areversible polymerization of the chromophores due to intermo-lecular interactions. The molecular exciton theory supplied theexplanation for the observed absorption spectra.3 Despite amultitude of studies and data available on optical and spectro-scopic properties of PIC and related dyes,4 which were obtainedover the more than sixty years of research, the knowledge ofthe supramolecular structure of the aggregates is only fragmen-tary. Recently, we could visualize for the first time directly therodlike morphology of the PIC-ClJ-aggregates in aqueoussolution.5 Using cryo-transmission electron microscopy (cryo-TEM) we estimated a rod diameter of 2.3 nm and proposedalternative structure models for the molecular packing within

the aggregate. In particular, a quasi-two-dimensional super-structure has been suggested, which could better explain theoptical properties of theJ-aggregates in solution than previousmodels.6

PIC formsJ-aggregates not only in homogeneous solutions,but also at interfaces7,8 and in crystals.9-11 The respectiveabsorption and fluorescence spectra are similar, which has beeninterpreted as the indication of a common entity that isresponsible for theJ-band. Kawasaki and Ishii12 as well asOwens and Smith13 have recently used scanning tunnelingmicroscopy to investigate PICJ-aggregates adsorbed at surfaces.Monolayers with a brickwork packing structure of individualmolecules,14 similarly well-ordered bilayers, or extended linearaggregates of a dimension corresponding to that of the rodlikeaggregates in solution,5 were found, respectively. These struc-tural studies demonstrate a surprising morphological variety evenon microscopic scale and it is not clear until now, if there reallyexists a unique molecular structure that is responsible for theformation of theJ-band. It is reported that the nature of thecounterion in solutions or at a solid/solution interface and thesolvent15-17 has a slight effect on the spectroscopic character-istics.

Disorder plays an important role in molecular aggregates.Exchange narrowing of static disorder is generally believed tobe responsible for the characteristic sharpness of theJ-band insolutions or in a glass.18,19 In addition, for the dynamics ofexcitons inJ-aggregates in solution, the dynamical nature ofthe environment becomes important and has to be taken intoaccount in the theoretical description.20 The situation is mucheasier when single crystals of the dye can be grown. Then theoptical properties are accessible basically by normal incidencereflectance measurements from different crystal faces, as it has

† Institut fur Chemie.‡ Fachbereich Physik der Philipps-Universita¨t.§ Max-Planck-Institut fu¨r Kolloid und Grenzflachenforschung.

5689J. Phys. Chem. B2001,105,5689-5699

10.1021/jp004581q CCC: $20.00 © 2001 American Chemical SocietyPublished on Web 05/24/2001

been shown recently for a series of dye crystals21-26 includingPIC.9-11 One should have in mind, however, that single crystalspectra will provide only some guidance on the packinggeometry inside theJ-aggregate in glassy host or in solution.

Nevertheless, reflectivity measurements on PIC single crystalsare of interest, because the published data are not in fullagreement and a complete theoretical description of the opticalproperties does not exist until now. Because of the anisotropyof the reflectivity of dye crystals, spectra from different faceshave to be taken, which requires single crystals with high surfacequality. Other shortcomings of the former studies are connectedwith the theoretical modeling of measurements. The success inmeasurement and interpretation of reflectance spectra of differentpolymethin dyes25,26gave us the motivation to reinvestigate thecrystal spectra of PIC. The dense packing of strong transitiondipoles of such dyes leads to stopping bands of high reflectancewhose width varies on different crystal faces. The opticalproperties are determined by polariton resonances that candeviate strongly from the transition energies of molecularexcitons due to the macroscopic polarization.22-27 PIC singlecrystals of optical quality with bromide or chloride as counter-ions were used for the present study. The X-ray structures areknown,28-30 showing isomorphism. In addition, crystallinePIC-Br layers of 100 nm typical thickness with domain widthsreaching hundreds ofµm, grown on solid substrates by a self-organization process (thin layer crystallization, TLC31), wereincluded in the study. Comparison with single-crystal spectraallowed to identify the structure of the different crystallinedomains.

This paper is organized as follows. Section II presentsexperimental details and methods. The results of structuralcharacterization of single crystals are given and the preparationof thin layers is described. After the description of opticalmeasurements the methods to simulate the reflectivity by apolariton model are briefly reviewed. Using these modelcalculations the reflectivity data are quantitatively analyzed insection III. The morphological structure of the thin layers isdiscussed in section IV. Finally, section V presents someconcluding remarks.

II. Experimental Section and Methods

1. Samples.Single Crystals.PIC-Br was a product (NK-1046)from Nippon Kankoh Shikiso Kenkyusho, Japan, and was usedwithout further purification. PIC-Cl was obtained as a gift fromAGFA AG and was used as received as well. Molar extinctioncoefficients of 7.71× 104 and 7.67× 104 L/mol cm weremeasured in methanol for PIC-Br and PIC-Cl, respectively.Taking into account the refractive index of the solvent, oscillatorstrengths of 1.06 were obtained by integrating the spectra.32

Single crystals of mm size were grown from solutions in drymethanol. The X-ray structures of PIC-Br and PIC-Cl areknown28,29 but they were newly determined30 for the presentcrystals to allow for a definite face assignment. Both structuresare isomorphous. PIC crystallizes in the monoclinic space groupP21/n with the lattice parameters given in Table 1, and fourmolecules per unit cell. Note that we have used different unitcells for PIC-Br (notation after ref 29) and PIC-Cl (our own30)crystals. The PIC molecule is nonplanar, i.e., the two quinolinerings are twisted around the central methin group by an angleof 50.6° (PIC-Cl28). Theπ-electrons are delocalized along themethin chain. Because of the twisting different transition dipolemoment vectors may be constructed, depending on which ofthe two polarization directions, i.e., either along theC2-C2′ oralong theN1-N1′ direction, is taken. It may be expected that

the effective dipole direction will be lying between these twotheoretical limits. The dipole moments are taken to be propor-tional to the vectors obtained as difference of the correspondingatom positions. The PIC crystal is built up of strands of stackedmolecules. Adjacent molecules forming one strand are arrangedalong the stacking axis (c-axis for PIC-Br,a-axis for PIC-Cl)with tilt angle τ. Figure 1 shows two single strands withoppositely oriented molecules forming a double strand withherringbone-like architecture. Two of the fourπ f π* transitiondipole moments in the unit cell are equivalent resulting in twoorthogonal Davydov componentsm+ andm- as even and oddcombinations of the nonequivalentπ f π* transitions, respec-tively. The smaller dipole component (m-) is aligned parallelto the monoclinicb-axis. The coordinates of the dipole momentsare included in Table 1.

