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Organic Electronics xxx (2013) xxx–xxx

ORGELE 2380 No. of Pages 11, Model 3G

25 November 2013

Contents lists available at ScienceDirect

Organic Electronics

journal homepage: www.elsevier .com/locate /orgel

Analyzing nanostructures in mesogenic host–guest systemsfor polarized phosphorescence

1566-1199/$ - see front matter � 2013 Published by Elsevier B.V.http://dx.doi.org/10.1016/j.orgel.2013.11.025

⇑ Corresponding author. Tel.: +886 2 33663636.E-mail address: [email protected] (C.-C. Wu).

Please cite this article in press as: Y.-T. Tsai et al., Analyzing nanostructures in mesogenic host–guest systems for polarized phoscence, Org. Electron. (2013), http://dx.doi.org/10.1016/j.orgel.2013.11.025

Yu-Tang Tsai a, Chien-Yu Chen a, Li-Yin Chen a, Su-Hao Liu a, Chung-Chih Wu a,⇑, Yun Chi b,Shaw H. Chen c, Hsiu-Fu Hsu d, Jey-Jau Lee e

a Department of Electrical Engineering, Graduate Institute of Electronics Engineering, Graduate Institute of Photonics and Optoelectronics, and InnovativePhotonics Advanced Research Center (i-PARC), National Taiwan University, Taipei 10617, Taiwan, ROCb Chemistry Department, National Tsing-Hua University, Hsin-Chu 30013, Taiwan, ROCc Chemical Engineering Department, University of Rochester, Rochester, NY 14623-1212, USAd Chemistry Department, Tamkang University, Taipei 25137, Taiwan, ROCe National Synchrotron Radiation Research Center, Hsin-Chu 30076, Taiwan, ROC

a r t i c l e i n f o a b s t r a c t

333435363738394041424344

Article history:Received 20 October 2013Received in revised form 5 November 2013Accepted 12 November 2013Available online xxxx

Keywords:OLEDsPhosphorescencePolarized emissionLiquid crystalGIXS

4546474849

Feasibility of polarized phosphorescent organic light-emitting devices (OLEDs) had beenpreviously demonstrated by combining a discotic Pt(II) complex with a glassy-nematicoligofluorene host to form a mesogenic host–guest phosphorescent emitting system. Pre-vious photophysical studies suggested that in the host–guest film, the Pt(II) complextended to aggregate into columnar stacks, exhibiting metal–metal-to-ligand charge trans-fer (MMLCT) emission. Both host molecules and guest aggregates in the host–guest filmscould be oriented by a conductive alignment layer, giving rise to polarized phosphores-cence from the Pt(II) aggregates. Nevertheless, film morphologies and nanostructures ofthe mesogenic host–guest systems have remained to be elucidated. In this work, grazingincidence X-ray scattering (GIXS) was carried out to analyze nanostructures in both neatfilms of the discotic Pt(II) complex and mesogenic host–guest mixture films. In addition,confocal laser scanning microscopy (CLSM) was also utilized for visualization of the mor-phologies of mesogenic host–guest systems. The columnar axes of nanostructured Pt(II)stacks lying on the alignment-treated surfaces were found to be preferentially orientedperpendicular to the rubbing direction, which is responsible for the observed linearlypolarized phosphorescence.

� 2013 Published by Elsevier B.V.

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1. Introduction

The ability to organize nanostructures in orderedorientations over large areas is of primary importance forthe bottom-up fabrication of nanostructure-based devices.By controlling the spatial arrangement and the degree ofordered nanostructures, it is possible to control polariza-tion of light for optical information processing, such asdisplays, optical communication, optical storage, and ste-reoscopic (3D) imaging systems etc. [1,2]. For instance,

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polarized electroluminescence devices would be usefulfor backlights of liquid–crystal displays (LCDs) to makethem more power efficient and for pixels of 3D displaysto simplify their configurations [3–5]. As such, there havebeen substantial efforts in developing polarized organiclight-emitting devices (OLEDs) [6–19]. With the strongintrinsic anisotropy in polymer chains, conjugatedpolymers that can form well aligned thin films [13–18,20], such as mesogenic polyfluorenes [21–24], repre-sent a common class of active materials for polarizedOLEDs. Meanwhile, with better control of molecular struc-tures and material purity, mesogenic conjugated oligomersthat can form well aligned films are another promising

phores-

http://dx.doi.org/10.1016/j.orgel.2013.11.025

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http://www.elsevier.com/locate/orgel


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class of active materials for polarized OLEDs [6–12]. Li-quid–crystalline (LC) oligofluorenes have been reportedas hosts to guide molecular orientation of guest emitters,forming host–guest emitting systems for highly polarizedand efficient OLEDs spanning the whole visible spectrumand white-light emission [8–11].

Most of previous efforts in polarized OLEDs, however,were mainly focused on fluorescence mechanisms. Yet,the development of OLEDs in recent years has largelyshifted toward phosphorescent OLEDs [25,26], since phos-phorescent OLEDs could effectively utilize both singlet andtriplet excitons and realize essentially 100% internal quan-tum efficiencies [27,28]. As such, it is of both scientific andtechnical importance to explore the possibility of achievingpolarized phosphorescent OLEDs. In a recent publication[29], we have reported an initial attempt to realize work-able and functional polarized phosphorescent OLEDs bymixing a discotic mesogenic phosphorescent metal (Pt(II))complex N200 (Fig. 1) with a glassy-nematic oligofluorenehost (F(MB)5, Fig. 1) to form the corresponding mesogenichost–guest (phosphorescent) emitting system.

