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Spectrochimica Acta Part A 70 (2008) 1180–1186

Role of coordinated metal ions on the orientationof phthalocyanine based coatings

Devendra Verma a, Rajalaxmi Dash a, Kalpana S. Katti a,∗,Douglas L. Schulz b, Anthony N. Caruso b

a Department of Civil Engineering, North Dakota State University, United Statesb Center for Nanoscale Science and Engineering, North Dakota State University, United States

Received 22 May 2007; received in revised form 21 October 2007; accepted 31 October 2007

bstract

In the present work, we have investigated the molecular orientation of phthalocyanine films deposited on polycrystalline gold. Three films builtrom the following molecules are investigated: phthalocyanine (H2Pc), cobalt phthalocyanine (CoPc) and copper phthalocyanine (CuPc). The filmsre prepared by spin coating and drop casting methods. Orientation analysis has been performed using polarization dependent Fourier transformnfrared (FTIR) spectroscopy using transmission and grazing angle reflectance mode. The FTIR study suggests that each phthalocyanine filmontains both �- and �-phases. H2Pc based films demonstrate deposition method dependence on the molecular orientation, while the CuPc andoPc films preserve their molecular orientation independent of deposition method. Grazing angle analysis also suggests that CoPc films show

egligible preferred orientation irrespective of film deposition methods. In literature, the band at 878 cm−1 in CuPc has been assigned to out-of-planeending of C–H. Our grazing angle experiments suggest that this band cannot be assigned to out-of-plane bending vibrations of C–H. Accurateand assignments are also described here for the phthalocyanine system.

2007 Elsevier B.V. All rights reserved.

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eywords: Phthalocyanines; FTIR; Polarization dependence; Molecular orient

. Introduction

Thin film organic semiconductors have attracted a significantmount of interest because of their technologically importantptical and electronic properties. Phthalocyanines (Pc) are espe-ially being investigated intensively because of their ease ofreparation, thermal and chemical stability and wide range oflectronic properties [1,2]. Traditionally, phthalocyanines arenown for their pigment properties. Also, phthalocyanine com-ounds have applications in many fields, such as medicine,ptical communication, gas sensors and most importantly theyre involved as stand-alone or composite semiconductor mate-ials [3–6]. The electronic properties of phthalocyanines can beodified by suitable chemical modifications and complex for-

ation with various metals and oxidation states. In recent years,

fforts have been made to synthesize Pc derivatives with alkoxy,lkyl and halogen substituted phthalocyanines [7–10]. The sub-

∗ Corresponding author. Tel.: +1 701 231 9504; fax: +1 701 231 6185.E-mail address: Kalpana.Katti@ndsu.edu (K.S. Katti).

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386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2007.10.050

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titutions significantly affect the orientation of the moleculesnd the electronic properties of the corresponding interface dueo altered molecule–molecule interactions [11] (Fig. 1).

Phthalocyanine films can be prepared by evaporation/ublimation, self-assembly, Langmuir–Blodgett techniques, ory spin coating [11–14]; each of these techniques has its owndvantages and disadvantages. Thin film preparation by spinoating offers a very low-cost production of thin oriented films.ne of most important requirements for thin film preparationy spin coating is the solubility of the material in a suitable sol-ent. Depending on the application of the material, the solubilityay be increased by increasing the constituents at their periph-

ral ring. The morphology of the thin film deposited is roughhen it is slowly deposited and smooth when it is deposited

t higher spinning rates. Significant research has been doneo relate the molecular orientation of the phthalocyanines ofnterest and substituted phthalocyanine films using X-ray pho-

oemission, or polarization dependent UV–vis spectroscopy andeflection–absorption infrared spectroscopy (RAIR) [9,15–18].AIR provides a nondestructive method to study the orientationf oligomers making up thin films. Thick samples on reflecting

D. Verma et al. / Spectrochimica Acta

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ig. 1. Substitution can be made at 1–4, 8–11, 15–18, 22–25 positions withlkoxy, alkyl, halogen moieties and metal ions at the centre.

urfaces are best analyzed by near-normal RAIR but for submi-ron films on reflective surfaces, grazing angle RAIR is the mostppropriate technique.

Several attempts [19–22] have been made to use this tech-ique in order to study the orientation of thin films where theolecular orientation was evaluated. Ogorodnik and Spektrosk

ave studied the orientation of sublimed �-CuPc to yield �-CuPclms [23]. The results show that a completely in-plane orienta-

ion of b-axis parallel or perpendicular to the substrate is possiblen 0.6 �m thick film. Cook has investigated the orientation ofelf-assembled substituted phthalocyanines by comparing nor-al incidence transmission and RAIR spectra and reports that

he molecules are arranged with the cores essentially perpen-icular to the surface [24]. Aroca found that polarization IRpectroscopy in transmission and reflection absorption geome-ry is a powerful technique for accurate and rapid determinationf molecular orientation [25]. They studied orientation for a largeumber of vacuum deposited metal phthalocyanines includinguPc and H Pc. From transmission and RAIR spectra, it was

2

ound that 25–200 nm thick CuPc films were organized in a face-n molecular orientation. Tokito et al. have studied the effect ofubstrates on the orientation of CuPc [26]. Thin films of CuPc

ig. 2. Schematic showing effect of substrate on molecular orientations in films.