The morphology of the investigated PIC-Br crystal with theindicated faces studied by normal incident reflectance is shownin Figure 2. The crystal faces were determined by comparingangles measured with a STOE reflection goniometer with thosecalculated from the X-ray data using the program SHAPE.33

The same procedure was applied to the PIC-Cl crystal. Becauseof the similarity details are omitted here, but one shouldremember, that, due to the different unit cells, the Miller indicesof equivalent faces differ too.

Thin Layers.The layers of PIC-Br were prepared by spin-coating as detailed described in ref 31. On applying a highacceleration of the spinning table (3000 rpm in 1.6s) layers withtypical thicknesses between 5 and 150 nm have been preparedfrom concentrated dye solutions in methanol (1.2 wt %). Thelayer thickness was estimated from the absorbance of redissolvedlayers. Prior to deposition dust particles were removed byfiltering. As substrates either quartz plates (22× 22 mm2), whichwere purified by concentrated H2SO4, rinsed by distilled waterand dried, or glass slides covered by a silicone rubber wereused. The rubber layers were prepared by casting a precursorsilicone polymer (“Sylgard 184” from Dow Corning) onto theslides and then cross-linked thermally. The hydrophobic surfacesof the covered substrates were then made hydrophilic byoxidation accomplished by a plasma cleaner (model PDC-32G,Harrick Scientific Corporation, USA) operating with ordinaryair.34 The procedure yielded substrates with smooth and cleansurfaces. Immediately after spin-coating the PIC-Br layers areglasslike but show the spectral characteristics ofJ-aggregates.The glassy state completely converts at room temperature within2 days of treatment with humid air of about 97% relative

TABLE 1: Crystal Structure Data of PIC-Br and PIC-Cl,Coordinates of Davydov Components m+ and m-, and TiltAngles between the Molecular Dipolesτ

PIC-Br

a ) 13.558 Å R ) 90.0°b ) 10.588 Å â ) 90.54°c ) 14.095 Å γ ) 90.0°N1-N1′: m+ ) (0.6554, 0.0, 8.7949)

m- ) (0.0, 3.8498, 0.0)τN1-N1′ ) 47.2°

C2-C2′: m+ ) (1.1719, 0.0, 4.6033)m- ) (0.0, 1.5607, 0.0)τC2-C2′ ) 36.4°

PIC-Cl

a ) 13.706 Å R ) 90.0°b ) 10.495 Å â ) 90.75°c ) 13.584 Å γ ) 90.0°N1-N1′: m+ ) (-8.7967,-0.0010, 0.7471)

m- ) (0.0014, 4.0185, 0.0)τN1-N1′ ) 48.9°

5690 J. Phys. Chem. B, Vol. 105, No. 24, 2001 von Berlepsch et al.

humidity (maintained in closed vessels containing a saturatedK2SO4 solution) into a highly organized state that is character-ized by the appearance of large and highly uniform crystallinedomains. In polarized light they show dichroic colors andcharacteristic absorption spectra. On silicone rubber domainsizes of the order of a few mm2 are often found, while thedomains are usually much smaller on quartz substrates. Thesame procedure of preparation yields for PIC-Cl again crystallinelayers, but with typical domain sizes only of the order of 10µm2, which is too small for absorption measurements.

Scanning force microscopy (SFM) was used to characterizethe surface morphology of the dye layers. The measurementshave been performed using a standard nanoscope (MultimodeIIIa, Digital Instruments, Santa Barbara, California). Themicroscope was operated in the TappingMode using silicon tipsat resonance frequencies of 290-310 Hz under ambientconditions. The cantilever was forced to oscillate near itsresonance frequency. The spring constant of the cantilevers usedwere in the range of 40 N/m with tips of a curvature radius>10 nm (Nanosensors, Jena, Germany).

2. Measurement and Evaluation of the Spectra.A universalmicroscope spectral photometer UMSP 80 (Carl Zeiss, Oberko-chen) with quartz optics was used to measure the normal inci-dence reflectance spectra between 1.5 and 5.0 eV. The diameterof the circular measuring area was between 3.1 and 10µm, themaximum resolution was 0.5µm. Spectra of two orthogonallinear polarizations were measured with one polarization selectedto yield maximum reflectivity at the low energy threshold ofπ

f π* transitions. Absolute reflectivity was obtained from thecomparison of measured spectra with a chromium standard ofknown reflectivity. The optical anisotropy of crystals can causesome ellipticity of polarization of the reflected light. Thisellipticity is small if only one of the two normal modes(polaritons) propagating into the crystal is excited. Directionaldispersion of the polariton can rotate this polarization as thespectrum is scanned over excitonic resonances.24 We did notfollow this rotation but maintained the polarization fixed. Theerrors introduced have little effect on the transition energiesderived for the excitonic states.25,26 The same UMSP 80 wasused to measure the absorption spectra of the thin crystallinelayers (from rear side). Absorption spectra of solutions andglassy layers as well as the reflection spectra of the glassy layerswere recorded by a Lambda 9 spectrometer (Perkin-Elmer)equipped with a 60 mm integrating sphere. All spectroscopicmeasurements were carried out at room temperature (21°C).

