J. Phys. Chem. C 2009, 113, 19251–19257
19251
Effect of Interface on the Orientation of the Liquid Crystalline Nickel Phthalocyanine Films Tamara V. Basova,*,† Mahmut Durmus¸,‡ Ays¸e Gu¨l Gu¨rek,‡ Vefa Ahsen,‡,§ and Aseel Hassan| NikolaeV Institute of Inorganic Chemistry, 3, LaVrentieV Pr., NoVosibirsk Russia, Gebze Institute of Technology, Department of Chemistry, P.O. Box 141, 41400, Gebze, Kocaeli Turkey, TUBITAK-Marmara Research Center, Materials Institute, P.O. Box 21, 41470, Gebze, Kocaeli Turkey, and Faculty of Arts, Computing, Engineering and Sciences, Sheffield Hallam UniVersity, FurniVal Building, 153 Arundel Street, Sheffield S1 2NU, United Kingdom ReceiVed: June 24, 2009; ReVised Manuscript ReceiVed: September 14, 2009
In this paper, the effect of interface on the orientation of the film of two derivatives of tetra-substituted nickel phthalocyanines NiPcR4-1 (R ) -OCH(CH2OC12H25)2) and NiPcR4-2 (R ) -SCH(CH2(OCH2CH2)2OC2H5)2) deposited on one substrate and between two identical substrates is studied by the method of polarized Raman spectroscopy. The methods of ultraviolet-visible (UV-vis) absorption spectroscopy and infrared (IR) spectroscopy were also used for the characterization of films deposited on quartz and KBr substrates, respectively. The orientation of columns in films confined between two substrates is characterized by polarized optical microscopy. It has been shown that the films of both compounds confined between two substrates are characterized by homeotropic alignment while no homeotropic alignments could be obtained with an air interface. The orientation is also independent of the type of substituents on the phthalocyanine ring. 1. Introduction The ability of discotic mesogens to self-assemble in columnar superstructures together with the self-healing of structural defects in the liquid crystalline columnar phase provide great potential for their application as semiconductors for organic electronic devices.1-4 To date, the number of discotic liquid crystals (LC) derived from more than 50 different cores comes to about 3000. Recent advances in basic design principles and synthetic approaches toward the preparation of most frequently encountered discotic liquid crystals, their relevant mesomorphic and physical properties, as well as some aspects of application are described in two recent critical reviews by Kumar5 and Laschat and coauthors.6 The first mesogenic phthalocyanines (Pc) were synthesized by Piechocki and co-workers in 1982.7 Since then, the mesophase structure of a wide variety of Pc’s and porphyrins have been investigated, including molecules that are substituted with eight linear alkyl,8 alkoxymethyl,9,10 and alkoxy10-12 chains. All of these compounds display a transition from the solid phase to the mesophase at elevated temperatures. Although many different side-chain-modified Pc’s and other discotic molecules have been synthesized, the thin film properties for only a limited number of phthalocyanines were reported. The exploitation of the desirable optical and electrical or electrochemical properties of phthalocyanines relies on the precise control over the molecular packing and ordering in the solid phase. Many Pc derivatives with flexible side chains make excellent candidates for Langmuir-Blodgett (LB) film fabrication as well as films produced by spin coating, and self-assembly.13 It is known that discotic molecules can adopt two characteristic orientations of columnar superstructures on surfaces, which * Corresponding author. E-mail:
[email protected]. Phone: (+7)3833302814. Fax: (+7)383-3309489. † Nikolaev Institute of Inorganic Chemistry. ‡ Gebze Institute of Technology. § TUBITAK-Marmara Research Center. | Sheffield Hallam University.