Spectral properties of the neat N200 spin-cast films sug-gest N200 molecules exhibit strong ground-state intermo-lecular interactions (instead of simply excited-stateinteraction, like excimers) and they tend to self-assembleto form aggregates in films [29]. Previous luminescencecharacterization of bulk samples of these Pt(II) complexesrevealed that they exhibited red emission only in thecolumnar mesophase, while monomer-like green emissionwas observed in isotropic liquid state or dilute solution[30]. Thus the appearance of red emission in N200 samplesis a good indication of columnar-like mesophase packing.Due to the columnar mesomorphic nature of N200 andthe resemblance of photophysical properties in spin-castfilms to those of bulk samples in the liquid crystal phase[30], presumably the square-planar N200 molecules alsopack into a one-dimensional columnar stacking arrange-ment in spin-cast films.

Fig. 1. The mesogenic materials, the Pt(II) complex N200 and the fluoreneoligomer F(MB)5, used in this work.

Please cite this article in press as: Y.-T. Tsai et al., Analyzing nanostructcence, Org. Electron. (2013), http://dx.doi.org/10.1016/j.orgel.2013.11.0

To account for the observed mesomorphism, photo-physical characteristics, and polarized transitions of thesematerials in various states and compositions, we proposedthat N200 self-assemble into columnar stacking in thehost–guest film, exhibiting metal–metal-to-ligand chargetransfer (MMLCT) emission of the Pt(II) complex N200[29,30]. Both the host molecules and guest aggregates inthe host–guest films were successfully aligned on therubbed conducting polymer alignment layer. With suchalignment and effective host-to-guest energy transfer,polarized red phosphorescence and electrophosphores-cence from the phosphorescent Pt(II) complex as aggre-gates were observed. Although this proposed scenarioappeared consistent with all the physical and photophysi-cal characterizations reported earlier, experimentalobservations are desired for film morphologies and nano-structures of the N200-oligofluorene host–guest systems.In this work, the grazing incidence X-ray scattering (GIXS)was performed to investigate morphologies and nano-structures in both neat N200 and its mixture withF(MB)5 films. In addition, the confocal laser scanningmicroscopy (CLSM) was also utilized for the visualizationof morphologies of mesogenic host–guest systems.

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2. Experiment

2.1. Materials

Fig. 1 shows the mesogenic materials, the Pt(II) complexN200 and the fluorene oligomer F(MB)5 used in this work.N200 is a Pt(N^N)2 complex, in which N^N = 2-(3-(3,4,5-trihexoxyphenyl)-1H-pyrazol-5-yl) pyridine. It adopts thedistinctive pyridyl azolate as ligands, which are known toform strong chelate bonding with Pt ions and render themolecule a square-plannar geometry, affording distinctiveemission (phosphorescent) properties [30–32]. Indeed,such Pt(II) complexes exhibit rather efficient green phos-phorescence at room temperature in dilute solutions [30].Further attaching alkyloxyphenyl groups with the alkylchain (i.e., 3,4,5-trihexoxyphenyl) onto the ligands impartscertain flexibility to the molecular core and mesomorphiccharacteristics, thereby yielding luminescent metallomes-ogens [30,33–36]. As studied by polarizing optical micros-copy and differential scanning calorimetry, N200 showsliquid–crystalline properties across a wide temperaturerange. Upon heating the crystalline samples of these Pt(II)complexes, a transition from the solid to the columnarmesophase (as verified by X-ray diffraction [30]) and atransition from the mesophase to the isotropic liquidoccurs at �98 �C and �342 �C, respectively. The details ofsyntheses and basic material properties of the Pt(II) com-plexes had been reported elsewhere [30].

Employed as the host, the fluorene oligomer F(MB)5consists of five fluorene units on the backbone and two2-methylbutyl substituents at C9 atoms [8,9]. F(MB)5belongs to a class of glass-forming nematic oligofluorenes,that is, materials that exhibit the nematic mesophase atelevated temperatures and yet also the stable glass phaseat room temperature [8–11]. As studied by polarizing opti-cal microscopy and differential scanning calorimetry,

ures in mesogenic host–guest systems for polarized phosphores-25


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Fig. 2. Schematic illustration of the GIXS measurement. hIN denotes theincident angle, 2h the diffraction angle (relative to the sample-detectoraxis). The propagation direction of the incident X-ray is defined as the Yaxis, the out-of-plane (OOP) direction as the Z axis, and in-plane (IP)direction perpendicular to the Y axis as the X axis. QX and QZ are thecomponents of the scattering vector along the IP (X axis) direction and theOOP (Z axis) direction, respectively. For samples subjected to rubbing andannealing treatments and thus having anisotropic in-plane characteris-tics, GIXS was conducted in two different configurations: (i) the propa-gation direction of the incident X-ray was parallel (aligned) with therubbing direction (x = 0�); (ii), the rubbing direction was rotated awayfrom the propagation direction of the incident X-ray by 90� (i.e., x = 90�),thus perpendicular to the incident beam.

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F(MB)5 exhibits a glass transition (Tg) at �89 �C and anematic-to-isotropic transition at 167 �C. It had been dem-onstrated that liquid–crystal glass of F(MB)5 could beformed by freezing its mesophase down to room tempera-ture without crystallization, and that the aligned glassynematic film of F(MB)5 could be formed by utilizing meso-phase-mediated alignment (e.g., assisted by the rubbedalignment layer of a polyimide or a conducting polymer)at elevated temperatures [8,9]. F(MB)5 is also an efficientfluorescent blue emitter on its own with a thin-film quan-tum yield of >50% [8,9]. Detailed syntheses and character-izations of F(MB)5 could be found in previous publications[8].