Kramers-Kronig transformation of the reflectivity of selectedcrystal faces provides the input data to simulate the dielectrictensorε(ω) by a polariton model.27,35,36Optical excitations wererepresented by Lorentz oscillators of transition energiespωjl ,

line widths pγjl, and a contributionøjl to the static dielectricsusceptibility and inserted into the Fresnel equation which yieldstwo electromagnetic waves of orthogonal polarization, thepolaritonsω(k) of the wavevectork, wheresj are the compo-nents of the unit vectork/k:

For near normal incident light the unit vectors is parallel tothe face normal. Diagonalization of the tensor succeeds only inthe case of orthogonal crystal axes, which does not apply here.As discussed previously24-26,36 the contribution of strongπ fπ* transitions to the dielectric susceptibility is much larger thanthe difference of the componentsεjl due to UV transitions whichare accounted for by a background dielectric constantεj(∞). Wetherefore use the two orthogonal Davydov componentsm+ andm- of theπ f π* transitions and a third direction perpendicularto both as principal axes of the dielectric tensorε(ω). Twodiagonal elementsεj result from the Davydov componentsm+andm- describing optical transitions to the energy levels|jl ⟩while the third component is only due to high-energy transitions:

Neglecting the small off-diagonal elements in this set ofcoordinates allows us to reduce the Fresnel equation to

Figure 1. Threadlike arrangement of PIC molecules in PIC-Br single crystals. Both stacks are symmetric to each other, but their inclination inrespect to the paper plane differs. The chinoline rings are sketched as rectangles. Crystal water and counterions are omitted.

Figure 2. Typical shape of a PIC-Br single crystal with the indicatedcrystal faces used for reflectivity measurements.

εijsisjn4 - ∑

l

(εijsisjεll - εilεljsisj) + |εij| ) 0 (1)

εj ) εj(∞) + ∑l

øjlωjl2

ωjl2 - ω2 - iγjω

(2)

Crystalline Pseudoisocyanine J. Phys. Chem. B, Vol. 105, No. 24, 20015691

the solutions of which provide the refractive indicesn(ω) ofeach polariton mode and allow calculating the reflectancespectra. The optical resonances depend on the anglesϑ+ andϑ- between the optical dipolesm+ andm- and the propagationvectors that is parallel to the face normal. The values of theseangles for the investigated faces are given in Table 2.

III. Results and Discussion

1. Single Crystals of PIC-Br and PIC-Cl. (1-10) and (101)Faces of PIC-Br Single Crystals.Best coupling of light to thestrong Davydov componentm+ is achieved on the (1-10) facebecause the dipole lies almost within this face and theprojections of both Davydov components onto the face arenearly orthogonal. The polaritons then couple primarily to onlythis componentm+ if the polarization of light is parallel to itsprojection with little perturbation by the second component, whatminimizes depolarization of the reflected light. The weakDavydov componentm- is directly accessible via the (100),(-101), and (101) faces because it is aligned in that case exactlyparallel to these faces. Indeed, the same spectra were observedfor all three faces. Figures 3 and 4 show the reflectance spectrafor light polarized parallel (|) and perpendicular (⊥) to theprojection ofm+ andm-, respectively, onto the (1-10) and (101)faces. Coupling of light to the strong dipolem+ leads on the(1-10) face to a strong 0.8 eV wide reflectance band, while forperpendicular polarization a reflectivity of only about 5% isobserved. Tanaka et al.10 obtained the same qualitative spectrabut with slightly lower intensity. The spectral behavior is typicalfor cyanine dye crystals and results from a narrow but verystrong excitonic resonance around 2.2 eV as Kramers-Kroniganalysis reveals. The high-energy edge around 3 eV correspondsto the transition energy of longitudinal excitons that do notcouple to light. The reflectivity of them- component measuredon the (101) face is only half that of them+ component. Thestill weaker reflectance band for perpendicular polarization(⊥m-) arises from them+ component. Because of the inclinationof them+ dipole with respect to this face the spectrum is blueshifted (directional dispersion of the polariton resonance36). Thesituation becomes more obvious when the imaginary part ofthe dielectric susceptibility Im(ε) obtained by Kramers-Kronigtransformation of the reflectance spectra are considered in Figure5. Because the wave vectork was orthogonal to both dipolemoments m+ or m- at these particular faces the peakscorrespond to transverse excitons. The Im(ε) spectra are verysimilar in shape, but differ in height by a factor of about 6.8.The weaker component is shifted by 30 meV to higher energy.The spectra show the same fine structure, indicating thecontribution of at least two vibronic satellites with little oscillatorstrength compared with the 0-0 transition. The half-width ofthe Im(ε) peaks are about 60 meV, which is still broader thanthat of the main absorption band (peak at 2.17 eV) in solutionat room temperature of about 20 meV (cf. Figure 10 below).37

This is a remarkable finding. While the extreme sharpness ofthe J-band in solution may be understood in terms of disorderinduced line narrowing effects,18,20the reason for the still largerhalf-width in the case of single crystal compared with solutionis unclear and requires theoretical foundation. A quantitativedescription of the experimental data is achieved by fitting thepolariton modes, i.e., the solutions of eq 2, to the correspondingreflectance spectra. The widths, amplitudes and positions of

peaks of Im(ε) provide the input parameters for the dielectricfunction. The obtained set of Lorentz oscillators representingthe contributions ofπ f π* excitations to the dielectric functionthereafter enables the calculation of all other spectra. Longitu-dinal excitons are obtained as resonances of Im(-1/ε). Theymark the high-energy cutoff of reflectivity bands. Because theirpositions are determined by the strength of transitions differentvalues at≈3.0 and 2.65 eV form+ and m- are found,respectively. Further experimental input is the reflectance at lowenergy, which together with the Lyddane-Sachs-Teller relationof transition energies of transverse and longitudinal excitonsprovides a value for the background dielectric constantε (∞)and the contributionΣløjl of the transitions to the staticsusceptibility. The fit of the strong reflectance spectra on the(1-10) and (101) faces with the values listed in Table 3 isexcellent (Figures 3 and 4).