are required for electronic devices with different geometries.14 In the films with planar (homogeneous) alignment, the edge-on orientation of the discotic molecules and columns parallel to the substrate surface is observed. Homeotropic alignment is characterized by a face-on orientation of the discs and columnar axes perpendicular to the substrate.14,15 Usually, homeotropic alignment can be generated by thermal annealing, that is, slow cooling of the isotropic melt confined between two substrates.1 The influence of the nature and number of solid interfaces on the alignment of the columns in a discotic liquid crystal was studied in ref 15. Models based on the different surface tensions are proposed by some authors16-18 to explain the mechanism inducing specific alignment of LC on the surfaces. Octakis(dialkoxyphenoxy)phthalocyaninatocopper(II) complexes [(CnO)2PhO]8PcCu with (n ∼9-14)) exhibited spontaneous uniform homeotropic alignment for the columnar tetragonal (Coltet) mesophase between and on soda-lime glass plates and quartz glass plates.19 Lone pairs of the phenoxy group in the present phthalocyanine derivatives may coordinate to the dangling bonds of silicon atoms on the surface of glass and/or quartz. A homeotropic alignment of 2,9(10),16(17),23(24)tetra(2-decyltetradecyloxy)-phthalocyanine is observed between two glass slides when this Pc was cooled from the isotropic melt to the columnar hexagonal (Colh) mesophase, as evidenced by the presence of millimeter-wide nonbirefringent domains.20 Nearly uniform alignment in which hexagonally packed columns of molecules were observed for the films of discotic liquid crystalline H2Pc (R ) -SC18H37) sandwiched between two clean glass substrates separated by about 3.8 µm.21 Cupere and co-workers have shown that no homeotropic alignmentcouldbeobtainedforthefilmsof2(3),9(10),16(17),23(24)tetra(2-decyltetradecyloxy)-phthalocyanine deposited on glass slides with an air interface.15 However, highly ordered homeotropically aligned thin films have been successfully obtained using liquid crystalline ethylhexyl pyrene tetracarboxylate and ethylhexyl triphenylene tetracarboxylate by controlling the kinetics of growth on the glass or silicon slide surface.16
10.1021/jp905907n CCC: $40.75 2009 American Chemical Society Published on Web 10/13/2009
19252
J. Phys. Chem. C, Vol. 113, No. 44, 2009
It has been demonstrated that a columnar liquid crystal layer can be kinetically stabilized in a homeotropic state, even though it is shown that the planar alignment is thermodynamically stable. Homeotropically aligned films are also obtained if a specific interaction between the LC molecule and the substrate surface can be induced. Hoogboom and co-workers22 have covered the surface of indium-tin-oxide by pyridine-functionalized siloxane, which resulted in the formation of an alignment layer with micrometer-sized grooves that was capable of binding the metalated dye zinc-phthalocyanine by coordination of the pyridine functions to the zinc centers. This coordination initiates the surface-templated epitaxial growth of perpendicular stacks of highly ordered ZnPc. The specific interaction between azacrown-substituted phthalocyanine and NaCl substrate was achieved by the ability of the pendant monoazacrown ether groups to coordinate alkali ions of NaCl substrate, triggering the homeotropic alignment of molecular columns.23 It has frequently been observed that attempting the deposition of films with planar alignment often leads to a random distribution of the columnar director in the plane parallel to the air interface. A uniaxial planar alignment is preferable for charge transport application. Some special techniques such as epitaxial growth,24,25 zone processing method,26-30 and deposition in magnetic field31 are used to achieve a uniaxial alignment of columns. The alignment in LC films deposited on one substrate is studied by X-ray diffraction32,33 and solid state nuclear magnetic resonance (NMR).34,35 In most cases, the investigation of the alignment of films deposited between two substrates is based only on polarized optical microscopy studies.15 In this paper, the effect of interface on the orientation of the film of two derivatives of tetra-substituted nickel phthalocyanines NiPcR4-1 (R ) -OCH(CH2OC12H25)2) and NiPcR4-2 (R ) -SCH(CH2(OCH2CH2)2OC2H5)2) (Figure 1) deposited on one substrate and between two identical substrates is studied by the method of polarized Raman spectroscopy. This technique allows investigating the preferential orientation of the molecules relative to the substrate surface not only for the films deposited on one substrate but also for films confined between two substrates. The methods of ultraviolet-visible (UV-vis) absorption spectroscopy and infrared (IR) spectroscopy were also used for the characterization of films deposited on quartz and KBr substrates, respectively. The orientation of columns in the films confined between two substrates is characterized by polarized optical microscopy. 2. Experimental Details 2.1. Films Preparation. The synthesis and liquid crystalline properties of NiPcR4-1 and NiPcR4-2 derivatives were described in earlier publications.36,37 Two sets of samples have been prepared and investigated in this work. 2.1.1. Deposition of the Films on One Substrate. A small volume of solutions of the NiPcR4 derivatives in chloroform (10 mg/mL) was dispensed via a glass pipet onto an ultrasonically cleaned substrate held onto a photoresist spinner (Microsystem model 4000). The speed of substrate rotation was 2000 rpm. Spinning was continued for 30 s during which time the solvent had evaporated to generate a film of the phthalocyanine derivative. The films were then heated to a temperature of 10-20 °C above the isotropic transition temperature (Table 1) and then cooled down to 80 °C at a rate of 10 °C min-1 and from 80 °C to room temperature at a rate of 1 °C min-1 for comparison with as-deposited layers. Thin quartz, silicon, and KBr plates were used as substrates.