2.2. Molecular Alignment

To conduct and study molecular alignment of themesogenic oligofluorene/mesogenic Pt(II) complex host–guest mixtures, different sample structures and alignmentapproaches were tested. In scheme I, the active thin filmsunder investigation were spin-coated onto the quartz orSi/SiO2 substrates and then uniaxially rubbed with a clothfor surface alignment. In scheme II, a conducting polymer,poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate)(PEDOT:PSS), was used as the alignment layer. PEDOT:PSSwas spin-coated onto quartz or Si/SiO2 substrates, dried at130 �C in air, and uniaxially rubbed with the cloth. The ac-tive thin films under investigation were then spin-coatedonto the rubbed PEDOT:PSS for surface alignment. In bothschemes I and II, the active layer to be aligned could be aneat film of F(MB)5, a neat film of the Pt(II) complexN200, or a host–guest mixture film of F(MB)5 and Pt(II)complex N200 (with 25 wt.% of N200). To induce meso-phase-mediated molecular alignment, all samples werefurther annealed at elevated temperatures for 1 h: 150 �Cfor neatN200 films and 120 �C for the neat F(MB)5 or theF(MB)5:N200 mixture films. With an aim to constructpolarized phosphorescent OLEDs using the mesogenicPt(II) complexes, we first tried to use the rubbed PED-OT:PSS film to align all material layers (i.e., scheme II),since PEDOT:PSS was to be used as the conductive align-ment layer in polarized OLED. However, since scheme II(the one preferred for device applications) failed to orientthe neat Pt(II) complex films, scheme I was followed toinvestigate fundamental properties of neat Pt(II) complexfilms [29]. Scheme II was successfully implemented toaccomplish orientation in neat F(MB)5 and theF(MB)5:N200 mixture films for both fundamental studiesand device applications.

2.3. Grazing incidence X-ray scattering

GIXS was conducted for the analysis of host–guestsystems. Compared to other conventional techniques formorphological characterizations (e.g. atomic force micros-copy, scanning or transmission electron microscopy etc.),GIXS has the particular advantage of being able to providestructural/morphological information of a thin film at dif-ferent scales [37–44], instead of being limited to localobservation. Fig. 2 illustrates the configuration of the GIXSmeasurement, which was conducted at the BL17A end sta-


tion of the National Synchrotron Radiation ResearchCenter, Hsinchu, Taiwan. The 9.3-keV (k = 1.33 Å) beamhaving a 1 mm beam diameter was directed at a 0.1� inci-dence angle (hIN). The 2D scattering images were collectedby a MAR345 CCD detector array 210 mm away. To avoidthe reflection of the incident beam from the substrate thatwas strong relative to the scattering signals, aluminumfoils were applied as the semitransparent beam stop. Thethin-film samples were kept at room temperature in airduring irradiation and GIXS image collection. In Fig. 2, hIN

denotes the incident angle, and 2h is the diffraction angle(relative to the sample-detector axis). The propagationdirection of the incident X-ray is defined as the Y axis,the out-of-plane (OOP) direction relative to the samplesurface as the Z axis, and in-plane (IP) direction perpendic-ular to the Y axis as the X axis. QX and QZ are the compo-nents of the scattering vector Q along the IP (X axis)direction and the OOP (Z axis) direction, respectively,where Q = 4p�sin(h)/k and k = the X-ray wavelength. Forsamples subjected to rubbing and annealing treatmentsand thus having anisotropic in-plane characteristics, GIXSwas conducted in two different configurations. In the firstconfiguration, the propagation direction of the incidentX-ray was parallel to the rubbing direction (i.e., the anglebetween these two directions x = 0�). In the second config-uration, the rubbing direction was rotated away from thepropagation direction of the incident X-ray by 90� (i.e.,the angle between these two directions x = 90�), thus per-pendicular to the incident beam.

2.4. Confocal laser scanning microscopy (CLSM)

CLSM was used as one of the techniques for visualiza-tion of morphologies and nanostructures of thin films[45,46]. CLSM of thin-film samples was conducted on a



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Leica TCS SP5 confocal laser scanning microscope system(Leica Microsystems) using an 100X oil-immersion objec-tive. The system used a linearly polarized diode UV laserat 405 nm as the excitation source and a spectral PMT asthe detector. The scanning rate is 400 Hz, and the step sizeis 40 nm. The optical resolution of CLSM can be optimizedto 200 nm. Three thin-film samples (either before or afterrubbing and annealing treatments) were investigated: neatfilms of F(MB)5 (host), neat films of Pt(II) complexes(guest), and mixture films of F(MB)5:Pt(II) complexes.The spectral window of the PMT was set at 420–460 nmfor collecting the (blue) luminescence CLSM image of thehost material [F(MB)5], and at 600–700 nm for collectingthe (red) luminescence CLSM image of the guest material[Pt(II) complex aggregates]. For oriented thin-film samples,polarization-dependent CLSM was studied by aligning thepolarization of the excitation laser either parallel (//) orperpendicular (\) to the rubbing direction.

2.5. Variable-angle spectroscopic ellipsometry

Variable-angle spectroscopic ellipsometry (VASE) in thereflection mode was used to study the optical constants ofthin films of neat N200 for verifying the in-plane and out-of-plane anisotropy absorption of the columnar stacking.Ellipsometry measures the change in polarization of lightas a function of incident angle and wavelength [6,47,48].The experimentally determined ellipsometric parametersare W and D, which are related to the ratio of Fresnelreflection coefficients Rp and Rs for p- and s-polarized light,respectively, by Rp/Rs = tan(W) eiD. Optical constants of thematerials are then determined by first constructing anoptical model of the sample with physically meaningfulstructural and optical parameters, and then by iterativelyadjusting these parameters to obtain the best fit to themeasured ellipsometric data by minimizing the meansquare error (MSE) [47]. Ellipsometry over a wavelengthrange of 270–1100 nm in 5 nm steps was performed inair using the J.A. Woollam V-VASE spectroscopic ellipsom-eter equipped with a xenon lamp source. The angles ofincidence used were between 65� and 75� relative to thesurface normal in steps of 5�. The nonlinear regressionanalysis of the measured ellipsometric data was performedusing the J.A. Woollam WVASE32 software.