For both Davydov components a strong exciton contains mostof the oscillator strength. The transition energies are only 30meV apart and their line width is nearly identical. Comparedwith monomer solution where the main transition is found at2.37 eV, the energy is lower. This is a common effect for

(n2 - ε2 )( n2 - ε3 )ε1s12 + (n2 - ε3 )( n2 - ε1 )ε2s2

2 +

(n2 - ε1 )( n2 - ε2 )ε3s32 ) 0 (3)

TABLE 2: Angles T+ and T- between the DavydovComponents m+ and m- and the Face Normals for PIC-Brand PIC-Cla

face ϑ+ (deg) ϑ- (deg)

PIC-Br,N1-N1′ (100) 85.2 90.0(-101) 50.6 90.0(101) 41.6 90.0(1-10) 87.1 38.0(11-1) 62.3 47.1(111) 56.6 47.4(01-1) 53.2 36.9

PIC-Br,C2-C2′ (100) 75.2 90.0(-101) 60.6 90.0(101) 31.6 90.0(1-10) 81.0 38.0(11-1) 68.9 47.4(111) 51.1 47.4(01-1) 54.4 36.9

PIC-Cl,N1-N1′ (001) 85.1 90.0(101) 50.5 90.0(10-1) 40.0 90.0(01-1) 87.0 37.7

a Note that the different choice of the molecular dipole direction(along N1-N1′ or C2-C2′) leads to considerable deviations in theorientation of Davydov components

Figure 3. PIC-Br: Reflectance spectra of the (110) crystal face withpolarization of light parallel and perpendicular to the projection of thestrong Davydov componentm+ onto this face. The spectrum for parallelpolarization (symbols) is well fitted (solid line) by the set of Lorentzoscillators given in the left column of Table 3.


strongly coupling dyes and assumed to be due to dipole-dipoleinteraction.36 Two vibronic satellites are well resolved for theweak transition, while five in the case of the strong transitionproduce the dips in the stopping band. The progression of the

Figure 4. PIC-Br: Reflectance spectra of the (101) crystal face withpolarization of light parallel and perpendicular to the weak Davydovcomponentm- . The spectrum for parallel polarization (symbols) iswell fitted (solid line) by the set of Lorentz oscillators given in theright column of Table 3.

Figure 5. PIC-Br: Imaginary part Im(ε) of dielectric constant of thestrong (m+) and weak (m-) Davydov components obtained byKramers-Kronig analysis from the reflectivity spectra of the (110) and(101) faces shown in Figures 3 and 4 (symbols).

Figure 6. PIC-Cl: Reflectance spectra (symbols) measured on (01-1)and (10-1) crystal faces with polarization of light parallel to theprojection of the strong (m+) and weak (m-) Davydov componentsonto the respective faces. The solid curves represent the fits by the setof Lorentz oscillators.

Figure 7. PIC-Br: Reflectance spectra (solid curves) for orthogonalpolarizations of light on different indicated crystal faces along withthe spectra calculated for the polariton (broken curves). Moleculartransition dipole alongN1-N1

′ direction: (- -). Molecular transitiondipole along theC2 - C2

′ direction: (- -).


vibronic states appears not strictly regular. A progression ofroughly 0.21 eV may be estimated, while the vibronic energyof the first state is markedly smaller. The additional shift hasbeen predicted by Philpott38 from studies of the vibroniccoupling in the polariton states. For the monomer spectrum asmaller vibronic energy of 0.174 eV is generally accepted,39

which is in agreement with the skeletal mode.40 A similardifference in the vibronic energies for crystal and solution hasalso been reported by Weiser et al.36 for another cyanine dyeand can be ascribed to the restricted motion in the crystal.

(01-1) and (10-1) Faces of PIC-Cl Single Crystals.Due tothe isomorphism of PIC-Br and PIC-Cl crystals the reflectancefrom equivalent faces can be expected to agree. The anglesϑ+andϑ- between the Davydov componentsm+ andm- and thenormals of equivalent faces listed in Table 2, are nearly thesame and the measured spectra of the (01-1) and (10-1) faces(Figure 6) are indeed identical with that of the (1-10) and (101)faces of PIC-Br. Note that even the absolute reflectivities agreewithin the accuracy of the measurement. The imaginary part ofthe dielectric susceptibility Im(ε) obtained by Kramers-Kronigtransformation shows good agreement with the PIC-Br crystalresult. Thus, it is not surprising, that also the Lorentz oscillatorsdescribing the reflectance data are identical within the errorrange. Because of their obvious agreement figures are not givenhere.

Summing up, it may be stated that the optical propertiesconfirm in a nearly perfect way the isomorphism in thecrystalline structure of PIC-Br and PIC-Cl single crystals. Hence,in the following we restrict the discussion solely on the PIC-Brcrystal.

Comparison of Reflectance Spectra with Solutions of theWaVe Equation.In crystals with strong molecular transitions,the normal modes of electromagnetic field are the polaritonmodes described by eqs 1 and 3. They are no longer transversewaves as in isotropic media. The coupling of light to theexcitonic resonances of the crystal opens a gap for electromag-netic waves between the transverse (T) and the longitudinal (L)exciton. The resonance energy of the polariton depends on theangle between thek vector of light and the transition momentof the Davydov components. It shifts through the gap from Tto L with decreasing angle,21,41an effect which becomes visibleas directional dispersion. Because of concomitant changes ofpolarization both polariton modes couple to both Davydovcomponents with varying strength for different crystal faces.The data sets of Tables 3 can be used to calculate the reflectanceof all other crystal faces taking into account only the different

angles between the wave vectork of incident light and thedirections of molecular dipoles,ϑ(, given in Table 2.