Basova et al.
Figure 1. Structure of NiPcR4-1 and NiPcR4-2 derivatives.
TABLE 1: Phase Transition Temperatures (°C) Measured by Differential Scanning Calorimetry for the NiPcR4 Derivatives derivative NiPcR4-1 NiPcR4-2
heating
cooling
C f Colh f I
I f Colh f C
14.4
116.9 230.5
112.2 220
7.5
ref 36 37
2.1.2. Deposition of the Films between Two Identical Substrates. Thicker films were prepared by depositing a few milligrams of NiPcR4 or one drop of a concentrated solution in chloroform on a substrate, which were heated on a hot stage to a temperature of 10-20 °C above the isotropic transition temperature. This was followed by depositing a second substrate on the molten liquid crystal, applying a slight pressure to spread the fluid film on as large an area as possible. Then, the sandwiched film was cooled to 80 °C at a rate of 10 °C min-1 and from 80 °C to room temperature at a rate of 1 °C min-1. Thin quartz and KBr plates were used as substrates. 2.2. Films Characterization. The thickness of deposited films was measured by ellipsometry. Spectroscopic ellipsometric measurements were performed on films deposited on silicon substrates using a Woolam M - 2000 V rotating analyzer spectroscopic ellipsometer in the spectral range 400-800 nm. Electronic absorption spectra of films deposited on quartz and KBr substrates were recorded with a UV-vis-NIR scanning spectrophotometer (UV-vis-3101PC, Shimadzu) in the range from 400 to 900 nm. Raman spectra were recorded with a Triplemate, SPEX spectrometer equipped with a CCD detector in backscattering geometry. The 488 nm, 40 mW line of an Ar laser was used for the spectral excitation.
Orientation of Nickel Phthalocyanine Films
J. Phys. Chem. C, Vol. 113, No. 44, 2009 19253
Infrared spectra of NiPcR4 in KBr pellets and their films on KBr substrates were recorded using a Vertex 80 FTIR spectrometer. Optical textures were observed with the polarizing microscope Leitz Wetzler Orthoplan-pol equipped with the hot stage Linkam TMS 93 and temperature-controller Linkam LNP. 3. Results and Discussion NiPcR4 derivatives were chosen for this research because they exhibit a hexagonal columnar structure over a wide temperature range including the room temperature. The temperatures of phase transitions of these phthalocyanines are presented in Table 1. NiPcR4-1 contains four 15-oxy-13,17-dioxanonacosane side chains on the phthalocyanine ring. NiPcR4-2 contains thiabridged polyoxyethylene substituents. 3.1. Principles of the Method for the Determination of Films’ Orientation. Polarized Raman spectroscopy was used to study the preferential orientation of the molecules relative to the substrate surface, because this technique allows investigating not only films deposited on one substrate but also films confined between two substrates. The methods of UV-vis absorption spectroscopy and IR spectroscopy were also used for characterization of films deposited on quartz and KBr substrates, respectively. The principles of polarized Raman spectroscopy for investigation of the molecular film orientation were described in detail in earlier publications.38-40 A phthalocyanine molecule is characterized by D4h group symmetry where the A1g, B1g, B2g, and Eg modes are Raman active.41,42 The detailed analysis of the Raman tensors for the D4h symmetry group and the determination of the molecular orientation are described in ref 38. In general, the intensity of Raman bands can be expressed as a function of crystal or molecular orientation and polarization geometry43 according to eq 1:
|
I ∝ ei · R · es
|
2
(1)
where I is the reflected Raman intensity, ei and es are the unit polarization vectors of the electric field for the incident and the scattered laser beam, respectively, and R represents the Raman scattering tensor of a specific vibrational mode. Therefore, the Raman intensities are proportional to the square of matrix elements. Generally, if the molecules are disordered relative to a certain axis, the elements of Raman tensors are defined by their average values in the range from 0 to π/2. In the case of a defined molecular orientation relative to a particular axis, the Raman tensors may be obtained by rotation of the initial Raman tensors through the corresponding angle. In terms of the phthalocyanine molecule, the designation of its molecular axes (index m) is presented in Figure 2, where the molecular z-axis coincides with the main axis of rotation (C4). Rotating the NiPcR4 molecule around the x- and y-axes with the angle R and β, respectively, the resulting molecular orientation exhibits an inclination angle of the molecule plane with respect to the substrate plane (Figure 2). By averaging the Raman tensor components obtained by rotation around the x-, y-, and z-axes, the dependence of the Iii/Iij ratio for each symmetry type of vibrations on the angle R and β (i.e., molecular orientation) may be deduced. Therefore, regarding the rotation around the z-axis, an averaging in the range from 0 to π/2 has to be carried out.