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Part A 70 (2008) 1180–1186 1181

n Au, Ag and Cu showed nearly parallel orientation whereashick films were found to be inclined to the plane of the metalubstrate. However, in case of a Cr substrate the molecular planef CuPc was found to be oriented perpendicular to the substratelane.

The present work focuses on the application of transmis-ion and reflection absorption FTIR spectroscopy to studyhe molecular orientations of phthalocyanine (H2Pc), copperhthalocyanine (CuPc) and cobalt phthalocyanine (CoPc) onold substrates. We have synthesized thick films of H2Pc, CoPcnd CuPc using two different techniques: spin coating and dropasting. As shown schematically in Fig. 2, it has been known thathe substrate affects molecular orientation in thin films [27]. Therientation of molecules in the film far from substrate is unaf-ected by the substrate and their orientation is purely governed byntermolecular interactions within the film. Thus, the substrateffect on orientation of molecules, is minimized by synthesizinghick films. We have investigated the preferential molecular ori-ntation of these films with respect to film deposition method.e have also investigated the effect of metallic ions substations

n molecular orientation of phthalocyanines.

. Experimental

Phthalocyanine, copper phthalocyanine and cobalt phthalo-yanine were purchased from Sigma–Aldrich (USA). Phthalo-yanine and metal phthalocyanine films were prepared on goldubstrates by spin coating and drop casting methods. The sur-ace of the substrates was cleaned using a liquid nitrogen jeto remove the dust particles; substrates and samples were storednside a desiccator. Tetrahydrofuran (THF) was used for makinghe solution of phthalocyanines. This solvent is commonly usedolvent for phthalocyanine compounds. Further, 0.01 g of thehthalocyanine was dissolved in 10 ml of the solvent. Further-ore, drops of the solution were allowed to fall on the rotating

ubstrate from a pipette placed closer to the substrate. The sub-trate was accelerated to a speed of 1000 rpm for 40 s. Copperhthalocyanine and cobalt phthalocyanine coatings were alsoade using the same procedure. We have also prepared the coat-

ng by drying out the solution of the respective phthalocyanineith THF on the gold substrates. These coatings were called

drop casting’ films. The concentration of the solution was keptame as spin coated sample.

Fourier transform infrared (FTIR) spectroscopy study wasone in transmission mode using Thermo Nicolet, Nexus 870pectrometer. Transmission spectra (500 scans) of all samplesere collected in the range of 4000–400 cm−1 by placing pow-er between two NaCl windows. For orientation analysis ofhthalocyanine thin films, spectra were collected using Spectra-ech model 500 variable angle accessory, in reflectance mode atrazing angle of 85◦ (Fig. 3). The angle of incidence is selectedepending on the thickness of the coating being studied.

According to the selection rules of RAIR, only those vibra-

ions which generate finite change in dipole moment along theerpendicular of the substrate can interact with the incident light.he signal is enhanced with p-polarized light. The p-polarized

ight implies that the electric vector of the incident light is

1182 D. Verma et al. / Spectrochimica Acta Part A 70 (2008) 1180–1186

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erpendicular to the sample surface and s-polarized light implieshat the electric vector of light is parallel to the sample surface.or determination of molecular orientation, the relative change

n intensity of out-of-plane and in-plane vibrations are comparedith corresponding bands in transmission spectra [15–18].

. Results and discussion

Phthalocyanines exist in different polymorphic forms, amonghem �, � and � are most commonly observed phases. The �-hthalocyanine is the most stable phase and �-phthalocyanine ismetastable phase. The structures of �- and �-phase are shown

n Fig. 4. Several studies have been conducted for identifica-ion of these phases by FTIR spectroscopy [28,29]. Fig. 5 showsTIR spectra of H2Pc, CuPc and CoPc. The experiments wereone in transmission mode. Band assignments have been sum-

stpb

Fig. 4. Schematic showing difference in struc

r grazing angle accessory with polarizer.

arized in Table 1. The band at 1003 cm−1 originates from N–Hending vibration. This band is absent in metal phthalocyanine.his is because in metal phthalocyanine hydrogen is replacedy metal cation. Metal ligand vibration band is observed inoth copper and cobalt phthalocyanine at 900 and 911 cm−1,espectively which is not found in H2Pc. The low metal ligandibrational frequency in CuPc could be attributed to the elec-rostatic repulsion between the outer electron of copper and thelectron pair donated by the ligand [30]. Fig. 6 shows FTIR spec-rum in transmission mode of Pc in the range of 800–650 cm−1.he presence of bands at 712 and 739 cm−1 suggest the pres-nce of �-phase and the presence of bands at 730 and 752 cm−1

uggest that �-phase is also present in H2Pc. The FTIR spec-roscopic analysis of CoPc and CuPc suggest that these metalhthalocyanines contain both �- and �-phases. The presence ofand at 729 cm−1 in CuPc spectrum suggests presence of �-

ture of �- and �-phthalocyanine phases.