The simplest case is that of the (100), (-101), and (101) faces,because here the weak Davydov componentm- lies completelyinside the face and the projection of the strong componentm+is strictly orthogonal, i.e., there is no coupling of light to thiscomponent. The experimental spectra alongm- agree for thedifferent faces within the accuracy of the measurement and thedata set of Table 3 obtained from the fit to the (101) spectrumwell reproduces the other two spectra (Figures 7a-c). Equally,a different choice of molecular dipole direction (alongN1-N1

′

or C2-C2′) has only a negligible effect on the weak component

(Figure 7a).The effect of decreasingϑ+ becomes visible as a blue shift

of the optical resonance. First we discuss the qualitative spectralfeatures while the quantitative fit will be considered below. Forlight polarized nearly parallel tom+, i.e., for the (100) face(ϑ+ ) 85.2°) , the resonance is located around 2.18 eV (cf. Table3 and Figure 5) close to the transverse exciton. Upon tiltingthe dipolem+ with respect tok, the peak of Im(ε), respectivelythe reflectivity maximum, is shifted up to∼2.7 eV for the (101)face (ϑ+ ) 41.6°, Figure 7c). For the (-101) face (ϑ+ ) 50.6°,Figure 7b) the peak of Im(ε) is located in between. In additionto the blue shift of resonances with decreasing angleϑ+ due to

Figure 8. Number of electronsneff that contribute to optical absorptionfor both Davydov components of PIC-Br, obtained by integration ofthe Im(ε) spectra plotted in Figure 5.

Figure 9. Absorption spectra of aqueous PIC solutions and glassylayers on Sylgard/glass, measured with unpolarized light at room tem-perature. PIC-Cl solutions of: 2.35× 10-5 mol/L (‚‚‚) and 8.3× 10-3

mol/L (- -). 60 nm thick layers of: PIC-Cl (- -) and PIC-Br (s).

Figure 10. Specular reflectivity (left ordinate) of glassy PIC-Br layerson Sylgard/glass as a function of layer thickness, measured withunpolarized light from the rear side. The specular reflectivity wasobtained as the difference of measured total and diffuse reflectivity.Layer thickness from bottom to top: 10, 50, 130, and 320 nm. Forclarity, the spectra were off-set by adding a constant. The broken curveis the absorption spectrum (right ordinate) of the 50 nm layer.


the directional dispersion of the polariton also the distributionof the oscillator strengths between the main transition and itsvibrational satellites is altered. The same effect was recentlyreported for another cyanine dye and has been explained byadditional microscopic coupling mechanisms.36 For checkingthe set of Lorentz oscillators we also compared measured andcalculated spectra of the (110) face, which is equivalent to the(1-10) face due to symmetry reasons (Figure 7d). The goodagreement supports the choice of parameters. Deviations arewithin the limits of variation on different faces. As an exampleof a crystal face where the projections of dipolesm+ andm-are not orthogonal to each other, the spectra of the (01-1) faceare presented in Figure 7e. Maximum reflectivity for the stronglyreflecting mode was observed when the polarizer was rotatedby 97° instead of 90° with respect to the weak mode. The reasonis the coupling of both dipoles to the same polariton modes,which become elliptically polarized.24 In the strongly reflectingmode two polariton resonances are observed, the first one near2.2 eV arises from the weak dipolem-, the second one near2.7 eV is related to the strong dipolem+; compare Figure 7c.The second polariton mode is weak and structureless. The mainfeatures of the measured spectra of both modes are wellreproduced by the model parameters, although rotation ofpolarization with photon energy was not accounted for.

Returning back to the quantitative comparison of measuredand calculated reflectance spectra we point out, that dependingon the choice of molecular dipole direction used for calculation(i.e., alongN1-N1

′ or C2-C2′), marked differences can appear,

as seen in Figures 7b,c. Note that the molecular dipole directioneffects the theoretically derived spectra via the modified anglesϑ+ and ϑ- as well as angleτ (cf. Tables 1 and 2). Thedifferences between the theoretical spectra may be neglectedin the case of the (100), (110), and (01-1) faces. However, thespectra of the (-101) face displayed in Figure 7b suggests thatthe PIC’s transition dipole moment is obviously aligned betweenthe two limiting directions taken for calculation (along theN1-N1

′ or C2-C2′ direction). The best fit of them+ component in

the case of the (101) face is achieved when the transition dipoleis oriented along theC2-C2

′ direction.Further support comes from a comparison of oscillator

strengths of the respective transitions. The tilt angleτ betweentwo nonequivalent molecular transition dipole moments in theunit cell determines the ratio of the oscillator strengths of bothDavydov componentsf+ and f- by the geometric relation:f+/f- ) 1/tan2(τ/2). The crystallographic data of Table 1 yield forthe f+/f- ratio values of 5.25 (N1-N1

′ direction) and 9.25 (C2-C2

′), respectively. The experimental oscillator strength can becalculated from the sum rule of the spectra of Im(ε):

whereneff(ω) is the number of electrons of a molecule, whichcontribute at photon energies belowpω to optical absorption.N is the density of molecules,ε0 the absolute permittivity,mthe mass of the electron, ande the electronic charge.neff(ω)was calculated for PIC-Br from the Im(ε) spectra of bothDavydov components displayed in Figure 5, and the resultingfunctions are plotted in Figure 8.neff(ω) strongly increases atthe absorption threshold and saturates at high energy when theabsorption band has been passed. Above 3 eV where residualabsorption is small the ratio of the saturation values is about6.7, which ranges between the corresponding limitingf+/f-values obtained from the crystallographic data along theN1-N1

′ or C2-C2′ direction. The oscillator strength can also be

obtained from the contributionsøjl to the susceptibility of theLorentz oscillators (Table 3) by the relation:26