Figure 2. Designation of molecular axes of the NiPcR4 molecule and scheme of its suggested orientation with respect to the substrate surface.
The expressions for Iii or Iij have the following overall form (eqs 2 and 3):
Iii )
2 · π
∫0π/2 fii2(R, β, γ) dγ
(2)
Iij )
2 · π
∫0π/2 fij2(R, β, γ) dγ
(3)
where fii(R, β, γ) and fij(R, β, γ) denote the Raman tensor components (ii and ij) of a specific Raman mode obtained by their rotation around the x-, y-, and z-axes on R, β, and γ, respectively. Since the expressions for f(R, β, γ) and, consequently, for Iii/ Iij are rather complex, they are not presented in this work. Knowing the experimental ratios of intensities Iii/Iij for the A1g, B1g, and B2g modes and using eqs 2 and 3, the R and β angles may be derived. 3.2. Films Deposited on One Substrate. Spectroscopic ellipsometry was used for the characterization of thickness, refractive index (n), and extinction coefficient (k) of the NiPcR4 filmsdepositedonsiliconsubstrates.UsingtheLevenberg-Marquardt multivariate regression algorithm, the measured ellipsometric data were fitted to the Cauchy model.44 Variation of the refractive index and extinction coefficient of NiPcR4 films deposited at 2000 rpm with incident photon wavelength is shown in Figure 3. The thickness of the films of NiPcR4-1 and NiPcR4-2 was 85 and 93 nm, correspondingly. The UV-vis spectra of the NiPcR4-1 and NiPcR4-2 films on quartz and their solutions in chloroform are shown in Figure 4. The characteristic absorption peaks observed in NiPcR4-1 and NiPcR4-2 solutions at approximately 675 and 681 nm have been assigned to π-π* transition and show that monomers dominate in chloroform solutions. Q-bands in the film spectra are broadened, and the maxima are blue-shifted to 604 and 622 nm for the NiPcR4-1 and NiPcR4-2 films, respectively. According to the molecular exciton theory,45 the blue shift points to stacking of the phthalocyanine molecules. The Raman spectra of NiPcR4-1 and NiPcR4-2 films deposited on quartz substrates in parallel (ii) and cross (ij) polarizations are shown in Figures 5a and 6a, respectively. The intensities of the strongest lines with known symmetry types are measured. It has already been shown that there are no intensive bands belonging to organic substituents in the range from 300 to 1650 cm-1 in Raman spectra of substituted phthalocyanines due to
19254
J. Phys. Chem. C, Vol. 113, No. 44, 2009
Figure 3. Extinction coefficients (k) and refractive indexes (n) as a function of wavelength of the NiPcR4-1 (solid line) and NiPcR4-2 (dotted line) films deposited on silicon substrate.