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Fig. 3. 2D GIXS images of the neat N200 films under three differentconditions: (a) as-deposited, without rubbing and annealing treatments,(b) after rubbing and annealing treatments, measured with the incidentX-ray parallel with the rubbing direction (x = 0�), and (c) after rubbingand annealing treatments, measured with the incident X-ray perpendic-ular to the rubbing direction (x = 90�).

3. Results and discussions

3.1. GIXS and ellipsometric analyses of neat films of Pt(II)complexes

Fig. 3 presents the 2D GIXS images of the neat N200films under three different conditions: (a) as-deposited,without rubbing and annealing treatments, (b) afterrubbing and annealing treatments, measured with theincident X-ray parallel to the rubbing direction (x = 0�),and (c) after rubbing and annealing treatments, measuredwith the incident X-ray perpendicular to the rubbing direc-tion (x = 90�). As can be seen in Fig. 3(a), even for the as-deposited film (without rubbing and annealingtreatments), there are already clear X-ray scattering pat-


terns, confirming the formation of N200 aggregates in theas-deposited neat film. Although not very strong, inFig. 3(a), one can observe a pattern representing the Pt–Pt stacking around Qx = 19 Å�1. It indicates that the N200Pt complexes tend to self-assemble into columnar stacksin the as-cast film. In addition, the Pt–Pt signal only ap-



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pears in the in-plane direction (i.e., along Qx), not observedin the out-of-plane direction (i.e., along Qz). It suggests thatthe columnar axis lies on the substrate surface. In Fig. 3(a),one also observes GIXS patterns at smaller Q’s, whichrepresent the inter-column assembly structure of theaggregates. The more ring-like GIXS patterns at smallerQ’s, however, indicate a somewhat random distributionof the inter-column structures.

As seen in Fig. 3(b) and (c), upon rubbing and thermalannealing, the signals representing Pt–Pt stacking becomesharper and stronger, indicating growth in aggregate size.Again, the Pt–Pt signals still appear only in the in-planedirection (i.e., at Qx � 19 Å�1), not observed in the out-of-plane direction (i.e., along Qz). As Fig. 3(b) is comparedwith Fig. 3(c), GIXS signals at smaller Q’s converge intospecific peaks (dots) and are significantly enhanced withrubbing and annealing. The observed GIXS patterns indi-cate that aggregates grow in size and adopt better orga-nized inter-column structures and orientation as a resultof rubbing/annealing. Moreover the better organized struc-tures are preserved upon subsequent cooling. Examiningthe GIXS patterns in Fig. 3(b) and (c) reveals that theassembly of the Pt complexes has the tetragonal columnarstructure (Colt), whose packing structure and assignmentof Miller indices are illustrated in Fig. 4. Note that Liaoet al. [30] had reported that similar Pt complexes, althoughexhibiting the hexagonal columnar mesophase at highertemperatures (>240 �C), did exhibit the rectangular/tetrag-onal columnar mesophase at lower temperatures (<240 �C,like the case here). For convenience, the [100] is assignedalong the Pt–Pt stacking direction, while [010] and [001]are assigned for the two inter-column packing directions.With the emerging assembly structure and indexing sys-tem, the GIXS peaks in Fig. 3(b) and (c) can now be unam-biguously assigned as such. Reflections occurring along theout-of-plane orientation that represent the out-of-planeinter-column packing can be assigned [001] and higher-order ones, while reflections occurring along the in-planeorientation that represent in-plane inter-column packingcan be assigned [010] and higher-order ones.

Anisotropic characteristics of the aggregates of Pt com-plexes in rubbed and annealed samples can be further ana-lyzed by considering different packing orientationsthrough comparing GIXS images detected with the inci-dent X-ray parallel to the rubbing direction (i.e., GIXS atx = 0�, Fig. 3(b)) to those detected with the incident

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426Fig. 4. The packing structure of the tetragonal columnar structure (Colt)and the assignment of Miller indices.


X-ray perpendicular to the rubbing direction (i.e., GIXS atx = 90�, Fig. 3(c)). Fig. 5 shows three possible columnarpacking orientations relative to the surface and therubbing direction. In case A, the columns (or intra-columnPt–Pt stacking) are vertical to the surface and the inter-column packing is along the surface. In such a case, ideallyin GIXS one would only observe the Pt–Pt stacking signalalong the OOP direction (i.e., along Qz) but not along theIP direction (i.e., along Qx). In addition, one would alsoobserve no inter-column packing signal (i.e., [001] or[010]) along the OOP direction. In case B, columns (or in-tra-column Pt–Pt stacking) are parallel to the surface andthe rubbing direction, while one inter-column packingdirection (i.e., [001]) is along the OOP direction andanother (i.e., [010]) is perpendicular to the rubbing direc-tion. In such a case, ideally in 2D GIXS one would observeno Pt–Pt stacking signal along the OOP direction, andwhether the Pt–Pt stacking signal along the IP directioncan be observed depends on x (0� or 90�). At x = 0�, onewould observe inter-column packing signals [001]/[010]along the OOP/IP direction, respectively, but no Pt–Ptsignal along the IP direction (i.e., along Qx) since thePt–Pt packing is along the Y direction under such a mea-surement configuration (thus cannot be easily resolvedby the 2D GIXS). On the other hand, by rotating the sampleto make x = 90�, one shall now observe (001)/Pt–Pt stack-ing signal along the OOP/IP direction, respectively, but no[010] signal along the IP direction since the [010] orienta-tion is now along the Y direction. In case C, columns (or in-tra-column Pt–Pt packing) are parallel with the surface butperpendicular to the rubbing direction, while one inter-column packing direction (i.e., [001]) is along the OOPdirection and another (i.e., [010]) is parallel with therubbing direction. In such a case, ideally in 2D GIXS onewould observe no Pt–Pt stacking signal along the OOPdirection, while whether the Pt–Pt stacking signal can beobserved again depends on x (0� or 90�). At x = 0�, onewould observe [001]/Pt–Pt stacking signal along theOOP/IP direction, respectively, but no [010] signal alongthe IP direction since the [010] orientation is along the Ydirection. On the other hand, at x = 90�, one shall nowobserve inter-column packing signals [001]/[010] alongthe OOP/IP direction, respectively, but no Pt–Pt signalalong the IP direction since the Pt–Pt packing is now alongthe Y direction.