The corresponding values aref+ ) 2.84 andf- ) 0.53, leadingto f+/f- ) 5.35 and to the total oscillator strength off+ + f- )ftot ) 3.37. Within the made approximations the agreementbetween all values is satisfying. Besides the ratio of oscillatorstrengths, also the total oscillator strength is an essentialparameter. In the models of Davydov splitting one usuallyassumes that the electronic states of the single molecules arenot altered by intermolecular interactions and the differencesin the spectra arise essentially from the interaction of transitiondipoles with the crystal field. In particular, it is supposed thatthe oscillator strength of a transition is not affected on goingfrom solution to crystalline state,9,23,25if a factor of 3 resultingfrom the random orientation of the molecules in solution isconsidered. For PIC-Brftot ) fs × 3 ) 3.18 is obtained fromthe oscillator strength in methanol (fs ) 1.06). The quantitativeagreement between the differently estimatedftot values is againsatisfying, what confirms the statement of conservation ofoscillator strength. Similar quantitative relations were obtainedfor the PIC-Cl crystal.

2. PIC-Br Layers. Immediately after preparation by spin-coating from methanolic solutions, PIC-Br and PIC-Cl layersshow isotropic absorption spectra, that indicate a glasslike state.The absorption bands are red-shifted with respect to monomerabsorption and they exhibit a well-resolved fine structurepointing out the formationJ-aggregates. The spectral featuresare the same for different counterions (Br-, Cl-, J-) andsolvents,42 yet the spectra differ from those of aqueous PIC-Clsolutions, cf. Figure 9. The shift in peak maxima of maintransition and vibrational satellites is obvious. However, theabsorption spectra of the layers are in surprising agreement withthose reported for polymer-bound PICJ-aggregates,43 whatindicates similarities in the aggregates structure. The layers showa metallic luster in reflection. To get quantitative values of theoptical constants normal incidence reflectance measurements(from the rear side through the glass support) were carried outfrom the NIR spectral region up to a photon energy of 3.5 eV.The spectra for different layer thicknesses between 10 and320 nm are plotted in Figure 10. It is obvious that thecorresponding absorption spectrum (broken curve) has itscounterpart in the reflectivity, but there are additional oscillationson the low energy side, the wavelength of which becomesshorter with increasing film thickness. This effect, that wasobserved by Marchetti et al.9 on their thin PIC-J crystals too,can be interpreted due to Philpott44 as the excitation of standingwaves inside a thin layer (virtual polariton modes). These modes

TABLE 3: Transition Energies pωjl , Line Widths γjl, andContributions øjl to the Static Susceptibility of theTransitions in PIC-Br a

strong componentm1 weak componentm2

ω1l (eV) øll pγ1l (meV) pω2l (eV) ø2l pγ2l (meV)

2.165 1.245 64 2.195 0.210 592.265 0.025 75 2.382 0.047 1052.385 0.230 115 2.605 0.025 2002.59 0.017 1702.61 0.042 2752.81 0.010 300

a ε(∞) ) 2.75 and is the background dielectric constant.

f )mε0

e2N∑j,l

ωjl2øjl (5)

neff(ω) )2mε0

e2πN∫0

ωω′Im(ω′) d ω′ (4)


prevent the calculation of optical constants of the thin films byKramers-Kronig analysis of the reflectance spectra.42

After treatment with humid air formerly glasslike layers ofPIC-Br show three different crystalline domains, which can beeasily identified by inspection with linearly polarized light.Dichroic colors due to light absorption become apparent.45 Intwo perpendicular directions of polarization, the domains appearred/pale red, ocher/pale red or yellow/pale red, respectively. Thecorresponding dichroic absorption spectra are depicted in Figure11. Measurements as a function of polarization angle showedthat both components in a domain are polarized strictlyperpendicular toward each other (90( 2°). Respective spectrataken from different positions inside the same domain, or fromdifferent domains of the same kind or different samples, arewell reproducible. The observed spectral behavior is very similarto that of recently studied31,45 crystalline streptocyanine dyelayers. PIC-Cl layers also show crystallization, but with muchfaster kinetics resulting in markedly smaller domain sizes, thatprevents spectroscopic investigations. Going from a red viaocher to a yellow domain is connected with a blue-shift of peakmaxima, analogous to the shift of the reflectivity spectra of the(100), (-101), and (101) faces of single crystal (cf. Figures 7a-c), while the weak component remains essentially unaltered inpeak position and intensity. To look for a an approximatecorrespondence of molecular dipole orientations in layers andsingle crystals, we measured the reflectance spectra of a red/pale red domain and compared these with those of the (100)face of the single crystal (Figure 12). Disregarding, at first, thelow energy range up to 2.2 eV, the agreement between bothspectra is remarkably good, even the absolute value of reflec-tivity, and a structural correspondence seems obvious. Theadditional maximum of the strong component at 2.1 eV andthe red-shift of peak maximum of the weak component has tobe considered as interference by virtual polariton modes. Thatthere also exists a similar correspondence of the dipole orienta-tions for the other two domains and certain single-crystal faceswill be demonstrated in the following.

Because of the discussed difficulties arising from virtual po-lariton modes, we compared in Figure 13 the measured ab-sorption spectra instead of reflectance spectra with the calculatedmodel spectra of the (100), (-101), and (101) faces. Agreementis excellent. With respect to their spectrum red/pale red, ocher/pale red, and yellow/pale red domains correspond to the (100),(-101), and (101) faces of the single crystal, indicating at leasta similar crystal structure. Remember, the respective single-crystal faces are unique in that the weak Davydov component

m- lies strictly inside the faces, while the orientation of thestrong componentm+ with respect to the face normals (ϑ+)differs (cf. Table 2). Thus, the same relations are expected tobe valid also for the domains of the thin layers. For the fit ofthe absorbance of the yellow/pale red domain ((101) face; Figure13c) it was again necessary to assume a transition dipole, thatis oriented along theC2-C2

′ direction of the molecule, whilefor the two other domains the orientation along theN1-N1

′

direction yielded the better fit. Reflection loss becomes visibleas weak maximum on the low energy side of the strong compo-nent absorption in Figure 13a. Correction would slightly shiftthe peak toward larger energies, but has generally not beenundertaken.