the resonance character of the Raman spectra excited by the lasers of the visible region.39 Thus, all bands in the Raman spectrum belong to the bending and stretching vibrations of the phthalocyanine macrocycle. The determination of the symmetry types of all observed modes was made on the basis of the polarized spectra of NiPcR4 solutions in CHCl3 and by analogy with Raman spectra of metal phthalocyanines with other substituents.40,46 The symmetry types of the most intensive bands used for the determination of film orientation are indicated in Figures 5 and 6. The average values of Iii/Iij ratios for each symmetry type of vibrations are listed in Table 2. The R and β angles were calculated to be 90 and 5°, respectively. Similar angles were obtained for the films of both phthalocyanines deposited on silicon and KBr substrates (Table 2). The molecules in the film have the same inclination angles, but they are azimuthally disordered. For confirmation of the results, the films deposited on KBr surfaces were studied by the method of IR spectroscopy. A comparison of the relative intensities of in-plane and out-ofplane modes in the IR spectrum of KBr pellet and films on KBr points out the molecular organization and preferential orientation of molecules in the film. In KBr pellet, there exists a random distribution of phthalocyanine molecules with no preferred orientation. Only transition dipoles parallel to the substrate surface are observed in a normal incidence transmission spectrum. The IR spectra of NiPcR4 derivatives are very complicated because of overlap of the modes belonging to the phthalocyanine ring and substituents; however, some bands can be good references for investigation of the film orientation. The significant difference is observed in the spectra of the pellet, and the film is C-H out-of-plane deformation modes (Figures 7a and 8a) at 744 (for NiPcR4-1) and 748 (for NiPcR4-2) cm-1. Their intensities increase in the film spectra (Figures 7b and 8b); meanwhile, in-plane pyrrole stretching modes (∼1460 cm-1) are less prominent. These features, where the out-of-plane modes in the transmission spectrum are clearly enhanced, correlate with an average preferential perpendicular orientation of the NiPcR4 molecules in the films deposited on KBr slides. Similar results were obtained for the films deposited on silicon substrates. 3.3. Films Deposited between Two Identical Substrates. The thickness of NiPcR4-1 and NiPcR4-2 films deposited between two substrates was in the range 0.3-3 µm. The optical microscopy images recorded between crossed polarizers for the
Basova et al. film of NiPcR4-2 formed between two quartz plates after annealing are presented in Figure 9. The aligned film does not exhibit any birefringence between cross-polarizers during polarized optical microscopy (Figure 9b). The lack of birefringence is a characteristic of the hexagonal homeotropic phase, which has a face-on arrangement of discs, as illustrated schematically in Figure 9c. With the optical axis oriented parallel to the incident light beam from the microscope, a black image is observed from polarizing optical microscopy, as the refractive index of the sample is uniform in all directions.15,47 In order to distinguish between the homeotropically aligned columnar and isotropic phases, an additional study using Raman spectroscopy has been carried out. The Raman spectra of NiPcR4-1 and NiPcR4-2 films deposited between two quartz substrates in parallel (ii) and cross (ij) polarizations are shown in Figures 5b and 6b, respectively. Similar spectra were also observed for the films deposited between KBr slides. The average values of Iii/Iij ratios for each symmetry type of vibrations and the calculated R and β angles are listed in Table 2. The inclination angles were approximately zero. The same angles were also obtained for the films of both phthalocyanines deposited between KBr substrates. The IR spectra of NiPcR4 films deposited between two KBr slides are presented in Figures 7c and 8c. The intensities of out-of-plane CH vibrations suffer a significant reduction in the film spectrum; meanwhile, in-plane pyrrole stretching modes (∼1460 cm-1) become really prominent. These features, where the in-plane modes in the transmission spectrum are clearly enhanced, correlate with an average preferential flat-on orientation of the NiPcR4 molecules in the films deposited between two KBr slides. Thus, a planar alignment with a random distribution of the columnar director in the plane parallel to the air interface has been observed for films of both NiPcR4-1 and NiPcR4-2 derivatives deposited on KBr, Si, and quartz surfaces. Films with homeotropic alignment however are found to form between two identical substrates (quartz and KBr). 3.4. Films Obtained after the Separation of the Two Substrates. In previous sections, we have described the alignment of thick films (0.3-3 µm) confined between two substrates and thin films (20 >20 3.4 3.6
3.4 3.6 1.0 1.0 3.4 3.5
1.2 1.4 1.0 1.0 1.2 1.3
90 85 ∼0 ∼0 90 85
5 8 ∼0 ∼0 5 6
first to 80 °C at a rate of 10 °C min-1 and then from 80 °C to room temperature at a rate of 1 °C min-1. The full annealing time of the NiPcR4-1 films was therefore estimated to be more than 1 h. This regime of annealing was selected in order to obtain the thermodynamically stable films which will keep their orientation at room temperature for a few days. In the current work, it is also demonstrated that films of both compounds confined between two substrates are characterized by homeotropic alignment, while no homeotropic alignment could be obtained for the films deposited on quartz, silicon, and KBr substrates with an air interface. However, some examples of successful homeotropic alignment of LC phthalocyanines on one substrate have been described in the literature.22,23,49,50 From all of the above cases, it can be inferred that the main driving force for the face-on orientation is the specific interaction between the disk and the surface.22,49 In ref 49, glass slides were treated by benzocyclobutene-based cross-linked polymer to
Figure 7. IR transmission spectra of NiPcR4-1 pellet in KBr (a) and films deposited on KBr substrate (b) and between two KBr substrates (c).