Since in GIXS images of Fig. 3(b) and (c) one sees clearinter-column packing signal [001] but no intra-columnPt–Pt stacking signal along the OOP direction (i.e., alongQz), the packing orientation case A can be excluded. Obser-vation of the Pt–Pt stacking signal mainly along the IPdirection suggests packing orientations of cases B and C.Cases B and C can be further distinguished by comparingGIXS images collected under x = 0� and x = 90� configura-tions. Fig. 6 shows the in-plane profiles (i.e., along Qx) ofthe three GIXS images in Fig. 3(a)–(c), with the Pt–Pt stack-ing signal (i.e., at Qx � 19 Å�1) being enlarged and shown inthe inset. In the inset of Fig. 6, one sees that for the samplesubjected to the rubbing and annealing treatments, thePt–Pt signal detected with x = 0� is stronger than that de-tected with x = 90�. In addition, for the sample subjectedto the rubbing and annealing treatments, the (010) signal



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Fig. 5. Three possible/representative column packing orientations (of the tetragonal columnar assembly) relative to the surface and the rubbing direction.

Fig. 6. In-plane profiles (i.e., along Qx) of the three GIXS images inFig. 3(a)–(c), with the Pt–Pt stacking signal (i.e., around Qx � 19 Å�1)being enlarged and shown in the inset.



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detected with x = 0� is significantly weaker than thatdetected with x = 90�. These results indicate that, withthe rubbing and annealing treatments: (i) the stacking col-umns of the Pt(II) complexes appear to have the preferen-tial orientation perpendicular to the rubbing direction; (ii)the in-plane inter-column packing (i.e., [010] orientation)is preferentially along the rubbing direction. That is, amongthe three possible column packing orientations relative tothe surface and the rubbing direction illustrated in Fig. 5,case C represents the preferential column packing orienta-tion, although the intra-column packing (i.e., Pt–Pt) andinter-column packing (i.e. [010]) perhaps are not perfectlyaligned relative to the rubbing direction and there is somedistribution between cases B and C. Such results are con-sistent with previous results of polarized optical spectros-copy (i.e., polarized photoluminescence and absorption) ofneat films of Pt complexes subjected to rubbing andannealing treatments [29]. Previous results of polarizedoptical spectroscopy showed that the polarization of theMMLCT absorption and emission (and thus the columnarPt–Pt stacking) were more along the direction perpendicu-lar to the rubbing direction, while the absorption associ-ated with the pyridyl azolate ligands (and thus themolecular plane of the discotic Pt complex) was morealong the rubbing direction.

From GIXS patterns of the rubbed and annealed samplesshown in Fig. 3(b) and (c), the d spacings along the intra-column Pt–Pt stacking direction (dPt–Pt), along the inter-


column [001] direction (d0 0 1), and along the inter-column[010] direction (d0 1 0) are calculated based on Bragg’s law[49] and are listed in Table 1. Furthermore, the averagedimensions of the aggregates/grains along the above differ-ent packing directions (i.e., LPt–Pt, L0 0 1, L0 1 0) are also esti-mated using the Scherrer grain-size analysis [50] and arealso listed in Table 1. Note that the calculated d0 1 0 of22.22 Å is close to that of d0 0 1 of 22.62 Å, and that bothare about

ffiffiffi

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times the d spacing along the [011] direction(15.48 Å, not listed in Table 1), consistent with theassignment of the packing structure to the tetragonalcolumnar structure. The calculated inter-column spacingof �22.4 ± 0.2 Å is shorter than the molecular length(�29.19 Å), but longer than the rigid core length of�16.24 Å, suggesting the discotic Pt complexes are assem-bled into the columnar organization with a tilted formatrelative to the inter-column [001] and [010] orientations.The intra-column Pt–Pt stacking distance (dPt–Pt) is calcu-lated to be 3.33 Å, which is well within the Pt–Pt distancesallowing strong MMLCT transitions to occur [51,52]. On theother hand, with LPt–Pt = 43.33 Å, L0 0 1 = 274.47 Å, andL0 1 0 = 206.48 Å in the rubbed and annealed neat films,one estimates the average size of the aggregates/grains tobe �13 Pt complexes along the Pt–Pt stacking direction,�12 columns along the inter-column [001] direction and�9–10 columns along the inter-column [010] direction.The Scherrer grain-size analysis also shows that the assem-bly size of Pt complexes had grown with rubbing andannealing treatment. For instance, LPt–Pt had grown from�26 Å of the as-deposited neat film to 43.33 Å of therubbed and annealed film.