After we could successfully describe the absorption spectraof PIC single crystals we will now compare them with thespectra of PICJ-aggregates in aqueous solution or in polymericmatrices.J-aggregates in solution or in glassy host generallyexhibit three peaks. While the characteristic narrowJ-bandappears always close to 2.17 eV, the intensity and position ofbroad absorption bands around 2.36 and 2.57 eV depend to someextent on the preparation conditions. Dichroic spectra forpolarization parallel (||) or perpendicular (⊥) to the aggregateaxis were measured on oriented samples.5,46,47 The narrowJ-band at 2.17 eV and a weak broad peak at about 2.38 eV arecharacteristic for parallel polarization, while the perpendicularabsorption shows peaks at about 2.17, 2.34, and 2.51 eV whoseintensity increases with increasing energy. The intensity of the2.17 eV peak for perpendicular polarization depends stronglyon the degree of orientation of the aggregates. It has beenconsidered as theJ-band of isotropically dispersed smallaggregates48 which are always present due to the polydispersityof aggregate lengths.5 The absorption spectra of solutions havealso been theoretically simulated.19,49-51 The theoretical ap-proaches differ in the approximations made in the excitonmodels and for exciton-phonon coupling, and the assumedmolecular structure of the aggregate. For a brickwork modelstructure with two molecules in the unit cell the dichroic absorp-tion spectra have been explained19,51in terms of Davydov com-ponents. While agreement with experimental data is good, thestructural basis of these model calculations can be questioned.On the basis of a morphological study of PIC-ClJ-aggregatesin aqueous solution and crystallographic data, we proposed apacking structure consisting of six single strands with oppositelyoriented molecules in a herringbone-like arrangement,5 wherefor the single strand a molecular packing like in single crystal(cf. Figure 1) was assumed. Under these assumptions dichroic

Figure 11. Set of absorption spectra for orthogonal polarizations oflight of an 80 nm crystalline PIC-Br layer from red/pale red (s), ocher/pale red (- -), and yellow/pale red (- -) domains.

Figure 12. Reflectance from a red/pale red domain of a 70 nm PIC-Br layer on quartz for orthogonal polarizations of light (solid curves)compared with the respective spectra on the (100) face of single crystal(broken curves).


spectra similar to that obtained for the (100) face of single crystal(Figure 13a) should be expected. While the spectrum along thestacking direction (m+ component) shows the main features ofthe corresponding solution spectrum (||), the m- componentdiffers. For perpendicular polarization the main peak is near2.2 eV and the strong bands at higher energies are either absentor markedly depressed and shifted with respect to the solutionspectrum. Because of the absence of conclusive structural dataon the packing of molecules in theJ-aggregate in solution itcan at present only be speculated that the structural differencesproducing the spectral deviations are due to the amphiphilicnature of the dye molecule. It is well established that self-aggregation for amphiphilic systems is driven by entropychanges in the aqueous phase, called the “hydrophobic effect”.52

The balance of enthalpically driven interactions between thedelocalizedπ-electron systems of the dye and the hydrophobic

interactions should ultimately determine the architecture of thedye assembly. The strong effect of the environment on molecularpacking has recently been demonstrated for other cyaninedyes.53,54 Here the side groups attached to the chromophore oftwo adjacent molecules in single crystal are oriented in oppositedirection, while they point into the same direction in aqueousenvironment. Such effect could produce a packing structurewhich is similar to that assumed in the theoretical studies ofthe PICJ-aggregate in solution, but which is different from thatin single crystals. The different ways to assemble the PICmolecule into a brickwork arrangement have already beendiscussed in the literature.15,55Further experimental and theoreti-cal studies are necessary to clear up these questions.

IV. Surface Morphology of Thin Layers

While the macroscopic properties of the dye layers have beenwell characterized by spectroscopy only little is known aboutdye thin film surface coverage, roughness, morphology, andpacking of molecules or aggregates on nanometer scale.Scanning force microscopy (SFM) has been proved to be asuitable method for surface characterization of organized dyelayers,43,56and has been applied to characterize the PIC layers.

While glassy layers appear optically isotropic within theresolution of light microscope,anisotropic structures are expectedto exist on nanometer scale due to the layers spectra, indicatingJ-aggregates. Indeed, a network of randomly oriented needlelikeparticles, as reproduced in Figure 14, was found for thin spin-coated PIC-Br layers (absorbance of 0.25 at 2.15 eV) onSylgard/glass substrate. The substrate is not completely coveredby these particles. They appear only weakly bent and thusobviously rather stiff. Their typical thickness is about 70 nmand they can reach lengths of several hundreds of nanometers.By appearance and optical spectra they differ from the threadlikeJ-aggregates formed in aqueous solution.5 Their morphologyalso differs from that of the flexible bundles of fibers formedby complexation of PIC-J with polyelectrolytes reported byHiggins et al.,43 whereby, however, either absorption spectraare very similar. In line with these findings it may be assumedthat the present needlelike particles are equally composed of

Figure 13. Measured absorption spectra (solid curves, left ordinate)for orthogonal polarizations of light in comparison with the absorptioncoefficientK (broken curves, right ordinate) calculated by the modelparameters of Table 3 for different crystalline domains of PIC-Br. (a):Red/pale red domain; (100) face; molecular transition dipole alongN1-N1

′ direction. (b): Ocher/pale red domain, (-101) face,N1-N1′ direction.

(c) Yellow/pale red domain, (101) face,C2-C2′ direction.