Figure 8. IR transmission spectra of NiPcR4-2 pellet in KBr (a) and films deposited on KBr substrate (b) and between two KBr substrates (c).
produce a hydrophilic surface. For such a surface, H-bonding between oligo(ethyleneoxy) substituents of the phthalocyanine molecule and residual hydroxy groups induces the face-on arrangement of the first layer of molecules. In the absence of such interactions on the hydrophobic surface of the cross-linked polymer, the edge-on orientation of phthalocyanine molecules is preferred on the surface of one substrate as in the case of the NiPcR4 films studied in the present work. NiPcR4-1 and NiPcR4-2 derivatives have different side chains (Figure 1); however, the interaction with the surface of quartz and oxidized
Figure 9. Optical (a) and polarizing optical microscopy (b) images with cross-polarizers of a homeotropically aligned film of NiPcR4-2 deposited between two quartz substrates; schematic illustration of the macroscopic alignment with respect to incident light (arrow) (c). The sample is placed perpendicular to the incident light beam.
Figure 10. Polarizing optical microscopy images with cross-polarizers of the NiPcR4-2 film confined between two quartz slides after the separation of the two substrates. The sample is placed perpendicular to the incident light beam.
Orientation of Nickel Phthalocyanine Films Si does not appear to be strong enough to induce a face-on arrangement of phthalocyanine molecules. Homeotropic alignment between two solid substrates has been reported for different discotic molecules and is independent of the aromatic core size, film thickness, and substrate material.14,15,51 In our case, the orientation is also independent of the type of substituents in the phthalocyanine ring. The introduction of a sulfur bridge and addition of more oxygen atoms to the substituents in the NiPcR4-2 derivative (Figure 1) leads to the increase of the isotropization temperature due to intermolecular hydrogen bonding between substituents (stronger interaction between substituents of the neighboring molecules in the stacks); however, it does not lead to change of the film alignment on the investigated substrates. Comparison of the results obtained here for the phthalocyanines with two different substituents with the data presented in previous publications for phthalocyanines with R ) -SC6H13, -SC12H25,40 R ) -OCH2CH(C12H25)(C10H21),15 and R ) -OPh(CnO)219 allows us to confirm the conclusion that homeotropic alignment of the films between two solid substrates and planar alignment of the films on one substrate is independent of the type of substituents on the phthalocyanine ring if there is no specific interaction between the phthalocyanine and the substrate. Conclusions Polarized Raman spectroscopy was used to study the preferential orientation of the molecules relative to the substrate surface for the films of NiPcR4 derivatives with different substituents deposited both on one substrate (Si, quartz, KBr) and the films confined between two identical substrates (quartz, KBr). It has been shown that the films of both compounds confined between two substrates are