From the above GIXS analyses, it has been establishedthat in the rubbed and annealed neat films of N200, thestructure and orientational alignment of the molecularassembly is inclined toward case C described in Fig. 5. Inaddition, it is deduced that the discotic Pt complexes areassembled into the columnar organization with a tilted for-mat relative to the inter-column [001] and [010] orienta-tions. Such anisotropic molecular assembly can be furtherverified by characterizing the anisotropic optical propertiesof rubbed and annealed neat films originating from theMMLCT transitions and ligand-centered transitions (utiliz-ing the variable-angle spectroscopic ellipsometry-VASE).Good fittings to the experimentally measured ellipsometricvalues W and D could be obtained by using a biaxial modelfor refractive indices and extinction coefficients, which inturn were constructed by a Kramers–Kronig consistent



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Table 1Packing parameters of N200 assemblies/aggregates in films with different compositions and processing/treatment conditions, including the d spacings alongthe intra-column Pt–Pt stacking direction (dPt–Pt), along the inter-column [001] direction (d0 0 1), and along the inter-column [010] direction (d0 1 0), and theaverage dimensions of the aggregates/grains along the above different packing directions (i.e., LPt–Pt, L0 0 1, L0 1 0).

dPt–Pt (Å) LPt–Pt (Å) d0 0 1 (Å) L0 0 1 (Å) d0 1 0 (Å) L0 1 0 (Å)

N200 no treatments 3.34 25.97 22.36 183.19 22.15 184.82N200 with treatments 3.33 43.33 22.62 274.47 22.22 206.48F(MB)5:N200 no treatments – – – – – –F(MB)5:N200 with treatments 3.35 18.96 23.68 143.21 22.10 121.62



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model using a combination of one Cauchy background andseveral Gaussian oscillators. Fig. 7 shows the extractedabsorption coefficients converted from the extracted aniso-tropic extinction coefficients for the rubbed and annealedneat film of N200. a//,IP, a\,IP, and aOOP represent the absorp-tion coefficients for the in-plane absorption coefficientalong the rubbing direction, the in-plane absorptioncoefficient perpendicular to the rubbing direction, and theout-of-plane absorption coefficient, respectively. One notesthat a//,IP and a\,IP agree well with the normal-incidenceabsorption spectra of the uniaxially rubbed N200 film mea-sured with the polarization parallel (//) and perpendicularto (\) the rubbing direction (inset of Fig. 7, [29]), respec-tively, inspiring additional confidence in the anisotropicoptical constants obtained. For the low-energy MMLCTtransition region (e.g., 520–600 nm), the strongest and clearabsorption peak in a\,IP, weaker a//,IP, and nearly zero aOOP

are consistent with the foregoing interpretation that thePt–Pt stacking orientation is most likely case C of depictedin Fig. 5, mingled with a bit of case B, and hardly any caseA, since the MMLCT transition is typically along the chainof the metal cores [53]. For the higher-energy ligand-cen-tered transition region (e.g., �300 nm), both a//,IP and aOOP

are larger than a\,IP, with the dichroic ratio of >2, which sug-gests that in the aligned neat films of Pt(II) complexes, thepyridyl azolate ligands with alkyloxyphenyl groups areroughly along the surface expanded by the rubbing and

Fig. 7. Extracted absorption coefficients converted from the extractedanisotropic extinction coefficients for the rubbed and annealed neat filmof N200. a//,IP, a\,IP, and aOOP represent the absorption coefficients for thein-plane absorption coefficient along the rubbing direction, the in-planeabsorption coefficient perpendicular to the rubbing direction, and theout-of-plane absorption coefficient, respectively. The inset shows thenormal-incidence absorption spectra of the uniaxially rubbed N200 filmmeasured with the polarization parallel with (//) and perpendicular to (\)the rubbing direction.


the OOP directions (with certain tilting relative to the sur-face). Again, the residual a\,IP suggests the alignment isnot perfect and there is certain mixture of case B and caseC (Fig. 5).

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3.2. GIXS analyses of host: guest mixture films

Fig. 8 shows the 2D GIXS images of the F(MB)5:N200mixture films coated over PEDOT:PSS under three differentconditions: (a) without alignment treatment, (b) withalignment treatment, measured with the incident X-rayparallel with the rubbing direction (x = 0�), and (c) withalignment treatment, measured with the incident X-rayperpendicular to the rubbing direction (x = 90�). InFig. 8(a), for the F(MB)5:N200 mixture film on PEDOT:PSSwithout alignment treatment, no specific GIXS patterns(e.g., peaks, rings etc.) associated with N200 assemblycan be detected, except for the cloudy background.Although previous photoluminescence studies indicatethere is Pt–Pt stacking and MMCLT emission of N200 evenin the pristine F(MB)5:N200 mixture films subjected to noalignment, the sizes and/or quantities of the N200 assem-blies are probably not sufficient and the orientations areprobably too random to be detected by GIXS. Comparisonof Fig. 8(a) and Fig. 3(a) also suggests the presence of theunoriented F(MB)5 host significantly disturbs the assem-bly/stacking of the Pt complex N200.

As seen in Fig. 8(b) and (c), for the mixture filmsubjected to alignment, the signal representing Pt–Ptstacking (at Qx � 19 Å�1) of N200 appears, indicatinggrowth of N200 aggregates. Again, the Pt–Pt signals stillappear only in the in-plane direction (i.e., at Qx � 19 Å�1),not observed in the out-of-plane direction (i.e., along Qz).Moreover, alignment produced signals representing the in-ter-column packing (i.e., peaks at smaller Q’s) that suggestgrowth of N200 aggregates and adoption of more orga-nized inter-column structures and orientation within theoriented F(MB)5 host. Overall, the GIXS pattern of thealigned F(MB)5:N200 mixture film is similar to that ofthe rubbed/annealed neat N200 film (Fig. 3(b) and (c)),indicating that N200 assemblies in both cases have similarPt–Pt stacking, inter-column structure (i.e., tetragonalcolumnar structure, Colt), and orientation/alignment. Assuch, Miller indices of GIXS peaks in Fig. 8(b) and (c) canbe assigned in a way similar to that in Fig. 3(b) and (c),with the [100] being for the Pt–Pt stacking direction,[001] and higher-order ones being for reflections andinter-column packing along the out-of-plane orientation,[010] and higher-order ones being for reflections andinter-column packing along the in-plane orientation.