D. Verma et al. / Spectrochimica Acta Part A 70 (2008) 1180–1186 1183

Table 1Band assignments for phthalocyanine and metal-phthalocyanines.

Phthalocyanine Copper phthalocyanine Cobalt phthalocyanine Band assignment

1608 1609 1606 C C macrocycle ring deformation1498 1508 1521 C N stretching1457 1462 1456 C–H in-plane bending1437 1420 1424 C–C stretching in isoindole1334 1334 1329 C–C stretching in isoindole1319 N–H deformation1300 N–H deformation1277 1287 1287 C–N stretching in isoindole1159 1166 1162 C–N in-plane bending1118 1122 1117 C–H in-plane bending1092 1089 1087 C–H in-plane deformation

1067 1069 C–H in-plane deformation1003 N–H in-plane bending vibration

900 911 Metal ligand vibration874 878 878 In-plane vibration780 780 781 C–N in-plane stretching vibration765 773 771 C–H out-of-plane deformation730 730 730 C–H out-of-plane deformation

721 C–H out-of-plane deformation

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hase. Some more bands which are signature of �-phase 956,80, 1178, 1100 and 984 cm−1 are also found, which suggestresence of �-phase also in CuPc. The characteristic bands of- and �-phases were observed in CoPc too.

In case of reflectance experiments, the incidence anglesed is 85 ◦C and light is p-polarized. The intensity of bandsn reflectance spectra is orientation dependent, because ineflectance spectrum only those bands appear which have finiteipole moment along perpendicular to the substrate. The trans-ission spectrum of the sample mixed with infrared transparentaterial as dilutant is used as a reference. The intensity in

ransmission spectrum is the average of interactions betweenandomly oriented dipole moment with the electric vector of theight. The observed changes between transmission spectrum and

Fig. 5. Transmission FTIR spectrum of (a) CoPc, (b) CuPc and (c) Pc.

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C–H out-of-plane deformation34 C–C out-of-plane ring deformation

eflectance spectrum can be attributed to molecular orientationn the film. One of the requirements for doing orientation studys the reliable assignment of at least two bands, which have theirransition dipole moment mutually normal to each other. Theommonly used bands for orientation study in phthalocyaninesre 733 cm−1 (C–H out-of-plane deformation) and 13334 cm−1

C C in-plane stretching) [25,31]. The qualitative investigationf molecular orientation of all phthalocyanine films on gold sub-trate was done by comparing the ratio of 733/1334 cm−1 ineflectance spectra (R) with the same of transmission spectra (T).his ratio (R/T) gives the preferential orientation of molecules

n gold substrate. The R/T value closer to 1 excludes the pos-ibility of molecular plane of phthalocyanines to be parallel orerpendicular to the substrate. The R/T value significantly higherhan 1 indicates that the C–H out-of-plane dipole moment has

ig. 6. Transmission FTIR spectrum of Pc in the range of 3360–3220 cm−1

howing bands for �- and �-Pc phases.

1184 D. Verma et al. / Spectrochimica Acta Part A 70 (2008) 1180–1186

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Fig. 9. FTIR spectra of CuPc in (a) transmission and (b) reflectance mode. Thereflectance spectrum is of spin coated thin film over gold substrate.

ig. 7. FTIR spectra of Pc in (a) transmission and (b) reflectance mode. Theeflectance spectrum is of spin coated thin film over gold substrate.

arger component along the electric vector of light (perpendicu-ar to the substrate) which makes molecules to be preferentiallyriented closer to plane of the substrate, whereas R/T lower thanindicates that molecular plane is perpendicular to the plane

f the substrate. Figs. 7–9 show comparisons between trans-ission spectra and reflectance spectra of spin coated films of2Pc, CoPc and CuPc, respectively. Figs. 10–12 show compar-

sons between transmission spectra and reflectance spectra ofrop casted films of H2Pc, CoPc and CuPc, respectively. Theeflectance spectra are for spin coated films. The reflectancend transmission spectra are normalized with respect to bandst 1334 cm−1. Areas under both of these peaks are determinedrom peak fitting using the GramsTM software.