characterized by homeotropic alignment, while no homeotropic alignments could be obtained on Si, quartz, and KBr substrates with an air interface. Instead, a planar alignment with a random distribution of the columnar director in the plane parallel to the air interface has been observed for films of both NiPcR4-1 and NiPcR4-2 derivatives deposited on KBr, Si, and quartz surfaces. The orientation is also independent of the type of substituents on the phthalocyanine ring. Acknowledgment. This work was financed by RFBRTUBITAK collaborative project N 09-03-91219-CT_a and NATO Collaborative Linkage Grant N CBP.NR.NRCLG.983171. References and Notes (1) Iino, H.; Hanna, J.; Bushby, R. J.; Movaghar, B.; Whitaker, B. J.; Cook, M. Appl. Phys. Lett. 2005, 87, 132102-1. (2) Deibel, C.; Janssen, D.; Heremans, P.; De Cupere, V.; Geerts, Y.; Benkhedir, M. L.; Adriaenssens, G. J. Org. Electron. 2006, 7, 495. (3) van de Craats, A. M.; Stutzmann, N.; Bunk, O.; Nielsen, M. M.; Watson, M.; Mu¨llen, K.; Chanzy, H. D.; Sirringhaus, H.; Friend, R. H. AdV. Mater. 2003, 15, 495. (4) Fujikake, H.; Murashige, T.; Sugibayashi, M.; Ohta, K. Appl. Phys. Lett. 2004, 85, 3474. (5) Kumar, S. Chem. Soc. ReV. 2006, 35, 83. (6) Laschat, S.; Baro, A.; Steinke, N.; Giesselmann, F.; Ha¨gele, C.; Scalia, G.; Judele, R.; Kapatsina, E.; Sauer, S.; Schreivogel, A.; Tosoni, M. Angew. Chem., Int. Ed. 2007, 46, 4832. (7) Piechocki, C.; Simon, J.; Skoulios, A.; Guillon, D.; Weber, P. J. Am. Chem. Soc. 1982, 104, 5245. (8) Engel, M. K.; Bassoul, P.; Bosio, L.; Lehmann, H.; Hanack, M.; Simon, J. Liq. Cryst. 1993, 15, 709. (9) Hanack, M.; Beck, A.; Lehmann, H. Synthesis 1987, 703. (10) Cho, I.; Lim, Y. Mol. Cryst. Liq. Cryst. 1988, 154, 9. (11) van der Pol, J. F.; Neeleman, E.; Zwikker, J. W.; Nolte, R. J. M.; Drenth, W.; Aerts, J.; Visser, R.; Picken, S. J. Liq. Cryst. 1989, 6, 577. (12) Ford, W. T.; Sumner, L.; Zhu, W.; Chang, Y. H.; Um, P.-J.; Choi, K. H.; Heiney, P. A.; Maliszewskyj, N. C. New J. Chem. 1994, 18, 495.
J. Phys. Chem. C, Vol. 113, No. 44, 2009 19257 (13) Smolenyak, P.; Peterson, R.; Nebesny, K.; To¨rker, M.; O’Brien, D. F.; Armstrong, N. R. J. Am. Chem. Soc. 1999, 121, 8628. (14) Sergeyev, S.; Pisula, W.; Geerts, Y. H. Chem. Soc. ReV. 2007, 36, 1902. (15) de Cupere, V.; Tant, J.; Viville, P.; Lazzaroni, R.; Osikowicz, W.; Salaneck, W.; Geerts, Y. H. Langmuir 2006, 22, 7798. (16) Grelet, E.; Bock, H. Europhys. Lett. 2006, 73, 712. (17) Mouthuy, P.-O.; Melinte, S.; Geerts, Y. H.; Nysten, B.; Jonas, A. M. Small 2008, 4, 728. (18) Mouthuy, P.-O.; Melinte, S.; Geerts, Y. H.; Jonas, A. M. Nano Lett. 2007, 7, 2627. (19) Hatsusaka, K.; Ohta, K.; Yamamoto, I.; Shirai, H. J. Mater. Chem. 2001, 11, 423. (20) Tant, J.; Geerts, Y. H.; Lehmann, M.; De Cupere, V.; Zucchi, G.; Laursen, B. W.; Bjørnholm, T.; Lemaur, V.; Marcq, V.; Burquel, A.; Hennebicq, E.; Gardebien, F.; Viville, P.; Beljonne, D.; Lazzaroni, R.; Cornil, J. J. Phys. Chem. B 2005, 109, 20315. (21) Sato, M.; Takeuchi, A.; Yamada, T.; Hoshi, H.; Ishikawa, K.; Mori, T.; Takezoe, H. Phys. ReV. E 1997, 56, R6264. (22) Hoogboom, J.; Garcia, P. M. L.; Otten, M. B. J.; Elemans, J. A. A. W.; Sly, J.; Lazarenko, S. V.; Rasing, T.; Rowan, A. E.; Nolte, R. J. M. J. Am. Chem. Soc. 2005, 127, 11047. (23) Basova, T.; Jushina, I.; Gu¨rek, A. G.; Atilla, D.; Ahsen, V. Dyes Pigm. 2009, 80, 67. (24) Zimmermann, S.; Wendorff, J. H.; Weder, C. Chem. Mater. 