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Fig. 8. 2D GIXS images of the F(MB)5:N200 mixture films coated overPEDOT:PSS under three different conditions: (a) without no alignmenttreatments, (b) with alignment treatments, measured with the incidentX-ray parallel with the rubbing direction (x = 0�), and (c) with alignmenttreatment, measured with the incident X-ray perpendicular to therubbing direction (x = 90�).

Fig. 9. In-plane profiles (i.e., along Qx) of the three GIXS images inFig. 8(a)–(c), with the Pt–Pt stacking signal (i.e., around Qx � 19 Å�1)being enlarged and shown in the inset.



25 November 2013

Fig. 9 shows the in-plane profiles (i.e., along Qx) of thethree GIXS images in Fig. 8(a)–(c), with the Pt–Pt stackingsignal (i.e., around Qx � 19 Å�1) being enlarged and shownin the inset. As in the case of the neat N200 film, for the


sample subjected to the rubbing and annealing treatments,the Pt–Pt signal detected with x = 0� is stronger than thatdetected with x = 90�. In addition, the inter-column [010]signal detected with x = 0� is significantly weaker thanthat detected with x = 90�. Thus in the mixture film, withthe alignment treatment, the N200 assemblies in theF(MB)5 host also preferentially adopt the column packingorientation (relative to the rubbing direction and the sur-face) like case C in Fig. 5, although the alignment is imper-fect (i.e., there may be some distribution between case Band case C). Such results are consistent with previousresults of polarized optical spectroscopy (i.e., polarizedphotoluminescence) of the F(MB)5:N200 mixture filmssubjected to alignment treatments [29]. Previous resultsof polarized optical spectroscopy showed that the polariza-tion of the MMLCT emission (and thus the columnar Pt–Ptstacking) of N200 guests were more perpendicular to therubbing direction, while the F(MB)5 host emission wasmore along the rubbing direction (i.e., F(MB)5 hostmolecules are aligned along the rubbing direction).Although F(MB)5 host molecules are oriented on the align-ment-treated surface, yet no GIXS signals associated withF(MB)5 can be detected. It is perhaps because F(MB)5possess a very short intermolecular correlation length notreadily detectable by GIXS.

From GIXS patterns of the rubbed and annealed samplesshown in Fig. 8(b) and (c), the d spacings dPt–Pt, d0 0 1, andd0 1 0 for the aligned mixture film are calculated and arelisted in Table 1. One notices that these d spacing valuesare similar to those in the N200 neat films, confirming sim-ilar assembly structures in both cases. The average dimen-sions of the N200 aggregates in mixture films along theabove different packing directions (i.e., LPt–Pt, L0 0 1, L0 1 0)are also estimated and are also listed in Table 1. On theother hand, with LPt–Pt = 18.96 Å, L0 0 1 = 143.21 Å, andL0 1 0 = 121.62 Å in the aligned mixture films, one estimatesthe average size of the aggregates at �5–6 Pt complexesalong the Pt–Pt stacking direction, �6 columns along theinter-column [001] direction and �5–6 columns alongthe inter-column [010] direction. From results of Table 1,one might speculate that although the nematic F(MB)5



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host can guide the alignment of the N200 aggregates in themixture film, the presence of the F(MB)5 hosts disturbsthe assembly of the Pt complex N200, thereby reducingthe sizes of N200 aggregates to nearly half.

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3.3. CLSM

Although the resolution of CLSM used might not besufficient to quantify the nanostructures encountered inthis study, the CLSM images were collected as supportiveinformation for aggregates of N200 and the nanoscalephase separation in the F(MB)5:N200 mixture films.

Fig. 10(a) and (b) show the luminescence CLSM imagesof the neat F(MB)5 film and the neat N200 film, respec-tively, either without (panel (i)) or with (panels (ii) and(iii)) rubbing/annealing treatments. For the alignedthin-film samples (i.e., with rubbing/annealing), polariza-tion-dependent CLSM images were taken by aligning thepolarization of the excitation laser either perpendicular(\, panel (ii)) or parallel to (//, panel (iii)) the rubbingdirection. In panel (i) of Fig. 10(a), one sees that CLSM im-age of the neat F(MB)5 film without rubbing/annealing ishomogeneous, as expected of an amorphous/glassy organicfilm. With rubbing and annealing, the image of the // polar-ization (panel (iii) of Fig. 10(a)) becomes significantlystronger than that of \ polarization in panel (ii) ofFig. 10(a) (as expected), and yet the CLSM images are stillgenerally homogeneous (except for a few spots associatedwith non-ideal rubbing/alignment). In contrast, in panel (i)of Fig. 10(b), one observes a grainy CLSM image even in the

Fig. 10. Luminescence CLSM images of (a) the neat F(MB)5 film and (b)the neat N200 film, either without (panel (i)) or with (panels (ii) and (iii))rubbing/annealing treatments. For the aligned thin-film samples (i.e.,with rubbing/annealing), polarization-dependent CLSM images weretaken by aligning the polarization of the excitation laser either perpen-dicular to (\, panel (ii)) or parallel with (//, panel (iii)) the rubbingdirection.


neat N200 film without rubbing/annealing, indicatingformation of aggregates/grains as coated. With rubbingand annealing (panels (ii) and (iii) of Fig. 10(b)), in additionto polarization dependence of CLSM images (due to align-ment), one clearly sees substantial growth of domains.