The R/T ratio for spin coated films is shown in Table 2. The/T ratio is around 0.5 in case of H Pc spin coated films, which

2uggests that the H2Pc molecules have their plane oriented closero perpendicular to plane of substrate. On the other hand, in CuPcpin coated films molecules have preferential orientation parallel

ig. 8. FTIR spectra of CoPc in (a) transmission and (b) reflectance mode. Theeflectance spectrum is of spin coated thin film over gold substrate.

Fig. 10. FTIR spectra of Pc in (a) transmission and (b) reflectance mode. Thereflectance spectrum is of drop casted thin film over gold substrate.

Fig. 11. FTIR spectra of CoPc in (a) transmission and (b) reflectance mode. Thereflectance spectrum is of drop casted thin film over gold substrate.

D. Verma et al. / Spectrochimica Acta

Fig. 12. FTIR spectra of CuPc in (a) transmission and (b) reflectance mode. Thereflectance spectrum is of drop casted thin film over gold substrate.

Table 2Comparison of 733/1334 cm−1 ratio of reflectance spectra of spin coated filmswith transmission

Reflectance (R) Transmission (T) R/T

H2Pc 1.48 2.95 0.5CC

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o plane of substrate. The R/T ratio was found to be close to 1n case of CoPc spin coated films, which indicates that CoPc

olecules have no preferential orientation on gold substrate.The R/T ratios for drop casting films over gold substrate are

hown in Table 3. The R/T was found to be around 1.4 in casef H2Pc films, which suggests that in drop casting H2Pc filmsave their molecular plane oriented closer to parallel to planef the gold substrate. In case of CuPc drop casting films, R/Talue is around 1.67, which suggests that CuPc molecules haveimilar preferential orientation as H2Pc drop casting films. The/T ratio was found to be close to 1 in case of CoPc drop castinglms, which indicates that CoPc molecules have no preferentialrientation on gold substrate.

The CoPc showed no preferential orientation in both spinoated films and drop casting films. On the other hand, H2Pcas different orientations on spin coated films and drop castinglms. The molecular orientation of H2Pc in spin coated films

s preferentially perpendicular to the gold substrate, whereas onrop casting films CoPc molecules have their plane preferen-ially oriented parallel to the substrate. The CuPc films showed

able 3omparison of 733/1334 cm−1 ratio of reflectance spectra of drop casting filmsith transmission

Reflectance (R) Transmission (T) R/T

2Pc 1.55 1.13 1.4oPc 0.81 1.03 0.8uPc 2.18 1.30 1.7

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Part A 70 (2008) 1180–1186 1185

o effect of film preparation method on their molecular orienta-ion. In both spin coated and drop casting films, CuPc moleculesre preferentially oriented parallel to the substrate.

The band assignment for CuPc exhibiting D4h symmetryroup has been described in literature [31]. In CuPc, 8 a2u and8 eu infrared active vibrations are observed. The change inipole moment for a2u is along the z direction of molecular planend for eu it is along x–y plane of the molecules. The band atround 731 cm−1 is the characteristic a2u band (out-of-plane–H vibration) with transition dipole moment along C4 axis

25]. In literature, the band at 878 cm−1 has been assigned to out-f-plane bending of C–H [1]. Since 878 cm−1 band is assignedo out-of-plane vibration mode, which is similar to assignmentf 731 cm−1, the change in intensity of both bands should followsimilar trend (i.e. should change according to orientation of theolecules). However, in the reflectance spectrum, the intensity

f 878 cm−1 band is higher as compared to that of the trans-ission spectrum, which is opposite to the trend shown by the

31 cm−1 band. This suggests that the 878 cm−1 band of CuPcay not be attributed to out-of-plane C–H vibrations.

. Conclusions

Thin films of phthalocyanines and metal phthalocyaninesre prepared by spin coating and drop casting method. TheTIR spectroscopy analysis indicates presence of both �- and-phases of phthalocyanines in all the samples. We have uti-

ized grazing angle FTIR spectroscopy to analyze molecularrientations in phthalocyanine films deposited over gold sub-trate. Grazing angle FTIR is a preferred technique to analyzeolecular orientation in thin films. The orientation analysis sug-

ests that H2Pc molecules attained perpendicular and parallelirections with respect to gold substrate in spin coated and dropasting thin film samples. The CoPc samples showed no pref-rential orientation in any of the thin films. The CuPc sampleshowed preferential orientation along parallel direction of theubstrate surface in both spin coated and drop casting thin films.ur results suggest that metal ions play an important role inefining molecular orientation in thin films.

cknowledgements

Authors would like to acknowledge financial support fromhe Defense Microelectronics Activity (DMEA) USA undergreement DMEA90-02-2-0218. Fourier transform infraredpectroscopy equipment used in this research was purchasedith grant (MRI 0320657) from National Science Foundation.

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