Figure 14. SFM image (TappingMode) of PIC-Br thin layer in glassystate on Sylgard/glass showing a random network of J-aggregates.Size: 5× 5 µm2.


smaller aggregates. Additional information on the structure couldbe gained from submicron spectroscopy,43,57 but has not beenobtained until now for the present system. SFM on thick glassylayers reveals a similar surface morphology as for the thinnerlayers. Again a random network of needlelike particles wasfound with a surface roughness of about 6 nm (rms). Forcomparison, the Sylgard/glass substrate has a typical roughness(rms) below 1 nm.

Before we discuss the changes occurring in morphology oncrystallization of glassy layers we inspect the two micropho-tographs, Figure 15a,b, taken with light microscope using eitherhorizontally or vertically polarized light. The images show fiveadjacent spherulite-like superdomains, each composed of severalof the three differently colored subdomains. All composingsubdomains of a super-domain show similarly brillant colorsor appear pale red, depending on the direction of polarization,respectively. This fact points to a uniform orientation of dipolesfor all composing subdomains of a super-domain and is inagreement with the measured angle dependence of respectivedichroic spectra. The weak Davydov components of the sub-domains are all parallel to each other, while the strongcomponents only differ by their angle with respect to the surfacenormalϑ+. In apparent contrast to this uniform dipole orientation

the boundaries between the (sub)domains are oriented mainlyalong the radial direction. Often additionally fine lines startingfrom the centers of the superdomains are seen. The nature ofthis texture became evident by SFM. For all domains typicalpictures like that shown for a red/pale red domain in Figure 16were obtained. On average about 0.5µm wide aggregates arevisible that are tightly packed and well aligned. The preferentialorientation coincides with the texture already seen by lightmicroscopy. The measured roughness (rms) of differentlycolored domains ranged always between 4.5 and 7 nm. Adefined tilt angle of composing aggregates with respect to thelayer surface, which might be used to characterize the differentdomains could not be derived. The fine texture obviouslyindicates the direction of growth of the heterogeneous layer anddoes not reflect a preferential orientation directly connected withmolecular order, i.e., in particular no correlation to the orienta-tion of the Davydov componentm+. The remarkable roughnesswas unexpected for layers, whose optical spectra are very similarto those of single crystals. The composing aggregates obviouslyexhibit the optical properties of single crystals, while the grainboundaries between seem to be unimportant on the scale of thereflectivity measurements.

V. Conclusions

The reflection spectra of PIC-Br and PIC-Cl single crystalsobtained on different crystal faces for two orthogonal polariza-tions of light have been quantitatively described by a polaritonmodel. The different reflection spectra result from directionaldispersion of polariton resonances as it is commonly observedfor dye-crystals with a strongπ f π* transition.23-26 Due tothe herringbone-like packing of the PIC molecules in the singlecrystal, the π f π* transition is split into two Davydovcomponents, with a small energy separation of about 30 meV.The half-width of the two transitions (Im(ε) spectra) of about60 meV is still larger than that of the main absorption band(peak at 2.17 eV) of PICJ-aggregates in solution of about 20meV (both at room temperature). While the extreme sharpnessof the J-band in solution is understood in terms of disorderinduced line-narrowing effects, the reason for the larger half-

Figure 15. Microphotographs of an 80 nm thick crystalline PIC-Brlayer showing five adjacent spherulite-like super-domains, each com-posed of several differently colored (sub)domains for horizontallypolarized (a) and vertically polarized light in transmission (b),respectively. Picture size: 550× 700 µm2.

Figure 16. SFM image (TappingMode) of a red/pale red domain ofan 80 nm thick crystalline PIC-Br layer showing a heterogeneoussurface with aligned texture. Size: 30× 30 µm2.


width in the case of the single-crystal still requires theoreticalfoundation. Moreover, the rather poor agreement between thedichroic spectra in solution and single crystals indicates differ-ences in molecular packing. Nevertheless, the almost identicalposition of the main exciton peak points to a quite similararrangement of molecules. Most of the oscillator strength ofthe π f π* transition in crystal is concentrated in the funda-mental exciton peaks, but a pronounced fine structure relatedto molecular vibrations has also been found for both Davydovcomponents. The fit of measured reflection spectra showed thatthe transition dipole moment of the PIC molecule is not strictlyaligned along theN1-N1

′ direction as for planar cyaninedyes,24-26 but slightly inclined toward theC2-C2

′ direction. Thewell-known structural isomorphism of PIC-Br and PIC-Cl singlecrystals28,29 is reflected in identical reflection spectra obtainedfor equivalent crystal faces, allowing for a description of thedielectric tensor with nearly the same Lorentz oscillators.

Thin spin-coated films of PIC-Br and PIC-Cl form glassylayers built up by a network of randomly distributedJ-aggregates. The excitation of standing waves inside the layers(virtual polariton modes) prevented the calculation of opticalconstants by Kramers-Kronig analysis of the reflectancespectra. The treatment of glassy PIC-Br layers with humid airleads to the formation of three different and highly orientedcrystalline domains, characterized by dichroic colors and well-resolved absorption spectra. The spectral differences discrimi-nating the domains originate from different dipole orientationswith respect to the layer normal. The similarity of the layerspectra to that of single crystals allowed the determination ofthe dipole directions in the layer and the assignment of thedomains to certain crystal faces. SFM revealed rough andheterogeneous surfaces composed of about 0.5µm wideaggregates, which exhibit the optical properties of single crystals.

Acknowledgment. We thank G. Reck for X-ray structureanalysis, E. Biller for reflectivity measurements, and A. Heiligfor the help with the SFM. L.D. thanks T. Pompe for numerousdiscussions on substrate wettability that led to the applicationof a silicon rubber. H.v.B. is grateful to G. Weiser for valuablediscussions related with the application of the polariton model.The work was supported by the Deutsche Forschungsgemein-schaft (DA 287/5-1) and a project of the INTAS foundation(INTAS No. 97-10434).

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