Fig. 11(a) and (b) show the luminescence CLSM imagesof the F(MB)5 emission and the N200 MMLCT emissiontaken from a same F(MB)5:N200 mixture film, respec-tively, either without (panel (i)) or with (panels (ii) and(iii)) rubbing/annealing treatments. For the aligned thin-film samples (i.e., with rubbing/annealing), polarization-dependent CLSM images were taken by aligning thepolarization of the excitation laser either perpendicular(\, panel (ii)) or parallel to (//, panel (iii)) the rubbingdirection. In panel (i) of Fig. 11(a) and (b), one observesgrainy CLSM images for both F(MB)5 emission and N200MMLCT emission even in mixture films without rubbing/annealing, indicating certain extent of phase separation.With rubbing and annealing (panels (ii) and (iii) ofFig. 11(a) and (b)), in addition to polarization dependenceof CLSM images (due to alignment), again one sees signifi-cant growth of phase-separated domains and N200 aggre-gates/grains, which is consistent with the observation inGIXS. In CLSM images of the aligned host–guest mixturefilms, //-polarization N200 MMLCT emission (panel (iii) ofFig. 11(b)) can be somewhat detected, although weakerthan the \-polarization emission (panel (ii) of Fig. 11(b)).It is perhaps due to imperfect alignment of guestaggregates as suggested by GIXS.

Fig. 11. Luminescence CLSM images of (a) the F(MB)5 emission and (b)the N200 MMLCT emission taken from a same F(MB)5:N200 mixture film,either without (panel (i)) or with (panels (ii) and (iii)) rubbing/annealingtreatments. For the aligned thin-film samples (i.e., with rubbing/anneal-ing), polarization-dependent CLSM images were taken by aligning thepolarization of the excitation laser either perpendicular to (\, panel (ii))or parallel with (//, panel (iii)) the rubbing direction.



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(a) Neat N200, no treatments (b) Neat N200, with treatments

(c) F(MB)5:N200, no treatments (d) F(MB)5:N200, with treatments

Fig. 12. Schematic representations of the morphologies and molecular alignments of: (a) N200 neat film without rubbing/annealing treatments, (b) N200neat film with rubbing/annealing treatments, (c) F(MB)5:N200 mixture film without rubbing/annealing treatments, and (d) F(MB)5:N200 mixture film withrubbing/annealing treatments.



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4. Conclusions

The GIXS and CLSM analyses were conducted to fur-nish new insight into molecular self-assembly, orienta-tion and the resultant nanostructures in four distinctcases: (a) N200 neat film without rubbing/annealingtreatments, (b) N200 neat film with rubbing/annealingtreatments, (c) F(MB)5:N200 mixture film without rub-bing/annealing treatments, and (d) F(MB)5:N200 mixturefilm with rubbing/annealing treatments. These combinedexperimental observations led to the film morphologiesas depicted in Fig. 12. In the as-cast neat N200 film(Fig. 12(a)), GIXS patterns suggest aggregates of N200have readily formed, although their sizes are relativelysmaller and orientations are somewhat random. Rubbingand annealing induce growth and better organized orien-tation of the N200 aggregates (Fig. 12(b)). Examining theGIXS patterns of rubbed/annealed N200 neat films re-veals that the assembly of the Pt complexes has thetetragonal columnar structure (Colt). With the rubbingand annealing treatments, the columnar stacks of thePt(II) complexes appear largely oriented perpendicularto the rubbing direction with the in-plane inter-columnpacking (i.e., [010] orientation) preferentially along therubbing direction. Such results are consistent with thosepreviously deduced from polarized optical spectroscopyof N200 films. For the F(MB)5:N200 mixture film without


rubbing and annealing treatments (Fig. 12(c)), althoughprevious photophysical studies indicate Pt–Pt stackingand MMCLT emission of N200, no specific signals associ-ated with N200 assembly can be detected in its GIXSpatterns. It suggests the presence of the (un-oriented)F(MB)5 host significantly disturbs the assembly of thePt(II) complex, so that the sizes and/or quantities ofthe N200 assemblies are probably not sufficient andthe orientations are probably too random for GIXS to de-tect. For the mixture film subjected to alignment(Fig. 12(d)), again the signals associated with assembliesof N200 (similar to those observed for rubbed/annealedneat N200 films) appear, indicating the growth of N200aggregates and adoption of more organized inter-columnstructures and orientation within the aligned F(MB)5host. The organization and alignment are presumably in-duced by the oriented F(MB)5 host oligomers on therubbed PEDOT:PSS alignment layer. Overall, the GIXSpattern of the aligned F(MB)5:N200 mixture film is sim-ilar to that of the rubbed/annealed neat N200 film. Itindicates that N200 assemblies in aligned mixture filmshave Pt–Pt stacking, inter-column structure (i.e., tetrago-nal columnar structure, Colt), and orientation/alignmentsimilar to those in aligned neat N200 films, althoughthe Scherrer grain-size analysis reveals that the aggre-gate sizes in the mixture films are smaller than thosein N200 neat films.



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Acknowledgements

The authors gratefully acknowledge the financialsupport from National Science Council and Ministry ofEducation of Taiwan (under the Grant NSC 98-2120-M-002-005, NSC 99-2221-E-002-118-MY3, NSC 102-2221-E-002-203-MY3, and NTU-CESRP-102R7607-2).

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http://www.olympusconfocal.com/theory/LSCMIntro.pdf

http://www.olympusconfocal.com/theory/LSCMIntro.pdf


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