2002, 14, 2218. (25) Bunk, O.; Nielsen, M. M.; Solling, T. I.; van de Craats, A. M.; Stutzmann, N. J. Am. Chem. Soc. 2003, 125, 2252. (26) Pisula, W.; Tomovic, Z.; Stepputat, M.; Kolb, U.; Pakula, T.; Mu¨llen, K. Chem. Mater. 2005, 17, 2641. (27) Breiby, D. W.; Hansteen, F.; Pisula, W.; Bunk, O.; Kolb, U.; Andreasen, J. W.; Mu¨llen, K.; Nielsen, M. M. J. Phys. Chem. B 2005, 109, 22319. (28) Breiby, D. W.; Bunk, O.; Pisula, W.; Solling, T. I.; Tracz, A.; Pakula, T.; Mu¨llen, K.; Nielsen, M. M. J. Am. Chem. Soc. 2005, 127, 11288. (29) Anokhin, D. V.; Rosenthal, M.; Makowski, T.; Tracz, A.; Bras, W.; Kvashnina, K.; Ivanov, D. A. Thin Solid Films 2008, 517, 982. (30) Tracz, A.; Makowski, T.; Masirek, S.; Pisula, W.; Geerts, Y. H. Nanotechnology 2007, 18, 485303. (31) Lee, J.-H.; Kim, H.-S.; Pate, B. D.; Choi, S.-M. Physica B 2006, 385-386, 798. (32) Wu, J.; Watson, M. D.; Zhang, L.; Wang, Z. H.; Mu¨llen, K. J. Am. Chem. Soc. 2004, 126, 177. (33) Lehmann, M.; Kestemont, G.; Aspe, R. G.; Buess-Herman, C.; Koch, M. H. J.; Debije, M. G.; Piris, J.; de Haas, M. P.; Warman, J. M.; Watson, M. D.; Lemaur, V.; Cornil, J.; Geerts, Y. H.; Gearba, R.; Ivanov, D. A. Chem.sEur. J. 2005, 11, 3349. (34) Lehmann, M.; Fischbach, I.; Spiess, H. W.; Meier, H. J. Am. Chem. Soc. 2004, 126, 772. (35) Fischbach, I.; Ebert, F.; Spiess, H. W.; Schnell, I. ChemPhysChem 2004, 5, 895. (36) Durmus¸, M.; Lebrun, C.; Ahsen, V. J. Porphyrins Phthalocyanines 2004, 8, 11756. (37) Gürek, A. G.; Durmus¸, M.; Ahsen, V. New J. Chem. 2004, 28, 693. (38) Basova, T. V.; Kolesov, B. A. Thin Solid Films 1998, 325, 140. (39) Basova, T. V.; Kolesov, B. A.; Gu¨rek, A. G.; Ahsen, V. Thin Solid Films 2001, 385, 246. (40) Basova, T.; Gu¨rek, A. G.; Ahsen, V. Mater. Sci. Eng., C 2002, 22, 99. (41) Jennings, C.; Aroca, R.; Hor, A.-M.; Loutfy, R. O. J. Raman Spectrosc. 1984, 15, 34. (42) Aroca, R.; Zeng, Z. Q.; Mink, J. J. Phys. Chem. Solids 1990, 51, 135. (43) Tourrell, G. In Practical Raman Spectroscopy; Gardiner, D. J., Graves, P. R., Bowley, H. J., Gerrard, D. L., Louden, J. D., Turrel, G., Eds.; Springer-Verlag: Berlin, 1989. (44) Arwin, H.; Aspnes, D. E. Thin Solid Films 1986, 138, 195. (45) Kasha, M.; Rawls, H. R.; El-Bayoumi, A. Pure Appl. Chem. 1965, 11, 371. (46) Basova, T.; Kol’tsov, E.; Gu¨rek, A. G.; Atilla, D.; Ahsen, V.; Hassan, A. Mater. Sci. Eng., C 2008, 28, 303. (47) Dierking, I. Texture of Liquid Crystal; Wiley-VCH: Weinheim, Germany, 2003. (48) Pouzet, E.; De Cupere, V.; Heintz, C.; Andreasen, J. W.; Breiby, D. W.; Nielsen, M. M.; Viville, P.; Lazzaroni, R.; Gbabode, G.; Geerts, Y. H. J. Phys. Chem. C 2009, 113, 14398. (49) Sergeyev, S.; Levin, J.; Balandier, J.-Y.; Pouzet, E.; Geerts, Y. H. MendeleeV Commun. 2009, 19, 185. (50) van Nostrum, C. F.; Picken, S. J.; Schouten, A.-J.; Nolte, R. J. M. J. Am. Chem. Soc. 1995, 117, 9957. (51) Einchhorn, H. J. Porphyrins Phthalocyanines 2000, 4, 88.
JP905907N
